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Storm Drainage Design
Storm Drainage Design

1. Rational Method:

a). The intensity of the rainfall is constant and is applied to the entire watershed
b). The runoff coefficient remains constant throughout the storm event
c). The frequency of the peak flow is equal to the frequency of the rainfall intensity

Rational Formula:

Q = 10 C.i.A

Q - peak runoff, m3/hr
C - runoff coefficient
i - rainfall intensity, mm/hr
A - Tributary area, hectares (104 m2)

runoff Coefficient:

Concrete pavement = 1.0
Asphalt = 1.0
Lawns = 0.2
Uncompacted soil = 0.2
Natural soil, grass cover = 0.4
Compacted sand soil = 0.4
Uncompacted gravel = 0.5
Natural bare soil = 0.6
Compacted gravel = 0.7
Oiled compacted soil = 0.8
Compacted impervious soil = 0.6

Composite runoff coefficient:

C = (A1.C1+ A2.C2 + ...) / (A1+A2 + ...)

2. time of Concentration

This is the time it takes for water to flow from the most remote point in the watershed to the point of discharge

Intial time of concentration for sheet flow:  tc = 7 n0.6 . L0.6 / ( i0.4 . s0.3)
tc - the time of concentration, minutes
n - Manning resistance coefficient
L - the distance from the upper end of the plane to the point of interest in meters
i - rainfall intensity, mm/hr
s - slope of the surface

Minimum time of concentration: for paved areas, 5 minutes; for unpaved area, 10 minutes

Manning's Resistance Coefficient (n):

Concrete,asphalt = 0.01-0.013
Sand = 0.01-0.016
Lawns = 0.2-0.3
Bare clay = 0.012-0.033
Gravel = 0.012-0.03

3. Intensity-Duration-Frequency Curve(IDF):

i = a / (t + b)

i - intensity of rainfall, mm/hr
t - duration of rainfall, hr

for example:
i1 = 72mm/hr, t1 = 0.25 hr,
i2 = 3 mm/hr, t2 = 24 hr;
then, a =72.2, b = 0.753;
If t = 1 hr, then, i = 72.2 /(1+0.753) = 41 mm/hr

4. Travel Time:

Time of flow in one reach + Initial time of concentration of the area

5. Storm Runoff at Junctions

For tributary area with longer time of concentration, Q1, T1, i1
For tributary area with shorter time of concentration, Q2, T2, i2

If T1 = T2,     Q = Q1 + Q2,     T = T1
If T1 > T2,     Q = Q1 + Q2.i1/i2,     T = T1
If T1 < T2,     Q = Q2 + Q1.i2/i1,     T = T2

Reference: Chow, Ven Te. Handbook of Applied Hydrology, Mcgraw Hill Inc. New York Chow Ven Te. Open-Channel Hydraulics, Mcgraw Hill Inc. New York