THE SIMPLEST METHOD TO CALCULATE DISCHARGE/PEAK FLOW/FLOOD

1. Khan Mujiburrehman, EIT (USA)
Senior Resources/Infrastructure Engineer
Bachelor in Civil, Master in Water Infrastructure
LEED® AP BD+C, LEED® AP ND
Mob:0537353488

3. NAME OF METHOD RATIONAL
FORMULA
Q = C * i * A /360
Q = Peak flow- m³/s
i = Rainfall intensity for the design storm- mm/h
A = Drainage area- hactare
C = Runoff coefficient
STANDARD RUNOFF
COEFFICIENT 'C'
Asphalt/Concrete Road/Roof/Commercial area- 0.95
Playground- 0.3
Residential- 0.75
ASSUMPTIONS
1. The duration of rainfall should be more than time of concentration (tc) so
that all catchment area is contributing to peak runoff.
2. The drainage area should be smaller than 80 hectares to able to satisfy
uniform rainfall over all catchment.
KIRPICH EQUATION TO
CALCULATE tc in minutes
L- length of longest water course in meter
S- Slope of water course in decimals
NOTE: Rainfall intensity 'i' will be estimated from intensity duration frequency
curve using 'tc'

4. EXAMPLE
IDF CURVE
CALCULATION
1. WEIGHTED VALUE OF "C":
(A1C1+A2C2+A3C3)/ (C1+C2+C3) = (0.9*30+0.7*20+0.5*15)/65= 0.75
2. CALCULATION OF TIME OF CONCENTRATION tC :
Length of water course: 2000m
Slope of water course: 740-700/2000 = 0.02
tc = 0.0195* ( L/S0.5
)0.77
= 0.0195* (2000/0.020.5
)0.77
= 30 minutes
3. ESTIMATION OF RAINFALL INTENSITY IN i "mm/hr":
for tc = 30 minutes, return period= 10 years i= 110 mm/hr
PEAK FLOW in m3
/sec Q = 0.75 * 110* 65/360 = 15 m3
/sec
prepared by: Mujib khan (mujibkhan87@gmail.com)

6. NAME OF METHOD SCS Runoff Curve Number Method
FORMULA Q = runoff (mm)
P = rainfall (mm)
S = potential maximum retention by ( vegetation, evaporation, and
infiltration etc.) after runoff begins (mm)
I a = initial abstraction (mm) (I a = 0.2S most common)
S in mm
CN: CURVE NUMBER
depends upon hydrologic soil group (HSG), cover type, treatment, hydrologic condition, and
antecedent moisture condition (AMC)
(ARC)
EXAMPLE
1. WEIGHTED CN
CN1 * A1 +CN2 * A2 + CN2 * A3 / (A1 + A2 + A3)
92* 30 + 77* 20 + 61* 15 / (30+ 20+ 15) = 81
Determine the peak discharge from the new
development area. Refer attached details.
URBAN AREA- 85% IMPERVIOUS AREA CONNECTED
RESIDENTIAL AREA- 65% IMPERVIOUS
AREA CONNECTED
OPEN SPACE- GRASS COVER > 75%

7. 2. KIRPICH EQUATION
TO CALCULATE tc IN
MINUTES
L- length of longest water course in meter
S- Slope of water course in decimals
NOTE: Rainfall intensity 'i' will be estimated from intensity duration frequency curve using 'tc'
3. IDF CURVE
4. CALCULATION OF
"P"
1. WEIGHTED VALUE OF "C":
(A1C1+A2C2+A3C3)/ (C1+C2+C3) = (0.9*30+0.7*20+0.5*15)/65= 0.75
2. CALCULATION OF TIME OF CONCENTRATION tC :
Length of water course: 2000m
Slope of water course: 740-700/2000 = 0.02
tc = 0.0195* ( L/S0.5
)0.77
= 0.0195* (2000/0.020.5
)0.77
= 30 minutes
3. ESTIMATION OF RAINFALL INTENSITY IN i "mm/hr":
for tc = 30 minutes, return period= 10 years i= 110 mm/hr
3. CALCULATION OF
"S"
((1000/CN) -10)* 25.4 = ((1000/81) - 10) *25.4 = 60mm
AMC
AMC I
– Lowest runoff potential soil dry enough for cultivation
AMC II
– Average condition
AMC III- MOST CRITICAL FOR FLOOD
– Highest runoff potential, practically saturated
Ia = initial
abstraction (mm)
0.2 * S = 0.2 * 60 =12mm
Q = runoff (mm) (100-12)*2
/ ((100-12)+60) = 53mm for 30 Minutes
Peak Runoff (m3
/sec) 20 m3
/sec
prepared by: Mujib khan (mujibkhan87@gmail.com)