Appendix F Preliminary Flood Hazard Assessments

Transcription

1 Appendix F Preliminary Flood Hazard Assessments

2 Final Preliminary Flood Hazard Assessment, Regulus Solar Project

3 REGULUS SOLAR PROJECT Kern County, California Preliminary Flood Hazard Assessment FINAL Prepared for FRV Regulus Solar, LP 44 Montgomery Street, Suite 2200 San Francisco, CA Prepared by RBF Consulting Alton Parkway Irvine, CA In conjunction with JE Fuller/Hydrology & Geomorphology, Inc South Kyrene Road, Suite 201 Tempe, AZ September 23, 2011 RBF JN

4 Regulus Solar Project H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc i

5 Regulus Solar Project Table of Contents 1 INTRODUCTION Objectives Hydrology and flood control design standards Watershed description Flood studies and floodplain mapping PRECIPITATION AND FLOOD FREQUENCY DATA Precipitation frequency-duration relationships Flood frequency relationships GEOMORPHIC SUMMARY Drainage and geomorphic features Flood processes Stable and unstable surfaces FLOOD ROUTING ANALYSES Background FLO-2D model development Grid system Precipitation Hydraulic roughness Hydraulic structures Boundary conditions Infiltration characteristics FLO-2D model simulation Conclusions ADDITIONAL CONSTRAINTS Other flood condition constraints General drainage design guidelines...32 REFERENCES H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc ii

6 Regulus Solar Project Tables Table 2-1. Watershed-average frequency-duration maximum precipitation depths...8 Table 4-1. Infiltration characteristics*...15 Table percent annual chance baseline volume accounting summary...16 Table percent annual chance baseline onsite flood depths...16 Table percent annual chance baseline onsite flood velocities...16 Table percent annual chance project onsite flood depths...17 Table percent annual chance project onsite flood velocities...17 Table percent annual chance cross-sectional peak flows and volumes...30 Table percent annual chance cross-sectional peak flows and volumes...30 Figures Figure 1-1. Site vicinity map...2 Figure 1-2. Site regional drainage map...3 Figure 1-3. Site local drainage map...4 Figure 1-4. Site aerial photograph...5 Figure 1-5. FEMA floodplain map...7 Figure 3-1. Preliminary map of drainage features and landform surfaces...10 Figure 3-2. NRCS soils map...11 Figure 3-3. Geologic map...12 Figure 4-1. Regional map of 1-percent annual chance baseline flood depths...18 Figure 4-2. Regional map of 1-percent annual chance baseline flood velocities...19 Figure 4-3. Local map of 1-percent annual chance baseline flood depths...20 Figure 4-4. Local map of 1-percent annual chance baseline flood velocities...21 Figure 4-5. Local map of 1-percent annual chance project flood depths...22 Figure 4-6. Local map of 1-percent annual chance project flood velocities...23 Figure percent annual chance peak flow distribution along XS Figure percent annual chance peak flow distribution along XS Figure percent annual chance peak flow distribution along XS Figure percent annual chance peak flow distribution along XS Figure percent annual chance peak flow distribution along XS Figure percent annual chance peak flow distribution along XS H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc iii

7 Regulus Solar Project 1 INTRODUCTION An assessment was conducted to evaluate the flood hazard environment of the area being considered for siting the location of the proposed Regulus Solar Project (Regulus Site) located in Sections 22 and 27, Township 30S, Range 29E, in unincorporated Kern County, California. The location of the Regulus Site is shown in Figures 1-1 and Objectives The primary objectives of this assessment include the following: Identify (1) drainage features, (2) flood processes, (3) stable and unstable surfaces, and (4) flood conditions that would constrain the future development of the Regulus Site Quantify project-related impacts specific to increases in flood distribution, runoff yield, and flow rates for the 85 th -percentile and 1-percent annual chance storm events. Determine the project-related impacts to the flood hazard environment. 1.2 Hydrology and flood control design standards The following publications related to hydrology, hydraulics, and sedimentation serve as the standard in Kern County: Kern County Hydrology Manual, Department of Planning and Development Services T.V. Hromadka II, California State University, Fullerton, 1992 Kern County Code of Building Regulations, Chapter 17.48, Floodplain Management Kern County Drainage Plan Check Compliance List 1.3 Watershed description The physical drainage area tributary to the Regulus Site is about 4.9 square miles based on U.S. Geological Survey (USGS) topographic data (Figure 1-3); however, the effective tributary drainage area may be much less as a result of several factors, which include the elongated nature of the drainage and the absence of a stream network. These factors serve to potentially reduce the area contributing to the onsite flood hazard. The Regulus Site, including interior areas that are not a part of this project (canal easement), encompasses approximately 778 acres. The canal channel section (as opposed to the canal easement) and the graded ponds located in the interior of the Regulus Site account for more than 12 acres. The Regulus Site, excluding interior areas that are not a part of the project is approximately 743 acres. 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 1

8 Figure 1-1. Site vicinity map Regulus Solar Project 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 2

9 Regulus Solar Project Figure 1-2. Site regional drainage map Source: USGS 24k H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 3

10 Figure 1-3. Site local drainage map Regulus Solar Project Source: USGS 24k 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 4

11 Figure 1-4. Site aerial photograph Regulus Solar Project 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 5

12 Regulus Solar Project 1.4 Flood studies and floodplain mapping Based on Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) Panel Number 06029C3150E, effective September 26, 2008, the Regulus Site is mapped as Zone X (Figure 1-5). Zone X is the flood insurance rate zone that corresponds to areas outside the 0.2- percent annual chance floodplain, areas within the 0.2-percent annual chance floodplain, and to areas of 1-percent annual chance flooding where the contributing drainage area is less than one square mile, and areas protected from the 1-percent annual chance flood by levees. No base flood elevations or depths are shown within this zone. 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 6

13 Figure 1-5. FEMA floodplain map Regulus Solar Project Source: National Flood Hazard Layer GIS 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 7

14 Regulus Solar Project 2 PRECIPITATION AND FLOOD FREQUENCY DATA 2.1 Precipitation frequency-duration relationships The synthetic 24-hour storm pattern adopted in the Kern County Hydrology Manual (KCHM) was developed using precipitation depth-duration-frequency spatial data from National Oceanic and Atmospheric Administration (NOAA) Atlas 14 (NWS, 2011). Specifically, the 1-percent annual chance as well as the annual watershed average maximum point precipitation depths for 5- and 30-minute, 1-, 3-, 6-, and 24-hour durations were determined and subsequently reduced based on the depth-areal reduction (DAR) relationships, which are dependent on storm duration and watershed size (KCHM Figure E-4), to account for the variability of the hydrologic processes generally experienced in larger watersheds (Table 2-1). The reduced values were used to develop the KCHM synthetic 24-hour storm pattern based on the format shown in KCHM Figure E-5. The 85th percentile 24-hour event was taken as 50 percent of the annual 24-hour event (0.47 inches). The synthetic 24-hour storm pattern for the 85 th percentile 24-hour event was based on the annual event precipitation depths. Table 2-1. Watershed-average frequency-duration maximum precipitation depths *DAR factors are based on 4.9 square miles 2.2 Flood frequency relationships No flood frequency relationships were identified or developed as part of this assessment. 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 8

15 Regulus Solar Project 3 GEOMORPHIC SUMMARY A field investigation of the Regulus Site and its tributary drainage was conducted on March 13, 2010, including a review of available topographic, aerial photographic, geologic, and soil information to identify drainage and geomorphic features, flood processes, stable and unstable surfaces, for the purpose of determining potential flood-related development constraints. 3.1 Drainage and geomorphic features The Regulus Site lies within California s Central Valley, a broad north-south trending alluvial plain that is almost completely covered by irrigated agricultural land. The Regulus Site is bounded on all sides by rectilinear agricultural parcels that have been mass graded to facilitate agricultural irrigation. Any remnants of the predevelopment natural drainage pattern are completely obscured by agricultural use, both within the Regulus Site boundaries and on surrounding parcels. An extensive system of canals, laterals, drains, and roads limit the potential for offsite runoff to impact the Regulus Site. There is little evidence that surface runoff traverses individual field boundaries past the berms and drains that bound the adjacent agricultural areas. Instead, storm water that does not infiltrate into the ground is most likely ponded onsite, is conveyed into the numerous drains, or collects along the roadways that parallel the field boundaries (Figures 1-3 and 1-4). Ponding will likely occur along the upstream face of the canal berms during major storms. 3.2 Flood processes The Regulus Site is subject to agricultural sheet flooding (Figure 3-1) that flows at low slopes toward the southwest. Sheet flooding consists of shallow unconcentrated flow, generally less than one foot deep (during extreme, rare events) that covers broad land areas rather than flowing along defined channels. It is unlikely that flooding within the Regulus Site boundaries is a significant concern for future development. 3.3 Stable and unstable surfaces The Regulus Site is underlain by sandy loam and loamy sand soils (Figure 3-2) with no areas of bedrock outcrop in the vicinity of the Regulus Site (Figure 3-3). No flood-related unstable surfaces were observed at the Regulus Site. 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 9

16 Regulus Solar Project Figure 3-1. Preliminary map of drainage features and landform surfaces 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 10

17 Figure 3-2. NRCS soils map Regulus Solar Project Onsite map units: 138 (Delano sandy loam, 0 to 2 percent slopes), 159 (Hesperia sandy loam, 0 to 2 percent slopes), and 246 (Whitewolf coarse sandy loam) 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 11

18 Figure 3-3. Geologic map Regulus Solar Project Onsite map units: Q (Quaternary alluvium) Source: California Geological Survey 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 12

19 Regulus Solar Project 4 FLOOD ROUTING ANALYSES 4.1 Background A review of available topographic and aerial photographic data indicates the floodwaters in the vicinity of the Regulus Site are generally unconfined. Therefore, a two-dimensional flood routing model was developed to evaluate the flood hazard environment using the computer application FLO-2D (O Brien, 2007), supported by several elements from the Kern County hydrologic standards, in pursuit of a reasonable estimate of the Baseline (without Project) and Project Conditions 85 th -percentile and 1-percent annual chance runoff characteristics. FLO-2D is currently a FEMA-accepted two-dimensional hydraulic model. 4.2 FLO-2D model development Grid system The FLO-2D grid system representing the tributary drainage (watershed) and fringe areas was subdivided into two model domains: (1) the offsite tributary drainage area (4.64 square miles), identified as the upper model (U100), was defined using 100 x 100 grid elements and excludes the areas on and immediately surrounding the Regulus Site, and (2) the local drainage (2.01 square miles) identified as the lower or local model (L025), was defined using 25 x 25 grid elements and includes areas on and immediately surrounding the Regulus Site, which were the areas specifically excluded from the upper model (U100). For the upper model (U100), the 10-meter USGS Digital Elevation Model (DEM) was used to interpolate the average elevation for each grid element. For the lower or local model (L025), onefoot contours compiled from recently flown aerial topography were used to interpolate the average elevation for each grid element to improve the recognition of local natural and anthropogenic features, which may influence flood conveyance. Project Conditions. A limited amount of grading is assumed for the purpose of addressing slope requirements associated with the design of the solar array. It is also assumed that this limited grading will not significantly alter the historical flood patterns within and adjacent to the Regulus Site and therefore, the grid element elevations determined for the Baseline Conditions were assumed to be the same for the Project Conditions Precipitation The KCHM synthetic 24-hour storm pattern was used in conjunction with the watershed-average maximum frequency precipitation depths from NOAA Atlas 14 (NWS, 2011), reduced based on the DAR factors (Table 2-1), to develop the required synthetic 24-hour storm patterns Hydraulic roughness The flood-wave progression was controlled by limiting the Froude number to a maximum value of 0.95, thereby precluding the occurrence of supercritical flow, which is not expected to occur under natural conditions. A general roughness coefficient of was assumed to represent the overland flow resistance. For shallow flow depths, the roughness coefficient typically ranges between and A roughness coefficient of was assumed for shallow flow conditions to limit the resistance during shallow flooding. 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 13

20 4.2.4 Hydraulic structures Regulus Solar Project The canal, which cuts through the far west portion of the Regulus Site, will likely cause ponding of the redirection of floodwaters along the upstream face of the canal berm. However, the canal and associated berms were not specifically defined. Instead, their influence was captured using a reduced grid cell size (25 x 25 ) in conjunction with grid cell elevations interpolated from 1 contours based on recent aerial topography. No other identifiable major hydraulic facilities or structures are located within the tributary drainage. Any influence related to anthropogenic features or disturbances such as transportationand utility-related alignments located within the modeled drainage were represented based on the level of detail captured by the 10-meter USGS DEM (U100 model) or the 1 contours developed from recently flown aerial topography (L025 model) Boundary conditions The outflow boundary conditions from the upper model were used to define the inflow boundary conditions of the lower model. The inflow grid elements from the lower or local model (L025) were overlapped with the outflow grid elements from the upper model (U100) along the shared boundary to facilitate the transfer of flow between the two models Infiltration characteristics The Green-Ampt infiltration relationships were used to account for precipitation losses in lieu of the County standard because of its direct relationship with physical soil properties, which can be easily correlated to changes in soil compaction (Saxton and Rawls, 2006). The physical soil parameters, which form the relationship for determining infiltration, are saturated hydraulic conductivity (XKSAT), wetting front capillary suction (PSIF), and volumetric soil moisture deficit (DTHETA). These parameters were estimated by relating the soil composition of the watershed based on National Resources Conservation Service (NRCS) soils mapping to average infiltration characteristics associated with soil texture classes for bare ground conditions (Rawls et al., 1983; Rawls and Brakensiek, 1983) assuming antecedent moisture conditions are near field capacity, which is consistent with the conditions immediately following a significant precipitation event. Each NRCS soil map unit is characterized by descriptive and numerical information such as (1) a representative profile, (2) engineering and physical properties, and (3) formation, morphology, and classification. This information was used in part to form the correlation between the soil composition and average infiltration characteristics. The Green-Ampt infiltration characteristics (Table 4-1) were determined based on the most restrictive soil layer with respect to infiltration and assuming the average infiltration characteristics associated with soil texture classes for bare ground conditions are representative of the watershed and fringe areas, regardless of land use, and are also aligned with the Baseline Conditions (without Project) soil compaction of 75 percent (assumed average value for natural and agricultural conditions). Project Conditions. The average soil compaction on the Regulus Site is assumed to increase to no more than 85 percent as a consequence of its intended development. To determine the infiltration characteristics for Project Conditions, the average infiltration characteristics associated with soil texture classes for bare ground conditions (Rawls et al., 1983; Rawls and Brakensiek, 1983) were adjusted to account for the increase in soil compaction (from 75 to 85 percent) and subsequent reduction in infiltration using the relationships developed by Saxton and Rawls (2006). 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 14

21 Regulus Solar Project The spatial land use assumptions based on the Kern County General Plan were intersected with the spatial soil information to estimate the spatial variation of effective imperviousness (RTIMP) for the watershed. The initial abstraction was assumed constant throughout the watershed at 0.15 inches implemented by assuming the threshold for flood routing (TOL) is equal to feet. Typical values for initial abstraction include 0.35 inches for flat-sloped desert and rangeland, 0.15 inches for Sonoran Desert hill slopes, 0.25 inches for mountains with vegetated surfaces, 0.20 inches for residential/commercial lawn and turf, 0.05 inches for pavement, and 0.50 inches for tilled fields and irrigated pasture. Table 4-1. Infiltration characteristics* *A soil compaction of 75 percent was assumed onsite for Baseline Conditions; and a soil compaction of 85 percent was assumed onsite for Project Conditions 4.3 FLO-2D model simulation The developed FLO-2D upper (U100) and lower (L025) models were analyzed for a simulation period of 36 hours. A maximum value of 0.25 was assigned to the numerical stability coefficient, which directly controls the maximum time step for full dynamic wave routing. Volume conservation was confirmed at each 0.1-hour time interval over the entire duration of the simulation. A summary of the volume accounting for the upper and lower model simulations for the 1-percent annual chance event only in Table 4-2, which indicates the rainfall and inflow volume balances with the combined volume from infiltration, storage, and outflow for each model and set of conditions. A breakdown of the 1-percent annual chance flood depths and velocities for each set of conditions is shown in Tables 4-3 through 4-6. Flood inundation maps showing the spatial variation of maximum flood depths and velocities throughout the watershed and Regulus Site are presented for the 1-percent annual chance event in Figures 4-1 (regional depths; baseline conditions), 4-2 (regional velocities; baseline conditions), 4-3 (local depths; baseline conditions), 4-4 (local velocities; baseline conditions), 4-5 (local depths; project conditions), and 4-6 (local velocities; project conditions). The maps depicting the local 50-percent annual chance event flood depths and velocities are presented as part the water quality assessment under separate cover (the 85 th -percentile event did not exhibit any project-related impacts). The 1-percent annual chance peak flow distribution along a cross section defined for each of the six outfall boundaries. These peak flow distributions are presented in Figures 4-7 through The cross sections (identified as XS-1 through XS-6) are depicted on each of the local maps presented in Figures 4-3 through 4-6. Outfall peak flows and runoff volumes are compared in Tables 4-7 (50-percent annual chance event) and 4-8 (1-percent annual chance event). 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 15

22 Regulus Solar Project Table percent annual chance baseline volume accounting summary Table percent annual chance baseline onsite flood depths Table percent annual chance baseline onsite flood velocities 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 16

23 Regulus Solar Project Table percent annual chance project onsite flood depths Table percent annual chance project onsite flood velocities 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 17

24 Regulus Solar Project Figure 4-1. Regional map of 1-percent annual chance baseline flood depths H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 18

25 Regulus Solar Project Figure 4-2. Regional map of 1-percent annual chance baseline flood velocities H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 19

26 Regulus Solar Project Figure 4-3. Local map of 1-percent annual chance baseline flood depths 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 20

27 Regulus Solar Project Figure 4-4. Local map of 1-percent annual chance baseline flood velocities 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 21

28 Regulus Solar Project Figure 4-5. Local map of 1-percent annual chance project flood depths 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 22

29 Regulus Solar Project Figure 4-6. Local map of 1-percent annual chance project flood velocities 23 September 2011 H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 23

30 Regulus Solar Project Figure percent annual chance peak flow distribution along XS Baseline Conditions Project Conditions Project-related changes Qp (cfs) Q p (cfs) Distance along XS-1 from south to north (feet) H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 24

31 Regulus Solar Project Figure percent annual chance peak flow distribution along XS Baseline Conditions Project Conditions Project-related changes Q p (cfs) Qp (cfs) Distance along XS-2 from north to south (feet) H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 25

32 Regulus Solar Project Figure percent annual chance peak flow distribution along XS Baseline Conditions Project Conditions Project-related changes Q p (cfs) Qp (cfs) Distance along XS-3 from north to south (feet) H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 26

33 Regulus Solar Project Figure percent annual chance peak flow distribution along XS Baseline Conditions Project Conditions Project-related changes Q p (cfs) Qp (cfs) Distance along XS-4 from east to west (feet) H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 27

34 Regulus Solar Project Figure percent annual chance peak flow distribution along XS Baseline Conditions Project Conditions Project-related changes Q p (cfs) Qp (cfs) Distance along XS-5 from east to west (feet) H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 28

35 Regulus Solar Project Figure percent annual chance peak flow distribution along XS Baseline Conditions Project Conditions Project-related changes Q p (cfs) Qp (cfs) Distance along XS-6 from east to west (feet) H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 29

36 Regulus Solar Project Table percent annual chance cross-sectional peak flows and volumes Table percent annual chance cross-sectional peak flows and volumes 4.4 Conclusions For the Project Conditions, the results demonstrated a general increase in the distributed flows and runoff volume across the affected outfalls, however, it is clear that the historic flood patterns (Figures 4-3 and 4-4) are preserved as shown in Figures 4-5 and 4-6. The canal and associated berms act to obstruct the conveyance of floodwaters, which leads to ponding along the upstream face of the canal berm during major storms, particularly on the north side of Panama Lane where the maximum flood depths occur onsite. Floodwaters are collected in the canal at locations where ponding exceeds the berm crest or where the crest of the canal embankment is at grade. For the 1- percent annual chance event, about 95 percent of the floodwaters that cross XS-2 (west property line to the south), exit the Regulus Site via the canal and almost all of the floodwaters that cross H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 30

37 Regulus Solar Project XS-6 (south property line to the west), exit the Regulus Site adjacent to the upstream face of the canal berm. The increase in 1-percent annual chance distributed flows along cross section XS-1, XS-3, XS-4, and XS-5 appear minimal. The 1-percent annual chance flood velocities encountered on the Regulus Site are less than 0.5 feet per second on average with a maximum of less than 5 feet per second, are generally associated with sheet or overland flooding, and do not present an erosion hazard for either set of project conditions, although localized erosion may occur in locations where floodwaters are altered and concentrated. More than 98 percent of the Regulus Site (excluding the canal and graded ponds) is subject to flood depths of less than 0.5 feet. The 50-percent annual chance event was used to determine project-related water quality impacts (Table 4-7) in lieu of the 85 th percentile event because the latter did not produce any impacts, which is consistent with the current language of the Statewide Construction General Permit. H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 31

38 Regulus Solar Project 5 ADDITIONAL CONSTRAINTS 5.1 Other flood condition constraints There are no other significant or unusual flood constraints that would preclude normal site development. 5.2 General drainage design guidelines If site development does not include habitable structures, more flexibility with regard to flood protection may exist, subject to the discretion of regulators and the absence of project-related adverse impacts to adjacent and/or downstream properties and structures. The canopy produced by the arrangement of solar power panels installed on the Regulus Site may have a tendency to concentrate runoff along a line below the bottom edge of each panel, which can lead to some measure of soil erosion under certain ground conditions, the significance likely influenced by the panel orientation and the topographic characteristics of the land below. The grading of access roads can lead to the concentration of runoff and subsequently result in localized erosion along and across these roads, and in some cases, trigger the formation of flowcut watercourses downstream. Local scour can occur around installed obstructions, e.g., panel supports and buildings, if exposed to concentrated runoff. Changes to the soil compaction conditions can influence runoff development. An increase in soil compaction can increase runoff development for a specific flood event; therefore, limiting increases in compaction will likely limit or preclude the necessity of mitigation associated with increased runoff volume impacts. Onsite grading should not significantly alter historic flood patterns to avoid increasing flood hazard impacts to adjacent and downstream properties. H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 32

39 Regulus Solar Project REFERENCES FEMA, 2008, Flood Insurance Study, Kern County, California, and Incorporated Areas, 06029CV001A, Federal Emergency Management Agency, Effective September 26, FEMA, 2003, Guidelines and Specifications for Flood Hazard Mapping Partners, Appendix G: Guidance for Alluvial Fan Flooding Analyses and Mapping, Federal Emergency Management Agency, April. NOAA, 2011, NOAA Atlas 14 Precipitation-Frequency Atlas of the United States, Volume 6: California, Version 2, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Silver Springs, MD, June. NRCS, 2008, Soil Survey Geographic (SSURGO) database for Kern County, Northwestern Part, California, CA666, Natural Resources Conservation Service, Fort Worth, Texas. NRCS, 2008, Soil Survey Geographic (SSURGO) database for Kern County, Northeastern Part and Southeastern Part of Tulare County, California, CA668, Natural Resources Conservation Service, Fort Worth, Texas. NRCS, 2008, Soil Survey Geographic (SSURGO) database for Kern County, Southwestern Part, California, CA691, Natural Resources Conservation Service, Fort Worth, Texas. O Brien, J.S., FLO-2D Computer Program and User Manual, Version , Nutrioso, AZ. Rawls, W.J. and D.L Brakensiek, 1983, A procedure to predict Green and Ampt infiltration parameters, Proceedings of the American Society of Agricultural Engineers Conference on Advances in Infiltration, Chicago, Illinois, Pages Rawls, W.J., D.L. Brakensiek, and N. Miller, 1983, Green-Ampt infiltration parameters from soils data, ASCE, Journal of Hydraulic Engineering, Volume 109, Number 1, Pages Saxton, K.E. and W.J. Rawls, 2006, Soil water characteristic estimates by texture and organic matter for hydrologic solutions, Soil Science Society of America Journal, Volume 70, Madison, Wisconsin, September/October. H:\pdata\ \Admin\reports\Regulus_PFHA_final_ doc 33

40 Final Preliminary Flood Hazard Assessment, Adobe Solar Project

41 ADOBE SOLAR PROJECT Kern County, California Preliminary Flood Hazard Assessment FINAL Prepared for FRV Adobe Solar, LP 44 Montgomery Street, Suite 2200 San Francisco, CA Prepared by RBF Consulting Alton Parkway Irvine, CA In conjunction with JE Fuller/Hydrology & Geomorphology, Inc South Kyrene Road, Suite 201 Tempe, AZ September 23, 2011 RBF JN

42 Adobe Solar Project H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc i

43 Adobe Solar Project Table of Contents 1 INTRODUCTION Objectives Hydrology and flood control design standards Watershed description Flood studies and floodplain mapping PRECIPITATION AND FLOOD FREQUENCY DATA Precipitation frequency-duration relationships Flood frequency relationships GEOMORPHIC SUMMARY Drainage and geomorphic features Flood processes Stable and unstable surfaces FLOOD ROUTING ANALYSES Background FLO-2D model development Grid system Precipitation Hydraulic roughness Hydraulic structures Boundary conditions Infiltration characteristics FLO-2D model simulation Conclusions ADDITIONAL CONSTRAINTS Other flood condition constraints General drainage design guidelines...37 REFERENCES H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc ii

44 Adobe Solar Project Tables Table 2-1. Watershed-average frequency-duration maximum precipitation depths...8 Table 4-1. Infiltration characteristics*...15 Table percent annual chance baseline volume accounting summary...16 Table percent annual chance Adobe Site baseline flood depths...17 Table percent annual chance Adobe Site baseline flood velocities...17 Table percent annual chance Adobe Site flood depths (Adobe Site only)...18 Table percent annual chance Adobe Site flood velocities (Adobe Site only)...18 Table percent annual chance Rigel Site baseline flood depths...19 Table percent annual chance Rigel Site baseline flood velocities...19 Table percent annual chance Rigel Site flood depths (Rigel Site only)...20 Table percent annual chance Rigel Site flood velocities (Rigel Site only)...20 Table percent annual chance Adobe Site flood depths (Adobe and Rigel Sites)...21 Table percent annual chance Adobe Site flood velocities (Adobe and Rigel Sites)...21 Table th-percentile cross-sectional peak flows and volumes...35 Table percent annual chance cross-sectional peak flows and volumes...35 Figures Figure 1-1. Site vicinity map...2 Figure 1-2. Site regional drainage map...3 Figure 1-3. Site local drainage map...4 Figure 1-4. Site aerial photograph...5 Figure 1-5. FEMA floodplain map...7 Figure 3-1. Preliminary map of drainage features and landform surfaces...10 Figure 3-2. NRCS soils map...11 Figure 3-3. Geologic map...12 Figure 4-1. Regional map of 1-percent annual chance baseline flood depths...22 Figure 4-2. Regional map of 1-percent annual chance baseline flood velocities...23 Figure 4-3. Local map of 1-percent annual chance Baseline flood depths...24 Figure 4-4. Local map of 1-percent annual chance Baseline flood velocities...25 Figure 4-5. Local map of 1-percent annual chance flood depths (Adobe Site only)...26 Figure 4-6. Local map of 1-percent annual chance flood velocities (Adobe Site only)...27 Figure 4-7. Local map of 1-percent annual chance flood depths (Rigel Site only)...28 Figure 4-8. Local map of 1-percent annual chance flood velocities (Rigel Site only)...29 Figure 4-9. Local map of 1-percent annual chance flood depths (Adobe and Rigel Sites)...30 Figure Local map of 1-percent annual chance flood velocities (Adobe and Rigel Sites)...31 Figure percent annual chance peak flow distribution along XS-1 (Adobe Site only)...32 Figure percent annual chance peak flow distribution along XS-2 (Rigel Site only)...33 Figure percent annual chance peak flow distribution along XS-1 (Adobe and Rigel Sites)...34 H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc iii

45 Adobe Solar Project 1 INTRODUCTION An assessment was conducted to evaluate the flood hazard environment of the area being considered for siting the location of the proposed Adobe Solar Project (Adobe Site) located in Section 34, Township 32S, Range 28E, Mount Diablo Meridian, in unincorporated Kern County, California. The Adobe Site shares a common property line along its south boundary with the proposed Rigel Solar Project (Rigel Site) evaluated under separate cover. The location of the Adobe Site as well as the Rigel Site is shown in Figures 1-1 and Objectives The primary objectives of this assessment include the following: Identify (1) drainage features, (2) flood processes, (3) stable and unstable surfaces, and (4) flood conditions that would constrain the future development of the Rigel Site Quantify project-related impacts specific to increases in flood distribution, runoff yield, and flow rates for the 85 th -percentile and 1-percent annual chance storm events. The project-related impacts to the flood hazard environment will be evaluated for two sets of conditions: (1) the post-construction conditions of the Adobe Site only, and (2) the combined post-construction conditions of the Adobe and Rigel Sites. 1.2 Hydrology and flood control design standards The following publications related to hydrology, hydraulics, and sedimentation serve as the standard in Kern County: Kern County Hydrology Manual, Department of Planning and Development Services T.V. Hromadka II, California State University, Fullerton, 1992 Kern County Code of Building Regulations, Chapter 17.48, Floodplain Management Kern County Drainage Plan Check Compliance List 1.3 Watershed description The physical drainage area tributary to the Adobe Site is approximately 3.2 square miles based on U.S. Geological Survey (USGS) topographic data (Figure 1-3); however, the effective tributary drainage area may be much less as a result of several factors, which include the elongated nature of the drainage, the absence of a stream network (only sheet flooding appears to occur), and the existence of an east-west canal across the drainage, which acts to obstruct floodwaters from being received from the upper two-thirds of the drainage. These factors serve to potentially reduce the area contributing to the onsite flood hazard. The onsite drainage area of the Adobe Site is 159 acres. The adjacent Rigel Site encompasses a very small portion of the offsite drainage tributary to the Adobe Site. Due to their location with respect to each other, the Adobe and Rigel Sites are more or less subject to the same tributary drainage. The onsite drainage of the Rigel Site is 157 acres. H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 1

46 Figure 1-1. Site vicinity map Adobe Solar Project H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 2

47 Figure 1-2. Site regional drainage map Adobe Solar Project Source: USGS 24k H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 3

48 Figure 1-3. Site local drainage map Adobe Solar Project Source: USGS 24k H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 4

49 Figure 1-4. Site aerial photograph Adobe Solar Project H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 5

50 Adobe Solar Project 1.4 Flood studies and floodplain mapping Based on the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) Panel Number 06029C3150E, effective September 26, 2008, the Adobe Site is mapped as Zone X (Figure 1-5). Zone X is the flood insurance rate zone that corresponds to areas outside the 0.2-percent annual chance floodplain, areas within the 0.2-percent annual chance floodplain, and to areas of 1-percent annual chance flooding where the contributing drainage area is less than one square mile, and areas protected from the 1-percent annual chance flood by levees. No base flood elevations or depths are shown within this zone. H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 6

51 Figure 1-5. FEMA floodplain map Adobe Solar Project Source: National Flood Hazard Layer GIS H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 7

52 Adobe Solar Project 2 PRECIPITATION AND FLOOD FREQUENCY DATA 2.1 Precipitation frequency-duration relationships The synthetic 24-hour storm pattern adopted in the Kern County Hydrology Manual (KCHM) was developed using precipitation depth-duration-frequency spatial data from National Oceanic and Atmospheric Administration (NOAA) Atlas 14 (NWS, 2011). Specifically, the 1-percent annual chance as well as the annual watershed average maximum point precipitation depths for 5- and 30-minute, 1-, 3-, 6-, and 24-hour durations were determined and subsequently reduced based on the depth-areal reduction (DAR) relationships, which are dependent on storm duration and watershed size (KCHM Figure E-4), to account for the variability of the hydrologic processes generally experienced in larger watersheds (Table 2-1). The reduced values were used to develop the KCHM synthetic 24-hour storm pattern based on the format shown in KCHM Figure E-5. The 85th percentile 24-hour event was taken as 50 percent of the annual 24-hour event (0.42 inches). The synthetic 24-hour storm pattern for the 85 th percentile 24-hour event was based on the annual event precipitation depths. Table 2-1. Watershed-average frequency-duration maximum precipitation depths *DAR factors are based on 3.2 square miles 2.2 Flood frequency relationships No flood frequency relationships were identified or developed as part of this assessment. H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 8

53 Adobe Solar Project 3 GEOMORPHIC SUMMARY A field investigation of the Adobe Site and its tributary drainage was conducted on March 13, 2010 in conjunction with a field investigation of the adjacent Rigel Site (flood hazard assessment evaluated under separate cover), including a review of available topographic, aerial photographic, geologic, and soil information to identify drainage and geomorphic features, flood processes, stable and unstable surfaces, for the purpose of determining potential flood-related development constraints. 3.1 Drainage and geomorphic features The Adobe and Rigel Sites lie within California s Central Valley, a broad north-south trending alluvial plain that is almost completely covered by irrigated agricultural land. The Adobe and Rigel Sites are bounded on all sides by rectilinear agricultural parcels that have been mass-graded to facilitate agricultural irrigation. Any remnants of the pre-development natural drainage pattern are completely obscured by the agricultural use, both within the property lines and on surrounding parcels. An extensive system of canals, laterals, drains, and roads limit the potential for offsite runoff to impact the Site. There is little evidence that surface runoff traverses individual field boundaries past the berms and drains that bound the adjacent agricultural areas. Instead, storm water that does not infiltrate into the ground is most likely ponded onsite, is conveyed into the numerous drains, or collects along the roadways that parallel the field boundaries (Figures 1-3 and 1-4). 3.2 Flood processes The Adobe and Rigel Sites are subject to agricultural sheet flooding (Figure 3-1) that flows at low slopes toward the south. Sheet flooding consists of shallow unconcentrated flow, generally less than one foot deep (during extreme, rare events) that covers broad land areas rather than flowing along defined channels. It is unlikely that flooding on the Adobe or Rigel Sites is a significant concern for future development. 3.3 Stable and unstable surfaces The Adobe and Rigel Sites are underlain by sandy loam and loamy sand soils (Figure 3-2) with no areas of bedrock outcrop in the vicinity of the Adobe and Rigel Sites (Figure 3-3). No floodrelated unstable surfaces were observed on the Adobe or Rigel Sites. H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 9

54 Adobe Solar Project Figure 3-1. Preliminary map of drainage features and landform surfaces H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 10

55 Figure 3-2. NRCS soils map Adobe Solar Project Onsite map units: 130 (Cerini sandy loam), 150 (Excelsior sandy loam), and 230 (Milagro loamy sand) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 11

56 Figure 3-3. Geologic map Adobe Solar Project Onsite map units: Q (Quaternary alluvium) Source: California Geological Survey H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 12

57 Adobe Solar Project 4 FLOOD ROUTING ANALYSES 4.1 Background A review of available topographic and aerial photographic data indicates the floodwaters in the vicinity of the Adobe and Rigel Sites are generally unconfined. Therefore, a two-dimensional flood routing model was developed to evaluate the flood hazard environment using the computer application FLO-2D (O Brien, 2007), supported by several elements from the Kern County hydrologic standards, in pursuit of a reasonable estimate of the Baseline (without Project) and Project Conditions 85 th -percentile and 1-percent annual chance runoff characteristics. FLO-2D is currently a FEMA-accepted two-dimensional hydraulic model. 4.2 FLO-2D model development Grid system The FLO-2D grid system representing the tributary drainage (watershed) and fringe areas was subdivided into two model domains: (1) the offsite tributary drainage area (2.62 square miles), identified as the upper model (U100), was defined using 100 x 100 grid elements and excludes the areas onsite and immediately surrounding the Adobe and Rigel Sites, and (2) the local drainage (0.73 square miles) identified as the lower or local model (L025), was defined using 25 x 25 grid elements and includes areas onsite and immediately surrounding the Adobe and Rigel Sites, which were the areas specifically excluded from the upper model (U100). For the upper model (U100), the 10-meter USGS Digital Elevation Model (DEM) was used to interpolate the average elevation for each grid element. For the lower or local model (L025), onefoot contours compiled from recently flown aerial topography were used to interpolate the average elevation for each grid element to improve the recognition of local natural and anthropogenic features, which may influence flood conveyance. Project Conditions. A limited amount of grading is assumed for the purpose of addressing slope requirements associated with the design of the solar array. It is also assumed that this limited grading will not significantly alter the historical flood patterns within and adjacent to the Adobe and Rigel Sites and therefore, the grid element elevations determined for the Baseline Conditions were assumed to be the same for the Project Conditions Precipitation The KCHM synthetic 24-hour storm pattern was used in conjunction with the watershed-average maximum frequency precipitation depths from NOAA Atlas 14 (NWS, 2011), reduced based on the DAR factors (Table 2-1), to develop the required synthetic 24-hour storm patterns Hydraulic roughness The flood-wave progression was controlled by limiting the Froude number to a maximum value of 0.95, thereby precluding the occurrence of supercritical flow, which is not expected to occur under natural conditions. A general roughness coefficient of was assumed to represent the overland flow resistance. For shallow flow depths, the roughness coefficient typically ranges between and A roughness coefficient of was assumed for shallow flow conditions to limit the resistance during shallow flooding. H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 13

58 4.2.4 Hydraulic structures Adobe Solar Project The east-west canal, which intersects the north-south drainage tributary to the Adobe and Rigel Sites, potentially limits or eliminates the contribution of runoff to the onsite flood hazard received from roughly the upper two-thirds of the tributary drainage. However, the influence of the canal was not considered to limit model complexity because the drainage upstream from the canal is only expected to have a minor contribution to the onsite flood hazard due to the narrowness and unconfined nature of the drainage. No other identifiable major hydraulic facilities or structures are located within the tributary drainage. Any influence related to anthropogenic features or disturbances such as transportationand utility-related alignments located within the modeled drainage were not specifically defined other than what may be captured by the 10-meter USGS DEM or the 1 contours developed from recently flown aerial topography Boundary conditions The outflow boundary conditions from the upper model were used to define the inflow boundary conditions of the lower model. The inflow grid elements from the lower or local model (L025) were overlapped with the outflow grid elements from the upper model (U100) along the shared boundary to facilitate the transfer of flow between the two models Infiltration characteristics The Green-Ampt infiltration relationships were used to account for precipitation losses in lieu of the County standard because of its direct relationship with physical soil properties, which can be easily correlated to changes in soil compaction (Saxton and Rawls, 2006). The physical soil parameters, which form the relationship for determining infiltration, are saturated hydraulic conductivity (XKSAT), wetting front capillary suction (PSIF), and volumetric soil moisture deficit (DTHETA). These parameters were estimated by relating the soil composition of the watershed based on (Natural Resources Conservations Service (NRCS) soils mapping to average infiltration characteristics associated with soil texture classes for bare ground conditions (Rawls et al., 1983; Rawls and Brakensiek, 1983) assuming antecedent moisture conditions are near field capacity, which is consistent with the conditions immediately following a significant precipitation event. Each NRCS soil map unit is characterized by descriptive and numerical information such as (1) a representative profile, (2) engineering and physical properties, and (3) formation, morphology, and classification. This information was used in part to form the correlation between the soil composition and average infiltration characteristics. The Green-Ampt infiltration characteristics (Table 4-1) were determined based on the most restrictive soil layer with respect to infiltration and assuming the average infiltration characteristics associated with soil texture classes for bare ground conditions are representative of the watershed and fringe areas, regardless of land use, and are also aligned with the Baseline Conditions (without Project) soil compaction of 75 percent (assumed average value for natural and agricultural conditions). Project Conditions. The average soil compaction on the Adobe and Rigel Sites is assumed to increase to no more than 85 percent as a consequence of its intended development. To determine the infiltration characteristics for Project Conditions, the average infiltration characteristics associated with soil texture classes for bare ground conditions (Rawls et al., 1983; Rawls and Brakensiek, 1983) were adjusted to account for the increase in soil compaction (from 75 to 85 H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 14

59 Adobe Solar Project percent) and subsequent reduction in infiltration using the relationships developed by Saxton and Rawls (2006). The spatial land use assumptions based on the Kern County General Plan were intersected with the spatial soil information to estimate the spatial variation of effective imperviousness (RTIMP) for the watershed. The initial abstraction was assumed constant throughout the watershed at 0.15 inches implemented by assuming the threshold for flood routing (TOL) is equal to feet. Typical values for initial abstraction include 0.35 inches for flat-sloped desert and rangeland, 0.15 inches for Sonoran Desert hill slopes, 0.25 inches for mountains with vegetated surfaces, 0.20 inches for residential/commercial lawn and turf, 0.05 inches for pavement, and 0.50 inches for tilled fields and irrigated pasture. Table 4-1. Infiltration characteristics* *A soil compaction of 75 percent was assumed onsite for Baseline Conditions; and a soil compaction of 85 percent was assumed onsite for Project Conditions 4.3 FLO-2D model simulation The developed FLO-2D upper (U100) and lower (L025) models were analyzed for a simulation period of 36 hours. A maximum value of 0.25 was assigned to the numerical stability coefficient, which directly controls the maximum time step for full dynamic wave routing. Volume conservation was confirmed at each 0.1-hour time interval over the entire duration of the simulation. A summary of the volume accounting for the upper and lower model simulations for the 1-percent annual chance event only in Table 4-2, which indicates the rainfall and inflow volume balances with the combined volume from infiltration, storage, and outflow for each model and set of conditions. A breakdown of the 1-percent annual chance flood depths and velocities for each set of conditions is shown in Tables 4-3 through Flood inundation maps showing the spatial variation of maximum flood depths and velocities throughout the watershed and Adobe Site are presented for the 1-percent annual chance event in Figures 4-1 (regional depths; baseline conditions), 4-2 (regional velocities; baseline conditions), 4-3 (local depths; baseline conditions), 4-4 (local velocities; baseline conditions), 4-5 (local depths; Project conditions Adobe Site only), 4-6 (local velocities; Project conditions Adobe Site only), 4-7 (local depths; Project conditions Rigel Site only), 4-8 (local velocities; Project conditions Rigel Site only), 4-9 (local depths, Project Conditions Adobe and Rigel Sites), and 4-10 (local velocities, Project Conditions Adobe and Rigel Sites). H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 15

60 Adobe Solar Project The maps depicting the local 85 th -percentile event flood depths and velocities are presented as part the water quality assessment under separate cover. The 1-percent annual chance peak flow distribution along the predominant Adobe Site outfall boundary (XS-1; north property line) is shown in Figures 4-11 (Adobe Site only) and Figure 4-12 (Adobe and Rigel Sites), and the 1- percent annual chance peak flow distribution along the predominant Rigel Site outfall Boundary (XS-2; north property line) is presented in Figure Outfall peak flows and runoff volumes are compared in Tables 4-13 (85th-percentile event) and 4-14 (1-percent annual chance event). Table percent annual chance baseline volume accounting summary H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 16

61 Adobe Solar Project Table percent annual chance Adobe Site baseline flood depths Table percent annual chance Adobe Site baseline flood velocities H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 17

62 Adobe Solar Project Table percent annual chance Adobe Site flood depths (Adobe Site only) Table percent annual chance Adobe Site flood velocities (Adobe Site only) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 18

63 Adobe Solar Project Table percent annual chance Rigel Site baseline flood depths Table percent annual chance Rigel Site baseline flood velocities H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 19

64 Adobe Solar Project Table percent annual chance Rigel Site flood depths (Rigel Site only) Table percent annual chance Rigel Site flood velocities (Rigel Site only) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 20

65 Adobe Solar Project Table percent annual chance Adobe Site flood depths (Adobe and Rigel Sites) Table percent annual chance Adobe Site flood velocities (Adobe and Rigel Sites) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 21

66 Adobe Solar Project Figure 4-1. Regional map of 1-percent annual chance baseline flood depths H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 22

67 Adobe Solar Project Figure 4-2. Regional map of 1-percent annual chance baseline flood velocities H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 23

68 Adobe Solar Project Figure 4-3. Local map of 1-percent annual chance Baseline flood depths H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 24

69 Adobe Solar Project Figure 4-4. Local map of 1-percent annual chance Baseline flood velocities H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 25

70 Adobe Solar Project Figure 4-5. Local map of 1-percent annual chance flood depths (Adobe Site only) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 26

71 Adobe Solar Project Figure 4-6. Local map of 1-percent annual chance flood velocities (Adobe Site only) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 27

72 Adobe Solar Project Figure 4-7. Local map of 1-percent annual chance flood depths (Rigel Site only) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 28

73 Adobe Solar Project Figure 4-8. Local map of 1-percent annual chance flood velocities (Rigel Site only) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 29

74 Adobe Solar Project Figure 4-9. Local map of 1-percent annual chance flood depths (Adobe and Rigel Sites) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 30

75 Adobe Solar Project Figure Local map of 1-percent annual chance flood velocities (Adobe and Rigel Sites) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 31

76 Adobe Solar Project 40 Figure percent annual chance peak flow distribution along XS-1 (Adobe Site only) Baseline Conditions Project Conditions Project-related changes Q p (cfs) 20 0 Q p (cfs) Distance along XS-1 increasing from west to east (feet) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 32

77 Adobe Solar Project 40 Figure percent annual chance peak flow distribution along XS-2 (Rigel Site only) Baseline Conditions Project Conditions Project-related changes Q p (cfs) 20 0 Q p (cfs) Distance along XS-2 increasing from west to east (feet) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 33

78 Adobe Solar Project 40 Figure percent annual chance peak flow distribution along XS-1 (Adobe and Rigel Sites) Baseline Conditions Project Conditions Project-related changes Qp (cfs) 20 0 Qp (cfs) Distance along XS-1 increasing from west to east (feet) H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 34

79 Adobe Solar Project Table th-percentile cross-sectional peak flows and volumes Table percent annual chance cross-sectional peak flows and volumes 4.4 Conclusions For all conditions, the 1-percent annual chance flood velocities encountered on the Adobe Site are less than 0.5 feet per second on average with a maximum of less than 3 feet per second, are generally associated with sheet flooding, and do not present an erosion hazard for either set of project conditions, although localized erosion may occur in locations where floodwaters are altered and concentrated. Roughly 99 percent of the Adobe and Rigel Sites are subject to flood depths of less than 0.5 feet. For all Project Conditions, the results demonstrated a general increase in the distributed flows and runoff volume across the the affected outfalls, however, it is clear that the historic flood patterns (Figure 4-3) are preserved as shown in Figures 4-5 and 4-11 (Adobe Site only) and Figures 4-7 and 4-12 (Rigel Site only) and Figures 4-9 and 4-13 (Adobe and Rigel Sites). Project Conditions Adobe Site only. The increase in the average and maximum 1-percent annual chance flood depth on the Adobe Site is limited to 0.01 and 0.21 feet, respectively. Similarly, the increase in the average and maximum 1-percent annual chance flood velocity on the Adobe Site is limited to 0.01 and 0.13 feet per second, respectively. The overall increase in discharge and runoff volume at the Adobe Site outfall (XS-1; north property line) is 18 cubic feet H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 35

80 Adobe Solar Project per second (cfs) and 21 acre-feet, respectively, distributed non-uniformly along the roughly 0.5- mile floodplain cross-section (Table 4-10). Project Conditions Rigel Site only. The increase in the average and maximum 1-percent annual chance flood depth on the Rigel Site is limited to 0.01 feet. Similarly, the increase in the average and maximum 1-percent annual chance flood velocity on the Rigel Site is limited to 0.01 and 0.04 feet per second, respectively. The overall increase in discharge and runoff volume at the Rigel Site outfall (XS-2; north property line) is 23 cfs and 21 acre-feet, respectively, distributed non-uniformly along the roughly 0.5-mile floodplain cross-section (Table 4-10). Project Conditions Adobe and Rigel Sites. The combined post-construction conditions of the Adobe and Rigel Sites results in outflow from the Rigel Site contributing to the impacts associated with the Adobe Site, however minor. The increase in the average and maximum 1- percent annual chance flood depth on the Adobe Site is limited to 0.02 and 0.21 feet, respectively. Similarly, the increase in the average and maximum 1-percent annual chance flood velocities on the Adobe Site are limited to 0.04 and 0.16 feet per second, respectively. The overall increase in discharge and runoff volume at the Adobe Site outfall (XS-1; north property line) is 44 cfs and 45 acre-feet, respectively, distributed non-uniformly along the roughly 0.5-mile floodplain crosssection (Table 4-10). H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 36

81 Adobe Solar Project 5 ADDITIONAL CONSTRAINTS 5.1 Other flood condition constraints There are no other significant or unusual flood constraints that would preclude normal site development. 5.2 General drainage design guidelines If site development does not include habitable structures, more flexibility with regard to flood protection may exist, subject to the discretion of regulators and the absence of project-related adverse impacts to adjacent and/or downstream properties and structures. The canopy produced by the arrangement of solar power panels installed on the Adobe Site may have a tendency to concentrate runoff along a line below the bottom edge of each panel, which can lead to some measure of soil erosion under certain ground conditions, the significance likely influenced by the panel orientation and the topographic characteristics of the land below. The grading of access roads can lead to the concentration of runoff and subsequently result in localized erosion along and across these roads, and in some cases, trigger the formation of flowcut watercourses downstream. Local scour can occur around installed obstructions, e.g., panel supports and buildings, if exposed to concentrated runoff. Changes to the soil compaction conditions can influence runoff development. An increase in soil compaction can increase runoff development for a specific flood event; therefore, limiting increases in compaction will likely limit or preclude the necessity of mitigation associated with increased runoff volume impacts. Onsite grading should not significantly alter historic flood patterns to avoid increasing flood hazard impacts to adjacent and downstream properties. H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 37

82 Adobe Solar Project REFERENCES FEMA, 2008, Flood Insurance Study, Kern County, California, and Incorporated Areas, 06029CV001A, Federal Emergency Management Agency, Effective September 26, FEMA, 2003, Guidelines and Specifications for Flood Hazard Mapping Partners, Appendix G: Guidance for Alluvial Fan Flooding Analyses and Mapping, Federal Emergency Management Agency, April. NOAA, 2011, NOAA Atlas 14 Precipitation-Frequency Atlas of the United States, Volume 6: California, Version 2, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Silver Springs, MD, June. NRCS, 2008, Soil Survey Geographic (SSURGO) database for Kern County, Southwestern Part, California, CA691, Natural Resources Conservation Service, Fort Worth, Texas. O Brien, J.S., FLO-2D Computer Program and User Manual, Version , Nutrioso, AZ. Rawls, W.J. and D.L Brakensiek, 1983, A procedure to predict Green and Ampt infiltration parameters, Proceedings of the American Society of Agricultural Engineers Conference on Advances in Infiltration, Chicago, Illinois, Pages Rawls, W.J., D.L. Brakensiek, and N. Miller, 1983, Green-Ampt infiltration parameters from soils data, ASCE, Journal of Hydraulic Engineering, Volume 109, Number 1, Pages Saxton, K.E. and W.J. Rawls, 2006, Soil water characteristic estimates by texture and organic matter for hydrologic solutions, Soil Science Society of America Journal, Volume 70, Madison, Wisconsin, September/October. H:\pdata\ \Admin\reports\Adobe_PFHA_final_ doc 38

83 Final Preliminary Flood Hazard Assessment, Rigel Solar Project

84 RIGEL SOLAR PROJECT Kern County, California Preliminary Flood Hazard Assessment FINAL Prepared for FRV Rigel Solar, LP 44 Montgomery Street, Suite 2200 San Francisco, CA Prepared by RBF Consulting Alton Parkway Irvine, CA In conjunction with JE Fuller/Hydrology & Geomorphology, Inc South Kyrene Road, Suite 201 Tempe, AZ September 23, 2011 RBF JN

85 Rigel Solar Project H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc i

86 Rigel Solar Project Table of Contents 1 INTRODUCTION Objectives Hydrology and flood control design standards Watershed description Flood studies and floodplain mapping PRECIPITATION AND FLOOD FREQUENCY DATA Precipitation frequency-duration relationships Flood frequency relationships GEOMORPHIC SUMMARY Drainage and geomorphic features Flood processes Stable and unstable surfaces FLOOD ROUTING ANALYSES Background FLO-2D model development Grid system Precipitation Hydraulic roughness Hydraulic structures Boundary conditions Infiltration characteristics FLO-2D model simulation Conclusions ADDITIONAL CONSTRAINTS Other flood condition constraints General drainage design guidelines...37 REFERENCES H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc ii

87 Rigel Solar Project Tables Table 2-1. Watershed-average frequency-duration maximum precipitation depths...8 Table 4-1. Infiltration characteristics*...15 Table percent annual chance baseline volume accounting summary...16 Table percent annual chance Rigel Site baseline flood depths...17 Table percent annual chance Rigel Site baseline flood velocities...17 Table percent annual chance Rigel Site flood depths (Rigel Site only)...18 Table percent annual chance Rigel Site flood velocities (Rigel Site only)...18 Table percent annual chance Adobe Site baseline flood depths...19 Table percent annual chance Adobe Site baseline flood velocities...19 Table percent annual chance Adobe Site flood depths (Adobe Site only)...20 Table percent annual chance Adobe Site flood velocities (Adobe Site only)...20 Table percent annual chance Adobe Site flood depths (Adobe and Rigel Sites)...21 Table percent annual chance Adobe Site flood velocities (Adobe and Rigel Sites)...21 Table th-percentile cross-sectional peak flows and volumes...35 Table percent annual chance cross-sectional peak flows and volumes...35 Figures Figure 1-1. Site vicinity map...2 Figure 1-2. Site regional drainage map...3 Figure 1-3. Site local drainage map...4 Figure 1-4. Site aerial photograph...5 Figure 1-5. FEMA floodplain map...7 Figure 3-1. Preliminary map of drainage features and landform surfaces...10 Figure 3-2. NRCS soils map...11 Figure 3-3. Geologic map...12 Figure 4-1. Regional map of 1-percent annual chance baseline flood depths...22 Figure 4-2. Regional map of 1-percent annual chance baseline flood velocities...23 Figure 4-3. Local map of 1-percent annual chance Baseline flood depths...24 Figure 4-4. Local map of 1-percent annual chance Baseline flood velocities...25 Figure 4-5. Local map of 1-percent annual chance flood depths (Rigel Site only)...26 Figure 4-6. Local map of 1-percent annual chance flood velocities (Rigel Site only)...27 Figure 4-7. Local map of 1-percent annual chance flood depths (Adobe Site only)...28 Figure 4-8. Local map of 1-percent annual chance flood velocities (Adobe Site only)...29 Figure 4-9. Local map of 1-percent annual chance flood depths (Adobe and Rigel Sites)...30 Figure Local map of 1-percent annual chance flood velocities (Adobe and Rigel Sites)...31 Figure percent annual chance peak flow distribution along XS-2 (Rigel Site only)...32 Figure percent annual chance peak flow distribution along XS-1 (Adobe Site only)...33 Figure percent annual chance peak flow distribution along XS-1 (Adobe and Rigel Sites)...34 H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc iii

88 Rigel Solar Project 1 INTRODUCTION An assessment was conducted to evaluate the flood hazard environment of the area being considered for siting the location of the proposed Rigel Solar Project (Rigel Site) located in Section 34, Township 32S, Range 28E, Mount Diablo Meridian, in unincorporated Kern County, California. The Rigel Site shares a common property line along its north boundary with the proposed Adobe Solar Project (Adobe Site) evaluated under separate cover. The location of the Rigel Site as well as the Adobe Site is shown in Figures 1-1 and Objectives The primary objectives of this assessment include the following: Identify (1) drainage features, (2) flood processes, (3) stable and unstable surfaces, and (4) flood conditions that would constrain the future development of the Rigel Site Quantify project-related impacts specific to increases in flood distribution, runoff yield, and flow rates for the 85 th -percentile and 1-percent annual chance storm events. The project-related impacts to the flood hazard environment will be evaluated for two sets of conditions: (1) the post-construction conditions of the Rigel Site only, and (2) the combined post-construction conditions of the Adobe and Rigel Sites. 1.2 Hydrology and flood control design standards The following publications related to hydrology, hydraulics, and sedimentation serve as the standard in Kern County: Kern County Hydrology Manual, Department of Planning and Development Services T.V. Hromadka II, California State University, Fullerton, 1992 Kern County Code of Building Regulations, Chapter 17.48, Floodplain Management Kern County Drainage Plan Check Compliance List 1.3 Watershed description The physical drainage area tributary to the Rigel Site is approximately 3.2 square miles based on U.S. Geological Survey (USGS) topographic data (Figure 1-3); however, the effective tributary drainage area may be much less as a result of several factors, which include the elongated nature of the drainage, the absence of a stream network (only sheet flooding appears to occur), and the existence of an east-west canal across the drainage, which acts to obstruct floodwaters from being received from the upper two-thirds of the drainage. These factors serve to potentially reduce the area contributing to the onsite flood hazard. The onsite drainage area of the Rigel Site is 157 acres. The adjacent Adobe Site is located at the outfall of the Rigel Site (Adobe Site south property line; Rigel Site north property line). Due to their location with respect to each other, the Adobe and Rigel Sites are more or less subject to the same tributary drainage. The onsite drainage area of the Adobe Site is 159 acres. H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 1

89 Figure 1-1. Site vicinity map Rigel Solar Project H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 2

90 Figure 1-2. Site regional drainage map Rigel Solar Project Source: USGS 24k H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 3

91 Figure 1-3. Site local drainage map Rigel Solar Project Source: USGS 24k H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 4

92 Figure 1-4. Site aerial photograph Rigel Solar Project H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 5

93 Rigel Solar Project 1.4 Flood studies and floodplain mapping Based on Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) Panel Number 06029C3150E, effective September 26, 2008, the Rigel Site is mapped as Zone X (Figure 1-5). Zone X is the flood insurance rate zone that corresponds to areas outside the 0.2- percent annual chance floodplain, areas within the 0.2-percent annual chance floodplain, and to areas of 1-percent annual chance flooding where the contributing drainage area is less than one square mile, and areas protected from the 1-percent annual chance flood by levees. No base flood elevations or depths are shown within this zone. H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 6

94 Figure 1-5. FEMA floodplain map Rigel Solar Project Source: National Flood Hazard Layer GIS H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 7

95 Rigel Solar Project 2 PRECIPITATION AND FLOOD FREQUENCY DATA 2.1 Precipitation frequency-duration relationships The synthetic 24-hour storm pattern adopted in the Kern County Hydrology Manual (KCHM) was developed using precipitation depth-duration-frequency spatial data from National Oceanic and Atmospheric Administration (NOAA) Atlas 14 (NWS, 2011). Specifically, the 1-percent annual chance as well as the annual watershed average maximum point precipitation depths for 5- and 30-minute, 1-, 3-, 6-, and 24-hour durations were determined and subsequently reduced based on the depth-areal reduction (DAR) relationships, which are dependent on storm duration and watershed size (KCHM Figure E-4), to account for the variability of the hydrologic processes generally experienced in larger watersheds (Table 2-1). The reduced values were used to develop the KCHM synthetic 24-hour storm pattern based on the format shown in KCHM Figure E-5. The 85th percentile 24-hour event was taken as 50 percent of the annual 24-hour event (0.42 inches). The synthetic 24-hour storm pattern for the 85 th percentile 24-hour event was based on the annual event precipitation depths. Table 2-1. Watershed-average frequency-duration maximum precipitation depths *DAR factors are based on 3.2 square miles 2.2 Flood frequency relationships No flood frequency relationships were identified or developed as part of this assessment. H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 8

96 Rigel Solar Project 3 GEOMORPHIC SUMMARY A field investigation of the Rigel Site and its tributary drainage was conducted on March 13, 2010, in conjunction with a field investigation of the adjacent Adobe Site (flood hazard assessment evaluated under separate cover), including a review of available topographic, aerial photographic, geologic, and soil information to identify drainage and geomorphic features, flood processes, stable and unstable surfaces, for the purpose of determining potential flood-related development constraints. 3.1 Drainage and geomorphic features The Adobe and Rigel Sites lie within California s Central Valley, a broad north-south trending alluvial plain that is almost completely covered by irrigated agricultural land. The Adobe and Rigel Sites are bounded on all sides by rectilinear agricultural parcels that have been mass-graded to facilitate agricultural irrigation. Any remnants of the pre-development natural drainage pattern are completely obscured by the agricultural use, both within the property lines and on surrounding parcels. An extensive system of canals, laterals, drains, and roads limit the potential for offsite runoff to impact the Site. There is little evidence that surface runoff traverses individual field boundaries past the berms and drains that bound the adjacent agricultural areas. Instead, storm water that does not infiltrate into the ground is most likely ponded onsite, is conveyed into the numerous drains, or collects along the roadways that parallel the field boundaries (Figures 1-3 and 1-4). 3.2 Flood processes The Adobe and Rigel Sites are subject to agricultural sheet flooding (Figure 3-1) that flows at low slopes toward the south. Sheet flooding consists of shallow unconcentrated flow, generally less than one foot deep (during extreme, rare events) that covers broad land areas rather than flowing along defined channels. It is unlikely that flooding on the Adobe or Rigel Sites is a significant concern for future development. 3.3 Stable and unstable surfaces The Adobe and Rigel Sites are underlain by sandy loam and loamy sand soils (Figure 3-2) with no areas of bedrock outcrop in the vicinity of the Adobe and Rigel Sites (Figure 3-3). No floodrelated unstable surfaces were observed on the Adobe or Rigel Sites. H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 9

97 Rigel Solar Project Figure 3-1. Preliminary map of drainage features and landform surfaces H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 10

98 Figure 3-2. NRCS soils map Rigel Solar Project Onsite map units: 130 (Cerini sandy loam), 150 (Excelsior sandy loam), and 230 (Milagro loamy sand) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 11

99 Figure 3-3. Geologic map Rigel Solar Project Onsite map units: Q (Quaternary alluvium) Source: California Geological Survey H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 12

100 Rigel Solar Project 4 FLOOD ROUTING ANALYSES 4.1 Background A review of available topographic and aerial photographic data indicates the floodwaters in the vicinity of the Adobe and Rigel Sites are generally unconfined. Therefore, a two-dimensional flood routing model was developed to evaluate the flood hazard environment using the computer application FLO-2D (O Brien, 2007), supported by several elements from the Kern County hydrologic standards, in pursuit of a reasonable estimate of the Baseline (without Project) and Project Conditions 85 th -percentile and 1-percent annual chance runoff characteristics. FLO-2D is currently a FEMA-accepted two-dimensional hydraulic model. 4.2 FLO-2D model development Grid system The FLO-2D grid system representing the tributary drainage (watershed) and fringe areas was subdivided into two model domains: (1) the offsite tributary drainage area (2.62 square miles), identified as the upper model (U100), was defined using 100 x 100 grid elements and excludes the areas onsite and immediately surrounding the Adobe and Rigel Sites, and (2) the local drainage (0.73 square miles) identified as the lower or local model (L025), was defined using 25 x 25 grid elements and includes areas onsite and immediately surrounding the Adobe and Rigel Sites, which were the areas specifically excluded from the upper model (U100). For the upper model (U100), the 10-meter USGS Digital Elevation Model (DEM) was used to interpolate the average elevation for each grid element. For the lower or local model (L025), onefoot contours compiled from recently flown aerial topography were used to interpolate the average elevation for each grid element to improve the recognition of local natural and anthropogenic features, which may influence flood conveyance. Project Conditions. A limited amount of grading is assumed for the purpose of addressing slope requirements associated with the design of the solar array. It is also assumed that this limited grading will not significantly alter the historical flood patterns within and adjacent to the Adobe and Rigel Sites and therefore, the grid element elevations determined for the Baseline Conditions were assumed to be the same for the Project Conditions Precipitation The KCHM synthetic 24-hour storm pattern was used in conjunction with the watershed-average maximum frequency precipitation depths from NOAA Atlas 14 (NWS, 2011), reduced based on the DAR factors (Table 2-1), to develop the required synthetic 24-hour storm patterns Hydraulic roughness The flood-wave progression was controlled by limiting the Froude number to a maximum value of 0.95, thereby precluding the occurrence of supercritical flow, which is not expected to occur under natural conditions. A general roughness coefficient of was assumed to represent the overland flow resistance. For shallow flow depths, the roughness coefficient typically ranges between and A roughness coefficient of was assumed for shallow flow conditions to limit the resistance during shallow flooding. H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 13

101 4.2.4 Hydraulic structures Rigel Solar Project The east-west canal, which intersects the north-south drainage tributary to the Adobe and Rigel Sites, potentially limits or eliminates the contribution of runoff to the onsite flood hazard received from roughly the upper two-thirds of the tributary drainage. However, the influence of the canal was not considered to limit model complexity because the drainage upstream from the canal is only expected to have a minor contribution to the onsite flood hazard due to the narrowness and unconfined nature of the drainage. No other identifiable major hydraulic facilities or structures are located within the tributary drainage. Any influence related to anthropogenic features or disturbances such as transportationand utility-related alignments located within the modeled drainage were not specifically defined other than what may be captured by the 10-meter USGS DEM or the 1 contours developed from recently flown aerial topography Boundary conditions The outflow boundary conditions from the upper model were used to define the inflow boundary conditions of the lower model. The inflow grid elements from the lower or local model (L025) were overlapped with the outflow grid elements from the upper model (U100) along the shared boundary to facilitate the transfer of flow between the two models Infiltration characteristics The Green-Ampt infiltration relationships were used to account for precipitation losses in lieu of the County standard because of its direct relationship with physical soil properties, which can be easily correlated to changes in soil compaction (Saxton and Rawls, 2006). The physical soil parameters, which form the relationship for determining infiltration, are saturated hydraulic conductivity (XKSAT), wetting front capillary suction (PSIF), and volumetric soil moisture deficit (DTHETA). These parameters were estimated by relating the soil composition of the watershed based on the Natural Resources Conservation Service (NRCS) soils mapping to average infiltration characteristics associated with soil texture classes for bare ground conditions (Rawls et al., 1983; Rawls and Brakensiek, 1983) assuming antecedent moisture conditions are near field capacity, which is consistent with the conditions immediately following a significant precipitation event. Each NRCS soil map unit is characterized by descriptive and numerical information such as (1) a representative profile, (2) engineering and physical properties, and (3) formation, morphology, and classification. This information was used in part to form the correlation between the soil composition and average infiltration characteristics. The Green-Ampt infiltration characteristics (Table 4-1) were determined based on the most restrictive soil layer with respect to infiltration and assuming the average infiltration characteristics associated with soil texture classes for bare ground conditions are representative of the watershed and fringe areas, regardless of land use, and are also aligned with the Baseline Conditions (without Project) soil compaction of 75 percent (assumed average value for natural and agricultural conditions). Project Conditions. The average soil compaction on the Adobe and Rigel Sites is assumed to increase to no more than 85 percent as a consequence of its intended development. To determine the infiltration characteristics for Project Conditions, the average infiltration characteristics associated with soil texture classes for bare ground conditions (Rawls et al., 1983; Rawls and Brakensiek, 1983) were adjusted to account for the increase in soil compaction (from 75 to 85 H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 14

102 Rigel Solar Project percent) and subsequent reduction in infiltration using the relationships developed by Saxton and Rawls (2006). The spatial land use assumptions based on the Kern County General Plan were intersected with the spatial soil information to estimate the spatial variation of effective imperviousness (RTIMP) for the watershed. The initial abstraction was assumed constant throughout the watershed at 0.15 inches implemented by assuming the threshold for flood routing (TOL) is equal to feet. Typical values for initial abstraction include 0.35 inches for flat-sloped desert and rangeland, 0.15 inches for Sonoran Desert hill slopes, 0.25 inches for mountains with vegetated surfaces, 0.20 inches for residential/commercial lawn and turf, 0.05 inches for pavement, and 0.50 inches for tilled fields and irrigated pasture. Table 4-1. Infiltration characteristics* *A soil compaction of 75 percent was assumed onsite for Baseline Conditions; and a soil compaction of 85 percent was assumed onsite for Project Conditions 4.3 FLO-2D model simulation The developed FLO-2D upper (U100) and lower (L025) models were analyzed for a simulation period of 36 hours. A maximum value of 0.25 was assigned to the numerical stability coefficient, which directly controls the maximum time step for full dynamic wave routing. Volume conservation was confirmed at each 0.1-hour time interval over the entire duration of the simulation. A summary of the volume accounting for the upper and lower model simulations for the 1-percent annual chance event only in Table 4-2, which indicates the rainfall and inflow volume balances with the combined volume from infiltration, storage, and outflow for each model and set of conditions. A breakdown of the 1-percent annual chance flood depths and velocities for each set of conditions is shown in Tables 4-3 through Flood inundation maps showing the spatial variation of maximum flood depths and velocities throughout the watershed and Adobe Site are presented for the 1-percent annual chance event in Figures 4-1 (regional depths; baseline conditions), 4-2 (regional velocities; baseline conditions), 4-3 (local depths; baseline conditions), 4-4 (local velocities; baseline conditions), 4-5 (local depths; Project conditions Rigel Site only), 4-6 (local velocities; Project conditions Rigel Site only), 4-7 (local depths; Project conditions Adobe Site only), 4-8 (local velocities; Project conditions Adobe Site only), 4-9 (local depths, Project Conditions Adobe and Rigel Sites), and 4-10 (local velocities, Project Conditions Adobe and Rigel Sites). H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 15

103 Rigel Solar Project The maps depicting the local 85 th -percentile event flood depths and velocities are presented as part the water quality assessment under separate cover. The 1-percent annual chance peak flow distribution along the predominant Rigel Site outfall Boundary (XS-2; north property line) is presented in Figure 4-11, and the predominant Adobe Site outfall boundary (XS-1; north property line) is shown in Figures 4-12 (Adobe Site only) and Figure 4-13 (Adobe and Rigel Sites). Outfall peak flows and runoff volumes are compared in Tables 4-13 (85th-percentile event) and 4-14 (1- percent annual chance event). Table percent annual chance baseline volume accounting summary H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 16

104 Rigel Solar Project Table percent annual chance Rigel Site baseline flood depths Table percent annual chance Rigel Site baseline flood velocities H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 17

105 Rigel Solar Project Table percent annual chance Rigel Site flood depths (Rigel Site only) Table percent annual chance Rigel Site flood velocities (Rigel Site only) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 18

106 Rigel Solar Project Table percent annual chance Adobe Site baseline flood depths Table percent annual chance Adobe Site baseline flood velocities H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 19

107 Rigel Solar Project Table percent annual chance Adobe Site flood depths (Adobe Site only) Table percent annual chance Adobe Site flood velocities (Adobe Site only) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 20

108 Rigel Solar Project Table percent annual chance Adobe Site flood depths (Adobe and Rigel Sites) Table percent annual chance Adobe Site flood velocities (Adobe and Rigel Sites) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 21

109 Rigel Solar Project Figure 4-1. Regional map of 1-percent annual chance baseline flood depths H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 22

110 Rigel Solar Project Figure 4-2. Regional map of 1-percent annual chance baseline flood velocities H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 23

111 Rigel Solar Project Figure 4-3. Local map of 1-percent annual chance Baseline flood depths H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 24

112 Rigel Solar Project Figure 4-4. Local map of 1-percent annual chance Baseline flood velocities H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 25

113 Rigel Solar Project Figure 4-5. Local map of 1-percent annual chance flood depths (Rigel Site only) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 26

114 Rigel Solar Project Figure 4-6. Local map of 1-percent annual chance flood velocities (Rigel Site only) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 27

115 Rigel Solar Project Figure 4-7. Local map of 1-percent annual chance flood depths (Adobe Site only) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 28

116 Rigel Solar Project Figure 4-8. Local map of 1-percent annual chance flood velocities (Adobe Site only) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 29

117 Rigel Solar Project Figure 4-9. Local map of 1-percent annual chance flood depths (Adobe and Rigel Sites) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 30

118 Rigel Solar Project Figure Local map of 1-percent annual chance flood velocities (Adobe and Rigel Sites) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 31

119 Rigel Solar Project 40 Figure percent annual chance peak flow distribution along XS-2 (Rigel Site only) Baseline Conditions Project Conditions Project-related changes Q p (cfs) 20 0 Q p (cfs) Distance along XS-2 increasing from west to east (feet) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 32

120 Rigel Solar Project 40 Figure percent annual chance peak flow distribution along XS-1 (Adobe Site only) Baseline Conditions Project Conditions Project-related changes Q p (cfs) 20 0 Q p (cfs) Distance along XS-1 increasing from west to east (feet) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 33

121 Rigel Solar Project 40 Figure percent annual chance peak flow distribution along XS-1 (Adobe and Rigel Sites) Baseline Conditions Project Conditions Project-related changes Qp (cfs) 20 0 Qp (cfs) Distance along XS-1 increasing from west to east (feet) H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 34

122 Rigel Solar Project Table th-percentile cross-sectional peak flows and volumes Table percent annual chance cross-sectional peak flows and volumes 4.4 Conclusions For all conditions, the 1-percent annual chance flood velocities encountered on the Adobe Site are less than 0.5 feet per second on average with a maximum of less than 3 feet per second, are generally associated with sheet flooding, and do not present an erosion hazard for either set of project conditions, although localized erosion may occur in locations where floodwaters are altered and concentrated. Roughly 99 percent of the Rigel and Adobe Sites are subject to flood depths of less than 0.5 feet. For all Project Conditions, the results demonstrated a general increase in the distributed flows and runoff volume across the the affected outfalls, however, it is clear that the historic flood patterns (Figure 4-3) are preserved as shown in Figures 4-5 and 4-11 (Rigel Site only) and Figures 4-7 and 4-12 (Adobe Site only) and Figures 4-9 and 4-13 (Adobe and Rigel Sites). Project Conditions Rigel Site only. The increase in the average and maximum 1-percent annual chance flood depth on the Rigel Site is limited to 0.01 feet. Similarly, the increase in the average and maximum 1-percent annual chance flood velocity on the Rigel Site is limited to 0.01 and 0.04 feet per second, respectively. The overall increase in discharge and runoff volume at the Rigel Site outfall (XS-2; north property line) is 23 cubic feet per second (cfs) and 21 acre-feet, H:\pdata\ \Admin\reports\Rigel_PFHA_final_ doc 35