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MANAGING DEVELOPMENT ON ALLUVIAL FANS It must be stressed that any development activity sus- tained on the active portion of an alluvial fan disrupts and alters the natural flood processes which perpetuate Its formation, and subjects any structure situated on the fan to unpredictable, erratic hazards during flood events. Furthermore, any new construction can redirect flood and debris flow to adjacent properties and thereby Increase flood hazards In other areas. A comprehensive approach Is therefore needed to manage development on fan areas such that the entire fan's natural flood processes and resulting hazards are taken Into account. The develop- ment and Implementation of a comprehensive approach Is best handled on the local government level through planning, zoning and building permit processes. Through these processes, future development can be planned and its effects on flood hazards adequately addressed. How Should a CommunIty Manage Development on Allu- vIal Fans? A comprehensive or master planning approach to managing growth on an alluvial fan considers fan conditions from apex to toe while guidingfuture development in a coordinated manner. The keystone of this planning process is the community's selection of flood/debris hazard management tools. The choice of tools will depend upon the nature and location of the hazards, and the location, timing, size and density of existing and future development. These tools can be structural (lev- ees, basins, channels) or nonstructural (zoning, public acqui- sition, subdivision regulations, building standards). The fan management plan may be incorporated as a separate element within the community's existing comprehensive plan, or may stand alone as a separate document. The planning process incorporates the following steps: I \ '~ 1. IDENTIFY THE HAZARDS. Using qualified geologists, hydrologists and engineers, conduct a detailed study of indi- vidual watershed and fan characteristics in orderto determine the severity and location of hazards to be expected on each fan. Where appropriate, active and inactive portions of the fan 8 should be mapped, as should the location of previous and potential debris flow paths. If the community has had its alluvial fan areas studied and mapped while preparing ~s Flood Insurance Rate Map (FIRM), some of this data may already be available. The FIRM will delineate the flood having a one percent chance of occurring in any given year (the 100-year flood), which can serve as the basis of more detailed hazard analysis and mapping. 2. PLAN FUTURE DEVELOPMENT. Identify the areas of the fan which require immediate flood protection, taking into accounlthe location and dens~y of existing development. Es- tablish the location, timing, and amount of future development in light of the delineated hazards. Development in the most hazardous portions of the fan should be restricted. 3. CHOOSE FLOOD MIllGATION TOOLS. Analyze the range of flood management/control tools available in consid- eration of identified hazards, present and planned dens~y and location of development, cost to locality, and public accep- tance. This is accomplished through the establishment of commun~y goals, and through coordination among local government officials, planners. engineers. residents, and the development community. The most widely-used alluvial fan flood management tools are presented in the next table. 4. ENFORCE REGULATIONS. Ensure the effective applica- tion and operation of the selected fiood management tools by adopting and enforcing regulatory controls: a. Zoning ordinances, subdivision regulations and open spaceAand acquis~ion plans can be used to prohibit or otherwise lim~ growth in high hazard areas, and would allow the safe passage of flood waters; b. Building codes can help ensure safe residential con- struction; c. Maintenancelinspection requirements can help guaran- tee the effective operation of structural flood control measures; 5. EDUCATE CITIZENS. Commun~ies should in~iate public awareness programs on alluvial fan hazards, install flood warning systems (where practical), and encourage the pur- chase of federal flood insurance. This comprehensive planning approach Is most effective when Implemented as early as possible during fan devel- opment. A community can then exercise maximum con- trol over future desired density levels and have the most flexible choice of flood management tools. It Is also best utilized on a multi-community or regional scale, espe- cially when multiple fans occupy range-fronts which may span several Jurisdictions. -