KATLIAN BAY ROAD PROJECT
Preliminary Hydraulic Site and Scour Analysis
Memorandum
09 February 2016
Prepared for:
Alaska Department of Transportation
& Public Facilities
6860 Glacier Highway
Juneau, AK 99801-7999
Agreement No. 02543017
AKSAS No. 67672
Prepared by:
LEI Engineering & Surveying
310 K Street, Suite, 200
Anchorage, Alaska 99501
KATLIAN BAY ROAD
HYDROLOGY AND HYDRAULIC MEMORANDUM
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KATLIAN BAY ROAD
HYDROLOGY AND HYDRAULIC MEMORANDUM
Table of Contents
Page
LIST OF ACRONYMS ......................................................................................................... III
1
INTRODUCTION .......................................................................................................... 1
2
PROJECT DESCRIPTION ........................................................................................... 1
3
REGULATORY STANDARDS ..................................................................................... 2
4
HYDROLOGY .............................................................................................................. 3
5
HYDRAULIC DESIGN.................................................................................................. 6
5.1 Waterfall Bridge ................................................................................................... 6
5.2 Gorge Bridge ....................................................................................................... 6
5.3 Horseshoe Bridge ................................................................................................ 7
5.4 South Katlian Bridge ............................................................................................ 7
5.5 Middle Fork Bridge............................................................................................... 8
5.6 Katlian River Bridge ............................................................................................. 9
6
HYDRAULIC MODELING .......................................................................................... 11
7
PROPOSED BRIDGES .............................................................................................. 11
8
FEMA COMPLIANCE ................................................................................................ 12
9
SCOUR ANALYSIS ................................................................................................... 12
10 RIPRAP ..................................................................................................................... 12
Tables
Table 1:
Table 2:
Table 3:
Table 4:
Hydrologic Condition Parameters ................................................................... 4
Recurrence Interval Regression Equations for Region 1 in Alaska ................. 5
Recurrence Interval Stream Flows ................................................................. 5
Structure Type and Length........................................................................... 12
Figures
Figure 1:
Figure 2:
Figure 3:
Figure 4:
Figure 5:
Figure 6:
Figure 7:
Figure 8:
February 2016
Basin Area Map for each Drainage ................................................ Appendix A
Waterfall Bridge TS&L ................................................................... Appendix A
Gorge Bridge TS&L ....................................................................... Appendix A
Horsehoe Bridge TS&L ................................................................. Appendix A
South Katlian Bridge TS&L ........................................................... Appendix A
Middle Fork Bridge TS&L .............................................................. Appendix A
Middle Fork Bridge TS&L Alternate ............................................... Appendix A
Katlian River Bridge TS&L ............................................................ Appendix A
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HYDROLOGY AND HYDRAULIC MEMORANDUM
Appendices
Appendix A:
Appendix B:
Basin Area and Type Size & Location Figures
Structure Location Photos
List of Acronyms
AASHTO
CFR
DOT&PF
FEMA
FIS
HEC
HEC-RAS
HPR
LEI
MP
USFS
USGS
American Association of State Highway and Transportation Officials
Code of Federal Regulations
Alaska Department of Transportation and Public Facilities
Federal Emergency Management Agency
Flood Insurance Study
Hydraulic Engineering Circular
Hydrologic Engineering Centers River Analysis System
Halibut Point Road
LEI Engineering & Surveying
Milepost or Milepoint
United States Forest Service
United States Geological Survey
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KATLIAN BAY ROAD
HYDROLOGY AND HYDRAULIC MEMORANDUM
1
Introduction
This memo presents an overview and preliminary analysis of the hydrology and hydraulics
engineering for the Katlian Bay Road project, including a discussion of preliminary stream
crossing scour analysis. Included in this memo is a discussion of the six major proposed
stream and river crossings included in the project, the types of hydraulic modeling to be used
for detailed crossing design; the planned type, size, and location of the six major stream
crossing structures; FEMA compliance, and stream crossing foundation scour analysis
2
Project Description
The Alaska Department of Transportation and Public Facilities (DOT&PF) proposes to
conduct a state-funded project for construction of a new road on Baranof Island in the vicinity
of Katlian Bay, in the City and Borough of Sitka, Alaska. The Katlian Bay Road project would
construct approximately 9 miles of new single lane, unpaved road with bridge crossings,
beginning at the northern terminus of Halibut Point Road (HPR), extending east along the
south shoreline of Katlian Bay to cross the Katlian River, and terminating 2.5 miles east of the
Katlian Bay estuary at the boundary between Shee Atika and United States Forest Service
(USFS) lands.
The project is located in Township 55 South, Range 63 East, Section 2 and 3; Township 54
South, Range 63 East, Sections 25, 26, 34, and 35; and Township 54 South, Range 64 East,
Sections 21, 22, 28, 29, and 30.
Six major stream crossings were identified along the proposed alignment as requiring bridge
crossings and are the focus of this report. These bridge crossings are located at mileposts
(MP) 2.63, 3.35, 4.94, 6.24, 7.39, and 8.49 respectively and are the focus of this report. The
proposed structures for these bridge crossings would all be pre-engineered, pre-fabricated
single-lane structural steel truss bridges with the exception being the structure at MP 2.63,
which would be a structural plate arch. The lengths of the proposed structures at the time of
this report are: 24 feet, 85 feet, 95 feet, 105 feet, 40 feet and 150 feet, respectively. Each
structure is anticipated to be a single-span. The abutments on three out of the six structures
are founded on bedrock. Of the remaining three structures, two will have at least one abutment
founded on exposed bedrock. The other abutments will be either in shallow gravels over
bedrock or much deeper gravel deposits which will be investigated in the spring of 2015 for
inclusion into the Hydraulic Report, expected for completion the summer of 2015.
This report summarizes the results of a hydraulic investigation conducted during the 2014 field
season to determine the structure type necessary to bridge the waterways along the 9.1mile
extension of HPR. Pictures and observations were completed and are summarized in this
report. A scour analysis and riprap size requirements will be evaluated in depth for the 2015
Hydraulic Report for the HPR Extension/Katlian Bay Road. This report summarizes the
observations necessary to estimate span lengths and substructure costs at the preliminary
stage.
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HYDROLOGY AND HYDRAULIC MEMORANDUM
3
Regulatory Standards
The bridges slated for construction on the HPR Extension/Katlian Bay Road do not currently
reside within any known detailed Flood Insurance Studies (FIS) prepared by the Federal
Emergency Management Agency (FEMA) for Baranof Island, near Sitka Alaska in Katlian Bay.
All FEMA maps indicated that the streams located in Katlian Bay are either Zone A or Zone D
in the upper reaches. Zone A on each of the attached FIS maps do not have base flood
elevations established.
Three of the six structures are located in uplands, having no backwater effects from previous
road construction activities. The other three structures are located in the Katlian valley and
will have some influence on floodplains, due to the development of the approaches to the
crossings.
For any road alignment alternative that encroaches on a floodplain or supports base floodplain
development, a Location Hydraulic Study is required per Code of Federal Regulations (23
CFR 650.111). Consequently, a location hydraulic study will be performed for the South
Katlian River and the Katlian River. The Middle Fork of the Katlian River has no evidence of
over topping its banks in the last 50+ years. Land adjacent to the planned crossing location
is not part of a FEMA floodplain, but is instead considered uplands adjacent to the stream.
All structures, whether culvert or bridge, will be designed to be fish passable and to minimize
adjacent environmental effects to riparian habitat. Site-specific survey data has been taken to
perform the required hydraulic analysis per Section 1120.5.4 of the Alaska Department of
Transportation and Public Facilities Alaska Highway Preconstruction Manual.
Construction of these structures would comply with Section 450.9.7, Hydrology and
Hydraulics, of the Alaska Department of Transportation and Public Facilities Alaska Highway
Preconstruction Manual, dated 15 November 2013. The manual outlines the procedure for the
approval of each hydrology and hydraulics report for any given bridge structure designed by
a consultant and constructed by DOT&PF.
The hydrologic methods used to determine flood flow frequencies shall conform to the
standards prescribed in the Alaska Department of Transportation and Public Facilities Alaska
Highway Drainage Manual.
Hydraulic design standards shall conform to the standards prescribed in the Alaska
Department of Transportation and Public Facilities Alaska Highway Drainage Manual,
DOT&PF Standard Specifications for Highway Construction and the American Association of
State Highway and Transportation Officials (AASHTO) Highway Drainage Guidelines. The 23
CFR, part 650.111, Location Hydraulic Studies, requires a site-specific hydraulic/location
design that addresses all construction projects that encroach on the 100-year floodplain.
February 2016
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HYDROLOGY AND HYDRAULIC MEMORANDUM
4
Hydrology
The hydrology will be broken down by separating the six bridge crossings into two discrete
types: 1) uplands with relatively steep channel gradients and 2) flat gradient valley bottom
crossings. The six sites and the specific hydrologic conditions are presented in Table 1 on
page 4 below.Upland crossings have average channel gradients ranging from 10 to 21
percent. These channels are also typified by at least one or more waterfalls of varying height
between the crossing location and Katlian Bay. The three crossings included in the upload
crossings are named Waterfall, Gorge and Horseshoe in the uplands. The one crossing with
a lake in the basin above is the “Waterfall” crossing. The other two crossings have no lakes
in the basins above.
Flooding typically seen in low gradient streams is all but absent in these upland stream
crossings. In the uplands, occurrences are due to flash events from large rainfall and also
rain-on-snow events. Due to the limited basin length in the upland basins, and the channel
gradient, high-water events are a very short-duration event and typically do not experience
backwater effects due to the relatively steep channel gradients. The planned bridges at these
crossings are located in steeply incised draws that are typically defined by exposed bedrock
in the channel and on the sides of the incisions
Valley crossings have average channel gradients from 0.3 to 0.4 percent. These stream
channels typically include woody debris log jams between the crossing and the Katlian Bay
delta and follow a meandering water course. The three valley crossings are named South
Katlian, Middle Fork and Katlian River. The Katlian River is the only valley drainage basin in
the project that has a lake and glaciers in its headwaters. All of the other valley basins only
have snowpack feeding into the main stem and tributaries.
The three valley structures span channels upstream of the delta in the Katlian Bay
headwaters. These crossings are located at stream elevations of 12 feet, 17 feet and 50 feet
respectively. The channel reaches are braided on the South Katlian River and the Katlian
River but not in the Sukkaheen (Middle Fork). These two streams are much larger than the
Middle Fork and, consequently are moving much more bedload material and woody debris.
The Middle Fork is much smaller than the other two valley crossings and has no lateral
movement, as evidenced by its well-defined banks. The Middle Fork does not appear to
overtop its banks and was observed to rise minimally during large rainfall events in the fall of
2014 that caused large slides in Starrigavan Bay as well as a location within one mile of the
mouth of the South Katlian River. Site-specific conditions for each stream crossing are
presented in Table 1 below.
Over 50 percent of the Middle Fork Basin occurs on the glacial rebounding flat east of the
Katlian Bay delta. The flat is believed to act like a large sponge, due to the gravel deposits
throughout this portion of the basin. These deposits absorb rapidly and release water much
more slowly into the Middle Fork, thereby reducing the large fluctuations in water surface
elevations that were observed on the South Katlian River and Katlian River but not on the
Middle Fork.
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HYDROLOGY AND HYDRAULIC MEMORANDUM
Table 1:
Bridge #
Bridge name
Milepost
Hydrologic Condition Parameters
1
Waterfall
2
3
Gorge
Horseshoe
4
South Katlian
5
Middle Fork
6
Katlian River
2.63
3.35
4.94
6.24
7.39
8.49
Length
24
85
95
105
40
150
Width
16
32
16
16
16
16
Foundation
Bedrock
Bedrock
Bedrock
Bedrock/Gravel
Gravel
Bedrock/Gravel
Crossing
Elevation(ft)
380
514
203
12
17
50
Channel
Length (mi)
1.20
2.02
2.05
7.26
2.85
12.48
Length
Above (mi)
0.85
1.30
1.67
6.58
1.78
10.08
Length Below
(mi)
0.35
0.72
0.38
0.68
1.07
2.40
Channel
Grade %
20.56
13.52
10.12
0.33
0.30
0.39
Maximum
Elevation(ft)
2,445
2,880
2,655
4,900
3,200
5,095
Minimum
Elevation(ft)
Sea Level
Sea
Level
Sea Level
Sea Level
Sea Level
Sea Level
0.242
0.813
1.094
12.813
1.219
37.656
Basin
(Sq. Mi.)
Flooding and high-water events usually occur between the months of October and April. The
larger flood events occur during the October and November storm events and to a lesser
extent, the spring melt-off in April and May. Discussions with local residents have led to this
observation.
The hydrology that will be used for the 2015 Hydraulic Analysis and Report will be obtained
from the United States Geologic Survey (USGS) report titled Estimating the Magnitude and
Frequency of Peak Streamflows for Ungaged Sites on Streams in Alaska and Conterminous
Basins in Canada (USGS) Water-Resources Investigations (Report 03-4188). Use of the
regression equations in this report will be the best analysis for deriving the expected stream
flows. In the hierarchy of determining peak discharges, gage data has first priority over other
methods when determining flow. There is a gage station that recorded events on Indian River.
Data from this station will be incorporated as is appropriate.
Drainage basin sizes for each stream crossing have been obtained from USGS published
quadrangle maps. This is shown on Figure 1 in Appendix A. None of the crossings have
gaging stations on them; consequently no correlation between gaging stations and the
respective rivers/streams crossed will be made. The following regression equations for the
specified recurrence interval (Qt) will be used in determining flow events. Regression
equations for Regions 1 and 3 for the State of Alaska will be used and are included in Table
2 below.
February 2016
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HYDROLOGY AND HYDRAULIC MEMORANDUM
Table 2:
Recurrence Interval Regression Equations for Region 1 in Alaska
Average
Standard
Error
Prediction
(Log Units)
Average
Standard
Error
Prediction
(Percent)
Ave. Equivalent
Years of Record
Q2 = 0.004119 A0.8361 (ST+1)-0.3590 P 0.9110 (J+32)1.635
0.158
38
0.88
Q5 = 0.009024 A0.8322 (ST+1)-0.3670 P 0.8128 (J+32)1.640
0.156
37
1.30
Q10 = 0.01450 A0.8306 (ST+1)-0.3691 P 0.7655 (J+32)1.622
0.157
37
1.80
Q25 = 0.02522 A0.8292 (ST+1)-0.3697 P 0.7165 (J+32)1.588
0.161
38
2.40
Q50 = 0.03711 A0.8286 (ST+1)-0.3693 P 0.6847 (J+32)1.559
0.166
40
2.80
Q100 = 0.05364 A0.8281 (ST+1)-0.3683 P 0.6556 (J+32)1.527
0.171
41
3.10
Q500 = 0.1209 A0.8272 (ST+1)-0.3646 P 0.5948 (J+32)1.449
0.188
45
3.60
Regression Equation for Recurrence Interval Qt
The regression equations will be plotted in the 2015 Hydraulic Report as a best fit line for the
natural logarithm of the recurrence interval, in years, and the flow, in cubic feet per second. A
log-log plot of the flood recurrence versus peak discharge will be shown on a figure in the
subsequent Hydraulic Report for the rivers and streams in Katlian Bay. A summary of peak
discharges at each stream/river crossing is listed in Table 3 below.
Table 3:
Recurrence Interval Stream Flows
Recurrence
Interval
Waterfall
Gorge
Horseshoe
South Katlian
Middle Fork
Katlian River
2-yr
62
170
218
1,703
141
4,195
5-yr
85
233
299
2,315
191
5,677
10-yr
101
276
353
2,724
225
6,669
25-yr
120
328
420
3,228
267
7,891
50-yr
134
367
469
3,604
299
8,805
100-yr
148
404
516
3,960
329
9,669
500-yr
181
492
629
4,816
403
11,748
Note:
Volume for each recurrence interval is measured in cubic feet per second.
No overtopping events are expected at proposed bridge crossings. However, this will be
verified through the use of Hydrologic Engineering Centers River Analysis System (HEC-RAS)
and will be used to model and determine the flow at which the road would be overtopped.
Regression analysis would be used to determine the year at which this flow would occur.
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HYDROLOGY AND HYDRAULIC MEMORANDUM
5
Hydraulic Design
Two different approaches will be used to determine flood elevations at proposed stream
crossings. Both methods rely on the flow volumes derived from the regression equations
established in Section 3 (Hydrology) above. The first method for the upland crossings will be
simple conveyance, where channel gradients and debris flows dictate structure size and
placement rather than structure capacity for flow volume. The second method is a backwater
analysis using HEC-RAS (Version 4.1, dated January 2010) and will be developed for the
three valley crossings located on the South Katlian River, the Sukkaheen (Middle Fork), and
the Katlian River.
The water surface elevations will be computed in HEC-RAS by using the flows calculated from
the regression equations, slope of the streambed, overbank reach lengths, and roughness
coefficients for each portion of the stream cross sections. The average stone size in the bed
of the stream will help to determine scour potential as well, pending the results of the HECRAS models.
A discussion of each crossing and their physical characteristics is provided below.
5.1
Waterfall Bridge
The Waterfall Bridge crossing is located on a tributary that flows directly into Katlian Bay
(Figure 1 Drainage Basin Map). This drainage is typified by a series of waterfalls and large
cobble to 36” in diameter in the substrate of the channel. At the crossing, the channel has
exposed bedrock with a 30-foot waterfall at the outfall of the proposed 24-foot structural plate
arch. The sides of this channel are rounded where the soil on either side of the channel has
eroded away leaving an incised channel. Failure of these incised channel banks leads to a
rounded slope near the bottom and steeper towards the upper slopes of the incision. There is
a fair amount of woody debris in the channel both upstream and downstream of the crossing.
An examination in the field led to the following observations: large debris flows have occurred
in the past, likely originating uphill from 1,000 to 2,000 feet based upon available orthographic
photos found online and flown in the field. Based on site observations, the observed historic
high water elevations spanned up to 24 feet. The observed ordinary high water is
approximately 20 feet in width at the crossing. Downstream where the stream flows over the
top of the waterfall, the stream narrows down to 12 feet in width with no observed back
watering effects. Photos of the crossing are included in Appendix B.
5.2
Gorge Bridge
The Gorge Bridge crossing is located on Clearcut Creek, which flows directly into Katlian Bay
(Figure 1 Drainage Basin Map). This drainage is typified by a series of waterfalls and large
cobble up to 48 inches in diameter in the channel substrate. At the crossing, the channel has
exposed bedrock with steeply incised channel walls. There is a fair amount of woody debris
in the channel both upstream and downstream of the crossing. An examination in the field led
to the following observations:

Large debris flows have occurred in the past and scoured the channel from top to
bottom;

The channel is stable and unmoving;
February 2016
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HYDROLOGY AND HYDRAULIC MEMORANDUM

Crossing foundations will be on bedrock; and

The minimum soffit elevations are expected to be at least 40 feet above the channel
bed.
The mouth of Clearcut Creek is a very large alluvial fan that resulted from upstream debris
flows. Some of this activity has occurred due to past logging activity but the majority of it has
come from near the basin headwaters. Historic high water elevations are roughly a 5-foot
maximum off of the channel bed. The minimum soffit 35 feet above this should yield no
problems with debris passage under the proposed structure. Photos were taken of the site but
are too dark to display in this report. Additional photos will be taken and submitted in the 2015
Hydraulic Report.
5.3
Horseshoe Bridge
Horseshoe Bridge crossing is located on a tributary that flows directly into Katlian Bay (Figure
1 Drainage Basin Map). This drainage is typified by a series of waterfalls and large cobble up
to 30 inches diameter in the channel substrate. At the crossing, the channel has exposed
bedrock with incised channel walls. There is a fair amount of woody debris in the channel both
upstream and downstream of the crossing. An examination in the field led to the following
observations:

Large debris flows have occurred in the past and periodically scour the channel from
top to bottom;

There was a large slope failure just upstream of the crossing on the east side;

The channel bed itself is stable and unmoving;

Crossing foundations will be on bedrock; and

Minimum soffit elevations are expected to be a minimum of 30-feet above the channel
bed.
The mouth of the tributary is an alluvial fan which is evidence of the debris flows from
upstream. Historic high water elevations are roughly a 5-foot maximum off of the channel bed.
The minimum soffit 30 feet above this area will yield no problems with debris passage under
the proposed structure. Even the recent debris avalanche, just upstream of the proposed
structure, would not have resulted in the loss of the structure due to impact, plugging and
scour. Photos of the crossing are included in Appendix B.
5.4
South Katlian Bridge
The South Katlian Bridge crossing is located on the South Katlian River, which flows directly
into Katlian Bay (Figure 1 Drainage Basin Map). This drainage is typified by a series of braided
channels and a very low gradient channel. The South Katlian has a longitudinal gradient of
just under 2 percent for the first 5 miles of the basin. The remaining portion of the basin rises
steeply to a little over 4,000 feet in elevation. The substrate is typically small gravel and sands,
with larger cobbles up to 8 inches in diameter. The D50 of the rock is approximately 2 inches
in diameter. The stream section itself is in a large flat that appears to have some influence
from side channels that failed in the past and pushed the stream to the east until the debris
flows were scoured out and the channel reasserted itself in its current location. There are two
–7–
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KATLIAN BAY ROAD
HYDROLOGY AND HYDRAULIC MEMORANDUM
known overflow channels within 300 feet of the planned crossing, located along the flat to the
south. The first will have a 10-foot span structural steel plate pipe arch for the conveyance of
the overflow and the other will be a 6-foot diameter corrugated metal pipe. The bedload
material here is less than 2.5 inches in diameter and has very low velocities.
Historically; there have been large debris flows on both sides of the basin, the most recent of
which occurred in November of 2014.
At the crossing, the channel has a well-graded substrate that is used as spawning gravel.
There is a fair amount of woody debris in the channel both upstream and downstream of the
crossing. An examination in the field led to the observation that large debris flows have
previously occurred on either side of the channel, which has recruited a significant amount of
woody debris to the stream channel. Due to the relative abundance of woody debris in the
channel, the minimum soffit elevation will be set a minimum of 5-feet above the 100-year event
water surface elevation, as modeled through the use of HEC-RAS and the regression
equations listed above. The channel bed itself is relatively stable and unmoving at the crossing
due to a large bedrock outcrop on the north side of the crossing. From the crossing, the South
Katlian River turns east and flows along the bedrock outcrop for roughly another 0.5 miles
before entering Katlian Bay. The crossing foundations will be on bedrock to the north and
potentially a deep foundation in gravel deposits to the south. Geotechnical explorations in
2015 will determine the substructure foundations to be discussed in the 2015 Hydraulic
Report. Historic high-water elevations are estimated to be roughly 5 feet above the current
channel bed elevation. The minimum soffit 5-feet above the computed high-water elevation
will minimize problems with debris passage under the proposed structure. It was noted during
the 2014 field season that the recent debris avalanche just upstream of the proposed structure
would not have put the structure at risk due to the avalanche having not reached the river.
This debris avalanche was well over 100 acres in size but, due to the glaciated slopes, the
avalanche ran out prior to reaching the river. Photos of the crossing are included in Appendix
B.
5.5
Middle Fork Bridge
The Middle Fork Bridge crossing is located on the Sukkaheen River that flows directly into
Katlian Bay (Figure 1 Drainage Basin Map). This drainage is typified by a meandering channel
with a very low gradient. The Sukkaheen River has a longitudinal gradient of just under 0.2
percent for the first 1.2 miles of the basin. The remaining portion of the basin rises steeply to
a little over 3,200 feet in elevation. The substrate is typically small gravel and sands with larger
cobbles up to 8 inches in diameter. The D50 of the bedload is approximately 0.5 inches in
diameter. The stream section itself is in a large flat that has been logged in the past and does
not appear to have moved laterally in the last 50+ years. The flat that comprises much of the
drainage basin for the Sukkaheen River is a combination of conifer forest, deciduous forest,
muskeg, and alpine shrub. The flat seems to act as a large sponge, due to the very porous
and gravelly nature of the ground, and releases water slowly into this river system. No large
water level fluctuations were observed during the heaviest events during the fall and winter of
2014. The stream does not appear to overtop its banks nor does it exhibit signs of scour. With
the very low stream gradients and the shallow flood depths, scour is almost non-existent even
with the small D50 in the bedload.
February 2016
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HYDROLOGY AND HYDRAULIC MEMORANDUM
Approaches from the north and south of the crossing make this crossing nearly perpendicular
to the flow of the stream. The approaches are not on a traditional floodplain but on what would
be considered minor uplands of the stream. Modeling this stream would demonstrate that the
overbank areas are not used for storage.
Upstream of the crossing is a relic log stringer bridge that has been in place since
approximately 1962. The structure is beginning to fail into the stream due to degradation of
the log stringers. The measured opening is 16 feet and does not appear to have been
overtopped. The bottom of beam is approximately 3-feet off of the channel bed. Pictures of
this structure are included in Appendix B. There are no overflow channels within several
hundred feet of this crossing.
At the crossing, the channel has a well-graded substrate that is used as spawning gravel.
There is a fair amount of woody debris in the channel both upstream and downstream of the
crossing. An examination in the field led to the observation that large alder on either side of
the channel have remained in place since logging in the early 1960s occurred. These alder
trees are being recruited to the stream as they are beginning to reach their age of decline. A
typical Red Alder forest reaches the age of maturity and subsequent decline at 60-years. In
approximately 7 years (2022), much of the alder within the basin can be expected to begin
dying off. This will result in substantial recruitment of woody debris to the stream, as these
trees fall into the stream and the adjacent riparian area. Given this knowledge, a minimum
soffit elevation of 3-feet above the top-of-bank would be preferred for this structure. The
minimum soffit elevation will be set by calculating the 100-year event water surface elevation
through the use of the regression equations listed above. The channel bed itself is stable and
unmoving at the crossing.
The crossing foundations will likely be on gravel deposits on both sides of the crossing. Due
to the low deck elevation of the structure and the low-volume road design, this structure is
considered a ‘low-risk’ structure and may well be suited to a spread footing rather than deep
piles. Geotechnical explorations in 2015 will help to better determine the substructure
foundations to be discussed in the 2015 Hydraulic Report. Historic high-water elevations are
roughly a 2.5 feet maximum above the channel bed. The minimum soffit elevation 3-feet above
this will minimize problems with debris passage under the proposed structure. Photos of the
crossing are included in Appendix B.
5.6
Katlian River Bridge
The Katlian River Bridge crossing is located on the Katlian River, which flows directly into
Katlian Bay (Figure 1 Drainage Basin Map). This drainage is typified by a series of braided
channels and a very low gradient channel. The Katlian has a longitudinal gradient of just 1.15
percent for the first 8.25 miles of the basin, rising to 500 feet in elevation at the far reach of
the valley floor. The remaining portion of the basin rises steeply up to a little over 5,000 feet
in elevation in under a mile.
The substrate is typically small gravel and sands with larger cobbles up to 8 inches in
diameter. The D50 of the rock is approximately 2 inches in diameter. The stream crossing is at
a confined portion of the river where a bedrock outcrop on the south side directs the flow to
the northwest where it impacts the bedrock outcrop on the north side of the river. The outcrop
on the south side will be used to protect the southerly abutment, and the northerly abutment
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will be founded on the bedrock outcrop. Historic aerial photography over the last 70 years
shows minimal to no lateral migration of this crossing, which can be attributed to these bedrock
outcrops on both sides of the river.
The approach from the south side of the river will cross a floodplain that will require three
overflow structures for overflow channels that bypass the bedrock outcrop on the south side
of the river. These overflow structures will be discussed in further detail in the 2015 Hydraulic
Report for this crossing. None of these overflow structures is anticipated to exceed 12 feet in
diameter. The approach fills for the main Katlian River Bridge are expected to be
approximately 15 feet in depth from the existing ground surface. The bedload material in the
overflow channels seem to vary, where to the south, small rock, and to a lesser extent woody
debris, are moving through two of the three overflow channels leading to the main bridge
crossing. The overflow channel closest to the southerly abutment appears to pass very little
rock, but does have larger quantities of woody debris. It can be inferred that the woody debris
is largely floating and the velocities are lower closer to the abutment, thereby accommodating
floating debris. Based upon the ground configurations and the modeling to be performed,
these variables will be accounted for to demonstrate any scour protection needed for the
various overflow structures. None of the rock in the overflow areas is over 2 inches in diameter.
Historically there have been large debris flows on both sides of the Katlian River basin. In an
aerial survey of the Katlian River basis performed in early 2015, photos were taken from an
aircraft (presented in Appendix A). These photos demonstrate the amount of woody debris
in the form of trees with rootwads attached that will eventually pass the Katlian River Bridge.
At the bridge crossing, the channel has a well-graded substrate that is used as spawning
gravel. There is a fair amount of woody debris in the channel both upstream and downstream
of the crossing. An examination in the field led to the observation that large debris flows on
either side of the channel that occurred in the past, resulted in the recruitment of a significant
amount of woody debris into the stream channel. Due to the relative abundance of woody
debris in the channel, the minimum soffit elevation will be set a minimum of 5-feet above the
100-year event water surface elevation, as modeled through the use of HEC-RAS and the
regression equations listed above. The channel bed itself is stable and unmoving at the
crossing. The crossing foundations will be on bedrock to the north and potentially a deep
crossing in gravel deposits to the south. Geotechnical explorations this spring will help to
better determine the substructure foundations to be discussed in the 2015 Hydraulic Report.
Historic high-water elevations are estimated to be approximately 10 feet above the thalweg in
the channel bed. The minimum soffit elevation 5-feet above this high-water elevation will
minimize problems with debris passage under the proposed structure. A deck elevation 18 to
20 feet above the thalweg elevation should be sufficient for debris passage. Photos of the
crossing are included in Appendix B.
February 2016
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6
Hydraulic Modeling
HEC-RAS will be used to model the bridges located on the South Katlian, Sukkaheen, and
Katlian River. To analyze the proposed bridges with HEC-RAS, two hydraulic models will be
developed: a natural conditions model and a proposed bridge model. The natural conditions
model represents the site conditions before any structures are built. This model is used to
compare the backwater effects caused by the proposed structures. The proposed model
represents the proposed bridge. After the models are run, the natural model and the proposed
models are compared to each other to determine if the proposed models increase the water
surface elevations by more than 1-foot. The project area is within Zone A on the FEMA FIS
map and does not fall within the limits of a no-rise zone. It is anticipated that, since each of
the structures is bridging the ordinary high-water elevations, that minimal backwater effects
would be realized due to the new structures. The HEC-RAS model results will be tabulated
in the 2015 Hydraulic Report and be submitted to the DOT&PF State Hydraulics Engineer for
his review.
It is expected that models of the proposed bridge conditions will show that an increase no
greater than 1-foot at the bridge will not be exceeded. It should be noted that this is a localized
increase that should not be exceeded.
The design flood event in all cases will be the 100-year event. Scour will be designed on all
structures for the 100-year event as well. The 500-year event will be checked at every crossing
to determine significant impacts to design should this event occur.
7
Proposed Bridges
A proposed bridge type, size, and location drawing for each of the six structures is included
as Figures 2, 3, 4, 5, 6, and 7 in Appendix A. Hydraulic Data sheets will be developed for
each bridge in the 2015 Hydraulic Report.
Two types of footings are anticipated at this time for the proposed bridges. These include
shallow spread footings over bedrock and driven pile foundations in deep gravel deposits.
With each footing type, the substructure design will be initiated, analyzed, and designed after
the geotechnical drilling work is completed in the spring of 2015. Substructure and
superstructure design will be performed simultaneously and supported by the hydraulic
modeling to help facilitate span lengths at each crossing. Currently all structures are
anticipated to be single-span structures. Table 4 below denotes the structure name, proposed
type of pre-engineered structure, and length.
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Table 4:
Structure Type and Length
Structure Name
Structure Type
Structure
Length (feet)
Waterfall
Structural Steel Plate Arch
24
Gorge
Cambridge Flat Truss
85
Horseshoe
Cambridge Flat Truss
95
South Katlian
Cambridge Flat Truss
105
Middle Fork
Cortez beam bridge*
40
Katlian River
Cambridge Flat Truss
150
Note:
* This structure will also be analyzed for a 19’6” Pipe Arch
8
FEMA Compliance
The hydraulic analyses for all six structures will be designed to pass the 100-year event.
Three of the structures are within Zone A on the Flood Insurance Study (FIS) maps. These
maps will be included as part of the 2015 Hydraulics Report. Each of these three structures
will be designed to pass the 100-year event while maintaining less than a 1-foot rise at each
location.
9
Scour Analysis
A scour analysis will be performed for each of the structures. The three structures located in
the Katlian Valley will potentially include some deep foundations. The median (D50) stone size
for each crossing has been noted. The D50 stone size will be used in determining the potential
scour depth at each crossing location. The South Katlian, the Middle Fork and the Katlian
River bridges all have at least one foundation in gravels. The South Katlian Bridge has one
foundation on the inside of the bend of the river. This foundation will not be subject to direct
scour due to the flow direction of the river, but is more likely to be subject to debris and
sediment accretion. The Middle Fork (Sukkaheen) has both foundations in gravel but
evidence suggests that there is very little scour potential in this river and at this crossing. The
Katlian River has one footing in gravels but is protected by an outcrop of bedrock from
upstream flows. The scour potential for this foundation will be significantly minimized due to
the physical geometry of the site. This foundation is also on the inside bend of the river. Each
of these structures will have the erodible gravels analyzed for contraction and clear water
scour depths. The bedrock is anticipated to not scour at this time unless the geotechnical
investigation discovers highly erodible soft bedrock. No midspan piers are anticipated for this
entire project.
10
Riprap
Riprap calculations will be performed for the bridge openings and their approaches in
accordance with the criteria in the Federal Highway Administration publication Hydraulic
Engineering Circular (HEC) 23, Bridge Scour and Stream Instability Countermeasures. The
Maximum Probable Flood Event will be used in the calculations to determine the riprap size
for the abutments.
February 2016
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Appendix A
Basin Area and Type, Size, and Location Figures
Appendix A
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Appendix B
Structure Location Photos
Appendix B
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Waterfall Bridge: View looking upstream at the crossing
Waterfall Bridge: View looking centerline of proposed road
– Appendix B-1 –
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Waterfall Bridge: View looking downstream at proposed headwall tie-in
Waterfall Bridge: View looking downstream from crossing towards proposed headwall
February 2016
– Appendix B-2 –
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Horseshoe Bridge: View looking downstream of proposed crossing
Horseshoe Bridge: View looking upstream at the crossing and debris
– Appendix B-3 –
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Horseshoe Bridge crossing: D50/northerly
abutment
February 2016
Horseshoe Bridge crossing: northerly
abutment
– Appendix B-4 –
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HYDROLOGY AND HYDRAULIC MEMORANDUM
South Katlian River Bridge: View looking downstream at the crossing
South Katlian River Bridge: View looking upstream from the crossing
– Appendix B-5 –
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South Katlian River Bridge: View looking at the northwesterly abutment
South Katlian River Bridge: View looking at the southeasterly abutment
February 2016
– Appendix B-6 –
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Middle Fork Bridge: View looking downstream to the crossing (log is the approximate
location)
Middle Fork Bridge: View looking upstream at the crossing (Log is at approximate location)
– Appendix B-7 –
February 2016
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HYDROLOGY AND HYDRAULIC MEMORANDUM
Middle Fork Bridge: View looking at the southerly abutment
Sukkaheen River: 16-foot log culvert crossing 300 feet upstream of the proposed Middle Fork
Bridge crossing
February 2016
– Appendix B-8 –
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Katlian River Bridge: View looking upstream from the crossing
Katlian River Bridge: View looking downstream from the crossing
– Appendix B-9 –
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Katlian River Bridge: View looking at the northerly abutment
Katlian River Bridge: View looking at the southerly abutment protection (bedrock outcrop)
February 2016
– Appendix B-10 –
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Katlian River: Upstream approximately 1.5 miles from crossing
Katlian River Bridge: D50 stone sizing picture
– Appendix B-11 –
February 2016