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<br />or where a drop may be considered undesirable, the apron <br />slab may be constructed on a slope to reduce or eliminate <br />the vertical face. <br /> <br />Caution must be exercised in attempting to gain the advantages <br />of a lowered inlet where placement of the outlet flowline <br />below the ,channel would also be required. Locating the entire <br />culvert flowline below channel grade 'may result in deposition <br />problems. <br /> <br />HEADWALLS AND WINGWALLS <br /> <br />Recessing the culvert into the fill slope and retaining <br />the fill by either a headwall parallel to the roadway or <br />by a short headwall and wingwalls does not produce signifi- <br />cant erosion problems. This type of design decreases the <br />culvert length and enhances the appearance of the highway <br />by providing culvert ends that approximately conform to <br />the embankment slopes. A vertical headwall parallel to <br />the embankment shoulder line should have sufficient length <br />so that the embankment spill cones remain clear of the <br />culvert opening. Normally riprap protection of these spill <br />cones is not necessary if slopes are sufficiently flat to <br />remain stable ~hen wet. <br /> <br />Wingwalls flared with respect to the culvert axis are commonly <br />used and are more efficient than parallel wingwalls. The <br />effects of various wingwall placements upon culvert capacity <br />are discussed in HEC Nos. 5, 10, and 13 (II-A-l,2,3). Use <br />of a minimum practical wingwall flare has the advantage <br />of reducing the area requiring protection against erosion. <br />The flare angle for the given type of culvert should be <br />consistent with the recommendations of HEC No. 13. <br /> <br />If flow velocity near the inlet indicates a possibility <br />of scour threatening the stability of wingwall footings, <br />erosion protection should be provided. A concrete apron <br />between wingwalls is the most satisfactory means for pro- <br />viding this protection. The slab has the further advantage <br />that it may be reinforced and used to support the wingwalls <br />as cantilevers. <br /> <br />It is not necessary to extend an inlet headwall (with or <br />without wingwallsl to the maximum design headwater elevation. <br />With the inlet and the slope above the headwall submerged, <br />velocity of flow along the slope is low. Even with easily <br />,erodible soils, a vegetative cover is usually adequate pro- <br />'tection in this area. <br /> <br />11-2 <br />