5.0 Hydrological Modelling
The SCS TR55 Model (USDA, 1986) presents the simplified procedures for estimating runoff
and peak discharges in small watershed. It gives the emphasizing procedures to urban and urbanizing
watersheds.
There are two method adopted from this model namely 1) Runoff Curve Number Method and
2) Graphical Peak Discharge Method.
5.1 Runoff Curve Number Method
Detailed on the NRCS Runoff Curve Number (CN) method is described in NEH-4 (SCS 1985).
The equation is;
Q = ((P-Ia)^2)/((P-Ia)+S) [eq. 5-1]
Where
Q = runoff (in)
P = rainfall
S = potential maximum retention after runoff begins (in) = (1000 / CN) - 10
I
a = initial abstraction (in) = 0.2 S
from [eq. 5-1], by substituting I
a = 0.2 S;
Q = (( P - 0.2 S )^ 2) / (( P + 0.8 S ) [eq. 5-2]
5.2 Graphical Peak Discharge Method.
To estimate the peak discharge, firstly we need to have the information on the time of
concentration and the travel time required by the water to flow from the distance point to the outlet.
5.2.1 Time of Concentration
To obtain the time of concentration;
Tc = Tt1 + Tt2 + .....+ Ttm [eq. 5-3]
Tt = L / 3600 V [eq. 5-4]
where:
T
c/T
t = time of concentration (hr) / travel time (hr)
m = number of flow segments
L = flow length (ft)
V = average velocity (ft/s)
3600 = conversion factor from seconds to hours.
For open channel flow velocity calculation, Manning’s equation are used;
V = ( 1.49 * r (2/3) * s(1/2)) / n [eq. 5-5]
where:
V = average velocity (ft/s)
r = hydraulic radius (ft) and is equal to a/p
w
a = cross sectional flow area (ft
2)
P
w = wetted perimeter (ft)
s = slope of the hydraulic grade line (channel slope, ft/ft)
n = Manning’s roughness coefficient for open channel flow.
5.2.3 Graphical Peak Discharge Method
The peak discharge equation used is;
qp = quAmQFp [eq. 5-6]
and log (qu) = C0 +
C1 log(Tc) + C2[(log(qu)]2 [eq. 5-7]
where:
q
p = peak discharge (ft
3 /s)
q
u = unit peak discharge (csm/in)
A
m = drainage area (mi
2)
Q = runoff (in)
F
p = pond and adjustment factor =1
T
c = time of concentration (hr)
C
0, C
1, C
2 = Coefficients
