Erosion and Sediment Pollution Control Program Manual.pdf

The principal spillway should be analyzed for three possible limiting flow types: weir flow, orifice flow
(top of pipe only, not through perforations), and pipe flow. The principal spillway discharge capacity is
the smallest of these three flow rates. Weir flow/orifice flow may be determined from Figure 7.3 or by
the following equations:
1. Orifice Flow: Q = CA (2gh) 0.5
Where: Q = discharge in cubic feet per second (cfs)
C = orifice coefficient, use C = 0.6 for corrugated metal pipe risers
A = cross-sectional area of the riser pipe in square feet
g = acceleration due to gravity, 32.2 ft/sec 2
H = head above riser crest in feet
2. Weir Flow: Q = CLH 1.5
Where: Q = discharge in cubic feet per second (cfs)
C = weir coefficient, use 3.1 for corrugated metal pipe risers.
L = circumference of the riser in feet
H = head above riser crest in feet
3. Pipe Flow: Q = a [(2gh) / (1 + Km + Kp L)] 0.5
Where: Q = discharge in cubic feet per second (cfs)
a = cross-sectional area of the barrel in square feet
g = acceleration due to gravity, 32.2 ft/sec 2
h = head above the centerline of the outlet end of the barrel
Km = coefficient of minor losses, can be assumed to be 1.0 for most principal spillway
systems
Kp = pipe friction coefficient = (5087 n 2 ) / di 4/3 (See Table 7.1 for Kp values for common
sizes of pipe.)
Where: n = Manning’s coefficient of roughness (0.025 for corrugated metal pipe, 0.015 for
reinforced concrete pipe)
Di = inside diameter of the barrel in inches
L = barrel length in feet
Pipe Flow may also be derived from Table 7.2 or 7.3.
363-2134-008 / March 31, 2012 / Page 175