Conceptual Plan for Intercepting
Sediment in the Lower Los Osos and
Warden Valleys, Morro Bay Area,
San Luis Obispo County, California
Prepared for:
Morro Bay National Estuary Program
Prepared by:
Barry Hecht
Daniel Malmon
Balance Hydrologics, Inc.
January 2003
TABLE OF CONTENTS
1. INTRODUCTION .................................................................................................................................................1
1.1
PROJECT BACKGROUND ............................................................................................................................1
1.2
STATEMENT OF THE PROBLEM .................................................................................................................1
1.2.1 Relation to the Chorro Flats Enhancement Project ................................................................................3
1.3
ORGANIZATION OF THIS REPORT .............................................................................................................4
1.4
ACKNOWLEDGMENTS................................................................................................................................5
2. ENVIRONMENTAL SETTING ..........................................................................................................................1
2.1
2.2
2.3
2.4
2.5
GEOLOGICAL SETTING AND QUATERNARY HISTORY ...........................................................................1
CLIMATE .....................................................................................................................................................2
SOILS AND LAND USE ................................................................................................................................2
GROUND WATER ........................................................................................................................................2
ANADROMOUS FISHERY ............................................................................................................................3
3. GEOMORPHIC CONTEXT................................................................................................................................5
3.1
SEDIMENTATION IN MORRO BAY ............................................................................................................5
3.2
SEDIMENT SOURCES ..................................................................................................................................5
3.2.1 Critical evaluation of past studies of sediment sources...........................................................................6
3.3
3.4
3.5
3.2.1.1
Soil Conservation Service studies ................................................................................................................... 6
3.2.1.2
Funk River Consultants................................................................................................................................... 6
3.2.1.3
TetraTech modeling ........................................................................................................................................ 7
3.2.1.4
Summary of previous estimates of sediment yield ........................................................................................... 9
ACCELERATED EROSION OVER THE PAST DECADES IN THE LOS OSOS WATERSHED ........................9
DEPOSITION DURING RECENT DECADES ...............................................................................................10
DIRECTIONS FOR SEDIMENT-YIELD ESTIMATION ................................................................................11
4. BASIS FOR DESIGN AND PLANNING..........................................................................................................12
4.1
CONCEPTUAL PLAN FOR PROPOSED FLOODPLAIN RESTORATION.....................................................12
4.2
PHYSICAL BASIS FOR PLANNING ...........................................................................................................14
4.3
RATIONALE FOR MODELING APPROACH ..............................................................................................15
4.4
HYDRAULIC MODELING ..........................................................................................................................18
4.4.1 Modeling approach................................................................................................................................18
4.4.2 Input data for the hydraulic model ........................................................................................................19
4.4.3 Hydraulic model results.........................................................................................................................20
4.4.3.1
Backwater limits from tidewater ................................................................................................................... 21
4.5
SEDIMENTATION CALCULATIONS AND INTERPRETATION..................................................................22
4.5.1 Model assumptions ................................................................................................................................22
4.5.2 Values used in the model .......................................................................................................................23
4.5.3 Results and interpretations of the sedimentation calculations ..............................................................24
5. TOWARD A MANAGEMENT PLAN..............................................................................................................27
5.1
PROGRAM ELEMENTS ..............................................................................................................................27
5.1.1 Cooperator’s preferred alternative .......................................................................................................27
5.1.2 Modified preferred alternative...............................................................................................................27
5.1.3 Possible supplemental future retention..................................................................................................29
5.2
PHASING ....................................................................................................................................................30
5.3
ANTICIPATED EFFECTIVENESS OF INDUCED SEDIMENTATION ..........................................................30
5.4
SEDIMENTATION MEASURES UNDER EXISTING CONDITIONS .............................................................31
5.4.1 Alternatives considered .........................................................................................................................31
5.4.2 Proposed project....................................................................................................................................31
5.4.3 Costs ......................................................................................................................................................32
5.5
SEDIMENTATION ROLE OF THE SITE FOLLOWING ANTICIPATED AVULSION ....................................32
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5.5.1 Problem statement .................................................................................................................................32
5.5.2 Avulsion effects and timing....................................................................................................................33
5.5.3 Preparing for avulsion...........................................................................................................................33
5.6
REQUIRED PERMITS .................................................................................................................................34
6. RECOMMENDED MONITORING..................................................................................................................36
7. CONCLUSIONS AND RECOMMENDATIONS ............................................................................................39
8. REFERENCES ....................................................................................................................................................46
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LIST OF TABLES
Table 1.
Summary of previous estimates of sediment yield from Los Osos Creek
Table 2.
Computed settling velocities for different particle size classes
Table 3.
Summary of results from HEC-RAS simulation
Table 4.
Estimated trap efficiency of the proposed floodplain
Table 5.
Floodplain sedimentation rates for high flow events
Table 6
Variable affecting sedimentation in the Warden Creek overbank area under
existing and combined flows
LIST OF FIGURES
Figure 1.
Location of Morro Bay on California Coast
Figure 2.
Large landslide scar in Los Osos Creek watershed
Figure 3.
Recent streambank erosion along Los Osos Creek
Figure 4.
Location map near the confluence of Los Osos Creek and Warden Creeks
Figure 5.
Cross-sectional topography of the valley floor
Figure 6.
Stream bed profiles and modeled water surface profile under project conditions
Figure 7.
Cross sections and modeled flow depths on Eto property, under existing and
project conditions
APPENDICES
A.
Observations and recollections of Alan Eto, lower Los Osos and Warden Valleys
B.
Memorandum of Notes from Los Osos Creek Management Plan 1/17/02 meeting –
A preferred alternative is found
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1. INTRODUCTION
1.1
Project Background
Sedimentation in Morro Bay threatens ecological, aesthetic, and economic resources in one of
the most significant estuaries along the central California coast. Studies funded by the State
Coastal Conservancy and the Coastal San Luis Resource Conservation District (“RCD”) over the
past 15 years have documented the sources and impacts of sediment delivery from the Morro
Bay watershed, and prescribed several potential measures for reducing sedimentation in the
Bay. Some of these measures have been adopted and implemented within the framework of a
Morro Bay Watershed Enhancement Project.
Among the proposed measures was to intercept as much sediment as might prove realistic in
the lowermost segments of the lower Los Osos Creek and Warden Creek valleys, while
concurrently allowing riparian vegetation to re-establish on a valley floor which is partly
converted to agricultural uses and is otherwise disrupted by other human uses. In part to
provide space for sediment storage in the valley floor, wetland reserve easements were
purchased along lower Los Osos Creek in 1995. This report describes pertinent geomorphic and
hydrologic background and presents preliminary analyses recommendations relevant to
realizing sedimentation in the wetland reserve easements to help fulfill this element of the
Morro Bay Watershed Enhancement Project.
1.2
Statement of the Problem
Morro Bay is a tidal lagoon located at the mouths of Chorro and Los Osos Creeks, in central
coastal California (Figure 1). Sediment is delivered primarily from the two major watersheds
discharging into Morro Bay – Chorro Creek (43 square miles) and Los Osos Creek (23 square
miles). The Bay is a local center of tectonic subsidence within an area which is generally
uplifting rapidly, making it a natural center of sediment deposition. The Chorro and Los Osos
watersheds are also underlain by mechanically weak rocks and are subjected to high intensity
storm rainfall, factors which contribute to naturally high rates of sediment production.
Furthermore, because the coastline is close to the mountain front, there is limited opportunity
for sediment storage within low-gradient valley floors. In addition, littoral (tidal and waverelated) and aeolian (wind-related) processes both transport sediment into Morro Bay (Haltiner,
1988). All these factors contribute to natural sedimentation in Morro Bay, even in the absence of
human intervention.
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Sedimentation in Morro Bay has accelerated during the past 150 years as a result to human use
of the land and the practices used in these watersheds. The Morro Bay Watershed
Enhancement Project was initiated in order to reduce the rate of sedimentation in Morro Bay.
From a geomorphological perspective, there are four possible groups of approaches to this
problem:
1. Reduce sediment eroded from hillslopes by using best management practices and
erosion-control measures.
2. Reduce sediment entrained directly into the stream from its bed and banks by
stabilizing the channels, inhibiting bank retreat and bed incision by various means.
3. Reduce the proportion of sediment transported in the streams which ultimately
reaches Morro Bay. Pragmatically, this calls for a change in the present sediment
balance on the valley floors towards deposition on beds of the channel or on the
valley floor.
4. Remove sediment from Morro Bay by dredging and dispersal to other upland
settings.
Of these four general strategies, the fourth approach is generally considered to be the most
expensive and ecologically destructive. The most beneficial approach to the problem is to retain
sediment within the watersheds of Chorro and Los Osos Creeks by employing the first two
approaches. While the first approach may provide significant benefits, it is beyond the scope of
the present study, although it is (in part) the subject of a concurrent investigation by others with
results not yet available. The Coastal Conservancy, RCD, and others have implemented, are
implementing, and will continue to implement projects that employ the first two approaches.
The focus of this project is to induce sediment storage in Los Osos Creek by restoring the role of
the floodplain as a site for storing sediment produced further up in the watershed. The site
chosen for floodplain restoration is the area within and adjacent to the Los Osos Creek Wetland
Reserve. This reserve includes 112 acres along the lowland valley of Los Osos and Warden
Creeks, extending upstream from a lower boundary approximately one-quarter mile upstream
of the head of tidewater at San Ysidro Avenue. Additional easements are expected to be
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negotiated as part of the present study. The goal of inducing floodplain sedimentation must be
accomplished within constraints imosed by economic, ecological, and regulatory
considerations, as well as topographic constraints, which are further elaborated below.
1.2.1 Relation to the Chorro Flats Enhancement Project
The Chorro Flats Enhancement Project (Central San Luis Resource Conservation District, 2000)
restored 83 acres of floodplain along lower Chorro Creek. The primary purpose of the project
was to reduce sediment loads entering Morro Bay by restoring the function of a portion of the
floodplain to trap sediment as a result of deposition during overbank flows. This was
accomplished by removing levees along the project site and allowing flows to spread across the
floodplain. Compared with the channelized flow, the shallower flow depth over the floodplain
(due to wider flows) and greater flow resistance (as a result of floodplain vegetation) promoted
floodplain sedimentation.
Deposition at Chorro Flats reduced the impact of several watershed perturbations on
sedimentation in Morro Bay. Since the acquisition of the Chorro Flats site in 1991, sediment
production rates in the Chorro Creek watershed temporarily increased as a result of: (1) the
1994 Highway 41 Fire and related erosion; (2) abnormally large floods during the post-fire
winter of 1994-95; and (3) floods during the El Nino winter of 1997-98. Between 1992 and 2000,
an estimated 188,000 cubic yards of sediment were deposited on the site (CSLRCD, 2000).
Developing a plan for depositing sediment within the valley of Los Osos and Warden Creeks
was recommended in the Morro Bay Watershed Enhancement Plan (SCS, 1989), and the success
of the Chorro Flats project has further encouraged the current investigation into the practicality
and usefulness of adopting a similar strategy in the lower portion of Los Osos Creek. The
George Martines property adjacent to Los Osos Creek, for which wetland easements have
already been partially obtained, is being given initial consideration for the purpose of reestablishing sediment storage in the floodplain.1 The proposed project area (Figure 4) is located
in the former floodplain, therefore the site is more likely to trap sediment than the valley floor
in narrower, steeper, upland reaches. It was the stated intent of the Morro Bay Watershed
Enhancement Project staff to draw heavily on the results of the Chorro Flats work. The Los
Osos restoration plan was budgeted at less than 10 percent of the planning effort applied to
We have learned during the week prior to preparation of the final report that the Coastal Conservancy
has not been successful in obtaining further easements, and that the focus of this project will be limited to
the Eto and Gota holdings; we have attempted to modify our findings and recommendations
accordingly.
1
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Chorro Creek, and with a proportionately shortened schedule. As it turns out, the lower Los
Osos/Warden valley is quite different both geomorphologically and with respect to current
land uses. Differences between Los Osos Creek and Chorro Creek include:
1.3
ƒ
Nearly all of the recent sedimentation in the lower Los Osos/Warden valley
originates from Los Osos Creek. Los Osos Creek has essentially developed a fan
extending across the valley, which is responsible for impounding Warden Lake. As
a result, the valley floor is tilted northward, such that any overflows from Los Osos
Creek move rapidly toward Warden Creek. Measures to store sediment within Los
Osos Creek increase the likelihood of a sudden change in stream course; measures to
store sediment overbank on the valley flat channel and floodplain in Los Osos using
virtually any natural process increase the gradient toward Warden Creek, and
aggravate the ‘tilted valley’ issue.
ƒ
Los Osos Creek is much steeper than Chorro Creek.
ƒ
While Chorro Creek is likely to remain more or less in the same location for the
geomorphically-foreseeable future, Los Osos may change course, a factor which we
believe must be considered in long-term planning for this corridor.
ƒ
Minimal space for storage of sediment is available upstream in the Los Osos Creek
watershed compared with that of Chorro Creek.
ƒ
Parts of the Los Osos Creek watershed are underlain by softer and more erosive
lithologic units than those in the Chorro Creek watershed, leading to greater
production of sand- and silt-sized sediment.
Organization of this Report
The hydrologic and environmental setting of the lower Los Osos/Warden valley is described in
Section 2. Chapter 3 discusses the information available, and its limitations. Alternatives
considered and our efforts to simulate peak flows under varying recurrences and conditions
are described in Chapter 4. The plan elements, phasing, and one recommended alternative are
discussed in Chapter 5. Chapter 6 outlines a monitoring program which appears needed to
guide long-term management of the project, given the unique conditions at the site. Our
conclusions and recommendations are summarized in Chapter 7, with the references cited and
other sources of information presented in Chapter 8.
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The appendices present (a) our notes of observations and recollections of Alan Eto, a thirdgeneration local grower, who is the primary operator and owner of agricultural land in this part
of the valley, and (b) output from the model runs. 2
1.4
Acknowledgments
In addition to project manager Bill Hoffman, Balance staff wish to express their gratitude to
Malcolm McEwen, Coastal San Luis RCD, and Tim Duff of the California Coastal Commission,
who were both central to developing and carrying through this innovative project. Malcolm’s
role in formulating, implementing, and developing the initial applications for funds for the
project were central. Linda Chipping, RCD board member, assisted at several stages, with field
measurements, first-hand accounts, crucial information not available to us from other sources,
as well as perspective. Landowners Alan Eto and Marla Morrissey provided essential
information, guidance, and suggestions at every step in plan formation, and went far beyond
the basic exchanges that we often encounter in restoration planning. Marla helped re-shape the
plan by showing us sites further upstream where channel-stability and sediment-source issues
will prevail for some time. Alan’s accounts of conditions during the past 50 years are reflected
in almost every aspect of our description of the project’s setting, and in formulation of the
alternatives discussed below. Margie Lundquist (USDA Natural Resources Conservation
Service) has helped the enhancement effort through numerous earlier stages and kept us filled
with background information. Balance staffers Don Song and Ed Ballman also contributed in
this study. Ed Wallace, who is leading Northwest Hydraulics’ assessment of erosion-control
and sediment-reduction opportunities, shared ideas regarding the sources and likely near-term
delivery of sediment from the upper, montane portion of the watershed (Clark Canyon) and
discussed the importance of sediment derived from the massive bank cuts upstream of Los
Osos Blvd.
To conserve paper and shelf space, the voluminous model runs are not included with most copies of
this report, but are available for inspection at the CSLRCD, and Coastal Conservancy offices.
2
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2. ENVIRONMENTAL SETTING
2.1
Geological Setting and Quaternary History
Morro Bay is a shallow lagoon located on the central California coast (Figure 1), about 100 miles
north of Santa Barbara. Two major drainages discharge water and sediment to Morro Bay –
Chorro Creek (43 mi2 drainage area) and Los Osos Creek (23 mi2). The two drainage basins are
separated by uplands consisting in part of a string of volcanic stocks and necks, the
Oligocene/Miocene Morros formation, which include the landmark Morro Rock. These
resistant plugs are the erosional remnants of dacite lavas that intruded rocks of the Mesozoic
Franciscan Complex, which underlie most of the Chorro Creek watershed. The Franciscan
contains a variety of variably-erodible volcanic, metamorphic, and sedimentary rocks, generally
more resistant than many of the younger sediments in mountains to the south but substantially
more erodible than the volcanic stocks and necks of the Morros formation.
Morro Bay is located within a subsiding structural trough extending southeastward through the
Los Osos Valley. The Los Osos Valley is fundamentally a tectonic ‘pull-apart basin’ bounded on
the south by the Los Osos Fault. South of the Los Osos Valley is the San Luis Range, an
actively-rising northwest-trending range underlain by durable Franciscan metasediments and
deformed younger and softer sedimentary rocks. Both the steeply-dipping Pismo (Pliocene)
and the Monterey formations are blocky siltsones and diatomaceous shales, susceptible to large,
deep-seated landslides (c.f., Nitchmann, 1988; see also, Figure 2). Upstream of the mountain
front, Los Osos Creek follows a course parallel with the structural axis of the San Luis Range,
into which it has eroded Clark Valley.
The trough which forms Los Osos Valley and Morro Bay is filled at its coastal edge by a sheet of
dune sands up to 300 feet deep, known as the Baywood formation, deposited mainly during the
past 5000 years, usually overlying Quaternary alluvial deposits eroded from the adjoining
uplands. As it emerges from the San Luis Range, Los Osos Creek cuts into the eastern edge of
the dune sheet, which recurrently collapses in sandy slabs up to several dozen feet high into the
creek. Much of this material is re-deposited in the lower Los Osos valley, in or near the project
site (Hall, 1973). No similar sand input occurs into Warden Creek, which is incised into the
alluvial and colluvial sediments of the Los Osos Valley floor, and transports predominantly
very fine grained silts and clays.
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2.2
Climate
Mean annual rainfall in the Los Osos watershed increases from about 16 inches at the mouth to
a maximum of nearly 30 inches near the crest of the San Luis Range. Historically, mean annual
rainfall has fluctuated from about 40 percent of mean during the driest years to about 250
percent of the long-term mean during the wettest winters.
Rainfall during dry years is often
not sufficient to generate runoff that is continuous through the winter months; in some of the
driest years, little or no runoff occurs. Rainfall can range up to 3.5 inches and up to 8 inches in
24 hours during hard rains at the site and in the heart of the San Luis Range, respectively (U.S.
Army Corps of Engineers, 1973). During the wettest years, flows in both Los Osos and Warden
Creeks can persist through the summer..
2.3
Soils and Land Use
Soils on the valley floor at the site generally become progressively finer northward toward
Warden Creek and northwestward toward Morro Bay. The soils along Warden Creek are
derived predominantly from silty clays; a distinct boundary is visible in the aerial photographs
approximately 500 to 800 feet south of Warden Creek separating persistently damp and clayey
soils from the sandier soils which have much better drainage. Soils adjoining Los Osos Creek
are loams or sandy loams derived from sandy overbank deposits. A similar but subtler sand-toclay gradient prevails downstream toward Morro Bay. Sandy soils at the upstream edge of the
study area along Los Osos Creek gradually become more clay-rich in the northwestern half of
the valley. In general, the wetter and heavier soils support a healthy riparian corridor, except
where cleared and/or farmed. Growers prefer the sandy soils, but cultivate some of the heavier
clays along Warden Creek and along the lower reaches of both streams. Tile drains are
generally required to support cultivation of the finer-grained soils. Ditches are used to help
drain most of the fields on the valley floor.
2.4
Ground Water
In the Los Osos Valley floor, ground water is occurs primarily in stringers of coarse sand and
gravel within the alluvium. The stringers may represent former channel deposits or may be
overbank or breakout flood deposits also known as crevasse splays (see discussion of channel
avulsion below). The water table is generally within 6 to 8 feet of the ground surface, probably
two or three feet lower than probably prevailed before the drainage associated with agricultural
production, which moved onto the valley floor during the late 1800s. During wet periods, the
water table rises, then falls after several days of no rain (Alan Eto, pers. comm.).
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Ground water is used to irrigate crops within the valley and on the adjoining terraces
surrounding Eto Lake. Significant production (> 30 to 50 gallons per minute) is limited to a few
areas of sandier aquifer materials, primarily along a line extending along and westward from
the Martines/Eto property lines along the valley axis.3 To identify these areas, early owners
(including Mr. Eto’s grandfather) drilled test holes along cross-valley transects at 50-foot
intervals. Only four of many dozens of test holes on the present-day Eto and Martines parcels
were eventually developed as wells.
Despite the limited extent of areas with easily-developed yields, water for irrigation is pumped
from the valley up to the terraces at the Eto property. The aquifer is pumped daily during the
summer to maintain supplies, but cannot be pumped around the clock without drawing water
levels down in wells between the southern edge of the Eto lands and Los Osos Blvd. Additional
information about the evolving ground-water conditions in this area can be found in Appendix
A, a record of Mr. Eto’s recollections regarding water supply, land-use practices, and channel
management during the past 50 years.
2.5
Anadromous Fishery
Los Osos Creek supports a reliable run of steelhead, a species listed as threatened in this
ecologically significant unit, and listed as endangered in the Santa Maria River watershed and
points further south, beginning only a few tens of miles away. The site is used for passage by
upmigrating adults (most commonly in January and February), and by downmigrating smolts
(typically in April and May). The proposed project must be designed and operated in a manner
which promotes steelhead passage through the site, both upstream toward the headwaters of
Los Osos Creek within the San Luis Range and downstream through Morro Bay to the Pacific
Ocean.
Several times during the 1990s, floodwaters in Los Osos Creek resulted in 4 to 5 feet of sediment
accumulation,filling the creek beyond bankfull depth, and leaving large bars and piles of sandy
debris. The combination of obstructions and a sedimented channel likely impeded or prevented
salmonid movement up- or down-stream during key life stages. The
RCD and landowners have been able to restore a passable channel in the worst-affected reaches
during the following summer, but it is likely that much of the year’s age class was lost, either
We interpret this sandy trend as being a former course of Los Osos Creek, and possibly the primary
location of the natural course of the creek prior to channelization.
3
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due to stranding of adults or (more likely) inability of the downstream-migrating smolts to
reach Morro Bay.
Steelhead are not reported from Warden Creek. It is not known whether steelhead use Eto Lake
as rearing or refuge habitat. California Department of Fish and Game biologist Dave Highland
has questioned whether salmonids can gain access through the existing culvert system (Bill
Hoffman, pers. comm.), but no seining or other sampling for trout has been conducted; if trout
were found in the lake, genetic testing would help establish steelhead linkages, if any.
Freshwater lakes and lagoons near the mouths of coastal streams are increasingly appreciated
as important nursery grounds, especially during droughts. They may also serve as winter
refuge during floods, and may have significant indirect roles.
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3. GEOMORPHIC CONTEXT
3.1
Sedimentation in Morro Bay
Based on bathymetric analyses of the bay, Haltiner (1988) suggested that sedimentation rates in
Morro Bay have increased by an order of magnitude over the past 150 years. While such
estimates may not be well constrained, it is clear that accelerated sedimentation associated with
human-induced (“anthropogenic”) change threatens the long term viability of Morro Bay as a
lagoon and a boat harbor.
At least three anthropogenic factors have likely contributed to accelerated sedimentation in
Morro Bay:
1. Increased rates of watershed erosion (sheet and rill erosion, gullies, and landslides),
as a result of land use and land cover change.
2. Increased rates of stream bank erosion (as documented by Funk River Consultants,
1998, and others), possibly as a result of increased runoff or removal of bankstabilizing vegetation; main-channel and tributary incision with related bank retreat
may be largest single source of sediment retained in the lower valley (see Figure 3).
3. A reduction in the volume of sediment which could be temporarily stored in the
lower valley, as a result of levees constructed in attempt to reclaim floodplains for
agricultural purposes. Much of the portion of the sediment loads of Los Osos Creek
which was previously stored in the floodplain now reaches Morro Bay and
contributes to sedimentation in the Bay.
3.2
Sediment Sources
A quantitative understanding of the sources, amount, and size distribution of sediment
produced within the Los Osos and Warden Creek watersheds is critical to designing
downstream sediment storage programs and assessing their likely longevity and maintenance
requirements. Thus, we have critically reviewed previous studies of sediment sources in the
watershed and summarize them in the context of field observations in this section.
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3.2.1 Critical evaluation of past studies of sediment sources
Several studies aimed at identifying and quantifying sediment production within the Los Osos
Creek watershed have been conducted. We summarize each of the main studies below and
discuss some of the limitations of each.
3.2.1.1 Soil Conservation Service studies
A study commissioned by the Coastal San Luis Resource Conservation district in the late 1980s
(SCS, 1989) implemented standard Soil Conservation Service techniques to assess erosion by
several sources. They estimated an annual erosion rate in Los Osos Creek watershed of 32,000
tons per year (Table H in that report). The SCS staff assumed that the particle-size distribution
of the material eroded was the same as that for the soils in the watershed. Based on arbitrarily
assigned ‘sediment delivery ratios’ (80% for fines, 25% for sands, 5% for gravel), they computed
that approximately 50% of the eroded sediment reached Morro Bay. Of this total, 60% was
estimated to derive from sheet and rill erosion, with streambank erosion contributing an
additional 20% and gully erosion less than 1%. Landslides were assumed to be negligible with
respect to sediment production.
The SCS study estimated that 75 percent of the sediment eroded from the entire Morro Bay
watershed derives from sheet and rill erosion, with 12 percent from streambank erosion, and
less than one percent from gully erosion. Landslides were assumed to be negligible (see p. 9 of
their report). These estimates are not consistent with our field observations in the Los Osos
watershed. We observed little evidence for sheet and rill erosion, but we have seen, on our trips
into the upper watershed, numerous large landslides and actively advancing gullies. Channel
incision and slab failures of the dune sheet where undercut by Los Osos Creek and its
tributaries are important sources. Discussions with land owners and members of the local
community, as well as our initial visit, have also convinced us that large, deep-seated landslides
may contribute significantly to the erosion rate of the study area (c.f., Figure 2). Erosion of
roadbeds or gullies generated from roadways or cleared areas also demonstrably contribute
sediment to the channels.
3.2.1.2 Funk River Consultants
In contrast with the SCS study, Funk River Consultants (1998) produced a report which
emphasized the importance of streambank erosion in the Los Osos Creek watershed. That
report measured several repeated cross sections and longitudinal profiles along Los Osos Valley
and extrapolated measured changes over the length of major creeks and tributaries, to arrive at
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an estimate of the annual streambank erosion rate. They considered the reaches they studied to
be more actively eroding than most in the watershed, and estimated a watershed-wide average
by assuming that the average bank erosion rate was one quarter of the rates they measured.
Still, their estimate of 3,200,000 cubic feet/year equates to more than 150,000 T/yr for
streambank erosion alone (assuming a bulk density of 95 lb/ft3), nearly 10 times the estimate in
the SCS report.
The report by Funk (1998) is based on field data and observations and emphasizes streambank
erosion as a clearly important source of sediment. However, this study focused data collection
on several reaches known to be rapidly eroding and extrapolated those data to the rest of the
watershed. While this point is noted by the authors, the scope of that study was not wide
enough to arrive at an accurate estimate of the amount of streambank erosion in the watershed.
3.2.1.3 TetraTech modeling
TetraTech (1998b) computed sediment delivery from Los Osos and Chorro Creeks for modeled
high magnitude events using a two-component sediment transport model (one for bed material
sediment transport and another for washload). Their event-based calculations of sediment
transport were based on a rainfall-runoff model which produced hypothetical 2-year, 5-year, 10year, 25-year, 50-year, and 100-year flood hydrographs in the Morro Bay watershed. Their
study estimated that an average of 70,000 T/y of sediment are delivered to Morro Bay, with 14
percent (9,600 tons/yr) coming from Los Osos Creek. Of the modeled 9,600 tons reaching
Morro Bay from Los Osos Creek, the contribution from the bed material sediment was
calculated to be only 188 tons, or about 2 percent.4 The remainder was considered to be fine
sediment (presumably silt and clay) derived from surface erosion in the watershed.
Calculations of transport rates of fine sediment were based on the Modified Universal Soil Loss
Equation, calibrated with limited data on sediment transport solely at two gaged subbasins in
the Chorro watershed. Coarse sediment transport rates were based on several sediment
transport models (one for bed load, one for bed forms, and one for bed material suspended
load) applied over modeled hydrographs. This strategy allowed them to derive estimates for a
range of flows. However, it should be noted that sediment transport formulae of the type used
in that study are frequently an order of magnitude different from instantaneous sediment
transport rates (Gomez and Church, 1989) compare measured and modeled sediment transport
4 By contrast, the RCD estimates that 70,000 tons of sediment were deposited at the site during the winter
of 1997-1998, much of it sand-sized, implying that much higher volumes of sediment were transported in
Los Osos Creek alone. (www. Coastalrcd.org/LOwetland.html)
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rates for several such models, including two of the equations used in the TetraTech study). The
coarse-sediment transport rates were not calibrated, nor were they compared with nearby
watersheds with long-term or directly-measured data, as is the normal practice for the purposes
of this report. No distinction was made between the Chorro and Los Osos watersheds despite
the very different conditions. Additional uncertainties are introduced by integrating these
formulae over hydrographs, which themselves are only modeled flows. Higher rates of
transport following fires, major storms, landslides, or droughts – all factors known to sharply
elevate sediment yields – were not considered.
According to their study less than 30 tons of total sediment would be delivered from Los Osos
Creek during the ‘2-year event’, while nearly 130,000 tons would be delivered during a ‘100year flood’. Thus, their computed ‘average’ sediment yield of nearly 10,000 tons/yr from Los
Osos Creek implies that, according to their calculations, practically all the ‘average’ sediment
yield is delivered during extreme events with return periods of more than 10 years; relatively
negligible quantities of sediment would be transported during most years. The latter finding,
in particular, does not accord with the accounts of knowledgeable observers of Los Osos Creek
or of our observations of changes in the bars over the period of study. Nonetheless, the
TetraTech report did not account for factors present in Los Osos Creek that are likely to increase
sediment yield, including:
(a) the channel of Los Osos Creek is actively incising,
(b) large, unsustainable volumes of sediment are entering the channel in the middle reaches
of Los Osos Creek upstream of Los Osos Valley Boulevard and
(c) similar channels throughout the region are known to have sediment rating curves (or
sediment production rates at a given flow) which differ by over an order of magnitude
depending upon the severity of a given storm or the proportion of the watershed
recently burned (see Knudsen and others, 1992, for a summary of the regional literature
on this issue).
The nature of the TetraTech (1998b) calculations is described in further detail later in this report,
where they are used to a limited degree only because they lend themselves to the management
questions raised, because we were asked to use them, and in the absence of credible data are the
only choice available.
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3.2.1.4 Summary of previous estimates of sediment yield
The studies cited above have arrived at widely varying estimates of sediment yields in the Los
Osos Creek watershed, from 9,800 to more than 150,000 tons/year (Table 1). Note that the
highest estimate only includes contributions from streambank erosion; the actual range of
estimates might be much larger if that study considered all the sediment sources in the
watershed . Significantly, the available studies do not agree on what processes provide the
most important sources of sediment. We note as well that none of these studies estimated the
magnitude of the uncertainties related to the estimates. While it should be remembered that all
authors note the difficulty of estimating sediment yields, the sponsoring agencies for this study
are left without a reliable estimate for sediment yields under both chronic (or ‘normal’) or
episodic watershed conditions, the distinction being an essential starting point for managing
sediment – and riparian vegetation -- in central and southern California streams (c.f., Hecht,
1993; Capelli and Keller, 1992). It is presently not feasible to meaningfully express the volume
of likely storage on the floodplain as so many years of sediment load, given the existing data.
Therefore, we emphasize that the values summarized in Table 1 should only be taken as general
guidelines of the long-term average erosion rates in Los Osos Creek, with uncertainties on the
order of at least several hundred percent, and those numbers should not be taken as realistic
expected values of the amount of sediment yield to expect from the watershed in any year or set
of years.
3.3
Accelerated erosion over the past decades in the Los Osos watershed
In addition to accelerated erosion over the past 150 years, additional sources of sediment have
contributed to increased sediment production from the Los Osos Creek watershed in the past
decade. Several recent sources are noted which may be contributing to accelerated erosion in
the Los Osos Creek watershed which have not been considered in the above reports. These
include:
1. A single recent landslide in the upper portion Los Osos watershed which has added to
the sediment supply since 1995 (Figure 2)
2. Recent bank retreat on the order of tens of yards in the lower portion of Los Osos Creek
(Figure 3); this -- and the associated incision -- were considered in the report by Funk
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River Consultants (1998) but the magnitude of these contributions were not compared
with other sediment sources in the watershed
3. Streambank erosion where Los Osos Creek undercuts dune deposits along the southern
valley wall downstream of its western-most bend.
4. Incision of larger tributaries, a process which appears likely to continue.
3.4
Deposition during recent decades
It appears that significant deposition has taken place on the Los Osos Creek valley floor in
recent decades. We believe that as much as 4 to 8 feet of deposition may possibly be recorded
on the valley floor at a location southeast of cross section 1 (see Figure 4), based on comparing
the elevations on the 1897 USGS topographic quadrangle with those on the photogrammetric
base prepared for this project.5 Such deposition rates are two to three orders of magnitude
greater than those reported for other valley floors in coastal California, but are credible if the
very high rates of bank retreat and incision a short distance upstream are considered (see Figure
3; also, Funk River Consultants, 1998).
This comparison is made upstream of the limit of the project area, and is based on the lowest
contour shown on 1897 map. Hence, it is not possible to develop a similar estimate of
aggradation over a similar period within the area proposed for easements as part of the present
project. We have identified this difference as a potentially-important element in developing a
plan for control of sediment sources in the two-mile reach upstream, should the cooperating
agencies wish to purse this urgent repair. Relative to the study area, one of the implications of
aggradation further upstream is that the valley gradient may be steepening, even over periods
as short as several decades, since indications are that elevations along Warden Creek near
Warden Lake may not have changed much during the past century.
The present land surface is a maximum of about 4 feet higher than shown on the older topographic
quadrangle. Additionally, the current base is referenced to mean sea level (as are present-day
topographic quadrangles), but we believe that the early map uses mean lower-low tide as its datum,
which is typically about 3.8 feet lower than mean sea level, meaning that a higher elevation would be
shown for a given point. Because the difference between the two maps is greatest on the valley floor and
occurs at a point on the natural levee where maximum deposition would be expected, it may well be a
valid and significant record of deposition, pending further analysis.
5
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3.5
Directions for sediment-yield estimation
Wide uncertainty over the volume, processes, and sizes of material in transport affects the
present study in many ways. Two of the most important are (a) without a meaningful estimate
of sediment yield, expectations regarding precision in other aspects of the analysis must
necessarily be reduced, and (b) it is difficult to estimate the ecological and cost effectiveness of
any measure to promote sediment retention. Additionally, much of Balance Hydrologics’
assignment involves developing a sediment-management plan for one segment of the lower
watershed without reliable – or even useful – estimates of sediment loads. If the sponsoring
agencies wish to understand the sediment yields of Los Osos and Warden Creek beyond an
order-of-magnitude estimate, a meaningful estimate specific to current field conditions in the
overall Los Osos watershed is needed, likely requiring a basic 2- or 3-year sediment monitoring
program measuring bedload and suspended-sediment transport rates and conducted in a
manner similar to those used in other streams in the region (c.f., Knudsen and others, 1992, for a
partial listing of such studies in other watersheds within the region) or a field-validated rapid
sediment budget of the type described in Reid and Dunne, 1996.6
6 See Knudsen and other, 1992, for a partial listing of similar field studies within the region; the Reid and
Dunne approach can perhaps best be read as an extension of the 1989 SCS study (which may be the most
valuable of the existing estimates, overall) to within-streambank processes.
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4. BASIS FOR DESIGN AND PLANNING
4.1
Conceptual Plan for Proposed Floodplain Restoration
The purpose of the proposed floodplain restoration is to induce sediment deposition on a
portion of the Los Osos Valley floor to reduce sedimentation in Morro Bay. The area shown in
Figure 4 was chosen by the cooperating agencies as a site deemed economically and potentially
physically feasible to accomplish this goal. Because reducing the upstream input of sediment to
this reach – although crucial to the Morro Bay plan’s success -- is beyond the scope of this
project, we conceived the basic technical problem is to provide the technical agencies with tools
to raise the proportion of sediment entering the reach which deposits on the floodplain. The
proportion of sediment influx which is trapped in7 the floodplain depends on two variables
which can potentially be manipulated: (1) the proportion of the sediment entering the reach
which is decanted over the banks and into the floodplain; and (2) the proportion of that
sediment which deposits in the floodplain, rather than reentering the channel and flowing
downstream (the ‘trap efficiency of the floodplain’). Concurrently, we also sought to establish
that increases in channel roughness were kept small, such that the project would not unduly
raise inundation levels during storm events.
The plan, discussed further in Section 5 and Appendix B, calls for setback levees along both Los
Osos Creek and Warden Creek to the existing farm roads, and allow willows to establish in the
intervening floodplain (see Figure 4). As noted above and also below, only Los Osos Creek is
expected to transport significant amounts of coarser sediment, as Warden Lake is expected to
intercept virtually all fine to medium sand and coarser material entering the lower Los Osos
Valley from Warden Creek. While our analysis also pertains in many respects to the Warden
Creek corridor, the focus is upon Los Osos Creek.
The specific restoration plan is subject to constraints imposed by agricultural land value and
fish habitat. As a result of geographical and economic constraints, the range of options which
can be explored is relatively limited. At a time when all properties in the lower Los Osos Valley
were thought to be part of the project, the cooperating agencies asked that we assess a proposed
levee setback as a possible scenario, as shown in Figure 4, with emphasis on the reach from
7 It is evident from historical analysis, field trenching of deposits from the 1998 storms, and from Alan
Eto’s recollections that sand deposited on the floodplain is rapidly incorporated into the floodplain, often
during the same overbank flooding event; hence, we use the preposition ‘in’ rather than ‘on’ in
subsequent discussions.
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cross-sections 4 to 6 on the Eto property. The area between the existing channel and the setback
levee would be allowed to establish as riparian vegetation. Growth of the willows and other
woody vegetation will increase hydraulic roughness and enhance sediment deposition
(discussed below) while converting some presently-cultivated land to riparian habitat.
This option provides approximately 11 acres for sediment storage north of Los Osos Creek.8
Assuming that sediment could potentially accumulate to a depth of 2 to5 feet (an arbitrary
value, and a planning maximum), theoretical maximum deposition of about 35,000 to 88,000
cubic yards might be provided, assuming flood flows can be contained (see below), or
approximately 30,000-70,000 tons of sediment, assuming a sediment bulk density of about 60 to
65 lbs/ft3 (1 metric ton/m3). For reasons discussed in Chapter 4 below, we doubt that more
than one third of this volume can be used without creating an appreciable risk of avulsion or
loss of agricultural land, or about 10,000 to 25,000 tons. By comparison, the TetraTech modeling
predicts a total sediment yield during the 100-year event of about 130,000 tons (a value which
believe to be reasonable, albeit low) and an ‘average’ sediment yield of 10,000 tons per year
(which consider to be a sharp underestimate). For the purposes of this analysis, it might
usefully be stated that the sedimentation potential is roughly equivalent to from less than one
year up to several years of sediment transport in Los Osos Creek.
We do not expect that this amount of sediment will be deposited there over any known time
period – this only provides an upper limit on the amount of sediment which could FIT in the
space. The rate at which sediment will actually accumulate will depend on how much sediment
is supplied to the reach, and on the trap efficiency for this sediment in the floodplain.
Calculations and discussions about realistic expectations of the sedimentation rate are discussed
later in this chapter.
8 Questions regarding sedimentation into Eto Lake and onto the area separating the lake from the channel
are considered in Chapter 5. Floodplain-level area south of Los Osos Creek is quite limited.
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Along Warden Creek, a setback levee – while not expected to retain as much sediment – will
allow a riparian corridor to develop in an area slowly accumulating fine silt and clay. No levees
are truly required along Warden Creek, because it already flows at the lowest point in the valley
floor. Levees would, however, demarcate the riparian and agricultural areas, and may serve to
slightly reduce flooding if their outboard sides are designed to drain to the channel.9
Our recommendations for the Warden Creek corridor on the project site are to set back the
existing minimal levees (where present) to the farm road paralleling Warden Creek along its
southern bank, and allow riparian scrub and willows to establish in the intervening area.
Portions of the southern bank, especially in the monitoring-well area near cross-section 11
already support low-growing riparian shrubs and vines outboard of a line of willows along the
stream.
4.2
Physical Basis for Planning
The amount of sediment which enters the floodplain at the site increases with decreasing
channel conveyance capacity; in order to maximize this quantity one would reduce the size of
the channel: a narrower, shallower channel would increase the proportion of the flow and
sediment which enters the floodplain. However, a channel must be maintained of a sufficient
size to provide for passage of anadramous fish through the reach. In the following calculations
we have assumed that the current channel width and depth remain the same (below the
inferred base of the levee). As this is a conceptual plan rather than a final design, the option of
further altering channel dimensions is left open for future consideration. In the plan considered
in this report, the amount of overbank discharge of sediment is increased by setting back the
levee along the Eto property from the channel edge to the back edge of the floodplain.
The floodplain restoration will also attempt to increase proportion of overbank sediment which
deposits in the floodplain. A parcel of water entering the floodplain and carrying sediment will
travel a certain distance before re-entering the channel. As the parcel travels over the
floodplain, particles in the water column settle downward at a rate which varies inversely with
the particle size. Larger, heavier particles such as sand settle at rates approaching one-half foot
While TetraTech (1998b) calculated that sediment yields from Warden Creek might be similar to those
in Los Osos Creek above the confluence, we believe sediment yields entering the site from Warden Creek
will be significantly lower because Warden Lake is likely to trap much of the sediment before reaching
the project reach. Each particle of sand or silt entering the site from Warden Creek has to pass through
Warden Lake, a reach of quiet water more than one-half mile long through which Warden Creek has less
sediment-transport capacity than through the proposed sediment storage site.
9
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per second, while silt and clay particles settle at rates less than one foot per hour (Table 2). The
trap efficiency therefore depends on the settling velocity, the water depth, and the amount of
time the sediment-laden water column spends over the floodplain.
If sediment is well-mixed in the water column when it enters the floodplain, then a simple
conceptual model of floodplain deposition can be expressed as:
etr (i ) =
ω i l ob ⎛ 1 ⎞
⎟
⎜
u ob ⎜⎝ d ob ⎟⎠
(1)
where etr is the floodplain trap efficiency of particle size class i (the proportion of sediment
entering the floodplain which deposits), ωi is the settling velocity of particle size class i, uob is
the downstream water velocity on the floodplain, lob is the distance a parcel of water travels
over the floodplain, and dob is the flow depth on the floodplain. Equation 1 expresses the trap
efficiency as the ratio of particle fall distance (as the water parcel crosses the floodplain) to the
overbank flow depth. In other words, equation 1 states that if a particle in the flow can settle
half the distance from the water surface to the floodplain, then 50% of the sediment in the water
column should settle out before the parcel of water re-enters the channel.
While equation 1 incorporates several assumptions which are not strictly valid, it provides a
simple physical basis for designing a conceptual plan for trapping sediment in the valley floor.
The floodplain restoration aims manipulate variables on the right side of equation 1 to increase
the trap efficiency. Particle settling velocities are fixed; the levee setback scenario proposed by
the project sponsors will:
(1) Increase lob from 0 to ~1250’ by choosing a configuration which maximizes the
overbank flow path given the land use constraints expressed by the sponsoring agencies;
(2) Reduce uob by increasing hydraulic roughness on the floodplain (by densely
vegetating the floodplain surface) and reducing the water surface slope (using a levee to
keep flow on the higher side of the valley floor); and
(3) Reduce dob by increasing flow width for a given flow.
4.3
Rationale for Modeling Approach
The calculations described next were made in order to respond to the cooperating agencies’
request to provide a quantitative evaluation of the proposed floodplain restoration strategy.
While the ideal product of such calculations would be an estimate of the average annual
deposition rate on the restored floodplain, we strongly believe that such a calculation would be
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misleading because the timescale over which an ‘average’ rate would be valid is much longer
than the design time scale. Instead, we suggest that the project be considered with respect to its
performance during individual events rather than its performance during a non-existent
‘average’ year. Such calculations are more far more realistic and can provide useful insights
about what is likely to occur.
First, the ‘average annual’ amount of sediment deposition on the floodplain would depend on
the ‘average annual’ sediment yield, its particle size distribution, and on the hydraulic
properties of flows carrying this material into the project reach. None of these quantities are
known with any degree of confidence, as evidenced by the order-of-magnitude discrepancies in
sediment yield estimates described in section 3 and Table 1. Furthermore, in any given year,
both the amount of sediment influx and the trap efficiency of the floodplain will vary due to the
occurrence of events of different magnitudes. For example, the sediment transport modeling by
TetraTech (1998b) predicts that 135,000 tons of fine sediment will be discharge from Los Osos
Creek during the ‘100-year flood’, and only 27 tons during the ‘2-year flood’ (see TetraTech,
1998b, Table 17).
The predicted sediment yield for the 2-year event is low by comparison with qualitative and
quantitative measures. A total sediment yield for a significant flood event from a steep, rapidly
eroding watershed in coastal California is likely to be several thousand tons, not several tens of
tons. The modeled peak discharge in Los Osos Creek during a 2-year event is 35 cfs; according
to the equation used by TetraTech to predict sediment concentrations (see p. 11 in their report),
the sediment concentration at peak discharge would be 31.5 mg/L, which is essentially clear
water. A more physically reasonable value of peak sediment concentration during a 2-year
flood event would be several thousand mg/L. Thus while we are skeptical about the absolute
magnitude of their estimates, they remain the best available input to a sedimentation model
because they include event-based predictions of sediment yield.
The TetraTech estimate of the ‘average annual’ sediment yield from the Los Osos Creek
watershed is about 10,000 tons/year10. While we do not necessarily support either the manner
in which this study was done nor its findings, it is the most recent estimate and is used in our
analysis. It suggests that over long periods of time most of the sediment transport occurs
Note: the TetraTech estimate of an average annual sediment yield of approximately 10,000 tons/year
from Los Osos Creek is based on applying their calibrated sediment transport model to 18 years of mean
daily discharge data from Chorro Creek. Since their hydraulic modeling estimates that flood
hydrographs last significantly less than 24 hours, and because in their model instantaneous sediment flux
increases with the 2.25 power of discharge, we believe that this underestimates sediment fluxes during
days with high flow (which transport nearly all the sediment according to their calculations).
10
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during events which occur on average less than once in 10 years, with relatively negligible
amounts of sediment transport in more than 90% of the years. Over geological time scales, it
may be reasonable to speak in terms of ‘average annual’ amounts of sediment transport in the
watershed. However, over human time scales and for the purpose of this project it is
misleading to present sediment yields for an ‘average’ year: in terms of the sediment yield
calculations by TetraTech, the ‘average’ year in Los Osos Creek would contain nearly 3 separate
10-year floods, 13 5-year floods, or 350 2-year floods. Alternatively, the ‘average year’ would
consist of less than 1/10 of a single 100-year flood. The concept of an ‘average’ year for
sediment transport is only valid if the time scale of interest is much longer than the recurrence
interval of the main sediment transporting events.
The problem of inter-annual variability becomes even more intractable for the purpose of
estimating the ‘average’ floodplain sedimentation rate over project time scales. Since the trap
efficiency will depend on the size of the flood event (uob and dob in equation 1), any estimate of
an average sedimentation rate will largely reflect an arbitrary decision about how to
characterize the hydrology of the nonexistent ‘average’ year. For example, consider if one were
to assume an annual average sediment yield of 10,000 tons/year for Los Osos Creek. A much
higher proportion of that 10,000 tons would be deposited in the project floodplain if it occurred
during several hundred 2-year events (an obviously impossible scenario) than if it were to occur
during one-tenth of a 100-year event (another impossible scenario).
Thus, while it is possible to model or measure long-term floodplain sedimentation rates (and
we have done both; e.g. Malmon, 2002), there is a limit to the usefulness of such a calculation for
a project in which the design life may be shorter than the return period of the main sediment
transporting events. One of the challenges of this project in lower Los Osos Creek is that
knowledge of sediment yields is not sufficient to characterize how many years of potential
transport into Morro Bay might be deposited at this site. Since knowledge of transport in the
channel is insufficient for some planning purposes, we chose to concentrate instead on the
amount and location of potential deposition
Therefore our strategy is to examine the potential performance of the project during individual
events, for which we can characterize the hydraulic conditions. To the extent that the TetraTech
calculations of sediment flux during individual events can be believed, we can make rough
predictions of floodplain sedimentation during different magnitude flows.
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If most of the sediment is transported to Morro Bay during infrequent, high-magnitude events
(TetraTech 1998b), it is the performance of the project during such events that will likely
determine its ultimate success. It is our opinion that considerable transport occurs at lower
flows, consistent with the experience of dredging sediment from the lower Los Osos channel
and that these should be considered as well.
4.4
Hydraulic Modeling
4.4.1 Modeling approach
The U.S. Army Corps of Engineers hydraulic modeling package HEC-RAS was employed to
examine the hydraulic impacts of floodplain restoration along the Los Osos Creek. HEC-RAS is
an integrated set of software which simulates one-dimensional steady and unsteady flow
hydraulics. This model is commonly used to predict water surface elevations and velocities
during high flow events. The basis for the calculations is the equations for conservation of mass
and momentum of flow in one-dimension, and an equation for flow resistance along the
channel and floodplain. For subcritical flow (which was assumed in this model
implementation), the calculations proceed upstream from the lower boundary using a stepbackwater procedure.11
The model was used to simulate flow hydraulics near the confluence between Los Osos and
Warden Creeks, for existing conditions and for the proposed project scenario. The spatial
extent of the model is shown in Figure 4, in which cross sections 2 to 6 were considered to be in
Los Osos Creek above the continuous willows on the Morrissey property, and cross sections 10
to 13 in Warden Creek above the six-acre farmstead. These sections were surveyed in the field.
We generated cross sections A to C from the map. Although the location of the stream channel
confluence is currently downstream of cross section C, and just upstream of the former Santa
Ysabel Avenue crossing, the entire area downstream of the wetland easement is known to be
flooded during high-flow events (Alan Eto, landowner, personal communication), and was
therefore considered to be downstream of the confluence.
The number of cross sections used in modeling was quite small, constrained by the limited funding
available for surveying. The CSLRCD staff were able to substantially supplement the surveyor’s efforts
in several ways, including clearing brush from several long sections. This effort allowed some
meaningful application of the model, even at cross-section intervals far below the recommended
distances.
11
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Cross section A, downstream of the project reach, contains two discrete channels at different
elevations (Figure 5). The higher elevation channel is the low flow channel from Los Osos
Creek and the lower channel is Warden Creek. Although there will be flow in both channels
during large runoff events, HEC-RAS cannot simulate two different water surface elevations in
the same cross section during the same flow. Including XS A as a single modeled cross section
below forced flow from XS 6 (above the confluence) into the lower channel in XS A, leading to
erroneous oversteepened water surface profiles within the project reach. Thus predicted
floodplain flow depths and velocities were not realistic. This problem was circumvented by
moving the modeled confluence downstream of XS A , including a copy of XS A in both
tributaries above the confluence, and forcing the flow from Los Osos Creek to fill the higher
elevation channel before overflowing and moving northwest across the floodplain.
In order to improve model stability, artificial cross sections were created at 100’ intervals,
between those indicated on the map. These cross sections were interpolated from upstream and
downstream cross sections, and the interpolated cross sections were checked and minor
adjustments made to improve the accuracy of the interpolated cross sections.
4.4.2 Input data for the hydraulic model
The hydraulic model requires two general types of data as input: (1) information about the
topography and hydraulic roughness along the study reach; and (2) a set of flows in both
tributaries and below the confluence. The following data were used in the model:
1. Cross Sections: We supplemented data from channel cross sections surveyed by
Vaughn Surveys (Figure 2, blue lines) with contour data from the topographic survey to
extend the cross sections across the floodplain (red lines, see below). The data from the
two sources did not always match. For example, the north endpoint in cross section 13
is located adjacent to the 30-foot contour on the topographic map, but the surveyed data
indicates an elevation of 20.61 feet. Most discrepancies were not as large as this, and we
attempted to combine the two data sets in the most reasonable way possible.
2. Longitudinal Data: the horizontal distances between cross sections were measured from
the topographic survey map. Distances were measured along the channel and along the
floodplain between each pair of cross sections. The vertical difference between cross
sections was determined by assuming that the elevations from the surveyed cross
sections were accurate.
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3. Downstream Boundary: we assumed flow conditions to be sub-critical in all instances.
Subcritical flow is probably a reasonable assumption and only requires the downstream
boundary condition to be defined. We computed the approximate downstream slope
between the surveyed thalweg elevation in cross section C (Figure 2) and another
surveyed cross section which is not indicated on the map (0.008). The topographic
information downstream of the wetland easement is imprecise because of dense tree
cover and low relief, so the boundary condition is poorly constrained. We performed a
sensitivity analysis and found that water surface elevations in the area of the proposed
modification were not affected by the downstream boundary condition. Similarly, the
model indicates that tidal influences do not propagate upstream to the project area even
during high-flow events.
4. Channel and Floodplain Roughness: the Manning roughness coefficient for the channel
was assumed to be 0.035 in all model runs. This value is representative of “winding,
natural streams” with considerable vegetative growth (c.f., Dunne and Leopold, 1978),
or “clean, straight, full [channels, with] no rifts or deep pools [but with some] stones and
weeds” (Chow, 1959). Floodplain roughness was assumed to be to 0.15, to characterize
“dense willows, summer” (HEC-RAS manual).
5. Flow Input Data: the model was run for the 2-year, 10-year, and 100-year discharges in
Los Osos Creek (“Los Osos Creek at Gage”, above Warden Creek) and in Warden Creek.
The discharge estimates were from TetraTech (1998a), who ran a hydrologic model of
the two sub-basins that considered watershed shape, soil conditions, and slope, given
assumed rainstorms of varying recurrence interval. We assumed that the peak
discharges from these modeled discharges reached the confluence concurrently. In
other words, the peak discharges from the two reaches above the confluence were
simply combined to estimate the discharge downstream of the confluence. This is not
likely to occur; therefore, our estimates of flow downstream of the confluence and
backwater effects can be considered conservative in the sense that they are probably
representative of flows with recurrence intervals greater than 2, 10, and 100 years.
4.4.3 Hydraulic model results
The topography at cross section A (Figure 5) illustrates the importance of the peculiar
topography of the project reach for the proposed project. The current levee keeps the Los Osos
Creek channel at an artificially high elevation, leading to a reduction in slope in the vicinity of
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the Eto property, and an increased slope downstream, where the contours are parallel to the
axis of the valley floor (Figure 4). This human-induced reduction in bed slope along Los Osos
Creek may partially explain historical bed material accumulation in the reach from which has
required repeated dredging to maintain a channel. Because Los Osos Creek does not flow at the
lowest point in the valley floor, a project which only removes the current levee in order to
restore the floodplain can potentially allow the channel to avulse, flow across the floodplain,
and occupy the lowest position in the valley (the Warden Creek channel). Therefore the
scenario that we have proposed is a setback levee. If the field in the area of cross sections 4 – 6
is to be used to store sediment, a new levee must be constructed as shown in Figures 4 and 7.
The red arrows in Figure 4 depict the trajectory of a parcel of water over the floodplain under
project conditions: sediment-laden water will be directed into the floodplain at the bend near
cross section 4. The setback levee will keep the flow on the west side of the valley and reduce
the water surface slope through the project reach (Figure 4), enhancing sedimentation.
The hydraulic model results for the existing and proposed conditions, in the vicinity of cross
sections 4 – 6, are summarized in Table 2 and illustrated graphically in Figure 7. According to
the model, under proposed project, overbank flooding is expected to occur at all three cross
sections during relatively small events, while expected overbank flow depths and velocities are
expected to remain relatively low (< 5’ and < 2 feet per second, respectively) during extreme
flow events. Frequent overbank inundation with relatively low depths and velocities are
conditions which are expected to enhance sediment deposition. Sedimentation calculations for
project conditions are discussed in the next section.
4.4.3.1 Backwater limits from tidewater
One of the concerns of Mr. Eto is the potential for backwater flooding of his property during
high flow events which coincide with high tide. A sensitivity analysis was performed to assess
the potential for backwater flooding on the Eto parcel. We simulated the effect of high tide by
running the hydraulic model after reducing the slope downstream of cross section C. We found
that reducing the slope an order of magnitude, from 0.008 to 0.0008, had no effect on predicted
water surface elevations on in cross sections 4-6, on the Eto property, for all three flow
magnitudes. Reducing the slope by another order of magnitude only affected water surface
elevations on the Eto property for the 100-year flow. However, the backwater effects were more
pronounced along Warden Creek, which is lower than Los Osos Creek.
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4.5
Sedimentation Calculations and Interpretation
4.5.1 Model assumptions
Because the absolute amount of sediment influx into the study reach is not well constrained, the
strategy we adopted is to first compute the proportion of the sediment load in each particle size
class expected to deposit on the restored floodplain surface. The calculations are based on
equation 1, which states that the floodplain trap efficiency is equal to the ratio of the particle
fall distance to the water depth on the floodplain. As with any model, this statement
incorporates several important simplifying assumptions :
ƒ
That sediment in all particle sizes is vertically well-mixed in the water column as the
flow enters the floodplain.
ƒ
That, once over the floodplain surface, sediment settles through the water column as it
would if the water were still; that is, that the intensity of turbulent mixing over the
floodplain is low compared with that over the channel bed.
ƒ
That vertical erosion of the floodplain by the overbank flow is negligible
These assumptions are probably not strictly valid, but are supported by the results of the
hydraulic modeling (Table 2): flow velocities over the floodplain are much lower than those
within the channel – thus one would expect that vertical mixing of sediment by turbulent eddies
is much more intense in the main channel, keeping the sediment well mixed in the flow. Where
the flow enters the floodplain, the intensity of turbulent mixing decreases rapidly, allowing
particles to settle out. Erosion of the floodplain is probably not likely because floodplain water
velocities are generally less than 1.5 foot/second. Furthermore, the floodplain surface will
likely be protected by vegetation.
Even if the above assumptions are violated, the model we have proposed to predict floodplain
sedimentation can provide qualitative insights into the problem. The model is simple,
transparent, and it contains the first-order variables which control the process: sediment size,
overbank flow depths, overbank flow velocities, and the size of the floodplain.
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4.5.2 Values used in the model
The calculation described above (equation 1) requires estimates of:
(1) particle settling velocity;
(2) the overbank path length of a parcel of water; and
(3) the water depth and velocity over the floodplain.
Settling velocities were computed using the method of Dietrich (1982), who presented an
empirical equation for settling velocity based on experimental studies. This equation accounts
for the effects of size, density, shape, and roundness. In making these calculations, we have
assumed a particle density of 2.65, plus typical values for grains and water and water viscosity
at 25 degrees Celsius. The computed settling velocities for different grain sizes are presented in
Table 2.12
The overbank trajectory of a ‘typical’ water parcel through the proposed floodplain is illustrated
by the red arrows in Figure 4. According to the topographic data, the flow path of water
entering the floodplain should be parallel to the setback levee to the point where the levee turns
across the valley toward Warden Creek. Beyond the edge of the levee, the water parcel will
turn northeastward across the valley, perpendicular to the contour lines. The slope of the water
surface is reduced in the vicinity of cross sections 4 – 6, and increases downstream where the
water is allowed to follow the lines of steepest topographic slope. Thus the opportunity for
sedimentation is maximized in the area adjacent to the setback levee. The length of the red
arrow parallel to the proposed setback levee is 1250’. This value was used in all the
sedimentation calculations below.
Water depths and velocities over the floodplain will vary with event magnitude: larger floods
will produce deeper and faster flows over the floodplain, reducing the amount of sediment
which will deposit on the floodplain. Depths and velocities over the floodplain for large using
the HEC-RAS model. Discharges for various recurrence intervals are based on the runoff
modeling by TetraTech (1998a), for the stations “Los Osos Creek at Gage”, and “Warden Creek”
(upstream of the confluence). Below the confluence, the discharges were combined. TetraTech
(1998b) reports modeled peak and average discharges for the 2-year, 5-year, 10-year, 25-year,
50-year, and 100-year flood events. We ran the HEC-RAS model for all 12 reported discharges,
We anticipate that floodflows will typically have temperatures of about 12 deg. C, and that some of the
diatomaceous sediments will have a particle density of less than 2.65, but used the higher values (which
will overestimate settling velocities) to offset the neglected role of vegetation which does increase
sedimentation rates.
12
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in order to examine how the floodplain trap efficiency would vary from moderate to extreme
events, and during events from the average to peak discharges. For each model run, the
average overbank flow depth and velocity from XS 4 –6 were noted; the values in the 2nd and 3rd
columns in Table 3 were determined by averaging the estimates at the three cross sections.
4.5.3 Results and interpretations of the sedimentation calculations
Predicted values of floodplain trap efficiency, for a range of discharge and particle size, are
presented in Table 3. According to the calculations, virtually all the sand transported overbank
at the head of the project reach should deposit on the restored floodplain. Nearly all the silt is
also expected to deposit, except during peak discharges of very large floods. The model
predicts that, for the hydraulic conditions modeled, nearly all the clay particles will traverse the
floodplain without being deposited. We believe that these calculations make intuitive sense
based on our field observations, a fact which increases our confidence in the underlying model.
However, the simple quantitative prediction that nearly all the sand and silt will deposit, while
nearly all the clay will not, would not have been possible from qualitative field observations.
These results demonstrate the importance of understanding the amount and character (i.e.,
timing and particle size distribution) of sediment entering the study reach to assessing the
potential success of the proposed project. The problems with estimating the ‘average annual’
sediment yield from the watershed at project time scales were discussed in section 4.3.
However, the TetraTech (1998b) report included estimates of sediment yield by event, and these
data can be analyzed in light of our model.
Because the TetraTech (1998b) predictions are central to our interpretations, we briefly outline
the nature of their sediment transport model, and our use of their numbers. Their calculations
considered the sediment load as consisting of two parts: (1) Bed material load, consisting of
particle size classes found in large quantities within the channel bed; and (2) Wash load,
consisting of particles which are generally not represented in the channel but which occur in
floodplain deposits. They make the reasonable assumption that the bed material load is
determined by the transport capacity of the flow (is transport-limited), while the washload is
controlled by the watershed erosion rate (supply-limited). They calculated the bed material
transport capacity of different particle size classes using hydraulically-based computations, and
integrated these equations over their modeled hydrographs for a range of large modeled flows.
To estimate the washload yield for each event, they used an empirical equation (Modified
Universal Soil Loss Equation), which was calibrated using: (1) suspended sediment data from
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two small watersheds – Walters Creek and Chumash Creek – in the Chorro watershed; and (2)
topographic data describing the length and slope of the Los Osos Creek watershed. The
TetraTech modeling does rest upon the assumption that erosion processes and substrate
(bedrock and soils) are similar in these three watersheds, an assumption with which we cannot
agree, even as a first approximation, and revisions of our modeling are warranted as soon as
data specific to the different geologic conditions of the Los Osos Creek come available (see
recommendations in Chapter 5,6 and 7, below)
Using this approach, they estimated that the bed material load accounted for only a few
percent of the total load to Morro Bay. TetraTech assumed that it accounts for a much larger
proportion of the total load during relatively moderate events and its contribution is essentially
negligible for very large events (Table 4, first 2 columns).13 For the sake of simplicity and
because of a lack of adequate data to be more precise, we assumed that the bed material load is
primarily sand, and the washload comprises equal parts silt and clay (data from soils in the
watershed support the second assumption, see Table 2 in TetraTech 1998b).
We made a very rough estimate of the proportion of the total sediment yield that enters the
floodplain in the project reach, by assuming it would be equal to the portion of the flow which
is flowing over the floodplain during the ‘average’ discharge during that event. The relative
proportions of water flowing over the channel and floodplain in the project reach was available
from the HEC-RAS model, and remained within a relatively narrow range (50 to 60%, see Table
4) over the range of modeled flows. Only this portion of the sediment load was considered to
have the potential to deposit in the floodplain.
We used the TetraTech (1998b) estimates of sediment discharge by event and our floodplain
sedimentation model to predict the amount of sediment expected to deposit within the project
reach during flood events of different sizes (Table 4). To the extent that the TetraTech
calculations accurately describe the sediment load14, our calculations suggest that the proposed
floodplain will trap relatively little sediment during small events (~10 tons during the 2-year
flood), but could potentially store up to 25,000 tons during extreme flood events.
While possibly true in the Chorro watershed, sediment influxes from large outcrop areas of both
ancient sand dunes (Hall, 1973) and sandstone (Nitchmann, 1988) are diagnostic of Los Osos Creek, and
substantial sand loads should be anticipated at least until the sand-influx reaches upstream of Los Osos
Valley Blvd are stabilized.
14 As noted above, while we these calculations to be imprecise at best. While they have not presented
uncertainty estimates, they are probably only useful to within an order-of-magnitude, especially for
storms with peaks of lower recurrences.
13
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Our calculations suggest that most of the sediment that will be trapped in the restored
floodplain during such large events will be in the silt size range – plus the additional sands
transported by Los Osos Creek due to the current destabilized conditions and not recognized in
the TetraTech calculations -- while the floodplain is not expected to trap much of the clay-sized
sediment during flows which inundate the restored floodplain.
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5. TOWARD A MANAGEMENT PLAN
Initial objectives outlined at the first field meeting of the project team included:
1. Trap sediment before it reaches Morro Bay, especially the finer fraction that is found in
suspended load, by restoring the floodplain and applying natural (geomorphic) stream
processes,
2. Design a bankfull (1.5-2 year recurrence) channel that is effective at: (1) transporting
bedload (coarse material), (2) spreads larger flood flows across the floodplain, and (3)
provides passage for upstream and downstream steelhead migration,
3. Project needs to coexist with other surrounding land uses, facilities, or structures,
4. Maintenance should be minimal, preferably self-maintaining; worst case would be
maintaining with hand crews and small pieces of equipment.
5. If possible, improve habitat for other threatened or endangered species found in the
immediate project area.
5.1
Program Elements
5.1.1 Cooperator’s preferred alternative
The preferred alternative identified by the project cooperators is described in Appendix B.
5.1.2 Modified preferred alternative
We suggest a phased modification to the proposed alternative, which (a) retains the
sedimentation initiative and essential elements of the program originally envisioned while also
recognizing (b) the late changes in the ownerships to be affected, (c) our recommendation to
incorporate the likelihood of future avulsion in the planning, (d) the value of easements already
negotiated for this program, the (e) insufficient geomorphic bases for planning sedimentation to
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control sediment contributions from the highly disturbed reach upstream of Los Osos Valley
Blvd. It also allows the cooperating (or sponsoring) agencies to consider the significant strategic
and resource-ethical questions remain as to when to use the current limited sedimentation
potential of the lower Los Osos Valley. It may be used now, when sediment from the highlydisturbed reach will be retained, or it may be used later to keep otherwise hard-to-settle silts
and clays out of Morro Bay, once this reach has been stabilized. It is also feasible to pursue a
combined strategy of commencing sediment management through restoration of the riparian
corridors now, followed by construction of structures to induce sedimentation once the current
load of excessive sediment has been reduced.
The modified preferred alternative can be best described as three phases:
•
Phase I. Establish the sediment-retention area, and (if necessary)15 the maximum levels
to which sediment accumulation can be tolerated within each sector and/or easement.
•
Phase II. (a) Conduct the additional basic investigations needed, including evaluating
the level of sediment-control which might be attained by stabilizing the channel between
its western-most bend and the Eto crossing, and the incremental hazards of channel
obstruction and avulsion which might be reduced by stabilizing these reaches. (b)
Further, re-define the project to serve in a ‘polishing role’ in which smaller volumes and
finer grain-sizes of sediment will settled, a more appropriate role for the site;
additionally, this role will extend the effective life of the project manifold, and allow it to
control finer sediment which is being deposited further out in the Bay.
•
Phase III. Re-assess the facilities and operations needed in light of reduced sediment
loads and the properties under easement for sediment retention, using the data and
models developed by Balance staff. If the re-assessment proves minor, the analyses
from this report may well prove sufficient; however, the possibility of a major rethinking of the project’s elements is possible. Changing the banks or existing berms and
constructing new facilities without this re-assessment could result in attributions of
responsibility for overflows or changes in flow patterns on the valley floor, particularly
if the Martines parcel is not included in the project.
Sediment accumulation maxima would be needed if (a) continued funding for clearance of the channel
cannot be assured, (b) the connection between Eto Lake and the stream should be maintained or
enhanced, and/or (c) all parcels subject to overflow or avulsion from Los Osos toward Warden Creeks
are not included in the sediment-retention program.
15
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5.1.3 Possible supplemental future retention
Additional sediment storage in the range of 100 to 200 acre feet might be achieved by emplacing
a berm across the valley at the present location of the access road to the farmstead. The main
function of this berm would be to provide a quiet-water environment in which finer-grained
sediment now transported into the heart of Morro Bay may be settled. This supplement should
not considered until upstream sediment sources have been substantially reduced, and periods
of heavy sediment transport have been reduced to those approximating the magnitude and
relatively-rapid recession typical of the responses to episodic events under the former natural
conditions.
Additional reasons for deferring consideration of bermed retention include the likelihood that
water would back up onto the Martines property, which we understand has now been excluded
from this analysis, the difficulty in making the bermed reach passable to steelhead until the
woodland established since 1994 on the Morrissey property has matured, questions regarding
long-term benefits to be achieved by directing flows into lower Los Osos and Warden Creeks
downstream from the farmstead, and the lack of pertinent geotechnical information, among
others.
Existing analogs are helpful when envisioning this conceptual possibility. We note that a berm
of similar nature constructed in 1929 at Searsville Lake at Stanford University’s Jasper Ridge
Biological Preserve. It supports a low-gradient deltaic willow woodland very similar to the one
now evolving on the Morrissey property and those downstream, and is considered a valuable
element of JBRP.
It should be noted that this approach is suited to control of fine sediment either from Los Osos
Creek’s present maintained channel, or from Warden Creek, either before or after Los Osos
Creek may avulse into Warden Creek.
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5.2
Phasing
The sponsoring agencies’ preferred alternative can be implemented in one phase to commence
when all easements and permits are in hand. The supplemental sediment retention element
(Sec. 5.1.3) may be considered once the willow woodland on the Morrisey property has
matured and the preferred alternative is deemed to have equilibrated.
Phasing for the modified preferred alternative has been described above.
5.3
Anticipated Effectiveness of Induced Sedimentation
noted in Chapter 3, sediment loads in Los Osos Creek are so much higher than those in Warden
Creek that the valley floor along Los Osos Creek is rapidly aggrading. With the progressively
steepening cross-valley slope, it is likely that Los Osos Creek will eventually spill across the
valley floor to the north side, where Warden Creek occupies the lowest portion of the valley.16
A spill of this type – called ‘avulsion’ in geomorphic parlance – may occur when water simply
rises over the top of the bank during large floods. More likely would be triggered by sudden
blockage of the creek (such as by the debris jam which often forms quickly after one or more
large trees fall into the stream during major storms) and/or by aggradation of the bed following
a large sediment influx upstream (such as a debris flow). Inducing sedimentation along Los
Osos Creek, as discussed below, may or may not hasten the timing of this geomorphic
eventuality, but it should be anticipated during the planning life of this project.17
The effectiveness of this project, then, in reducing sedimentation in Morro Bay and its
contributing waters should reasonably be evaluated both before and after the anticipated
change in the channel’s course. Measures designed to address the near-term conditions are
discussed in section 5.4, and their costs and effectiveness approximated. The role of the site in
reducing post-avulsion sedimentation is considered in Sec. 5.5.
There is some indication in both the topographic map and aerial photography for this project of prior
avulsive events of this type both just upstream of Warden Lake and in the western portion of the
Martines property. Alan Eto has mentioned that he can recall flow from Los Osos to Warden Creek
during major storms of the past, perhaps during the 1950s or 1960s. See also Schmit (1992), reporting
valley-wide overflows and 0.5 feet of deposition on the valley floor during the storms of February 1992.
17 An earlier 3-page feasibility study by Jeff Haltiner (Philip Williams & Associates, 1993) for earlier
phases of this project identified the likely ‘capture’ of Warden Creek by Los Osos Creek near the
farmstead access road on the Morrissey property, although apparently not further upstream.
16
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5.4
Sedimentation measures under existing conditions
5.4.1 Alternatives considered
Alternatives considered by the sponsoring agencies during selection of the preferred alternative
are described in Appendix B. Additionally, Balance staff advanced another alternative in June
2001 including a planned overbank channel from the Eto Crossing, downhill through the
Martines property to the vicinity of cross-section 13. The modified preferred alternative
described above is a new approach, first described in this report, in part in response to the
likelihood that the Martines property will not be included.
5.4.2 Proposed project
The proposed project was outlined by Central San Luis RCD, Morro Bay Estuary Program and
their cooperators in a meeting held on January 17, 200218. It includes a number of different
elements:
1. Setting back the existing (eastern) levee along Los Osos Creek some appropriate
distance, beginning just downstream of the low-water crossing (cross-section X-3),
allowing Los Osos Creek to flood onto lands between the existing creek and the setback
levee. The intervening area is to be allowed to revert to floodplain vegetation, with the
expectation that the creek will eventually migrate into (and eventually out of) the new
floodplain.
2. A new levee will be installed at some appropriate distance west of Warden Creek, with
the upper portion derfined by the southwestern boundary of the existing wetland
easement. This new levee is intended to allow Warden Creek to flood while still
protecting agricultural operations in the center of the valley, and to separate riparian
and agricultural uses..
3. The lands between the setback Los Osos levee and the new levee southwest of Warden
Creek would be built up with material excavated from the channels of Los Osos and
Warden Creeks.
18
Documented in a memo of January 21, 2002 by Malcolm McEwen (Appendix B)
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4. Overflows from Warden Creek (and presumably also from Los Osos Creek) toward the
central sump on the Martines property would be halted by construction of the levees.
A map of these elements is shown in Figure 4. The cooperators’ committee also expressly chose
to not pursue a number of other alternatives, including:
A. Diverting Los Osos Creek to Warden Creek, at a point slightly downstream from
Warden Lake
B. Removing the levees and allowing the entire area between the levees to flood
C. Clearing sediment mechanically from Los Osos Creek (or ‘mining’ without sale of
material)
D. Clearing sediment mechanically from Los Osos Creek while allowing Warden Creek to
flood.
The committee also encouraged that the plan include forward-looking elements, such as
easements which might allow purchase of the land in the future, perhaps when the existing
owners may choose to stop farming. The full text of this important memorandum is included as
Appendix B.
5.4.3 Costs
Costs will be considered separately by the cooperating agency, in part through use of a
spreadsheet provided by Balance.
5.5
Sedimentation role of the site following anticipated avulsion
5.5.1 Problem statement
It is likely that sediment retention will occur in two stages – before and after a likely channel
change. This section of the report describes effectiveness following the fundamental changed
condition following a channel change.
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5.5.2 Avulsion effects and timing
Effects of a major channel change will be (a) erosion of sediment from the valley floor between
the turnout point on Los Osos Creek and the re-entry point on Warden Creek, (b) release of a
wedge of sediment from Los Osos Creek immediately upstream of the diversion, as the channel
incises, (c) expansion of the Warden Creek channel to accommodate the Los Osos flows,
sediment, and woody debris.
A useful question is can be asked as to how effective sedimentation will be within the lower Los
Osos Valley once the two channels have joined. This will depend, of course, on the location of
avulsion and the new confluence. Recognizing the tremendous uncertainties involved, we reran the sedimentation model for combined flows to assess whether velocities would
substantially increase – meaning that sedimentation effectiveness would diminish. Results are
presented in Table 6, and show relatively small increases in simulated velocities at peak flows,
exaggerating the actual peak by assuming that the post-avulsion is computed by adding the
pre-avulsion peaks of the two streams. We concluded that the lower Los Osos valley can
continue to function as a long-range sedimentation reach, even if the major anticipated shifts in
channel do occur.
We do not know when avulsion will occur, only that it is likely in the foreseeable geomorphic
future. It could be triggered by an individual event, collapse of a wooded bank into the stream,
or movement a potential ‘pulse’ of sediment originating in the actively disturbed reach into the
project area. Other catalytic events can be envisioned. It might be noted that the relatively
minor floods of 1992 are reported to have flowed overbank out of Los Osos Creek at Eto
Crossing (Schmitt, 1992).
5.5.3 Preparing for avulsion
There will be some questions as to whether or not to accept a partial or temporary change in
channel course. Written criteria should be developed to guide sedimentation-reach managers
as to when to acknowledge a permanent and along-term change in course, and when to
consider re-directing a partially-changed channel into the existing Los Osos Creek channel.
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We suggest that environmental and operational costs incurred in operating under existing, preavulsion conditions be tracked, such that post-avulsion costs may be anticipated. This may
affect protocols for a decision as to whether to accept a channel change.
5.6
Required permits
Three groups of permits will be required as part of the proposed project:
a. Permits associated with recurring excavation and grading to remove accumulated sediment
The CSLRCD and the site owner have conducted dredging of the channel on multiple occasions
in the past, and are familiar with the permitting requirements, which include:
ƒ
An individual permit under Section 404 of the Clean Water Act (CWA), to be
sought from the Regulatory Section of the Corps of Engineers, San Francisco
District.
ƒ
A streambed alteration agreement under section 1603 of the California Fish and
Game Code, to be sought from the Department of Fish and Game.
ƒ
Approval by the California Regional Water Quality Control Board, Central Coast
Region, under Section 401 of the Clean Water Act.
b. Permits associated with one-time grading
The proposed project is of a size requiring a grading permit from the County of San Luis
Obispo.
The proposed project will not materially affect the closed County landfill immediately to the
northeast. Results of the HEC-RAS simulations (Chapter 4) show that allowing revegetation of
the Warden Creek corridor will not discernibly raise water levels during periods of inundation.
To the extent that the project induces deposition or delays or deflects westward whatever future
channel changes may occur, only beneficial effects of the proposed project are expected on the
landfill and its remnant water-quality influences. No reason to amend the closure plan for the
site can be identified.
c. Permitting associated with extracting aggregate for sale
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Under California’s Surface Mining and Reclamation Act (SMARA), the removal of earth
materials for sale may be subject to obtaining a use permit and preparation of an approved
reclamation plan. Sites where extraction occurs intermittently and where removal is incidental
to another primary use, such as will occur in lower Los Osos Creek, are subject to the discretion
of the local agency charged with implementing SMARA, which in San Luis Obispo County is
planning agency. It is our understanding that a permit and reclamation plan will not be
required as part of a project which clears accumulated habitat-impairing sediment and sells a
portion of the material removed to defray some of the related costs.
Beyond the individual permits, the project will require compliance with the California
Environmental Quality Act, which is not a part of the scope of this assessment. The CEQA
process will allow resource agencies and jurisdictions without specific permitting
responsibilities to comment on the project and functions and values of its setting.
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6. RECOMMENDED MONITORING
A program promoting sedimentation calls for periodic monitoring of the channel and
floodplain areas to anticipate future changes in channel course. In the case of Los Osos Creek,
this need is magnified by:
(a) its elevation 6 to 10 feet above corresponding segments of Warden Creek on the
other side of the valley, and
(b) the very high rates of sediment (including wood) delivery to the lower valley.
If the program proposed by the sponsoring agencies is implemented, we suggest that the
program initiate a basic but regular program of monitoring, to include the following elements,
plus other deemed needed by biologists, growers, and estuary managers. Sedimentation at this
site should be monitored annually and after storms with a recurrence of about 2 years or more,
with several objectives:
ƒ
The depth of sediment accumulation in the straight reach downstream of the Eto
crossing should be checked for potential blockage of steelhead passage or for filling
of the channel to the extent that conveyance is appreciably reduced.
ƒ
Logjams temporarily obstructing flows should be reduced in size such that no more
than 15 percent of the channel conveyance is reduced; progressively lower threshold
may be considered in future years if the present channel instabilities upstream of Los
Osos Boulevard and current rates of incision persist. In some years, this may require
walking Los Osos Creek from tidewater to the southern edge of the present Eto
lands upstream of cross-section 1.
ƒ
Long-term changes in the stream profile should be monitored to assess whether
significant and persistent changes are occurring, and whether these may be related
to sedimentation promoted at this site. We suggest that re-surveys of cross-sections
1, 4, 6, 12, 14, 17, and 22 be performed at intervals of approximately 5 years or
following events with recurrences of about 10 years or more in either or both creeks.
The program should purchase and/or review recent or post-flood aerial
photography from slightly upstream of cross-section 1 as far downstream as Santa
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Ysabel Avenue (tidewater) to assess if avulsion or other significant change in
channel course may be occurring, and what effects these changes may have on
sediment delivery to and steelhead passage to our from Morro Bay. Locations of
knickpoints in the Los Osos and Warden Creek channels downstream of crosssections 6 and 14 should be mapped and their heights noted. The 5-year monitoring
should be conducted by a specialist in channel behavior or sedimentation, with
experience in steelhead passage, and with State of California registration in an
appropriate profession.
ƒ
The essential assumptions of the HEC-RAS models affecting flow profiles should be
qualitatively re-examined at intervals of five years or (if first occurring) following
events with recurrences of 10 years on either stream. At a minimum, width and
Manning’s ‘n’ of the riparian corridor should be compared with those assumed in
the model. If substantially different and if differences may materially increase
inundation levels or risk of avulsion, the model should be re-run and modifications
to the project once implemented should be considered.
At least once a year, growers or other lessees should meet with program managers regarding
the condition of wells, water-supply lines, major drains and other infrastructure to assess
compatibility of the program with ongoing agricultural operations.
Because it is likely that significant changes in the channels may occur, including shifts in the
locations of the channels, program managers should develop contingency plans for actions
which they may wish to take following potential (a) major avulsion of Los Osos Creek to the
north (Warden Creek) side of the valley, and/or (b) recurrent minor avulsion of Warden Creek
between sections 12 and 13, with specific attention to:
ƒ
Steelhead passage
ƒ
Effects on infrastructure and access
ƒ
Potential damage to the County’s monitoring-well network immediately below the
closed landfill.
Since existing data on flood flows and sediment transport into the reach are either not quite
adequate or subject to question, the program should consider contributing/participating in
some manner in a professional sediment-transport monitoring program or a standard ‘rapid
sediment budget.’ Findings of such an effort should be incorporated into the management
program for the lower Los Osos Valley.
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The sponsoring agencies should also regularly monitor the costs and benefits of conducting this
program, as fundamental changes in the lower Los Osos Valley are expected should Los Osos
Creek avulse. Managers should periodically review the costs and benefits, updating a
contingency program which anticipates avulsion with prior criteria for deciding whether to
incur the costs of returning Los Osos Creek to its present channel or accepting its eventual shift
into the existing Warden Creek alignment.
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7. CONCLUSIONS AND RECOMMENDATIONS
1. This report summarizes some of the key opportunities and constraints affecting
sedimentation in lower Los Osos and Warden Creeks, and analyzes effects of a proposed
preferred alternative on flood levels and velocities during design storms. It also explores
factors affecting steelhead migration as Los Osos Creek passes through the heavily-wooded or
intensively-farmed valley floor. This analysis was funded by the California Coastal
Conservancy, and directed by the Morro Bay Natural Estuary Program, with very substantial
assistance from the Central San Luis Resource Conservation District.
2. The present study was motivated in large part by the identified success of planned deposition
of floodplains along lower Chorro Creek, as the main element in the Chorro Flats Enhancement
and Management Plan, a 12-year effort directed by the Central San Luis Resource Conservation
District and funded by the Coastal Conservancy, with substantial work by the USDA Soil
Conservation Service and several consulting firms. Authors of the Chorro Flats study
recommended, based on a rapid and limited assessment that the lower Los Osos/Warden
Creek Valley also be considered for a depositional area to protect Morro Bay,
3. While it was hoped when this program was initially authorized in the mid-1990s that a very
limited study ‘piggybacking’ on the Chorro Flats work would allow development of a detailed
plan for the lower Los Osos/Warden Creek area, conditions and constraining factors have
proven to be fundamentally different, and the approach to inducing deposition in this area
must necessarily be different.
4. Primary project objectives adopted by the sponsoring agencies include:
ƒ
restoring riparian and alluvial-valley functions while also retaining agricultural uses
ƒ
reducing the volume of sediment entering Morro Bay from Los Osos and Warden
Creeks
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39
ƒ
maintaining, if not enhancing, steelhead passage between the Bay and Clark Valley
or other montane portions of the Los Osos watershed, and managing the lower Los
Osos Valley in a manner consistent with sensitive-species needs.
ƒ
Minimizing economic and environmental costs of maintaining flow and sediment
transport through the two channels, especially through Los Osos Creek, which has
been mined or had sediment removed to depths of four to five feet over much its
length within the study area.
5. Geomorphic Conditions: Slopes, substrate, and sediment transport in the Los Osos and
Warden arms of the system differ fundamentally. Nearly all of the sands and gravels
transported to Lower Los Osos Creek/Warden Creeks originate from the steep, incising Los
Osos Creek portion of the watershed. Los Osos Creek has deposited a fan-like alluvial apron,
actually damming Warden Creek to form Warden Lake. Sediments passing through Warden
Lake from Warden Creek are almost entirely silts and clays. Most of the deposition in this part
of the valley has been and will be from the Los Osos arm. The proposed project will help retain
coarse sediment within the valleys upstream of Morro Bay, keeping much of the hard-toremove coarse sediment from entering the Bay.
6. Land Use Considerations: The valley has been farmed continuously since the late 1800s, and
continued agricultural use is sought by both the project sponsors and growers who own and
lease most of the land.
7. Sediment yields from Los Osos Creek to Morro Bay, which likely have increased by an order
of magnitude during the past 150 years, are currently even greater, due to four sources which
have developed recently, probably during the past decade or two:
ƒ
A large landslide dating to the 1995 storms which obstructed and fundamentally
disrupted the channel of Los Osos Creek, near approximately one-half mile upstream of
the mountain front, in an eastward draining unnamed tributary near the horseshoe bend
at the western-most point of the creek’s trace;
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ƒ
Several feet of channel incision and up to many tens of feet of bank retreat, in a highlydisturbed reach extending more than 2 miles downstream from the western-most bend
just upstream of the mountain front to the head of the depositional reach;
ƒ
Erosion of dune deposits on the west bank of Los Osos Creek, especially upstream of
Los Osos Valley Boulevard, but also downstream as well;
ƒ
Incision of gullies through the dune deposits of the Baywood formation, likely
associated with intensifying land uses; Downstream from this highly-disturbed reach,
incision of tributaries and more limited bank retreat along Los Osos Creek
ƒ
Downstream from the highly-disturbed reach, incision of tributaries and more limited
bank retreat along Los Osos Creek.
A separate study into sediment sources has been underway, directed by the Morro Bay
National Estuary Program. Results of that study will provide information essential to refining
the plan for the lower portions of Los Osos and Warden Creeks.
8. Increasing rates of sediment delivery from upstream are evidenced in the study area by (a) a
bed with significantly more sand and gravel than appear in the beds of older channels exposed
in the cutbanks of the creek. Similarly, the proportion of white ‘chalkrock’ gravels and cobbles
(the source formation for the 1995 landslide) is presently higher than in most (but not all) older
channels.
9. The high and recently-increased rates of sediment delivery from Los Osos Creek, unless
reduced or mitigated by continued dredging, will tend to promote a shallower, broader, and
steeper stream geometry, possibly resulting eventually in a sudden major change in course,
with Los Osos Creek cutting a new channel across the valley to join Warden Creek.
10. The proposed project was formulated by the cooperators. It includes four main elements:
ƒ
Setting back the levee downstream of the main crossing,
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ƒ
Ceasing cultivation of a setback corridor along Warden Creek from Warden Lake
downstream to the farmstead hill where the two creeks come closest together,
ƒ
Allowing the new setback areas along both Los Osos and Warden Creek to revert to
riparian corridors, inducing sedimentation,
ƒ
Continuing ‘mining’ or removing sediment and debris from the channel as needed
following major storms to maintain (a) conveyance, and (b) a continuous channel, such
that steelhead are able to move upstream and downstream.
ƒ
Balance Hydrologics presented a number of alternatives which were considered briefly
at several stages of program development.
11. We modeled water levels during design flows of varying magnitude using the conventional
HEC-RAS program. Modeling results show that it will be feasible to allow riparian vegetation
to develop within the setback corridors without increasing the risk of flooding during a given
storm, at least initially. Eventually, the desired sedimentation in the setback corridors will
increase the level of the floodplain within the levees to the point that water levels may be higher
during at least some design storms. In actuality, the aging riparian vegetation will gradually
become less rough as the individual trees – although larger – become progressively less dense
and smoother, which in turn will reduce the rate of deposition in the overbank areas. Hence,
the duration of increased conveyance will depend largely upon the rate of sediment
accumulation, which will depend upon the duration of the current accelerated sedimenttransport rates.
12. We also developed a sedimentation model specific to lower Los Osos Creek which
established that (a) virtually all sand and the vast preponderance of silts transported by Los
Osos Creek may be settled in the project area, (b) the volume of deposition will be constrained
by slanted valley, and (c) when Los Osos Creek eventually changes course and joins the muchlower alignment of Warden Creek, the proposed project will still be effective in retaining sands
and most silts.
13. Overbank deposition along Los Osos Creek associated with the proposed project may
eventually total about 10,000 to 25,000 cubic yards, under assumptions described in the report,
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equivalent to from less than one year up to several years of sediment transport in Los Osos
Creek. Sedimentation along Warden Creek is expected to be relatively minor, as most coarse
silt and sands settle in Warden Lake, just upstream of the lower Los Osos corridor.
14. Sedimentation potential is currently limited by the presence of high value agriculture on the
middle of the slanted valley floor, generally lower than the bed of Los Osos Creek during and
following storms. Any program to induce sedimentation runs the risk of contributing to an
sudden channel change, which may be forthcoming even absent the proposed project. If Los
Osos Creek does shift to the lower alignment of Warden Creek, much greater sedimentation
potential may be achieved.
15. Strategic and ethical questions remain as to when to use the current limited sedimentation
potential. It may be used now, when sediment from the highly-disturbed reach will be
retained, or it may be used later to keep otherwise hard-to-settle silts and clays out of Morro
Bay, once this reach has been stabilized. It is also feasible to pursue a combined strategy of
beginning restoration of the riparian corridors now, followed by construction of structures to
induce sedimentation once the current rain of sediment has been reduced.
16. While the scope of the current report is restricted to a preliminary investigation of the
potential for sediment storage on this site, we recommend that other potential sites within the
watershed be identified for floodplain restoration in order to reduce sediment loads to Morro
Bay and increase the longevity of the project currently under consideration.
17. Eventually, further sediment storage may be created by building a low berm across the
valley along the existing road to the farmstead hill on the Morrissey property, near cross-section
14. A two-foot berm in this location may induce deposition of an additional 150 to 200 acre feet
of sediment and woody material; a lower berm would retain less. Structures of this type have
been used elsewhere in the Central Coast counties, ranging from the causeway at Stanford’s
Searsville Lake to the Mono Creek sediment-retention facility just upstream of Gibraltar
Reservoir in Santa Barbara County. The Stanford facility, constructed in 1929, is a particularly
apt analog.
18. Sedimentation at this site should be monitored, annually and after storms with a recurrence
of about 2 years or more, with several objectives:
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ƒ
The depth of sediment accumulation in the straight reach downstream of the Eto
crossing should be checked for potential blockage of steelhead passage or for filling of
the channel to the extent that conveyance is appreciably reduced.
ƒ
Logjams temporarily obstructing flows should be reduced in size such that no more than
15 percent of the channel conveyance is reduced; progressively lower thresholds may be
considered in future years if the present channel instabilities upstream of Los Osos
Boulevard and current rates of incision persist. In some years, this may require walking
Los Osos Creek from tidewater to the southern edge of the present Eto lands upstream
of cross-section 1.
ƒ
Long-term changes in the stream profile should be monitored to assess whether
significant and persistent changes are occurring, and whether these may be related to
sedimentation promoted at this site. We suggest that re-surveys of cross-sections 1, 4, 5,
6, 12, 14, 17, and 22 be performed at intervals of approximately 5 years and following
events with recurrences of about 10 years or more in either or both creeks. The program
should purchase and/or review recent or post-flood aerial photography from slightly
upstream of cross-section 1 as far downstream as Santa Ysabel Avenue (tidewater) to
assess if avulsion or other significant change in channel course may be occurring, and
what effects these changes may have on sediment delivery to and steelhead passage to
our from Morro Bay. Monitoring should be conducted by a specialist in channel
behavior or sedimentation, with experience in steelhead passage, and state-registered in
an appropriate profession.
ƒ
At least once a year, growers or other lessees should meet with program managers
regarding the condition of crossings, wells, water-supply lines, major drains and other
infrastructure to assess compatibility of the program with ongoing agricultural
operations.
Many decisions will be limited by lack of information on the volume of sediment to be managed
under various flow and watershed conditions. If the sponsoring agencies wish to resolve
existing discrepancies and to understand the sediment yields of Los Osos and Warden Creek
beyond an order-of-magnitude estimate, a basic 2- or 3-year sediment-monitoring program
measuring bedload- and suspended-sediment transport rates and conducted in a manner
similar to those used in other nearby streams; alternatively, a rapid sediment budget may be
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developed. While there have been many estimates of sediment yield or sediment-transport
dynamics for the Los Osos portion of the Morro Bay watershed, they are not based on data from
this portion of the watershed. Existing reports acknowledge differences exceeding a factor of 10,
and little agreement on when and what type of sediment is mobilized. Especially since gaging
has been initiated a short distance upstream, it would be a a responsible course of action to
concurrently measure sediment transport before recommending sediment-management
measures.
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8. REFERENCES
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resources bulletin, v. 28, no. 5, p. 813-832.
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Coastal San Luis Resource Conservation District (CSLRCD), 2000, Chorro Flats Enhancement Project:
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Crawford, Multari, and Star; Jones and Stokes Associates; Philip Williams & Associates; Habitat
Restoration Group, 1992, Appendices to the background report, prepared for the Coastal San Luis
Resource Conservation District and the California State Coastal Conservancy.
Crawford, Multari, and Star; Jones and Stokes Associates; Philip Williams & Associates; Habitat
Restoration Group, 1993, Analysis of options and alternatives report, prepared for the Coastal San
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Management Plan, prepared for the Coastal San Luis Resource Conservation District and the
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Crawford, Multari, and Star; Jones and Stokes Associates; Philip Williams & Associates; Habitat
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Funk River Consultants, 1998, River morphological conditions within the Morro Bay Watershed, Morro
Bay, California: Consulting report prepared for the Morro Bay National Estuary Program, 27 p.
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Geological Survey Water-Resources Investigation Report 77-56.
Gomez, B., and M. Church, 1989, An assessment of bed load sediment transport formulae for gravel bed
rivers: Water Resources Research, v. 25, no. 6, p. 1161-1186.
Hall, C.A., 1973, Geologic map of the Morro Bay South and Port San Luis quadrangles, San Luis Obispo
County, California: U.S. geological survey misc. field Studies Map MF 511.
Haltiner, J.P., 1991, Sedimentation processes in Morro Bay, California, in proceedings from Coastal
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Hecht, B., 1984, Sequential changes in bed habitat conditions in the upper Carmel River following the
Marble-Cone fire of August 1977: in Warner, R., and Hendrix, K. (eds), California Riparian Systems:
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Hecht, B., 1993, South of the spotted owl -- Restoration strategies for episodic channels and riparian
corridors in Central California: Society of Wetland Scientists 1993 Conference Proceedings, Davis,
CA., p. 104-121
Hecht, B., and Owens, J., 1996, Approaches to in-situ calculation of floodplain roughness: Proceedings of
the Association of State Floodplain Managers, 20th annual conference, June 10-14. San Diego, CA, p.
142-148.
Knudsen, K., Hecht, B., Holmes, D.O., and Flaschka, I., 1992, Hydrologic and geomorphic factors
affecting management of the lower Sisquoc River alluvial corridor, Santa Barbara County, California:
Balance Hydrologics, Inc., consulting report prepared for SP Milling Company. 72 p.
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Philip Williams & Associates, Ltd., 1993, Recommendations on the potential acquisition of property or
conservation easements on the G. Martines property, San Luis Obispo County (Morro Bay Area),
California: Consulting report prepared for the California State Coastal Conservancy, 7 p.
Philip Williams & Associates (PWA); Crawford, Multari, and Star; Jones and Stokes Associates (JSA);
Habitat Restoration Group, 1996, Conceptual Plan Refinement/Final Design Issues, Chorro Flats
Enhancement and Management Plan, prepared for the Coastal San Luis Resource Conservation
District and the California State Coastal Conservancy, 44 p. plus appendices.
Philip Williams & Associates, Ltd., 1997, George Martines property: Observations/recommendations:
Memorandum of February 24, 1997 to Karyn Gear, California State Coastal Conservancy, 4 p.
Reid, L.M., and T. Dunne, 1996, Rapid evaluation of sediment budgets, Catena Verlag, Reiskirchen,
Germany, 164 p.
Schmit, E., 1992, Coastal San Luis Resource Conservation District file memo, “George Martines WRP
Applicant”, dated August 1, 1992, 1 page.
Tetra Tech, Inc., 1998a, Watershed streamflow model, Morro Bay, California: Consulting report prepared
for the Morro Bay National Estuary Program, 26 p., appendices and addendum.
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the Morro Bay National Estuary Program. 40 p.
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Coastal San Luis Resource Conservation District and the California State Coastal Conservancy, 51p.
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TABLES
FIGURES