IMPINGING JET CALIBRATION OF EXCESS SHEAR SEDIMENT DETACHMENT PARAMETERS

An excess shear sediment detachment relation is commonly used in erosi on prediction models. Traditional field methods for calibration of soi l-dependent empirical constants are both expensive and conceptually su spect. The subprocesses of sediment detachment and transport cannot be physically separated, therefore they cannot be calibrated independent ly. Impingement of a clear-water jet on a soil bed produces detachment without transport at the point of maximum scout: In this article, a m athematical description of the impinging jet scour process is used to calibrate an excess shear detachment relation. Four calibration models are developed based on variants of the jet-scour rate equations (diff erential and integral form, each evaluated using arithmetic and logari thmic values for dependent variables). All models assume detachment is linearly related to excess shear. The validity of this calibration te chnique has been tested in the laboratory by measuring scour depth ver sus time for six different soils, all evaluated in a disturbed, satura ted condition. A total of 14 replicates on the soils were conducted wh ere hydraulic inputs were varied with replicate. Statistical analyses were used to place confidence limits about the optimal sediment detach ment values for each of the four models. Only the arithmetic different ial model produced results considerably different than the other three . The integrated log model produced the tightest confidence intervals and this is suggested for further use. The size of the confidence inte rvals demonstrates the replicability of the technique. The technique p roduced detachment rate constants that are similar to field calibratio n studies for soils of similar properties. Critical shear values are a n approximate order of magnitude lower but all soils were evaluated on ly in the highly erosive, disturbed unconsolidated, and saturated cond ition. The technique can be applied to soils in other conditions and i t is possible that this discrepancy is due to different soil condition s.