Hydrographs and Estimates of Scour Depth Excess for Pier Scour Prediction

A practical procedure for using the Scour Rate in Cohesive Soils (SRICOS) method to predict bridge pier scour depth in ungauged streams is presented. The procedure is illustrated by the Interstate 90 westbound bridge over Split Rock Creek in South Dakota. Instead of generating a continuous hydrograph for the design service life of a bridge, the simplified approach considered a series of maximum annual floods. The parameters of peak flow distribution were calculated with estimates of peak flow magnitude obtained from regional regression equations. Flood duration was estimated with a rainfall atlas and a Soil Conservation Service runoff curve number based on a design storm. A set of equally probable future hydrographs was generated and used with the SRICOS method to compute the final scour depth. The predicted final scour depth followed the normal distribution. The risk values associated with different design scour depths were calculated with the mean and standard deviation of the normal distribution. The scour predictions were consistent with a bridge founded on highly erosion-resistant cohesive soils. The cumulative effect of scour from the small floods in a long sequence of floods could be represented by an equivalent time; the scour depth produced by the whole sequence could be modeled by using only the large floods if the equivalent time were small compared with the combined duration of the large floods.