DATA COLLECTION AND CONCEPTS MODEL
DEVELOPMENT AND VALIDATION
Prepared by
Eddy J. Langendoen
Watershed Physical Processes Research Unit
U.S. Department of Agriculture – Agricultural Research Service
National Sedimentation Laboratory,
P.O. Box 1157,
Oxford, MS. 38655
April 2013
Prepared as part of agreement #60-6408-8-088 (Enhanced Stream-Corridor Modeling Tools for
Adaptive Management of Tahoe Basin Streams; P003) with:
USDA Forest Service
Pacific Southwest Research Station
1731 Research Park Dr
Davis, CA 95618
This research was supported using funds provided by the Bureau of Land Management through
the sale of public lands as authorized by the Southern Nevada Public Land Management Act.
TABLE OF CONTENTS
TABLE OF CONTENTS ................................................................................................................. I
LIST OF FIGURES ........................................................................................................................ II
LIST OF TABLES ........................................................................................................................ IV
1. INTRODUCTION AND BACKGROUND ............................................................................... 1
1.1 Objectives and Scope ................................................................................................................ 2
1.2 Approach, Methodology, and Geographic Location of Research ............................................. 3
1.2.1 Quantifying the Effects of Vegetation and Bio-Engineered Treatments ............................... 5
1.2.2 Near-Bank Groundwater Model: Dynamic Pore-Water Pressure .......................................... 5
1.2.3. Lateral Migration of Meanders ............................................................................................. 5
1.2.4 Evaluating Performance of Restoration Treatments and Validating Model Results ............. 6
2. FIELD DATA COLLECTION ................................................................................................... 8
2.1 Streambank hydrology .............................................................................................................. 8
2.1.1 Upper Truckee River.............................................................................................................. 8
2.1.2 Trout Creek .......................................................................................................................... 15
2.2 Resistance to Erosion of Channel Boundary Materials .......................................................... 20
2.2.1 Trout Creek .......................................................................................................................... 21
2.3 Channel Geometry of Trout Creek ......................................................................................... 25
3. CONCEPTS MODEL ENHANCEMENTS ............................................................................. 40
3.1 Near-bank Groundwater Model .............................................................................................. 40
3.2 Lateral Migration Model ......................................................................................................... 41
4. CONCEPTS MODEL VALIDATION ..................................................................................... 43
4.1 Model Setup ............................................................................................................................ 43
4.2 Model Results ......................................................................................................................... 47
REFERENCES ............................................................................................................................. 54
I
LIST OF FIGURES
Figure 1 Map showing locations of study reaches on the Upper Truckee River and Trout Creek.
......................................................................................................................................................... 4
Figure 2 Map of monitoring sites on the Upper Truckee River. Symbols: blue triangle, water
surface transducer; red plus, tensiometer; orange circle, groundwater transducer; pink square,
snow staff gage; white square, acoustic distance sensor; blue pentagon, soil moisture probe. ...... 9
Figure 3 Photos of the monitoring sites on the Upper Truckee River: (a) Site 1, (b) Site 2, (c)
Site 3, and (d) Site 4. ..................................................................................................................... 10
Figure 4 Observed precipitation, groundwater elevation (GWE), and pore-water pressure (PWP)
at Site #1 on the Upper Truckee River.......................................................................................... 12
Figure 5 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and
pore-water pressure (PWP) at Site #2 on the Upper Truckee River. ............................................ 13
Figure 6 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and
pore-water pressure (PWP) at Site #3 on the Upper Truckee River. ............................................ 14
Figure 7 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and
pore-water pressure (PWP) at Site #4 on the Upper Truckee River. ............................................ 15
Figure 8 Map of monitoring sites on Trout Creek. Symbols: blue triangle, water surface
transducer; red plus, tensiometer; orange circle, groundwater transducer; pink square, snow staff
gage; white square, acoustic distance sensor; blue pentagon, soil moisture probe....................... 16
Figure 9 Photos of the monitoring sites on Trout Creek: (a) Site 1, (b) Site 2, (c) Site 3, and (d)
Site 4. ............................................................................................................................................ 17
Figure 10 Observed precipitation, surface water (SWE) and groundwater elevations (GWE),
and pore-water pressure (PWP) at Site #1 on Trout Creek. .......................................................... 18
Figure 11 Observed precipitation, surface water elevation (SWE), and pore-water pressure
(PWP) at Site #3 on Trout Creek. ................................................................................................. 19
Figure 12 Observed precipitation, surface water and groundwater elevations, and pore-water
pressure at Site #4 on Trout Creek. ............................................................................................... 20
Figure 13 Location of data collection sites on Trout Creek to characterize the resistance to
erosion of channel boundary materials. . ...................................................................................... 22
Figure 14 Grain-size distribution of bank-material samples collected along Trout Creek ......... 24
Figure 15 Grain-size distribution of riffles along Trout Creek. The riffles labeled PCx-x xx
were sampled by SHG (2004). The first number indicates monitoring reach (cf. Figure 17), the
second number indicates cross section, and the last number indicates year of sampling. The
riffles labeled Trout #x were sampled in 2008; the number indicates sampling site (cf. Figure
13). ................................................................................................................................................ 24
Figure 16 Grain-size distribution of pools along Trout Creek. The numbers in the legend
indicate data collection site (cf. Figure 13). .................................................................................. 25
Figure 17 Location of reaches and cross sections on Trout Creek monitored by Swanson
Hydrology + Geomorphology (2004). . ........................................................................................ 26
Figure 18 Observed changes in thalweg elevation of the reconstructed Trout Creek channel
between 2001 and 2008. ............................................................................................................... 29
Figure 19 Observed changes in thalweg elevation along the reaches monitored by Swanson
Hydrology + Geomorphology (2004) of the reconstructed Trout Creek channel between 2001
and 2008. ...................................................................................................................................... 30
Figure 20 Observed changes in cross section geometry along Reach 1 on Trout Creek. ........... 34
II
Figure 21 Observed changes in cross section geometry along Reach 2 on Trout Creek. ........... 35
Figure 22 Observed changes in cross section geometry along Reach 3 on Trout Creek. ........... 36
Figure 23 Observed changes in cross section geometry along Reach 4 on Trout Creek. ........... 37
Figure 24 Observed changes in cross section geometry along Reach 5 on Trout Creek. ............ 38
Figure 25 Observed changes in cross section geometry along Reach 6 on Trout Creek. ........... 39
Figure 26 Definition sketch of variables used to formulate the CONCEPTS sub-model of
groundwater dynamics. ................................................................................................................. 40
Figure 27 Trout Creek modeling reach with model sections. Cold Creek discharges into cross
section 152+05. ............................................................................................................................. 44
Figure 28 List of soil profiles used in the CONCEPTS model of Trout Creek. For example, the
soil profile between river stations 201+00 and 192+00 comprised four layers ranging from a
medium loamy sand near the floodplain surface to a silt loam layer at depth. ............................. 45
Figure 29 List of soils used in the CONCEPTS model of Trout Creek. The figure shows the
properties for the “fine loamy sand” soil. ..................................................................................... 45
Figure 30 List of sediment profiles used in the CONCEPTS model of Trout Creek. For
example, the riffle profile between river stations 182+00 and 162+00 comprises three layers
ranging from a coarse riffle layer at the surface to a lacustrine layer at depth. ............................ 46
Figure 31 List of sediments used in the CONCEPTS model of Trout Creek. The figure shows
the properties for the “riffle 182+00 – 162+00” sediment............................................................ 46
Figure 32 Hourly discharge record used in the CONCEPTS model. Horizontal lines indicate,
bankfull, five-year, and ten-year discharge................................................................................... 47
Figure 33 Simulated water surface profile for bankfull flow discharge of 2.5 m3/s. .................. 48
Figure 34 Simulated change in bed elevation for bankfull flow discharge of 2.5 m3/s. ............. 49
Figure 35 Simulated change in channel top width for bankfull flow discharge of 2.5 m3/s ....... 49
Figure 36 Simulated water surface profile for 5-year flow discharge of 5.6 m3/s. ..................... 50
Figure 37 Simulated change in thalweg elevation for 5-year flow discharge of 5.6 m3/s. ......... 51
Figure 38 Simulated change in channel width for 5-year flow discharge of 5.6 m3/s. ............... 51
Figure 39 Simulated change in thalweg elevation between Oct 1, 2010 and Sep 30, 2009. ...... 52
Figure 40 Simulated change in channel top width between Oct 1, 2010 and Sep 30, 2009. ...... 53
III
LIST OF TABLES
Table 1 Discharges used in the CONCEPTS simulations. .......................................................... 47
IV
1. INTRODUCTION AND BACKGROUND
Substantial progress has been made on understanding and quantifying the contribution of stream
channel erosion to sediment loading of Lake Tahoe (Simon et. al. 2003, Simon 2006).
Streambank erosion has been estimated to account for about 25% of the total fine-sediment load
entering Lake Tahoe (Simon, 2006). Much of this material emanates from unstable reaches of the
Upper Truckee River, while Blackwood and Ward Creeks are also large contributors. Stream
restoration has been accepted as an appropriate method to mitigate bank and channel instability,
improve water quality, and contribute to other Basin Thresholds, with significant investment in
planning, design, and implementation over several years. However, there are continuing needs
for improved predictive modeling of channel processes with tools validated in the Lake Tahoe
Basin at both watershed and project-specific scales, including representation of typical
restoration features (channel, bank, and floodplain morphology, materials, hydrology, and
vegetation). Modeling tools are needed that: 1) simulate channel response and quantify water
quality performance of alternative restoration designs prior to approval and construction; and 2)
estimate channel adjustments following implementation to inform adaptive management
decisions.
As open systems, alluvial streams operate in a balance (dynamic equilibrium) between the
upstream delivery of sediment and the transport capacity. Changes to this balance, either through
direct modification of the channel or by indirect changes in sediment delivery or hydrology (i.e.
land use changes), adjust the stream’s ability to transport sediment and consequently cause
erosion and deposition. Thus, the best intended projects still represent “disturbances” to the
fluvial system and must be designed/constructed to maintain a balance of flow energy and
sediment transport so that stabilization of one reach does not lead to instability in adjacent
reaches. This is referred to as sediment continuity. For instance, if sediment delivery from an
upstream reach is cut off, it is possible that the imbalance between transport capacity and
sediment delivery to the downstream reach will induce renewed streambed incision and
associated streambank erosion.
On the Upper Truckee River for example, at least three projects along adjacent reaches were in
the planning stages by different design groups in 2008, yet none of these projects explicitly
analyzed for streambank stability or account for sediment contributed by bank failures in
calculations of sediment continuity. Estimating future channel stability, migration, bank erosion,
and resultant water quality for the project designs is hindered by the lack of geotechnical
information and accepted tools. Similarly, restoration projects implemented on Angora Creek
and Trout Creek have been monitored, but lack quantified information on performance of
streambank and other restoration treatments, leading to the following questions regarding
potential fine-load reductions from streambank erosion:
1.
2.
3.
4.
Are the restoration treatments performing as designed?
Why are some treatments more successful than others?
Have fine-loadings to the lake been reduced, and by what magnitude?
Have projects resulted in a properly functioning channel that supports habitat for
native species?
1
There is a critical need for a comprehensive planning and prediction tool that deterministically
incorporates bed and bank processes. Furthermore, with the widespread use of vegetation and
bio-engineering techniques in restoration projects, data and numerical tools are required to
account for the effects of riparian vegetation on geotechnical and hydraulic resistance of channel
materials.
Two such tools were used previously in the Lake Tahoe Basin. The conservational channel
evolution and pollutant transport system (CONCEPTS) model (Langendoen and Alonso, 2008;
Langendoen and Simon, 2008) was used successfully to simulate sediment transport and channel
changes on three streams tributary to Lake Tahoe: Upper Truckee River, Ward Creek, and
General Creek (Simon et al., 2003). The deterministic, numerical model is unique in accounting
for hydraulic and geotechnical processes that control streambank erosion in addition to bed
processes and the routing of flow and sediment. The bank-stability algorithms are similar to
those included in the Bank-Stability and Toe-Erosion Model (BSTEM) (Simon et al., 2000;
2011; Simon and Pollen, 2005), which was used along a reach of the Upper Truckee River to
predict critical conditions for bank stability and the reinforcing effects of riparian vegetation
(Simon et al., 2006).
These existing numerical tools are ideal for addressing the types of critical issues concerning
stream-restoration design and performance aimed towards reducing fine-sediment loadings to
Lake Tahoe. Model enhancements to address load reductions from specific treatments and
projects would provide long-term benefits to basin agencies. Recent research has shown that the
two most critical variables to accurately predicting bank strength, stability and channel-widening
rates are cohesion and pore-water pressure (Langendoen and Simon, 2008). Vegetation plays a
large role in controlling these variables. Work by Pollen and Simon (2005) using fiber-bundle
theory and direct shear experiments of root-permeated soils showed that predictions of the
magnitude of cohesion provided by root reinforcement were being overestimated by as much as
100% using conventional techniques. Enhancements to the CONCEPTS and BSTEM models to
include a near-bank groundwater model, and quantification of the role of riparian roots on
hydraulic resistance to erosion would improve predictive abilities. Addition of a lateral migration
component to CONCEPTS would then complete a state-of the-art set of tools for adaptive
management of streams tributary to Lake Tahoe. Field experiments of erosion resistance of bioengineered treatments and numerical simulations of channel and treatment response could be
validated with time-series data collected from selected streams in the Tahoe Basin.
1.1 Objectives and Scope
The primary objectives of the proposed study are to enhance and further validate the predictive,
numerical models CONCEPTS and BSTEM to fully realize their potential as state-of the-art
tools for stream management, in the Lake Tahoe Basin and elsewhere. The project-scale data
collection and resultant model enhancement will specifically inform adaptive management
strategies for local restored streams, and the validated models will be available for use in
evaluating design and expected performance of proposed restoration projects. Specific study
objectives include:
2
Quantifying the effects of riparian vegetation and bio-engineered treatments on the
resistance of bank materials to hydraulic erosion and bank undercutting for inclusion into
both models;
Developing a near-bank groundwater model to integrate with CONCEPTS and BSTEM
for the purpose of simulating pore-water pressures dynamically;
Developing algorithms for CONCEPTS to simulate lateral migration of meandering
channels in a deterministic fashion by accounting for hydraulic and geotechnical controls;
and
Validate the use of the CONCEPTS and BSTEM models at the project-scale for existing
and restored reaches of selected Tahoe basin streams using time-series historical data on
flow, sediment transport and channel geometry.
The underlying hypothesis of this research effort is that with improved definition of the effects of
vegetation and other bio-engineered treatments on boundary resistance coupled with enhanced
algorithms to simulate channel response, resource managers in the Lake Tahoe Basin will have
the ability to accurately evaluate restoration strategies to reduce fine-sediment loadings to the
lake with state-of the-art numerical tools.
1.2 Approach, Methodology, and Geographic Location of Research
The geographic scope of the presented research is selected reaches of the Upper Truckee River
and Trout Creek (Figure 1). Trout Creek is included because it contains well documented
restoration efforts that have been installed for several years.
The approach combines field and numerical experiments on the effects of vegetation, bioengineered treatments and hydraulics in meander bends with the development and validation of
process-based algorithms to better simulate pore-water pressure distributions, streambank
erosion, lateral migration of meanders, and fine-sediment loadings. The approach builds on prior
field data and modeling efforts in the same stream systems (Simon et al., 2003; Simon, 2006;
Simon et al., 2006), and validation opportunities provided by post-project monitoring of
implemented projects.
3
Figure 1 Map showing locations of study reaches on the Upper Truckee River and Trout Creek.
4
1.2.1 Quantifying the Effects of Vegetation and Bio-Engineered Treatments
To accurately evaluate the susceptibility of root-permeated and bio-engineered streambanks to
erosion, channel widening and lateral migration, the hydraulic and geotechnical resistance of
these features must be quantified. A series of field experiments were conducted using a root
tensile-strength testing device (Abernethy and Rutherford, 2001) in conjunction with root
mapping to quantify root reinforcement (Simon and Collison, 2002; Pollen and Simon, 2005) of
meadow and other riparian vegetation throughout the bank profiles. Submerged jet-test devices
(Hanson, 1990; Tolhurst et al., 1999) were used to determine hydraulic resistance (critical shear
stress and erodibility) of bank sediments with and without riparian roots as well as bioengineered treatments. Results of these field experiments provide values of critical shear stress
( c; stress for incipient motion of particles) and the erodibility coefficient (kd; erosion volume per
unit of stress, per unit of time) that are important input parameters into both numerical models
for predicting bank-toe erosion and undercutting (Hanson and Simon, 2001). This study is
documented in the accompanying report by Bankhead et al. (2013).
1.2.2 Near-Bank Groundwater Model: Dynamic Pore-Water Pressure
Sub-models were developed or enhanced for BSTEM and CONCEPTS to simulate: (1) the
spatial distribution of pore-water pressure in a streambank and along a stream-riparian corridor;
and (2) the effectiveness of different riparian species in controlling streambank erosion by
increasing the resistance of the bank to hydraulic and geotechnical forces. Data were collected at
selected sites on Trout Creek and the Upper Truckee River to determine: permeability, soil
texture, soil water retention curve, above- and below-ground plant biomass, volumetric soil
water content, pore-water pressure, and groundwater table elevation. Data on groundwater levels
and pore-water pressure distributions are available for some reaches (EDAW, 2006; Simon et al.,
2006). Additional pore-water pressure data for model validation were obtained using digital
tensiometers installed at various depths and distances from the channel along selected reaches
representing a range of riparian-buffer systems (i.e., meadow vegetation, woody species). This
approach was successfully used for Lemmon’s willow and Lodgepole pine along the Upper
Truckee River (Simon et al., 2006). The data were used to: (1) develop species-specific data
bases for BSTEM, and (2) test and validate the groundwater sub-models of BSTEM and
CONCEPTS. Validation runs were conducted to compare simulated pore-water pressure
distributions with time-series of pore-water pressure data obtained with the digital tensiometers
along the study streams. The effects of different vegetation and bio-engineered treatments were
then simulated with the validated models to evaluate and compare the effectiveness of various
treatments. This report presents the CONCEPTS groundwater sub-model. The BSTEM model
enhancements are documented in the accompanying report by Bankhead et al. (2013).
1.2.3. Lateral Migration of Meanders
To accurately predict channel evolution in sinuous streams and to evaluate restoration strategies
that include re-meandering of channels, a model was developed to be used together with
CONCEPTS to simulate the lateral migration and sediment loads of meandering streams. The
RVR Meander model (Abad and Garcia, 2006) was enhanced with the process-based streambank
erosion algorithms of CONCEPTS to provide a comprehensive, physically-based model of
5
lateral channel migration at engineering time scales. The combined model improves upon
current, commonly used quasi two-dimensional models derived using perturbation analysis (e.g.,
Larsen, 1995; Sun et al., 2001) that are limited to cross sections of simple geometry with vertical
banks linear transverse bed slope, at constant width. These models relate lateral retreat to nearbank velocity through an empirical erosion coefficient, and, therefore, are less useful for
simulating lateral erosion rates of specific bends where materials and treatments may vary. By
simulating the lateral distributions of flow and boundary shear stress, the enhanced model is able
to predict the divergence of sediment-transport rates and bank-toe scour within sinuous streams.
Shear stress distributions calculated by RVR Meander can be used to improve bank erosion
estimates by CONCEPTS in meandering streams. The enhancements to the RVR Meander model
and its application to Trout Creek are documented in the accompanying papers of Motta et al.
(2011; 2012).
1.2.4 Evaluating Performance of Restoration Treatments and Validating Model Results
Validation of simulations with deterministic, numerical models such as CONCEPTS and
BSTEM is best conducted with high-quality field data to define the available forces and
boundary resistance that control the specific processes being simulated. Because the models
simulate erosion by both hydraulic and geotechnical forces, field data quantifying channel
dimensions, hydraulic resistance (particle-size for coarse sediments and jet tests for cohesive
sediments) and geotechnical resistance (cohesion, friction angle bulk unit weight, pore-water
pressure and root reinforcement) will be collected along the selected reaches. Fortunately, a
wealth of data already exists along a number of the proposed study reaches of Trout Creek and
on the Upper Truckee River, having been collected as part of earlier studies (Walck, 2001;
Walck 2004; Simon et al., 2003; Swanson Hydrology and Geomorphology, 2004). Rapid
geomorphic assessments were conducted along all of the study reaches to determine the relative
stability and dominant geomorphic processes that are active along channels and treatments. This
technique, which has been used at more than 300 sites in the basin (Simon et al., 2003) provides
an index of channel stability and the performance of treated reaches.
Bed- and bank-material particle size data were collected at cross sections along previously unsampled reaches as were geotechnical data to define shear strength (cohesive and frictional) of
the bank materials. The latter were conducted in situ for at least two layers in each bank profile
using a borehole shear-test (BST) apparatus (Lutenegger and Hallberg, 1981). Critical shear
stress and the erodibility coefficient of fine-grained bank materials were measured in situ using
submerged jet-test devices (Hanson, 1990; Tolhurst et al., 1999). Both of these types of
instruments have been used successfully in previous data-collection efforts along Tahoe basin
streams.
BSTEM simulations provide site specific predictions of appropriate streambank design and
performance criteria using selected geometry, vegetation or other treatments. In contrast,
CONCEPTS simulations provide quantifiable evaluations of the cumulative impact of all
treatments within the reach on fine-load reduction as well as any downstream impacts on channel
processes and morphology. The CONCEPTS simulations of the Trout Creek reach are for the
purpose of validating model enhancements and for evaluating the performance of various
restoration treatments. BSTEM simulations on Trout Creek and the Upper Truckee River were
6
conducted to validate the near-bank groundwater model and to test performance of specific bioengineered treatments on streambank stability and reduction of fine-sediment loads. This report
documents the CONCEPTS simulations. The BSTEM simulations are documented in the
accompanying report by Bankhead et al. (2013).
7
2. FIELD DATA COLLECTION
Field data were collected to accurately characterize the forces acting on the channel boundary
materials (represented by channel slope and cross-sectional geometry) and the resistance to
erosion (boundary material texture, erodibility, shear-strength, and degree of saturation) of the
channel boundary. The resistance to erosion of failed bank-material blocks is reported by
Bankhead et al. (2013). Because of previous boundary material characterization studies (Simon
et al., 2003; Simon, 2006), fewer data were collected along the Upper Truckee River study
reaches.
2.1 Streambank hydrology
Streambank erosion can be greatly enhanced by soil water content of the bank soils. Soil water
content is determined by the processes of infiltration from precipitation and snowmelt through
the floodplain surface, lateral inflows from upland and stream, and evapotranspiration.
Vegetation has been found to greatly modify local soil water content by both increasing
evapotranspiration and infiltration (Simon and Collison, 2002). Stream restoration measures in
the Tahoe Basin increasingly include management of the riparian zone through vegetative
measures. Hence, four sites each on the Upper Truckee River and Trout Creek were
instrumented to measure the movement of riparian water for various vegetation types. Sites were
selected based on the presence of nearby existing groundwater wells. Instruments consisted of
combinations of tensiometers to measure pore-water pressure, transducers to measure stream
water surface elevation and groundwater elevation, soil moisture probes, and acoustic sensors
and staff gages to measure depth of snowpack. Instruments were installed in October 2007, and
data were collected to August 2009.
2.1.1 Upper Truckee River
Two sites (#1 and #2) were located on the Golf Course Reach and two sites (#3 and #4) were
located on the Sunset Stables Reach of the Upper Truckee River (see Figure 2). Figure 3 shows
a photograph of each site.
Site #1
SHG (2003) classified the vegetation at Site 1 as Lodgepole Pine Forest. The instrument
installation consisted of (Figure 2): (1) a nest of four tensiometers located at 1 and 3 m from the
bank edge and at depths of 0.3 and 1 m below the floodplain elevation, and (2) a transducer at
well #14 monitored by the State of California Parks and Recreation.
Figure 4 compares the measured groundwater elevation and pore-water pressure. The pore-water
pressure at a depth of 0.3 m is strongly correlated with precipitation events, whereas the porewater pressure at a depth of 1.0 m is more strongly correlated with the groundwater elevation.
During the spring snowmelt period groundwater elevation and pore-water pressure head are
similar. During the dry summer and fall period, evapotranspiration provides almost 5 m of
suction, which is equivalent to approximately 10 kPa of added cohesive strength to the bank
material.
8
Figure 2 Map of monitoring sites on the Upper Truckee River. Symbols: blue triangle, water
surface transducer; red plus, tensiometer; orange circle, groundwater transducer; pink square,
snow staff gage; white square, acoustic distance sensor; blue pentagon, soil moisture probe.
9
Site #2
SHG (2003) classified the vegetation at Site 2 as Dry Meadow. The instrument installation
consisted of (
Figure 2): (1) a nest of four tensiometers located at 1 and 3 m from the bank edge and at depths
of 0.3 and 1 m below the floodplain, (2) a transducer at well #19 monitored by the State of
California Parks and Recreation, (3) a snow staff, and (4) a surface water elevation transducer.
Figure 5 compares the measured surface water and groundwater elevations and pore-water
pressure. The pore-water pressure at a depth of 0.3 m is strongly correlated with precipitation
events and the water surface elevation, whereas the pore-water pressure at a depth of 1.0 m is
more strongly correlated with the groundwater elevation. During the spring snowmelt period
groundwater elevation and pore-water pressure head are similar, and the streambank profile is
completely saturated. During the dry summer and fall period, the pore-water pressure at 0.3 m is
in equilibrium with the surface water elevation, whereas the soil remains fairly wet at a depth of
1 m especially during and immediately following the snowmelt season. Evapotranspiration
seems less important here, probably because the bank faces northeast.
(a)
(b)
(c)
(d)
Figure 3 Photos of the monitoring sites on the Upper Truckee River: (a) Site 1, (b) Site 2, (c)
Site 3, and (d) Site 4.
10
Site #3
The vegetation at Site 3 can be classified as Dry Meadow. The instrument installation consisted
of (Figure 2): (1) a nest of four tensiometers located at 1 and 3 m from the bank edge and at
depths of 0.3 and 1 m below the floodplain, (2) a tensiometer located 3 m from the bank edge at
a depth of 1.5 m below the floodplain, (3) a transducer at a well monitored by the Tahoe
Conservancy, (4) a soil moisture probe at a depth of 0.3 m below the floodplain, (5) a staff and
acoustic distance sensor to measure depth of snowpack, and (6) a surface water elevation
transducer.
Figure 6 compares the measured surface water and groundwater elevations and pore-water
pressure. During the observation period groundwater elevation is strongly correlated and similar
to the surface elevation except for the month of March. Ice dynamics may affect the surface
water elevations during this period. The pore-water pressure at a depth of 0.3 m is strongly
correlated with precipitation events and the groundwater elevation in Winter and Spring. The
pore-water pressures at a depth of 1.0 and 1.5 m are mainly determined by the groundwater
elevation. During the spring snowmelt period the top of the streambank profile remains
unsaturated. During the dry summer and fall period, evapotranspiration provides about 4 m of
suction (ca. 8 kPa of added cohesive strength) at a depth of 0.3 m and about 2 m (ca. 4 kPa of
added cohesive strength) at a depth of 1 m. The significant increase in apparent cohesion at a
depth of 1 m can be probably attributed to the fact that the bank faces west.
11
Figure 4 Observed precipitation, groundwater elevation (GWE), and pore-water pressure (PWP)
at Site #1 on the Upper Truckee River.
Site #4
The vegetation at Site 4 can be classified as Dry Meadow Mixed with Willow. The instrument
installation consisted of (Figure 2): (1) a nest of four tensiometers located at 1 and 3 m from the
bank edge and at depths of 0.3 and 1 m below the floodplain, (3) a transducer at a well installed
fairly close to the tensiometers, and (4) a surface water elevation transducer.
Figure 7 compares the measured surface water and groundwater elevations, and pore-water
pressure. As at Site 3, groundwater elevation is strongly correlated and similar to the surface
elevation during the observation period except for the month of March. The pore-water pressure
at a depth of 0.3 m is strongly correlated with precipitation events and the groundwater elevation
in Winter and Spring. The pore-water pressure at a depth of 1.0 is mainly determined by the
groundwater elevation. During the spring snowmelt period the top of the streambank profile
remains unsaturated. During the dry summer and fall period, evapotranspiration provides about
5 m of suction (ca. 10 kPa of added cohesive strength) throughout the top 1 m of the bank
profile. The significant increase in apparent cohesion at a depth of 1 m compared to that at Site 3
may be caused by the increased transpiration provided by the willows.
12
Figure 5 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and
pore-water pressure (PWP) at Site #2 on the Upper Truckee River.
13
Figure 6 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and
pore-water pressure (PWP) at Site #3 on the Upper Truckee River.
14
Figure 7 Observed precipitation, surface water (SWE) and groundwater elevations (GWE), and
pore-water pressure (PWP) at Site #4 on the Upper Truckee River.
2.1.2 Trout Creek
Three sites were located upstream of the Cold Creek tributary and one site was located
downstream of the Cold Creek tributary (see Figure 8). Figure 9 shows a photograph of each
site. All sites can be classified as Wet Graminoid Meadow. Site 3 has some willow at the
bank’s edge.
15
Figure 8 Map of monitoring sites on Trout Creek. Symbols: blue triangle, water surface
transducer; red plus, tensiometer; orange circle, groundwater transducer; pink square, snow staff
gage; white square, acoustic distance sensor; blue pentagon, soil moisture probe.
16
(a)
(b)
(c)
(d)
Figure 9 Photos of the monitoring sites on Trout Creek: (a) Site 1, (b) Site 2, (c) Site 3, and (d)
Site 4.
Site #1
The instrument installation consisted of (Figure 8): (1) a nest of four tensiometers located at 1
and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain, (2) a soil
moisture probe at a depth of 1 m below the floodplain, (3) a surface water transducer, (4) a
transducer at installed at a nearby groundwater well, and (5) a staff gage and acoustic distance
sensor to measure snowpack depth.
Figure 10 compares the measured surface water and groundwater elevations and pore-water
pressure. The tensiometers at a depth of 1 m did not work properly for most of the observation
period (from April 2008). The tensiometer 1 m from the bank’s edge at a depth of 0.3 m was not
working properly during the summer of 2008. Groundwater elevation is strongly correlated and
similar to the surface water elevation except for the month of March. The pore-water pressure at
a depth of 0.3 m is strongly correlated with precipitation events and the groundwater elevation in
winter and spring. During the spring snowmelt period the top of the streambank profile remains
unsaturated. The wet meadow vegetation does not provide any transpiration-induced strength
(through suction) to the bank soil.
17
Figure 10 Observed precipitation, surface water (SWE) and groundwater elevations (GWE),
and pore-water pressure (PWP) at Site #1 on Trout Creek.
Site #2
The instrument installation consisted of a nest of four tensiometers located at 1 and 3 m from the
bank edge and at depths of 0.3 and 1 m below the floodplain (see Figure 8). The tensiometers
did not work correctly, therefore the measured pore-water pressures are not reported here.
Site #3
The instrument installation consisted of (Figure 8): (1) a nest of four tensiometers located at 1
and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain, and (2) a surface
water transducer.
Figure 11 compares the measured surface water and pore-water pressure. The pore-water
pressure at a depth of 0.3 m is strongly correlated with precipitation events and the water surface
elevation. During the winter of 2008 the entire bank profile was saturated. The pressure head
nearest to the stream (1 m from the bank’s edge) is almost identical to the water surface
elevation, indicating that the soil water content is controlled by the water surface elevation in the
stream and movement of water into the bank. During the summer of 2008, further away from the
18
bank, about 30 cm of suction (or approximately 0.5 kPa of added cohesive strength) is provided
by the meadow vegetation in the upper part of the soil profile.
Figure 11 Observed precipitation, surface water elevation (SWE), and pore-water pressure
(PWP) at Site #3 on Trout Creek.
Site #4
The instrument installation consisted of (Figure 8): (1) a nest of four tensiometers located at 1
and 3 m from the bank edge and at depths of 0.3 and 1 m below the floodplain, and (2) a staff
gage to measure depth of snowpack.
Figure 12 shows the measured pore-water pressure. The pore-water pressure at a depth of 0.3 m
is strongly correlated with precipitation events and remains fairly high during the wet season and
when snowpack is present. At a depth of 1 m, pressure head is generally lower than in the upper
part of the streambank, indicating it is controlled by the water surface or groundwater elevation.
During the spring snowmelt the upper part of the bank profile remains unsaturated. During the
dry period, the meadow vegetation provides about 5 m of suction (approximately 10 kPa of
added cohesive strength) in the upper part of the soil profile. The lower part of the Trout Creek
floodplain seems to be drier than the area upstream of the Cold Creek confluence. This could be
19
attributed to a combination of: (1) patches of willow trees across the floodplain; and (2) lowering
of the water surface elevation caused by channel incision (see section 2.3).
Figure 12 Observed precipitation, surface water and groundwater elevations, and pore-water
pressure at Site #4 on Trout Creek.
2.2 Resistance to Erosion of Channel Boundary Materials
Validation of simulations with deterministic, numerical models such as CONCEPTS and
BSTEM is best conducted with high-quality field data to define the available forces and
boundary resistance that control the specific processes being simulated. Because the models
simulate erosion by both hydraulic and geotechnical forces, field data quantifying channel
dimensions, hydraulic resistance (particle-size for coarse sediments and jet tests for cohesive
sediments) and geotechnical resistance (cohesion, friction angle bulk unit weight, pore-water
pressure and root reinforcement) were collected along selected reaches on the Upper Truckee
River and Trout Creek.
Geotechnical data to define shear strength (cohesive and frictional) of the bank materials was
measured in situ for at least two layers in each bank profile using a borehole shear-test (BST)
apparatus (Lutenegger and Hallberg, 1981). Critical shear stress and the erodibility coefficient
of fine-grained bank materials was measured in situ using submerged jet-test devices (Hanson,
1990; Tolhurst et al., 1999).
20
The data collected along the Upper Truckee River are reported by Simon et al. (2003), Simon
(2006), Simon et al. (2006), and Bankhead et al. (2013).
2.2.1 Trout Creek
Bank and bed-material samples, and data on resistance to erosion and failure were collected at 12
sites on Trout Creek (see Figure 13). Data are available on CD-ROM in the Microsoft Excel file
named “Physical data.xlsx.”
The texture of the collected bank soil samples ranges from silt loams to coarse loamy sands
(Figure 14). The soils are resistant to erosion by mass failure. Measured effective cohesion
ranges from 0.7 kPa to 28.8 kPa (18 tests), with a mean cohesion of 11.3 kPa and a median
cohesion of 10.2 kPa. The resistance to erosion is characterized by a critical shear stress and
erodibility. The critical shear stress ranged from 0.2 Pa to 9.1 Pa (20 tests), with a mean value of
2.2 Pa and a median value of 1.4 Pa. This critical shear stress is similar to a stress needed to
move a very coarse (1-2 mm) sand particle. The erodibility coefficient had a fairly constant
value of 1E-5 m s-1 Pa-1.
SHG (2004) reported grain-size distribution of the riffles in 2001 and 2003 located within their
six monitoring reaches (cf. Figure 17). Selected riffles were resampled in 2008. Grain-size
analysis of the riffles along Trout Creek shows a downstream fining (Figure 15). The most
upstream riffles from Site 1 through halfway Site 4 and 5 have a mean diameter of 38.1 mm (red
line #1 in Figure 15). From this location to the confluence with Cold Creek the riffles have a
mean diameter of 27.9 mm (green line #2 in Figure 15), whereas the riffles downstream of the
confluence with Cold Creek have a mean diameter of 17.1 mm (blue line #3 in Figure 15).
Bed material in the pools is mainly comprised of medium and coarse sands (0.25 – 2 mm).
There is little fine-grained (<63 m) sediment, indicating that these sediments are mainly
transported as wash load. At certain locations where the bed has incised, such as near the
confluence with Cold Creek and in the reach downstream of the Cold Creek confluence, a
lacustrine layer containing about 25% clay has been exposed.
21
Figure 13 Location of data collection sites on Trout Creek to characterize the resistance to
erosion of channel boundary materials. (Continued on next page).
22
Figure 13 Location of data collection sites on Trout Creek to characterize the resistance to
erosion of channel boundary materials.
23
Figure 14 Grain-size distribution of bank-material samples collected along Trout Creek
Figure 15 Grain-size distribution of riffles along Trout Creek. The riffles labeled PCx-x xx
were sampled by SHG (2004). The first number indicates monitoring reach (cf. Figure 17), the
second number indicates cross section, and the last number indicates year of sampling. The
riffles labeled Trout #x were sampled in 2008; the number indicates sampling site (cf. Figure
13).
24
Figure 16 Grain-size distribution of pools along Trout Creek. The numbers in the legend
indicate data collection site (cf. Figure 13).
2.3 Channel Geometry of Trout Creek
The restored channel of Trout Creek was activated in 2001. Swanson Hydrology +
Geomorphology monitored the geomorphic adjustment between 2001 and 2003 (SHG, 2004).
Annual surveys were conducted of thalweg profile and four cross sections at six reaches (Figure
17). SHG (2004) found no significant adjustment of the channel had occurred by 2003. Some
sand had accumulated along Reach #5.
In August 2008 the thalweg of the entire reconstructed channel and the 24 cross sections
monitored by Swanson Hydrology + Geomorphology were resurveyed. Figure 18 and Figure 19
compare this survey with those reported in SHG (2004) and the channel design. It can be
observed that the riffles downstream of the confluence with Cold Creek have been eroded
between 2003 and 2008. This most likely occurred during the snow melt in 2006 when
discharge consistently exceeded the five-year discharge (~ 5.6 m3/s at USGS gage #10336775 on
Pioneer Trail, see also Figure 32). The riffles upstream of the confluence have remained fairly
stable. Only some scour on the edges of the riffles has taken place. Pools have generally
deepened by 0.3 to 0.6 m.
25
Figure 17 Location of reaches and cross sections on Trout Creek monitored by Swanson
Hydrology + Geomorphology (2004). (Continued on next page).
26
Figure 17 Location of reaches and cross sections on Trout Creek monitored by Swanson
Hydrology + Geomorphology (2004). (Continued on next page).
27
Figure 17 Location of reaches and cross sections on Trout Creek monitored by Swanson
Hydrology + Geomorphology (2004).
28
Figure 18 Observed changes in thalweg elevation of the reconstructed Trout Creek channel
between 2001 and 2008.
29
REACH 1
REACH 2
Figure 19 Observed changes in thalweg elevation along the reaches monitored by Swanson
Hydrology + Geomorphology (2004) of the reconstructed Trout Creek channel between 2001
and 2008. (Continued on next page).
30
REACH 3
REACH 4
Figure 19 Observed changes in thalweg elevation along the reaches monitored by Swanson
Hydrology + Geomorphology (2004) of the reconstructed Trout Creek channel between 2001
and 2008. (Continued on next page).
31
REACH 5
REACH 6
Figure 19 Observed changes in thalweg elevation along the reaches monitored by Swanson
Hydrology + Geomorphology (2004) of the reconstructed Trout Creek channel between 2001
and 2008.
32
Figure 20 to Figure 25 show the changes in cross-sectional geometry between 2001 and 2008 of
the reaches monitored by SHG (2004). Channel width adjustment is minimal in the riffle
sections. Only cross section 5A, which is located just downstream of the confluence with Cold
Creek, experienced significant erosion of approximately 0.5 m, which appears to have occurred
during the snowmelt of 2006.
Only three out of the twelve surveyed pool sections show significant erosion of the outer bank.
Cross section 1B experienced about 1.5 m of erosion, cross section 1D experienced about 2 m of
erosion, and cross section 4D experienced almost 2.5 m of erosion. The pool sections where
widening occurred also experienced infilling of their pool. Deposition is also occurring in pool
sections 2B, 4B, and 5D. The probability that streambank will occur at these sections may
therefore increase. Further, these sections are located upstream of the bendway apex. Note that
bank erosion on strongly curved bends (such as the Trout Creek bends) typically occurs at their
upstream section.
33
Figure 20 Observed changes in cross section geometry along Reach 1 on Trout Creek.
34
Figure 21 Observed changes in cross section geometry along Reach 2 on Trout Creek.
35
Figure 22 Observed changes in cross section geometry along Reach 3 on Trout Creek.
36
Figure 23 Observed changes in cross section geometry along Reach 4 on Trout Creek.
37
Figure 24 Observed changes in cross section geometry along Reach 5 on Trout Creek.
38
Figure 25 Observed changes in cross section geometry along Reach 6 on Trout Creek.
39
3. CONCEPTS MODEL ENHANCEMENTS
Two sub-models were developed for CONCEPTS to simulate the effects of riparian groundwater
and lateral channel migration on long-term channel morphology and sediment loads. The
collected field data presented in the previous section were used to test these enhancements.
3.1 Near-Bank Groundwater Model
Figure 4 through Figure 7 and Figure 10 through Figure 12 show that: (1) groundwater and
surface water elevations are closely correlated, and (2) soil water content above the groundwater
table is in equilibrium with the water table except for the dry summer period. Little erosion of
the channel boundary occurs during the summer and early fall period as flows are near base flow;
hence the enhanced shear strength contributed by matric suction in the upper portion during this
period is not important to the overall channel stability. As a result, the groundwater model for
use in the Lake Tahoe Basin can be simplified and only needs to account for exchanges between
surface water and groundwater through the bank face. In addition such model is very efficient
for long-term simulations.
Figure 26 shows a definition sketch of the variables used to formulate the groundwater submodel of CONCEPTS. Assuming a surface water elevation hs and groundwater elevation hg, the
mass flux of water between the stream and riparian zone, q, is a function of the difference in
elevations h = hg – hs. Seepage occurs if h > 0 and infiltration occurs if h < 0. Following
Darcy, the flux of water between stream and riparian zone is defined as:
(1)
where K is the saturated hydraulic conductivity of the bank soil. Assuming that this mass flux
drains or fills the riparian zone over a length L, the water balance for the riparian zone is:
Figure 26 Definition sketch of variables used to formulate the CONCEPTS sub-model of
groundwater dynamics.
40
(2)
Substituting Eq. (1) into Eq. (2) yields:
(3)
where = K / L has the unit of time-1. The time scale 1 / can be perceived as a lag time of the
groundwater table with respect to changes in surface water elevation. As surface water
calculations are decoupled from the groundwater calculations, Eq. (3) can be written as:
(4)
Eq. (4) has an analytical solution. For a model time step t, the change in groundwater elevation
can be calculated from:
(5)
3.2 Lateral Migration Model
To accurately predict channel evolution in sinuous streams and to evaluate restoration strategies
that include re-meandering of channels, the RVR Meander model was enhanced with the
CONCEPTS streambank erosion algorithms to accurately simulate the lateral migration and
sediment loads of meandering streams (see accompanying paper of Motta et al., 2012). The
enhanced RVR Meander model was tested on the observed migration of Trout Creek (Motta et
al., 2011). One-dimensional (1D) sediment transport models such as CONCEPTS are not
capable of simulating flow and sediment transport rates accurately in meander streams.
However, CONCEPTS can simulate the correct bank erosion rates at the reach scale when used
in combination with RVR Meander.
CONCEPTS uses an excess shear stress equation to calculate the rate of erosion of fine-grained
streambank materials by flowing water:
(6)
where E is erosion rate, kd is erodibility or detachment coefficient, is boundary shear stress, and
c is critical shear stress below which no erosion occurs. The detachment coefficient and critical
shear stress are soil properties, which can be measured in situ with devices such as the jet tester.
Their values for the bank material on Trout Creek were reported in section 2.2.1. In the
41
following analysis we assume, for simplicity, homogeneous bank soils, that is kd and
vary along the streambank. The only parameter that varies is boundary shear stress .
c
do not
The erosion rate along the outer bank of a meander bend, Ebend, is
(7)
where bend is the actual boundary shear stress acting on the bank material. The shear stress
can be accurately calculated by the RVR Meander computer model. Assuming the ratio
bend
(8)
where 1D is the bank shear stress predicted by a 1D model such as CONCEPTS, and substituting
Eq. (8) into Eq. (7) yields
(9)
Eq. (9) shows that CONCEPTS can simulate the correct fluvial erosion in meander bends by
modifying the resistance-to-erosion properties of the bank soils proportionally to the ratio , that
is use:
(10)
The ratio
can be calculated using RVR Meander.
42
4. CONCEPTS MODEL VALIDATION
4.1 Model Setup
Modeling Reach
The capabilities of the classic, one-dimensional version of CONCEPTS to evaluate the
performance of stream restoration measures to reduce sediments eroded from the channel
boundaries, were assessed for the Trout Creek Meadow Restoration Project completed in 2001.
The modeled reach extended from river station 102+70 to 200+70 (Phase II stationing), see
Figure 27. The reach comprises 172 model sections, which are typically located at each riffle
and pool along the reach.
Boundary Materials
The data presented in Section 2.2.1 were used to characterize the boundary material properties.
The data were analyzed to identify channel sections with similar characteristics. Figure 28 to
Figure 31 show the bed and bank materials and their properties used by CONCEPTS. The soil
erodibility properties for cross sections located in meander bends were modified using Eq. (10)
in section 3.2.
Flows
The flow records at USGS gages 10336775 (TROUT CK AT PIONEER TRAIL NR SOUTH
LAKE TAHOE, CA), 10336778 (COLD CK AT PIONEER TRAIL NR SOUTH LAKE
TAHOE, CA), and 10336780 (TROUT CK NR TAHOE VALLEY, CA) located at the bridge
crossing on Martin Avenue were analyzed to determine (1) hourly flow, (2) bankfull flow, and
(3) five-year flow conditions at the upstream end of the modeling reach and Cold Creek tributary
inflows for water years 2002 through 2009. Gages 10336775 and 10336780 bound the restored
channel reach of Trout Creek.
The tributary inflow from Cold Creek is on average 65% of the discharge reported at gage
10336775. A discharge record for Cold Creek was then constructed by multiplying the flows at
10336775 by 0.65. Table 1 lists the discharge with 2- and 5-yr return period and its probability
of exceedence. The bankfull discharge has a return period of about two years.
An hourly flow record at the upstream boundary of the modeling reach was constructed from the
instantaneous flow record at gage 10336775 (Figure 32). Largest discharges (about 5-year
recurrence interval) occurred during late December 2005 and the 2006 snowmelt season.
Bankfull flow was exceeded during the snowmelt seasons of 2003, 2005, and 2006 only.
43
Figure 27 Trout Creek modeling reach with model sections. Cold Creek discharges into cross
section 152+05.
44
Figure 28 List of soil profiles used in the CONCEPTS model of Trout Creek. For example, the
soil profile between river stations 201+00 and 192+00 comprised four layers ranging from a
medium loamy sand near the floodplain surface to a silt loam layer at depth.
Figure 29 List of soils used in the CONCEPTS model of Trout Creek. The figure shows the
properties for the “fine loamy sand” soil.
45
Figure 30 List of sediment profiles used in the CONCEPTS model of Trout Creek. For
example, the riffle profile between river stations 182+00 and 162+00 comprises three layers
ranging from a coarse riffle layer at the surface to a lacustrine layer at depth.
Figure 31 List of sediments used in the CONCEPTS model of Trout Creek. The figure shows
the properties for the “riffle 182+00 – 162+00” sediment.
46
10-yr flood
5-yr flood
bankfull
Figure 32 Hourly discharge record used in the CONCEPTS model. Horizontal lines indicate,
bankfull, five-year, and ten-year discharge.
Table 1 Discharges used in the CONCEPTS simulations.
Return period
2-year
5-year
Discharge in m3/s
Trout Creek upstream
Cold Creek inflow
2.5
1.6
5.6
3.6
%Exceedence
3.3
0.05
4.2 Model Results
Bankfull Flow
Figure 33 to Figure 35 show the water surface profile, change in bed elevation, and channel
widening for bankfull flow. Note that the model results are plotted in model simulation days.
As the %exceedence of the bankfull discharge is 3.3%, model simulation days need to be
adjusted to reflect actual, real days. For example, an 80 day simulation of bankfull flow
represents 80/0.033 ≈ 2,400 days (or 6.6 years). Such duration would approximately cover the
2001-2008 study period documented in section 2.
47
The predicted water surface profile is aligned with overbank elevation indicating that the used
Manning n values, which range from 0.03 to 0.06, properly represent boundary roughness.
Channel grade along the riffle sections shows negligible change, whereas the pool sections show
significant scour. Most pool scour ranges from 0.3 to 0.6 m, which is similar to that observed
and documented in section 2. Channel widening is zero except at a few locations where
deposition has taken place on riffles. This agrees reasonably well with the observed widening
documented in section 2.
The simulations show that the bankfull flow was unable to erode the riffles as was observed.
This could partly explain why very little widening has been observed.
Figure 33 Simulated water surface profile for bankfull flow discharge of 2.5 m3/s.
48
Figure 34 Simulated change in bed elevation for bankfull flow discharge of 2.5 m3/s.
Figure 35 Simulated change in channel top width for bankfull flow discharge of 2.5 m3/s
49
5-Year Discharge
Figure 36 to Figure 38 show the water surface profile, change in bed elevation, and channel
widening for 5-year flow discharge of 5.6 m3/s. The entire floodplain near the channel is
covered by approximately 0.3 m of water. Channel grade along the most upstream riffle sections
between river stations 200+70 and 163+15 shows negligible change, whereas the riffle sections
downstream and immediately upstream of the confluence with Cold Creek have been completely
removed causing the downstream reach to incise by approximately 1 m. This is very similar to
that observed (cf. Figure 18). It seems that about 20 model simulation days were needed,
however the gage record shows that the 5-year discharge was only exceeded for about 3 days. It
can be concluded that the size of the bed material comprising the riffles was sized too small. The
pool sections show significant scour. Even with the significant incision, channel widening is
limited to a few locations where deposition has taken place on riffles. The bank soils and sod
cover are sufficiently strong to withstand a discharge with a five-year return period for a limited
time.
Figure 36 Simulated water surface profile for 5-year flow discharge of 5.6 m3/s.
50
Figure 37 Simulated change in thalweg elevation for 5-year flow discharge of 5.6 m3/s.
Figure 38 Simulated change in channel width for 5-year flow discharge of 5.6 m3/s.
51
Hourly Flow Record
Simulating pool-riffle systems, where the riffle bed-material is significantly larger than that of
the pool, with a 1D model such as CONCEPTS may yield unrealistic results for long-term
simulations. The simulated scour of the pools at bankfull conditions is too large, causing
unrealistic deposition on the very coarse riffles. Therefore, it was decided too evaluate the
restored reach on Trout Creek for the observed discharge record between 2001 and 2009 by only
including the riffle sections. These sections are responsible for the vertical stability of the
channel. Also, the above simulations have shown that simulated channel widening is minimal.
Figure 39 and Figure 40 show the simulated adjustment of bed elevation and channel top width.
Both the simulated changes in bed elevation and channel top width are small. Significant
incision was predicted in the non-restored section connecting two reconstructed reaches around
model kilometer 2.5, where bed-material grain size is significantly smaller than that of the
adjacent riffles. The difference in simulated bed elevation adjustment for the 5-year flow
discharge (Figure 37) and that shown in Figure 40 is probably related to the duration at which the
higher discharges were active.
Figure 39 Simulated change in thalweg elevation between Oct 1, 2010 and Sep 30, 2009.
52
Figure 40 Simulated change in channel top width between Oct 1, 2010 and Sep 30, 2009.
53
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