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I I I I I I I I I I I I I I I I I I il 2.4 Culverts and Storm Water Sewer Systems The function of a drainage culvert is to pass the design storm flow under a roadway, railroad or other embankment section without causing excessive backwater and creating excessive downstream flow velocities. Flow velocities are considered excessive when erosion or safety of downstream structures and embankments are endangered. Consideration must be given to the effect of high velocities, local eddy currents, or other turbulence on the channel, surroundi ng property and roadway embankment. Flow vel ociti es greater than 2 ft/sec in culverts must be maintained at the design flow in all drainage structures to limit siltation. Where local silting or scour is determined to be a threat, protection shall be installed to ensure that damage to or failure of the structure will not occur. The hydraul ic design of any culvert may be affected by its cross-sectional area, shape, entrance geometry, length, slope, construction material, and depth of ponding at the inlet (headwater) and outlet (tailwater). Culvert flows are classified as having either inlet or outlet control. Inlet control means that the discharge capacity of a culvert is controlled at the culvert entrance by the depth of headwater and the entrance geometry, including the barrel shape and cross-sectional area, and the type of inlet edge. In inlet control, the roughness and length of the culvert barrel and outlet conditions, including depth of tailwater, are not factors in determining culvert capacity. Outlet control means that the discharge capacity of a culvert is controlled at the culvert exit. This usually results from a downstream flow constriction causing the tailwater to back up into the barrel, thus reducing the flow. The general criteria for culvert design shall be: 1. Culvert inlets shall be designed for entrance and friction losses. 2. Outlets shall be designed to avoid potential sedimentation and erosion of the downstream channel. Outlet erosion control devices may be required where excessively high discharge velocities occur. 3. The minimum size culvert shall be 15 inches in diameter or its hydraulic equivalent. The use of culverts less than IS" in diameter for driveways must have the approval of the governing jurisdiction. 4. All culverts shall be designed for H-20 loading with the appropriate embankment considerations. 5. Culverts, designed for the la-year peak discharge, will be designed for fl owi ng fu 11 wi th headwater depth no greater than the cu 1 vert height, and culverts designed for the lOa-year peak discharge will be des i gned under all owab 1 e headwater and ta ilwater conditi ons (see Chapter _). 2-4