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HYDRAULIC STRUCTURES DRAINAGE CRITERIA MANUAL (V. 2) 2.7 SimDlified Minimum Desian AoDroach for Boatable Channels e Due to the fact that a special standard of care for the design of drops and iow-head dams on boatable channels is required, the following design approach for boatable channels is limited to suggestions for the experienced hydraulic structure designer once the channel has been determined to be a boatable one. 1. Contact reliable whitewater boating experts to discuss general design objectives and boater safety concerns. 2. Select maximum height of individual drops--generally 4 feet. If they are more than 4 feet, a physical hydraulic model may be necessary. 3. Determine basic drop characteristics to be compatible with public safety and recreational boating. Suggestions are as follows: . Use a Froude number, F" less than 1.5 at the toe of the drop. . Avoid reverse rollers under all conditions of flow. . Assess stability of the structure taking into account expected downstream channel degradation. . Consider the slope of the downstream face of a sloping drop; 10(H) to 1 (V) is common. e . Provide boat chute with pilot rocks for routine boat passage of drop. . Do not use an energy dissipating basin; instead, continue the sloping surface at least 5 feet below the downstream thalweg of the stream. . Provide adequate warning signs and portage area. . Use grouted sloping boulder or appropriately sized large ungraded sloping boulder structure. . Consider vertical cutoff walls at the upstream end for seepage control. 4. Obtain peer review on the preliminary design. 5. Allow for follow-up rock adjustment after completion, especially for boat chutes. 6. Observe for proper functioning over a range of discharges in case further adjustment of rocks is required to achieve a normal standard of care for design of the drop. e HS-36 06/2001 Urban Drainage & Flood Control District