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II. EROSION HAZARDS II-A - EROSION HAZARDS AT CULVERT INLETS Erosion from vortexes, flow over wingwalls, and fill sloughing at culvert inlets is generally not a major problem. There are some exceptions, however. For example, where a confined approach channel is not aligned with the culvert axis, some degree of protection may be required. The area of greatest potential damage is on the outside of a sharp bend where the flow must turn to enter the culvert. At design nischarge, water will normally pond at the culvert inlet and flow from this pool will accelerate over a relatively short distance. Significant increases in velocity only extend upstream from the culvert inlet a distance about equal to the height of the culvert. Velocity near the.inlet may be approximated by dividing the flow rate by the area of the culvert opening. The risk of channel erosion should be judged on the basis of this approach velocity. It is essential that any protection provided also be adequate for flow rates less than the maximum design rate, since depth of ponding at the inlet is less and greater velocities may occur. This is especially true in channels with steep slopes where high velocity flow prevails. DEPRESSED INLETS Culvert inverts are sometimes placed below existing channel grades to increase culvert capacity or to meet minimum cover requirements. The depression may result in progressive degradation of the upstream channel unless resistant natural material or channel protection is provided. Hydraulic Engineering Circular No. 13 (II-A-3) discusses the advantages of providing a depression or fall at the culvert entrance to increase culvert capacity. Culvert invert depressions of-lor 2 feet are usually adequate to obtain minimum cover, and may be reanily provided by modification of the concrete apron. The drop may be provided in two ways. A vertical wall may be constructed at the upstream edge of the apron, from wingwall to wingwall, II-I