CA FTBL DUMONT DUNES (1) Dumont Dunes Road Hydraulics Report.pdf

Final Hydraulics Report Dumont Dunes Road

CA FTBL Dumont Dunes (1) San Bernardino County, California

Federal Highway Administration Central Federal Lands Highway Division

12300 West Dakota Avenue Lakewood, Colorado 80228

Project Location:

Western Extent: 35.6925° / 116.2519° Eastern Extent: 35.6956° / 116.2403°

Prepared by:

Douglas P. Stewart, PE

August 2021

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No. 62468 i

Contents

Acronyms and Abbreviations ......................................................................................................................................... iii

1. Introduction and Purpose

1.1 Project Location

1.2 Project Description

1.3 Previous Studies

1.4 Field Survey

1.5 Floodplain Investigation

1.6 Wetlands Investigation

2. Drainage Design Criteria and Methodology

2.1 General

2.2 Hydrologic Analysis

2.3 Hydraulic Analysis

2.4 Roadside Ditches

2.5 Erosion Control

2.6 Miscellaneous

3. Hydrologic and Hydraulic Analyses

3.1 Existing Low Water Crossing

3.2 Proposed Low-water Crossing

3.3 Culverts

3.4 Ditches

3.5 Riprap Analysis

3.6 Waters of the US Crossing Dumont Dunes Road

4. Final Design

4.1 Low-water Crossing

4.2 Dumont Dunes Road

4.3 Little Dumont Dunes Road

5. Erosion and Sediment Control

5.1 Construction Best Management Practices

5.2 Permanent Best Management Practices

6. References

Appendixes

A FEMA FIRMette B Riprap Sizing C FlowMaster Cross Sections ii

Table

1 USGS StreamStats Peak Flow Events on the Armargosa River at the Low Water Crossing

Figures

1 Vicinity Map 2 USGS StreamStats – Delineated Watershed 3 Existing Low Water Crossing 4 Existing LWC Capacity FlowMaster Analysis 5 Proposed LWC Capacity FlowMaster Analysis 6 Map of WOUS Crossings iii

Acronyms and Abbreviations

BMP best management practice

CFLHD Central Federal Lands Highway Division

FEMA Federal Emergency Management Agency

FIRM Flood Insurance Rate Map ft3/s cubic feet per second

HEC hydraulic engineering circular

LWC low-water crossing

OHV off-highway vehicles

PDDM Project Development and Design Manual

Project Dumont Dunes Road

WOUS waters of the U.S.

1. Introduction and Purpose

This preliminary hydraulics report describes the hydrologic and hydraulic design recommendations for improvements to the low-water crossing (LWC) on Dumont Dunes Road in the Dumont Dunes Off- Highway Vehicles (OHV) Recreation Area. Dumont Dunes Road and Little Dumont Dunes Road are proposed to be paved. There was evidence during the site visit that portions of the road had been previously paved with asphalt pavement, but it has broken down over the years, with mostly gravel visible.

As there are no culverts along either of the roadways, no hydraulic analysis is included in the scope.

Proposed drainage across Dumont Dunes Road will sheet flow generally to the south while drainage across Little Dumont Dunes Road will sheet flow to the north, similar to existing conditions.

The overall purpose of the Dumont Dunes Road project (Project) is to enhance public safety and access to the recreation area by creating a uniform width and paving Dumont Dunes Road.

1.1 Project Location

The OHV Recreation Area is located in the Mohave Desert approximately 33 miles north of Baker, California along State Highway 127. Refer to Figure 1 for a general location of the Project.

1.2 Project Description

The Project is located in a desert environment. Rainfall along the roadways typically percolates into the sandy soils. If there is any excess runoff, it sheet flows across the roadway. There are no formalized ditches along the roadways, but there are windrows created by grading and maintenance. Run-out cuts and berms have been placed periodically through the windrows on the south side of Dumont Dunes Road to direct any runoff collected along the windrows away from the roadway.

The Armargosa River crosses the roadway at an existing concrete LWC. The crossing appears to have been constructed in two adjacent segments: one segment being shorter in length than the other. There appears to be a scour hole about 2 feet deep at the downstream side of the LWC.

Figure 1. Vicinity Map

1.3 Previous Studies

No known previous studies have been identified for this area.

1.4 Field Survey

The field survey was completed by Jacobs staff in November 2020 in the vicinity of the LWC area.

1.5 Floodplain Investigation

There are no Federal Emergency Management Agency (FEMA) -regulated floodplain crossings within the project limits as shown in Effective FEMA Flood Insurance Rate Map (FIRM) Panel 06071C0750H dated August 28, 2008, for San Bernardino County Unincorporated Areas. A FIRMette map for this area can be found in Appendix A.

1.6 Wetlands Investigation

There are no wetlands within the project limits.

2. Drainage Design Criteria and Methodology

2.1 General

The drainage design for the Project will be in accordance with the Central Federal Lands Highway Division’s (CFLHD’s) Project Development and Design Manual (PDDM) Chapter 7 (2017). The scope of this Project includes increasing the flow-over capacity of the LWC to eliminate damage at each end of the crossing caused by more frequent storm events.

2.2 Hydrologic Analysis

StreamStats (USGS n.d.) and the Regional Regression Equations (USGS 2018) was used to calculate flows for 2-, 5-, 10-, 25-, 50-, and 100-year flood events in the Armargosa River. Per project scope, no other hydrologic analyses was performed for the Project.

2.3 Hydraulic Analysis

Recommended hydraulic design standards and criteria can be found in Exhibit 7.1-A of the PDDM (CFLHD 2017). Published information for LWCs recommends that they be designed for a 25-year flood event.

Because of the flat topography of the Armargosa River overbanks, the hydraulic capacity of the proposed LWC will be calculated for its maximum feasible length within the lower conveyance area of the river. No other hydraulic analyses will be performed for the Project.

Because of the difference in elevation between the downstream end of the LWC and the channel of over 2 feet, riprap will be designed using Hydraulic Engineering Circular (HEC) 23 Design Guideline 5 – Riprap Design for Embankment Overtopping (FHWA 1997) to best estimate rock size for the steeper longitudinal slope of the placed riprap along the channel.

2.4 Roadside Ditches

Roadside ditch flows and capacity will not be analyzed as part of this Project.

2.5 Erosion Control

Temporary construction best management practices (BMPs), as shown in the standard CFLHD details, will be used for control of erosion during construction in the vicinity of the LWC and staging areas.

2.6 Miscellaneous

The Project is not within a mapped FEMA Floodplain. Neither a Conditional Letter of Map Revision nor a Letter of Map Revision will be needed. Fish passage will not be accommodated since the Armargosa River is dry throughout most of the year.

As rainwater generally percolates into the area’s surrounding soils, runoff generated from the added pavement surface will mostly sheet flow to the south side of the road and percolate into the adjacent soils.

There is a large expanse of desert sands between most of the roadway and the Armargosa River downstream of the roadway. As there is only a small section of roadway near the Armargosa River thalweg, which is currently paved as a LWC, there would be insignificant adverse impacts of increased cumulative flows to the channel.

3. Hydrologic and Hydraulic Analyses

3.1 Existing Low Water Crossing

The Amargosa River crosses over Dumont Dunes Road at an existing LWC. Its watershed stretches across the Amargosa Desert, terminating in Death Valley into the ground aquifer. StreamStats (USGS n.d.) was used to delineate the basin, which resulted in a drainage area of 810,240 acres (Figure 2). This area can produce flows up to 50,100 cubic feet per second (ft3/s) for the 100-year storm event (Table 1). As the topography is generally flat, a bridge or culvert is not feasible for conveyance of the flow. There is an existing concrete LWC located on Dumont Dunes Road within the project area, which extends across the river bottom for approximately 149 feet along the roadway (Figure3). The original width of the LWC is approximately 24 feet with a newer section added 12 feet upstream.

Figure 2. StreamStats – Delineated Watershed Source: USGS n.d.

N

Figure 3. Existing Low-water Crossing

Table 1. StreamStats Peak-flow Events on the Armargosa River at the Low-water Crossing

Statistic Value Desert (Region 6) Eqn.

2-year peak flood 383 ft3/s 10.3(DRNAREA)0.506

5-year peak flood 2,230 ft3/s 60.0(DRNAREA)0.506

10-year peak flood 5,610 ft3/s 151(DRNAREA)0.506

25-year peak flood 15,000 ft3/s 403(DRNAREA)0.506

50-year peak flood 28,200 ft3/s 760(DRNAREA)0.506

100-year peak flood 50,100 ft3/s 1,350(DRNAREA)0.506

200-year peak flood 84,300 ft3/s 2,270(DRNAREA)0.506

500-year peak flood 15,9000 ft3/s 4,280(DRNAREA)0.506

Peak-flow Statistics Flow Report (100 percent [1,270 square miles] 2012 5113 Region 6 Desert)

Source: USGS n.d.

3.2 Proposed Low-water Crossing

During the initial field visit, the Bureau of Land Management stated that flows in the river frequently exceeded the capacity of the existing LWC during the spring runoff season. It was also observed that scour was occurring at each end and just downstream of the LWC. Based on this information, field observations, and existing concrete conditions, it is recommended to replace the existing LWC with a new LWC that expands farther across the Amargosa River and will convey flows from larger storm events.

Various geometries for the LWC could be created to allow increased flows across its surface, including increasing its depth and/or lengthening the crossing. The capacity of the river approaching the LWC is generally controlled by channel topography and established embankments. Therefore, increasing the low-flow area of the LWC to be wider than the channel conveyance width would not improve hydraulic conditions. Instead it would slow down velocities, allowing sediment to settle out onto the road. As the goal is to expand the crossing to convey more flows, thereby reducing maintenance, it was decided to extend the length of the LWC by following generally the existing roadway vertical alignment, terminating its end points at higher elevations along the roadway than the elevations of the existing LWC end points.

This would increase the capacity of the LWC, reducing maintenance generally required for events exceeding the capacity of the existing LWC.

CFLHD PDDM (2017) criteria do not give specific rainfall event criteria to adhere to for the design of LWCs, so the extents of the LWC were determined by existing roadway topography. Each end of the LWC terminated at an elevation matching that near a localized high point along the road at the west end of the crossing. This design produced a LWC with a 232-foot length, while extending the width of the proposed roadway section to 32 feet to match the roadway. A hydraulic analysis was performed using FlowMaster (Bentley Systems 2009) to model the LWC as an irregular trapezoidal channel based on using Manning’s formula and approximating the existing roadway/LWC geometry. These parameters yielded a proposed LWC that can convey over three times the capacity of the existing LWC, allowing the proposed LWC to accommodate the 5-year rainfall event (Figures 4 and 5).

Figure 4. Existing LWC Capacity FlowMaster Analysis

Figure 5. Proposed LWC Capacity FlowMaster Analysis

3.3 Culverts

No culverts were analyzed for this Project.

3.4 Ditches

No ditches were analyzed for this Project.

3.5 Riprap Analysis

Riprap will be used to fill the scour hole downstream of the proposed LWC and to stabilize the channel. A longitudinal riprap channel slope of 1:20 was used to connect the LWC to the downstream channel in approximately 52 feet. The extents of the riprap along the downstream face of the LWC will be to the locations where it was observed that scour was beginning. Although the proposed LWC only conveys approximately the 5-year flow, it is recommended that the riprap be sized to be stable for a 25-year event.

In HEC-23 Design Guideline 5 – Riprap Design for Embankment Overtopping (FHWA 1997), the following parameters and assumptions were made:

• FlowMaster was used to determine the 4.14-foot, 25-year depth of flow for an irregular section representing the channel cross section at the LWC.

• Unit discharge and overtopping depth in the HEC-23 Design Guideline 5 calculations were determined based on an average top width to obtain the 25-year flow depth of 4.14 feet obtained in FlowMaster.

• Calculated rock size was increased from 9 inches to 12 inches to meet the adequate thickness check in Step 8 of the calculations.

It was determined that riprap with a d50 of 12 inches and a depth of 24 inches would be stable for the 25-year storm event. See calculations in Appendix B.

3.6 Waters of the US Crossing Dumont Dunes Road

Maps prepared by CFLHD indicate that eight other Waters of the U.S. (WOUS) drainageways, in addition to the Armargosa River, cross Dumont Dunes Road. Although they are indicated on a map, they are not readily apparent in the field. There were no visible signs of channels approaching or crossing Dumont Dunes Road at these five locations. Very shallow sand dune undulations were observed, which could serve to create rivulets during high rainfall events, but no identifiable locations where concentrated flow would cross the road were observed.

The WOUS drainageway closest and parallel to Highway 127 was more identifiable as a potential waterway since the construction of the highway added berms on the east side of the road, which could divert and concentrate runoff from larger storm events. The existing Dumont Dunes Road crosses this potential flow path at the same elevation of the surrounding ground, creating a large flat area where water could accumulate and spread out.

The Jacobs and CFLHD environmental teams determined that the WOUS path crossing Dumont Dunes Road closest to Highway 127 was to be further evaluated for impacts to the WOUS. The Dumont Dunes roadway improvements will not significantly impact the other seven identified WOUS crossings, and they will drain similar to existing conditions. See Figure 6 for map of WOUS crossings.

Figure 6. Map of WOUS Crossings

4. Final Design

4.1 Low-water Crossing

The existing LWC will be replaced with a 232-foot-long by 32-foot-wide LWC with a depth of 1.64 feet to convey more flows than the existing LWC and reduce potential maintenance for the higher flows it conveys.

D50=12-inch riprap constructed at a depth of 24 inches will be placed at the downstream end of the crossing to fill the scour hole and stabilize the channel. The riprap will extend approximately 52 feet downstream of the LWC to tie into the existing channel.

4.2 Dumont Dunes Road

The Dumont Dunes Road improvements near its connection to Highway 127 will be designed to not negatively impact runoff in the WOUS crossing drainageway for higher flow events.

Dumont Dunes Road will have a cross slope that drains generally to the south in the same direction as the general overall topography. Runoff will sheet flow across the roadway with no ditches or culverts placed to concentrate flows.

4.3 Little Dumont Dunes Road

No drainage improvements will be made at Little Dumont Dunes Road. As the existing terrain is very flat along this road, the proposed road will be constructed with a crown to shed runoff to each side of the road.

5. Erosion and Sediment Control

5.1 Construction Best Management Practices

Riprap berms and/or gravel bag check dams will be used around the disturbed areas in the vicinity of the LWC to minimize the amount of sediment that could potentially leave the project site. Silt fencing or fiber rolls will be used at the base of disturbed slopes in the staging area to minimize sediment leaving the Project via sheet flow. It is not anticipated that stabilized construction exits or other BMPs along the remaining roadway will be required.

5.2 Permanent Best Management Practices

Riprap will be placed along the downstream end of the LWC to stabilize the channel and protect against scour.

6. References

Bentley Systems, Incorporated. 2009. OpenFlows FlowMaster Hydraulic Calculator Software. Haestad Methods Solution Center. https://www.bentley.com/en/products/product-line/hydraulics-and-hydrology-software/flowmaster.

Central Federal Lands Highway Division (CFLHD). 2017. Project Development and Design Manual.

Federal Highway Association (FHWA). 1997. Bridge Scour and Stream Instability Countermeasures.

HEC-23. Archived. July. https://www.fhwa.dot.gov/engineering/hydraulics/pubs/hec/hec23.pdf.

U.S. Geological Survey (USGS). n.d. StreamStats. Version 4.5. https://streamstats.usgs.gov/ss/.

U.S. Geological Survey (USGS). (2018). Regional Regression Equation Publications by State.

https://water.usgs.gov/osw/programs/nss/pubs.html.

U.S. Department of Transportation Federal Highway Administration (FHWA). 2009. Hydraulic Engineering Circular No. 23, Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance. Third edition, Volume 2. Design Guideline 5. September.

https://www.bentley.com/en/products/product-line/hydraulics-and-hydrology-software/flowmaster https://www.bentley.com/en/products/product-line/hydraulics-and-hydrology-software/flowmaster https://www.fhwa.dot.gov/engineering/hydraulics/pubs/hec/hec23.pdf https://streamstats.usgs.gov/ss/ https://water.usgs.gov/osw/programs/nss/pubs.html

Appendix A FEMA FIRMette

Appendix B Riprap Sizing

Sheet No. 1

Calculcation No. 1

PROJECT: CA FTBL DUMONT DUNES Project No.

CFL Calc By DPS Date 5/13/2021 Rev

SUBJECT: Checked By Date

Hydraulic Engineering Circular No. 23

Federal Highway Adiministration

September 2009

Volume 2

1.0 Determine Unit Discharge

Use

Where:

q = Discharge per unit width of the channel, cfs/ft

Q = Design flow (25-year event), cfs 15,000 cfs from StreamStats Wave = Average channel width, ft

Wave Q25 qf

(ft) (cfs) (cfs/ft)

628.00 15000.00 23.89

2.0 Determine Overtopping Depth

Use

Where:

Q = Total discharge, cfs

L = Embankment Overtopping length, ft

C = Weir flow coefficient, 2.84 for Broad-crested weirs, ft0.5/S

H = Depth above weir, ft

Q25 C L H

(cfs) (ft0.5/S) (ft) (ft)

15,000 2.84 628.00 4.14 L=628' selected to reach H of 4.14 per FlowMaster

CFL CA FTBL DUMONT DUNES

Riprap Size Design For Downstream of Low Water Crossing

Reference:

Rock Size per Design Guideline 5 - Riprap Design for Embankment Overtopping

Broad-Crested Weir Equation

Used 628' average top width to obtain FlowMaster flow depth below in Section 2

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3.0 Compute Rock Size

Use

Where: d50 = Particle size for which 50% is finer by weight, ft

Ku = Riprap sizing equation coefficient, S0.52/ft0.04 qf = Unit discharge, cfs/ft

Cu = Coefficient of uniformity of the riprap, d60/d10

S = Slope of the embankment, ft/ft

Sg = Specific gravity of the riprap α = Slope of the embankment, degrees ϕ = Angle of repose of the riprap, degrees

Ku qf Cu S Sg α ϕ d50 d50

(S0.52/ft0.04) (cfs/ft) (ft/ft) (degrees) (degrees) (ft) (in)

0.525 23.89 2.1 0.05 2.65 2.86 42 0.53 6.38

Choose next largest size class from Table 4.1: d50 (inches) = 12 Use 12" v.s. 9" to meet Step 8 check

4.0 Interstitial Velocity

Use

Where:

Vi = Interstitial velocity, ft/s g = Acceleration due to gravity, 32.2 ft/s2 d50 = Particle size for which 50% is finer by weight, ft

Cu = Coefficient of uniformity of the riprap, d60/d10

S = Slope of the embankment, ft/ft

Vave = Average velocity, ft/s η = Porosity of the rock g d50 Cu S Vi η Vave

(ft/s2) (ft) (ft/ft) (ft/s) (ft/s)

32.2 1.00 2.1 0.05 0.48 0.45 0.21

5.0 Compute Average Flow Depth

Use

Where:

y = Average flow depth, ft qf = Unit discharge at failure, cfs/ft

Vave = Average velocity, ft/s

Vave qf y

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

0.21 23.89 111.29 i.e. Since y > 2d50, all the flow is not contained within the thickness of the riprap layer. Go to Step 6.

Equation 5.1

Equation 5.2

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6.0 Find Allowable Flow Depth Above Riprap

Use

Where: h = Allowable flow depth over riprap, ft

Sg = Specific gravity of the riprap d50 = Particle size for which 50% is finer by weight, ft ϕ = Angle of repose of the riprap, degrees

S = Slope of the embankment, ft/ft

Sg d50 ϕ S h

(ft) (degrees) (ft/ft) (ft)

2.65 1.00 42 0.05 1.84

7.0 Calculate Mannings Roughness and Unit Discharge Over and Through Riprap

Use

Where: n = Manning roughness coefficient d50 = Particle size for which 50% is finer by weight, ft y = Average flow depth, ft (use h)

S = Slope of the embankment, ft/ft q1 = Unit discharge which can flow over riprap, cfs/ft q2 = Required interstitial flow through the riprap, cfs/ft qf = Unit discharge, cfs/ft

Vave = Average velocity, ft/s q2d50 = Unit flow divided by a 2d50 riprap thickness, cfs/ft q4d50 = Unit flow divided by a 4d50 riprap thickness, cfs/ft d50 n y S q1 qf q2 Vave q2d50 q4d50

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

1.00 0.034 1.84 0.05 26.95 23.89 -3.07 0.21 0.43 0.86

8.0 Check

2d50 adequate thickness? YES

4d50 adequate thickness? YES

Equation 5.3 since, q2 < q2d50 since, q2 < q4d50

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632.50

4.14' Q25 = 15,000 cfs

Appendix C FlowMaster Cross Sections