CA ERFO NP DEVA 2022-1(2) Final Report_Signed.pdf

Salt Creek Interpretive Trail Final Hydraulics Report

CA ERFO NP DEVA 2022-1(2)

In partnership with:

Death Valley National Park

CA

Federal Highway Administration Central Federal Lands Highway Division

July 16, 2024

2 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Signature Sheet

Report prepared by: ____________________________________________________

Amanda Peters, Hydraulic Engineer

Report reviewed by: ____________________________________________________

Luis Calderón, Lead Hydraulic Engineer

Approved for distribution by: ____________________________________________________

Luis Calderón, Lead Hydraulic Engineer

Distribution

Electronic:

N:\CA\erfonp2022-1(2)\Hydraulics\9_Final-Report Nate Allen, Project Management Jeff Felling, Project Development, Lead Designer Henry Castillo, Project Development, Designer Casey Balthrop, Environmental Protection Specialist Devin Dixon, Geotechnical Karl Eikermann, Bridge Steve Belcher, Bridge

3 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Executive Summary

Hydrologic and hydraulic analyses were conducted for the CA ERFO DEVA 2022-1(2), Salt Creek

Interpretive Trail project located in Death Valley, CA. The scope of this project is to evaluate the alignment and rebuild the interpretive trail (boardwalk) that was destroyed on August 5, 2022 due to flash flooding.

In addition to the 2022 storm event, this same area saw significant precipitation and associated sediment movement in August 2023 from Tropical Storm Hilary. The design flows used for analysis on this project were computed using a flood frequency analysis based on a gage located in the project vicinity. Hydraulic performance was evaluated using the two-dimensional hydraulic modeling software SRH-2D. The following report summarizes proposed hydraulic design criteria, computational methods, and final hydraulics recommendations.

In this report, hydraulic conditions for various return periods are reported and compared to the proposed boardwalk elevation. Due to the nature of this project (pedestrian trail), hydraulic design criteria will not follow PDDM standards for bridges. Figure 1 below provides a visual of the final proposed boardwalk alignment. The boardwalk profile was set to be able to pass the 25-yr storm event without pressure flow.

For scour evaluation, the 50-yr storm was designated as the design event and the 100-yr as the check event. A sediment transport model was created to evaluate global sediment movement throughout the system (aggradation/degradation), and pier scour equations were used to determine local scour at the helical pile locations. The boardwalk was split into different zones of scour to identify locations of similar expected scour conditions. Table 1 below provides scour depths for each scour zone (station ranges for each zone are identified). The scour depth is referenced to the existing ground elevation at each pier location. This table also provides maximum velocity values found in each scour zone for the scour design and check events based on the results of the hydraulic capacity analysis.

In addition to the boardwalk scour analysis, scour was calculated for the embankment protection being placed along the 4R section of roadway, portions of the parking lot, and the location where proposed sidewalk ties into the boardwalk. The scour depth for the embankment protection along the 4R section of roadway was determined to be 3 ft. A minimum 3 ft of embedment is recommended to be applied along the length of the embankment protection, which is recommended at a minimum from 56+60-62+50 LT

(along Salt Creek). Along a portion of the roundabout section of the parking lot, the recommended embankment protection has a scour depth of 2 ft. Class 3 riprap is recommended to be placed with type c, class 1, non-woven geotextile for all embankment protection. For the sidewalk, a 2 ft deep cutoff wall is recommended where tying into the boardwalk. Additional embankment protection is also recommended from 59+40-62+40 RT to protect the new parking lot from any water that may impact the parking lot from the north side, however, ERFO funding is not available for this addition.

All stationing in this report references the 95% boardwalk alignment.

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Figure 1: Proposed Boardwalk Alignment

Figure 2: Proposed Roadway Alignment

Sta. 128+13.37

Sta. 100+00

Sta. 120+00

Salt Creek

Boardwalk Parking Lot and

Rodway Reconstruction

Roadway Reconditioning to intersection with CA-190

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Table 1: Boardwalk Hydraulic Summary Table

Hydraulic Data

Station Range 50-yr Storm Total

Scour (Ft)

50-yr Maximum

Velocity (fps)

100-yr Storm Total

Scour (Ft)

100-yr Maximum

Velocity (fps)

100+00-104+20 6.0 6.7 7.0 7.9

104+20-104+80 5.5 4.3 7.0 5.4

104+80-113+20 5.5 10.0 7.5 12.3

113+20-120+00 6.0 4.6 7.0 5.8

120+00-121+60 5.5 7.1 7.0 7.9

121+60-128+13 1.0 1.0 1.5 6.5

Note: See Hydraulics Report for additional information on hydraulic capacity and stability analysis.

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Table of Contents

1 Project Background Information

1.1 Project Information

1.2 Former Boardwalk

1.3 Flood Insurance Study

1.4 Aerial Imagery

2 Hydraulic Design Criteria

3 Hydrology

3.1 Hydrologic Setting and Basin Data

3.2 Available Hydrologic Data

3.3 Regression Equation Estimates

3.4 Flood Frequency Analysis

3.4.1 Model Parameters

3.4.2 Results

4 Hydraulic Analysis

4.1 Hydraulic Setting/Site Observations

4.2 Hydraulic Model Development

4.3 Topographic Information

4.4 Boundary Conditions

4.5 Land Cover

4.6 Project Mesh

4.7 Hydraulic Results (Capacity Design)

4.7.1 WSEL Profile Locations

4.7.2 SRH-2D Hydraulic Model Results

4.7.3 SRH-2D Hydraulic Model Results Discussion

4.8 Proposed Roadway Profile Discussion

5 Scour Analysis

5.1 Sediment Transport Model Development

5.2 Pre-Hilary Sediment Transport Comparison

5.3 Sediment Transport Model Results

5.4 Pier Scour Development

5.5 Pier Scour Results

5.6 Total Boardwalk Scour Results

5.7 Roadway Embankment Protection

5.8 Sidewalk Scour

5.9 Parking Lot Embankment Protection

6 Recommendations

6.1 Capacity Design

6.2 Stability Design

7 References

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Tables Table 1: Boardwalk Hydraulic Summary Table

Table 2: PDDM Hydraulic Design Criteria

Table 3: Gage Information and General Flow Statistics

Table 4: HEC-SSP Results (Recommended Design Flows)

Table 5: Boardwalk Pressure Flow Locations

Table 6: Total Scour Results

Figures Figure 1: Proposed Boardwalk Alignment

Figure 2: Proposed Roadway Alignment

Figure 3: Project Location Map

Figure 4: September 2015 Google Earth Aerial Imagery

Figure 5: March 2023 CFLHD Aerial Imagery

Figure 6: October 2023 Google Earth Aerial Imagery

Figure 7: April 2023 CFLHD LiDAR Data

Figure 8: October 2023 CFLHD LiDAR Data

Figure 9: Boundary Conditions

Figure 10: Existing Conditions Land Cover and Manning’s Roughness

Figure 11: Proposed Conditions Land Cover and Manning’s Roughness

Figure 12: Project Mesh Elements (Hydraulic Capacity Analysis)

Figure 13: Project Mesh Elements (Sediment Transport)

Figure 14: Boardwalk Profile WSEL Profile Locations

Figure 15: P1, Various Storm Events

Figure 16: P2, Various Storm Events

Figure 17: P3, Various Storm Events

Figure 18: P4, Various Storm Events

Figure 19: P5, Various Storm Events

Figure 20: P6, Various Storm Events

Figure 21: Proposed Roadway/Parking Lot Topography

Figure 22: Scour Zone Locations

Figure 23: Sediment Coverage

Figure 24: Tropical Storm Hilary Topographic Changes

Figure 25: Pre-Tropical Storm Hilary Sediment Transport (100-yr Storm)

Figure 26: Embankment Protection (Riprap Revetment)

Appendices Appendix A: Background Data

Appendix B: Hydrology

Appendix C: SRH-2D Hydraulic Capacity Results

Appendix D: Scour/Stability

Appendix E: 95% Boardwalk Plan Sheets

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1 PROJECT BACKGROUND INFORMATION

1.1 PROJECT INFORMATION

The Central Federal Lands Highway Division (CFLHD) in cooperation with Death Valley National Park

(DEVA) plan to replace the Salt Creek Interpretive Boardwalk that was destroyed in a flash flood event on

August 5, 2022. Heavy rains impacted this area again in August 2023 due to Tropical Storm Hilary. This boardwalk allows visitors to observe the salt creek pupfish that live in small pools that form in the creek.

The trailhead to the boardwalk is located at 36.59055, -116.99073 in Inyo County, CA. Figure 3 below identifies the location of the project.

The current proposed boardwalk alignment has been modified since the 30% and 50% plans. One reason for this shift is to avoid paleontologically sensitive areas as identified by DEVA in a report dated January

12, 2024. In addition to this consideration, the boardwalk length has been modified so the length is no longer than the existing boardwalk to ensure the ERFO program can fund the proposed boardwalk. The alignment shown in this report is the alignment associated with the 95% submittal. The boardwalk alignment is not anticipated to be modified from the alignment shown in this report.

https://www.google.com/maps/place/36%C2%B035'26.0%22N+116%C2%B059'26.6%22W/@36.5905543,-116.9933049,17z/data=!3m1!4b1!4m4!3m3!8m2!3d36.59055!4d-116.99073?entry=ttu

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Figure 3: Project Location Map

1.2 FORMER BOARDWALK

The former boardwalk had a total length of 2,871 ft and a width of 6 ft. In 1997 the boardwalk was removed and replaced. The former boardwalk was composed of cast in place concrete piers with a treated

Douglas Fir deck. The boardwalk was considered a total loss from the August 2022 floods and a full replacement is being proposed. There are no inspection reports available for the boardwalk.

1.3 FLOOD INSURANCE STUDY

The Federal Emergency Management Agency (FEMA) provides Flood Insurance Rate Maps (FIRMs) that detail special flood hazard areas. Lands inundated by the base flood (100-year) are subject to National

Flood Insurance Program (NFIP) floodplain management regulations. The FIRMs indicate that Zone A, Zone

D, and Zone X encompass different portions of the proposed boardwalk location. Inyo County was contacted to ensure that all local floodplain permitting requirements were met. Inyo County indicated that they do not require any permitting for this project. FIRMs have been included in Appendix A.

1.4 AERIAL IMAGERY

The floods of 2022 and 2023 altered the topography of Salt Creek at the boardwalk location. To show a comparison between the pre- and post-flood landscapes, aerials before and after are shown below.

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Figure 4: September 2015 Google Earth Aerial Imagery

Figure 5: March 2023 CFLHD Aerial Imagery

Figure 6: October 2023 Google Earth Aerial Imagery

Begin Ex. Boardwalk

End Ex. Boardwalk

Begin Pr. Boardwalk

End Pr. Boardwalk

End Pr. Boardwalk

Begin Pr. Boardwalk

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2 HYDRAULIC DESIGN CRITERIA

Due to the uniqueness of this project, the Federal Lands Highway Project Development and Design Manual

(PDDM) (Federal Lands Highway 2012) does not provide specific design criteria to use as a part of this project. For hydraulic capacity, CFLHD recommends the boardwalk be able to pass the Q25 event with no pressure flow along the length of the boardwalk to the greatest extent practicable. Scour design and check flood events were determined from guidance in Table 2.1 of HEC-18 (FHWA 2012). This table provides recommended minimum scour design and check flood frequencies based on the hydraulic design flood frequency.

For the entrance road portion of the project, the roadway is assumed to have an ADT of less than 100 vehicles per day, a speed limit of 30 mph, and is not a critical access road. Therefore, this road is considered a low-standard roadway.

Embankment protection will be placed along a portion of the roadway that runs parallel to the creek close to the parking lot, as well as portions of the parking lot. To ensure the stability of the embankments being placed, a scour analysis was conducted to ensure adequate embedment depths are met. In addition, the parking lot and 4R section of roadway were raised to ensure embankments were not overtopped for the

25-yr storm event. However, to ensure minimal water surface elevation (WSEL) rise as well as minimizing the footprint of the embankments, 2-ft of freeboard is not met at all locations along the proposed parking lot and roadway.

Table 2: PDDM Hydraulic Design Criteria

Design Frequency

Check Frequency

Design Criteria

Capacity Design Recommendation: Pass Q25 without pressure flow

Stability Design 50-yr 100-yr Use the worst case scour up to and including these events

Floodplain Encroachment

(Rodway/Parking Lot)

100-year Overtopping (≤ 500-year)

Unregulated base floodplain: rise ≤ 1.0 ft.

Regulated floodplain with no detailed FEMA study: rise ≤ 1.0 ft.

Longitudinal Embankments

25-year

Greater of 100-year and overtopping

Analyze when encroachment on base floodplain is unavoidable

Minimum 2 ft. of freeboard at the design flood

Embankments stable at the design flood

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3 HYDROLOGY

3.1 HYDROLOGIC SETTING AND BASIN DATA

The project is located in the Salt Creek drainage within DEVA. The contributing drainage basin was estimated using data from the National Watershed Boundary dataset and manual editing using ArcGIS

(ESRI 2019). The Salt Creek watershed headwaters originate in the Cucomungo Mountains and drain south-southeast via a system of intermittent and ephemeral streams that become Salt Creek north of the

SR 190 crossing. Salt Creek continues to flow generally south-southeast through the project site to its outfall at the Badwater Basin. The outlet of this basin is located below sea level by approximately 200 ft.

This basin drains an area of approximately 1500 sqmi. Drainage basin delineation and additional hydrologic information have been included in Appendix B.

This area has experienced two major precipitation events in the last two years. According to data documented by the National Oceanic and Atmospheric Administration’s (NOAA) National Centers for

Environmental Information located at Furnace Creek in DEVA, the August 2022 event produced 1.7 in of rain and the August 2023 event produced 2.2 in of rain in a single day. These values are only for reference and do not represent the entire basin as the precipitation could vary significantly throughout the basin.

The August 2023 rainfall event was caused by the landfall of Hurricane Hilary on Southern California. By the time the storm reached DEVA, the storm classification had been modified to a tropical storm.

3.2 AVAILABLE HYDROLOGIC DATA

One USGS streamgage located in the project vicinity was used to determine the flows seen at the boardwalk location for various return periods. Table 3 below summarizes the station location, period of record and general flow statistics at the gage location (Wolock 2003). Water data reports list the drainage area as indeterminate, likely due to uncertainty in infiltration vs surface flow for portions of the basin. The information from the gage was used to conduct a flood frequency analysis, as will be described in Section

3.4.

Table 3: Gage Information and General Flow Statistics

Station Number 10251100

Station Name SALT C NR STOVEPIPE WELLS CA

Latitude 36.59944°

Longitude -117.01278°

Drainage Area Indeterminate

Period of Record 1974-1988

Total Days of Record 5343

Status Inactive

Maximum Daily Flow 363 cfs

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3.3 REGRESSION EQUATION ESTIMATES

The state of California provides regression equations to estimate peak flows for various return periods

(USGS 2012). The project limits are in hydrologic region 6 (Desert). The equations developed for this region relate peak discharge to drainage area. These equations were developed using basins with drainage areas ranging from 0.04-173 sqmi. The drainage basin draining to the project is much larger than the maximum area used for development in the regression equations. Therefore, the California regression equations were not used to estimate peak flows for this project.

3.4 FLOOD FREQUENCY ANALYSIS

3.4.1 Model Parameters

A flood frequency analysis was conducted to estimate flows associated with various return periods using the peak flow information collected at gage 10251100. A Bulletin 17C analysis was conducted using HEC-

SSP (USACE 2019). The hydrology of the drainage basin contributing flow collected at the boardwalk location is unique due to the effects of groundwater contributing flow to Salt Creek. Because of the complex hydrologic conditions contributing flow to the gage site, no drainage area is reported by USGS as a part of the streamgage data. However, the gage is located just upstream of the boardwalk location. Due to the very close proximity of the gage to the boardwalk, the flows provided at the gage are taken to represent the flows at the project location. No drainage area ratio was completed to transpose the gage data to the project location as the drainage basins are taken to be virtually the same.

In the flood frequency analysis, both station skew and weighted skew were analyzed to best estimate flows. A regional skew of zero with a mean squared error (MSE) value of 0.2 was used in accordance with the findings reported in the document Methods for Determining Magnitude and Frequency of Floods in

California, based on data through water year 2006, Page 39, for the California Desert Region. In an effort to provide conservative estimates of flows at the boardwalk, the results using station skew analysis were used for hydraulic analysis. These values are reported in the next section. Additional information can be found in Appendix B.

During a site visit to the project in January 2023, markers placed by the USGS were found delineating high-water marks from the August 2022 storm event. These were compared to the results of the hydraulic model to help validate the results from the flood frequency analysis. When existing conditions were mapped, the high-water marks from USGS fell between the flood extents of the 200-yr and 500-yr storm events. The results of this mapping can be found in Appendix B. These results validate the results of the

HEC-SSP analysis used to determine design flows on this project.

3.4.2 Results

Table 4 below shows the flood frequency analysis results for the stream gage determined using HEC-SSP to conduct a Bulletin 17C analysis. This information was taken from the station skew analysis. Additional information can be found in Appendix B.

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Table 4: HEC-SSP Results (Recommended Design Flows)

Annual Exceedance Probability (%)

Return Period (Year)

Computed Flow (cfs)

50 2 20

20 5 100

10 10 240

4 25 630

2 50 1,180

1 100 2,110

0.5 200 3,620

0.2 500 7,040

4 HYDRAULIC ANALYSIS

4.1 HYDRAULIC SETTING/SITE OBSERVATIONS

The former Salt Creek boardwalk was built close to ground level to provide visitors a way to see the pupfish in their natural habitat. Salt Creek flows generally east-southeast through the project towards Badwater

Basin. The channel is braided and lies in a very wide floodplain, providing a high potential for lateral migration of the main channel, which is what occurred during both the 2022 and 2023 storm events. There is evidence of bank failures on bends upstream and downstream of the boardwalk. Local scour pools have developed due to the large storm events as well as local aggradation and degradation.

4.2 HYDRAULIC MODEL DEVELOPMENT

Hydraulic conditions were evaluated using the two-dimensional hydraulic modeling software SRH-2D (U.S.

Bureau of Reclamation 2017). Two-dimensional hydraulic simulations were conducted using SRH-2D to determine hydraulic data for the site such as water surface elevations, water depth and flow velocities.

The SRH-2D computational mesh generation and model pre- and post-processing was performed using the SMS 13.2.17 interface (Aquaveo 2023).

4.3 TOPOGRAPHIC INFORMATION

LiDAR data was originally collected by CFLHD in April 2023 and again in October 2023 to capture the effects of Tropical Storm Hilary. Both datasets are shown below; however, the October 2023 LiDAR data was used as the sole source of topographic data for the capacity and stability design of project. Figure 7 and Figure

8 below provide information on both sets of LiDAR data.

In addition to this existing topography, proposed parking lot and roadway elevations were incorporated into the project surface and used to determine proposed conditions, as will be described in section 4.8.

The boardwalk itself was not modeled into the topographic data, but low chord elevations of the boardwalk were utilized to determine hydraulic capacity.

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Figure 7: April 2023 CFLHD LiDAR Data

Figure 8: October 2023 CFLHD LiDAR Data

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4.4 BOUNDARY CONDITIONS

The upstream boundary conditions in the model were specified as Inlet-Q constant discharge using peak flow values. The downstream boundary conditions were specified as Exit-H constant water surface elevation, obtained using Manning’s equation with a slope at the model boundary of 0.009 ft/ft. The hydraulic toolbox (FHWA 2021) was utilized to determine downstream WSEL values used for the boundary conditions. These boundary condition locations were applied to all simulation runs and can be found in Figure 9 below.

For the inlet boundary condition, a sensitivity analysis was conducted to determine the most appropriate distribution at the inlet. The results of conveyance and velocity distributions were compared. The results of the hydraulic model were very similar throughout for both Inlet-Q configurations. In addition, the results of the sediment transport model (which will be discussed later in this report) indicated erosion in the same locations with no meaningful differences in values. For these reasons, the default inlet distribution of conveyance was used for all simulations.

Figure 9: Boundary Conditions

4.5 LAND COVER

SRH-2D uses Manning’s N values to compute friction corresponding to the roughness of the streambed and floodplain. Land use was delineated using aerial imagery and topographic information. Figure 10 provides a breakdown of roughness values used within the project domain. The project is located in a desert environment with minimal vegetation within the reach. The streambed material is generally sandy.

Inlet-Q

Exit-H

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Figure 10: Existing Conditions Land Cover and Manning’s Roughness

Figure 11: Proposed Conditions Land Cover and Manning’s Roughness

4.6 PROJECT MESH

A mesh was created encompassing the entire floodplain width of Salt Creek and the location of the proposed boardwalk. Proposed topography of the roadway and parking area was incorporated into the mesh in conjunction with LiDAR data collected by CFLHD. The mesh extends approximately 1,000 ft

18 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report upstream of the west side of the boardwalk and 750 ft downstream of the east side of the boardwalk.

Figure 12 below provides a view of mesh elements within the vicinity of the proposed boardwalk and roadway/parking area for capacity analysis. To complete the sediment transport analysis, an additional mesh was created with fewer elements to increase the speed of simulation runs while still providing sufficient detail of the topography. This mesh can be seen in Figure 13.

Figure 12: Project Mesh Elements (Hydraulic Capacity Analysis)

Figure 13: Project Mesh Elements (Sediment Transport)

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4.7 HYDRAULIC RESULTS (CAPACITY DESIGN)

The results of the hydraulic model are summarized in the following sections. The existing post Tropical

Storm Hilary topography of this area was used in conjunction with proposed roadway and parking lot elevations to complete the hydraulic capacity analysis of the boardwalk for proposed conditions. These results were compared to the proposed boardwalk profile and the results are shown in this section.

4.7.1 WSEL Profile Locations

WSEL profiles were taken along the length of the boardwalk. Figure 14 below provides locations of WSEL profiles taken along the boardwalk. Appendix C includes additional profiles taken perpendicular to the boardwalk profile.

Figure 14: Boardwalk Profile WSEL Profile Locations

4.7.2 SRH-2D Hydraulic Model Results

The figures presented in this section provide a comparison of the proposed boardwalk elevation to water surface elevations for various storm events. Proposed boardwalk finished and low chord elevations are shown in the figures below. WSEL profile locations are shown above in Figure 14. Additional profile views can be found in Appendix C.

P1 P2

P3

P4

P5

P6

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Figure 15: P1, Various Storm Events

Figure 16: P2, Various Storm Events

Proposed Boardwalk

Low Chord

Existing Streambed

Existing Streambed

Proposed Boardwalk

Finished Elevation

Proposed Boardwalk

Finished Elevation Sta. 100+00

Sta. 104+20

Sta. 104+20

Sta. 104+80

Proposed Boardwalk

Low Chord

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL

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Figure 17: P3, Various Storm Events

Figure 18: P4, Various Storm Events

Existing Streambed

Proposed Boardwalk

Finished Elevation

Proposed Boardwalk

Finished Elevation

Existing Streambed

Sta. 104+80

Sta. 113+20

Sta. 120+00

Sta. 113+20

Proposed Boardwalk

Low Chord

Proposed Boardwalk

Low Chord

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL

Blue: 2-yr WSEL Red: 25-yr WSEL

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Figure 19: P5, Various Storm Events

Figure 20: P6, Various Storm Events

4.7.3 SRH-2D Hydraulic Model Results Discussion

As can be seen in section 4.7.2 above, the proposed boardwalk elevation is able to pass storm events up to and including the 25-yr storm event without pressure flow. This is assuming that existing topography will remain the same as the post-Hilary conditions used for modeling, however, there is potential for lateral migration of the stream as well as aggradation and degradation of the streambed as the result of additional storm events.

Portions of the boardwalk are under pressure flow for the scour design and/or check floods. Table 5 below provides locations where there is pressure flow for the 50- or 100-yr storm events. Information on the depth of pressure flow is included for the 100-yr storm event as this information was needed for structural analysis of the boardwalk.

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL Existing Streambed

Proposed Boardwalk

Finished Elevation

Sta. 121+60

Proposed Boardwalk

Low Chord

Sta. 120+00

Sta. 121+60 Proposed Boardwalk

Finished Elevation Proposed Boardwalk

Low Chord

Sta. 128+13.37 (End of

Boardwalk)

Existing Streambed

Blue: 2-yr WSEL Red: 25-yr WSEL

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Table 5: Boardwalk Pressure Flow Locations

Storm Event

Pressure

Flow

Location

Pressure

Flow Length

(Ft)

Avg. Depth of

Boardwalk

Under Pressure

Flow (ft)

Overtopping?

Max. Velocity for Storm Event

(fps)

50-yr 100+00-

101+40 140 ---- No 6.4

50-yr 102+27-

103+33 106 ---- No 4.7

50-yr 107+30-

109+82 252 ---- No 5.9

50-yr 110+86-

112+72 186 ---- No 9.6

50-yr 113+63-

116+14 251 ---- No 4.6

100-yr 100+00-

104+59 459 0.49 No 7.4

100-yr 106+48-

108+22 174 0.64 No 7.6

100-yr 108+22-

109+52 130 ---- Yes 6.1

100-yr 109+52-

113+64 412 0.75 No 11.8

100-yr 113+64-

116+24 260 ---- Yes 5.8

100-yr 116+24-

116+51 27 0.80 No 3.0

100-yr 127+69-

128+13 44 0.43 No 6.5

4.8 PROPOSED ROADWAY PROFILE DISCUSSION

In addition to the capacity of the boardwalk, the parking lot and road leading down to the parking lot were raised to avoid being inundated by storm events up to and including the 25-yr storm. Portions of the roadway are very close to Salt Creek and increases in roadway grade will cause embankments to spill further into the creek, which is not desirable. To minimize the increase in roadway height while providing additional resiliency, the parking lot and road were raised to be outside of the 25-yr storm event, but 2 ft

24 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report of freeboard is not provided along the entire length of the proposed roadway. The proposed road is a significant improvement from existing conditions and provides additional resiliency with the installation of embankment protection to protect the roadway. Figure 21 below provides the proposed topography of the road and parking lot. Cross sections were taken approximately every 50-ft to compare water surface elevations before and after this change. This information can be found in Appendix C. Water surface elevation rise was ensured to be less than 1 ft for the 100-yr storm event.

Figure 21: Proposed Roadway/Parking Lot Topography

5 SCOUR ANALYSIS

Scour was determined for the boardwalk using two methods, the results of a sediment transport model combined with the results of pier scour at different locations along the boardwalk. The boardwalk was split into different zones of similar expected scour depths. These zones (shown in Figure 22) correspond to the same location for which WSEL profiles were cut to analyze the capacity of the boardwalk, shown in section 4.7.2. A single scour value for the scour design and scour check flood were determined for each zone to ease helical pile design and construction. Scour depths are recommended to be measured from existing ground elevation at each helical pile location.

Proposed Roadway (with

Embankment Protection)

Proposed Parking

Area

Roundabout

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Figure 22: Scour Zone Locations

5.1 SEDIMENT TRANSPORT MODEL DEVELOPMENT

To complete a sediment transport model for this site, information on streambed material needed to be input into SMS, in addition to all the information and coverages utilized in the capacity analysis. During the site visit conducted in January 2023, a streambed sample was collected by CFLHD. The information on this gradation can be found in Appendix A. To create and execute a sediment transport model using SRH-

2D, a sediment material coverage was created. This coverage details the area of the model where sediment transport will occur and the gradation of material that will be transported. The area of sediment transport in the model was limited to the floodplain of Salt Creek. The bedrock outcroppings that were seen in the field, which contain the creek on both sides throughout the boardwalk area, were modeled as zones of no sediment transport. For the area of sediment transport, the thickness of transportable material was chosen to be 32-ft to ensure that the scour seen in the model was less than the sediment thickness layer. Figure 23 below provides a visual of the sediment coverage utilized in the sediment transport model. All other coverages remain the same as those shown in section 4 (capacity analysis).

Each simulation in the sediment transport model was run for 400 hours to allow the model sufficient time to complete the analysis and ensure useable results are produced. Guidance set forth in the Bureau of

Reclamation’s (BOR) SRH-2d User’s Manual: Sediment Transport and Mobile-Bed Modeling document

(Bureau of Reclamation 2020) in conjunction with guidance from Aquaveo (Aquaveo 2023) were followed to provide input parameters. This is a sand-bed system (d50<2 mm). Based on the BOR’s guidance in the document, the Engelund-Hansen equation is the recommended equation for use. This equation is applicable to total load transport in sandy-bed rivers. Multiple sensitivity analyses were conducted varying input parameters within the ranges recommended by the BOR. The results of these analyses can be found in Appendix D. In addition to these sensitivity analyses, time steps were varied from 1- to 5-seconds to

Scour Zone 1 (Sta.

100+00 -104+20)

Scour Zone 5 (Sta.

120+00-121+60) Scour Zone 4 (Sta.

113+20-120+00)

Scour Zone 3 (Sta.

104+80-113+20)

Scour Zone 2 (Sta.

104+20-104+80)

Scour Zone 6 (Sta.

121+60-128+13)

26 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report determine how this would affect sediment transport results. There was not a meaningful difference in results from varying the time step. For the results in this report, a time step of 5 seconds was used.

Figure 23: Sediment Coverage

5.2 PRE-HILARY SEDIMENT TRANSPORT COMPARISON

The new topography (post-tropical storm) was compared against the results of the sediment transport model created for pre-tropical storm conditions. The results of sediment transport using the topographic surface associated with the pre-tropical storm conditions. The magnitude of results was not compared as the storm events modeled do not necessarily correlate with the storm event caused by Tropical Storm

Hilary. Rather, locations of expected erosion were compared.

In Figure 24 below, positive values indicate that the post-tropical storm elevation is lower than the pre-tropical storm conditions (erosion occurred). Some of these major bed elevation changes are likely attributed to the propagation of a headcut (shown in the image below) upstream that was observed during the scoping site visit. As these sediment transport models are depth-averaged, local scour is difficult to accurately predict. Therefore, this area was not well predicted by the sediment transport model. However, other portions of the sediment transport model were compared to the difference in surveyed topographic surfaces. These comparisons showed that the sediment transport model was predicting degradation in the areas where sediment movement was observed between the two storm events. In Figure 25 below, the results of sediment transport modeling prior to Tropical Storm Hilary are shown. This model produced greater degradation results than what was observed when comparing the two topographic surfaces from the 2022 and 2023 floods. However, areas of degradation/aggradation shown in the model represent areas of sediment movement that reasonably could occur and did occur from 2022 to 2023. The sediment transport model appears to provide conservative results of degradation.

This comparison provides confidence that input parameters for sediment transport modeling produce useable results to predict degradation.

27 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Figure 24: Tropical Storm Hilary Topographic Changes

Figure 25: Pre-Tropical Storm Hilary Sediment Transport (100-yr Storm)

Former Headcut

Location

Current Headcut

Location

28 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

5.3 SEDIMENT TRANSPORT MODEL RESULTS

Results of the sediment transport model can be found in Appendix D. Note, all information presented is for post-Hilary conditions. The expected sediment transport varies along the length of the boardwalk.

Within each scour zone (show in Figure 22 above), multiple values were taken to determine average and maximum degradation values. Areas of aggradation were not used in determining maximum scour values knowing that the channel could migrate, and zones predicated as aggrading may degrade in the future, and using degradation values provides conservative scour estimates. Table 6 below provides the adopted values for degradation.

5.4 PIER SCOUR DEVELOPMENT

To determine total scour, local scour at helical pile locations needs to be considered. The results of the sediment transport model only provide data on the global movement of sediment throughout the system but does not investigate the effects of obstructions within the floodplain (i.e. helical piles). To complete this analysis, the HEC-18 pier scour equation was utilized (equation 7.1) (FHWA 2012). The results of the hydraulic capacity analysis for the 50- and 100-yr storm events were used to extract hydraulic information (velocities and water depths) for use in the pier scour equation. The information required for the pier scour equation needs to be taken from upstream of the pier. Individual helical piles were not modeled, but the hydraulic information taken from the capacity analysis was taken to provide reasonable estimates of velocity and water depth along the boardwalk with piles installed. The column center to column center spacing of the helical piles is 4.67 ft along the width of the boardwalk and 10 ft along the length of the boardwalk. The width of the helical piles is 2.875 in, 5.5 in, or 7 in depending on the location. Portions of the boardwalk that cross the main channel of Salt Creek will be equipped with the larger helical piles. For scour zones 1, 2, 3, and 4 a pile diameter of 5.5 in was used, for zone 5 a 7-in diameter was used, and for zone 6 a 2.875-in pile diameter was used. Per HEC-18 section 7.6, if multiple circular columns are spaced 5 diameters or greater apart, scour depths should be limited to 1.2 times the local scour of a single column.

The spacing in both directions is greater than 5 diameters apart for both helical pile sizes. Therefore, the factor of 1.2 was applied to the scour depth of a single column for all pier scour calculations for the 50-yr storm event.

Debris was also considered in the estimation of pier scour. It is possible that the area between piles could become filled with debris which could increase the effects of local scour. To account for this, the two piles associated with each pier cap were modeled as a single wall pier, assuming that the 4.67 ft distance between the piers would be filled with debris. This wall pier scenario is highly sensitive to the angle of attack at which water is hitting the pier. Knowing that the channel could migrate in the future, a 15-degree angle of attack was applied to all scour zones along the boardwalk, assuming that the entire width between the helical piles would be filled with debris. A round nose pier width equal to the distance between piles, was used, which equates to a wall pier width of 4.67 ft. This information was used to estimate pier scour for the scour check flood (100-yr storm) to ensure that the boardwalk would be designed to withstand scour associated with debris loads.

All input values used to determine pier scour are based on the topographic data available post-Tropical Storm Hilary. Future storm events could change topographic and hydraulic conditions at the boardwalk location. Additional information on pier scour can be found in Appendix D.

29 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

5.5 PIER SCOUR RESULTS

The results of the pier scour analysis provide local scour results in each of the scour zones. Average values were taken by analyzing many locations within each zone. These locations and a full list of results can be found in Appendix D.

5.6 TOTAL BOARDWALK SCOUR RESULTS

Total scour results combine both degradation (erosion) throughout the system and local (pier) scour.

Contraction scour was not considered as the channel is not being constrained by the implementation of the boardwalk. The results of the total scour analysis produced a value for both the scour design and check floods in each scour zone for which all helical piles in that zone should be designed. This value was determined by adding the average of sediment transport and pier scour results. This value was then rounded up to the nearest 0.5 ft. At each helical pile location, scour elevation is the total scour depth subtracted from the ambient bed elevation at that location. A minimum scour depth of 1 ft was assumed for all scour zones. Table 6 below provides a summary of total scour for each zone.

Table 6: Total Scour Results

Zone

Scour Design Flood (50-yr Storm) Scour Check Flood (100-yr Storm)

Degradation (Sediment

Transport) (Ft)

Local Scour

(Pier Scour)

(Ft)

Total

Scour

(Ft)

Degradation (Sediment

Transport) (Ft)

Local Scour

(Pier Scour)

(Ft)

Total

Scour

(Ft)

1 4.2 1.5 6.0 3.9 3.1 7.0

2 3.5 1.9 5.5 3.6 3.4 7.0

3 3.8 1.6 5.5 4.0 3.5 7.5

4 3.6 2.1 6.0 3.6 3.2 7.0

5 3.2 1.9 5.5 2.7 4.0 7.0

6 0.0 0.0 1.0 0.0 1.2 1.5

5.7 ROADWAY EMBANKMENT PROTECTION

In addition to determining scour along the boardwalk, a stability analysis was conducted along the portion of the parking lot and roadway that interacts with Salt Creek. This is recommended from sta. 56+60-62+50, LT. To protect the roadway, embankment protection will be placed with a specified embedment depth to protect against scour. Cross sections were taken along the location of the embankment protection and hydraulic data was extracted from the results of the SRH-2D model results. Multiple methodologies were utilized to determine the scour depth of the embankment protection. One equation used for this analysis is equation 4.1 from HEC-23, Volume 2 (FHWA 2009). The other two equations used were taken from the

HEC-RAS user manual that specifies bend scour equations, as the proposed roadway is located in a stream bend. The two equations utilized are the Maynord and Zeller equations (USACE 2024). The Maynord equation provided the most conservative scour results. A value of 3 ft has been adopted for the scour depth along 4R section of roadway with embankment protection.

30 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

The size of riprap needed for embankment protection was also analyzed. Hydraulic data was taken from

SMS and used in the hydraulic toolbox. A sensitivity analysis was conducted varying values for the curvature of the channel bend. A radius value of 100-ft was chosen to be used. Current conditions show a radius larger then 100-ft, however since the channel may migrate as a result of future storm events, a conservative radius value was chosen. Class 3 riprap is recommended for all applications for ease of construction.

In addition to the location on the LT side of the embankment adjacent to Salt Creek, a section of the RT side of the roadway is recommended to be protected. From ~sta. 59+40-60+40 RT, there is a depression created at the outlet of a small drainage. To protect this section of embankment during a storm event, riprap embankment protection (using the same configuration as that along Salt Creek) is recommended to be placed. The embankment protection along the LT side of the embankment (along Salt Creek) is recommended to be prioritized over the embankment protection on the RT side of the road. This work will not be completed under the ERFO program, but it is recommended this be placed at a later date.

Additional information on embankment protection can be found in Appendix D.

Figure 26: Embankment Protection (Riprap Revetment)

5.8 SIDEWALK SCOUR

In addition to the scour analysis conducted for the boardwalk and roadway, a scour analysis was conducted along the proposed sidewalk where the boardwalk ties in. A concrete cutoff wall is recommended to be installed along a portion of the proposed sidewalk to protect against scour. To complete this analysis, equation 4.3 from HEC-23, volume 1 was used to determine scour depths.

Hydraulic data was extracted from SMS to determine a scour depth for the 25-year event. Flow was assumed to be parallel to the sidewalk. Scour was found to be largest at the interface of the boardwalk and sidewalk, and to protect this section, scour depth was determined at this location and applied around the proposed section of sidewalk. The cutoff wall is recommended to be 2-ft deep. Additional information can be found in Appendix D.

2-ft

3-ft (Roadway)

2-ft (Parking Lot)

Extend riprap to top of slope

Cover Riprap with

Embankment Material

31 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

5.9 PARKING LOT EMBANKMENT PROTECTION

Along a portion of the proposed parking lot, the area is inundated by the 25-year storm event and embankment protection is recommended to ensure the stability of the lot. The location of proposed armoring can be found in Appendix D. Class 3 riprap is recommended to match the embankment protection being placed on the 4R section of roadway. Equation 4.3 from HEC-23, volume 1 was used to determine scour depths. Hydraulic data was extracted from SMS. Flow was assumed to be parallel to the embankment. The revetment is recommended to be embedded 2 ft for scour protection. The riprap is recommended to tie into the concrete cutoff wall being installed along the sidewalk. Additional information can be found in Appendix D.

6 RECOMMENDATIONS

6.1 CAPACITY DESIGN

The recommended design criterion for hydraulic capacity is to keep the boardwalk out of pressure flow for the 25-yr storm event based on hydraulic results using the most current topographic data. In addition, the roadway and parking lot are designed to be above the 25-yr WSEL. Appendix E includes information from the 95% plan set showing the boardwalk alignment in comparison to the finished boardwalk elevation.

6.2 STABILITY DESIGN

All helical piles are recommended to be designed for scour based on the results of sediment transport modeling and pier scour computations. The scour design flood is the 50-yr storm, and the scour check flood is the 100-yr storm. Scour depths can be found in the Executive Summary for all scour zones.

32 | Page CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

7 REFERENCES

Aquaveo. 2023. SMS: SRH-2D Sediment Transport Parameters. https://www.xmswiki.com/wiki/SMS:SRH-

2D_Sediment_Transport_Parameters.

Aquaveo. 2023. Surface-water Modeling System (SMS), version 13.2.17. Provo, UT.

Bureau of Reclamation. 2020. "SRH-2D User's Manual: Sediment Transport and Mobile-Bed Modeling."

ESRI. 2019. ArcGIS Desktop: Release 10.7.1. Redlands, CA: Environmental Systems Research Institute.

ESRI. 2020. "ArcMAP, version 10.8."

Federal Lands Highway. 2012. "Project Development and Design Manual, Chapter 7 Hydrology and

Hydraulics." https://flh.fhwa.dot.gov/resources/design/pddm/.

FEMA. n.d. "Definitions of FEMA Flood Zone Designations."

https://snmapmod.snco.us/fmm/document/fema-flood-zone-definitions.pdf.

FHWA. 2012. "Evaluating Scour at Bridges, Hydraulic Engineering Circular No. 18, Fifth Edition, FHWA-HIF-

12-003."." Washington, D.C.

FHWA. 2009. "Hydraulic Engineering Circular No. 23."

FHWA. 2021. Hydraulic Toolbox 5.1.1.0.

U.S. Bureau of Reclamation. 2017. SRH-2D, Sedimentation and River Hydraulics – Two-Dimensional model.

Version 3.2.

U.S. Geological Survey. 2018. The National Map. https://viewer.nationalmap.gov/advanced-viewer/.

USACE. 2024. "HEC-RAS User's Manual."

USACE. 2019. "HEC-SSP." June 19.

USGS. 2012. "Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water Year 2006."

Wolock, D.M. 2003. Flow characteristics at U.S. Geological Survey streamgages in the counterminous

United States. Open-File Report 03-146, digital data set, U.S. Geological Survey.

A | Appendix CA ERFO DEVA 2022-1(2) – Draft Hydraulics Report

Appendix A

Background Data

A1 | Background Data CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Pre-Tropical Storm Hilary Photos

Salt Creek, Facing Southeast

Salt Creek, Facing Northwest

A2 | Background Data CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Salt Creek, Facing West

Existing Boardwalk, Facing West

A3 | Background Data CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Existing Boardwalk & Bank Erosion, Facing Southwest

A4 | Background Data CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Post-Tropical Storm Hilary Photos

A5 | Background Data CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

A6 | Background Data CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

FEMA Firmette

A7 | Background Data CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Streambed Gradation

B | Appendix CA ERFO DEVA 2022-1(2) – Draft Hydraulics Report

Appendix B

Hydrology

B1 | Hydrology CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Drainage Basin

Gage 10251100 Data

B2 | Hydrology CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

HEC-SSP Results

USGS 10251100 Salt Creek near Stovepipe Wells Weighted Skew

Percent Chance Exceedance (%) Return Period (yr) Computed Flow (cfs)

Confidence Limits

0.5 0.95

99 1 0 1 0 95 1 1 3 0 90 1 2 5 1 80 1 5 11 2 50 2 21 50 9 20 5 102 321 43 10 10 234 1012 93 4 25 577 3992 202 2 50 1038 10503 325 1 100 1767 26517 489

0.5 200 2885 64610 701

0.2 500 5247 201266 1064

B3 | Hydrology CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

USGS 10251100 Salt Creek near Stovepipe Wells Station Skew Percent Chance Exceedance

(%) Return Period (yr) Computed Flow (cfs) Confidence Limits

0.5 0.95 99 1 0 1 0 95 1 1 3 0 90 1 2 5 1 80 1 5 11 2 50 2 20 50 9 20 5 100 346 41 10 10 240 1293 91 4 25 625 7814 207 2 50 1178 32304 341 1 100 2105 137916 523

0.5 200 3615 433992 762

0.2 500 7039 1957305 1177

B4 | Hydrology CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

High-Water Mark Mapping (Pre-Hilary)

200-yr Storm Event

500-yr Storm Event

USGS High Water Mark Locations

USGS High Water Mark Locations

B5 | Hydrology CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Furnace Creek Precipitation Data

B6 | Hydrology CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

C | Appendix CA ERFO DEVA 2022-1(2) – Draft Hydraulics Report

Appendix C

Capacity Analysis

C1 | SRH-2D Hydraulic Capacity Results CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

Additional WSEL Profile Locations

Note: All cross sections are taken facing downstream

A

U

Existing Ground

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL

Boardwalk Finished Grade

Boardwalk Low Chord

C2 | SRH-2D Hydraulic Capacity Results CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

B

C

D

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL

Existing Ground

Existing Ground

Existing Ground

Boardwalk Finished Grade

Boardwalk Finished Grade

Boardwalk Finished Grade

Boardwalk Low Chord

C3 | SRH-2D Hydraulic Capacity Results CA ERFO DEVA 2022-1(2) – Final Hydraulics Report

E

F

G

Green: 50-yr WSEL Black: 100-yr WSEL

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL

Blue: 2-yr WSEL Red: 25-yr WSEL Green: 50-yr WSEL Black: 100-yr WSEL

Existing Ground

Existing Ground

Existing Ground

Boardwalk Finished Grade

Boardwalk…