COMMENTARY ON FLOOD CONTROL : February Rain Posts a Santa Ana River High-Water Warning : Storms in 1862 and 1938 would have overcome Prado Dam. Will a new upstream dam be ready before the next deluge?

Early February’s storms have shown Southern California again that while we live in a desert climate that averages only about 15 inches of rain per year, we get that rain in often-intense, short-term deluges. Great property destruction and interruption of people’s lives and businesses result from these deluges, not to mention the tragic loss of life, such as recently happened in Los Angeles.

Although Los Angeles and the rest of Southern California is not the first experiment in desert living, it is, perhaps, the grandest. Most Southern Californians get their water from pipes and canals traveling long distances. Orange County, however, gets more than half of its water from the deep, porous sands that underlie the orange groves, strawberry fields and new homes of the county.

The Santa Ana River is the source of most of this water; we depend on its flow annually to replenish the 265,000 acre-feet of water pumped from the ground annually. (An acre-foot is about 326,000 gallons.) The county buys supplemental water yearly from outside agencies such as the Metropolitan Water District. When we take more water from the ground than the river can resupply, we buy extra water to put back into the ground.

We live in a state of balance among drought, flood, expensive water purchases and new-growth needs. The only facility that allows us to manage these forces is Prado Dam, up the Santa Ana River bordering Corona.

Before the dam was completed in 1941, floodwaters rolled unchecked from the high mountain areas of the San Bernardino, San Gabriel and San Jacinto Mountains to the ocean. If, as in 1938, the rainfall comes in intense storm cells, the normally dry river and stream channels are unable to contain the large bulk flows, and the water spreads out onto the flood plain, or flat nearby low ground.

In our case in Orange County, these flood plains include most of Fullerton, Anaheim, Orange, Santa Ana, Fountain Valley, Garden Grove, Stanton, Costa Mesa, Newport Beach and Huntington Beach. Floodwaters several feet deep flowed over these town sites in 1938, when the late February-early March rains came with an intensity thought to be unparalleled in Southern California history. Prado Dam was built as a flood protection structure to prevent such an event from damaging the county again.

Regrettably, the first test of Prado Dam showed a serious weakness in its ability to control flows equal to those in 1938: Again great damage, although little flooding, occurred along the downstream path of the Santa Ana River.

Unfortunately, history tells us that twice before, storms up to three times the fury of 1938 have occurred; geologic theory tells us that these storms will occur again at any time; and common sense tells us to do something about it as quickly as possible.

Cooperation between the Orange County Water District, which manages the river and various diversions that add floodwaters to the ground, and the Army Corps of Engineers, which manages Prado Dam, can do a great deal to soften the blow of intense storms, such as those of early February this year, 1969 and 1938.

Recreational lakes, large holding pits, golf courses and other potentially expendable and low-cost resources can be maintained along the flood paths.

However, what about a storm such as that of 1862, when virtually all of the county was under water? For a storm of that immensity, Prado could fail. If waters top an earthen dam, rapid erosion and undercutting make short work of the structure; it may fail in a matter of hours.

Prado contains a concrete-lined spillway, should the flow into the area behind the dam be too great for the pipe that lets water out. Once water is in the spillway, the Santa Ana River is again an unchecked torrent in flood to the ocean.

During a storm, it becomes a management nightmare to check the volume of water flowing in behind the dam, the water being allowed out and the damage that discharges might cause to downstream structures, all versus the water building up in a matter of hours to enter the spillway.

If more water floods behind the dam than can safely exit the spillway, it will top the dam and cause the dam to fail. This has never happened, but in 1969 the outlet pipe to drain the water was near maximum capacity to prevent spillage over the large concrete spillway near the Riverside Freeway.

Immediately after the 1969 storms, the Corps of Engineers determined that Prado Dam was not big enough to control an 1862-type flood. By 1975, the corps released a report recommending an “all-river plan” for what has become known as Project Mainstem.

Project Mainstem calls for a new dam upstream of Prado (Seven Oaks Dam in the San Bernardino Mountains), raising the height of Prado and improving the channels and crossings of the streams and Santa Ana River in Orange County. Congress approved the project in 1986, requiring that up to 50% of the cost be borne by local sponsors--in our case, the Orange County Flood Control District.

In 1988, the California Legislature approved the project and agreed to reimburse local sponsors for 70% of the costs. In 1989, the Corps of Engineers and the flood control districts of Orange, San Bernardino and Riverside counties agreed jointly to build the Mainstem Project.

About $85 million has been approved by Congress and activities have begun on the 10-year project. Orange County has approved $192 million of its total allotment of $427 million.

So what do we get?

These improvements are designed to prevent up to $15 billion in damage and extreme hazard and loss to 3.3 million people in Orange County alone. In addition, the project considers wildlife habitats, recreation, conservation and transportation.

You will see these improvements through the years, as Pacific Coast Highway is widened and improved where it crosses the river and as channels are moved and lined and other engineering feats are accomplished.

History shows us that we have been lucky here in Orange County. The hazards that happened in Los Angeles a few weeks ago are small compared to the potential for disaster here. We have five to six years to wait it out until Seven Oaks Dam is finished.

The entire Mainstem river project hinges on that dam as the controlling structure; the rest of the project merely modifies and handles the flow checked by this new, large dam.

Despite our recent warm weather, we are entering a wet cycle, and we will recover from the drought. Let’s hope our luck holds and we don’t get too wet!