Erosion and Sediment Pollution Control Program Manual.pdf

8. Calculate CDAFN using the following equation:
2 (KP KD)
CDAFN = ----------------------
1.94 VA 2
(43.7 YO 2 ) WO
Where: KP = (1.94VO Q) + ------------------
2
(1.94VD Q) + 62.4Q 2
KD = -------------------------------
2VD 2 (WD)
VD = Q/ (WD TW)
9. Compute CDAFN from the values derived in steps 6 and 7 above.
10. If CDAFN computed in step 9 above is equal to or greater than that calculated in step 8,
the design is satisfactory. If it is less, return to step 4 and select a new configuration
from Figure F.7.
11. Determine the Riprap Protection requirement downstream of the Dissipater using
Figure F.12.
EXAMPLE - An 8 8’ Box Culvert, length = 235’, slope = 0.02 ft/ft, Q design = 1400 cfs, n =
0.013, critical depth (yc) = 9.8’, flow depth (d) = 6.0’, Tailwater (TW) = 3.3’, natural channel
width = 41’
1. Discharge Flow Depth (YO) = flow depth (d) = 6.0’
2. Discharge Velocity (VO) = Q/A = 1400 / (6 8) = 29.1 fps
3. Froude Number (Fr) = VO / (32.2 YO) 0. 5 = 29.1/ (32.2 6.0) 0. 5 = 2.1
4. Dissipater Configuration from Figure 9.17
a) Expansion Ratio (WD/WO): Channel Width/Culvert Width = 41/8 = 5.14
therefore W1/WO = 0.63
b) Number of Rows (N r), 5 is chosen because 4 rows is found to be
insufficient
Number of Elements = 19
c) h/yA = 0.71 and L/h = 6 are chosen
5. Approach Velocity (VA) - Roughness field begins at a distance 2WO from the
Culvert. Since WO = 8’, the field begins 16’ from the culvert.
363-2134-008 / March 31, 2012 / Page 414