FloodingFlooding is a natural attribute of rivers. Flooding is defined as flow that exceeds the capacity of the channel, i.e., when flow inundates the floodplain. A flood is a streamflow event where there is more water flowing in the stream than the channel can handle. Under these flood conditions, water spills over the streambanks onto the adjacent floodplain, which can be considered as part of the natural channel that is periodically used by the stream and that has been constructed by the stream over millennia. Floods represent the upper extreme of runoff generation and produce most of the sediment erosion, transport, and deposition within a watershed. Stormwater runoff is the "excess" rainfall that exceeds the infiltration capacity and flows over the ground surface. It is the portion of runoff that causes the initial rise in a storm hydrograph and usually causes the peak flow in the receiving stream. In the urban context, stormwater runoff often contributes to flooding because the streets, parking lots, and storm drains generate far more (often 10 to 1,000 times more) surface runoff than would have occurred prior to development and paving. The collection and conveyance of stormwater runoff in efficient storm drains also delivers water to streams much faster than a natural channel would. Therefore, the lag between rainfall and runoff is greatly reduced, and the peak flow is usually increased. The creation of large impervious surfaces and storm drains in urban areas may generate flooding (overbank flows onto the floodplain) from much smaller storms than would have occurred in the absence of development. Planning and Management City and County general plans must evaluate flood hazards and develop strategies for floodplain management, as noted in the OPR General Plan Guidelines. Flooding is often a regional problem that crosses multiple jurisdictional boundaries. Policies are best developed cooperatively with local, state, and federal agencies, including special districts, to create feasible solutions, Guidelines for the preparation of an optional floodplain management element are provided in Appendix C of California's General Plan guidelines. The Department of Water Resources’ Division of Flood Management can provide floodplain management and flood control information, including floodplain maps, where available. In addition, the state Cobey-Alquist Floodplain Management Act encourages local governments to plan, adopt, and enforce floodplain management regulations through an ordinance or other means (Water Code §8400, et seq.). Los Angeles conducts flood planning and management, which you can read about on their Web site. Floodplain and Management In our channel-centric view of rivers, the floodplain is often a neglected component of the fluvial system. This view has been prevalent historically in developed areas of California, including the San Fernando Valley, where attempts to convey flood flows within existing channels are common. Prior to urbanization, natural river channels had evolved to accommodate the "bank full flow" (flows with a range of recurrence intervals of about 1 to 5 years) while higher magnitude flood-flows inundated the adjacent floodplain and riparian zone on average every few years. The current assumption that the channel itself, without its floodplain, can convey a full range of flood flows is unrealistic in some places, and has led to considerable flood hazards. Moreover, the hazard worsens with increases in watershed development that reduce infiltration and increase runoff and peak flows. In the Tujunga watershed there may be places and flows where this is true -- the channels are incapable of delivering the precipitation pouring into the drain and channel system. Floodplain urban development has led to a situation where there is often no riparian buffer between the top of the channel bank and the adjacent development. In this case, attempts are made to prevent the natural processes of bank erosion and channel migration, processes integral to the storage and transfer of sediment within a river system, as well as to health and succession of the riparian vegetation. Levees and bank protection are employed to prevent natural geological and hydrological processes acting between channels and their floodplains, thus creating static river shapes and structure. Los Angeles, including the lower Tujunga Was watershed is an extreme example of this, where there is no natural floodplain or riparian zone and the channel-floodplain connection has been destroyed. Flooding and the Watershed A watershed approach to reduce flood hazards must consider cumulative effects of past and proposed floodplain changes. The most successful approach to minimize flood hazards it to minimize floodplain development, and to instead preserve the natural flood storage capacity of the floodplain. Flood hazard reduction and floodplain management is encouraged by many, including the California Department of Water Resources and the professional, non-profit educational organization, the Floodplain Management Association. Tujunga Watershed Flood History There were severe floods in the watershed in 1914, 1934, and 1938. Following these floods, several key projects were undertaken that now define the lower watershed and waterways. These were the lower Tujunga Wash channel, the Pacoima Wash channel, the Big Tujunga Dam, and the Hansen Dam.
References and Resources Philip Williams & Associates, Ltd. 2002. Tujunga Wash: An Investigation of Channel Hydraulic Conditions and Potential Restoration Alternatives. A report to WaterCycle LLC and The River Project. |