State of California Department of Water Resources Floodplain Management Branch draft alluvial Fan Floodplain Evaluation and Delineation Program Scoping Report October 2010 Report Preparation Support Provided by: table of contents

Appendix A – Dale Lake Area A-1

Figure A-1. Dale Lake Area Preliminary Flood Hazard Areas A-2

Figure A-2. Dale Lake Area Preliminary Sediment Hazard Areas A-3

Appendix B – Shavers Valley Area B-1

Figure B-1. Shavers Valley Area Preliminary Flood Hazard Areas B-2

Figure B-2. Shavers Valley Area Preliminary Sediment Hazard Areas B-3

Appendix C – Indio Area C-1

Figure C-1. Indio Area Preliminary Flood Hazard Areas C-2

Figure C-2. Indio Area Preliminary Sediment Hazard Areas C-3

Acronyms and Definitions

AFFED Alluvial Fan Floodplain Evaluation and Delineation

AFTF Alluvial Fan Task Force

CGS California Geological Survey

DEM Digital Elevation Model

DWR Department of Water Resources

FEMA Federal Emergency Management Agency

GIS Geographic Information System

LiDAR Light Detection and Ranging

NED National Elevation Dataset

NFIP National Flood Insurance Program

NOAA National Oceanic and Atmospheric Administration

NRCS National Resources Conservation Service

NWS National Weather Service

PBS&J Post, Buckley, Schuh, and Jernigan

SCS Soil Conservation Service

USGS United States Geological Survey

Definitions

100-year Flood Hazard – Area subject to flooding during an event that has a one-percent chance of occurring in a given year.

Active – Refers to the portion of an alluvial fan where deposition, erosion, and unstable flow paths are possible.

Active Alluvial Fan Flooding – Flooding that occurs only on alluvial fans and is characterized by flow path uncertainty so great that this uncertainty cannot be set aside in realistic assessments of flood risk or in the reliable mitigation of the hazard. An active alluvial fan flooding hazard is indicated by three related criteria: (1) flow path uncertainty below the hydrographic apex; (2) abrupt deposition and ensuing erosion of sediment as a stream or debris flow loses its ability to carry material eroded from a steeper, upstream source area; and (3) an environment where the combination of sediment availability, slope, and topography creates an ultra hazardous condition for which elevation on fill will not reliably mitigate the risk. (Federal Emergency Management Agency 2003)

Alluvial Fan – Gently sloping fan-shaped landforms that are created over long periods of time by the natural deposition of eroded sediment from an upland source.

Alluvial Fan Apex – Highest point on the alluvial fan where there exists physical evidence of channel bifurcation and /or significant flow outside the defined channel.

Alluvial Fan Task Force (AFTF) – Stakeholder-driven task force directed by 2004 Assembly Bill 2141 to develop a Model Ordinance and planning tools to mitigate flood hazards associated with alluvial fan flooding.

FloodSAFE California – A collaborative statewide effort designed to accomplish five broad goals to reduce the chance of flooding, reduce the consequences of flooding, sustain economic growth, protect and enhance ecosystems, and promote sustainability.

Inactive – Refers to the portion of an alluvial fan surface no longer subject to unstable flow paths and deposition and erosion that normally builds alluvial fans.

Inactive Alluvial Fan Flooding – Occurs only on alluvial fans and is characterized by flow paths with a higher degree of certainty in realistic assessment of flood risk or in the reliable mitigation of the hazard. (FEMA 2003)

Sediment Hazard Area – Area subject to an increased hazard level due to the potential for erosion as a result of high velocities or deposition of alluvium from upstream sources.
1.0 Introduction

The Alluvial Fan Floodplain Evaluation and Delineation (AFFED) Program is one of the many programs of the FloodSAFE California initiative being implemented by the Department of Water Resources (DWR). The AFFED program was also a major recommendation in the recently completed Alluvial Fan Task Force (AFTF) Findings and Recommendations Report, dated July 2010 (AFTF, 2010). The Task Force concluded that one of the most significant challenges related to future development in Southern California is an insufficient understanding of the flood risks and hazards on alluvial fan areas. The Task Force recommended that work continue on AFFED to identify and map alluvial areas projected for growth.

The goal of the AFFED Program is to improve the quality and accuracy of alluvial fan flood hazard data and mapping information available to local Southern California communities. The AFFED Program will provide local agencies with the necessary tools to understand the characteristics and potential hazards of flooding and sediment from alluvial fans, when making land use planning decisions. The AFFED Program study area is limited to the following ten counties in Southern California, where planning development on alluvial fans is considered prevalent:

• 
  Imperial County
  
• 
  Kern County
  
• 
  Los Angeles County
  
• 
  Orange County
  
• 
  Riverside County
  
• 
  San Bernardino County
  
• 
  San Diego County
  
• 
  San Luis Obispo County
  
• 
  Santa Barbara County
  
• 
  Ventura County
  

The AFFED Program will initially focus on the alluvial fan areas that are subject to future development over the next twenty-five years, however the methodology outlined in this document will allow for modeling and mapping of the entire ten-county study area. The information provided by the AFFED Program may serve to reduce future losses on alluvial fans due to flooding and to minimize or eliminate the need for future investments in flood control projects.

The philosophy of the AFFED Program is similar to DWR’s Awareness Floodplain Mapping Program which is intended to fill the gap of flood risk information for areas not currently mapped under FEMA’s National Flood Insurance Program (NFIP).

1.1 Program Objectives

The primary objectives of the AFFED Program are:

• 
  Identify priority alluvial fan areas projected for growth that would need future NFIP mapping
  
• 
  Prepare preliminary hazard area maps that display potential flood and sediment hazard areas for multi-hazard planning
  
• 
  Provide information to local communities to support land use decision making
  
• 
  Provide a cost effective product within a short time frame
  

Flood and sediment hazard simulations will be conducted to identify areas of potential flooding and elevated sedimentation hazard in support of needs identified by the AFTF, local communities, FEMA, and California Geological Survey (CGS). The preliminary data developed from this process and provided to local communities along with the procedures described in this document are advisory but establish a technical foundation that may be refined by local agencies for understanding the potential risks associated with alluvial fan areas.

The AFFED program intends to only provide flood risk information to complement ongoing FEMA programs. Analysis will only be conducted for areas that are not mapped under FEMA's NFIP and thus will not provide conflicting information but provide the communities and residents a tool to understand potential flood hazards currently not regulated. The data produced by the AFFED program will provide a starting point for understanding the potential hazards that can be refined with detailed studies that can be included in the NFIP. Ultimately, communities that require regulatory maps should develop them based on the 100-year flood hazard boundary utilizing the FEMA process.

1.2 Program Requirements

As a result of the general lack of existing flood hazard data on alluvial fans throughout Southern California, it is of great importance that during the AFFED Program constant communication exists between the modelers and the local communities to provide an understanding of the methods being applied. The use of an analytical approach to determine the relative potential hazard expected for any fan or part of a fan provides a predictive process that can be easily updated by local communities beyond the AFFED Program.

Preliminary potential flood and sediment hazard data created from the AFFED Program will be provided to the local communities in both digital and hard copy format as Geographic Information Systems (GIS) shapefiles and overlaid on base USGS quadrangle maps, respectively. Computer models may also be provided to the communities. The shapefiles, maps, and computer models may be used by local communities to assist them in making informed land use decisions.

2.0 Program Approach

The AFFED Program approach consists of four major components: (1) Data gathering and model setup, (2) Flood hazard simulation, (3) Sediment hazard simulation, and (4) Preliminary hazard mapping. Figure 1 shows the overall program approach, including potential refinements beyond the AFFED hazard simulations and preliminary hazard mapping.

The AFFED Program begins with inquiry of and data gathering from the affected communities to identify the existing baseline data and to determine if improved data exists from alternative sources. Hydrologic and topographic data will be incorporated into the modeling process if it is determined to provide more detailed information than the previously collected base data and pertain to a geographic area that will improve the determination of the hazard. The best available hydrology and topography data for the analysis is processed to ensure the data is of a consistent standard with the rest of the program and accurately represents the actual community conditions, as understood by the local agencies. Hydrologic boundaries determined from the topography will be used to determine individual model domains for analysis. During the data gathering communication with the local agencies will assist in the identification of areas of high importance that may be given priority during simulation and hazard identification or used as sample regions to demonstrate the methods and results of the AFFED Program.

Figure 1

The AFFED Program intends to identify flood hazard and sediment hazard areas for alluvial fans within the ten-county study area, using the FLO-2D (FLO-2D Software, Inc., 2009) computer model. The FLO-2D is a two-dimensional computer simulation model that can be used for flood routing and modeling unconfined flows on floodplains and alluvial fans. The model has the ability to analyze unconfined flows as clear water, mudflow, or a complete sediment transport simulation. The use of the sediment transport function allows for the additional delineation of an area with an elevated potential for sediment related hazard (i.e., scour or deposition) within the preliminary floodplain delineation area. Although debris flows will not be explicitly modeled within FLO-2D the presence of high levels of deposition and erosion in the sediment transport simulation may indicate an increased probability of the occurrence of debris flows.

The FLO-2D model results will be used for the creation of preliminary hazard maps based on the 100-year flooding extents and potential for sediment transport on active and inactive alluvial fans. This is an analytical process that will examine both the potential for delivery of available upstream alluvium to the fan surface and the stability of the materials that currently make up the fan surface based on the amount of deposition and erosion within the upstream stream channels and on the fan surfaces. Sensitivity can be assessed through the variation of input parameters such as Manning n, sediment size, bulking factor applied to rainfall, and storm duration to define the potential hazard.

With the proper topographic and hydrologic data inputs, the FLO-2D flood simulations are run to determine the 100-year flood hazard. The depths of flooding reported from FLO-2D are used to identify a preliminary flood hazard area, for which the flood depths will be sufficient to result in significant damage. Model results are then reviewed to check consistency with evidence reflected in the geomorphology as seen in topographic maps, aerial photographs, and soils maps. Following the identification of the preliminary flood hazard area, a second FLO-2D simulation is conducted utilizing sediment transport capabilities of the model. Input for the sediment model is verified with local agencies where possible and estimated for remaining areas. The sediment simulation provides guidance in determining an additional hazard area that is not only subject to flooding but also a greater hazard due to the potential for deposition and erosion. The maximum depths of deposition and erosion reached throughout the simulation will be reviewed for the determination of the preliminary sediment hazard area.

The model results from both simulations are reviewed thoroughly and engineering judgment is applied to create delineations of the complete flood and sediment hazard areas. Delineations of preliminary flood and sediment hazard areas are combined into preliminary hazard maps. These maps will be provided to communities to assist in planning development.

Further refinement of the analysis and the preliminary flood and sediment hazard maps may be conducted beyond the AFFED Program by the local agencies should they desire to do so. The datasets from the flood and sediment hazard simulations along with the methodology used to create them will be provided to communities for their review. As more detailed or improved input data is made available this additional refinement may provide great value to the local communities.

Figure 2 provides a more detailed graphic illustration of the AFFED Program approach. The program approach calls for a process to make certain that the delineation of hazard areas is limited only by the quality of the input data. The approach provides for the opportunity for incorporating input and revision at multiple levels, and multiple simulations can be run to ensure a full understanding of the sensitivity to parameters and the overall relative level of risk for all areas of the alluvial fans is clear. Each step of the AFFED Program approach listed in Figure 2 is discussed in detail in the Sections 3.0 through 7.0.


Figure 2
Detailed AFFED Program Approach
3.0 Data Gathering and Model Setup

Figure 3 shows the detailed steps required to complete the data gathering and model setup program component.

Figure 3

3.1 Data Gathering and Community Inquiry

Prior to model development, inquiries will be made to local communities to determine if they have access to more detailed datasets than those readily available from federal agencies such as the United States Geological Survey (USGS) and the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS). Communities will also be informed on the AFFED Program approach and its advantages. Any detailed data or previous studies from communities will serve to provide more accurate inputs for the FLO-2D model simulations, and in turn, a better understanding of the potential hazards specific to each community. Possible data contributions that counties and local agencies may be able to provide fall into three categories: (1) topographic, (2) hydrologic, and (3) sediment data. Data from previous studies may be extracted and incorporated into the AFFED Program or results, such as floodplain delineations may be used for comparison of results.

3.2 Best Available Topographic Data

Hydrologic basin boundaries are to be determined through the use of USGS National Elevation Dataset (NED) 10 meter Digital Elevation Model (DEM) topographic data, which has already been acquired for the entire study area. Initially, this topography data will be used for the flood simulations; however, it may be replaced at a later date, if more detailed topographic data, such as LiDAR (Light Detection and Ranging) or detailed contours, is available publically or provided by local agencies. Additional topographic data will be incorporated into the analysis provided that it results in an increase in the improvement of the overall program. In some instances the inconsistencies associated with combining topography may outweigh the increased accuracy provided by those data sets, evaluation will be done on a case by case basis.

3.3 Best Available Hydrologic Data

The use of the FLO-2D model to generate runoff from a storm event requires careful application of available hydrologic data and relies upon the ability of the model to accurately represent the actual processes occurring. If historic hydrologic data is available and preferred, or a design storm has been adopted by a local agency it will be incorporated into the analysis. In the absence of any data from communities, NOAA Atlas 14 gridded precipitation data (Bonnin, et al. 2006) will be used to generate a 100-year storm event.

Gridded precipitation data for multiple durations is already available for the eastern portion of the ten-county study area that is considered southeastern California by NOAA, as illustrated by Figure 4. Data for the remainder of California is scheduled for release in September 2010. If the release of this data is delayed, available 6- and 24-hour rainfall depths can be used to interpolate values for the required durations utilizing NOAA procedures (Frederick and Miller, 1979).

Figure 4

3.4 Hydrologic Basins and Units of Study

Two dimensional modeling (e.g., FLO-2D model) is extremely sensitive to model domain size, or more accurately the number of elements or grid cells. Thus, two dimensional modeling requires a balance in the size of the grid cells and the model domain size. Sensitivity analysis has shown that the maximum grid cells sizes for an acceptable level of accuracy in approximate boundary delineation for both the flood hazard and sediment hazard areas are between 200 and 400 feet. At this grid cell size, the maximum model domain size is approximately 50 square miles. For models covering larger areas, the simulations perform poorly and often struggle to reach completion. The use of a rainfall-runoff model dictates that distinct hydrologic basins be used as ‘units of study,’ incorporating hydrologic boundaries when determining model limits will prevent modeling error from double counting runoff, or accounting for the same flow across multiple models. With a “unit of study size” of 50 square miles the ten-county study area will need to be divided into approximately 1,100 distinct models for simulation.

4.0 Flood Hazard Simulation

Figure 5 shows the detailed steps required to complete the flood hazard simulation program component.

Figure 5

4.1 Run FLO-2D Flood Simulation

Following the division of the ten-county study area into the hydrologic units of study the FLO-2D model is used to compute the flood simulation. Each of the approximately 1,100 - 50 square mile basins will be modeled. For the determination of the 100-year flood hazard area no sediment modeling will be conducted, the clear water mode in the model is used over a stable bed (no deposition or erosion is computed).

Some of the basins, whole or in part may be excluded from the analysis. The AFFED Program will exclude alluvial fans (in the ten-county study area) from the analyzes, if the fan area (1) is below a flood control structure for which hydrology cannot be provided or (2) already has FEMA floodplain mapping. The program will study areas subject to man-made disturbances (e.g. crop farming or urbanization) assuming that available topography reflects current conditions.

Hydrology used for FLO-2D simulations will be intended to simulate a 100-year rainfall event. This 100-year event will have a total storm duration of 12 hours and be created from the 30-minute, 1-, 2-, 3-, 6-, and 12-hour intensities such that the rainfall intensities corresponding to durations shorter than 12 hours are contained. For example, the maximum rainfall input in a one hour period corresponds to the 100-year 1-hour rainfall depth.

The total storm duration of 12-hours and peak rainfall intensity (30-minute interval) were selected following sensitivity analysis because these parameters generated floodplains that closely reproduced expected flood extents as seen in aerial photography in the pilot study areas. Extending the total storm duration to 24-hours did not have a significant impact on simulation results and only served to create excessive simulation times. It is also important to note that the proposed methodology does not include any abstraction of the precipitation; that is, all the precipitation is converted directly into runoff. This assumption may be overly conservative but can be considered to represent a worst-case post-fire condition.

The distribution of rainfall is spatially uniform across the modeled watershed and follows a temporal pattern similar to that of the National Resources Conservation Service (NRCS) Type II rainfall distribution, the recommended distribution for the eastern most portion of the ten-county study area Soil Conservation Service (USDA SCS, 1986). This distribution is symmetric with the greatest intensity occurring at approximately the midpoint of the simulation. Application of a consistent rainfall distribution across the entire study area is unlikely to have a significant impact on floodplain delineation; however, if it is requested by a local agency, incorporating additional or specific rainfall distribution patterns can be done.

In the flood simulation it is recommended that a bulking factor be applied to increase the total runoff volume to account for the additional volume that would be provided by sediment. Following the analysis of the sample study areas, it is recommended that the precipitation be bulked a minimum of 40 percent to ensure flood extents reflect those seen in examination of aerial photos. This bulking factor will be adjusted based on the recommendation of local agencies, many of which have developed jurisdiction-specific bulking factors.

4.2 Review of Flood Simulation Results

The maximum water depth recorded during the simulation at each element is summarized into polygon shapefiles that depict areas of increasing flood depths. For the clear water simulations, polygons shapefiles showing flood depths greater than 0.5 feet and one-foot will be created. Following review of the FLO-2D results, input parameters such as Manning n, bulking factor and possibly initial abstraction can be adjusted and simulations will be re-run to ensure reasonable results.

4.3 Identification of Preliminary Flood Hazard Areas

The preliminary flood hazard area delineation is intended as a guide only for understanding the basic area at risk of flooding from the 100-year event. Delineation of the preliminary 100-year flood hazard areas will be conducted by experienced engineers utilizing accepted geomorphic principles to modify boundaries created from the FLO-2D simulations. Any modifications to the preliminary flood hazard areas will be based on topographic data and will include additional areas of the fan surfaces that were not indicated as having substantial flood risk in the FLO-2D simulation but still appear to be subject to significant flooding hazard (e.g., areas downstream of possible avulsion locations and current or recent flow paths recognized from review of aerial photographs).

Figure 6 demonstrates an example delineation of the preliminary floodplain overlaid on FLO-2D simulation results.
FLO-2D flood simulation results are shown to assist the communities in recognizing the relative distribution of the flood hazard across the fan surface and the potential severity of that hazard.

Figure 6

5.0 Sediment Hazard Simulation

Figure 7 shows the detailed steps required to complete the sediment hazard simulation program component.

Figure 7

After completion of the delineation of the preliminary flood hazard area, a second FLO-2D simulation will be conducted utilizing the sediment transport capabilities of FLO-2D model to determine the portion of the flood hazard area that is subject to a higher degree of hazard because of the potential for excessive sediment deposition or erosion. The results of the FLO-2D sediment simulation will be used to delineate a preliminary sediment hazard area, in addition to the previously detailed preliminary flood hazard area.

5.1 Best Available Sediment Data

The use of the rainfall-runoff modeling option reduces the required input for the sediment data to only the median bed material size (d₅₀) for the recommended sediment transport function, discussed in the following section. Widespread sediment size gradation data is not currently readily available, communication with local communities will be critical in obtaining actual sediment sample data (if available) or recommendation on a representative mean bed material size for use within the units of study.

In the absence of additional sediment sample data or recommendations that can be extrapolated throughout the region the d₅₀ will initially be set at 2mm or the threshold between sand and gravel, which are both abundant deposits on alluvial fans in Southern California.
5.2 Run FLO-2D Sediment Simulation

The FLO-2D model determines the carrying capacity of the flow as it begins to collect and then determines the amount of deposition or scour required to balance that capacity. Because the volume associated with the transported sediment is also accounted for by the model, bulking of the precipitation is not required in the sediment transport simulation. The FLO-2D model domain established for the flood hazard simulations will be used for the sediment simulations. For FLO-2D sediment simulation, Yang’s Method is suggested as the sediment transport function based on recommendations within the FLO-2D Reference Manual which states that the use of the Yang transport function results in moderate sediment transport quantities and that it can be applied to sand and gravel. (FLO-2D, 2009) Sensitivity analysis in multiple sample areas confirmed that reasonable amounts of sediment transport were reported by the model. This method uses hydraulic computations in conjunction with input sediment parameters to determine the amount of sediment that can be transported and thus the amount of deposition or scour at each grid cell over time.

5.3 Review of Sediment Simulation Results

In a similar manner to the flood depth results (see Section 4.2), polygon shapefiles will be generated for the flood hazard boundary delineation and the sediment transport simulations for those showing maximum deposition or scour depths greater than 0.5 feet and one-foot.

Similar to the clear water simulation, following the review of the sediment hazard simulation results, sediment input parameters such as median sediment size may be adjusted and the simulations will be repeated, to help ensure that the results are reasonable. Once the simulation results are acceptable, the information will be used to delineate a preliminary sediment hazard boundary.

5.4 Identification of Preliminary Sediment Hazard Areas

The preliminary sediment hazard areas will be delineated within the 100-year flood hazard boundary for which there is significant amount of sediment transport (e.g. greater than 0.5 feet of scour or deposition). This area will typically but not exclusively be limited to the upstream areas of the alluvial fan nearest the apex as shown in the example delineations (see Figure 8) and in Appendices A, B, and C. Delineation of the sediment hazard area will also take into consideration the quality of the topographic information used in the simulation, as it may impact the reported deposition or scour. Topography that is developed with insufficient breaklines may result in the FLO-2D simulations reporting isolated areas of scour and deposition that would not otherwise exist, application of engineering judgment is necessary to interpret model results and accurately identify preliminary sediment hazard areas. This boundary is provided as a guide in understanding areas at risk for sediment hazard and is an effective aid in identifying active alluvial fans.

Figure 8 demonstrates an example delineation of the preliminary sediment hazard area generated from the FLO-2D sediment transport simulation results for deposition and scour, which are also shown.

Figure 8

Figure 9 shows the detailed steps required to complete the preliminary hazard mapping component.

Figure 9

6.1 Preliminary Hazard Maps

Following the delineation of the preliminary flood and sediment hazard areas, the data sets may be combined and assembled into a single preliminary hazard map similar to the one shown on Figure 10.

Figure 10

6.2 Comparison with CGS Quaternary Maps

Quaternary mapping indicates the relative levels of risk of flooding (i.e., low, medium, and high) on alluvial fan surfaces. Where available within the ten-county study area, Quaternary mapping will be provided by CGS for use as an additional verification tool of preliminary boundary map delineations along with other related sources of information such as previous mapping studies conducted by FEMA or the local agencies that may be available.
6.3 Local Agency Review

After verification against CGS Quaternary mapping and hazard determination (if available), identified preliminary flood and sediment hazard areas will be reviewed by the appropriate local agencies. Following the local agency review, if required, the FLO-2D input parameters will be adjusted and simulations be repeated to develop new flood and sediment hazard boundaries to reflect the local agency review. Local agencies may request that adjustments be made to the bulking factor applied to rainfall, amount of rainfall abstraction, median sediment size used in the sediment hazard determination, or any other parameter to adhere to local guidelines or to produce more or less conservative hazard delineations.

6.4 Distribution of Preliminary Hazard Maps and Datasets

Following the acceptance of digital datasets of FLO-2D model results, 100-year preliminary flood hazard and sediment hazard area identification, local agencies will be provided hard copies of the preliminary hazard maps using USGS 7.5 minute quadrangle data as a base map. The FLO-2D models and model results as GIS shapefiles may also be provided to the local agencies that regulate planning and land use decisions.

7.0 Refinement of Hazard areas and Simulations

Figure 11 shows the possible steps that may be conducted by local communities to add further refinement to hazard areas and simulations beyond the AFFED Program.

Figure 11

7.1 Refinement of Hazard Areas by Local Agencies

Following the delivery of digital datasets and preliminary boundary maps, communities may elect to perform additional refinement of the hazard boundaries beyond the AFFED Program. Communities may also elect to build on the work of the AFFED Program and perform similar analysis as more detailed input datasets become available.

7.2 Implementation of Hazard Areas for Future Development

The use of the hazard areas developed from the AFFED Program by local agencies to assist in the planning of future development or in any regulatory manner is beyond the AFFED Program and only subject to the restraints placed by the agencies themselves.

8.0 AFFED Program Implementation Summary

The AFFED Program’s intent is to provide a tiered methodology and procedural framework from which additional detailed analysis can be built upon by local agencies as improved topographic, hydrologic, and/or sediment data becomes available.

The AFFED Program is set apart from current mapping methods through the creation of multiple hazard areas and the reporting of depths of flooding determined by two dimensional model simulations. The program is helpful in allowing local planning and land use personnel to see the relative levels of risk across larger areas containing several alluvial fans. Finally, the preliminary flood and sediment hazard delineations that result from the AFFED Program will assist local agencies in identifying areas that may require a more detailed analysis prior to approval of planned future development.

9.0 Pilot Studies

Three pilot sites have been selected for sample delineation using the approach outlined in this document. These sites have been chosen because they contain numerous alluvial fans of different sizes with various degrees of activity.

Appendices A, B, and C contain example delineations showing the results of the initial FLO-2D simulations and the preliminary flood hazard and sediment hazard delineations that are created through the interpretation of the FLO-2D results and examination of aerial photographs and topographic maps. The sample preliminary hazard boundary delineations shown in Figures 6, 8, and 10 within this document were developed in the study of the Shavers Valley area.

Review of the example delineations shows a dramatic difference in the extents of the sediment hazard area between the three regions. The overall slope of the fan surfaces in the Indio sample area (Appendix C) appears to be greater than either the Dale Lake or Shavers Valley areas (Appendices A and B, respectively) and is the likely cause for the increased sediment hazard computed on the fan surfaces within the Indio sample area.

10.0 Program Milestones

The schedule of major program milestones is shown below in Table 1. The upfront tasks prior to beginning the hazard delineation process consist of the finalization of the scoping document and coordination with both FEMA and the AFTF. Coordination meetings with FEMA personnel are tentatively scheduled for October 2010, with meetings with key individuals of the AFTF to follow in November 2010. Input from these agencies will be incorporated into the methodology of the AFFED program and reflected in the final version of the scoping document, scheduled for completion in November 2010.

Following the completion of the scoping document initial meetings with the communities can begin in December of 2010 and will continue through February 2011. These initial meeting will allow for communities to be educated about the AFFED program and provide input into the studies such as any recommendations for parameters, sharing of available data, and areas that may be given priority for delineation. Concurrent with the initial meetings with communities beginning in January 2011 will be the model preparation and planning stages. During this time the necessary tools to create and execute the models will be developed in addition to the determination of hydrologic boundaries for model domains and the prioritization of the study areas.

As areas become ready for modeling the actual model runs will be executed beginning as soon as January 2011 and continuing through October 2012. Following the preliminary delineation of the flood and sediment hazards follow up meetings with the communities to share model results will begin, scheduled for February 2011 through November 2012. Incorporation of input from communities from these meetings will allow for the adjustment to the models and deliverables to be finalized for distribution beginning in March 2011 and continuing to the end of the program scheduled for December 2012.

Table
Schedule of Major Program Milestones

Milestone	Dates
Revise Scoping Document	Current – November 2010
FEMA Coordination	October 2010
AFTF Coordination	November 2010
Initial Meetings with Communities	December 2010 – February 2011
Model Preparation and Planning	January 2011 – March 2011
Model Execution and Delineation	January 2011 – October 2012
Follow-up Meetings with Communities	February 2011 – November 2012
Model Adjustment and Preparation of Deliverables	March 2011 – December 2012

11.0 References

Alluvial Fan Task Force, Findings and Recommendations Report, Water Resources Institute, California State University San Bernardino, July 2010.

Bonnin, G.M., Martin, D., Lin, B., Parzybok, T., Yekta, M., Riley, D., NOAA Atlas 14 Volume 1, Version 4.0: Semiarid Southwest (Arizona, Southeast California, Nevada, New Mexico, Utah), Precipitation-Frequency Atlas of the United States.

Federal Emergency Management Agency, Guidelines and Specifications for Flood Hazard Mapping Partneers, Appendix G: Guidance for Alluvial Fan Flooding Analyses and Mapping, April 2003.

FLO-2D Software, Inc., FLO-2D Reference Manual Version 2009, 2009.

Frederick, Ralph H., Miller, John F., “Short Duration Rainfall Frequency Relations for California,” Office of Hydrology, National Weather Service, NOAA. Silver Spring, Maryland, 1979.