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aggraded rapidly with increasing discharge and maintained its level near the crest height of the structure. (Trace the values of p with time). Opservations made during the field investigation of the Rio Grande indicated that the level of sediment deposition upstream of several exist- ing weir structures was at an appreciable depth below the tops of the weirs. The apparent difference in aggradation between laboratory studies and prototype can be qualitati.vely explained by comparing the two systems. In the laborator"J studies, a recirculating flume of both sand and water was used. The supply of sediment to the upstream end of the flume was depen- dent upon the amount of sediment transported into the tailbox (see Fig. 1). Generally, all of the sediment entering the tailbox is pumped to the head of the flume. If there is an unbalance in total sediment transport up- stream and downstream of the weir in the flume, an adjustment will occur. With the existence of a control structure in the flume, the velocity up_ stream of the structure for flows not fully submerging the wier will generally be less than the velocity downstream. (At least, it was initially when the sand bed thickness was the same throughout the flume). More sedi- ment is therefore transported from the downstream channel into the tailbox (and subsequently to the headbox) than is transported over the structure, thus aggradation will result. This was evident in Figs. 10 and 11 in the decreaSing values of p with time. If, however, the sediment supply to the upstream end of the flume is somehow restricted, increase in diSCharge and velocity will cause degradation of the upstream channel. Actually, during these tests the former condition prevailed. The latter condition was enforced for the studies in the 8 ft flume to avoid the difficulty experienced in the smaller flume. ": ",9L- ~' ~ V...1.w .'- -17 - .