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Laguna Canyon Flood Investigation Project Location: Project Budget: Year Completed: Client: Orange County, California $24,000 1995 California Corridor Constructors Heavy precipitation on the Laguna Canyon drainage basin on January 6 and January 10, 1995, caused extensive flooding of properties in the canyon and in the City of Laguna Beach. The California Corridor Constructors Company asked Boy Ie to investigatl~ the two floods to determine what led to their severity. The investigation consisted offhree tasks: 1) perfonn a flood surwy, 2) evaluate the soil erosion in the basin, and 3) prepare a hydrologic analysis of the two events. The first task involved a field visit to collect data concerning the extent of the flooding, including high water marks, flow patterns, and other documentation. Data collection included taking photographs, interviewing flood witnesses, obtaining precipitation and flow data, and acquiring newspaper articles. The collected data was evaluated and incorporated into a preliminary report with an annotated photo album. The second task involved an evaluation of the soil,~rosion potential for the basin just prior to the January 1995 storms. Due to extensive wildfires in Octobt,r 1994 and disturbed soils in the construction area of the San Joaquin Hills Transportation Corridor, significant soil erosion occurred. Existing soil survey information was combined with mapping of the disturbed areas to develop estimates of the potential soil loss that contributed to the high sediment loads in the two floods. The third task involved modiJying the existing hydrology model of Laguna Canyon to determine the effect of the changed land use on the peak flows. The model developed by the Orange County Environmental Management Agency, uses the Rational Method for small subareas and routes and combines the subarea flows. By modeling various scenarios for each of the two storms with and without the burned areas and with and without the transportation corridor areas, the relative impact of each disturbance was evaluated. 11 ANdERSON CoN5UlTiN<j EN<jiNEERS, INC.