Time of Concentration, tc = to + td
Where,
L = Overland sheet flow path length (m) = 59.70 m
n = Manning's roughness value for the surface = 0.035 (Table 2.2 MSMA 2)
S = Slope of overland surface (%) = 256.000
107 x 0.011 x 8.801/3
=
3.331/5
to= 4.83 min
Where,
L = Length of reach (m) = 6 m
n = Manning's roughness coefficient = 0.035 (Table 2.3 MSMA 2)
S = Friction slope (m/m) = 0.0033
R = Hydraulic radius (m) = 1.104 m
Flow time in channel, td = n x Ld
60 x R2/3 x S1/2
= 0.011 x 83
60 x 0.5332/3 x 0.0021/2
= 0.06 min
tc = tcs = to + td
= 4.89 min (Assume 10 min)
Major storm = 10 years
Rainfall intensity = 232.83 mm/hr
Runoff coefficient, C = 0.35 (Open space - Bush Cover, 0.40 - Grass cover)
= 0.90 (Commercial & Business Centre)
Qy = cxixA
Rational Method
360
Development Impervious Area Pervious Area I Q
Status CA
C A (m2) C A (m2) (mm/hr) (l/s)
Pre-development 0.9 - 0.4 122,200.00 48,880.0 232.8 3,161.3
Post-development 0.9 97,760 0.5 24,440.00 100,204.0 232.8 6,480.6
Qpost > Qpre, SUSToM facilities is required
Using Equation 19.1 with Equation 19.1a and 19.1b for below ground storage (MSMA)
a - (a2 - 4b)
PSD =
2
a=
= 5539.66 * ( 1.62 + 3.50 + 6.50 )
= 64,395.15
b = 8.548 Qa Qp
= 8.55 * 6480.65 * 3,161.29
= 175,124,755.76
PSD = 64,395.15 - (64,395.15 - 4 x 175,124,755.76)
2
= 2,845.25 l/s
70% PSD is adopted for tank sizing, PSD70 = 2845.25 * 0.7
= 1991.67 l/s
