Minimizing Floods Imperils Beaches : Environment: Sand carried to the shore replenishes the coast and protects it from erosion.

Recent floods have created consternation among many Southern California residents because of mud flows, closed roads and home damage from landslides. But there is an up side to our floods: Flood waters are a natural and needed part of the environmental processes in a mountainous region with a semi-arid climate. Efforts to reduce flood runoff by trapping waters upstream threaten our beaches.

Our sand beaches are valuable for more than aesthetics and recreation. As long as we have wide beaches, we have a buffer zone that absorbs the impact of storm-generated waves. But the sand on these beaches has to be replaced constantly. Sand taken by storm waves moves into deep water, then is funneled to the ocean basins that lie offshore.

When beaches don’t receive sand from flooding rivers because of damming or drought, wave attack advances inland, destroying the houses on the beach first and ultimately eroding the bedrock under inland structures and roads.

Geologists have long argued that river flood plains should remain undeveloped to allow for runoff water to infiltrate the ground-water reservoirs while the remaining flood waters are allowed to carry sediment to the coast to nourish beaches. The Los Angeles and San Gabriel rivers have had so many flood-control measures placed in their drainage basins that they deliver little sediment.

The Santa Clara, Ventura and Santa Ana rivers and the creeks along the Malibu, Orange County and San Diego coasts still deliver significant quantities of sand. But flood control work on these rivers and streams is under way or proposed, so prospects are not good for saving them as sand suppliers for the beaches.

If we continue to minimize the vital role that flooding plays in the Southern California environment by damming too many rivers, then we must be prepared to take over the role that rivers have played in nourishing beaches.

This means going to the dam sites, digging up the trapped sediment and trucking it to the coast.

Southern Californians have to understand that if the natural forces operating in our environment are altered too greatly, then severe and unwanted environmental and monetary penalties are the inevitable result. Rock walls will be needed to protect expensive coastal real estate once the beaches are gone. It will be too expensive and impractical to haul sand from the mountain flood control dams to the coast to maintain the beaches. This is work that has been done effectively and at no cost by our streams and rivers.