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UDFCD EXPERIENCE SINCE 1990 UDFCD's experience since it published its new criteria for four types of drop structures in a revised "Hydraulic Structures" chapter to the Urban Storm Drainage Criteria Manual (USDCM) in November, 1990 has been mostly positive. The versatility and aesthetic features of the Grouted Sloping Boulder (GSB) design have made it the most popular design with both developers and local governments, and the UDFCD. The height of the drop is limited only by the topography in which it is placed. The tallest drop seen to date is approximately twenty feet high. On the other end of the spectrum, the design has proven valuable as a low flow check structure, one to two feet high, placed across only the low flow channel of the drainageway. UDFeD's maintenance program has used the GSB to "hook on" to existing structures to extend their vertical distance to arrest degradation and undennining of the original structures. The basic geometry of the GSB has been modified somewhat on occasion to avoid buried utility lines or to fit into a tight topographic situation. From the standpoint of aesthetics, the boulders which form the structure are more visually pleasing to most people than concrete structures. In a drainageway with a base flow, the GSB can form a waterfall pleasing to both the eye and ear, if proper attention is given to spreading the base flow across the structure. The structure geometry can also be modified, as noted above, to provide a more interesting looking structure in a high traffic area. Finally, portions of the structure can be buried and vegetated, with the understanding that infrequent flows will damage the vegetation and require repair. UDFCD is aware of only one case where a developer used the Vertical Hard Basin design, and that was in an industrial park; and no cases where the Vertical Riprap Drop was used. The general feedback received by UDFCD was that the vertical drop presents a hazard which is not acceptable in most settings, particularly residential; and the height limitation restricts its application. Aesthetics are also a consideration, although various architectural treatments are certainly possible, at added cost. UDFCD is not aware of any use of the Baffle Chute design since the new chapter was released, although UDFCD has used the application on a few occasions in the past. Aesthetics are the major reason why no one seems to want to use what is otherwise a fine hydraulic structure. DROP STRUCTURE RELATIONSHIP WITH TRICKLE CHANNELS AND OVERALL STREAM ST ABI1.ITY The geometric shape of the control crest and hydraulic characteristics of the drop structure are critical to the ultimate channel stability. Numerous drop examples exist in Denver and other regions where the lack of a compatible trickle channel and shaping of the overall drop crest often result in aggradation and loss of major flood capacity in the upstream channel. Also, since the numerical criteria is solely oriented to 100 year conditions, the frequent runoff events are not really addressed. Most flood channels indicate aggradation and local erosion problems. More research is needed to explore sediment transport and stable channel design in an urbanizing situation along vegetated waterways. Nevertheless, experience has shown that drop crest geometry and hydraulics should be matched to the channel geometry and 5 Taggart, et al