Erosion and Sediment Pollution Control Program Manual.pdf

ROUGHNESS ELEMENT DISSIPATER - This type of dissipater, shown in Figure F.1, makes use of
hydraulic jump to dissipate energy and reduce velocity from a pipe outfall.
DESIGN LIMITATIONS
FIGURE F.1
Typical Roughness Element Dissipater
USDOT, FHWA HEC 14
1. Maximum Pipe Diameter (WO) < 1/4 Downstream Channel Width
2. Slope from Pipe Outfall to end of Dissipater < 10 percent
3. Roughness Elements should be anchored sufficiently to withstand the anticipated drag forces.
The Anticipated Drag Force (FD) is given by the formula:
FD = 1.94 AF Va 2
Where: AF = Frontal Area of roughness element
Va = Approach Velocity (average velocity in expansion area)
The Required Anchoring Force (FA) can be calculated by the formula:
FA = 0.97 (h/LC) AF Va 2
Where: h = Height of roughness element
LC = Distance from the downstream edge of roughness element
to its centroid, as shown in Figure F.2
363-2134-008 / March 31, 2012 / Page 411