ALLUVIAL FAN FLOODING HAZARDS: AN ENGINEERING GEOLOGIC APPROACH FOR REGIONAL PLANNING AND PRELIMINARY DESIGN

Alluvial fans form where streams emerge from mountain fronts onto relatively flat valley bottoms. Within a mountain range, particularly in areas experiencing tectonic uplift, a stream is often steeply inclined and confined to a single channel by narrow canyon walls. Once a stream reaches the mountain front its gradient typically flattens and waters may spread into a distributary network of channels, both of which reduce the depth and velocity of stream waters and reduce size and volume of sediment that the stream is capable of carrying. Debris flows and hyperconcentrated flows are a common component of actively building alluvial fans. They respond to these diverging channel patterns and lack of channel confinement by avulsing or splaying on to the surface of the alluvial fan, inundating areas that are not typically perceived to be hazardous. To assist regional planning efforts it is important to identify the general distribution and age of alluvial fans, and to characterize the potentially hazardous processes in advance, as development continues to encroach on alluvial fan surfaces. However, the analysis of flood hazards on alluvial fans is complex. A three stage analysis approach developed by the National Research Council (NRC, 1996) uses geologic information, including the review of historic imagery, to assist in establishing alluvial fan boundaries, and relative state of activity. These data provide a physical basis for detailed hydrologic/hydraulic modeling of the design flood on the active fan surfaces.

To address the both necessary information for planning, and in part the first two stages of the NRC analysis approach, the California Geological Survey has developed a engineering geologic approach for assessing the alluvial fan depositional environment as a preliminary means of identifying areas susceptible to alluvial fan flooding. Engineering geologic maps developed from surficial geologic maps and field investigations are used to identify the general distribution of alluvial fans, the relative age of alluvial deposits, and the relative likelihood of alluvial fan flooding. These maps may then form the engineering geologic basis for communicating the potential distribution of alluvial fan flooding from both a planning and preliminary design perspective as well as help frame more detailed geologic studies.