Role of Precipitation Patterns on the Bankfull Hydraulic Geometry of Alluvial Channels

Abstract:

Universal modeling of bankfull hydraulic geometry based on basic watershed variables is an area of ongoing interest. Watershed characteristics affecting bankfull channel geometry include drainage area, valley slope, precipitation, elevation, vegetation, and others. While drainage area is an important predictor for bankfull channel geometry across watersheds, the effect of the amount and nature of precipitation is important to channel morphology and needs to be further explored. Average annual precipitation has been used with some success in modeling bankfull channel geometry, but it does not sufficiently capture the effect of varying rainfall intensity and duration between different climatic regions with similar average annual precipitation. Bankfull channel discharge is often defined to be a morphologically dominant discharge and is associated with moderately frequent flooding events with an average return period of about 1.5 years. Consequently, rainfall of a given intensity, duration and frequency may be attributed to the bankfull discharge and to the bankfull hydraulic geometry of a given channel. This study seeks to identify a possible ‘dominant precipitation’ that is important for the evolution of bankfull hydraulic geometry. We employ precipitation intensity information of uniformly defined duration and frequency as one of the predictors of bankfull hydraulic geometry for over 500 channel reaches. Preliminary results of regression analysis suggest that rainfall intensity is a significant predictor for bankfull discharge and average channel depth but not for bankfull channel width.