Laserfiche WebLink
<br />III. CULVERT OUTLET VELOCITY AND VELOCITY MODIFICATION <br /> <br />Culvert outlet velocity is one of the primary indicators of <br />erosion potential. Outlet velocities are seldom less than <br />10 fps and range up to 30 fps for culverts on small or mild <br />slopes and can exceed this for culverts on steep slopes. <br />Under these conditions, it is reasonable to investigate measures <br />to modify or reduce velocity within the culvert before con- <br />sidering an energy dissipator. Several possibilities exist, <br />but the degree of velocity reduction is, in most eases, <br />limited and must always be weighed against the increased <br />costs which are generally involved. <br /> <br />The continuity equation, Q = AV, can be utilized in all <br />situations to compute culvert velocities, either within the <br />barrel or at the outlet. Since ~ischarge will generally be <br />known from culvert design, determining the flow area will <br />define the velocity. <br /> <br />CULVERTS ON MILD SLOPES <br /> <br />Figure III-I, taken from HEC No.5 (III-I), indicates the <br />four types of flow for culverts on mild slopes, i.e., culverts <br />flowing with outlet control. <br /> <br />Figure lII-lA indicates a condition where high tailwater <br />controls the culvert outlet velocity. In this case, outlet <br />velocity is determined using the full barrel area. With <br />this flow condition, it is possible to reduce the velocity <br />by increasing the culvert size. The degree of reduction <br />is proportional to the reciprocal of the culvert area. <br />Selecting several culvert diameters provides a specific <br />example: <br /> <br />CULVERT DIAMETER (ft.) <br /> <br />3 <br /> <br />4 <br /> <br />5 <br /> <br />6 <br /> <br />Percentage reduction of <br />Outlet Velocity (V = Q/A) 44% 36% 31% <br /> <br />For high tailwater conditions, erosion may not be a serious <br />problem. It may be more important to determine if tailwater <br />will always control, or if the conditions shown in figures <br />III-IB, C, or D might occur under some circumstances. <br /> <br />III-l <br />