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I I I I I I I I I I I I I I I I I I I prevent future reduction of hydraulic capacity. Channels must be maintained. 6. Drops are used to limit flow velocity and thereby control ero- sion where justified. Grass-Lined Channels 1. The improved channel closely follows the existing natural channel alignment and invert. 2. Flow velocity in a grass-lined channel does not exceed 7 feet per second in straight stretches. Velocities at curves must be analyzed separately. 3. Channel capacity must be adequate for 100-year flood with six detention dams. 4. Channel side slopes are to be 4:l in new development. In already built-up areas 3:1 side slopes may be permitted to reduce land acquisition costs. In densely urbanized areas, the minimum slope permitted would be 2~ to 1. 5. A uniform, dense stand of suitable grass is required. Blue grass with adequate provision for sprinkling or other equally effective grass is recommended. 6. At least 0.5 feet and preferably 1.0 feet of freeboard is recom- mended for the lOO-year flood design discharge. 7. Drops are used to limit flow velocity if necessary. 8. Channel centerline curvature is at least twice the channel top width, but not less than 100 feet. 9. Froude number of channel flow does not exceed 0.8 to prevent excessive wave generation. 10. The channel section includes a well-defined low flow channel to maintain non-silting low flow velocities. 11. Riprap or other slope protection is provided at transitions and sharp bends to prevent erosion. 12. On-site detention of flood water must not be reduced by new develop- ments. Concrete-Lined Channels l. The concrete-lined channel invert generally follows the existing channel invert. -51-