22_-_191210__Ash_Meadows__H_H_Report.pdf

Prepared for U.S. Fish and Wildlife Service Region 9

Division of Engineering Dam, Bridge and Seismic Safety Branch Falls Church, Virginia

DECEMBER 2019

HYDROLOGY AND HAZARD

CLASSIFICATION EVALUATION FOR

THREE INVENTORY DAMS ON

ASH MEADOWS NATIONAL

WILDLIFE REFUGE

Page i

EXECUTIVE SUMMARY

This report was prepared by W. W. Wheeler & Associates, Inc. (Wheeler) for the U.S. Fish & Wildlife Service (FWS), Division of Engineering, Dam, Bridge and Seismic Safety Branch. The purpose of this report is to develop inflow hydrology and evaluate the hazard classification of three Inventory dams owned and operated by the FWS on the Ash Meadows National Wildlife Refuge (Refuge) in southwest Nevada.

The three Inventory dams, summarized in Table No. ES-1, were visited by Wheeler during field inspections that were performed during the week of November 11, 2018. Key hydrologic, hydraulic, and hazard information is provided in Table No. ES-1. Wheeler recommends that Crystal Springs Dam, Horseshoe Dam and Lower Crystal Dam be classified as low hazard-potential dams based on FEMA 333 (FEMA, 2014) guidelines;

hazard classification depth versus velocity curves from ACER TM No. 11 (USBR, 1988) and incremental depth constraints from FEMA 94 (FEMA, 1998).

Table No. ES-1: Hazard Classification and Hydrology Summary

Results Category Crystal Springs

Dam Horseshoe

Dam Lower Crystal

Dam1 100-year Peak Inflow (cfs) 987 54 292 100-year Storm Volume (acre-feet) 400 22 497 100-year Overtopping Depth (feet) 0.0 0.0 0.0 100-year Overtopping Duration (hrs) -NA- -NA- -NA- 100-year Freeboard (feet) 2.7 5.3 0.0 PMF Peak Inflow (cfs) 25,328 1,448 27,462 PMF Storm Volume (acre-feet) 3,443 171 4,224 PMF Overtopping Depth (feet) 1.3 0.0 4.2 PMF Overtopping Duration (hours) 3.3 -NA- 55.3 Recommended Hazard Classification Low Low Low

1 – Flood routing results include spillway releases from Crystal Springs Dam and Horseshoe Dam.

The hazard classifications documented in this report are based on conditions that existed during Wheeler’s November 2018 site inspections, ground topography as represented by a 1/3rd arc-second (10-meter) resolution Digital Elevation Model (DEM) obtained from the United States Geologic Survey (USGS) for the area downstream of the refuge up to the confluence with the Amargosa River (USGS, 2013) and the inclusion of a 2-foot-high road berm at the State Line Road in the hydraulic model. The DEM was not of sufficient resolution to capture the road berm at State Line Road, so the minimum road berm height of 2-foot that was measured during the 2018 field inspection was included in the hydraulic model to provide results that are as representative of the existing field conditions as possible. If additional potential downstream hazards are identified during future Safety Evaluation of Existing Dams (SEED) inspections or if more accurate topographic data becomes available, the hazard evaluations documented in this report should be revisited.

Page ii

HYDROLOGY AND HAZARD CLASSIFICATION REPORT FOR THREE DAMS LOCATED

ON THE ASH MEADOWS NATIONAL WILDLIFE REFUGE

TABLE OF CONTENTS

EXECUTIVE SUMMARY ................................................................................................. i TABLE OF CONTENTS ................................................................................................. ii

INTRODUCTION

1.1 OBJECTIVE

1.2 AUTHORIZATION

1.3 SCOPE OF WORK

1.4 PROJECT PERSONNEL

BACKGROUND

2.1 PROJECT AREA

DATA COLLECTION

INFLOW HYDROLOGY

4.1 DRAINAGE BASIN CHARACTERISTICS

4.2 BASIN SOILS AND INFILTRATION

4.3 PRECIPITATION

4.4 UNIT HYDROGRAPH

4.5 RESERVOIR CAPACITY AND SPILLWAY CURVES

4.6 INFLOW HYDROLOGY SUMMARY

RESERVOIR ROUTING AND DAM BREACH ANALYSIS

5.1 DAM BREACH PARAMETERS

5.2 HAZARD MODELING APPROACH

5.3 HEC-HMS MODEL RUN RESULTS

WATER SURFACE PROFILE MODELS

6.1 WATER SURFACE PROFILE MODEL DEVELOPMENT

6.2 WATER SURFACE PROFILE RESULTS

HAZARD CLASSIFCIATION EVALUATION

7.1 HAZARD CLASSIFICATION GUIDELINES AND APPROACH

7.2 HAZARD CLASSIFICATION RESULTS

RECOMMENDATIONS

REFERENCES

Page iii

LIST OF TABLES

Table No. ES-1 – Hazard Classification and Hydrology Summary Table No. 1 – Summary of Key Inventory Dam Pertinent Data Table No. 2 – Summary of Key Drainage Basin Parameters Table No. 3 – Summary of Soil and Infiltration Parameters Table No. 4 – 100-year Point Rainfall Depths Table No. 5 – Local Storm PMP Rainfall Depths Table No. 6 – General Storm PMP Rainfall Depths Table No. 7 – Summary of Unit Hydrograph Characteristics Table No. 8 – Summary of Inflow Hydrology Table No. 9 – Dam Breach Parameter Summary Table No. 10 – Flood-Induced Breach Triggers and Results where Crystal Springs Dam is Primary Table No. 11 – Flood-Induced Breach Triggers and Results where Horseshoe Dam is Primary Table No. 12 – Flood-Induced Breach Triggers and Results where Lower Crystal Dam is Primary Table No. 13 – HEC-HMS Sunny Day Breach Results Summary Table No. 14 – Summary of Flood Inundation Results at State Line Road

LIST OF FIGURES

Figure No. 1 – Vicinity Map Figure No. 2 – Dams of Interest for Study Figure No. 3 – Hazard Classification Drainage Basin Characteristics Figure No. 4 – HEC-HMS Schematic (No-Breach and Flood-Induced Breach Scenarios) Figure No. 5 – HEC-HMS Schematic (Sunny-day Breach of Crystal Springs Dam) Figure No. 6 – HEC-HMS Schematic (Sunny-day Breach of Lower Crystal Dam) Figure No. 7 – Hydrographs from Lower Crystal Dam (Sunny Day Breach of Indicated Dam) Figure No. 8 – Hydrographs from Lower Crystal Dam (% PMF Flood Routing and Flood-Induced

Breaches of Crystal Springs Dam) Figure No. 9 – Hydrographs from Lower Crystal Dam (% PMF Flood Routing and Flood-Induced

Breaches of Horseshoe Dam) Figure No. 10 – Hydrographs from Lower Crystal Dam (% PMF Flood Routing and Flood-Induced

Breaches of Lower Crystal Dam) Figure No. 11 – Potential Downstream Hazard Figure No. 12 – Areas with Higher Flood Depths at State Line Road Figure No. 13 – Sunny Day Hazard Evaluation (Dam Failure of Crystal Springs Dam with subsequent failure of Lower Crystal Dam) Figure No. 14 – PMF Plus Breach Hazard Evaluation (Dam Failure of Horseshoe Dam at Max WSE and

Crystal Springs Dam and Lower Crystal Dam at 1-foot over the dam crest)

LIST OF APPENDICES

Appendix A – HEC-HMS Input & Output Appendix B – Basin Soils, Precipitation and Unit Hydrograph Calculations

Appendix B1 – Basin Soils Appendix B2 – Precipitation Appendix B3 – Unit Hydrographs

Appendix C – Reservoir Capacity and Spillway Discharge Rating Curves Appendix D – Detailed Breach Parameter Selection Appendix E – Representative Photos of Dams and Potential Hazard Areas Appendix F – Site Maps of Dams

R:\1700\1772\1772.23_2018SEEDProgram\Ash_Meadows\7_Documents\Hazard_Classification\190724_AshMeadows_H&H_Report.docx

INTRODUCTION

1.1 OBJECTIVE

The objective of this report was to evaluate the hazard classifications for three dams located within the Ash Meadows National Wildlife Refuge (Refuge). The three dams are: Crystal Springs Dam, Horseshoe Dam and Lower Crystal Dam. The location of the Refuge and the three dams evaluated in this report are shown on Figure Nos. 1 and 2. As a necessary prerequisite to hazard classification, this report also documents development of the inflow hydrology and reservoir routing simulations for the three dams.

1.2 AUTHORIZATION

This study was performed by W. W. Wheeler and Associates, Inc. (Wheeler) for the FWS Dam, Bridge and Seismic Safety Branch. The work documented in this report was authorized as part of Modification No. 1 to Task Order No. 140F0518F0084 under Contract No. F15PC00157 on April 1, 2019.

1.3 SCOPE OF WORK

Wheeler performed the following work as part of this hydrology and hazard classification evaluation.

1. A dam inventory assessment was performed for several impoundment structures located at the Refuge that included a reconnaissance site visit to collect field measurement, confirm dam structures and downstream features that would impact the hydrology and hazard evaluation. The reconnaissance site visit was completed November 13 – 15, 2018.

A hand level and tape survey was performed during the site visit to measure dimensions of key features of the dams and respective downstream structures. Survey results, specifically, 1-meter resolution Light Detection and Ranging (LIDAR) data (FWS, 2007) was used to recommend that both Lower Crystal Dam and Horseshoe Dam be classified as Inventory dams. Crystal Springs Dam was already classified as an Inventory dam.

2. Individual drainage characteristics were developed for tributary basin areas along with design hydrology for storms including the 100-year average recurrence interval (ARI) flood for Crystal Springs Dam, Horseshoe Dam and Lower Crystal Dam. The Probable Maximum Flood (PMF) was also developed for the three Inventory dams.

3. Reservoir and spillway capacity curves for the three Inventory dams evaluated in this report were developed.

4. Dam breach parameters were developed for the three Inventory dams evaluated in this report.

5. A HEC-HMS watershed model was developed for the three Inventory dams and their contributing basins. This model was then utilized for performing reservoir routing and dam breach analyses.

6. A HEC-RAS two-dimensional water surface profile model of the floodplain downstream of Lower Crystal Dam was developed for assessing inundation and potential hazards for a distance approximately 13 miles downstream of the dam along Carson Slough to its confluence with the Amargosa River.

7. A hazard classification evaluation of the three Inventory dams referenced in this report was made following FEMA 333 (FEMA, 2014) guidelines; hazard classification depth versus velocity curves from ACER TM No. 11 (USBR, 1988) and incremental depth constraints from FEMA 94 (FEMA, 1998).

8. The capacities of the dams to safely convey their respective Inflow Design Floods (IDF) were assessed.

9. This hydrology and hazard classification evaluation report was prepared in order to document the key analysis assumptions, analysis methods and results of the evaluation.

1.4 PROJECT PERSONNEL

The following personnel completed the work documented in this report:

Project Manager Danielle Hannes, P.E.

Project Engineer Todd Lewis, P.E.

Project Engineer Christine Mugele, P.E.

Assistant Engineer Andrea Fasen, P.E.

Technical Reviewer Steve Jamieson, P.E.

BACKGROUND

2.1 PROJECT AREA

The Ash Meadows National Wildlife Refuge encompasses roughly 23,000 acres of the Amargosa Valley of southern Nye County, in southwestern Nevada. The Refuge is located 90 miles west-northwest of Las Vegas and the nearest town is Pahrump, as illustrated on Figure No. 1.

Runoff from a 6.7 square-mile drainage area flows through the three dams of interest and continues downstream 13 miles along the Carson Slough to the southwest before its confluence with the Amargosa River.

Refuge lands in the contributing drainage areas for the three dams range in elevation from 2140 to 21911 feet. Average annual precipitation is approximately 3 inches. Extreme thunderstorms can cause flooding along the drainages in the Refuge. A map of the Refuge is provided on Figure No. 1 and a plan view of the key dams located in the study area is provided on Figure No. 2. Pertinent data associated with the three Inventory dams is summarized in Table No. 1 with additional details included in the SEED Inspection Reports (Wheeler, 2018-1;

Wheeler, 2018-2; and Wheeler, 2018-3). Site maps of each dam are provided in Appendix F.

Table No. 1 – Summary of Key Inventory Dam Pertinent Data

Dam Name Dam Type

Dam Height (feet)

Crest Length (feet)

Dam Crest

Elevation (feet)

Maximum Normal

Pool Elevation

(feet)

Reservoir Storage Maximum

Normal Pool Capacity

(acre-feet)

Dam Crest Capacity

(acre-feet) Crystal Springs Dam

Earth Embankment 30.0 4,350 2186.0 2181.0 1,102.0 1,926.3

Horseshoe Dam

Earth Embankment 9.0 1,385 2158.0 2151.0 9.7 77.5

Lower Crystal Dam

Earth Embankment 6.1 2901 2146.0 2143.0 76.4 296.0

1 – Crest length along main embankment; the crest length along the right embankment is 2005 feet. Refer to Appendix F, Figure F3.

1 Elevations in this report are provided in feet above the North American Vertical Datum of 1988 (NAVD 88).

DATA COLLECTION

Andrea Fasen and Danielle Hannes conducted low hazard potential formal SEED inspections of Crystal Springs Dam, Horseshoe Dam, and Lower Crystal Dam during the week of November 11, 2018 (Wheeler, 2018-1; Wheeler, 2018-2; and Wheeler, 2018-3). Their inspections included confirming pertinent data for each dam and using a hand level and cloth tape to verify the dimensions of the service spillway and emergency spillway for each dam. In addition to the dam inspections, their field investigation included visiting and measuring (with hand level and cloth tape) key structures downstream of Lower Crystal Dam along the existing channel alignment until its confluence with the Amargosa River. The only key structure identified was the State Line Road.

In conjunction with the field inspections, 1-meter resolution Light Detection and Ranging (LIDAR) data was obtained from U.S. Fish and Wildlife Service Las Vegas office (FWS, 2007). The LIDAR data was flown on January 1, 2007, for the entire Ash Meadows National Wildlife Refuge. In addition to the LiDAR data, 1/3rd arc-second (10-meter) resolution Digital Elevation Model (DEM) data was obtained from the United States Geologic Survey (USGS) for the area downstream of the refuge up to the confluence with the Amargosa River (USGS, 2013). This area includes a potential downstream hazard identified as the State Line Road.

The Average Annual Daily Traffic (AADT) on State Line Road is 750 (Caltrans, 2017) and Wheeler also took two traffic counts along State Line Road in the location of potential inundation during the formal SEED inspections; the resulting AADT was in excess of 400. These traffic count numbers justify that further analysis is necessary to determine the hazard classification associated with the upstream dams.

INFLOW HYDROLOGY

This section of the report summarizes the key steps used in developing the inflow hydrology. A comprehensive hydrologic rainfall-runoff computer simulation model of the area of concern for this study was developed with U.S. Army Corps of Engineers (USACE) HEC-HMS software (USACE, 2017) to estimate runoff from a PMF and a 100-year ARI, 24-hour duration storm from three Inventory dams within the Ash Meadows National Wildlife Refuge. Discharges from Horseshoe and Crystal Springs Dams are stored in Lower Crystal Reservoir. Releases from Lower Crystal Reservoir flow downstream approximately 13 miles to the southwest where they join with the Amargosa River.

Pertinent HEC-HMS input and output are provided in Appendix A. Key steps in the development of these inflows included:

1. Determine drainage characteristics for each contributing sub-basin;

2. Develop Probable Maximum Precipitation (PMP) data, specifically: depth vs. duration relationship and temporal distributions;

3. Update reservoir and spillway capacity curves; and

4. Develop synthetic unit hydrographs to simulate the transformation of rainfall to runoff in the sub-basins reporting to each of the three dams of interest.

4.1 DRAINAGE BASIN CHARACTERISTICS

The boundaries for the sub-basins contributing runoff to the three dams under evaluation were developed in ArcGIS 10.7 using 1-meter LIDAR data (FWS, 2007). Figure No. 3 shows the different contributing sub-basins that were modeled for Crystal Springs, Horseshoe and Lower Crystal Dams.

Table No. 2 summarizes the key drainage basin parameters for the sub-basins, including: areas, calculated longest flow paths, longest flow path from centroids, changes in elevation from sub-basin maximum elevations to sub-basin minimum elevations and average sub-basin slopes, that were calculated in ArcGIS with the LIDAR topography. Figure No. 4 is a schematic of the resulting HEC- HMS model.

Table No. 2 – Summary of Key Drainage Basin Parameters

Dam Name Drainage Area, A (square miles)

Longest Flowpath, L (miles)

Longest Flowpath to

Centroid, Lca (miles)

Change in Elevation Along

Longest Flowpath, ∆Z (feet)

Average Slope, S (feet/mile) Crystal Springs Dam 5.27 4.24 1.83 615 145.1 Horseshoe Dam 0.26 1.85 0.89 52 28.1 Lower Crystal Dam 1.19 2.55 0.76 84 32.9

4.2 BASIN SOILS AND INFILTRATION

The soil characteristics for sub-basins reporting to the three dams of interest were obtained from the United States Department Agriculture (USDA) National Resources Conservation Service (NRCS).

The NRCS soils data indicated that onsite soils consist of a mixture of sands, silts and clays with rock outcrops (NRCS, 2018). The NRCS soils data also indicated the dominant hydrologic soil group for each soil type identified. Within the Crystal Springs Dam sub-basin, soil types B, C and D are present.

In the Horseshoe Dam and Lower Crystal Dam sub-basins, only soil type C is present. These soil groupings were used to determine recommended infiltration rates (for each soil type) following U.S.

Bureau of Reclamation guidelines (USBR, 1989). Once infiltration rates were determined, a weighted average infiltration rate was calculated for each basin. The average infiltration rate and percent imperviousness of each sub-basin are summarized in Table No. 3. Impervious areas were assumed to correspond to the extents of the maximum normal pool present in each sub-basin. Soil maps and infiltration calculations are provided in Appendix B.1.

Table No. 3 – Summary of Soil and Infiltration Parameters

Dam Name

% Sub-Basin Area Corresponding to Soil Group1

Weighted Average

Infiltration Rate (inches/hour)

Sub-Basin Percent

Impervious A B C D Crystal Springs Dam 0% 28% 48% 24% 0.117 4.8% Horseshoe Dam 0% 0% 100% 0% 0.100 6.9% Lower Crystal Dam 0% 0% 100% 0% 0.100 15.9% 1 – Excluding impervious areas in each sub-basin.

4.3 PRECIPITATION

The 100-year, 24-hour precipitation for each basin was estimated following procedures documented in National Oceanic and Atmospheric Administration (NOAA) Atlas 14, Volume 1, Version 5.0 (DOC, 2011). The 100-year, 24-hour precipitation depth versus duration values for basins within the study area are provided in Table No. 4. Detailed calculations for the areal reduction and temporal distribution of the various 100-year precipitation depth-duration values are documented in Appendix B.2.

Table No. 4 – 100-year Point Rainfall Depths

Dam Name 5-min (inch)

15-min (inch)

1-hr (inch)

2-hr (inch)

3-hr (inch)

6-hr (inch)

12-hr (inch)

24-hr (inch)

Crystal Springs Dam1, 2 0.45 0.85 1.42 1.61 1.68 1.97 2.29 2.75 1 – Results show that the 100-year, 24-hour precipitation depth at both the Horseshoe Dam and Lower Crystal Dam sub-basin centroids was 2.72 inches. Estimates from the Crystal Springs Dam sub-basin were assumed to apply to all three dams.

2 – For runoff modeling with HEC-HMS, 100-year rainfall is applied using a 1st Quartile, 50% Probability of Occurrence temporal distribution described in NOAA Atlas 14.

The PMP depths and associated temporal distribution were estimated following procedures documented in National Weather Service (NWS) Hydrometeorology Report No. 49 (HMR-49) (DOC, 1984). Site-specific application of the methods are documented in Appendix B.2. The PMP depth-duration values were estimated from monthly 24-hour convergence PMP isohyet maps (HMR-49 Figures 2.5 – 2.16) for General Storms and with a 1 square mile (HMR-49 Figure 4.5) isohyet map for Local Storms along with other procedures for the total combined watershed area (6.7 square miles) based on procedures outlined in HMR-49. Table No. 5 summarizes the Local Storm PMP values for the sub-basins reporting to all three dams, noting that these values apply to the entire area considered in this assessment and Table No. 6 does the same for the General Storm PMP values.

PMP values do not change for individual sub-basins because a single storm is assumed to cover the entire 6.7 square mile basin.

Table No. 5 – Local Storm PMP Rainfall Depths

Dam Name1 5-min (inch)

15-min (inch)

30-min (inch)

1-hr (inch)

2-hr (inch)

3-hr (inch)

4-hr (inch)

5-hr (inch)

6-hr (inch)

Crystal Springs Dam, Horseshoe Dam & Lower Crystal Dam

3.07 5.42 7.33 8.94 10.70 11.48 12.04 12.55 12.90

1 – For runoff modeling with HEC-HMS, Local Storm PMP is applied using the EM 1110-2-1411 temporal distribution as recommended in Standard Project Flood Determination (USACE, 1965).

Table No. 6 – General Storm PMP Rainfall Depths

Dam Name1 5-min (inch)

15-min (inch)

30-min (inch)

1-hr (inch)

2-hr (inch)

3-hr (inch)

6-hr (inch)

12-hr (inch)

18-hr (inch)

Crystal Springs Dam, Horseshoe Dam & Lower Crystal Dam

1.50 2.19 2.77 3.51 4.45 5.04 6.24 8.36 9.69

Dam Name1 24-hr (inch)

36-hr (inch)

48-hr (inch)

72-hr (inch)

Crystal Springs Dam, Horseshoe Dam & Lower Crystal Dam

10.70 12.19 13.36 14.59

1 – For runoff modeling with HEC-HMS, General Storm PMP is applied using the 2/3-weighted alternating block temporal distribution as recommended in Flood Hydrology Manual (USBR, 1989).

4.4 UNIT HYDROGRAPH

Unit hydrographs were developed for each dam’s sub-basin using the dimensionless unit hydrograph technique as documented in Chapter 4 of the United States Bureau of Reclamation (USBR) Flood Hydrology Manual (USBR, 1989). Detailed unit hydrograph calculations are provided in Appendix B.3. The USBR unit hydrograph procedure is based on the observation that unit hydrograph lag time is a function of measurable basin parameters. For the dimensionless unit hydrograph, the lag time is defined as the time from the center of unit rainfall excess to the time at which 50 percent of the runoff has passed the concentration point. The following equation was used to determine the lag time for each basin:

𝐿𝐿𝑔𝑔 = 𝐶𝐶𝑡𝑡(𝐿𝐿𝐿𝐿𝑐𝑐𝑐𝑐/𝑆𝑆0.5)0.33

where:

Lg = Lag time (hours);

Ct = a constant equal to (26 ⋅ Kn);

L = length of the longest watercourse (miles);

Lca = length along the longest watercourse from the point of interest to a point opposite the centroid of the basin, (miles); and

S = overall slope of the longest watercourse from the point of concentration to the drainage basin divide, (feet / mile).

Ct values are approximated as 26 times Kn, where Kn is estimated as the average Manning’s roughness value for the principal watercourse(s) in the drainage basin.

Kn values were selected based on review of similar drainage basins in the region as documented in Table No. 4.3 of (USBR, 1989). Kn values for the Southwest Desert, Great Basin or Colorado Plateau area (Type 4) range between 0.029 and 0.078; the values selected were considered appropriate for the development of reasonable lag times in each sub-basin. Kn along with associated Ct and lag time (Lg) values used for each sub-basin are summarized in Table No. 7.

Table No. 7 – Summary of Unit Hydrograph Characteristics

Dam Name Kn Ct Lag Time, Lg

(hours) Crystal Springs Dam 0.050 1.30 1.12 Horseshoe Dam 0.050 1.30 0.88 Lower Crystal Dam 0.050 1.30 0.91

4.5 RESERVOIR CAPACITY AND SPILLWAY CURVES

Documentation of the methods used to develop the reservoir storage capacity relationship along with the discharge capacity relationships of service and emergency spillways for each dam used in the HEC-HMS hydrologic reservoir routing model is provided in Appendix C.

4.6 INFLOW HYDROLOGY SUMMARY

Wheeler used the HEC-HMS model to simulate the runoff associated with the PMP events for the dams being evaluated in this report (USACE, 2017). The HEC-HMS model was developed using the sub-basin, reservoir and rainfall characteristics described in Sections 4.1 through 4.5.

A schematic of Wheeler’s HEC-HMS rainfall-runoff model is shown on Figure No. 4. The model automatically combines runoff hydrographs at junctions. Channel routing wasn’t performed between the junctions as the distances involved are relatively short and well-defined drainage pathways aren’t present. Flood wave attenuation between the junctions was omitted in the model.

Peak runoff results of the 100-year and PMP storms from the HEC-HMS model include upstream spillway releases and exclude any releases due to dam breaches. Results are summarized in Table No. 8.

Table No. 8 – Summary of Inflow Hydrology

Dam Name

100-year, 24-hour Frequency Storm

6-hour Local PMP Storm

72-hour General PMP Storm

Peak Inflow (cfs)

Inflow Volume

(acre-feet)

Peak Inflow (cfs)

Inflow Volume

(acre-feet)

Peak Inflow (cfs)

Inflow Volume

(acre-feet) Crystal Springs Dam 987 400 25,328 3,443 8,492 2,331 Horseshoe Dam 54 22 1,448 171 456 125 Lower Crystal Dam1 292 497 27,462 4,224 9,988 2,923

1 – Flood routing results include spillway releases from Crystal Springs Dam and Horseshoe Dam.

The Probable Maximum Flood (PMF) was determined to be the result of a 6-hour Local PMP storm.

RESERVOIR ROUTING AND DAM BREACH ANALYSIS

The HEC-HMS model and inflows described in Section 4.0 were used to develop the Sunny Day breach hydrographs and flood event breach (and no-breach) hydrographs for each Inventory dam being evaluated in this report. The flood event hydrographs were based on 10% increments of the PMF. The HEC-HMS model was used as a base model that was expanded through incorporation of different dam breach parameter scenario assumptions as an aid for the evaluation of hazards associated with various Sunny Day and flood event breach scenarios. This section of the report documents reservoir routing and dam breach assumptions along with key HEC-HMS simulation results.

5.1 DAM BREACH PARAMETERS

The dam breach parameters used to simulate the Sunny Day and flood-induced failure of each of the dams in the HEC-HMS models are summarized in Table No. 9. Detailed analysis of the dam breach parameter selection and reasoning is provided in Appendix D and representative photos of the three Inventory dams are provided in Appendix E. The breach parameters for the embankment dams were based on a review of several empirical breach relationships as discussed in: (Froehlich, 2008), (MacDonald and Langridge-Monopolis, 1984), (Susilo, 1997), (Von Thun and Gillette, 1990) and (Washington State, 2007) along with a sensitivity analysis of the breach development time and breach width as discussed in (Jamieson and Ferentchak, 2008). The sensitivity analysis also considered the expected range of breach parameters as given in (FEMA, 2013b) and (FERC, 1987). Additional details regarding the breach parameters associated with the different modeled runs are provided in Section 5.2.

The Sunny Day failure of each of the dams was initiated at the reservoir maximum normal pool level.

Table No. 9 – Dam Breach Parameter Summary

Description Crystal Springs

Dam Horseshoe

Dam8 Lower Crystal

Dam Breach Bottom Width (feet)1 105 3.5 | 31.5 21.3 Breach Development Time (hours)1 0.50 0.13 | 0.50 0.25 Breach Side Slopes (H : 1V)2 0.50 0.00 | 0.50 0.50 Breach Bottom Elevation (feet)3 2156.0 2149.0 2139.9 Sunny Day Dam Breach Initiation Elevation (feet)4 2181.0 2151.0 2143.0 Sunny Day Dam Breach Piping Elevation (feet)5 2171.05 2149.06 2139.96

Flooding Event Dam Breach Initiation Elevation (feet) Varies: see Table No. 10

Varies: see Table No. 11

Varies: see Table No. 12

Dam Crest Elevation (feet)7 2186.0 2158.0 2146.0 1 – A breach sensitivity analysis was completed to determine the breach bottom width and breach development time that produce the maximum discharge with realistic breach parameters that lie within expected ranges given in (FEMA, 2013b) and (FERC, 1987) while also resulting in an erosion rate that falls within the expected range given in (Von Thun and Gillette, 1990).

2 – Breach side slopes are considered typical for an earthen embankment.

3 – Breach bottom elevation taken from SEED report as dam crest elevation minus dam height.

4 – Sunny Day Breach initiation elevation taken from SEED report as the Maximum Normal Pool. The maximum normal pool was established based on the FWS criteria of the lesser of 1-foot below the emergency spillway crest or 3 feet below the dam crest. In the case of Horseshoe and Lower Crystal Dam, the service spillways at the time of the inspections were configured at or below the elevations to meet the maximum normal pool requirements. For Crystal Springs Dam, the service spillway is uncontrolled; however, it is 5 feet below the dam crest elevation and 4 feet below the emergency spillway elevation.

5 – Piping elevation at initiation of Sunny Day Breach taken as the midpoint between the dam crest elevation and the dam bottom elevation as recommended for piping breaches in (Colorado, 2010).

6 – Piping elevation at initiation of Sunny Day Breach taken as the downstream toe of the dam.

7 – Dam crest elevation taken from SEED report.

8 – For Horseshoe Dam, two sets of breach parameters are given. The first value in each pair refers to the value used for a Sunny Day Partial Breach determined using the procedure described in (Colorado, 2010), the second value in each pair refers to the value used for an overtopping breach initiated by a Flooding Event.

The primary dam in each hazard classification scenario is the one for which hazard classification is being evaluated; the other two dams in each hazard classification scenario are referred to as secondary.

Flood-induced breach trigger elevations are set for the primary dam, at each PMF storm increment, as the minimum of either one foot above the dam crest or the maximum water surface elevation attained under the no-breach scenario. Flood-induced breach triggers elevations for the secondary dams in a hazard classification scenario are set at one foot above the dam crest.

This procedure results in three sets of dam breach triggers, each differing by which dam is considered the primary and which dams are considered secondary. Table Nos. 10, 11 and 12 summarize the flood-induced breach trigger elevations used for all considered scenarios.

Table No. 10 – Flood-Induced Breach Triggers and Results where Crystal Springs Dam is Primary

PMP Storm Type &

Duration (hr)

PMP Storm Type &

Duration (hr)

PMP Storm Increment

Precipitation (in)

Breach Trigger Elevations & Breach Results for:

Crystal Springs1

Dam Horseshoe2

Dam Lower Crystal3

Dam Elev.

(ft)

Result

(BR/NB)4

Elev.

(ft)

Result

(BR/NB) 4

Elev.

(ft)

Result

(BR/NB) 4

6-hr Local 10% 1.29 2182.5 BR 2159.0 NB 2147.0 BR 6-hr Local 20% 2.58 2184.2 BR 2159.0 NB 2147.0 BR 6-hr Local 30% 3.87 2185.5 BR 2159.0 NB 2147.0 BR 6-hr Local 40% 5.16 2186.1 BR 2159.0 NB 2147.0 BR 6-hr Local 50% 6.45 2186.4 BR 2159.0 NB 2147.0 BR 6-hr Local 60% 7.74 2186.6 BR 2159.0 NB 2147.0 BR 6-hr Local 70% 9.03 2186.8 BR 2159.0 NB 2147.0 BR 6-hr Local 80% 10.32 2187.0 BR 2159.0 NB 2147.0 BR 6-hr Local 90% 11.61 2187.0 BR 2159.0 NB 2147.0 BR 6-hr Local 100% 12.90 2187.0 BR 2159.0 NB 2147.0 BR

1 – Crystal Springs Dam Crest Elevation = 2186.0 feet.

2 – Horseshoe Dam Crest Elevation = 2158.0 feet.

3 – Lower Crystal Dam Crest Elevation = 2146.0 feet.

4 – BR indicates dam breach; NB indicates no breach.

Table No. 11 – Flood-Induced Breach Triggers and Results where Horseshoe Dam is Primary

Type &

Duration (hr)

PMP Storm Type &

Duration (hr)

PMP Storm Increment

Precipitation (in)

Breach Trigger Elevations & Breach Results for:

Crystal Springs1

Dam Horseshoe2

Dam Lower Crystal3

Dam Elev.

(ft)

Result

(BR/NB)4

Elev.

(ft)

Result

(BR/NB)4

Elev.

(ft)

Result

(BR/NB)4

6-hr Local 10% 1.29 2187.0 NB 2152.6 BR 2147.0 NB 6-hr Local 20% 2.58 2187.0 NB 2153.8 BR 2147.0 NB 6-hr Local 30% 3.87 2187.0 NB 2154.4 BR 2147.0 BR 6-hr Local 40% 5.16 2187.0 NB 2154.9 BR 2147.0 BR 6-hr Local 50% 6.45 2187.0 NB 2155.5 BR 2147.0 BR 6-hr Local 60% 7.74 2187.0 NB 2156.0 BR 2147.0 BR 6-hr Local 70% 9.03 2187.0 NB 2156.4 BR 2147.0 BR 6-hr Local 80% 10.32 2187.0 NB 2156.8 BR 2147.0 BR 6-hr Local 90% 11.61 2187.0 BR 2157.2 BR 2147.0 BR 6-hr Local 100% 12.90 2187.0 BR 2157.6 BR 2147.0 BR

1 – Crystal Springs Dam Crest Elevation = 2186.0 feet.

2 – Horseshoe Dam Crest Elevation = 2158.0 feet.

3 – Lower Crystal Dam Crest Elevation = 2146.0 feet.

4 – BR indicates dam breach, NB indicates no breach

Table No. 12 – Flood-Induced Breach Triggers and Results where Lower Crystal Dam is Primary

Type &

Duration (hr)

PMP Storm Type &

Duration (hr)

PMP Storm Increment

Precipitation (in)

Breach Trigger Elevations & Breach Results for:

Crystal Springs1

Dam Horseshoe2

Dam Lower Crystal3 Springs Dam

Elev.

(ft)

Result

(BR/NB)4

Elev.

(ft)

Result

(BR/NB) 4

Elev.

(ft)

Result

(BR/NB) 4

6-hr Local 10% 1.29 2187.0 NB 2159.0 NB 2145.0 BR 6-hr Local 20% 2.58 2187.0 NB 2159.0 NB 2146.4 BR 6-hr Local 30% 3.87 2187.0 NB 2159.0 NB 2147.0 BR 6-hr Local 40% 5.16 2187.0 NB 2159.0 NB 2147.0 BR 6-hr Local 50% 6.45 2187.0 NB 2159.0 NB 2147.0 BR 6-hr Local 60% 7.74 2187.0 NB 2159.0 NB 2147.0 BR 6-hr Local 70% 9.03 2187.0 NB 2159.0 NB 2147.0 BR 6-hr Local 80% 10.32 2187.0 NB 2159.0 NB 2147.0 BR 6-hr Local 90% 11.61 2187.0 BR 2159.0 NB 2147.0 BR 6-hr Local 100% 12.90 2187.0 BR 2159.0 NB 2147.0 BR

1 – Crystal Springs Dam Crest Elevation = 2186.0 feet.

2 – Horseshoe Dam Crest Elevation = 2158.0 feet.

3 – Lower Crystal Dam Crest Elevation = 2146.0 feet.

4 – BR indicates dam breach, NB indicates no breach

5.2 HAZARD MODELING APPROACH

The HEC-HMS model described in Section 4.6 was expanded to incorporate the different dam breach parameters described in Table No. 9 to simulate various Sunny Day and flood-induced dam failure scenarios in order to estimate peak routed flow rates downstream of Lower Crystal Dam. Figure No.

4 shows a schematic of the comprehensive Wheeler HEC-HMS model used for evaluating the area of concern with rainfall events, both for no-breach and flood-induced breach scenarios.

Wheeler developed HEC-HMS model scenarios to assess hazard impacts for the three Inventory dams of interest in the Ash Meadows National Wildlife Refuge with the exception of a Sunny Day breach of Horseshoe Dam, which was assessed differently. The scenarios include three Sunny Day dam failures (Scenarios 1 through 3) and three flood-induced breach failures (Scenarios 4 through 6).

With Scenarios 4 through 6, flow rates were varied in 10% increments of the PMF storm. These specific HEC-HMS models were executed both with and without their specific flood-induced breach triggers, as given in Table Nos. 10, 11 and 12, to estimate the incremental impacts of the various flow rates for hazard classification. Each modeled scenario uses different breach parameters, initial assumptions and breach initiation triggers as described below.

• Scenario 1 Scenario 1 evaluates the Sunny Day breach of Crystal Springs Dam using breach parameters described in Table No. 9. This scenario includes one downstream dam:

Lower Crystal Dam which has a flood-induced overtopping breach trigger set at the dam crest elevation plus one foot (EL 2147.0). Breach parameters for Lower Crystal Dam are also taken from Table No. 9. Figure No. 5 shows a schematic for the Wheeler HEC-HMS model used with Scenario 1. This scenario is referred to as Max Sunny in Sections 6 and 7.

• Scenario 2 Scenario 2 evaluates the Sunny Day breach of Horseshoe Dam using breach parameters described in Table No. 9. This scenario includes the downstream dam:

Lower Crystal Dam which has a flood-induced overtopping breach trigger set at the dam crest elevation plus one foot (EL 2147.0). Breach parameters for Lower Crystal Dam are also taken from Table No. 9. With the Sunny Day breach of Horseshoe Dam, Lower Crystal Dam does not breach.

As discussed in Appendix D, a Sunny Day breach for Horseshoe Dam would involve a partial piping failure that does not completely breach the embankment due to the small reservoir volume and low head available under maximum normal pool conditions. A HEC-HMS model wasn’t created to model this scenario because it is not currently possible to model this type of partial failure with HEC-HMS software.

To explicitly model (generate an outflow breach hydrograph for) this condition it would be necessary to do so using a HEC-RAS 2D model. Instead, a Sunny Day breach of Horseshoe Dam was evaluated by adding the entire volume impounded by Horseshoe Dam (9.72 acre-feet) to the Lower Crystal Reservoir under Maximum Normal Pool conditions, conservatively neglecting timing effects. This results in pool elevation raise of 0.2 feet which isn’t sufficient to even engage the emergency spillway. Predicted maximum downstream outflow through the combined service spillway and low-level outlet of Lower Crystal Dam is 1.2 cfs for this scenario.

Lower Crystal Dam has sufficient capacity to store a Sunny Day breach of Horseshoe Dam without overtopping; no adverse downstream consequences are expected with Scenario 2. Consequently, downstream hazard classification modeling of Scenario 2 is not necessary.

• Scenario 3 Scenario 3 evaluates the Sunny Day breach of Lower Crystal Dam using breach parameters described in Table No. 9. No downstream dams are included with this scenario. Figure No. 6 shows a schematic for the Wheeler HEC-HMS model used with Scenario 3.

• Scenario 4 Scenario 4 was used to evaluate flood flows and hazard classification for Crystal Springs Dam. This model includes flood-induced breach triggers for the three dams of interest as given in Table No. 10 along with breach parameters as described in Table No. 9.

For 10% to 70% PMF flood increments in Scenario 4, the primary dam for this scenario: Crystal Springs Dam is assumed to breach at its maximum PMF pool level, which is lower than its crest elevation plus one foot. For 80% to 100% PMF increments, Crystal Springs Dam is assumed to breach at its crest elevation plus one foot.

For the PMF flood increments in Scenario 4, the secondary dams: Horseshoe Dam and Lower Crystal Dam are assumed to breach only if their respective pool levels reach their crest elevations plus one foot. Figure No. 4 shows a schematic for the Wheeler HEC-HMS model used with Scenario 4.

• Scenario 5 Scenario 5 was used to evaluate flood flows and hazard classification for Horseshoe Dam. This model includes flood-induced breach triggers for the three dams of interest as given in Table No. 11 along with breach parameters as described in Table No. 9.

For all PMF flood increments in Scenario 5, the primary dam for this scenario:

Horseshoe Dam is assumed to breach at its maximum PMF pool level, which is lower than its crest elevation plus one foot.

Similarly, for all PMF flood increments in Scenario 5, the secondary dams: Crystal Springs Dam and Lower Crystal Dam are assumed to breach only if their respective pool levels reach their crest elevations plus one foot. Figure No. 4 shows a schematic for the Wheeler HEC-HMS model used with Scenario 5. This scenario is referred to as Max Flood in Sections 6 and 7.

• Scenario 6 Scenario 6 was used to evaluate flood flows and hazard classification for Lower Crystal Dam. This model includes flood-induced breach triggers for the three dams of interest as given in Table No. 12 along with breach parameters as described in Table No. 9.

For 10% to 20% PMF flood increments in Scenario 6, the primary dam for this scenario: Lower Crystal Dam is assumed to breach at its maximum PMF pool level, which is lower than its crest elevation plus one foot. For 30% to 100% PMF increments, Lower Crystal Dam is assumed to breach at its crest elevation plus one foot.

Similarly, for all PMF flood increments in Scenario 6, the secondary dams: Crystal Springs Dam and Horseshoe Dam are assumed to breach only if their respective pool levels reach their crest elevations plus one foot. Figure No. 4 shows a schematic for the Wheeler HEC-HMS model used with Scenario 6.

Results of the HEC-HMS scenarios, along with the different dams that breach for each scenario are described in Section 5.3.

5.3 HEC-HMS MODEL RUN RESULTS

Table Nos. 10, 11 and 12 summarize the flood-induced breach results of Scenarios 4 through 6, while Table No. 13 summarizes the Sunny Day breach results of Scenarios 1 through 3.

Table No. 13 – HEC-HMS Sunny Day Breach Results Summary

Scenarios Evaluated Breach Trigger Type

Breach Results (BR/NB) 2 Crystal Springs

Dam Horseshoe Dam Lower Crystal Dam

Scenario 1 – Sunny Day Breach of Crystal Springs Dam1 Sunny Day BR NB BR

Scenario 2 – Sunny Day Breach of Horseshoe Dam1 Sunny Day NB BR NB

Scenario 3 – Sunny Day Breach of Lower Crystal Dam Sunny Day NB NB BR

1 – Flood-induced breach triggers are included in the model for downstream dams.

2 – BR indicates dam breach, NB indicates no breach

Result hydrographs taken immediately downstream of Lower Crystal Dam, which serve as the primary hydraulic input to the HEC-RAS 2D water surface profile model and subsequent hazard classification evaluation are illustrated as follows:

• Figure No. 7 for Sunny Day breaches (Scenarios 1 – 3);

• Figure No. 8 for Flood-Induced breaches of Crystal Springs Dam (Scenario 4);

• Figure No. 9 for Flood-Induced breaches of Horseshoe Dam (Scenario 5); and

• Figure No. 10 for Flood-Induced breaches of Lower Crystal Dam (Scenario 6).

Inspection of these results reveals that the worst-case peak downstream flow due to a Sunny Day breach of Crystal Springs Dam, which includes a cascading overtopping breach of Lower Crystal Dam, is 14,816 cubic feet per second (cfs). Similarly, for a Flood-Induced breach the worst-case peak downstream flow is 48,931 cfs; this flow rate corresponds to a 100% PMF breach of Horseshoe Dam, which also includes overtopping breaches of both Crystal Springs and Lower Crystal Dams (Scenario 5). Wheeler notes that the breaches of Crystal Springs and Lower Crystal Dams in this case are not caused by the flood-induced breach of Horseshoe Dam but are triggered due to flood runoff raising their pool levels more than one foot above their respective dam crests.

For the downstream water surface profile modeling (discussed in Section 6) and subsequent hazard classification evaluation (discussed in Section 7), Wheeler’s approach is to evaluate downstream hazards for two worst-case conditions:

1. The worst-case Sunny Day breach (Scenario: Max Sunny) is the result of a Sunny Day breach of Crystal Springs Dam and includes a consequent overtopping breach of Lower Crystal Dam.

Peak outflow downstream of Lower Crystal Dam resulting from (Scenario: Max Sunny) is 14,816 cfs.

2. The worst-case incremental (breach vs. no-breach) 100% PMF scenario is the result of a 100% PMF storm and includes breaches of all three dams: Crystal Springs Dam, Horseshoe Dam and Lower Crystal Dam. Peak outflow downstream of Lower Crystal Dam from (Scenario: Max Flood) is 22,447 cfs if no breaches were to occur and 48,931 cfs if breaches were to occur.

As documented in Section 7, the two worst-case breach scenarios: Max Sunny and Max Flood do not result in lethal flooding conditions at the State Line Road. As a result, documentation of incremental flooding results at lower % PMF storm increments was not required.

WATER SURFACE PROFILE MODELS

6.1 WATER SURFACE PROFILE MODEL DEVELOPMENT

A water surface profile model was developed to estimate flood depths and velocities downstream of Lower Crystal Dam. The model included both overland and channel routing of the dam breach hydrographs obtained from the HEC-HMS model described in Sections 4 and 5. The water surface profile model was simulated using the two-dimensional (2D) U.S. Army Corps of Engineers (USACE) HEC-RAS 5.0.7 River Analysis System computer model (USACE, 2019). The model was terminated approximately 13 miles downstream at the confluence with Amargosa River because the attenuated breach flows downstream of this point were less than 100-year ARI peak discharge of 27,300 cfs

(USGS, 2019).

The 2D geometry flow area used in the water surface profile model was generated with a 100-foot by 100-foot computational grid that extended from the Lower Crystal Dam to where the model was terminated; downstream of the area of concern for hazard classification evaluation. The 2D flow area grids extracted elevation from a terrain file made up of two sources:

1. 1-meter LiDAR data flown January 1, 2007 (FWS, 2007) for the Refuge; and

2. 1/3 arc-second (10-meter) DEM data from USGS with publication date November 1, 2013 (USGS, 2013) for areas downstream of the Refuge.

Two-dimensional surface roughness was also part of the 2D water surface profile model. Australian Rainfall & Runoff Revision Projects (AR&RRP), Project 15, Two-Dimensional Modelling in Urban and Rural Floodplains (AR&RRP, 2012) provides ranges of 2D roughness parameters for various land use types; it was used to determine appropriate surface roughness values for the model. The values documented by AR&RRP were used because AR&RRP roughness parameters reflect 2D roughness only, omitting additional modeling factors that are otherwise incorporated in more traditional Manning’s values used for one-dimensional analyses. Because breach inundation flows were routed in a 2D environment, the model accounts for the loss of energy associated with the flow changing directions and the flow magnitude in certain areas. 1D models are not able to account for this loss of energy in the model itself, so traditional 1D Manning’s values used for surface roughness are often higher to account for the loss of energy that cannot be captured in the 1D environment. However, the Manning’s values are often similar between 1D vs. 2D, with 1D often being slightly (0.01 – 0.02) higher. The following 2D roughness values were incorporated in the model coincident with the inundation footprint as follows:

• Vegetated waterways – 0.06;

• Emergent vegetated wetlands – 0.06;

• Open Area with shrubs – 0.06;

• Open Area with low grass – 0.03;

• Barren ground – 0.03; and

• Paved roads – 0.02.

A sensitivity analysis was performed by modifying the Manning values to be higher and lower, within the recommended range for the existing land types, which encompassed the range of expected Manning values for 1D and 2D modeling. The sensitivity analysis indicated that modifying the Manning values, within the reasonable range, did not change the results of the hazard classification analysis.

One major roadway: State Line Road, is located downstream of Lower Crystal Dam. The culvert dimensions that were identified were measured by Wheeler with a hand-level and tape during the formal SEED inspections for the three dams. Several culverts are located along the roadway and range between 2 feet and 2.5 feet in diameter. The measured coverage between the road and the top of culverts also varied between no coverage and approximately 2 feet of coverage. Based on the size of these culverts compared to magnitude of dam breach flows, Wheeler conservatively assumed that these culverts were completely blocked for all modeled scenarios.

Also, the DEM was not of sufficient resolution to capture the road berm at State Line Road, so the minimum road berm height of two-foot that was measured in the 2018 field inspection was included in the hydraulic model to provide results that are as representative of the existing field conditions as possible. The two-foot high berm was input into the 2D computational grid above the ground elevation extracted from the 10-meter DEM data along the roadway using a weir structure.

The “Full Momentum” set of Saint Venant equations were used to run the 2D water surface profile model with a computational timestep of one second. The inflow hydrograph was applied to the grid cells at the downstream toe of Lower Crystal Springs Dam for all breach scenarios. The downstream boundary, which was located downstream of the confluence of the drainage in question with the Amargosa River, was established in the model perpendicular to the flow field and used an assumed normal depth boundary condition, computed using the longitudinal slope of the channel bed at that location.

6.2 WATER SURFACE PROFILE RESULTS

The water surface profile results were collected from RAS Mapper which is a tool included with HEC-RAS 5.0.7. RAS Mapper extrapolates the simulated water surface elevation (WSEL), depths based on the WSEL minus the ground elevation and velocities located at each point in the 2D computational grid. The resulting water surface elevations along State Line Road were used to determine the flooding depth above the roadway berm. Associated depths and velocities for each sunny day and flood-induced breach scenario were used to assess the hazard classification for the three dams, as described in Section 7.

HAZARD CLASSIFCIATION EVALUATION

This section of the report documents Wheeler’s hazard classification evaluations for Crystal Springs Dam, Horseshoe Dam and Lower Crystal Dam analyzed in this report.

7.1 HAZARD CLASSIFICATION GUIDELINES AND APPROACH

At the time this report was prepared, the FWS used federal guidelines for hazard systems (FEMA, 2004) and the hazard classification depth versus velocity curves in ACER TM No. 11 (USBR, 1988) to classify the hazard potential for its dams as discussed in (FWS, 2008). In addition, FWS also refers to the FEMA 94 for incremental consequences of dam failures during flood events (FEMA, 1998). The 2D HEC-RAS water surface profile model, discussed in Section 6, was used to identify any potential downstream hazard areas. Figure No. 11 shows State Line Road as the one potentially hazardous area downstream of the three dams. Based on field traffic counts and information from Caltrans (Caltrans, 2017), State Line Road has an Annual Average Daily Traffic (AADT) in excess of 400 which is the threshold as given by FWS guidance for potential lethal flooding at roadways within downstream inundation limits.

In order to streamline the hazard classification process for this report, Wheeler initially evaluated the worst-case sunny day breach scenario and flood-induced dam breach scenario to determine if the associated downstream depths and velocities generate hazardous conditions. These two scenarios were assumed to produce the most hazardous potential depths and velocities for a sunny day and flooding dam failure. These scenarios were shown to generate low hazard results, so it was concluded that scenarios with lesser flow rates would also produce low hazard results.

Depths and velocities attributed to each scenario were evaluated using the hazard evaluation curve taken from USBR ACER TM No. 11, Depth vs. Velocity flood danger level relationship for passenger-vehicles. Figure No.