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I 'I I I I I I I I I I 37 Specific planning and design criteria applicable to the South Boulder study area are: 1. Grass Lined Artificial Channels: Design will generally be based on channels having natural waterway hydraulic and appearance characteristics. They will be esthetically designed to the local environment and useful for local recreation continuous along the channel. Criteria include: a. A maximum design velocity of 8 fps for the 100-year storm, b, Design depth of up to 4 feet except where top width to depth ratio exceeds 30 , and then not more than 5 feet. c. Side slopes generally of 4:1, acceptable from 3:1 to 5:l, or more. d. Centerline curvature of twice the top width, but not less than 100 feet. e. Freeboard of from I to 2 feet except where overbank flow does not effect or cause damage. f. The channel bottom may be lined with concrete when velocities are low enough that siltation might occur on a grass lined bottom. g. Hydraulics roughness in accordance with DRCOG criteria. h. Grass cover of crested wheat grass or equal. I I I I I I I I i. Use trickle channels for low flows with capacities of about one per cent of design flow, but not to exceed 20 cfs. Use riprap, concrete or other protection to prevent erosion. j. The bottom of channel shall have a 1% slope towards the trickle channel. k. Froude Number (turbulence factor) shall be less than 0.80. l. Design top width to range between 10 and 30 times the design depth. 2. Natural Channels. Natural waterways will be used wherever feasible. a. Channel and overbank capacity must be adequate for design runoff. b. Design velocItIes as computed should not generally exceed critIcal velocity as Indicated by backwater curve computa- tions. Exceptions permitted in short reaches where variances are reasonable environmentally.