EROSION & SEDIMENTATION
The process of erosion and sedimentation is inevitable. For as long as there have been wind and water, there has been erosion. Erosion has shaped our valleys and mountains and will continue to, despite the best efforts of man. Erosion and the resulting sedimentation is a natural process that is the major factor in the ever-changing face of the earth.
Although the weather in Southern California has often been described as the best to be found anywhere, it is capricious and capable of producing enormous flood devastation. Storms, moving in from the Pacific generally from October to March, come up against the numerous steep mountain ranges ring Malibu, the Los Angeles Basin, and the Antelope, San Fernando, San Gabriel, and Santa Clarita Valleys. These storms frequently produce enormous amounts of runoff that spill out onto the flatlands leading to the Pacific Ocean or Mojave Desert. And, if the age-old fire sequence has occurred along the hillsides during the dry months, the rate, composition and amount of runoff are substantially increased. Since the geological composition of the mountain ranges erodes quite easily, a heavy mix of debris is added to the descending storm waters.
Beginning with the severe mudflows and loss of life that occurred in the La Crescenta and Montrose communities in the 1934 New Year's Day Storm, erosion has become an increasingly significant issue as development has expanded in the valley areas and mountainous areas of Los Angeles County. Homes and businesses now line the rivers and streams that convey storm flows to the ocean and desert. Homes dot the hillsides from Malibu to Claremont, and from Santa Clarita to La Canada Flintridge. Erosion is a key factor in the design of channels, drains and their upstream components. Debris retention structures, such as Public Works' debris basins, are designed to collect the eroded material (sediment) and prevent it from damaging property downstream, as well the channels and drains themselves. Very large volumes of sediment also collect in Public Works' reservoirs. Without continued maintenance of these facilities, the sediment will build up until such a time that the debris basins and reservoirs will no longer offer protection to downstream properties and improvements.
Therefore, the sediment collected in Public Works' reservoirs and debris basins, which originally would have been carried to the ocean, now must be removed by other means. One method of sediment removal is trucking. Depending on the severity and frequency of winter storms, the process of reservoir clean-outs could be repeated several times during a decade period. Due to their smaller size, debris basins could be cleaned out several times during a given storm season.