DickeyCreekBridge_Waterway_Hydraulics_Memo.pdf

DICKEY CREEK BRIDGE REPLACEMENT

WATERWAY DESIGN HYDRAULICS MEMO

To: Michael Traffalis, WFLHD Project Manager From: Matthew Dillin, P.E., WFLHD Hydraulics Engineer Date: October 7, 2020 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. Recommendations for the initial bridge waterway width, minimum low chord elevation, and riprap erosion control are presented.

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 elevation 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.

Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661-3801

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. For large storm events, 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. For the 2-year event, Manning’s n values of 0.015, 0.02, 0.045, 0.055, 0.055, and 0.08 were utilized for the paved roadway, gravel roadway, bare gravel bars, main channel, grassy floodplain benches, and forested floodplain benches, respectively. The models used a two second time-step and a total run time of one hour, allowing the model to reach a steady-state condition. 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 constant discharge, conveyed across the main channel. 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 5). The 2-year event and 100-year events were modeled to evaluate the impacts of the new crossing on the channel floodplain, channel stability, and stream continuity. For the preliminary scour analysis the 100-year and 500-year event were modeled.

A summary of the hydraulic analysis results can be found in the appendix of this memo (Figures 6 to 13).

North Fork Teanaway River Floodplain

The site is located within a Federal Emergency Management Agency (FEMA) special flood hazard Zone A1 (FIRM 530095 0164 B, Figure 14). A detailed FEMA mapping study was completed for the site and base flood elevations (BFEs) are depicted on the FIRM. The FIRM base flood elevations are based on the NGVD 29 Datum and a datum shift of +3.793-feet is necessary to convert the elevations to the NAVD 88 Datum. There are slight differences in water surface elevations between the existing conditions model and the printed FIRM BFEs. The difference in elevations can likely be attributed to updated surface mapping, updated hydrology, and an improved 2-dimenional hydraulic analysis.

The proposed bridge is over two times longer than the existing bridge and provides a significant reduction in the 100-year water surface elevation (WSE) upstream of the existing crossing and a slight increase in the WSE downstream of the existing crossing. Upstream of the existing bridge crossing, the 100-year WSE is decreased roughly 2.5-feet and the decrease extends approximately 300-feet upstream. Downstream of the existing bridge crossing, the wider bridge actually causes a slight increase in the 100-year WSE and the increase extends roughly 100-feet downstream. The increase in WSEs can be attributed to the water going supercritical through the narrow opening of the existing bridge and the realignment of the proposed bridge slightly downstream of the existing bridge. In general, the larger proposed structure decreases the 100-year WSE and floodplain extents. A comparison profile of the existing and proposed conditions models can be found in the appendix (Figure 6).

Stream Continuity

The existing bridge is an undersized structure with an opening that is less than the observed width of the OHW.

The undersized opening has interrupted the natural channel processes through the structure and over time has created a disconnect between upstream and downstream channel processes which is evident in the channel profile, cross-sectional geometry, and sediment gradation. An important design consideration for the new bridge was to improve the natural function of the stream through the crossing by providing a properly sized structure to restore stream continuity. This has multiple wide-ranging benefits including reduced maintenance, improved biological function, and improved health of the overall stream corridor.

The field observed OHW set the minimum opening width to improve stream continuity; however, this width generally corresponds with stream function at relatively small and frequent flow events. To improve continuity for larger more infrequent events, several hydraulic analyses were conducted for a range potential opening sizes and return events. For each opening size and return event, the alternative analysis considered four hydraulic indicators water surface depth, velocity, shear stress, and stream power within the channel both upstream and downstream of the crossing.

Ultimately a 125-foot span was chosen for the design opening as the analysis indicated a continuity through the structure that was evident in the water surface profile, water depths, velocities, shear stresses, and stream power.

Values for the four hydraulic indicators near the structure were within the range of values found in the natural channel outside of the influence of the bridge. The wider proposed opening provides a drastic improvement in stream continuity over the existing condition. A comparison of the existing conditions and proposed conditions for the 2-year and 100-year return events is available in the appendix (Figures 10 to 13).

Recommendations

Bridge

Bridge span and low chord recommendations are based on a preliminary hydraulic analysis.

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.

Design should allow for a 9.5-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.

Design should allow for 105-feet for waterway width.

Scour

Scour estimates are based on a preliminary hydraulic analysis of the proposed 125-foot bridge opening during the 100-year and 500-year events. Long-term degradation was estimated based on 17-years of cross-section data at the existing crossing. It was assumed that the bridge would be supported by deep piles on spread footings. A more detailed scour analysis will be conducted after the geotechnical investigations are complete and the structure size and alignment are established.

100-year abutment scour: 2.6-feet 500-year abutment scour: 3.3-feet Long-term degradation: 7.2-feet Total scour elevation: 2372.0-feet, in reference to project datum (11.0-feet of total scour depth)

Abutment Erosion Protection

Preliminary design based on the hydraulic analysis of the existing bridge opening:

A spill-through abutment is recommended at this location.

Install riprap Class 5 (FHWA FP-14), 4-feet thick, and sloped 2(h):1(v) to an invert elevation of 2376.7-feet (project datum), approximate depth of contraction scour plus long term degradation.

Set riprap crest elevation to 2390.0-feet (project datum) (100-year water surface plus 1-feet of freeboard).

Install geotextile filter under all placed riprap.

Place conserved gravel-cobble streambed material over placed riprap.

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 – 2D Hydraulic Model Mesh

Figure 6 – Flow Depth Profile

Figure 7 – Flow Depth

Figure 8 – Flow Velocity

Figure 9 – Shear Stress

Figure 10 – Flow Continuity 2-year event

Figure 11 – Flow Continuity 2-year event

Figure 12 – Flow Continuity 100-year event

Figure 13 – Flow Continuity 100-year event

Figure 14 – FEMA FIRM

Figure 14 – Preliminary Bridge Scour Plot 100-year Event

Figure 15 – Preliminary Bridge Scour Plot 500-year Event

Cc: Sven Leon, Hydraulics Team Lead

Project Site Location Bridge Location

Map from Google Earth Pro, image date 7/2017

N

FIGURE 1

Dickey Creek Bridge

SITE LOCATION

Cle Elum

Dickey Creek Bridge

0 650 feet

Map from Google Earth Pro

FIGURE 2

Dickey Creek Bridge

DRAINAGE AREA

N

0 2 mi

Dickey Creek Bridge

Approximate Drainage Area 82 sq. mi.

FIGURE 3

Sourdough Bridge

HYDROLOGY SUMMARY

SRH2D - Two-Dimensional hydraulic model surface

N

FIGURE 4

Dickey Creek Bridge

MAPPED SURFACE

Dickey Creek Bridge

N F

T ea n aw ay

R iv er

SRH2D - Two-Dimensional hydraulic model mesh

N

FIGURE 5

Dickey Creek Bridge 2D Hydraulic Mesh

SRH2D 2-dimensional hydraulic model results

FIGURE 6

Dickey Creek Bridge

FLOW DEPTH PROFILE

Design Low Chord = 2392.0'

100yr WSE = 2389.0'

100yr WSE =2389.0'

N e w B ri d ge

Design Low Chord= 2392.0' Existing WSE

Proposed WSE

Ground

Ex . B ri d ge

SRH2D 2-dimensional hydraulic model

FIGURE 7

Dickey Creek Bridge

FLOW DEPTH

Bridge, 7'

Depth in feet for 100yr Event

FIGURE 8

Dickey Creek Bridge

FLOW VELOCITY

Bridge, 11 fps

Velocity in feet per second for 100yr Event

FIGURE 9

Sourdough Bridge

SHEAR STRESS

Bridge, 3 lbs/sf

Shear in lbs/SF for 100yr Event

FIGURE 10

Dickey Creek Bridge

FLOW CONTINUITY 2-YEAR

Existing Profile

Proposed Profile

Existing Bridge LocationProposed Bridge Location

FIGURE 11

Dickey Creek Bridge

FLOW CONTINUITY 2-YEAR

Existing Profile

Proposed Profile

Proposed Bridge Location

Existing Bridge Location

FIGURE 12

Dickey Creek Bridge

FLOW CONTINUITY 100-YEAR

Existing Profile

Existing Bridge Location

Proposed Bridge Location

Proposed Profile

FIGURE 13

Dickey Creek Bridge

FLOW CONTINUITY 100-YEAR

Existing Profile

Proposed Profile

Existing Bridge Location

Proposed Bridge Location

FIGURE 14

FEMA FIRM

FIGURE 15

PRELIMINARY BRIDGE SCOUR

100YR EVENT

FIGURE 16

PRELIMINARY BRIDGE SCOUR

500YR EVENT