1. Flooding in San Diego County Deserts Designed 11/99 by Rand Allan Flood Control Hydrology

2. The Desert…For Many, a Desirable Place to Live • Ideal Winter Home…cool, relatively dry • Relatively Isolated, Some Level of Solitude • Beautiful Environment

3. However desirable it may be to live in the desert, there are certain considerations one needs to know about when deciding to build in the desert.

4. Desert Geology/Climate • Very Dry - less than 6” rain per year • Very little vegetation to prevent erosion • Runoff from mtns transports debris to desert floor creating alluvial fans • Flatness of alluvial fan makes ideal location for construction Carrizo Wash

5. The Creation of Alluvial Fans • While rainfall is not plentiful, it frequently comes in heavy bursts from thunderstorms and tropical storms • Flood flows move down the mountain canyons in a flash flood, rapidly moving out onto the alluvial fan and down to the desert floor Flash Flood Borrego Palm Canyon 8/16/79

6. The Creation of Alluvial Fans An alluvial fan is built up by repeated floods occurring over time Runoff may stay in one wash for several years, but can create new washes in subsequent floods

7. Mapped Alluvial Fans • Flood Control has extensively studied the hydrological flows of alluvial fans in the region • Has determined with reasonable accuracy: area of affect of 100-yr flows, depth of 100-yr flows

8. As development occurs on the desert floor, it becomes vulnerable to the processes of alluvial fan creation

9. This can lead to property damage De Anza Flooding

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11. What Can Be Done to Minimize Flood Damage? • The home can be elevated above the 100-year flood level • Orient structure to present minimum obstruction to flow • The rest of property should remain free of obstructions to allow free flow of water around home

12. Future Concerns Increased development can interfere with the efficient transport of water and debris by constricting flows to increasingly smaller spaces between homes

13. Possible Solutions • Prohibit any development in certain high- risk areas • Divert flow around development and reintroduce flow to alluvial flow below development • Build large open spaces within a development to allow relatively free transport of water in direction of natural flow

14. Example - Rams Hill Flood flows are intercepted upstream from development Channels divert flow around development Channel at bottom of development redirects flow back to original alluvial fan Requires dedicated channel maintenance funding source

15. Open Space Drainage • Open belts of undeveloped land established along major flood flows • Flood flows from properties and roads gradually diverted into open belts

16. If homes and developments can be designed to accommodate the forces of nature rather than resist them, potential damage to both homes and the desert can be minimized