Erosion and Sediment Pollution Control Program Manual.pdf

Head
TABLE 7.8 - GRASS-LINED EMERGENCY SPILLWAY CAPACITY IN CFS
(Side Slopes 4H:1V)
Bottom Width
(ft)
(ft)
8 10 12 14 16 18 20 22 24 26 28 30
0.5 7 8 9 10 12 14 16 18 19 21 23 25
0.6 9 11 12 14 17 19 21 23 25 28 30 32
0.7 11 14 16 19 21 24 26 29 31 34 37 39
0.8 14 17 20 23 26 30 32 35 38 42 45 48
0.9 19 22 25 27 32 36 40 45 49 51 55 59
1.0 22 27 31 35 39 45 48 53 57 61 65 70
1.1 27 32 36 41 47 52 57 60 66 71 76 81
1.2 34 38 45 50 57 61 67 72 78 84 90 96
1.3 38 45 50 57 64 73 79 89 91 98 106 110
1.4 43 51 58 66 75 81 89 95 102 109 117 125
1.5 50 58 67 74 84 93 100 110 118 125 134 143
1.6 57 71 75 83 92 100 113 121 131 141 149 159
1.7 63 75 85 95 106 115 124 133 144 155 164 175
1.8 73 88 95 106 116 129 137 149 160 173 183 194
1.9 80 92 104 114 128 141 152 165 176 188 200 213
2.0 89 102 113 127 140 153 167 180 191 205 217 230
2.1 96 112 125 140 154 168 186 195 207 223 238 253
2.2 105 122 137 153 168 183 198 216 229 244 261 274
2.3 119 134 149 166 183 200 215 232 247 266 282 297
2.4 129 146 162 179 198 217 232 252 266 284 302 319
USDA NRCS Engineering Field Manual
2. An acceptable alternative is the equation for a broad-crested weir:
Q = CLH 1.5
Where this option is used, the maximum value of C should be 2.8. — relatively smooth liners. In
many cases, a smaller C value should be used. The appropriate C value will vary depending
upon the type of liner and anticipated flow depth — usually 2.4 to 2.6. Smaller values should be
used for shallower flows over rough surfaces, larger values for deeper flows over smoother
surfaces. The designer should indicate the standard reference used to determine the C value. L
is the bottom width of the spillway at the crest, and H is the depth of flow above the spillway crest.
Note: Manning’s channel equation should not be used to size the control section of the
emergency spillway. However, it should be used to design the outlet channel below the spillway
crest. Include all outlet channel dimensions and specifications on the plan drawings and provide
supporting calculations in the narrative.
3. The total of the emergency and principal spillway capacities for temporary basins should equal or
exceed the required 2 cfs/acre discharge capacity, or be capable of discharging the routed
25-year, 24-hour storm event without encroaching upon the required freeboard. Permanent basins
may require spillways with capacities exceeding 2 cfs/acre. An acceptable alternative to the 2
cfs/acre discharge capacity with 24” freeboard is a discharge capacity capable of handling the
peak flow from a 100-year, 24-hour storm event — using maximum runoff conditions — while
maintaining 12” of freeboard. The principal spillway should be used to handle as much of the
required discharge capacity as possible. At a minimum, the principal spillway should be designed
to convey the calculated peak flow from a 10-year, 24-hour storm event. Principal spillways of
permanent basins may be required to have greater discharge capacities.
363-2134-008 / March 31, 2012 / Page 198