SCOUR BELOW AN OVERFALL: PART II. PREDICTION

Abstract

The impinging jet is the driving mechanism causing scour below gradecontrol 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 submergedcircular jet; (3) determine and compare excess stress parameters for both overfall and submerged circular jet scour testresults; and (4) compare erodibility results for each experimental system. Largescale overfall tests were conducted in anoutdoor flume 1.8m wide and 29m long with 2.4m walls. Smallscale submerged circular jet tests were conducted in thelaboratory on sample from the largescale overfall tests.<br><br>The scour tests were conducted on a cohesive soil, prepared at various average compaction moisture contents (12% to16%) and densities (1.66 to 1.80 Mg/m3). Critical stress ranged from 0 to 9 Pa, and the detachment coefficient ranged from0.1 to 42 cm3/Ns. Comparisons of the excess stress parameters determined from the overfall tests and circular jet tests wereof the same order of magnitude. The results indicated that the laboratory sample tests can be used to indicate trends in thevariation of erodibility with compaction moisture content but can only make crude estimates of the actual scour. In order toimprove predictive capabilities, more laboratory sample tests need to be conducted to determine the largescale flume testvariability, or smallscale insitu tests in the impingement zone need to be conducted.