Debris Basins are instrumental components of the flood control system. Typically located at the mouths of canyons, debris basins capture the sediment, gravel, boulders, and vegetative debris that are washed out of the canyons during storms. The debris basins capture the material and allow the water to flow into the downstream storm drain system, thereby protecting the downstream drainage system. If these materials were permitted to flow downstream, blockage could occur in the system, causing localized flooding and property damage. Increased sediment loads in storm flows also accelerate surface wear and thus shorten the service life of the system's concrete channels and drains, causing large-scale, multi-million dollar re-construction projects. These projects can have their own environmental impacts, such as traffic delays and noise. By capturing the debris in basins we ensure that the drainage systems remain unplugged and operate smoothly during storm events with an extended service life.
Once debris basins are full of sediment, it is imperative to remove the sediment as soon as possible to provide storage capacity for sediment from future storms. Debris basins clean-outs range from 1,000 to 300,000 cubic yards. (One cubic yard is roughly the size of your washing machine at home. The Rose Bowl in Pasadena would hold about 400,000 cubic yards.) Depending on a basin's size, clean-outs can take anywhere from two days to several months. If the debris is not removed, the basin would not provide the necessary flood or debris protection and homes downstream of the facility may be damaged by debris flows during future storms.
Moving the sediment out of a debris basin requires excavation equipment and numerous dump trucks. Due to a debris basin's function to protect neighborhoods, most often residential ones, these dump trucks have to utilize the local streets to transport the sediment to a Public Works' sediment placement sites or some other authorized disposal sites. This presents certain regrettable but unavoidable consequences such as dust, noise, and increased traffic and travel times along the streets compromising the haul route. However, if the basins were not cleaned out, even worse consequences of debris flows and localized flooding would cause significant damage and endanger local residents. Local residents would experience noise, dust and traffic delays during the repair and re-construction of the facilities and structures damaged by uncaptured debris flows. Understanding these possibilities hopefully helps residents understand and better endure short-term inconveniences along a haul route.
FLOOD CONTROL AND THE ECONOMY
Although Public Works endeavors to minimize impacts from its debris basin clean-out and sediment hauling work, regrettably the work causes unavoidable short term disruptions to nearby residents. However, the efforts are vital to provide flood control services to facilitate a stable economy. With this necessary maintenance work, the debris basins, along with the drains and channels of the flood control system, operate as designed to safeguard the communities in which the 10 million residents of Los Angeles County live and the businesses the provide goods and services to them and others. Even during the storms of the 2004-05 storm season, the second wettest year on record, the vast majority of employees drove to work along passable roads; businesses shipped their products to market; students attended school; and umbrella toting residents didn't have to give a second thought about the integrity of their houses or places of work during storms. This could not occur during major storm events without a properly maintained and reliable flood control system.
The topography and location of Los Angeles County create a high potential for flooding. With the Pacific Ocean providing a perpetual source of moisture, storms can drop several inches of rain over a relatively short period of time. The Los Angeles County flood control system was designed to convey run-off from streets and occupied properties as fast as possible. The flood control channels we drive over on a daily basis are able to quickly move vast amounts of run-off, thereby protecting property and lives in the process.
At the same time, however, as Southern California is primarily a desert environment, Public Works strives to conserve storm water runoff generated during storm events. To accomplish this, Public Works operates 27 groundwater recharge or spreading facilities, which are adjacent to some of these channels. Public Works directs the storm water from the channels to percolate back into the groundwater table. Local water companies or water rights holders can then pump the water to the surface for water supply purposes.