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I I I I II I 10) Public Acceptability Finally, the scope of work required a discussion of research needs. This includes specific guidance related to the GSB and the VHB drops which are already being implemented, but for which research would be especially useful. Other research related to rock and riprap is discussed. ADDITIONAL INVESTIGATIONS COMPLETED During the course of the study, four topics emerged which warranted more detailed investigat.ions than originally conceived in the study scope. The following paragraphs highlight these four topics. I I I I I I I I I I I I I II Trickle Channels Many or most of the existing drops do not provide for a significant trickle fluw conveyance through the crest. In the field, this resulted in aggradation and loss of channel conveyance upstream of the drop. The design implication of providing a contiguous trickle channel is that much higher unit discharges will occur in this area, causing a jet or a portion of the flow in the basin to remain in a supercritical flow condition and potentially force a portion of the jump further downstream. On the other hand, a portion of the vertical height of a design trickle channel could be used to create a deeper tail water in the main portion of the basin. Hydraulic Analysis of the Drop Profiles through the Trickle Channel and Main Drop Area A computer program was developed to allow simultaneous hydraulic analysis of the trickle channel zone through the drop and the main portion of the drop. It provided analysis of the supercritical flow down the face of a sloping drop, and determination of the location where the jump would begin. Rock Sizing Criteria There is an extremely high failure incidence for sloping riprap drops. It was apparent that there were major construction problems. It was also apparent that a more conservative approach needed t.o be developed and evaluated against (refined by) the successes and failures in the field. A fairly extensive literature search (part of 1-3