Page 413 - Planning and Design of Airports

358 Airp o r t D e sign

coefficient n for each pipe by use of the Manning formula. For this
example the pipes were assumed to be concrete with a roughness
coefficient n of 0.015 and laid on a 1 percent slope. The discharge in
cubic feet per second multiplied by 3600 s is the discharge capacity
ordinate in cubic feet at the 60-min abscissa in Fig. 9-7. Each discharge
capacity curve must pass through the origin of coordinates, and one
point as determined above will define the straight-line relationship.
The significance of the cumulative runoff and discharge capacity
curves as plotted in Fig. 9-7 is that the difference in ordinates (cumulative
runoff minus discharge capacity) represents the amount of ponding at
any instant after the beginning of the storm. The maximum amount of
ponding is determined by scaling the largest difference between the
cumulative runoff curve and the discharge capacity curve.
It is considered essential that all ponding area edges be kept at
least 75 ft from the edges of pavements. In this example, this would
mean that the pond should not reach a level above elevation 88.0. The
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storage capacity below this elevation is 161,000 ft . If a 12-in-diameter
pipe were used, the maximum ponding would amount to 99,260 ft ,
3
3
considerably less than the available 161,100 ft . For practical consider-
ation a pipe of lesser diameter is not recommended.
Although not shown in this text, computations were also made
for a 10-year-frequency storm. With a 12-in-diameter pipe such a
storm would develop a pond of 123,000 ft , still less than the available
3
capacity of 161,000 ft .
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Determining the Amount of Runoff
by the Corps of Engineers Procedure
For determining runoff, the Corps of Engineers uses a relationship
for overland flow developed by R. E. Horton [17]. This relationship,
as modified by the Corps of Engineers, is as follows:

⎡ ⎛ σ ⎞ 12 ⎤
/
2
/
q = (σ tan h ⎢ ) . 0 922 t ⎜ ⎟ S 14 ⎥ (9-4)
⎢ ⎣ ⎝ nL⎠ ⎥ ⎦
where q = rate of overland flow at lower end of elemental strip of
turfed, bare, or paved surface, in/h of ft /s per acre of
3
drainage area
Q = total discharge from a drainage area, ft /s; Q equals product
3

of q and drainage area in acres
S = slope of surface or hydraulic gradient, absolute, i.e.,
1 percent = 0.01
t = time or duration, min; time from beginning of supply
(storm); total time t = t + t
c d
t = duration of supply which produces maximum rate of out-
c
flow from a drainage area but not in a pipe