DickeyCreekBridge_Scour_Memo.pdf

DICKEY CREEK BRIDGE REPLACEMENT

SCOUR HYDRAULICS MEMO

To: Michael Traffalis, WFLHD Project Manager From: Matthew Dillin, P.E., WFLHD Hydraulics Engineer Date: June 1, 2021 Project: WA Kittitas 5880(1) - Dickey Creek Bridge Replacement

Background

The project will be replace The North Fork River Bridge (MP 10.06 Dickey Creek) located in Kittitas County, WA (Figure 1). The existing bridge, constructed in 1963, is a 52-foot prestressed girder bridge that is functionally obsolete and currently load restricted. The bridge is owned and maintained by Kittitas County (County). 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, Location (Drawings H.1 and H.2)

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

Bridge low chord should be set to a minimum elevation of 2392.0-feet, in reference to the project datum, which is the approximate 100-year water surface elevation plus 3-feet of freeboard for large woody debris passage, which is expected to be moderate.

Design should allow for a 9-foot clearance between stream bed and the low chord.

The design bridge should have a minimum span of 125-feet with 2H:1V sloping abutments.

Skew bridge abutments 18-degrees Shift bridge to the new alignment, to improve horizontal curves at approaches.

Design should allow for 105-feet for waterway width.

Scour

Scour estimates are based on the hydraulic analysis of the proposed 125-foot bridge opening during the 200-year and 500-year events. Long-term degradation was estimated based on 20-years of cross-section data at the existing crossing. The bridge will be supported by drilled shaft on rock.

200-year abutment scour: 2.5-feet 500-year abutment scour: 2.8-feet Long-term degradation: 3.0-feet Total scour elevation: 2377.0-feet, in reference to project datum (5.5-feet of total scour depth)

Pier scour calculations were made using main channel hydraulics and provide an estimate for scour at the abutment supports in the event that the drilled shafts are exposed during a large event.

200-year pier scour: 4.9-feet 500-year pier scour: 5.0-feet

Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661-3801

Memo to: Mike Traffalis, WFLHD Project Manager June 1, 2021

Abutment Erosion Protection

Design based on the hydraulic analysis of the proposed 125-foot bridge opening:

A spill-through abutment is recommended at this location.

Install riprap Class 5 (FHWA FP-14), 6-feet thick, and sloped 2(h):1(v) Set riprap crest elevation to 2390.0-feet (project datum) (100-year water surface plus 1-feet of freeboard).

Set riprap bottom elevation to 2377.0-feet (project datum), approximate depth of abutment scour plus long term degradation.

Install geotextile filter under all placed riprap.

Place conserved gravel-cobble streambed material over placed riprap.

All construction activities will take place outside of the ordinary-high-water (OHW).

Construction

The project will have the necessary environmental approvals to allow the contractor to conduct in-water work to remove the existing bridge and riprap only. Work within the limits of the ordinary-high-water to install proposed bridge structure and proposed riprap is not allowed without a variance to the environmental permits.

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 and keep excavations from impacting the ordinary-high-water. Constructing the proposed bridge may require heavy track-mounted equipment operating near the stream bed.

Operation and Maintenance

Riprap placed for controlling bank erosion 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.

Stream Conditions

The North Fork of the Teanaway River originates within Cascade Mountain Range. The basin for the project site encompasses 82.4 square miles near the crest of the Cascade Mountain Range (Figure 2). The project reach is located in a moderately steep and moderately confined valley. Through the project reach the river is a single thread meandering channel with a sinuosity of 1.4. The valley slope through the project reach is approximately 0.013 ft/ft.

The ordinary-high-water (OHW) was observed to be roughly 65-feet, which is wider than the existing bridge span.

Pool-riffle is the dominant bed form. Sand, gravel, cobble, and small boulders comprise the banks and streambed.

The stream banks appear moderately stable. Based on aerial imagery, the channel has not migrated appreciably over the past decade. The adjacent floodplain is mostly well vegetated with grasses, shrubs, and mature trees. Based on the available woody debris in the system, bank-side vegetation, and expected flood flow depths, the stream can carry a large amount of large woody debris.

Just upstream of the crossing, the river travels through a bend. The bridge is located just downstream of a bend in the river and there are signs of the river eroding the abutment on the outside of the bend. The river has deposited large material upstream of the bridge and scoured around the existing bridge abutments. Downstream of the existing bridge, the adjacent floodplain is roughly 2-feet above the base flow elevation and appears well connected to the channel. Upstream of the existing bridge, the adjacent floodplain is roughly 6-feet above the base flow and appears disconnected from the channel. Bedrock was observed in multiple locations upstream of the bridge, arresting any further channel degradation.

Hydrology

The nearest gaging station to the project site is the United State Geological Survey (USGS) gage Station 12480000

(Teanaway River Below Forks Near Cle Elum, WA). The gage was operated by USGS for nine years, from 1968 to 1976. After 1976, the gage was operated by United States Bureau of Reclamation (USBR). The gage has two additional major tributaries (Middle Fork Teanaway River and West Fork Teanaway River) in the intervening reach between the bridge site and the gage site. Additionally, the contributing watershed for the gage is significantly larger (greater than 25%) than the contributing watershed for the bridge site. Therefore, the transposition of the gage data to the project site is not recommended by FHWA technical guidance (Publication No. FHWA-NHI-02- 001, October 2002) as the hydrology at the gage site is likely not representative of the hydrology at the bridge site.

A hydrologic analysis was performed using USGS regional regression equations to compute peak flow estimates for the ungagged project site. The methodology for calculating peak flows using the regional regression equations is outlined in the Scientific Investigations Report 2016-5118, Version 1.2, November 2017 by USGS. With the lack of gage data for the project site, the regression analysis represents the best available data for the project site.

However, the computed regression analysis flows appeared to underestimate peak flow values when compared to personal accounts from County staff and Yakima Nation Tribal staff who live and work in the basin.

Flows from the FEMA Flood Insurance Study (FIS) were also evaluated as part of this study. The FEMA flows are 50-percent, 60-percent, and 180-percent greater than the flows calculated using the USGS regional regression equations for the 500-year, 100-year, and 2-year flows, respectively. The FEMA flows appear to provide a conservative estimate that is consistent with the personal accounts of flooding at the bridge site over the past 30-years; therefore, the FEMA flows were utilized for the design of the Dickey Creek Bridge Project.

A summary of the hydrologic analyses results can be found in the appendix of this memo (Figure 3).

Mapping

LiDAR mapping for the project was available through the Washington State Department of Natural Resources Lidar portal. The LiDAR data was collected in 2015 and included channel bathymetry. LiDAR mapping was supplemented with some additional ground survey within the project limits collected by Western Federal Lands.

The surface utilized in this study can be found in the appendix (Figure 4).

Hydraulic Model

A two-dimensional, non-uniform, steady-state hydraulic analysis was performed using SRH-2D. The model was developed using Aquaveo’s Surface-Water Modeling System (SMS) interface Version 13.0. The model was used for estimating flow depths, flow velocities, shear stress, and water surface elevations within the North Fork of Teanaway River through the project reach. Manning’s n values of 0.015, 0.02, 0.035, 0.04, 0.05, and 0.06 were utilized for the paved roadway, gravel roadway, bare gravel bars, main channel, grassy floodplain benches, and forested floodplain benches, respectively. Both the upstream and downstream boundary conditions were located roughly 3,000-feet from the bridge crossing. The upstream boundary condition was modeled as a hydrograph that includes the 2, 10, 50, 100, 200, and 500-year flood flows, conveyed across the main channel. Each flow was allowed to run until an equilibrium state was reached. The total simulation run time was 6-hours using a 3-second computation interval. The downstream boundary condition was modeled as a constant water surface elevation, corresponding to the normal water surface depth. The mesh within the main channel and immediately adjacent floodplain generally consists of equilateral triangles and squares with approximately 5 to 6-foot lengths for each side (figure 9). Flow profiles, velocities, depths, and water surface elevations are summarized in Figures 5, 6, 7, and 8, 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 columns for the same events, to represent a situation where the roadway approach is washed out and the stream is flowing around the abutment columns. Channel profile is controlled by the coarse cobble-boulder bed material and appears stable. Long-term channel degradation was assumed to be 3-feet based on long term trends from 1998 to 2020. Streambed material gradations were developed from visual observation of gravel bar and floodplain surfaces immediately upstream of the bridge. 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 200-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 and right abutments, Class 4 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 6-feet thickness was selected.

Stream Impacts

All work is to be performed outside the limits of the ordinary-high-water; therefore, construction impacts to active flows are expected to be minimal. The wider span and additional freeboard will help pass debris during large events. Additionally, the larger span will help the channel function more naturally when transporting sediment during smaller, more frequent events.

Floodplain and Flood-rise Limitations

The larger bridge structure will have a significant impact on the existing floodplain. A separate floodplain memo will be developed to document the overall impacts of the proposed improvements.

A final Hydraulics TSL Report will be prepared when the Bridge TSL is completed.

Attachments: Figure 1 – Site Location Map

Figure 2 – Drainage Area Map

Figure 3 – Hydrology Summary

Figure 4 – Mapped surface

Figure 5 – Water Surface Profile

Figure 6 – Flow Velocity

Figure 7 – Flow Depth

Figure 8 – Water Surface Elevation

Figure 9 – 2D Hydraulic Model Mesh

Figure 10 – Streambed Gradation

Figure 11 – Historic Cross-Sections

Figure 12 – Preliminary Bridge Scour Plot 100-year Event

Figure 13 – Preliminary Bridge Scour Plot 200-year Event

Figure 14 – Preliminary Bridge Scour Plot 500-year Event

Table 1 – Scour Summary Table

Photographs 1 to 12

Preliminary Plan Sheets H.1 & H.2

Cc: Sven Leon, Hydraulics Team Lead

Bridge Location Project Site

Image from Google Earth, 7/24/2018

N

FIGURE 1

DICKEY CREEK BRIDGE

PROJECT LOCATION

Dickey Creek Bridge

Cle Elum

0 650 feet

Image from Google Earth, 7/24/2018

FIGURE 2

DICKEY CREEK BRIDGE

DRAINAGE AREA

Fish Creek Bridge

14.2 sq. miles

N

Dickey Creek Bridge

Approximate Drainage Area 82 sq. mi.

0 2 mi

FIGURE 3

DICKEY CREEK BRIDGE

HYDROLOGY SUMMARY

SRH2D - Two-Dimensional hydraulic model surface

N

Dickey Creek Bridge

N F

T ea n aw ay

R iv er

FIGURE 4

MAPPED SURFACE

FIGURE 5

WATER SURFACE PROFILE

yr y yr

SRH2D - Two-Dimensional hydraulic model.

Design Value Location

FIGURE 6

DICKEY CREEK BRIDGE

100-YEAR FLOW VELOCITY

2-yr

500-yr

100-yr

50-yr

10-yr

200-yr

Design Value Location

FIGURE 7

DICKEY CREEK BRIDGE

100-YEAR FLOW DEPTH

2-yr

500-yr

100-yr

50-yr

Design Value Location

FIGURE 8

DICKEY CREEK BRIDGE

100-YEAR WATER SURFACE

2-yr

500-yr

100-yr

50-yr

US Hydro Boundary

Mesh

FIGURE 9

DICKEY CREEK BRIDGE

SRH2D MESH MODEL

DS Normal Depth Boundary

Bridge Mesh

FIGURE 10

STREAMBED GRADATION

FIGURE 11

HISTORIC CROSS-SECTIONS

FIGURE 12

100-YEAR SCOUR ANALYSIS

FIGURE 13

200-YEAR SCOUR ANALYSIS

FIGURE 14

500-YEAR SCOUR ANALYSIS

To: By: Matthew Dillin, P.E.

Project: Dickey Creek Bridge Scour Evaluation WFLHD Hydraulics Engineer

Bridge: Date:

BRIDGE STRUCTURE SUMMARY

Low Chord Elevation 2392.0 feet (Project Datum)

Minimum Design Freeboard 3.0 feet above the 100-year flood * Needed for woody debris passage.

Stream Bottom Elevation 2382.5 feet

Clearance to Stream Bot. 9.5 feet

Waterway Width 121 feet 3.1 feet (vertical at 100-yr WS)

Skew 18 degrees relative to stream Bridge Length (feet) 135.5

208+17.1 207+49.3 208+84.8

Abutment Type Sloped Abutments

HYDRAULIC & HYDROLOGIC SUMMARY Hydraulic Model: SRH2D

Flood Frequency Yr 2 10 50 100 200 500

Exceedance Probability % 50 10 2 1 0.5 0.2

Discharge cfs 2366 2850 3600 3900 4200 4560

Modeled Water Surface Elevation Elev 2387.8 2388.2 2388.7 2388.9 2389.1 2389.4

Elevation Datum Conversion Ft 0.0 0.0 0.0 0.0 0.0 0.0

Project Water Surface Elevation Elev 2387.8 2388.2 2388.7 2388.9 2389.1 2389.4

Clearance above WS Ft 4.2 3.8 3.3 3.1 2.9 2.6

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 9.0 9.5 10.0 10.5 11.0 11.5

Scour Base Elevation (Channel Thalweg) Elev 2382.5 2382.5 2382.5

Contraction Scour Depth Ft 0.0 0.0 0.0

Stream Degradation Depth Ft 3.0 3.0 3.0

Vertical Contraction Scour Depth Ft 0.0 0.0 0.0

Left Abutment Scour Depth Ft 2.3 2.5 2.8

Right Abutment Scour Depth Ft 2.3 2.5 2.8

Total Left Abutment Scour Elevation ** Elev 2377.2 2377.0 2376.7

Total Right Abutment Scour Elevation ** Elev 2377.2 2377.0 2376.7

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 18.7 8.5 4.9

Scour Design Basis

Project Elevation Datum

Bridge StartBridge/Stream CL Sta

BRIDGE HYDRAULICS & SCOUR ANALYSIS SUMMARY

Michael Traffalis, WFLHD Project Manager

5/27/2021WA Kittitas 5880(1)

NAVD 88

HEC-18

Bridge End

DickeyCreekBridge-Hydraulics-Scour_Summary.xls

Photos - Dickey Creek Dickey Creek Bridge

PHOTO 1

PHOTO 2

PHOTO 3

9/30/2020

Dickey Creek Bridge approach looking south

Looking upstream from bridge.

Downstream channel roughly perpendicular to roadway. Large woody debris within channel roughly 500' downstream of bridge.

Bedrock outcrop, roughly 200' upstream of bridge crossing, located on the outside of the bend. Channel is steep riffle/cascade coming into the bridge opening.

Looking downstream from bridge deck.

PHOTO 4

PHOTO 5

PHOTO 6

9/30/2020

Downstream channel aligned roughly perpendicular to roadway. Moderate scour hole at the crossing.

Bridge inlet looking downstream.

Bridge located on a bend. Approach angle roughly 20-degrees coming into the bridge inlet. South (right) abutment is located on the outside of bend and recent riprap repair visible along right bank just upstream. Channel is very steep riffle/cascade approaching the bridge crossing.

Bridge outlet looking upstream.

Looking downstream at bridge inlet.

PHOTO 7

PHOTO 8

PHOTO 9

9/30/2020

Dickey Creek Bridge north (left) abutment.

No erosion visible on the left abutment.

Dickey Creek Bridge south (right) abutment.

Upstream of bridge (looking upstream).

Stable banks, armored with cobble. Pool-riffle dominant bed form. Boulder-Cobble-gravel streambed material with areas of bedrock.

Erosion observed at the right abutment.

Fairly new riprap bank repair visible.

PHOTO 10

PHOTO 11

PHOTO 12

9/30/2020

Banks fairly stable with some areas of instability located at outside bends.

Boulder-cobble stream bed with pool-riffle dominant bed form. Well vegeted floodplain.

Exposed gravel bar upstream of bridge.

Streambed material typically transported by stream flows. Tape showing rougly 5'.

D50 approximately 5" and D100 approximately 18" to 24".

Downstream of bridge (looking upstream).

Streambed material typically transported by stream flows. Tape showing roughly 5'. D50 approximately 5" and D100 approximately 18" to 24".

Exposed gravel bar downstream of bridge.

N

10 200510

SCALE IN FEET

T e a n a w a y R iv e r

N o r t h F o r k existing bridge

Remove centerline

Proposed roadway

2383'

2393'

2383'

2383'

2383'

2383'

2383'

2390'

2390'

2393'

2383'

2390'

2383'

2383'

2383'

2383'

2383'

2383'

2383'

2383'

2383'

2390'

2393'

2393'

2393'2393' method B, class 5

Placed riprap, method B, class 5

Placed riprap, disturbance

Limits of disturbance

Limits of

25'

25'

25'

25'

125'

6'

14'14'

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' 5'

XX

° 18°

:3

A

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

\D ic k e y

C r e e k

R ip r a p

.d g n

RIPRAP PLAN

DICKEY CREEK BRIDGE

STATE PROJECT

NUMBER

SHEET

WA Kittitas 5880(1) H.1

2,400

2,410

2,390

2,380

2,370

2,360

2,420

2,430

2,400

2,410

2,390

2,380

2,370

2,360

2,420

2,430

2376.72377.2ElevTotal Scour Elevation

0.00.0FtPier Scour Depth

2.82.3FtAbutment/Contraction Scour Depth

3.03.0FtStream Degradation Depth

2382.52382.5ElevScour Base Elevation (Channel Thalweg)

11.510.59.0Ft/secDesign Flow Velocity

2.63.14.2FtClearance to Low Chord

2389.42388.92387.8ElevWater Surface Elevation

456039002366cfsDischarge

0.0020.010.5%Exceedence Probability

5001002YearFlood Frequency

HYDRAULIC & HYDROLOGIC SUMMARY

Elev. 2383' Elev. 2383'

Elev. 2390' Elev. 2390'

1 OHW Elev = 2,383.5'

100-year W.S. Elev = 2389.0' geotextile, class 1, type A

Geotextile, class 1, type A

Existing grade method B, 6-feet thick

Placed riprap, class 5, method B, 6-feet thick

Placed riprap, class 5, Conserved streambed material

Conserved streambed material

10 200510

SCALE IN FEET

RIPRAP ELEVATIONS

DICKEY CREEK BRIDGE

Limits of disturbance Limits of disturbance

8' 6'

:4

A

M

J u n e

U n n a m e d P r o f il e

R ip r a p P r o f il e

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

\d

\D ic k e y

C r e e k

R ip r a p

.d g n

STATE PROJECT

NUMBER

SHEET

WA Kittitas 5880(1) H.2