Regional bankfull geometry relationships for southern California mountain streams and hydrologic applications

Abstract This study develops and intercompares regional relationships for bankfull channel width, hydraulic depth, and cross-sectional area for southern California mountain streams based on several data sources: surveyed streams, US Geological Survey stream survey reports, and existing literature. Although considerable uncertainty exists in estimating bankfull conditions, the relationships developed from the varying data sources show significant agreement. For small watersheds with drainage area ranging from 15 to ~ 2000 km 2 , the estimates of bankfull top width ranged from 7.2 to 44.5 m and hydraulic depth estimates ranged from 0.35 to 1.15 m. The utility of the developed bankfull geometry regional curves is demonstrated for southern California catchments through (a) the computation of the bankfull discharge and (b) the estimation of the surface runoff response necessary to produce bankfull conditions in the streams at the outlet of these catchments. For selected locations with instantaneous flow records, the occurrence frequency of events exceeding bankfull flow was examined for the available 10–15 year span of observational records. Bankfull discharge estimates for all small watersheds in the region ranged from 1.3 to 74 m 3 /s, while the range at the selected gauged stream locations was from 2.6 to 16.4 m 3 /s. Stream locations along the Transverse Mountains of southern California showed an average occurrence frequency of less than 1 year, whereas along the Peninsular Mountains the average return period tended to be greater than 1 year. The application of the regional curves to the estimation of the surface runoff response necessary to produce bankfull conditions at the channel outlets of small catchments may be used as an index for conditions of minor flooding with saturated soils. This surface runoff response index ranges from 2.0 to 5.5 mm for a 3-hour rainfall duration for southern California watersheds greater than 15 km 2 in area. Differences between the values for the Peninsular and Transverse Mountain Ranges are linked to geological, climatic, and geomorphologic differences. The developed regional geometry relationships are suitable for use in various hydrologic modeling applications, including distributed modeling with high resolution pertinent to flash flood forecasting.