Flooding in San Diego County Deserts

Designed 11/99 by Rand Allan

Flood Control Hydrology

The Desert…For Many, a Desirable Place to Live

• Ideal Winter Home…cool, relatively dry

• Relatively Isolated, Some Level of Solitude

• Beautiful Environment

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.

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

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

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

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

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

This can lead to property damage

De Anza Flooding

Click mouse for next picture

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

Future Concerns

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

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

Example - Rams Hill

Requires dedicated channel maintenance funding source

Channel at bottom of development redirects flow back to original alluvial fan

Channels divert flow around development

Flood flows are intercepted upstream from development

Open Space Drainage

• Open belts of undeveloped land established along major flood flows

• Flood flows from properties and roads gradually diverted into open belts

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