AK BLM GFO 2019(1)_ScourMemo.pdf

Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA

98661-3801

SOURDOUGH CAMPGROUND BRIDGE

SCOUR HYDRAULICS MEMO

To: Nathan Jones, WFLHD Project Manager From: Matthew Dillin, P.E., WFLHD Hydraulics Engineer Date: June 24, 2021 Project: AK BLM GFO 2019(1)

Background

Sourdough Campground Bridge (No. 000000000L59815) is located 250-feet west of Richardson Highway at MP 147.5 within the Valdez-Cordova Census Area, Alaska. The existing bridge is a 50-feet long two-lane structure, installed in 1990 with timber glulam girders and a timber deck. The bridge currently provides access to Sourdough Campground; however, it has been showing signs of foundation movement and needs replacement. The bridge is owned and maintained by the Bureau of Land Management (BLM). Western Federal Lands Highway Division (WFLHD) is designing the new bridge.

Final scour, skew angle, minimum low chord elevation, riprap erosion control, and abutment type recommendations for Dickey Creek Bridge are presented.

Recommendations

Bridge Type, Size, and Location (Drawings H.1 and H.2)

Remove existing bridge and bridge abutments to open up the waterway.

Minimum low chord elevation 1888.0-feet , in reference to the project datum, which is the approximate 100-year water surface elevation (upstream of the drawdown) plus 2.5-feet freeboard for large woody debris passage, which is expected to be moderate.

Design should allow for a 12-foot clearance between stream bed and the low chord.

The design bridge should have a minimum span of 85-feet with 2H:1V sloping abutments.

Perpendicular bridge abutments (0-degree skew).

Bridge to remain on same alignment.

Design should allow for 70-feet for waterway width.

Scour

Scour estimates are based on the hydraulic analysis of the proposed 85-foot bridge opening during the 100-year and 500-year events. Long-term degradation was estimated based on 19-years of cross-section soundings at the Richardson Highway Bridge, located approximately 700-feet upstream of the project crossing. The bridge will be supported by deep piles.

100-year Scour

• Long-term degradation: 1.5-feet

• Contraction Scour: 1.5-feet

• Abutment Scour: 9.6-feet

• Total Scour Elevation Main Channel: 1,872.3-feet (3.0-feet)

• Total Scour Elevation Abutment: 1,864.2-feet (11.1-feet)

• Pier Scour (assuming 18-inch pier width & 0-degree skew angle): 7.9-feet

Memo to: Nathan Jones, WFLHD June 24, 2021

500-year Scour

• Long-term degradation: 1.5-feet

• Contraction Scour: 4.0-feet

• Abutment Scour: 11.3-feet

• Total Scour Elevation Main Channel: 1,869.8-feet (5.5-feet)

• Total Scour Elevation Abutment: 1,862.5-feet (12.8-feet)

• Pier Scour (assuming 18-inch pier width & 0-degree skew angle): 10.9-feet

Abutment Erosion Protection

Design based on the hydraulic analysis of the proposed 85-foot bridge opening:

A spill-through abutment is recommended at this location.

Install riprap Class 5 (FHWA FP-14), 5-feet thick, and sloped 2(h):1(v) Set riprap crest elevation to 1885.5-feet (project datum) (100-year water surface plus approximately 1-foot of freeboard).

Set riprap bottom elevation to 1868.0-feet (project datum), just below the anticipated long-term degradation plus the contraction scour in the main channel.

Install geotextile filter under all placed riprap.

Place conserved gravel-cobble streambed material over placed riprap.

Construction

The project will have the necessary environmental approvals to allow the contractor to conduct in-water work. Dewater as needed for installing bridge structure, riprap, and conserved streambed material. Working in standing water is allowed for riprap placement if CO can verify installation is consistent with the plans. Restore existing stream bank and floodplain topography. Temporary excavation stability measures may be necessary to isolate the work area. Constructing the proposed bridge may require heavy track-mounted equipment operating near the stream bed.

Operation and Maintenance

Countermeasures, including placed riprap for controlling scour, should be inspected annually and after each extreme flood event. Replace or reposition dislodged stone. Water deflected around trapped woody debris could potentially dislodge riprap stone. The presence of woody debris trapped upstream, beneath, and downstream of the bridge should be inspected annually and after each extreme flood event. Remove woody debris as needed for maintaining flow conveyance capacity and preventing logs deflecting flow towards the riprap or abutments.

Site Conditions

The existing bridge is a 50-feet long two-lane structure, installed in 1990 with timber glulam girders and a timber deck. The bridge currently provides access to Sourdough Campground; however, it has been showing signs of foundation movement and needs replacement. The existing bridge is approximately one mile upstream of the Sourdough Creek confluence with the Gulkana River (Fig. 1). The stream is aligned approximately perpendicular to the road with bends just upstream and downstream of the crossing (Photos 2 and 3). Clearance under the bridge from the low-chord to the channel thalweg is approximately 8-feet (Photos 4 and 5). Channel bed appears stable through the crossing with no signs of significant long term degradation; however, some areas of local scour were present near the face of abutment walls. The bridge is starting to lose embankment material from underneath the right abutment (Photo 4).

Sourdough Creek reach encompassing the bridge crossing appears relatively stable. Site photos depict well vegetated banks both upstream and downstream of the crossing (Photos 2 and 3). The channel is highly sinuous (>1.5) with a gentle valley slope (0.006 ft/ft). Channel bankfull width is approximately 40-feet. Median streambed material (D50) estimated to be roughly 1.2-inch based on site photos near bridge and field measurements obtained using split barrel sampler (3-inch O.D.).

Based on potential flood depths and available wood upstream of the channel, the stream can likely carry a moderate amount of small to moderate sized woody debris. Floodplain is well vegetated with grass, shrubs, trees, and other upland plants (Photos 2 and 3).

Hydrology

USGS quadrangle maps were used to delineate the watershed and calculate an approximate drainage area of 75 sq. miles (Fig. 2). Peak discharges for the 100-year and 500-year return events were taken from the Methodology and Estimates of Scour at Selected Bridge Sites in Alaska (USGS Scientific Investigations Report 00-4151, 20 p.) prepared by USGS with the cooperation with the Alaska Department of Transportation & Public Facilities. The report evaluated 325 bridge sites in Alaska to determine peak flows and scour. Richardson Highway bridge over Sourdough Creek (ADOT&PF #575), located just several hundred feet upstream from the project site, was one of the bridges evaluated. Other return event flows were taken from the USGS StreamStats Data-Collection Station Report at gage station number 16200270 (Sourdough Creek at Sourdough AK), located near the Richardson Highway crossing. A summary of the flows used in the hydraulic analysis can be found in the attached Figure 3.

Water Surface Elevations and Flow Velocities

Surface-water Modeling System (Aquaveo, SMS 13.0.3) was used to develop and run SRH-2D, a two-dimensional, unsteady flow hydraulic model. The two-dimensional model was used for estimating the existing bridge water surface elevations, flow depths, and flow velocities. Terrain for the hydraulic model was created from existing LiDAR survey data. LiDAR was supplemented with ground topography obtained by WFL survey group in 2011. Elevations based on NAVD88 GEOID 12b and provided in Alaska Zone 3 State Plane Coordinate System NAD 83 2011. The model uses a gridded mesh with 3.5-foot sides under the bridge and an equilateral triangle mesh with roughly 8-foot sides in the main channel and overbanks. The model extends roughly 2,000-feet upstream and 1,300-feet downstream (Fig. 8).

Normal flow depth was set for the downstream boundary condition. The upstream boundary condition uses a hydrograph that includes the 2, 5, 10, 25, 100, 200, and 500-year flood flows. Each flow was allowed to run until an equilibrium state was reached. The total simulation run time was set at 7-hours using a 3-second computation interval. Manning’s Roughness Coefficients of 0.035, 0.065, and 0.02 were selected for the main channel, floodplain, and roadway areas, respectively. The model was calibrated with apparent bankfull flows. Flow profiles, velocities, depths, and water surface elevations are summarized in Figures 4, 5, 6, and 7, respectively.

Scour

A HEC-18 (5th ED., April 2013) scour analysis was completed using the SRH-2D two-dimensional hydraulic model results (attached). The analysis includes contraction and abutment scour for the 100-year, 200-year, and 500-year flood events. The analysis also includes pier scour at the abutment piles for the same events, to represent a situation where the roadway approach is washed out and the stream is flowing around the abutment piles. Long-term degradation was estimated to be 1.5-feet, based on 19-years of cross-section soundings at the Richardson Highway Bridge, located just upstream of the project crossing. Median streambed material (D50) was estimated to be roughly 1.2-inch based on site photos near bridge and field measurements obtained using split barrel sampler (3-inch O.D.). Calculations assume spill-through abutments and flow impingement angles based on the two-dimensional modeling. Predicted scour is summarized in Table 1.

Riprap Sizing

Riprap design for the bridge abutments is based on Hydraulic Engineering Circular No. 23 (FHWA-NHI- 09-112) using equations 14.1 and 14.2. Calculations were performed using the local depth and velocity at the bridge abutment for the 100-year flood event. Specific gravity of the stone was assumed to be 2.65.

Slope of finished riprap surface is 2(h):1(v). For the left abutment Class 4 riprap was calculated as the adequate design riprap size. For the right abutment Class 5 riprap was calculated as the adequate design riprap size. To account for unknown flow impingement angles caused by shifting debris and mid-channel sediment deposits, Class 5 (FHWA, FP-14) riprap, minimum 5-feet thickness, was selected for both abutments.

Stream Impacts

A moderate amount of turbidity and sediment is expected to be released when the stream diversions are removed. The amount of turbidity and sediment will be reduced by removing the flow barriers gradually introducing the stream flow over an extended two to three-hour time.

Floodplain and Flood-rise Limitations

The site is not mapped by the Federal Emergency Management Agency (FEMA) as being in a special flood hazard area. A detailed FEMA mapping study has not been completed for the site. No base flood elevations have been determined. The proposed bridge structure is significantly larger than the existing structure; therefore, the new structure is not expected to negatively impact flooding of private property or insurable structures (Fig. 4).

Attachments: Figure 1 – Site Location Figure 2 – Drainage Area Figure 3 – Hydrology Summary Figure 4 – Water Surface Profile Figure 5 – Flow Velocity Figure 6 – Flow Depth Figure 7 – Water Surface Elevations Figure 8 – 2D Hydraulic Model Figure 9 – Bridge Soundings BN 575 Figure 10 – 100-year Scour Figure 11 – 200-year Scour Figure 12 – 500 year Scour Table 1 – Scour Summary Photographs 1 to 6 Preliminary Plan Sheets H.1 & H.2

Cc: Sven Leon, WFLHD Hydraulics Functional Manager

Project Site Location Bridge Location

Map from Google Earth Pro, image date 7/19/2014.

N

FIGURE 1

Sourdough Campground Bridge

SITE LOCATION MAP

Location of Sourdough Campground BridgeSourdough

Campround

0 450 feet

Map from Google Earth Pro

FIGURE 2

Sourdough Bridge

DRAINAGE AREA MAP

Rustic Falls

N

0 2 mi

Sourdough Campground Bridge

Approximate Drainage Area 75

sq. mi.

The 100 & 500 year flow rates were take from the 2001 WRIR 00-4151 report.

Other return event flows were taken from the USGS analysis of gage station number 16200270.

FIGURE 3

Sourdough Bridge

HYDROLOGY SUMMARY

SRH2D 2-dimensional hydraulic model

FIGURE 4

Sourdough Bridge 100-Year Water Surface Profile

Design Low Chord = 1888.0'

100yr WSE = 1885.5'

100yr WSE = 1884.0'

C ro ss in g

Design Low Chord= 1888.0'

Upstream100yr WSE = 1885.5'

FIGURE 5

Sourdough Bridge

100-YEAR FLOW VELOCITY

500-yr 200-yr

100-yr

25-yr

10-yr

5-yr

2-yr

Design Value Location

N

FIGURE 6

Sourdough Bridge

100-YEAR FLOW DEPTH

500-yr

200-yr

100-yr

25-yr

10-yr

5-yr 2-yr

FIGURE 7

Sourdough Bridge

100-YEAR WATER SURFACE

500-yr

200-yr

100-yr

25-yr

10-yr

5-yr 2-yr

SRH2D - Two-Dimensional hydraulic model.

FIGURE 8

Sourdough Bridge 2D Hydraulic Model

Sourdough Bridge

Bridge Mesh

DS Boundary

US Boundary

Data from Alaska DOT for Richardson Highway Bridge crossing Sourdough Creek

FIGURE 7

SOUNDINGS BN 575

FIGURE 10

100-YEAR SCOUR

FIGURE 11

200-YEAR SCOUR

FIGURE 12

200-YEAR SCOUR

To: By: Matthew Dillin, P.E.

Project: Sourdough Creek Bridge Scour Evaluation WFLHD Hydraulics Engineer

Bridge: Date:

BRIDGE STRUCTURE SUMMARY

Low Chord Elevation 1888.0 feet (Project Datum)

Minimum Design Freeboard 2.5 feet above the 100-year flood * Needed for woody debris passage.

Stream Bottom Elevation 1875.3 feet

Clearance to Stream Bot. 12.7 feet

Waterway Width 73.3 feet 4.0 feet (vertical at 100-yr WS)

Skew 0 degrees relative to stream Bridge Length (feet) 92

2+72.9 2+27.39 3+18.39

Abutment Type Sloped Abutments

HYDRAULIC & HYDROLOGIC SUMMARY Hydraulic Model: SRH2D

Flood Frequency Yr 2 10 25 100 200 500

Exceedance Probability % 50 10 4 1 0.5 0.2

Discharge cfs 664 1120 1720 2330 3280 3570

Modeled Water Surface Elevation Elev 1881.3 1882.3 1883.2 1884.0 1885.0 1885.3

Elevation Datum Conversion Ft 0.0 0.0 0.0 0.0 0.0 0.0

Project Water Surface Elevation Elev 1881.3 1882.3 1883.2 1884.0 1885.0 1885.3

Clearance above WS Ft 6.7 5.7 4.8 4.0 3.0 2.7

Existing Water Surface Elevation Elev NA NA NA NA NA NA

Predicted Flood Rise Ft NA NA NA NA NA NA

Design Flow Velocity Ft/sec 4.7 6.7 8.7 9.6 10.6 10.8

Scour Base Elevation (Channel Thalweg) Elev 1875.3 1875.3 1875.3

Contraction Scour Depth Ft 1.5 3.4 4.0

Stream Degradation Depth Ft 1.5 1.5 1.5

Vertical Contraction Scour Depth Ft 0.0 0.0 0.0

Left Abutment Scour Depth Ft 9.6 10.9 11.3

Right Abutment Scour Depth Ft 9.6 10.9 11.3

Ft 1872.3 1870.4 1869.8

Total Left Abutment Scour Elevation ** Elev 1864.2 1862.9 1862.5

Total Right Abutment Scour Elevation ** Elev 1864.2 1862.9 1862.5

Pier 1 Scour Depth Ft

Pier 2 Scour Depth Ft

Total Pier 1 Scour Elevation ** Elev*

Total Pier 2 Scour Elevation ** Elev*

* or to bedrock when present.

** includes CS+LTD or VCS for SCF

Streambed material grain size (inches) D100/D85/D50 0.1

Scour Design Basis

Project Elevation Datum

Bridge StartBridge/Stream CL Sta

BRIDGE HYDRAULICS & SCOUR ANALYSIS SUMMARY

Nathan Jones, WFLHD Project Manager

6/25/2021AK BLM GFO 2019(1)

NAVD 88

HEC-18

Bridge End

Total Scour Main Channel

SourdoughBridge-Hydraulics-Scour_Summary.xls

Photos - Sourdough Bridge Sourdough Bridge

PHOTO 1

PHOTO 2

PHOTO 3

6/6/2018

Sourdough Bridge approach looking east.

Richardson Highway in background.

Looking upstream from bridge deck.

Downstream channel roughly perpendicular to roadway. Bend just downstream of bridge. Well vegetated and stable banks.

Upstream channel roughly perpendicular to roadway. Some woody debris visible on channel banks. Bend just upstream of bridge. Well vegetated and stable banks.

Looking downstream from bridge deck.

Photos - Sourdough Bridge Sourdough Bridge

PHOTO 4

PHOTO 5

PHOTO 6

6/6/2018

Looking at left abutment from right bank.

Riprap protection still in place. Utility attached to bridge.

Looking at right abutment from left bank.

Looking upstream from bridge deck.

Riprap no longer protecting abutment.

Exposed geotextile. Bottom of foundation visible.

C r e e k

S o u r d o u g h

10 200510

SCALE IN FEET

1873'

1873'

1885.5'

1885.5'

1873'

1885.5'

1885.5'

1873'

1873'

1873'

1885.5'

1885.5'

1873'

1885.5'

1885.5'

1873'

1873' 1873'

1873' 1873'

1885.5'

1885.5' 1885.5'

1885.5'

Ordinary High Water

Placed riprap, class 5, method B

Placed riprap, class 5, method B

85.3'

20'

20' 20'

20'

X

X

X

X

X X

X X

X

X

X

X X

X

X X

X

X

X

X

X

X

X X

X

X X

X

X X

X X

5' 25' 6' 5'6' 25'

:4

P

M

J u n e

M a in li n e

P la n

S h e e t] c p w w o r k

\d

\S o u r d o u g h

B r id g e

R ip r a p

M o d e l.

d g n

RIPRAP PLAN

SOURDOUGH BRIGE

STATE PROJECT

NUMBER

SHEET

AK BLM GFO 2019(1) H.2

1,870

1,860

1,880

1,890

1,900

1,910

1,850

Elev. 1885.5' Elev. 1885.5'

Elev. 1873.0'

100-year W.S. Elev = 1885.0'

Geotextile, class 1, type C Geotextile, class 1, type C

Conserved streambed material method B, 5-feet thick

Placed riprap, Class 5, method B, 5-feet thick

Placed riprap, class 5, 5 1002.55

SCALE IN FEET

1862.51864.2ElevTotal Scour Elevation Abutment

1869.81872.3ElevTotal Scour Elevation Channel

--FtPier Scour Depth

11.39.6FtAbutment Scour Depth

4.01.5FtContraction Scour Depth

1.51.5FtStream Degradation Depth

1875.31875.3ElevScour Base Elevation (Channel Thalweg)

10.89.64.7Ft/secDesign Flow Velocity

2.74.06.7FtClearance to Low Chord

1885.31884.01881.3ElevWater Surface Elevation

35702330664cfsDischarge

0.0020.010.5%Exceedence Probability

5001002YearFlood Frequency

HYDRAULIC & HYDROLOGIC SUMMARY

Elev. 1875.3

OHW Elev = 1877.0'

Elev. 1873.0'

5' 5'

6' 6'

:4

P

M

J u n e

U n n a m e d P la n

P la n

S h e e t] c p w w o r k

\d

\S o u r d o u g h

B r id g e

R ip r a p

M o d e l.

d g n

RIPRAP ELEVATIONS

SOURDOUGH BRIDGE

STATE PROJECT

NUMBER

SHEET

AK BLM GFO 2019(1) H.1