SPATIAL DISTRIBUTION OF RAINFALL CHARACTERISTICS ASSOCIATED WITH DEBRIS FLOWS IN THE 2004-2005 SEASON IN SOUTHERN CALIFORNIA

High rainfall rates and seasonal totals for the 2004-2005 winter season in southern California have caused fewer debris flows than might have been expected; however, some areas were affected by significant numbers of debris flows. To some extent, this spatial distribution is probably a result of spatial variations in intensity and duration of rainfall. The spatial distribution of cumulative rainfall (and derivatives such as hourly rates and indexed cumulative rainfall) for the principal storm episodes in the 2004-2005 season can be examined quantitatively using hourly rain gage records. Although it has not been possible to make comprehensive inventories following each storm period, repeated reconnaissance showed that significant numbers of debris flows occurred in the San Timoteo badlands (10/20/04 and 1/7-10/05), the Sunland-San Fernando area (1/7-10/05), in tributaries to San Juan Capistrano Creek (1/7-10/05), the Ventura-Santa Paula area (1/7-10/05), and some other smaller areas.

The rainfall characteristics, and their spatial distributions at critical times, can be compared for the 2004-2005 events, as well as with a multi-year database of archival rainfall in southern California covering storm periods known to have produced significant debris-flow activity. Rain gage distribution is not uniform across the area, and reporting practices vary from county to county; CDEC and NCDC also use different reporting formats. Still, maps using station locations and graphs of cumulative season rainfall, variations of intensity with time, ratios of cumulative seasonal rainfall to mean annual precipitation, and rain accumulations in excess of selected thresholds provide a basis for interpreting reconnaissance observations. Most of the debris flows occurred during the October and December-January storm episodes. Although rains in February and March contributed to exceptional seasonal totals, few additional debris flows were observed during repeats of earlier traverses.