DickeyCreekBridge_Floodplain_Memo.pdf

DICKEY CREEK BRIDGE REPLACEMENT

FLOODPLAIN HYDRAULICS MEMO

To: Michael Traffalis, WFLHD Project Manager From: Matthew Dillin, P.E., WFLHD Hydraulics Engineer Date: September 17, 2021 Project: WA Kittitas 5880(1) - Dickey Creek Bridge Replacement

Purpose

The project will replace the North Fork Teanaway River Bridge (North Fork Teanaway Road, MP 2.83) located in Kittitas County, WA (Figure 1). Western Federal Lands Highway Division (WFLHD) will provide the proposed bridge design and the bridge will be owned and maintained by Kittitas County (County). The existing bridge is a 52-foot prestressed girder bridge constructed in 1963, that has become functionally obsolete. In addition to being load restricted, the existing bridge is hydraulically undersized with insufficient span length and height. The proposed bridge will have a 125-foot span and a low chord elevation roughly 3-feet higher than the existing bridge.

Because the proposed bridge structure will significantly increase the hydraulic capacity at the crossing, Kittitas County (the local floodplain administrator) is not requesting a CLOMR be completed prior to construction. The purpose of this memo is to document the impacts of the proposed structure on the existing floodplain.

Background

Detailed hydrologic and hydraulic analyses for the project reach were originally completed by the USGS in 1978.

The hydrologic analysis utilized a log-Pearson Type III distribution of annual peak-flow gage data in accordance with the guidelines set forth in Bulletin 17. The hydraulic analysis was completed using USGS’s step-backwater model Computer Program E431, an obsolete program that was a precursor to WSPRO. The results from these analyses form the basis for FEMA’s effective FIRM panel (530095-0164-B) and FIS report with effective dates of May 1981 and October 1981, respectively (References 1 & 2). Since that time, the same data has been redelineated to match updated topographic data based on the computed water surface elevations from the effective model. As part of the redelineation, the vertical datum was updated from NGVD29 to NAVD88. The results of the redelineation were published by FEMA in a preliminary FIS study that was released in March 2018 (Reference 3).

The redelineation effort was also incorporated into a new FEMA FIRM map (53037C0411D) with an effective date of September 24, 2021 (Reference 4). The newer redelineated FEMA documents were utilized for comparison purposes in this document.

Study Limits

The hydraulic model limits extend roughly 3,000-feet upstream and downstream of the existing crossing to capture the full impacts of the new proposed bridge crossing and to limit the influence of boundary conditions at the bridge site. The model limits roughly span FIS sections AP through BE.

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 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). Survey data was provided in the geographic coordinate system NAD83 Washington State Plane, South Zone horizontal, US foot on the NAVD88 vertical datum.

Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661-3801

Memo to: Mike Traffalis, WFLHD Project Manager September 17, 2021

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 rare flooding events observed 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).

Hydraulic Model

Hydraulic models were developed to analyze the existing and proposed conditions through the study limits. The Existing Conditions Model (ECM) utilized updated mapping data to model the existing 52-foot span bridge. The Proposed Conditions Model (PCM) was created by updated the existing surface to include the proposed grading, roadway realignment, and larger bridge structure.

Two-dimensional, non-uniform, steady-state hydraulic analyses were performed using SRH-2D. The models were developed using Aquaveo’s Surface-Water Modeling System (SMS) interface Version 13.0. The models were used for estimating flow water surface elevations within the North Fork of Teanaway River through the project reach.

Manning’s n values of 0.015, 0.02, 0.04, 0.05, and 0.06 were utilized for the paved roadway, gravel roadway, 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 constant 100-year flood flow, conveyed across the main channel. The downstream boundary condition was modeled as a constant 100-year flood flow, conveyed across the main channel and floodplain. The flow was allowed to run until an equilibrium state was reached. The total simulation run time was 0.5-hours using a 0.5-second computation interval. 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).

Floodplain Impacts

Effect vs ECM There are significant differences in water surface elevations (WSEs) between the Effective FIS profile and the ECM profile. In general, the ECM depicts an increase in the WSEs. Downstream of the crossing the existing model depicts a rise of roughly 2-feet that persists for over 1,000-feet before converging near FIS section AS. Upstream of the crossing the profiles are much closer and converge more quickly. The WSEs are mostly within 1-foot and the profiles converge before FIS section AZ (Figure 6). Significant differences between the two profiles can be expected since the original effective model was completed using older topographic data and a one-dimensional analysis. One-dimensional models can be very accurate for channelized flows; however, they are not inherently equipped to correctly model streams, like the one in the project reach, that overtop their channel banks and activate the adjacent floodplain. In these instances, switching to a more suitable two-dimensional model can result in substantial differences when compared to a one-dimensional model.

Although the ECM shows a significant increase in water surface elevations, the area mapped within the 100-year floodplain decreases within the study limits. The effective FIRM map has roughly 44-acres mapped within the 100-year floodplain and the existing conditions delineation has 36-acres mapped within the 100-year floodplain. The realized decrease in inundated areas is likely a consequence of the more precise delineation using new detailed topography and two-dimensional modeling (Figure 7).

ECM vs PCM There are significant differences in WSEs between the ECM profile and the PCM profile. In general, the PCM depicts a decrease in WSEs upstream of the crossing and an increase in WSEs downstream of the crossing.

Upstream of the crossing, the PCM shows a reduction in the 100-year water surface of roughly 2-feet for the first 120-feet. After the first 120-feet, the existing and proposed WSEs begin to converge. At roughly 400-feet upstream the existing and proposed WSEs converge near FIS section AY. Immediately downstream of the existing crossing, the PCM has a WSE elevation that is roughly 2-feet higher than the ECM; however, this difference persists for a much shorter distance and the two profiles converging near FIS section AV (Figure 6).

The discrepancy between upstream and downstream comparison of the two models is due to the hydraulically undersized existing structure. The existing 52-foot span bridge impedes flows upstream of the crossing creating deeper slower moving subcritical flow. To pass through the crossing, the flows become pressurized creating shallower high velocity supercritical flows. These supercritical flows are not naturally sustainable so a hydraulic jump forms within the channel, abruptly raising the WSE just downstream of the bridge. In contrast, the larger proposed structure allows the flows to pass through the structure relatively unimpeded.

Although the profile comparison shows significant changes in water surface elevations upstream and downstream of the crossing, the area mapped within the 100-year floodplain remains relatively unchanged with a slight reduction in inundated areas in the proposed condition. The existing condition delineation has 36-acres mapped within the 100-year floodplain and the proposed condition delineation has 35-acres mapped within the 100-year floodplain (Figure 7).

Conclusions

The proposed condition has a net overall decrease in floodplain inundation when compared to the effective FIRM and existing condition. The project does not negatively impact private properties or any insurable structures.

Additionally, the proposed project will eliminate the harsh hydraulics created by an existing hydraulically undersized crossing; improving public safety and decreasing maintenance costs.

References

1. Federal Emergency Management Agency. (May 5, 1981). Flood Insurance Rate Map, Kittitas County, Washington and Incorporated Areas, Community-Panel Number 5300950164B.

2. Federal Emergency Management Agency. (October 15, 1981). Flood Insurance Study, Kittitas County Washington and Incorporated Areas, Community Numbers 530096-530263-530234-530098-530095.

3. Federal Emergency Management Agency. (Effective September 24, 2021). Flood Insurance Rate Map, Kittitas County, Washington and Incorporated Areas, Community-Panel Number 53037C0411D.

4. Federal Emergency Management Agency. (Preliminary March 26, 2018). Flood Insurance Study, Kittitas County Washington and Incorporated Areas, Community Numbers 530096-530234-530098-530095- 530299-530263.

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 – Annotated FIS Flood Profile

Figure 7 – Annotated FIRM

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

DICKEY CREEK BRIDGE

MAPPED SURFACE

SRH2D - Two-Dimensional hydraulic model.

N US Hydro Boundary

Mesh

FIGURE 5

DICKEY CREEK BRIDGE

SRH2D MESH MODEL

DS Normal Depth Boundary

Bridge Mesh

Existing 1% Chance Flood

Proposed 1% Chance Flood P r o p o s e d

N o r th

F o r k

T e a n a w a y

R d

Existing Floodplain

Proposed Floodplain

Proposed Water Surface Elev.

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