FLOOD03286 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I o Hydraul ic roughnpss in accordance with the Urban Storm Drainagp. Cri- tf'ria Manual and othp.r suitable criteria. o Grass cover chosen for h)draul ic retardancf' and ease of maintenance. o Use low flow channels for low flows with capacities of about 1 percent of laO-year flow. Use riprap or other protection to prevent erosion. Maximum flood depth over low flow channel - 8 feet; maximll11 velocity in low flow channel zone - g fps. o The bottom of the channel shall have a slope toward the trickle chan- nel. o Froude nlJTlber shall be less than 0.8. 2. Natural Channels. Natural waterways will be used wherever feasible. o Channel and overbank capacity must be adequate for design runoff. o Design velocities, as computed, should not generally exceed critical velocity as indicated by backwater curve computations. Exceptions may be permitted in short reaches where variances are environmentally reasonab 1 e. o The floodpl ain 1 imits must be defined. o Filling of floodplain fringes should be limited in order to maintain channel storage capabil ity. o Hydraul ic roughness should be representative of reasonably maintained channel cond it ions. o Perform backwater curve computations based on existing conditions with C-2 assumption that floodplain limits will be revised as bridges are re- placed or new roadways are constructed. 3. Riprapped or Other Hard-Lined Channels. nels will be used with .discretion, and development for minimum reaches. Higher velocity artificial chan- then only to accomodate existing . Riprap size, design velocity and On" values are to be based upon the Urban Drainage and Flood Control District Riprap Criteria. . Side slopes from 3:1 to 2:1 may be riprapped with 2-112:1 being the maximum recommended slope. Steeper slopes must be solidly lined with an anchored wall. The preferable section for steeper side slopes is 0.5:1 with a retaining wall structure embedded with natural stone. Care must be taken for freeze, thaw, groundwater, and expansive soil effects. · The bottom of channels must generally be lined when either the velo- city exceeds 7 fps or the Froude number exceeds 0.8. · Use low flow channels for low flows with capacities of about I percent of 100-year flow. Riprap, concrete, or other protection to prevent erosion is needed. 4. Bridges. Bridges should be designed for overtopping in accordance with the criteria of the Urban Storm Drainage Criteria Manual and according to the designated street use. 5. Drops and Checks. Ch anne 1 drDps wi 11 be u sed as needed to contra 1 the hydraulic grade line and velocity to within reasonable slopes and limits. Drops will be of placed or grouted rock riprap, and be properly notched for the low flow channel. Design must account for potential vandalism.