Laserfiche WebLink
<br />which indicate the effect on brink depth of tailwater for <br />culverts on mild or horizontal slopes. Values of Q/BD3/2 and <br />Q/DS/2 for use with figures 111-9 and 10 are included as <br />table III-3. <br /> <br />When the tailwater depth is low, culverts on mild or horizontal <br />slopes will flow with critical depth near the outlet. This <br />is indicated on the ordinate of figures 111-9 and 10. As <br />the tailwater increases, the depth at the brink increase, <br />at a variable rate, along the Q/BD3 2 or Q/DS 2 curve, until <br />a point where the tailwater and brink depth vary linearly, <br />the 450 line on figures 111-9 and 10. Using these figures, <br />the effects of changing culvert size may be determined. For <br />example, <br /> <br /> Q = 60 cfs (constant) <br /> TW = 24 inches (constant) <br />D D sl2 Q/D S 12 TW/D YolD <br />42 22.9 2.6 .57 .63 <br />48 32.0 1.9 .50 .54 <br />54 43.0 1.4 .44 .46 <br />60 55.9 1.1 .40 .41 <br />YolD Yo Yc A/D2 A V = Q/A D <br /> .63 2.2 2.4 0.52 6.4 9.4 42 <br /> .54 2.16 2.3 0.43 6.9 8.7 48 <br /> .46 2.1 2.3 0.35 7.1 8.5 54 <br /> .41 2.05 2.2 0.30 7.5 8.0 60 <br /> <br />Changing culvert diameter from 3.5 to 5.0 feet, a 43 percent <br />increase, results in a decrease of only 15 percent in the <br />outlet velocity. <br /> <br />For culvert shapes other than rectangular and circular, <br />the brink depth for low tailwater can be approximated from <br />the critical depth curves figures 111-4 through 8. Since <br />critical depth is larger than brink depth, determining brink <br />depth in this manner is not conservative,. but is acceptable. <br /> <br />CULVERTS ON STEEP SLOPES <br /> <br />For the situation shown in A and B of figure 111-2, <br />it is convenient to determine normal flow conditions by the <br />use of Manning's equation. The charts and tables of <br />reference 111-1 provide rapid solutions under these circum- <br />stances". <br /> <br />Increasing the barrel size for a given discharge and <br />slope has little effect on velocity if the flow reaches <br />normal depth, as it will within most culverts on steep slopes. <br />For example, using a 5-foot diameter concrete pipe with <br /> <br />III-3 <br />