Attachment 9 - 2021-11-11 McChord H-H Analysis Report.pdf

Hydrologic and Hydraulic Analysis Report McChord Field Bridges/Design J84004, J84011, J84018 at JBLM, WA

Contract W912DW-18-D-1011 Task Order W912DW19F2166 Design PKG No. CE0668

November 11, 2021

Hydrologic and Hydraulic Analysis Report McChord Field Bridges/Design J84004, J84011, J84018 at JBLM, WA

November 11, 2021 | i

Contents

1 Introduction

2 Survey Data

3 Existing Conditions

3.1 Historic Conditions

3.2 Site Visit

3.2.1 Crossing 84004 at Barnes Boulevard

3.2.2 Crossing 84011 at C-Street

3.2.3 Crossing 84018 at Perimeter Road

3.3 FEMA Floodplain

4 Hydrology

5 Hydraulics

5.1 HEC-RAS Model

5.1.1 Corrected Effective Model

5.1.2 Natural Conditions Model

5.1.3 Proposed Conditions Model

5.2 Downstream Analysis

5.3 Manning’s n-value Sensitivity Analysis

5.4 Hydraulic Results

5.4.1 Barnes Boulevard

5.4.2 Perimeter Road

5.5 Scour Analysis

Figures

Figure 1. Vicinity Map Figure 2. Clover Creek Vertical Profile Figure 3. Lower Clover Creek Reach HEC-RAS Model Geometry Figure 4. Natural Conditions HEC-RAS Model Geometry Representation Figure 5. January 2009 Representative Hydrograph Figure 6. 2D Water Surface Elevation Results at Perimeter Road (Corrected Effective) Figure 7. Mannings Sensitivity Analysis Range of Events Figure 8. Manning’s Sensitivity Analysis Figure 9. Pier Scour Example Figure 10. Pier Scour Example Figure 11. Abutment Scour Example Figure 12. Pressure Flow Example

Tables

Table 1. LiDAR Resolution and Accuracy – Specifications and Achieved Values Table 2. Summary of Existing Crossing Geometry Table 3. Summary of FEMA Hydrologic Information ii | November 11, 2021

Table 4. Summary of Bulletin 17C Hydrologic Information Table 5. Summary of FEMA and Bulletin 17C Flow Differences Table 6. Downstream Analysis Hydraulic Results Table 7. Mannings Sensitivity Analysis Range of Events Table 8. HEC-RAS Existing Conditions Results Table 9. HEC-RAS Proposed Conditions Results Table 10. Total Scour for 100-year AEP

Photos

Photo 1. 84004 Inlet, Looking Downstream Photo 2. 84004 Outlet, Looking Upstream Photo 3. 84011 Upstream Face, Looking Downstream Photo 4. 84011 Downstream Face, Looking Upstream Photo 5. 84011 Channel Substrate Photo 6. 84018 Outlet, Looking Upstream Photo 7. 84018 Security Grate on Left Culvert taken from inside, Upstream Side Photo 8. 84018 Outlet, Looking Downstream Photo 9. 84018 Security Grate on Right Culvert taken from inside, Upstream Side

Appendices

USGS Maps (1987) and Aerial Imagery (1957)

Photographs

FEMA Flood Map Panels 53053C0313E and 53053C0314E

Bulletin 17C Results

Mannings Sensitivity Analysis

HEC-RAS Standard Table 1 Results

Static Water Surface Elevation of the Upstream and Structure Face Cross Sections and 2D Model Results

Proposed Conditions Scour Results

HEC-RAS Cross Section Locations

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Acronyms

1D one-dimensional 2D two-dimensional CFR Code of Federal Regulations cfs cubic feet per second CMP Corrugated metal pipe DTM Digital Terrain Model FEMA Federal Emergency Management Agency FIS Flood Insurance Study JBLM Joint-Base Lewis McChord LiDAR light detection and ranging USAF U.S. Air Force USGS U.S. Geological Survey WDFW Washington Department of Fish and wildlife WSEL Water Surface Elevation

November 11, 2021 | 1

1 Introduction The intent of the Design for Repair McChord Field Bridges project is to repair or replace three existing crossings of Clover Creek within Joint-Base Lewis McChord (JBLM). The three crossings, from upstream to downstream, are referred to as 84018, 84011, and 84004 (Figure 1). As part of this analysis, bridge structures will be assessed for each of the crossings and a downstream analysis performed to detail potential downstream consequences of structural changes to the crossings. This report discusses the effects of the proposed conditions at the three crossing locations in reference to Pierce County and Federal Emergency Management Agency (FEMA) requirements. The crossing locations are within a FEMA Zone AE detailed study area, and a detailed no-rise analysis is required. Currently, a no-rise can be achieved with the proposed designs.

The HEC-RAS corrected effective model was developed from the FEMA effective model to incorporate survey information collected in 2018 and address several issues present in the FEMA effective model. A natural conditions model was then created based on the corrected effective model to analyze the system without the influence of the study crossings. The proposed conditions model incorporates the structural changes to the crossings. When constructed, the selected alternatives will be built at the same time or in close succession.

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Figure 1. Vicinity Map

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2 Survey Data Survey information collected in 2018 at each of the three crossings was provided by Reid Middleton Engineering Consultants. Digital Terrain Models also were provided. The 2011 Pierce County Light Detection and Ranging (LiDAR) was downloaded from the Washington LiDAR Portal. The LiDAR was collected and processed by Watershed Sciences, Inc. on January 3, 2011, February 8-10, 2011, and February 20, 2011 and is summarized in Table 2. This information was incorporated into the FEMA effective model to create a corrected effective model (Section 3.3).

Table 1. LiDAR Resolution and Accuracy – Specifications and Achieved Values

3 Existing Conditions Within JBLM, Clover Creek flows in a low-gradient engineered channel with heavily vegetated side slopes. Crossing 84018 is located at the upstream boundary of the base, Crossing 84011 is located approximately 3,000 feet downstream of the runway, and crossing 84004 is approximately 200 feet upstream of the western base boundary where the creek exits the base. The existing crossing geometry is summarized in Table 2.

Table 2. Summary of Existing Crossing Geometry

Crossing Location Structure Type Dimensions HEC-RAS Station

84004 Barnes Boulevard

Three Arch Culverts

3 x 13-foot span by 8-foot rise 3.849

84011 C-Street Clear-Span Bridge 57-foot span 2.655

84018 Perimeter Road Two Arch Culverts

2 x 12-foot span by 8-foot rise 2.125

The creek has an average channel gradient of approximately 0.08 percent between the 84004 and 84018 crossings (Figure 2). Outside JBLM, between Pacific Lutheran University and Steilacoom Lake, the channel gradient is approximately 0.14 percent including the steep drop to Steilacoom Lake (Figure 2).

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Figure 2. Clover Creek Vertical Profile

Additional information collected during the site visit is provided in Section 3.2.

3.1 Historic Conditions

Historic U.S. Geological Survey (USGS) maps from 1897 indicate development in areas around Clover Creek, and show Clover Creek flowed in a natural channel that had not been straightened (Appendix A). USGS aerial imagery from 1957 shows Clover Creek in the engineered conveyance channel that it flows through today (Appendix A). The document Historic Flows, Flow Problems and Fish Presence in Clover Creek – 1924 – 1942: Interviews with Early Residents indicates that the culvert crossing beneath McChord Field was installed in the late 1930s, which is likely when the rest of the stream within JBLM was realigned.

3.2 Site Visit

A site visit was conducted on October 25, 2017 by HDR staff accompanied by a fisheries biologist from Washington Department of Fish and Wildlife (WDFW) and a U.S. Air Force (USAF) representative. Observations and information collected in the field are discussed below. Additional photos are provided in Appendix B.

3.2.1 Crossing 84004 at Barnes Boulevard

At crossing 84004, Clover Creek is conveyed beneath Barnes Boulevard in three, 13-foot span by 8-foot rise corrugated metal pipe (CMP) culverts located in a heavily vegetated (tall grasses and other aquatic plants) channel (Photo 1 and Photo 2). Near the channel banks soft silty soils were dominant, however near the center of the channel cross section there were some smaller cobbles (up to 4 inches in size) and gravels across the channel section. The channel depth upstream of the culverts was approximately 2.5 feet in the center. Upstream of the crossing, the right bank was not densely vegetated with some trees and exposed ground, and the left bank was heavily vegetated with tall

0 5000 10000 15000 20000 25000 30000

El ev at io n (ft

, N

AV

D

8)

Station (ft)

Lower Clover Creek Vertical Profile

Stream Profile Crossing Structures Alternative Locations

84004

84011

84018

Steilacoom Lake

10th Ave S

McChord Field

November 11, 2021 | 5 grasses. Downstream of the crossing both banks were heavily vegetated with tall grasses and blackberries. At B-Street approximately 350 feet upstream of Barnes Boulevard, there appeared to be a riffle that quickly transitioned to a low velocity backwater. Downstream of the crossing there is a grate across the stream, beneath a foot bridge, and over the channel. Just upstream of the grate there is a boom in the channel that appears to be diverting debris into a box culvert located on the left bank of the channel. Between this diversion and the 84004 crossing there is a second box culvert on the left bank that also appears to be used as a debris diversion. A bankfull width measurement of 47.5 feet was taken approximately 100 feet upstream of the 84004 crossing.

Photo 1. 84004 Inlet, Looking Downstream

Photo 2. 84004 Outlet, Looking Upstream

3.2.2 Crossing 84011 at C-Street

At crossing 84011, Clover Creek is conveyed beneath C-Street through a 56.6-foot span bridge (Photo 3 and Photo 4). This is the only existing bridge crossing that is part of the project, compared to the culvert crossings at 84004 and 84018. The depth of flow here was deeper than at 84018, approximately 4 feet at the thalweg beneath the bridge.

Channel substrate was composed of cobbles up to 6 inches across and gravels in the center of the channel, with smaller grained silt and muck on the left and right sides.

There were broken concrete slabs placed beneath the bridge on the left and right banks presumably to provide scour protection. Downstream of the crossing, the right bank was heavily vegetated with tall grasses while the left bank had a sparser mix of grasses, blackberries, and trees. Upstream of the crossing both banks were steep and heavily vegetated with grasses and blackberries. A bankfull width measurement of 35 feet was taken approximately 100 feet downstream of the crossing.

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Photo 3. 84011 Upstream Face, Photo 4. 84011 Downstream Face, Photo 5. 84011 Channel Substrate

3.2.3 Crossing 84018 at Perimeter Road

At crossing 84018, Clover Creek is conveyed beneath Perimeter Road in two, 12-foot span by 8-foot rise CMP culverts (Photo 6 and Photo 8). The upstream end of the culverts is near the fenced JBLM boundary, and there is a grate with 6-inch-wide by 13-inch-tall openings across the mouth of both culverts (Photo 7 and Photo 9). Debris has accumulated across the grate of the left side culvert, causing an approximately 10-inch to 12-inch water surface elevation drop from the exterior of the grate to the interior of the grate and culvert. The entrance to the right side culvert is completely obstructed with debris, while the left hand culvert is approximately 50 percent obstructed. Depth of flow within the left side culvert was approximately 1.5 feet, while 75 feet downstream of the crossings the depth increased to approximately 2.5 feet.

The right and left banks of the channel on the downstream end of the crossing are both heavily vegetated with tall grasses, and some shrubs and trees. There is rocky substrate

November 11, 2021 | 7 in the channel, with some 6-inch to 8-inch cobbles. Two bankfull width measurements were taken at this crossing, these were a 44-foot width approximately 50 feet downstream of the crossing, and a 40-foot width approximately 100 feet downstream of the crossing.

Photo 6. 84018 Outlet, Photo 7. 84018 Security Grate on Left Culvert taken from inside, Upstream Side

Photo 8. 84018 Outlet, Photo 9. 84018 Security Grate on Right Culvert taken from inside, Upstream Side

3.3 FEMA Floodplain

Within the limits of the proposed project, Clover Creek is mapped as a FEMA Zone AE detailed study area with a floodway. This project is within FEMA Flood Map Panels 53053C0313E and 53053C0314E (Appendix C). The FEMA effective hydraulic model was provided by Pierce County. The current effective model and Flood Insurance Study (FIS) report were approved in March 2017. Several corrections and updates have since been made to the effective model, including incorporation of the 2018 survey data at the study structures and 2011 LiDAR for the overbanks, resulting in a corrected effective model. These changes are discussed in greater detail in Section 5.1.1.

The Code of Federal Regulations (CFR), Title 44, Chapter 1, Subchapter B, Part 60 stipulates that new construction within a FEMA Zone AE Floodway shall not result in any increase in flood levels during the base flood discharge. Pierce County Code Chapter 18E.070.030 also requires that a zero-rise analysis be conducted once the County determines that the project area is located within a regulatory flood hazard area. Per Pierce County Code requirements, this analysis must demonstrate the proposed project results in a 0.01 foot or less rise in water surface elevations outside the FEMA mapped

8 | November 11, 2021 floodway, and a 0.00 foot or less rise in water surface elevations inside the FEMA mapped floodway.

4 Hydrology Hydrologic information for the 100-year flow event was included in the FEMA effective hydraulic model provided by Pierce County. Additional hydrologic data is available in the effective FIS report (effective March 2017) for the 10-year, 50-year, 100-year, and 500-year peak flow events. It should be noted the hydrologic analysis for Clover Creek was performed using Bulletin 17B methods with a period of record from 1949 to 1999.

Table 3 provides a summary of the FEMA design flows.

Table 3. Summary of FEMA Hydrologic Information

Location 10-year (cfs) 50-year (cfs) 100-year

(cfs) 500-year

(cfs)

Peak Flow (cfs) Gravelly Lake Drive

291 434 501 677 cfs = cubic feet per second

Because an additional 20 years has passed from the initial analysis and the USGS Bulletin 17B has been updated to Bulletin 17C, an updated hydrologic analysis was performed as verification. USGS 12090500 Clover Creek near Tillicum, Washington was used for this analysis with a period of record from 1950 to 2016 and these results can be seen in Appendix D. Table 4 provides a summary of the Bulletin 17C design flows and Table 5 shows the difference from FEMA flows. Although the locations are different, they are close enough in proximity to be reasonably comparable for this analysis. For this analysis, the FEMA flows will be used because the Bulletin 17C flows are within 10 percent and well within the bounds of uncertainty.

Table 4. Summary of Bulletin 17C Hydrologic Information

Location 10-year (cfs) 50-year (cfs) 100-year

(cfs) 500-year

(cfs)

Peak Flow (cfs) Pacific Highway

331 478 543 696

Table 5. Summary of FEMA and Bulletin 17C Flow Differences Location 10-year 50-year 100-year 500-year

Peak Flow (cfs) Gravelly Lake Drive /Pacific Highway

14% 10% 8% 3%

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5 Hydraulics

5.1 HEC-RAS Model

The FEMA effective hydraulic model for Clover Creek provided by Pierce County was updated with recent survey and LiDAR information to a corrected effective hydraulic model, which was used to perform the hydraulic analysis for each alternative. The effective model was completed in Version 3.13 of HEC-RAS in June 2006. Figure 3 shows the Lower Clover Creek Reach and HEC-RAS cross sections (Appendix I). The project area is located within the Lower Clover Creek Reach in the HEC-RAS model.

Reaches upstream of the Lower Clover Creek Reach were truncated to decrease model run times. HEC-RAS version 6.0 was used for hydraulic modeling of the corrected effective and natural conditions runs.

5.1.1 Corrected Effective Model

Changes to the effective model included updating expansion and contraction coefficients to 0.3 and 0.5, respectively, at the two cross sections upstream and one cross section downstream of all bridge or culvert crossings within the Lower Clover Creek HEC-RAS project reach. Peak flow events at all bridge and culvert crossings within the Lower Clover Creek reach were reviewed to determine whether the high-flow bridge modeling approach should use the pressure or energy/weir method and updated accordingly.

Cross sections within the limits of the 2018 Digital Terrain Models data and 2011 LiDAR data were updated to reflect the new topography, and Manning’s n-values were adjusted based on site photos. Culvert, bridge, and roadway geometry also was updated based on the 2018 survey information. Ineffective flow areas, bank stations, and encroachments were adjusted at cross sections, which changed based on the new topography datasets. In the effective model, lateral structures located downstream of Interstate 5 use a weir coefficient of 2.0 to represent natural ground. Typically, weir coefficients representing natural ground range between 0.1 and 0.5. Because water surface elevations within the project reach were not affected by these lateral structures, no changes were made during the analysis. The hydrology in the effective model between the base-flow 100-year flow profile and the encroachment 100-year flow profile differs downstream of section 4.412. To compare these two events, their hydrology must be similar. Similar to the lateral structure weir coefficients, the hydrology differences do not affect water surface elevations through the project area and no changes were made during the analysis.

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Figure 3. Lower Clover Creek Reach HEC-RAS Model Geometry

5.1.2 Natural Conditions Model

The Natural Conditions Model was created to analyze the existing and design the proposed structures effect on the system. Changes to the corrected effective model included deleting the crossings at Barnes Boulevard and Perimeter Road. Two additional cross sections were placed at the existing structure faces to represent the internal cross sections associated with the HEC-RAS structure and an example of this at Barnes Boulevard can be seen in Figure 4. The elevation profile was then interpolated from the existing upstream and downstream cross sections.

XS 1.2918;

USGS 12090500 CLOVER

CREEK NEAR TILLICUM, WA

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Figure 4. Natural Conditions HEC-RAS Model Geometry Representation

5.1.3 Proposed Conditions Model

The Proposed Conditions Model was created to analyze the structural changes and their effect on base flood elevations at Barnes Boulevard and Perimeter Road. Further analysis needs to be completed at C Street, but these design changes for slope protection are not anticipated to affect the no-rise analysis. No other modeling changes were made.

5.2 Downstream Analysis

A downstream analysis was performed to consider the effects a larger proposed structure may have on downstream flooding using two-dimensional (2D) unsteady model runs. Hydrographs for the 25- and 100-year return event were scaled from the January 2009 flooding event which was the third largest ever recorded in the watershed. This hydrograph is provided in Figure 5. The analysis found that flow attenuated upstream of Perimeter Road and that the one-dimensional (1D) steady (no-rise) model is more conservative in reported water surface elevations. At Perimeter Road, the water surface elevations were within 0.25 foot for the 1D steady (no rise) and 2D unsteady model for both existing and proposed conditions which highlights the validity of the analysis.

Different results should be expected due to different computational methods use, but a major divergence would indicate a potential issue in model setup and flow applications.

Based on the analysis, no downstream flooding effects are expected due to the proposed larger structures. The results can be seen in Table 6 and an image of the 2D water surface elevation and flowpaths can be seen in Figure 6. In Table 6, the 1D and 2D models used the same corrected effective and proposed conditions, with the only difference being steady versus unsteady simulations.

HEC-RAS structure symbology

Natural Conditions at Barnes Blvd (no structure)

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Figure 5. January 2009 Representative Hydrograph

Table 6. Downstream Analysis Hydraulic Results

Location Analysis

1D Steady (no-rise) 2D Unsteady Delta

Corrected Effective Proposed

Corrected Effective Proposed

Corrected Effective Proposed

Barnes Boulevard

Flow (cfs) 501 501 464.4 469.55 -36.6 -31.5

Water Surface Elevation (feet) 277.80 277.48 277.64 277.39 -0.16 -0.09

Perimeter Road

Flow (cfs) 501 501 464.5 464.5 -36.5 -36.5

Water Surface Elevation (feet) 286.40 286.00 286.17 285.87 -0.23 -0.13

November 11, 2021 | 13

Figure 6. 2D Water Surface Elevation Results at Perimeter Road (Corrected Effective)

5.3 Manning’s n-value Sensitivity Analysis

Because the selected roughness value can significantly affect computed model water surface elevations, a sensitivity analysis was performed using the downstream analysis geometry, which consisted of a 1D channel and 2D overbanks. Peak flow events were analyzed at USGS 12090500 Clover Creek near Tillicum, Washington. The events considered for the sensitivity analysis are shown in Table 7 and Figure 7.

Table 7. Mannings Sensitivity Analysis Range of Events

Date USGS Flow

(cfs) USGS Stage (ft-NGVD 29)

Water Surface Elevation

(ft-NAVD88)

5-Mar-00 119 16.05 268.46 11-Jun-01 56 15.48 267.89 16-Dec-01 119 16.3 268.71 12-Mar-03 84 15.74 268.15 31-Jan-04 111 15.93 268.34 16-Apr-05 79 15.63 268.04 13-Jan-06 286 17.18 269.59

WSEL, ft NAVD88

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Date USGS Flow

(cfs) USGS Stage (ft-NGVD 29)

Water Surface Elevation

(ft-NAVD88)

7-Jan-07 332 17.26 269.67 3-Dec-07 91 15.95 268.36 9-Jan-09 284 17.03 269.44

15-Jan-10 183 16.66 269.07 5-Apr-11 144 17.06 269.47

31-Mar-12 142 17.03 269.44 25-Dec-12 134 16.94 269.35 11-Mar-14 246 18.25 270.66 8-Feb-15 135 17.11 269.52

10-Dec-15 230 18.22 270.63

Figure 7. Mannings Sensitivity Analysis Range of Events

Events for analysis were then selected based on the availability of 15-minute observed hydrograph data and event magnitude. The data range was selected because prior data appeared recorded on a different gage datum. Seasonality played a major factor in stream stage with the three largest events in the last 20 years occurring in January with a lower relative stage than other smaller events that occurred in December or March. This seasonal variation is most likely due to vegetation in the channel with more vegetation resulting in higher water surface stages in March and December and less vegetation resulting in lower water surface stages in January. Manning’s Flow Roughness Factors were then utilized within HEC-RAS to calibrate the n-value for the full range of flows (Figure 7 and Appendix D). All events exhibited the same trend with higher roughness values with lower flows and decreasing roughness values as flows become greater.

These trends are reasonable considering the effects that depth and vegetation can have

267.50

268.00

268.50

269.00

269.50

270.00

270.50

271.00

0 50 100 150 200 250 300 350

W

SE

L (ft

-N

AV

D

8)

Flow (cfs)

Jan-09

Mar-14

Mar-12

Dec-15

November 11, 2021 | 15 on velocities. It was determined to use the roughness values as is and not apply any adjustment factor for the corrected effective or proposed conditions models based on the cumulative trends presented in Figure 7 and Figure 8 because the flooding events, or 100-year event, is the design driver. In Figure 7, the March 2014 and December 2015 trended higher than normal so less weight was given to their calibration. The January 2009 and March 2012 events appeared more in line with the trend of most recorded peak flows so more weight was placed in those roughness calibrations. Defining an extrapolation to the 100-year flow was impossible due to the uncertainty of the hydraulics within the system because data for higher flows was not available but using a Mannings Roughness Factor of 1 is reasonable given the weight of the January 2009 and March 2012 events.

Figure 8. Manning’s Sensitivity Analysis

5.4 Hydraulic Results

Table 8 provides a comparison of base flood elevations between the corrected effective model and natural conditions for locations where structural changes occurred. Table 9 provides a comparison of base flood elevations between the corrected effective model and proposed conditions. The HEC-RAS Standard Table 1 results are provided in Appendix F.

Table 8. HEC-RAS Existing Conditions Results

Location Profile WSEL (ft)

Location Profile WSEL (ft)

Existing Natural Delta Existing Natural Delta

Barnes Boulevard

P010yr 276.30 276.20 -0.10

Perimeter Road

P010yr 284.16 283.95 -0.21

P050yr 277.38 277.14 -0.24 P050yr 285.69 285.36 -0.33

P100yr 277.80 277.48 -0.32 P100yr 286.40 285.99 -0.41

P500yr 278.88 278.26 -0.62 P500yr 288.63 287.78 -0.85

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Table 9. HEC-RAS Proposed Conditions Results

Location Profile WSEL (ft)

Location Profile WSEL (ft)

Existing Natural Delta Existing Natural Delta

Barnes Boulevard

P010yr 276.30 276.21 -0.09

Perimeter Road

P010yr 284.16 283.97 -0.19

P050yr 277.38 277.14 -0.24 P050yr 285.69 285.38 -0.31

P100yr 277.80 277.48 -0.32 P100yr 286.40 286.00 -0.40

P500yr 278.88 278.27 -0.61 P500yr 288.63 287.77 -0.86

5.4.1 Barnes Boulevard

At Barnes Boulevard, the water stays in the channel up to at least the 500-year annual exceedance probability (AEP) for the existing conditions model. However, flooding caused by downstream conditions backup waters in the channel close to the project.

The right overbank elevations in this downstream section are approximately 3 feet lower than in the immediate vicinity of Barnes Boulevard contributing to the flooding.

Consequently, the low chord elevation on the structure does not drive flooding.

Appendix G details the static water surface elevation of the upstream and structure face cross sections along with 2D model results from the downstream analysis for the 100-year AEP which shows flooding downstream of the project.

Flooding is similar for the proposed conditions model, albeit 0.32 feet lower for the 100-yr

AEP.

5.4.2 Perimeter Road

Water overtops Perimeter Road to the north and floods between the 10- and 50-year AEP for the existing conditions model. The roadway elevation at this location is

284.30 feet while the tops of the culverts are approximately 286.30 feet. However, the volume of flow overtopping is small compared to the overall conveyance of the channel.

Debris accumulation on the security grate also may factor into the actual water surface elevations/flooding. Appendix G details the static water surface elevation of the upstream and structure face cross sections along with 2D model results from the downstream analysis for the 100-year event.

Water still overtops at a low spot roughly 650 feet to the north between the 50- and the 100-year AEP for the proposed conditions model, but water surface is 0.40 feet lower for the 100-year AEP.

5.5 Scour Analysis

The potential for scour to develop at Barnes Boulevard and Perimeter Road was investigated. The scour computations performed by the HEC-RAS program used for this analysis are based on the fourth edition of the Hydraulic Engineering Circular No. 18 (HEC-18). In HEC-18, scour is defined as the combined effects of:

1. Long-term aggradation and/or degradation

2. Contraction scour

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3. Local scour at piers and abutments

Long-term aggradation and/or degradation can be calculated using the sediment transport module of HEC-RAS which can then be exported to update the geometry, and the scour module run using the updated geometry. However, the conservative and general best practice is to only adjust the geometry if degradation occurs and not if aggradation occurs. These calculations can also be performed by following the procedure in HEC-18 and adding or subtracting the aggradation or degradation from the total scour computed by HEC-RAS. For any further information required for scour computations, more definitions and illustrations can be found in Chapter 10 of the HEC- RAS Hydraulic Reference Manual, Chapter 12 of the HEC-RAS User’s Manual, and the fourth edition of HEC-18.

• Contraction scour, in a natural channel or at a bridge crossing, is defined as the removal of material from the bed and banks across all or most of the channel width. This component of scour results from a contraction of the flow area at the bridge which causes an increase in velocity and shear stress on the bed at the bridge. The contraction can be caused by the bridge or from a natural narrowing of the stream channel. Figure 9 illustrates the contraction of flow through a bridge opening.

Figure 9. Pier Scour Example

Contraction Scour (HEC-18 2012)

• Local scour involves the removal of material from around piers, abutments, spurs, and embankments caused by an acceleration of flow and resulting vortices induced by obstructions to the flow. Figure 10 and Figure 11 illustrate pier and abutment scour, respectively.

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Figure 10. Pier Scour Example

Pier Scour (HEC-18 2012)

Figure 11. Abutment Scour Example

Abutment Scour (HEC-18 2012)

• Pressure (Orifice) scour, commonly referred to as vertical contraction scour, results from flow impinging on bridge superstructure elements (e.g., low chord). A combination of pressure flow and weir flow is illustrated in Figure 12. It is important to note that HEC-RAS cannot model this situation.

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Figure 12. Pressure Flow Example

Pressure (orifice) flow (HEC-18 2012)

• Clear-water scour occurs when there is no movement of the bed material in the flow upstream of the bridge crossing or the bed material being transported in the upstream reach remains in suspension through the scour hole at the pier or abutment. Therefore, no upstream bed material is being deposited in the scour hole.

• Live-bed scour occurs when there is a transport of bed material from the upstream reach into the scour hole, creating a cyclic process that creates and refills the scour hole during a flood event.

• Critical Shear Stress is the minimum amount of shear stress required to initiate soil particle motion.

• Critical Velocity is the minimum velocity of the reach required to initiate soil particle motion. Note that critical velocity varies based on water depth for soil particles.

The U.S. Army Corps of Engineer’s 2019 geotechnical investigation reported a channel probe depth up to 120 plus feet at locations immediately downstream of the existing Barnes Boulevard culverts (B-1 and B-2 from Attachment B, Geotechnical Exploration Plan, McChord Field Bridges) and immediately upstream and downstream of the existing Perimeter Road culverts (B-3 and B-4 from Attachment B, Geotechnical Exploration Plan, McChord Field Bridges). This is the nearest channel probe location on the downstream side of the existing culverts. A D50 particle size of 9.5 millimeter was used for the scour analysis based on these probes.

HEC-RAS allows the user to select a specific equation (either the HIRE or Froehlich equation), or select the default mode. When the default mode is selected, the program chooses the equation that is the most applicable to the situation. The selection is based on computing a factor of the embankment length divided by the approach depth. If this

20 | November 11, 2021 factor is greater than 25, the program automatically uses the HIRE equation. If the factor is equal to or less than 25, the program automatically uses the Froehlich equation.

Table 10 provides a comparison of base flood elevations between the corrected effective model and natural conditions for locations where structural changes occurred. The appropriate scour depth protection should be further analyzed as design progresses.

Scour results are provided in Appendix H.

Table 10. Total Scour for 100-year AEP

Location HIRE Elevation

(feet) Froehlich Elevation

(feet)

Barnes Boulevard 260.11 263.23*

Perimeter Road 264.91* 274.40

*Default HEC-RAS selection

USGS Maps (1987) and Aerial Imagery (1957)

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pwurden Rectangle pwurden Callout Historic Clover Creek Channel pwurden Text Box USGS Historic Map - 1897

Callout Crossing 84004 pwurden Callout Crossing 84011 pwurden Callout Crossing 84018 pwurden Text Box USGS Historic Aerial - 1957

Photographs

84004 debris boom downstream of Barnes Boulevard from atop pedestrian bridge.

84004 debris boom downstream of Barnes Boulevard from atop box culvert structure.

84004 looking upstream (east) towards Barnes Boulevard. 84004 debris barrier below pedestrian bridge downstream of

Barnes Boulevard.

84004 zoom view of second box culvert structure located on right bank downstream of Barnes Boulevard.

84004 second box culvert structure located on right bank downstream of Barnes Boulevard.

84004 looking downstream (west) to Barnes Boulevard.84004 looking upstream (east) from Barnes Boulevard.

84004 left bank at Barnes Boulevard, upstream side.84004 right bank at Barnes Boulevard, upstream side.

84011 channel, looking upstream (south). 84011 channel, looking downstream(north).

84011 right bank protection and large wood piece. 84011 right bank protection and large wood piece viewed from left bank.

84018 channel, looking downstream (south).

84018 channel, looking downstream (south) from Perimeter Road.

84018 right culvert, debris racked against security grate at upstream end.

84018 left culvert, debris racked against security grate at upstream end.

FEMA Flood Map Panels 53053C0313E and 53053C0314E

Crossing 84004 pwurden

Crossing 84011

Crossing 84018

73P

Crossing 84004

74P

Crossing 84011

75P

Crossing 84018

Bulletin 17C Results

Mannings Sensitivity Analysis

HEC-RAS Standard Table 1 Results

Reach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Ch

(cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft)

Clover‐Low 5.2118 P100yr 396 289.88 295.84 292.86 296.01 0.00128 3.54 129.96 111.63 0.27

Clover‐Low 5.2118 Floodway 396 289.88 295.87 292.86 296.05 0.001249 3.51 131.06 33.08 0.26

Clover‐Low 5.2118 P010yr 178 289.88 293.95 291.79 294.05 0.001233 2.62 74.25 27 0.24

Clover‐Low 5.2118 P050yr 316 289.88 295.25 292.5 295.4 0.001251 3.24 111.8 45.23 0.26

Clover‐Low 5.2118 P500yr 644 289.88 297 293.83 297.28 0.001622 4.53 166.25 221.88 0.31

Clover‐Low 5.2049 P100yr 396 289.81 295.83 292.73 295.96 0.000952 3.05 147.06 35.43 0.23

Clover‐Low 5.2049 Floodway 396 289.81 295.87 292.73 295.99 0.000929 3.03 148.15 35.43 0.23

Clover‐Low 5.2049 P010yr 178 289.81 293.93 291.73 294 0.000926 2.26 88.98 32.05 0.21

Clover‐Low 5.2049 P050yr 316 289.81 295.24 292.43 295.34 0.000921 2.78 128.91 34.37 0.22

Clover‐Low 5.2049 P500yr 644 289.81 297 293.55 297.21 0.001252 3.99 182.76 37.56 0.27

Clover‐Low 5.201 Bridge

Clover‐Low 5.198 P100yr 396 290.14 295.8 292.67 295.91 0.000828 2.7 148.73 34.99 0.22

Clover‐Low 5.198 Floodway 396 290.14 295.83 292.67 295.94 0.000807 2.68 149.87 34.99 0.21

Clover‐Low 5.198 P010yr 178 290.14 293.9 291.77 293.96 0.000863 1.99 90.08 31.17 0.2

Clover‐Low 5.198 P050yr 316 290.14 295.21 292.4 295.3 0.000811 2.45 130.3 33.45 0.21

Clover‐Low 5.198 P500yr 644 290.14 296.95 293.35 297.14 0.001078 3.55 184.68 38.39 0.25

Clover‐Low 5.1925 P100yr 396 289.88 295.71 292.78 295.87 0.001303 3.33 126.9 31.32 0.26

Clover‐Low 5.1925 Floodway 396 289.88 295.75 292.78 295.91 0.001268 3.3 128.11 31.32 0.26

Clover‐Low 5.1925 P010yr 178 289.88 293.84 291.74 293.93 0.001345 2.46 73.71 26.14 0.25

Clover‐Low 5.1925 P050yr 316 289.88 295.13 292.45 295.27 0.001275 3.03 109.27 29.27 0.26

Clover‐Low 5.1925 P500yr 644 289.88 296.82 293.65 297.1 0.001681 4.32 180.95 93.07 0.31

Clover‐Low 5.0653 P100yr 396 288.82 294.69 294.9 0.001625 3.74 133.85 85.88 0.29

Clover‐Low 5.0653 Floodway 396 288.82 294.71 294.93 0.001679 3.81 115.94 33.1 0.29

Clover‐Low 5.0653 P010yr 178 288.82 292.86 292.97 0.001504 2.72 67.06 22.23 0.26

Clover‐Low 5.0653 P050yr 316 288.82 294.12 294.3 0.001629 3.47 98.21 30.33 0.28

Clover‐Low 5.0653 P500yr 644 288.82 296.01 296.15 0.001104 3.57 357.76 233.49 0.24

Clover‐Low 4.9101 P100yr 396 287.37 293.39 290.78 293.59 0.001563 3.86 125.68 39.87 0.29

Clover‐Low 4.9101 Floodway 396 287.37 293.41 290.78 293.61 0.001542 3.84 126.28 39.88 0.29

Clover‐Low 4.9101 P010yr 178 287.37 291.56 289.6 291.69 0.001625 2.97 68.06 28.15 0.28

Clover‐Low 4.9101 P050yr 316 287.37 292.81 290.39 292.99 0.00157 3.58 106.64 36.15 0.29

Clover‐Low 4.9101 P500yr 644 287.37 294.76 291.79 295.03 0.001668 4.64 172.78 94.76 0.32

Clover‐Low 4.9048 P100yr 396 287.62 293.37 290.71 293.54 0.001357 3.56 130.14 33.31 0.28

Clover‐Low 4.9048 Floodway 396 287.62 293.39 290.71 293.56 0.001339 3.54 130.71 33.31 0.28

Clover‐Low 4.9048 P010yr 178 287.62 291.54 289.75 291.64 0.001388 2.68 75.52 30.02 0.26

Clover‐Low 4.9048 P050yr 316 287.62 292.79 290.41 292.94 0.001338 3.26 112.39 32.27 0.27

Clover‐Low 4.9048 P500yr 644 287.62 294.73 291.53 294.98 0.001513 4.39 173.07 87.59 0.3

Clover‐Low 4.901 Bridge

Clover‐Low 4.898 P100yr 396 287.26 293.36 290.21 293.49 0.0009 3.07 145.48 38.45 0.23

Clover‐Low 4.898 Floodway 396 287.26 293.38 290.21 293.51 0.00089 3.06 146.02 38.45 0.23

Clover‐Low 4.898 P010yr 178 287.26 291.52 289.14 291.59 0.000765 2.18 92.31 33.72 0.2

Clover‐Low 4.898 P050yr 316 287.26 292.78 289.87 292.89 0.000848 2.78 128.71 36.96 0.22

Clover‐Low 4.898 P500yr 644 287.26 294.7 290.97 294.91 0.00111 3.93 184.29 86.36 0.26

Clover‐Low 4.8932 P100yr 396 287.07 293.25 290.31 293.45 0.001444 3.83 123.27 31.11 0.29

Clover‐Low 4.8932 Floodway 396 287.07 293.27 290.31 293.47 0.001425 3.81 123.89 31.11 0.28

Clover‐Low 4.8932 P010yr 178 287.07 291.46 289.19 291.56 0.001197 2.7 73.1 24.95 0.24

Clover‐Low 4.8932 P050yr 316 287.07 292.68 289.93 292.85 0.001366 3.47 106.24 29.16 0.27

Clover‐Low 4.8932 P500yr 644 287.07 294.55 291.29 294.86 0.00173 4.82 166.71 85.82 0.32

Clover‐Low 4.8048 P100yr 396 286.4 292.91 289.64 292.98 0.000635 2.5 254.29 274.82 0.19

Clover‐Low 4.8048 Floodway 396 286.4 292.91 289.64 292.99 0.000678 2.58 243.95 103 0.19

Clover‐Low 4.8048 P010yr 178 286.4 290.94 288.53 291.03 0.001069 2.43 84.49 161.54 0.22

Clover‐Low 4.8048 P050yr 316 286.4 292.24 289.3 292.33 0.000858 2.67 182.25 254.77 0.21

Clover‐Low 4.8048 P500yr 644 286.4 294.65 290.83 294.66 0.000089 1.12 1100.16 321.52 0.07

Clover‐Low 4.7899 P100yr 396 286.29 292.8 289.51 292.92 0.000913 3.03 185.39 123.46 0.22

Existing Model Results

Clover‐Low 4.7899 Floodway 396 286.29 292.8 289.51 292.92 0.000929 3.05 175.26 63.45 0.23

Clover‐Low 4.7899 P010yr 178 286.29 290.86 288.42 290.95 0.00102 2.42 78.84 28.31 0.22

Clover‐Low 4.7899 P050yr 316 286.29 292.12 289.15 292.25 0.001067 3.01 134.87 69.41 0.24

Clover‐Low 4.7899 P500yr 644 286.29 294.59 290.47 294.64 0.000405 2.42 561.33 274.03 0.16

Clover‐Low 4.7842 P100yr 396 286.48 292.77 289.27 292.89 0.00082 2.85 151.58 99.37 0.21

Clover‐Low 4.7842 Floodway 396 286.48 292.77 289.27 292.89 0.000818 2.85 151.68 31.65 0.21

Clover‐Low 4.7842 P010yr 178 286.48 290.85 288.34 290.91 0.000726 2.02 92.35 51.26 0.19

Clover‐Low 4.7842 P050yr 316 286.48 292.11 288.98 292.21 0.000809 2.6 130.95 85.69 0.21

Clover‐Low 4.7842 P500yr 644 286.48 294.58 290.1 294.63 0.00035 2.24 539.09 250.85 0.15

Clover‐Low 4.783 Bridge

Clover‐Low 4.7808 P100yr 396 286.41 292.6 289.4 292.75 0.001094 3.4 150.95 115.27 0.25

Clover‐Low 4.7808 Floodway 396 286.41 292.6 289.4 292.76 0.001092 3.39 151.08 48.39 0.25

Clover‐Low 4.7808 P010yr 178 286.41 290.81 288.31 290.9 0.000904 2.41 82.63 34.92 0.21

Clover‐Low 4.7808 P050yr 316 286.41 292.03 289.03 292.17 0.00106 3.12 125.35 87.88 0.24

Clover‐Low 4.7808 P500yr 644 286.41 294.03 290.36 294.23 0.001113 3.97 231.7 164.27 0.26

Clover‐Low 4.7763 P100yr 396 285.32 292.57 288.71 292.73 0.00092 3.38 150.76 79.25 0.23

Clover‐Low 4.7763 Floodway 396 285.32 292.58 288.71 292.73 0.000919 3.38 150.86 38.91 0.23

Clover‐Low 4.7763 P010yr 178 285.32 290.8 287.49 290.88 0.000597 2.22 93.28 27.02 0.18

Clover‐Low 4.7763 P050yr 316 285.32 292.02 288.29 292.14 0.000821 3.01 130.52 66.84 0.21

Clover‐Low 4.7763 P500yr 644 285.32 293.97 289.79 294.2 0.00114 4.26 220.32 130.09 0.26

Clover‐Low 4.6603 P100yr 396 286.62 291.61 291.86 0.002372 4.22 107.66 33.36 0.36

Clover‐Low 4.6603 Floodway 396 286.62 291.61 291.87 0.002362 4.21 107.84 33.37 0.35

Clover‐Low 4.6603 P010yr 178 286.62 290.09 290.24 0.002204 3.06 62.17 26.84 0.32

Clover‐Low 4.6603 P050yr 316 286.62 291.13 291.34 0.002311 3.85 92.13 31.32 0.34

Clover‐Low 4.6603 P500yr 644 286.62 292.83 293.19 0.002502 5.1 157.42 47.68 0.38

Clover‐Low 4.54 P100yr 396 285.21 288.9 287.97 289.43 0.007052 5.97 71.9 28.11 0.58

Clover‐Low 4.54 Floodway 396 285.21 288.89 287.97 289.43 0.007082 5.97 71.79 28.09 0.58

Clover‐Low 4.54 P010yr 178 285.21 287.76 286.93 288.03 0.006122 4.21 43.17 22.48 0.5

Clover‐Low 4.54 P050yr 316 285.21 288.52 287.62 288.97 0.006859 5.43 61.73 26.25 0.56

Clover‐Low 4.54 P500yr 644 285.21 289.92 288.89 290.65 0.007046 7.13 103.15 33.83 0.6

Clover‐Low 4.5342 P100yr 396 285.2 288.85 287.68 289.2 0.004845 4.77 84.86 30.08 0.48

Clover‐Low 4.5342 Floodway 396 285.2 288.84 287.68 289.19 0.00487 4.77 84.72 30.07 0.48

Clover‐Low 4.5342 P010yr 178 285.2 287.64 286.82 287.84 0.005215 3.58 50.08 27.57 0.46

Clover‐Low 4.5342 P050yr 316 285.2 288.45 287.39 288.75 0.004954 4.39 73.26 29.4 0.48

Clover‐Low 4.5342 P500yr 644 285.2 289.91 288.42 290.41 0.004657 5.7 115.96 31.91 0.5

Clover‐Low 4.527 Bridge

Clover‐Low 4.5196 P100yr 396 284.48 288.51 287.23 288.83 0.004447 4.58 87.7 33.3 0.46

Clover‐Low 4.5196 Floodway 396 284.48 288.5 287.23 288.82 0.004482 4.59 87.49 33.26 0.46

Clover‐Low 4.5196 P010yr 178 284.48 287.26 286.26 287.44 0.004836 3.45 51.59 26.96 0.44

Clover‐Low 4.5196 P050yr 316 284.48 288.1 286.91 288.37 0.004561 4.22 75.64 31.05 0.45

Clover‐Low 4.5196 P500yr 644 284.48 289.59 287.98 290.05 0.004286 5.48 119.54 38.05 0.47

Clover‐Low 4.5042 P100yr 396 283.98 286.92 286.92 288.06 0.022026 8.6 47.6 22.97 0.97

Clover‐Low 4.5042 Floodway 396 283.98 286.94 286.94 288.06 0.021397 8.52 48.06 22.86 0.96

Clover‐Low 4.5042 P010yr 178 283.98 285.89 285.89 286.61 0.028 6.78 26.26 18.51 1

Clover‐Low 4.5042 P050yr 316 283.98 286.57 286.57 287.58 0.023886 8.07 39.82 21.46 0.99

Clover‐Low 4.5042 P500yr 644 283.98 287.83 287.83 289.31 0.019098 9.91 70.05 26.06 0.96

Clover‐Low 4.412 P100yr 501 278.02 286.51 281.24 286.51 0.000039 0.55 2173.59 1580.83 0.04

Clover‐Low 4.412 Floodway 501 278.02 287.42 281.25 287.43 0.000133 1.09 714.72 130 0.07

Clover‐Low 4.412 P010yr 291 278.02 284.61 280.53 284.62 0.000181 0.97 775.32 1393.6 0.07

Clover‐Low 4.412 P050yr 434 278.02 285.84 281.04 285.84 0.000062 0.65 1666.87 1526.67 0.04

Clover‐Low 4.412 P500yr 677 278.02 288.68 281.8 288.68 0.000013 0.37 3905.27 1920.03 0.02

Clover‐Low 4.2908 P100yr 501 277.52 286.48 281.48 286.48 0.000027 0.46 3045.02 2126.9 0.03

Clover‐Low 4.2908 Floodway 501 277.52 287.28 281.48 287.3 0.00028 1.58 535.12 100 0.1

Clover‐Low 4.2908 P010yr 291 277.52 284.46 280.62 284.47 0.000254 1.15 850.42 1657.09 0.08

Clover‐Low 4.2908 P050yr 434 277.52 285.8 281.23 285.8 0.000049 0.58 2285.25 2087.91 0.04

Clover‐Low 4.2908 P500yr 677 277.52 288.67 282.63 288.67 0.000008 0.29 5494.88 2814.63 0.02

Clover‐Low 4.1278 P100yr 501 278.21 286.45 282.1 286.46 0.000054 0.65 1788.84 2021.14 0.04

Clover‐Low 4.1278 Floodway 501 278.21 286.9 282.01 286.97 0.000625 2.3 291.59 45 0.15

Clover‐Low 4.1278 P010yr 291 278.21 284.28 281.03 284.29 0.000218 1.02 729.16 1000.97 0.08

Clover‐Low 4.1278 P050yr 434 278.21 285.75 281.97 285.75 0.000079 0.73 1432.47 1257.98 0.05

Clover‐Low 4.1278 P500yr 677 278.21 288.66 282.42 288.66 0.000022 0.49 2952.13 2953.94 0.03

Clover‐Low 4.0063 P100yr 501 278.51 286.44 281.58 286.44 0.000018 0.37 2579.23 1001.58 0.03

Clover‐Low 4.0063 Floodway 501 278.51 286.54 281.58 286.6 0.000581 2.15 266.57 42 0.14

Clover‐Low 4.0063 P010yr 291 278.51 284.21 280.87 284.22 0.0001 0.68 969.59 814.43 0.06

Clover‐Low 4.0063 P050yr 434 278.51 285.73 281.37 285.73 0.000027 0.43 2051.41 924.31 0.03

Clover‐Low 4.0063 P500yr 677 278.51 288.66 282.13 288.66 0.000005 0.23 5366.1 1084.92 0.01

Clover‐Low 3.9318 P100yr 501 276.81 286.44 278.75 286.44 0.000016 0.42 2253.95 869.84 0.02

Clover‐Low 3.9318 Floodway 501 276.81 286.46 278.75 286.49 0.000154 1.28 400.72 45 0.07

Clover‐Low 3.9318 P010yr 291 276.81 284.2 278.18 284.2 0.000039 0.53 1073.1 585.99 0.04

Clover‐Low 3.9318 P050yr 434 276.81 285.72 278.58 285.73 0.000021 0.45 1852.35 658.58 0.03

Clover‐Low 3.9318 P500yr 677 276.81 288.65 279.15 288.65 0.000008 0.33 3523.97 1004.81 0.02

Clover‐Low 3.8935 P100yr 501 276.66 286.43 278.6 286.43 0.000032 0.58 1466.98 906.62 0.03

Clover‐Low 3.8935 Floodway 501 276.66 286.43 278.6 286.46 0.000132 1.18 454.71 55 0.07

Clover‐Low 3.8935 P010yr 291 276.66 284.18 278.03 284.19 0.000066 0.69 715.91 471.52 0.05

Clover‐Low 3.8935 P050yr 434 276.66 285.72 278.42 285.72 0.000039 0.61 1228.36 796.61 0.04

Clover‐Low 3.8935 P500yr 677 276.66 288.65 279.02 288.65 0.000017 0.49 2209.81 1046.06 0.03

Clover‐Low 3.8576 P100yr 501 278.06 286.4 280.44 286.42 0.000129 1.06 561.17 915.98 0.07

Clover‐Low 3.8576 Floodway 501 278.06 286.41 280.44 286.43 0.000133 1.08 560.54 98.69 0.07

Clover‐Low 3.8576 P010yr 291 278.06 284.16 279.96 284.17 0.000172 0.96 344.31 375.79 0.07

Clover‐Low 3.8576 P050yr 434 278.06 285.69 280.29 285.7 0.000144 1.05 489.75 795.3 0.07

Clover‐Low 3.8576 P500yr 677 278.06 288.63 280.78 288.64 0.000086 1.03 783.61 969.88 0.06

Clover‐Low 3.849 Culvert

Clover‐Low 3.8402 P100yr 501 277.8 285.97 280.39 286.01 0.00016 1.64 355.6 927.16 0.11

Clover‐Low 3.8402 Floodway 501 277.8 285.98 280.39 286.02 0.000166 1.68 332.11 55.76 0.11

Clover‐Low 3.8402 P010yr 291 277.8 283.93 279.79 283.96 0.000175 1.37 230 56.23 0.11

Clover‐Low 3.8402 P050yr 434 277.8 285.35 280.21 285.38 0.000166 1.57 314.86 667.24 0.11

Clover‐Low 3.8402 P500yr 677 277.8 287.82 280.83 287.86 0.000126 1.7 488.78 1012.74 0.1

Clover‐Low 3.7929 P100yr 501 276.74 285.86 280.57 285.94 0.000375 2.17 234.7 39.5 0.15

Clover‐Low 3.7929 Floodway 501 276.74 285.88 280.57 285.95 0.000373 2.17 235.05 38.83 0.15

Clover‐Low 3.7929 P010yr 291 276.74 283.84 279.78 283.89 0.000388 1.8 161.56 32.83 0.14

Clover‐Low 3.7929 P050yr 434 276.74 285.25 280.36 285.31 0.00039 2.08 210.88 37.62 0.15

Clover‐Low 3.7929 P500yr 677 276.74 287.73 281.05 287.81 0.000297 2.28 313.61 358.18 0.14

Clover‐Low 3.7489 P100yr 501 277.58 285.73 281.03 285.83 0.00049 2.65 211.28 42.58 0.18

Clover‐Low 3.7489 Floodway 501 277.58 285.74 281.03 285.85 0.000486 2.65 211.84 42.47 0.18

Clover‐Low 3.7489 P010yr 291 277.58 283.7 280.13 283.78 0.000576 2.26 133.5 34.22 0.18

Clover‐Low 3.7489 P050yr 434 277.58 285.11 280.76 285.21 0.000518 2.56 185.6 40.01 0.18

Clover‐Low 3.7489 P500yr 677 277.58 287.62 281.64 287.73 0.000367 2.71 297.79 81.5 0.16

Clover‐Low 3.6992 P100yr 501 277.23 285.66 279.91 285.73 0.000273 2.09 255.15 42.4 0.14

Clover‐Low 3.6992 Floodway 501 277.23 285.68 279.91 285.74 0.000271 2.09 255.73 42.32 0.14

Clover‐Low 3.6992 P010yr 291 277.23 283.63 279.14 283.67 0.000276 1.7 174.77 36.57 0.13

Clover‐Low 3.6992 P050yr 434 277.23 285.04 279.68 285.1 0.000277 1.99 229.2 40.61 0.13

Clover‐Low 3.6992 P500yr 677 277.23 287.57 280.45 287.64 0.000224 2.21 341.26 325 0.13

Clover‐Low 3.662 P100yr 501 276.97 285.56 280.57 285.66 0.000459 2.53 210.6 133.95 0.17

Clover‐Low 3.662 Floodway 501 276.97 285.57 280.55 285.67 0.000455 2.53 211.17 39.22 0.17

Clover‐Low 3.662 P010yr 291 276.97 283.52 279.63 283.6 0.000533 2.14 137.43 32.58 0.18

Clover‐Low 3.662 P050yr 434 276.97 284.93 280.3 285.02 0.000483 2.44 186.62 40.78 0.17

Clover‐Low 3.662 P500yr 677 276.97 287.48 281.2 287.59 0.000346 2.6 291.46 659.83 0.16

Clover‐Low 3.6291 P100yr 501 276.73 285.52 280.2 285.58 0.0003 2.11 260.25 173.32 0.14

Clover‐Low 3.6291 Floodway 501 276.73 285.53 280.2 285.6 0.000298 2.11 260.97 48.46 0.14

Clover‐Low 3.6291 P010yr 291 276.73 283.46 279.32 283.51 0.00035 1.8 166.59 41.31 0.15

Clover‐Low 3.6291 P050yr 434 276.73 284.89 279.94 284.95 0.000317 2.04 230.04 100.76 0.14

Clover‐Low 3.6291 P500yr 677 276.73 287.46 280.78 287.53 0.000228 2.18 359.17 714.7 0.13

Clover‐Low 3.5948 P100yr 501 276.49 285.43 280.69 285.52 0.000428 2.42 233.18 50.9 0.17

Clover‐Low 3.5948 Floodway 501 276.49 285.45 280.69 285.53 0.000425 2.41 233.97 50.79 0.17

Clover‐Low 3.5948 P010yr 291 276.49 283.36 279.77 283.43 0.00056 2.13 140.34 38.4 0.18

Clover‐Low 3.5948 P050yr 434 276.49 284.79 280.44 284.88 0.000466 2.35 201.84 47.71 0.17

Clover‐Low 3.5948 P500yr 677 276.49 287.4 281.31 287.48 0.000296 2.4 341.45 204.78 0.15

Clover‐Low 3.5486 P100yr 501 276.17 285.32 280.16 285.41 0.000418 2.44 222.08 97.96 0.17

Clover‐Low 3.5486 Floodway 501 276.17 285.34 280.16 285.43 0.000415 2.43 222.77 41.85 0.16

Clover‐Low 3.5486 P010yr 291 276.17 283.24 279.2 283.3 0.000501 2.09 141.78 34.23 0.17

Clover‐Low 3.5486 P050yr 434 276.17 284.68 279.89 284.77 0.000445 2.35 195.66 40.29 0.17

Clover‐Low 3.5486 P500yr 677 276.17 287.31 280.81 287.41 0.000307 2.48 311.09 455.66 0.15

Clover‐Low 3.5127 P100yr 501 275.92 285.21 280.43 285.32 0.000549 2.72 202.1 42.11 0.19

Clover‐Low 3.5127 Floodway 501 275.92 285.23 280.43 285.34 0.000544 2.71 202.83 42.06 0.19

Clover‐Low 3.5127 P010yr 291 275.92 283.1 279.42 283.19 0.000698 2.37 124.65 31.76 0.2

Clover‐Low 3.5127 P050yr 434 275.92 284.56 280.14 284.67 0.000593 2.64 175.72 39.14 0.19

Clover‐Low 3.5127 P500yr 677 275.92 287.23 281.1 287.34 0.000383 2.72 296.53 322.22 0.16

Clover‐Low 3.4796 P100yr 501 275.69 285.1 280.31 285.22 0.000563 2.97 206.99 42.19 0.19

Clover‐Low 3.4796 Floodway 501 275.69 285.11 280.31 285.24 0.000558 2.96 207.77 42.23 0.19

Clover‐Low 3.4796 P010yr 291 275.69 282.97 279.32 283.07 0.000655 2.55 126.66 33.17 0.2

Clover‐Low 3.4796 P050yr 434 275.69 284.44 280.04 284.56 0.000596 2.87 180.21 39.59 0.2

Clover‐Low 3.4796 P500yr 677 275.69 287.15 280.94 287.27 0.000408 2.98 301.93 233.74 0.17

Clover‐Low 3.4447 P100yr 501 275.44 285.02 279.45 285.13 0.00042 2.67 221.5 41.34 0.17

Clover‐Low 3.4447 Floodway 501 275.44 285.04 279.45 285.15 0.000416 2.67 222.3 41.39 0.17

Clover‐Low 3.4447 P010yr 291 275.44 282.89 278.43 282.97 0.000433 2.21 141.95 32.65 0.16

Clover‐Low 3.4447 P050yr 434 275.44 284.36 279.16 284.46 0.000432 2.56 195.02 39.16 0.17

Clover‐Low 3.4447 P500yr 677 275.44 287.09 280.12 287.2 0.000325 2.74 313.85…