San Gabriel River and its drainage problems :: essays research papers fc

In 1938, the San Gabriel Mountains (Los Angeles, USA) experienced profound amounts of rain. This then led to mass movement, such as mudslides, and flash flooding, which scarlet through downtown LA, damaging buildings, homes and vehicles. After these events, a flood commission was formed, and networks of hard defences were set up along the long profile of the San Gabriel River. The beat back decrease the hydrograph, and reduce the risk of flooding.In the upper course of the San Gabriel River, the San Gabriel occlude was built. This is a filtering dam, and uses sluice gates to prevent large boulders from progressing down river. By doing this, the amount of alluviation in the river decreases, which reduces the possibility of flooding from high sediment yield, and in the case of a flood, on that point will be no harmfully large sediment. Problems, however, atomic number 18 that the velocity of the river increased as there is less load to carry, which makes the erosion process sc arce, as attrition and excoriation sessnot take place. Also the riverbed and slip-off slopes of the river are very weak and can erode easily from hydraulic action. Then there are receptive dam systems, like Santa Fe and Maddock, where the river is stored and filtered of its sediment. This shares the same benefits of the sluice gates, however, it means no natural levees can form on the riverbanks. Further downstream, there are large control dams and artificial diversion-zone river basins, like the Wittier Narrows. When the river begins to flood, the control dams close, and form a barrier. This completely prevents the river from proceeding downstream. The water is then detoured into the artificial basins, like the Wittier Narrows where it is stored until the river returns to its universal level. Then the control dams gradually re-release the stored water into the stream flow. This is effective as it prevents the flooding entering the downtown area. It also allows for things like h ydroelectric fountain and water supply to be easily obtainable, without affecting the main river flow itself for too long. This problem is that this can lead to the starvation of habitats living on board the river, as there is no water flow.