Erosion and Sediment Pollution Control Program Manual.pdf

IMPACT DISSIPATERS are box-like structures used at culvert or channel outlets to dissipate
energy without reliance upon tailwater.
DESIGN RESTRICTIONS
1. Maximum Discharge Rate (Q) = 400 cfs Note: Two or more Impact Dissipaters may be
constructed side by side where needed.
2. Maximum Discharge Velocity (VO) = 50 fps.
3. Maximum Tailwater = h 3 + (h 2/2)
4. Maximum Culvert Slope = 0.27 ft/ft For culverts with slopes greater than 0.27 ft/ft, a
horizontal section of at least 4 conduit widths long should be provided immediately
upstream of the dissipater.
5. In Soils that are not Erosion-Resistant (i.e. having a k factor 0.37), the Alternative End
Sill and 45 O Wingwall Design should be used.
6. A Cutoff Wall should be provided at the end of the dissipater to prevent undermining.
7. Riprap should be provided downstream of the dissipater for a distance equal to at least
4 culvert widths. The size of the riprap should be determined from Figure F.12.
8. Flows should be free of debris or substantial ice build-up.
DESIGN PROCEDURE
1. Compute Flow Area (A) from the Design Discharge (Q) and the Anticipated Velocity
(VO) using the equation:
A = Q / VO
2. Calculate Ye for a rectangular section of equivalent area twice as wide as depth of flow:
3. Compute the Froude Number (Fr):
Y e = (A / 2)
0. 5
Fr = VO / (g Ye) 0.5
4. Calculate the Energy Head (HO) at the end of the conduit:
HO = Ye + VO 2 / 2g
5. Using Figure F.13, Determine Basin Width (W).
6. From Table F.1, determine the remaining dissipater dimensions indicated in Figure
F.14.
363-2134-008 / March 31, 2012 / Page 426