Rational Area Method
General
• The Rational Formula is one of the most frequently used urban hydrology methods in Malaysia. It gives satisfactory results for small catchments only
• The formula is:
C . y It . A
Qy =
360
where,
Qy =
y year ARI peak flow (m3/s)
C=
dimensionless runoff coefficient
yI t = y year ARI average rainfall intensity over time of concentration, tc , (mm/hr)
A=
drainage area (ha)
Or
Q=CiA
i = average rainfall intensity (in/hr)equals to tc
A = catchment area ( acre)
Q = Peak flow ( cusecs)
Rational Method
1
Qp
tc
tc +td
• Experience has shown that the Rational Method can provide satisfactory estimates of peak discharge on small catchments of up to
80 hectares. For larger catchments, storage and timing effects become significant, and a hydrograph method is needed.
• Assumptions used in the Rational Method are as follows:
1. The peak flow occurs when the entire catchment is contributing to the flow.
2. The rainfall intensity is the same over the entire catchment area.
3. The rainfall intensity is uniform over a time duration equal to the time of concentration, tc..
4. The ARI of the computed peak flow is the same as that of the rainfall intensity, i.e., a 5 year ARI rainfall intensity will produce a 5 year ARI peak flow. • Experience has shown that when applied properly, the Rational Method can provide satisfactory estimates for peak discharges on small catchments where storage effects are insignificant. The Rational
Method is not recommended for any catchment where:
1. the catchment area is greater than 80 hectares
2. ponding of stormwater in the catchment might affect peak discharge.
3. the design and operation of large (and hence more costly) drainage facilities is to be undertaken, particularly if they involve storage.
Rational Method
2
Estimation of Time of Concentration
• Overland flow to =
to
L
n
S
=
=
=
=
107. n . L 1 / 3
S 0.2
overland sheet flow
• The Rational Formula is one of the most frequently used urban hydrology methods in Malaysia. It gives satisfactory results for small catchments only
• The formula is:
C . y It . A
Qy =
360
where,
Qy =
y year ARI peak flow (m3/s)
C=
dimensionless runoff coefficient
yI t = y year ARI average rainfall intensity over time of concentration, tc , (mm/hr)
A=
drainage area (ha)
Or
Q=CiA
i = average rainfall intensity (in/hr)equals to tc
A = catchment area ( acre)
Q = Peak flow ( cusecs)
Rational Method
1
Qp
tc
tc +td
• Experience has shown that the Rational Method can provide satisfactory estimates of peak discharge on small catchments of up to
80 hectares. For larger catchments, storage and timing effects become significant, and a hydrograph method is needed.
• Assumptions used in the Rational Method are as follows:
1. The peak flow occurs when the entire catchment is contributing to the flow.
2. The rainfall intensity is the same over the entire catchment area.
3. The rainfall intensity is uniform over a time duration equal to the time of concentration, tc..
4. The ARI of the computed peak flow is the same as that of the rainfall intensity, i.e., a 5 year ARI rainfall intensity will produce a 5 year ARI peak flow. • Experience has shown that when applied properly, the Rational Method can provide satisfactory estimates for peak discharges on small catchments where storage effects are insignificant. The Rational
Method is not recommended for any catchment where:
1. the catchment area is greater than 80 hectares
2. ponding of stormwater in the catchment might affect peak discharge.
3. the design and operation of large (and hence more costly) drainage facilities is to be undertaken, particularly if they involve storage.
Rational Method
2
Estimation of Time of Concentration
• Overland flow to =
to
L
n
S
=
=
=
=
107. n . L 1 / 3
S 0.2
overland sheet flow