Two-Dimensional Hydraulic Model of Folsom Dam Breach Under The

TWO-DIMENSIONAL HYDRAULIC MODEL OF FOLSOM DAM BREACH UNDER THE

MAXIMUM PROBABLE FLOOD

A Project

Presented to the faculty of the Department of Civil Engineering

California State University, Sacramento

Submitted in partial satisfaction of the requirements for the degree of

MASTER OF SCIENCE

Civil Engineering

(Water Resources Engineering)

Michael Pantell

SPRING 2017

Michael Pantell

TWO-DIMENSIONAL HYDRAULIC MODEL OF FOLSOM DAM BREACH UNDER THE

MAXIMUM PROBABLE FLOOD

A Project

Michael Garland Pantell

Approved by:

______, Committee Chair Dr. Saad M. Merayyan

______, Second Reader Dr. Cristina Poindexter

______Date iii

Student: Michael Garland Pantell

I certify that this student has met the requirements for format contained in the University format manual, and that this project is suitable for shelving in the Library and credit is to be awarded for the project.

______, Department Chair ______Dr. Benjamin Fell Date

Department of Civil Engineering

Abstract

TWO-DIMENSIONAL HYDRAULIC MODEL OF FOLSOM DAM BREACH UNDER THE

MAXIMUM PROBABLE FLOOD

Michael Garland Pantell

Folsom Dam and 11 earthen structures make up Folsom Reservoir, located on the

American River east of Sacramento, California. These structures were constructed by the United

States Army Corps of Engineers (USACE) in 1956 to create a reservoir that can hold 1.38 billion cubic meters (1.12 million acre-feet) which protects approximately 1.5 million people as well as the State’s seat of government from flooding. In recent years, updated hydrologic information has been used to better estimate the probable maximum flood (PMF) that can be expected to inflow into Folsom Reservoir. It was found that this PMF inflow was greater than the outflow capability of the current dam structure and could put the earthen structures that surround the reservoir at risk of failing. A spillway has been constructed to increase the outflow capabilities of the reservoir and is slated to be operational in 2017.

As a part of spillway design process, the USACE developed a hydraulic model to estimate the flooding that results from scenarios with and without the spillway. However, this model and the resulting inundation areas are not available to the public and many local agencies.

This project used Hydrologic Engineering Center River Analysis Software Version 5.0 (HEC

RAS 5.0) two-dimensional hydraulic modeling capabilities and publicly available information to model multiple flooding scenarios with and without the spillway during a PMF event. Since

without the spillway present, massive overtopping would occur which would likely lead to a breach along one of the earthen structures, the model also estimated multiple breaching scenarios.

Two breach methods were applied to earthen structure on the north and south sides of the reservoir to better estimate the range of possible flows and spatial differences in the floodplain.

The resulting floodplain for all worst-case scenarios, produced high flooding depths along the American River, Downtown Sacramento and the Pocket and Natomas regions of greater than 4 meters (13.12 feet). Due to the breaches along the north and south earthen structures, the without spillway scenarios produced much greater flooding extents and depths than the with spillway scenario. The northern breach diverted flows into Arcade, Linda and Dry creeks which produced flooding depths of greater than 4 meters (13.12 feet) along the creeks and increases the extent of flooding in the Natomas regions. The southern breach produced flooding depths of greater than 4 meters (13.12 feet) in Folsom and increases the floodplain south of the American

River.

Using peer-reviewed papers and information provided by the State of California, mortality and single family home property damage rates were estimated for each of the worst- case scenarios. Both mortality and property damage rates closely followed floodplain depths with the highest risk and costs being along the American River, Downtown Sacramento and in the

Pocket and Natomas regions. Although this information is not 100% accurate, it does provide helpful guidelines for emergency response and planning purposes. To prepare for a PMF event, evacuating areas along the American River, the Pocket area, the Natomas area and Downtown

Sacramento should be considered high priority for saving the most lives. To prepare for the uncertainty of which earthen structure will breach, areas in Folsom near the reservoir and areas

along natural creeks such as Dry Creek, Linda Creek and Arcade Creek should also be evacuated.

Other areas that have predicted flooding, should also be evacuated to high ground.

______, Committee Chair Dr. Saad M. Merayyan

______Date

vii

ACKNOWLEDGEMENTS

I would like to first thank God for providing me the patience and perseverance necessary to complete this project. I would also like to thank the following people:

• My wife, Cheryl Pantell, for having the patience and love to put up with me during this

time.

• Stanford Gibson for encouraging me to become a civil engineer and providing me

opportunities to learn hydraulic modeling.

• My adviser, Dr. Merayyan, and committee member, Dr. Poindexter, for their feedback

and guidance on this project.

• Peterson Brustad Inc. for providing me technical support and an environment where I can

learn and advance in my career.

• Finally, all of my friends and family who have supported me and encouraged me along

the way.

viii

TABLE OF CONTENTS

Acknowledgements ...... viii

List of Tables ...... xi

List ot Figures ...... xii

Chapter

1. INTRODUCTION ...... 1

2. BACKGROUND ...... 4

Location ...... 4

Characteristics of Folsom Dam and Reservoir ...... 6

Dam Breach ...... 8

Breach Modeling ...... 10

Hydrology ...... 13

Inflow ...... 13

Outflow ...... 15

Hydraulic Analysis ...... 17

Mass Conservation ...... 17

Momentum Conservation ...... 17

HEC RAS modeling ...... 20

Mortality and Property Damage ...... 21

Mortality ...... 21 ix

Property Damage ...... 23

3. ANALYSIS ...... 25

Project Datum and Coordinate System ...... 25

Model Parameters ...... 25

Terrain ...... 25

Boundary Conditions ...... 27

Folsom Reservoir ...... 28

Breaches ...... 29

Scenarios ...... 30

4. RESULTS AND DISCUSSION ...... 32

Breach Timing ...... 32

Breach Method...... 32

Mortality ...... 39

Property Damage ...... 43

5. CONCLUSION AND RECOMMENDATIONS ...... 47

APPENDIX A. Floodplain Maps ...... 49

REFERENCES ...... 58

LIST OF TABLES

Tables Page

1. Concrete dam characteristics ...... 6

2. Earthen structures characteristics ...... 7

3. D values based on mixing intensity and geometry ...... 19

4. Mean and standard deviation of the zones defined by Jonkman et. al. (2008) ...... 23

5. Manning n values based on land use type ...... 26

6. Breach parameters for each location and breaching method ...... 30

7. Organization of scenarios based on variable ...... 31

8. Peak inflows and outflows of the Folsom Reservoir ...... 32

9. Peak outflow and total volume discharged for both breach locations and methods ...... 35

LIST OF FIGURES

Figures Page

1. Overview of Folsom Reservoir manmade structures ...... 2

2. Study area of the dam breach model ...... 5

3. Overtopping breach dimensions ...... 12

4. American River watershed ...... 14

5. PMF inflow at Folsom Reservoir ...... 15

6. Flow out of Folsom Dam based on elevation of water, with- and without- spillway ...... 16

7. Damage-depth curve for residential homes in Sacramento Region ...... 24

8. Folsom Reservoir volume-elevation curve ...... 28

9. Worst-case with-spillway scenario ...... 34

10. Breach hydrographs for each scenario ...... 35

11. Without spillway, worst-case modeled breach scenario for the northern earthen

structures using MacDonald et. al. breach method ...... 37

12. Without spillway, worst-case modeled breach scenario for the southern earthen

structures using MacDonald et. al. breach method ...... 38

13. With spillway worst-case scenario mortality ...... 40

14. Without spillway worst-case scenario mortality from the northern earthen structure

breach ...... 41

15. Without spillway, worst-case scenario mortality from the southern earthen structure

breach ...... 42

16. With spillway, worst-case scenario percent damage done to residential areas ...... 44

xii

17. Worst-case scenario percent damage done to residential areas from northern earthen

structure breach ...... 45

18. Worst-case scenario percent damage done to residential areas from southern earthen

structure breach ...... 46

xiii

xiv

CHAPTER 1 INTRODUCTION

Folsom Reservoir is located on the American River east of Sacramento, California (CA).

The reservoir, which can hold 1.38 billion cubic meters (1.12 million acre-feet) of water was constructed by the United States Army Corps of Engineers (USACE) in 1956 to mitigate flooding potential in the Sacramento region and to store water for use in summer months (US Bureau of

Reclamation, 2000). The construction consisted of building Folsom Dam, a 104 meter (340 feet) tall concrete dam on the American River, and a total of 11 earthen manmade structures to the north and the south of the dam (US Bureau of Reclamation, 2000). The eleven earthen structures, shown in Figure 1, include two wing dams that are directly adjacent to the concrete dam, one auxiliary dam, and eight dikes.

Directly downstream of Folsom Dam lies Sacramento and its surrounding suburbs with approximately 1.5 million people as well as the State’s seat of government (United States Census

Bureau, 2010). If Folsom Dam or any of the surrounding earthen structures were to fail, the flooding extent and damage could be great.

Dam failures throughout the world in the 1970’s and 1980’s, prompted the federal government to pass the National Dam Safety Program Act, 1996, which funds dam improvements, training and research (United States Army Corps of Engineers, 1997). As a part of that research, failure risk studies were performed on dams across the nation including Folsom

Dam and the surrounding earthen structures (Wahl, et al., 1989). Wahl, et al. (1989) found that

Folsom Dam and the surrounding earthen structures were at low risk of structural failure due to hydrostatic forces, earthquakes or design issues. However, earthen structures are more susceptible

2 to failure if sustained overtopping were to occur. The Sacramento District of the USACE, in

2001, calculated that the probable maximum flood (PMF) for the American River basin into

Folsom Reservoir would cause overtopping (USACE Sacramento District, 2001). Thus, many of the earthen structures to are still at risk of failure.

Locations of Potential Overtopping

Figure 1 Overview of Folsom Reservoir manmade structures (USBR, 2016)

In the last three decades, hydraulic modeling has allowed for more accurate predictions of the effects of large scale dam breach flooding. One-dimensional (1D) modeling made it possible to predict the effects of flooding in rivers, levee breaches and smaller dam breaches (USACE

Hydrologic Engineering Center, 2016). With the recent development of two-dimensional (2D) modeling, large dam breaches can be modeled more accurately (USACE Hydrologic Engineering

Center, 2016). These 2D models can help emergency and planning services direct people to safe zones, allow for more response time, faster evacuation, and educate the public of flooding risk; ultimately saving lives.

This study, will construct a 2D hydraulic model of the area downstream of Folsom

Reservoir using the USACE Hydrologic Engineering Center River Analysis Software (HEC

RAS) version 5.0. The model will investigate the worst-case flooding extents and depths in the

Sacramento Region due to overtopping breaches on the earthen structures during a PMF scenario.

The resulting worst-case floodplains will be used to estimate mortality and property damage within the Sacramento Region.

CHAPTER 2

BACKGROUND

Location

Folsom Reservoir is located on the American River east of Sacramento at the base of the

Sierra Nevada foothills. Downstream of the reservoir the American River joins into the

Sacramento River, just south of the Sacramento Weir. The confluence of these rivers is surrounded by community of Natomas to the North, West Sacramento to the Southwest and

Downtown Sacramento to the Southeast. There are also multiple small creeks that feed into both the Sacramento and American Rivers near the confluence including Linda Creek, Dry Creek and

Arcade Creek. Levees line these rivers and creeks to protect the people in the Sacramento Region from a 200-year flooding event.

The Sacramento Region, shown in Figure 2, includes Sacramento, the Pocket area in the south, Natomas in the North and many other large suburbs including Roseville, Citrus Heights,

Elk Grove, Folsom and Rancho Cordova. In total, there are approximately 1.5 million people living in the region (United States Census Bureau, 2010). A breach along the Folsom Reservoir during a PMF has the potential of causing extensive flooding in the region putting many of these people at high risk. Although the flooding will occur well beyond the Sacramento Region, only this region will be considered in this project.

Figure 2: Study Area of the Dam Breach model

Characteristics of Folsom Dam and Reservoir

Folsom Reservoir is created by Folsom Dam, a concrete gravity dam, and eleven earthen structures, shown in Figure 1. The concrete dam and earthen structures were built in 1956 by the

USACE and is operated by the U.S. Bureau of Reclamation (USBR) (Wahl, et al., 1989). The outflow through Folsom Dam consists of three small pipe outflows to the power station and eight tainter gates at the top of the dam (Hall, et al., 1989). Physical details of the concrete dam are summarized in Table 1.

Table 1: Concrete dam characteristics

Height meters (feet) 104 (340) Crest length meters (feet) 426 (1400) Crest Width meters (feet) 10 (32) 147.25 Overflow Elevation meters (feet) (483.1)

The eleven earthen structures include two wing dams that are directly adjacent to the concrete dam, one auxiliary dam, and eight dikes. Although all the earthen structures vary in size, length and side slope, all were built to engineering standards and include clay cores to prevent through-seepage (Wahl, et al., 1989). The wing dams act as earthen transitions for the center concrete dam and together with the concrete dam cover the entirety of the American River channel. The auxiliary dam was built on an ancient channel of the American River called Blue

Ravine (Wahl, et al., 1989). The eight dikes are used to fill in gaps within the terrain to create the whole of the Folsom Reservoir. All the dikes are similar in construction with Dike 5 being the largest and most damaging if it were to breach (Wahl, et al., 1989). The physical characteristics of the wing dams, the auxiliary dam and dike 5 are presented in Table 2.

Table 2 Earthen structures characteristics

South Auxiliary North Wing Dike 5 Wing Dam Height meters (feet) 59 (195) 51 (167) 50 (165) 20 (66) Crest length meters 2042 (6700) 640 (2100) 1469 (4820) 150 (feet) Crest Width meters 8 (26) 8 (26) 8 (26) 8 (26) (feet) Overflow Elevation NAVD88 meters 146 (479) 146 (479) 146 (479) 148 (486) (feet)

Folsom Reservoir, which drains the 4856-square-kilometer (1,875-square-mile)

American River basin, has a maximum capacity of 1.38 billion cubic meters (1,120,000 acre- feet.) (US Bureau of Reclamation, 2000) The amount of water in the reservoir changes based on inflows and storage capacity rules developed by the USBR (U.S. Bureau of Reclamation, 2012).

The storage capacity rules divide the reservoir into three categories: Dead Space, the constant minimum amount of volume in the reservoir due to the height of the lowest discharge facilities;

Flood Space, the volume required to stay empty for flood waters based on the time of year; and

Active Space, the volume in the reservoir that is not taken for Flood Space or Dead Space. For

Folsom Reservoir, the Dead Space is 0.111 billion cubic meters (90,000 acre-feet), the Active

Space is at minimum 0.269 billion cubic meters (217,000 acre-feet) and the maximum Flood

Space is 1 billion cubic meters (813,000 acre-feet) (USACE Sacramento District, 2013).

When the reservoir was built, the population downstream was small with Sacramento having only 135,000 people (Brunsman, 1952). Since the completion of Folsom Dam, population has grown rapidly and the understanding of the hydrology has changed drastically. Due to these

8 changes, the USACE decided that improvements to the dam were needed to better protect the lives and property downstream.

The first improvement is to build an auxiliary spillway that would increase the amount of flow that the dam could release. The auxiliary spillway will discharge from lower in the reservoir which decreases the Dead Space and increases the Flood Space. Construction of the auxiliary spillway is completed and it is scheduled to be operational in 2017 (USACE Sacramento District,

2016).

The second improvement will be to increase the height of the dam and the surrounding earthen structures by 3.5 feet, increasing the total storage capacity. This will allow for more water to come into the reservoir and increasing the flooding buffer the reservoir provides. This improvement has not yet begun but is set to be finished by 2030 (USACE Sacramento District,

2016).

Dam Breach

Due to lack of data and the dynamic nature of the problem, dam failure mechanics in both earthen and concrete dams are not well understood. Initial dam failure modeling attempts assumed that a dam failure was total and instantaneous but this scenario is not realistic and was not considered in this study. The total dam failure scenario is only viable if a major earthquake

(exceeds the design earthquake magnitude) is to hit the Sacramento region. Currently, it is understood that dams more often fail slowly and only partially (USACE Hydrologic Engineering

Center, 2014). This is especially true for earthen dams.

In general, there are seven type of failure modes for any type of dam; overtopping, foundation, piping, sliding, structural, spillway and earthquake (Tatalovich, 1998). In every case,

9 the likelihood of this event occurring is very small, on the order of 0.001% (Tatalovich, 1998).

The seven failures modes can be divided into two general scenarios: overtopping and normal pool.

In an overtopping failure scenario, the water level rises to the level of the crest of the structure because the inflow into the reservoir exceeds the outflow through the dam. When earthen structures are overtopped, erosion occurs at both the top and dryland face of the structure

(MGS Engineering Consultants, Inc., 2007; Tatalovich, 1998). Given enough time, earthen structure will erode with one foot of sustained water and catastrophic failure will occur (Bureau of Reclamation, 2013). Concrete dams are generally designed to discharge from gates at the top of the structure, are built to withstand a great deal of overtopping pressure and cannot easily erode. Because of these things, well maintained concrete dams, such as Folsom Dam, don’t generally fail from overtopping (USDOI, USBR & USACE, 2015).

Normal pool failures, also known as “sunny day” failures, do not occur due to large inflows but rather other reasons such as: erosion, earthquakes, piping or other structural issues (USACE

Hydrologic Engineering Center, 2014). As the name suggests, these can occur at any time, not necessarily during a storm event. Since Folsom Dam and the surrounding earthen structures are exceptionally designed, constructed and maintained, they are unlikely to fail due to most sunny day failure modes (Wahl, et al., 1989; USDOI, USBR & USACE, 2015). The USACE also performed an earthquake safety analysis which concluded the fault lines near the dam are not capable of causing a failure on Folsom Dam or the surrounding earthen structures (Wahl, et al.,

1989; Hall, et al., 1989).

Therefore, a normal pool failure along any of the Folsom Reservoir structures or an overtopping failure on the Folsom Dam are both highly unlikely. The only plausible failure scenario would be an overtopping failure along the earthen structures that create Folsom

Reservoir.

Breach Modeling

There are two approaches to modeling a dam breach: empirical models based on historic data or numerical models based on the physics of the process. The USACE has developed dam breach modeling capabilities in HEC-RAS (USACE Hydrologic Engineering Center, 2016). As a part of the model, HEC-RAS calculates multiple dam breach widths, timings and side slopes based on the physical properties of the dam using five different peer-reviewed methods. For this study, the MacDonald et al. (1984) and Von Thun & Gillete (1990) methods will be used because these methods provide the most extreme cases; with MacDonald et. al. being the largest and slowest breach method and Von Thun & Gillette being the smallest and fastest breach method

(MacDonald, et al., 1984; Von Thun, et al., 1990).

MacDonald et al. (1984) used 42 data sets of mostly earthen fill dams to develop an equation that relates the volume of eroded material, 푉푒푟표푑푒푑, with the volume of water that comes out of a breach, 푉표푢푡, for earthen dams with clay cores (MacDonald, et al., 1984). The equation for the volume eroded based on the height of the water behind the dam, 풉풘, in the metric system is:

0.852 푉푒푟표푑푒푑 = 0.0261(푉표푢푡 ∗ ℎ푤)

The time of the formation of the breach, 푡푓, in hours, is calculated from the volume of eroded material.

0.364 푡푓 = 0.0179(푉푒푟표푑푒푑)

The breach is assumed to be trapezoidal in shape with 0.5 H: 1 V side slopes. The bottom width of the breach, 푊푏, will then be calculated using

2 0.5ℎ푏푍3 푉푒푟표푑푒푑 − ℎ푏(0.5퐶 + ) 푊 = 3 푏 ℎ 푍 ℎ (퐶 + 푏 3) 푏 2

Where ℎ푏 is the height from the top of the dam to the bottom of the breach, C is the crest width and 푍3 is the sum of the horizontal slopes (Z:1) of the upstream, 푍1, and downstream, 푍2, face of the dam.

The volume of water out of the breach is not known before the analysis; therefore, it is assumed to be the volume of reservoir at breach for the initial run. The volume discharge from the results of the initial analysis will be applied to the volume discharged and reanalyzed. This process will be done iteratively until a solution is converged upon.

Von Thun and Gillette (1990) used 57 dams to develop equations for both the average width of the breach, 퐵푎푣푒, and the breach formation time, 푡푓. The breach is assumed to be a trapezoidal shape with side slopes of 1.0 H: 1.0 V (Von Thun, et al., 1990). The equation for average breach width in the metric system is:

퐵푎푣푒 = 2.5ℎ푤 + 54.9

Where ℎ푤 is the height of the water. The equation for the breach formation in hours is:

푡푓 = 0.02ℎ푤 + 0.25

With the final breach width, height and timing parameters, HEC RAS 5.0 will model the progression of the breach during a flood event. Once overtopped, HEC RAS 5.0 erodes at the

12 center location at the top of the earthen structure and progresses down the height of the structure as shown in Figure 3.

Figure 3: Overtopping breach dimensions (USACE Hydrologic Engineering Center, 2016)

Where h0 is the breach top elevation at time t, hd is the elevation at the top of the dam, hb is the breach bottom elevation at time t, hbm is the lowest breach bottom elevation at time t, hf is the specified center-line elevation of the pipe, h is the height of the water at time t and b is the maximum bottom breach width. The side slope, Z, will determine the width of the top of the breach. This creates a larger opening which will allow more water through the breach than if the side slopes were vertical.

At each time step during the breach, the weir equation is used to calculate the amount of flow, Q, that passes through the breach.

3 푄 = 퐶퐿퐻2

Where C is the weir coefficient of 1.44 for trapezoidal weirs in the metric system, L is average width of the breach, which is equal to b in Figure 3, and H is the hydraulic head upstream of the breach (USACE Hydrologic Engineering Center, 2016).

Hydrology

The hydrologic variables of concern for a Folsom Reservoir earthen structure breach are the inflow from the American River and the outflow through the control structures. Other inflows and outflows such as precipitation over the reservoir and groundwater seepage are assumed to be negligible because the area of the reservoir is small which causes there to be very little rain deposited or groundwater lost in comparison to the amount of surface water inflows and outflows.

This section will discuss, in detail, the inflow and outflow parameters of Folsom Reservoir.

Inflow

The inflow into Folsom Reservoir is completely controlled by the American River

Watershed. The American River Watershed, shown in Figure 4, is in the Sierra Nevada and the outlying foothills (USACE Sacramento District, 2001). The watershed consists of the three forks of the American River and all the minor offshoots covering approximately 1,850 square miles

(USACE Sacramento District, 2001). Along the rivers there are 17 reservoirs of varying capacities used for flood control and water storage. Using existing stream and precipitation gages, the USACE developed a hydrologic model that predict the amount of flow that will enter Folsom

Reservoir (USACE Sacramento District, 2001).

Figure 4: American River watershed (USACE Sacramento District, 2001)

The USACE used existing precipitation data to estimate the probable maximum precipitation (PMP) for the American River Watershed (USACE Sacramento District, 2001). The

PMP is the greatest depth of precipitation that is physically possible over the watershed for a duration, location and time of year. The PMP, which was calculated to be 29.62”, was inputted into the USACE hydrologic model to calculate the probable maximum flood (PMF). The PMF is the likely worst-case flood scenario considering the PMP, basin snowmelt and basin characteristics that result in the maximum runoff (USACE Sacramento District, 2001). Figure 5 shows the PMF inflow hydrograph at Folsom Reservoir.

30000

25000

20000

15000

Flow Flow (m^3/s) 10000

5000

0 0 50 100 150 200 Time (hours)

Figure 5: PMF inflow at Folsom Reservoir

In a later study, the USACE used stochastic methods to calculate the annual exceedance probability (AEP) of the PMF to be 1:25000 (.004%) (MGS Engineering Consultants, Inc. ,

2005). The AEP of overtopping the reservoir during any event was calculated to be 1:5000

(.02%) (MGS Engineering Consultants, Inc. , 2005). Although both scenarios are unlikely, the possibility of flooding still exists and the results could be catastrophic. The resulting floodplains can be used to better prepare for a worst-case scenario. therefore, this study will use the PMF to consider the worst-case flooding scenario.

Outflow

When Folsom Dam was originally built, the outflow from the Reservoir was controlled solely by the 8 tainter gates located at the top of the dam and the 3 small powerhouse pipes located about halfway up the dam (Wahl, et al., 1989). The bottoms of the gates are at an elevation 128 meter (420 feet) in the North American Vertical Datum 1988 (NAVD88) causing

16 the outflow to be limited to the powerhouse pipes when the reservoir level is below 128 meter

(394 feet) NAVD88 (USACE Sacramento District, 2001). This design makes it difficult to maintain lower reservoir levels and limits outflow response times which negatively impacts the level of flood protection.

To solve this issue, an auxiliary spillway will be completed with 6 more tainter gates that are positioned at an elevation of 112 meter (367 feet) NAVD88 (USACE Sacramento District,

2016). This auxiliary spillway, slated to open in 2017, will allow for the reservoir to release water more easily, increasing the overall flood fighting storage capacity. This will increase the level of flood protection afforded by Folsom Dam from 100-year protection to 200-year protection

(USACE Sacramento District, 2016). Figure 6 shows the discharge elevation curve for Folsom reservoir both with and without the spillway.

150 145 140 135 130 125 120 115 110

Elevation(NAVD88 meters) 105 100 1000 4000 7000 10000 13000 16000 19000 22000 25000 28000 Outflow (CMS) Without Spillway With Spillway

Figure 6: Flow out of Folsom Dam based on elevation of water, with- and without- spillway

Hydraulic Analysis

For the purposes of modeling hydraulic flow, mass and momentum conservation equations are used. The underlying assumptions of the model will determine the mass and momentum conservation equations are solved. This section will discuss the equations and assumptions used by HEC RAS 5.0 to model 2D flow.

Mass Conservation

The mass conservation equation for an incompressible fluid and unsteady flow in 2D is as follows (USACE Hydrologic Engineering Center, 2016):

휕퐻 휕(ℎ푢) 휕(ℎ푣) + + + 푞 = 0 휕푡 휕푥 휕푦

Where H is the hydraulic head and h is the height of the water, q is the source/sink flux and u and v are the velocities in the x and y direction, respectively. The equation for hydraulic head is given as

퐻 = ℎ + 푧

Where z is the elevation at that point.

Momentum Conservation

The Navier-Stokes equations use momentum conservation to describe the motion of a fluid in three-dimensions (USACE Hydrologic Engineering Center, 2016). The Navier-Stokes equations are simplified to the shallow wave (SW) equations which describe the motion of water in two dimensions. The SW equations assume the fluid is incompressible and has uniform density; that there is uniform hydrostatic pressure; that the vertical scale is much smaller than the horizontal; and that the turbulent motion can be approximated using eddy viscosity (USACE

Hydrologic Engineering Center, 2016). These simplifications imply that the vertical velocity is nearly zero. The vector form of the SW equation is shown in the following equation

휕푉 + 푉 ∙ ∇푉 = −푔∇퐻 + 푣 ∇2푉 − 푐 + 푓푘×푉 휕푡 푡 푓

휕푉 Where + 푉 ∙ ∇푉 is the unsteady flow and convection acceleration term; −푔∇퐻 is the 휕푡

2 barotropic pressure term; 푣푡∇ 푉 is the eddy diffusion term; 푐푓 is the bottom friction term; and

푓푘×푉is the Coriolis term. Here, the k term is the unit vector in the vertical and V is the velocity term.

Due to equatorial bulge that results from the rotation of the earth, latitudinal location can affect the gravitational constant by as much as 0.3% (USACE Hydrologic Engineering Center,

2016). To correct for this change, the Somigliana formula is used:

1 + 푘 sin2 휑 푔 = 푔0 ( ) √1 − 푒2 sin2 휑

2 Where 휑 is the latitude, 푔0 = 9.7803267715 m/s is the gravitational acceleration at the equator and k and e are constants that equal 0.0019318514 and 0.0066943800, respectively.

The Eddy viscosity term takes into account the loss of energy due to turbulent fluid motion and eddies from very small to very long lengths (USACE Hydrologic Engineering Center,

2016). Since the smaller scale phenomenon are too small to be resolved by a discrete numerical model, a gradient diffusion process is used. The diffusion rate is defined as the eddy viscosity, 푣푡.

The eddy viscosity is calculated in HEC RAS using the following equation:

푣푡 = 퐷ℎ푢∗

Where D is a non-dimensional mixing coefficient, h is the head and 푢∗is the shear velocity computed as:

푛√푔 푢∗ = 1 |푉| 푅6

Where n is the Manning’s n value, R is the hydraulic radius, g is the corrected gravitational constant and V is the velocity of the water. The mixing coefficient is based on the surface geometry and the bottom/side surfaces. Table 3 describes the range of values of D

(USACE Hydrologic Engineering Center, 2016).

Table 3: D values based on mixing intensity and geometry

D Mixing Intensity Geometry and Surface

0.11 to 0.26 Little Mixing Straight channel, smooth surface

0.30 to 0.77 Moderate Mixing Gentle meanders, moderate surface irregularities

2.0 to 5.0 Strong Mixing Strong meanders, rough surface

The bottom friction coefficient, 푐푓, is calculated using the Chezy formula (USACE

Hydrologic Engineering Center, 2016). The Chezy formula is approximated using the Manning’s formula as follows:

푛2푔|푉| 푐푓 = 4 푅3

Where n is the Manning’s n value, R is the hydraulic radius, g is the corrected gravitational constant and V is the velocity of the water.

The Coriolis Effect is the horizontal force due to a reference frame on a rotating planet

(USACE Hydrologic Engineering Center, 2016). This force is dependent on the angular velocity of the Earth and the location on the planet. The Coriolis Effect is defined as

푓 = 2휔 sin 휑

Where ω= 0.00007292115855306587/s is the angular velocity of Earth and φ is the latitude.

HEC RAS modeling

For both the mass and momentum conservation equations, the hydraulic head, H, and velocity, V, are unknown. To solve for these variables, HEC RAS 5.0 uses a 2D mesh that is divided into any number of cells. These cells are generally squares of specified width but can also be an irregular polygon up to eight sides. For each cell, a volume-elevation curve and topographic cross sections for each face are calculated based on the underlying terrain (USACE Hydrologic

Engineering Center, 2016). These curves allow for the hydraulic head to be determined quickly and efficiently within the program.

Since the velocity and the hydraulic head are dependent on each other, HEC RAS 5.0 uses an iterative approach to converge on a solution for both values. Using the initial conditions of each cell, HEC RAS 5.0 will iteratively solve for both the hydraulic head and velocity terms using the mass and momentum conservation equations described earlier (USACE Hydrologic

Engineering Center, 2016). This is done for each cell and face at each time step of the model run.

The solution will create smooth water elevation and velocity profiles across the mesh area.

The USACE has developed a hydraulic model of Folsom Reservoir and performed dam breach analyses with it (Shultz, 2016). Unfortunately, both the model and the resulting estimated

21 floodplain require a security clearance to observe and cannot be shared with the public. Currently, no other hydraulic models have been developed to estimate a breach along Folsom Reservoir that are available within the public domain. Thus, the results of this model cannot be compared or verified due to lack of available comparative studies.

Mortality and Property Damage

The two results that are of most concern when considering the effect of a dam break are mortality and property damage. In the case of a Folsom Dam breach, there are approximately 1.5 million people, 600,000 homes, numerous high rises, commercial and industrial centers, and historical buildings in the possible floodplain (United States Census Bureau, 2010). This section will discuss the methods that will be used in this study to estimate the mortality and property damage during a Folsom Dam breach scenario.

Mortality

In the last half century, scientist have tried to develop models to predict mortality due to different types of flooding events including: dam breaches, levee breaches, tsunamis and hurricanes. In each case, empirical data is categorized into similar event types and fit to best estimate the data (Jonkman, et al., 2008). In the case of dam breach, the empirical data is slim due to the few number of dam breach failures in recorded history. Thus, there are many interpretations of these data and many equations that follow. For the purposes of this study, the method proposed by Jonkman et al. (2008) is used.

Jonkman et al. used previous research and dam breach scenarios to develop different mortality, 퐹퐷, curves based on the water depth, h, and velocity, v. The flooded area is broken down into three zones: the breach zone, the rapidly rising zone and the remaining zone. The

22 breach zone is defined as the area in which the maximum velocity of the water is greater than 2 m/s and the maximum depth times the maximum velocity is greater than 7 m2/s (Jonkman, et al.,

2008). In the breach zone, it is assumed that the mortality will be 100%

2 퐹퐷 = 100% 𝑖푓 ℎ푣 ≥ 7 푚 ⁄푠 푎푛푑 푣 ≥ 2 푚⁄푠

This high mortality rate in the breach zone is due to the high velocity and shear stress applied to structures which causes building to collapse and increases loss of life. The rapidly rising zone applies to areas that have a maximum depth greater than 2.1 meter (6.88 feet) and the water rises faster than 0.5 meter (1.64 feet) per hour (0.00014 m/s). The rapidly rising zone produces higher mortality because the water rises quickly enough and is deep enough to make it difficult for people to respond (Jonkman, et al., 2008). The remaining zone is all other areas that are not within the rapidly rising zone or breach zone and thus has much lower mortality. The rapidly rising zone and the remaining zone both use a normal distribution function based on the natural log of the height of water.

ln(ℎ) − 휇 퐹 (ℎ) = Φ ( ) 퐷 푁 휎

Table 4 shows the mean () and standard deviation () for both the rapidly rising and remaining zones as defined by Jonkman et. al. (2008). Since the units for both variables are in the natural log of meters, the 50% mortality would be the exponential value to the power of the mean. For example, in the case of the rapidly rising zone, the 50% mortality would be the exponential function to the power of 1.46 which equals 4.30 meters.

Table 4 Mean and standard deviation of the zones defined by Jonkman et. al. (2008)

Zone Mean Standard Deviation [ln(meters)] [ln(meters)] Rapidly Rising 1.46 0.28 Remaining 7.6 2.75

Property Damage

To predict amount of damage due to flooding, the Federal Emergency Management

Association (FEMA) and other government agencies use the Hydraulic Engineer Center Flood

Damage Reduction Analysis software. This model will calculate expected flood damage for an event by comparing the damage-depth curves to every building in the areas by type and value to produce an accurate economic impact. This process requires detailed information for every building within the flooding area.

To estimate and evaluate the property damage, damage-depth curves will be applied to the resulting floodplain. These curves estimate the percent damage to the structure of a building at different depths by considering the height the building, the type of building, the number of basements in the area and other factors. For the Sacramento region, the Central Valley Flood

Protection Board (CVFPB), created these curves for many types of residential homes and types of businesses (Central Valley Flood Management Planning Program, 2012). For the purposes of this study, the average of all residential damage-depth curves, shown in Figure 7, will be used to evaluate flood damage done to properties.

% Damage % 30

0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Depth (Meters)

Figure 7: Damage-depth curve for residential homes in Sacramento Region

CHAPTER 3

ANALYSIS

Project Datum and Coordinate System

Northing and easting coordinates are based on the Universal Transverse Mercator

(UTM10N) coordinate system in meters and the 1983 North American Datum (NAD83). The elevation reference is in meters and is based on the North American Vertical Datum of 1988

(NAVD88).

Model Parameters

The HEC RAS 5.0 model developed to simulate Folsom Reservoir breaches consisted of a single 2D mesh that covered an area from the Feather River to the North, Modesto to the South,

Folsom Reservoir to the East and Antioch to the West. The mesh covers over 1800 sq. mi. which is the extent of flooding that occurs from Folsom Reservoir breaching during a PMF event. The model also included Folsom Reservoir as a storage area, storage area connections that represent the earthen structures and the dam and a 2D connection at the mouth of the Delta to allow for water to leave the system.

Terrain

The terrain data for the HEC RAS 5.0 model was obtained from the Central Valley

Floodplain Evaluation and Delineation (CVFED) program (CVFED Program, 2014). CVFED was a California State funded program with the task of modeling the floodplain extents throughout the Central Valley. As a part of this, CVFED acquired Light Detection and Ranging

(LiDAR) at 1 point per square meter resolution for the entire Central Valley. This LiDAR was digitized to a digital elevation model (DEM) for use in the project.

Unfortunately, LiDAR does not penetrate the water surface and bathymetric data is not easily accessible. The DEM created from the LiDAR models water surface as terrain. Thus, when modeling flow over the area, water will flow over the surface of water as if it were solid. This introduces some error into the model by decreasing the flow capacity of channels and altering the motion of the water. This error is greatest in areas with large amounts of water and where flood depths are minimal. This is true mostly in shallow waterways like the Delta. Although the Delta is a part of this model, it is far enough downstream that it will minimally impact the study area of this project.

The other aspect of CVFED was to determine Manning’s n values for land use types.

CVFED assigned Manning’s n values to dozens of different land use categories. A team of hydrologic and hydraulic experts produced these Manning’s n values based on literature, experience and professional judgement (CVFED Program, 2014). For the purposes of this study the CVFED Manning’s n values were simplified to 4 broad land use categories of Urban,

Agriculture, Mixed Use and Native Vegetation shown in Table 5.

Table 5: Manning n values based on land use type

Land Use Manning’s n Urban 0.04 Agriculture 0.12 Mixed Use 0.10 Native Vegetation 0.20

The Manning’s n value for Urban areas is based on flow over a smooth impermeable layer such as asphalt or concrete. This Manning’s n value does not consider flow obstructions such as buildings, walls or fences. Although these obstructions may increase the n value, a

27 previous 2D hydraulic model of areas along the American River performed a Manning’s n value calibration which resulted in an optimal Manning’s n value of 0.04 (Kalyanapu, et al., 2013).

This information supports the CVFED findings of an Urban Manning’s n value of 0.04 and thus will be used in this study.

Geographic Information System (GIS) land use shapefiles were acquired from local counties and cities. CVFED Manning’s n values were applied to the GIS land use shapefiles which were combined and imported into the HEC RAS 5.0 model.

Since the motion of water is determined along the face of each cell, break lines were applied at all hydraulically significant structures. Break lines force the mesh to apply a face along the specified line to make sure that flow impeding structures will be recognized in the model.

Break lines were applied to all levees along the American River, Yolo Bypass and much of the

Delta. These were applied as far south as required to no longer affect the results within the greater

Sacramento region.

Boundary Conditions

Two boundary condition locations are applied for outflow and inflow of the model. For the outflow, the boundary condition is applied at the mouth of the Delta near Antioch. This boundary condition outflows into an infinite basin which approximates the ocean. Due to the constant water surface elevation because of the lack of bathymetry data, the outflow could not be modeled as a variable tidal stage boundary. Regardless, because the boundary condition is far enough away, this will have minimal effect on the flooding extents in the Sacramento Region.

For the inflow into the model, the PMF hydrograph into Folsom Reservoir is used. For the purposes of this model, the Yolo bypass is assumed to be empty and the Sacramento River is

28 assumed to have the water stage height of the DEM surface. During a PMF, the Yolo bypass and

Sacramento River would likely have a relatively high stage but the lack of bathymetric data makes this difficult to model properly. Regardless, the amount of water from a Folsom Dam breach would make this error small in the Sacramento Region.

Folsom Reservoir

Folsom Reservoir is modeled as a 1D storage area with the PMF inflow via a lateral inflow hydrograph. The reservoir water surface elevation and volume are related by the curve shown in Figure 8. For every scenario, the starting water surface elevation of the model is the maximum Flood Space volume which occurs at 116 meters (380 feet) NAVD88. This water surface elevation allows for the maximum flood capacity in the reservoir at the beginning of the flood event.

160

140

120

100

Elevation(NAVD88 Meter) 60

40 0 200000 400000 600000 800000 1000000 1200000 1400000 1600000 Volume (1000 m^3)

Figure 8: Folsom Reservoir volume-elevation curve

The reservoir is connected to the 2D mesh area by 3 connections: One at Folsom Dam, one along the northern wing dam and one along the auxiliary dam. Two user-defined elevation- flow curves, shown in Figure 6, were used to model the outflow at Folsom Dam with and without spillway scenarios. These elevation-flow curve scenarios will dictate the timing of the start of the breach. The northern and southern earthen structures were modeled using the weir equation and could both overtop and breach.

Breaches

The locations of the breaches were chosen based on the proximity around the reservoir and the resulting size of the breaches. Because the northern wing dam (northern earthen structure) and the auxiliary dam (southern earthen structure) are the largest structures and are on opposite sides of the reservoir, the resulting breach sizes would be the largest and would produce very different inundation areas. The southern wing dam was not chosen because it would produce similar flooding to the auxiliary dam but to a lesser extent. The dikes are small and would not produce major flooding if failed.

Breach widths, heights and formation times for the northern and southern earthen structures were chosen based on MacDonald et. al. and Von Thun & Gillette as discussed previously in Chapter 2. These methods provide the most extreme cases; with MacDonald et. al. being the largest and slowest breach method and Von Thun & Gillette being the smallest and fastest breach method. Table 6 provides the final calculated breach width, height and formation times used at each location for both breach methods.

Table 6: Breach parameters for each location and breaching method

North Earthen structure South Earthen structure MacDonald Von Thun MacDonald Von Thun & et. al. & Gillette et. al. Gillette Width (m) 929 114 1194 101 Height (m) 14.3 14.3 23.3 23.3 Formation 4.41 0.83 4.14 0.72 Time (hrs)

For each case, the breach began when the water surface elevation of the reservoir was just overtopping the earthen structure. In both cases, this happened at an approximate elevation of

146.3 meters (480 feet) NAVD88.

Scenarios

In all, there are 3 variables, creating 8 scenarios in total that will be analyzed for this study as presented in Table 7. The variables are dam outflows, breach locations and breach methods. The dam outflows will dictate the timing of the breach; the breach location will dictate the location of the flooding; and the breach method will determine the flooding extents and depths in those regions.

Table 7: Organization of scenarios based on variable

Dam Breach Breach method Outflow Location North MacDonald et. al. Earthen With structures Von Thun & Gillette Spillway South MacDonald et. al. Earthen structures Von Thun & Gillette North MacDonald et. al. Earthen Without structures Von Thun & Gillette Spillway South MacDonald et. al. Earthen structures Von Thun & Gillette

CHAPTER 4

RESULTS AND DISCUSSION

This section will present the flood depth result of the HEC RAS 5.0 Folsom Reservoir breach model for each scenario. Only the worst-case scenarios will be used to analyze the mortality and property damage. All the resulting floodplains are presented in Appendix A.

Breach Timing

Since the water surface elevation that triggers the breach and the inflow into the reservoir are identical for each scenario, the dam outflow is the only variable that controls the timing of the start of the breach. If the maximum inflow is less than the maximum outflow through the control structures, the breach will not be triggered. Table 8 shows the maximum inflow (PMF) into the

Folsom Reservoir and the maximum outflow from all control structures for the with and without spillway scenarios. Since the maximum inflow into the reservoir is less than the maximum outflow through the dam and spillway, the breach is never triggered for the with spillway scenarios.

Table 8 Peak inflows and outflows of the Folsom Reservoir

Peak Flows [cms (cfs)] Inflow Outflow Outflow Probable Maximum Without Spillway With Spillway Flood 17,500 (618,000) 28,000 (988,800) 25,600 (904,000)

Although the with spillway scenario has enough outflow to keep the reservoir from overtopping any of the earthen structures, the outflow from spillway and the dam are enough to overtop levees and cause flooding in the Sacramento region. Figure 9 shows the modeled

33 floodplain for the with spillway scenarios. Since all the water flows through the dam and the spillway, this produces high flooding depths along the American River of greater than 4 meters

(13.12 feet). Floodwater overtops levees along the north and south banks of the American River and flows to the lowest elevation regions. Along the north side of the American River, the

Natomas region experiences flooding depths of greater than 4 meters (13.12 feet). On the south side of the American River, the Pocket area and portions of downtown Sacramento are flooded to depths of greater than 4 meters (13.12 feet).

Although East Sacramento, Rosemont, Florin and other areas to southeast of Downtown do not experience flooding at depths of greater than 4 meters (13.12 feet), major flooding still occurs. These areas experience flooding depths between 1 meter (3.28 feet) and 3 meters (9.84 feet) which still has the potential to cause major damage and life loss. In contrast, West

Sacramento experience very shallow flooding of less than 1 meter (3.28 feet). This is due to the

Sacramento River eastern levees retaining the floodwater on the landside which alleviates the pressure on the western levees.

Figure 9 Worst-case with-spillway scenario

Breach Method

Since the with spillway scenarios do not breach, the breach method is only of concern for the without spillway scenarios. Figure 10 shows the breach hydrographs through the northern and southern earthen structure breaches for both the MacDonald et. al. and Von Thun & Gillette methods and Table 9 summarizes the information derived from the breach hydrographs.

Table 9: Peak outflow and total volume discharged for both breach locations and methods

Northern Earthen Southern Earthen Inflow structure structure PMF MacDonald Von Thun MacDonald Von Thun et. al. & Gillette et. al. & Gillette Peak Flow 25,600 85,000 24,000 72,000 15,000 [Cubic Meters per Second(cfs)] (904,000) (3,001,746) (847,522) (2,542,656) (529,720) Total Volume Discharged 4.129 2.633 2.088 2.408 1.621 [billion cubic meters (acre- feet)] (3,347,439) (2,135,036) (1,693,134) (1,952,277) (1,314,638)

100000 90000 80000 70000 60000 50000

40000 Flow Flow (m^3/s) 30000 20000 10000 0 0 20 40 60 80 100 120 140 160 Time from Beginning of PMF (hrs)

North Breach- MacDonald North Breach- Von Thun & Gillette South Breach- MacDonald South Breach- Von Thun & Gillette

Figure 10: Breach hydrographs for each scenario

For both the northern and southern earthen structure breaches, the MacDonald et. al. method produces the greatest peak flow and total volume discharged. The floodplains associated with the northern and southern earthen structure breaches, shown in Figure 11 and Figure 12, respectively, are the worst-case scenarios for each breach location.

Both scenarios produce deep flooding of 4 meters (13.12 feet) or more along the

American River, in the Natomas Area, in the Pocket Area and in Downtown Sacramento. The northern breach diverts a portion of the flow away from the American River and into Arcade,

Linda and Dry creeks. This produces flooding depths of greater than 4 meters (13.12 feet) along the creeks and increases the extent of flooding in the Natomas regions. The southern breach produces flooding depths of greater than 4 meters (13.12 feet) in Folsom and increases the floodplain South of the American River.

Figure 11: Without spillway, worst-case modeled breach scenario for the northern earthen structures using

MacDonald et. al. breach method

Figure 12: Without spillway, worst-case modeled breach scenario for the southern earthen structures using

MacDonald et. al. breach method

Mortality

The mortality rate equations discussed in Chapter 2 were applied to the estimated maximum depths and velocities of the northern and southern earthen structure worst-case breach scenarios. Because the mortality rate equations are a normal distribution function based on depth, depths of less than 2.4 meters (7.2 feet) produce low mortality rates of less than 20%, depths of

4.3 meters (14.1 feet) produce moderate mortality rates of 50% and depths greater than 5 meters

(16.4 feet) produce high mortality rates greater than 80%. The mortality rate for the with spillway scenario is shown in Figure 13 and the without spillway worst-case scenarios are shown in Figure

14 and Figure 15.

For the with spillway scenario, there is a predicted high mortality rate along the

American River and in the Pocket and Natomas regions. The high mortality along the American

River is due to a combination of the high flow velocities and great depths while the Pocket and

Natomas regions areas have high mortality due only to the extremely deep flooding. Outside of the high mortality regions, the mortality rate drops quickly to less than 20% due to a decrease in flooding depth and the nature of the normal distribution function.

For the without spillway scenarios, both figures show a higher mortality rate along the

American River and in the Pocket and Natomas regions than the with spillway scenario. The breaches cause an increased amount of volume to escape from Folsom Reservoir and pond in the low-lying regions. The northern breach scenario creates high mortality rates on the north side of the American River through Citrus Heights because of the deep flooding along Linda Creek and

Dry Creek. The southern breach scenario increases the mortality rate along the south side of the

American River in areas such as Rosemont and Folsom.

Figure 13 With spillway worst-case scenario mortality

Figure 14: Without spillway worst-case scenario mortality from the northern earthen structure breach

Figure 15: Without spillway, worst-case scenario mortality from the southern earthen structure breach

Property Damage

Much like the mortality, property damage is also a function of depth and thus the percentage of damage done to any given single family residence follows closely to the mortality rate. Figure 16 show the percentage of damage done to single family residences for the with spillway scenario while Figure 17 and Figure 18 show the percentage of damage done to single family residences for the northern and southern breach scenarios, respectively.

For all the scenarios, homes along the American River and in the Pocket and Natomas areas are estimated to have damages greater than 80%. The northern and southern breach scenarios produce much greater flooding in these regions than the with spillway scenario due to increased outflows from their respective breaches. The northern breach scenario predicts high damage to residential homes on the north side of the American River through Citrus Heights because of the deep flooding along Linda Creek and Dry Creek. The southern breach scenario predicts high damage along the south side of the American River in areas such as Rosemont and

Folsom.

The areas with the greatest percent damage to homes correspond directly to the mortality rate. Where the mortality rate and percent damage to property diverge is in area with shallower depths. The property damage functions produced by CVFPB estimate a 50% property damage at

2 meters (6.56 feet) of depth which is much lower than the corresponding 50% mortality rate.

Since both breach scenarios produce more than 2 meters of flooding depth within the flooding area, most of the inundated homes would be at least 50% destroyed.

Figure 16: With spillway, worst-case scenario percent damage done to residential areas

Figure 17: Worst-case scenario percent damage done to residential areas from northern earthen structure

breach

Figure 18: Worst-case scenario percent damage done to residential areas from southern earthen structure

breach

CHAPTER 5

CONCLUSION AND RECOMMENDATIONS

The results, discussed in Chapter 4, indicate that all scenarios modeled during a PMF, regardless of breach, result in catastrophic flooding for much of the Sacramento Region. All the floodplain maps, presented in Appendix A, show deep flood of greater than 4 meters (13.12 feet) along the American River and in the Pocket and Natomas areas. Because the scenarios with the spillway did not breach, the flooding extent was limited as compared to the scenarios that did breach. The MacDonald et. al. breach method, which produced the worst-case scenarios for both the northern and southern earthen embankments, creates even more extensive flooding in these deep areas and extends further into Downtown Sacramento. Both the northern and southern breach scenarios create unique flooding; the northern breach scenario estimates deep flooding miles north of the American River along Linda and Dry creeks and the southern breach scenario estimates deep flooding in parts of Folsom.

Since both the mortality and property damage are functions of depth, areas with high mortality also have high property damage. The areas discussed with the greatest amount of flooding also result in very high mortality and residential property damage of greater than 80%.

To be clear, this does not mean that more than 80% of the people who live in these areas will perish in a flood. Mortality indicates the likelihood of death for people in the area at the time of the flooding. With proper evacuation time and methods, most people could be evacuated from the region which would greatly decrease the number of people exposed to the flood.

For the purposes of developing evacuation methods and procedures, these results can be used guidelines in the case of a PMF with the spillway operation or an overtopping breach along

48 the northern or southern earthen structures. Although these results can be used as a guideline, they should not be taken as absolute fact. Many variables and unknowns are not or cannot be accurately predicted such as: breach locations, breach extents, breach timing, Folsom Reservoir inflow, debris, erosion, local flood barriers and much more. Regardless of this uncertainty, the results can still be used as an approximate flood inundation map for emergency purposes.

Evacuating areas along the American River, the Pocket area, the Natomas area and

Downtown Sacramento should be considered high priority for saving the most lives. To prepare for the uncertainty of which earthen structure will breach, areas in Folsom near the reservoir and areas along natural creeks such as Dry Creek, Linda Creek and Arcade Creek should also be evacuated. Other areas that have predicted flooding, should also be evacuated to high ground.

For this model to be used for official purposes, improvements and further research is required. It is recommended that future studies include the bathymetry of the American and

Sacramento Rivers and improve on the number and location of the break lines in the model to improve the overall accuracy. Further studies should also consider flows down the Sacramento

River and Yolo Bypass which could increase the amount of flooding due to decreased storage capacity in the channels. More failure scenarios should also be considered including normal pool failures and breaches on different structure around the reservoir. These improvements to the model and further studies would help emergency response in the case of various Folsom Dam breach scenarios.

APPENDIX A

FLOODPLAIN MAPS

North breach without spillway using MacDonald breach method

North breach without spillway using Von Thun & Gillette breach method

North breach with spillway using MacDonald breach method

North breach with spillway using Von Thun & Gillette breach method

South breach without spillway using MacDonald breach method

South breach without spillway using Von Thun & Gillette breach method

South breach with spillway using MacDonald breach method

South breach with spillway using Von Thun & Gillette breach method

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