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<br />HYDRAULIC STRUCTURES <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />2.1.2 Boatable Channels. Channels that are known to be boatable, either now or that will be in the <br /> <br />future, and those others that are classified by the Colorado Water Quality Control Commission for Class 1 <br /> <br />or 2 Recreation, but are not presently judged to be boatable, should have hydraulic structures designed <br /> <br />with public safety as a special consideration. The designer should not set the stage for hazardous <br /> <br />hydraulics that would trap a boater, such as at a drop structure having a reverse roller that may develop <br /> <br />as the hydraulic jump becomes submerged. <br /> <br />e <br /> <br />Design criteria for boatable channels, grade control structures, and low-head dams include avoidance of <br />submerged hydraulic jumps, a sloped or stepped downstream face, and avoidance of a deep stilling basin <br />that would encourage the creation of a submerged hydraulic jump. One design approach is to direct the <br />hydraulic momentum into the channel bottom at a relatively flat angle to help avoid a reverse roller. A <br />downstream face of large boulders with high roughness sloped at 10(H) to l(V) has been used with <br />success on several projects along the South Platte River and on Clear Creek in conjunction with boat <br />chutes. <br /> <br />Drop structures or low-head dams in boatable channels should incorporate a boat chute designed in <br />accordance with carefully planned components that are consistent with recreational requirements for <br />boater safety. Often, physical model studies are used to verify the efficacy of the proposed design. <br /> <br />Hydraulic structures on boatable channels should not create obstructions that would pin a canoe, raft or <br /> <br />kayak, and sharp edges should be avoided. <br /> <br />e <br /> <br />2.1.3 Grass and Wetland Bottom Channels. Structures for grass and wetland bottom (i.e., <br />non boatable ) channels are described in detail on the following pages and are represented by a variety of <br />choices and shapes to suit the particular site and related hydraulics. <br /> <br />Based on experience, the sloped drop has been found to be more desirable than the vertical wall drop <br /> <br />that has a free water surface drop with a hardened energy dissipation basin because this can create a <br /> <br />reverse roller and backfloweddies. Because of safety concerns, vertical drops are considered less <br /> <br />desirable than sloping drops in urban areas. Other disadvantages of a vertical drop include the <br /> <br />turbulence and erosive effect of the falling water on the drop structure, necessitating high maintenance. <br /> <br />It is desirable to limit the height of most drops to 3 to 5 feet to avoid excessive kinetic energy and to avoid <br />the appearance of a massive structure, keeping in mind that the velocity of falling water increases <br />geometrically with the vertical fall distance. <br /> <br />2.1.4 Basic ADDroach to DroD Structure Desian. The basic approach to design of drop structures <br />includes the following steps: <br /> <br />1. Determine if the channel is, or will be, a boatable channel. If boatable, the drop or check <br />structure should use a standard of care consistent with adequate public safety. <br /> <br />e <br /> <br />HS-8 <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br />