SCOUR BELOW AN OVERFALL: PART II. PREDICTION

The impinging jet is the driving mechanism causing scour below grade–control structures, dam spillways, gullies, pipe outlets, etc. The objectives of this study were to: (1) utilize a previously developed excess stress parameter approach, with small modifications, for the free overfall jet; (2) develop similar excess stress parameter approaches for the submerged circular jet; (3) determine and compare excess stress parameters for both overfall and submerged circular jet scour test results; and (4) compare erodibility results for each experimental system. Large–scale overfall tests were conducted in an outdoor flume 1.8–m wide and 29–m long with 2.4–m walls. Small–scale submerged circular jet tests were conducted in the laboratory on sample from the large–scale overfall tests. The scour tests were conducted on a cohesive soil, prepared at various average compaction moisture contents (12% to 16%) and densities (1.66 to 1.80 Mg/m3). Critical stress ranged from 0 to 9 Pa, and the detachment coefficient ranged from 0.1 to 42 cm3/N–s. Comparisons of the excess stress parameters determined from the overfall tests and circular jet tests were of the same order of magnitude. The results indicated that the laboratory sample tests can be used to indicate trends in the variation of erodibility with compaction moisture content but can only make crude estimates of the actual scour. In order to improve predictive capabilities, more laboratory sample tests need to be conducted to determine the large–scale flume test variability, or small–scale in–situ tests in the impingement zone need to be conducted.