Post on 24-Aug-2020
27-5-2014
Challenge the future
DelftUniversity ofTechnology
CIE4491Lecture. Quantifying stormwater flow –Rational method
Marie-claire ten Veldhuis, Watermanagement Department
2CIE4491 Lecture Rational Method
Storm event dynamic modelling
Rainfall series dynamic modelling
Robust method stationary modelling
Design storms Standard rainfall seriesIDF curves
Branched networks (few loops)
Rational method
Detailed branched and looped networks
Hydrodynamic model calculations
Hydrodynamic model calculations
Simplified branched and looped networks
Stationary hydraulic calculations
Dynamic hydraulic calculations
Dynamic hydraulic calculations
Water levels in nodesQ-t diagram per node
per storm event
Q-t diagram per node for series of storm events
Rainfall runoff modelling Rainfall runoff modelling Rainfall runoff modelling
3CIE4491 Lecture Rational Method
Rational method for stormwater
drainage system design
Rational method: 𝑄 𝑡 = 𝐶. 𝐼. 𝐴
Q = inflow into manhole (m3/s)
C = Runoff coefficient, representing runoff losses (-)
I = rainfall intensity (mm/h or l/s/ha)
A = catchment area connected to urban drainage system (m2)
4CIE4491 Lecture Rational Method
Rational method for stormwater
drainage system design
Rational method: 𝑄 𝑡 = 𝐶. 𝐼. 𝐴
1. Determine size and characteristics of subcatchment areas
2. Determine runoff coefficient C for subcatchment areas
3. Calculate concentration time at critical inflow points in drainage
system
4. Determine critical rainfall intensity based on concentration time and
IDF curve
5. Calculate Q at all critical inflow points
5CIE4491 Lecture Rational Method
Runoff coefficient C: dependent on
catchment type
Flat roofsInclined roofsImpervious areaPervious area
6CIE4491 Lecture Rational Method
Inflow points
7CIE4491 Lecture Rational Method
Contributing area per inflow point (manhole)
8CIE4491 Lecture Rational Method
Catchment areas connected to sewer system
Example: Bomenwijk Delft
9CIE4491 Lecture Rational Method
Time of concentration
Time required for surface runoff to flow from remotest part of catchment to inflow point under consideration
Two components time of entry (te) time of flow (tf)
Return period (y)
Time of entry (min)
1 4-8
2 4-7
3 3-6
c e ft t t
10CIE4491 Lecture Rational Method
Time of entry & time of flow
In reality, time of entry varies with• surface roughness
• slope & length of flow path
• rainfall intensity
Time of flow depends on• Hydraulic properties of pipe – flow velocity
• Length of flow path
11CIE4491 Lecture Rational Method
Rational method for urban drainage
design
Steady state stormwater flow:
Qn = flow at location with n upstream sections (l/s)
i = critical rainfall intensity (l/s/ha (or mm/h)) for return period T and concentration time t
Cm = runoff coefficient of catchment discharging tosection m
Am = catchment area discharging to section m (ha)
Q i C An m m
m
n
1
12CIE4491 Lecture Rational Method
Assumptions:
Rainfall is uniform over the whole catchment area Catchment imperviousness remains constant throughout
storm Flow in the system is at constant velocity throughout tc Steady state flow reached when tstorm tc
Critical rainfall intensity for at points in systemat time t = tc
Rational method
13CIE4491 Lecture Rational Method
Rational method: stationary
conditions
Hydrograph response to continuous rainfall (graph b)
14CIE4491 Lecture Rational Method
Critical rainfall intensity
50
10 403020 8070600 10090500
100
150
duration (min)t=tc
i
inte
nsity (
l/s.h
a)
Intensity duration curve
15CIE4491 Lecture Rational Method
Summary rational method for
stormwater system design
• Choose return period of rainfall (T )
• Estimate contributing areas for each pipe and run-off coefficient
• Assume flow velocity of 1 m/s
• Estimate maximum time of concentration (tc) for connected
catchment areas
• Read critical rainfall intensity from IDF curve
• Calculate flow rate in each pipe according to:
• Adjust diameters of pipes if necessary
• Check flow velocity and concentration time
Q i C An m m
m
n
1
16CIE4491 Lecture Rational Method
Example: calculate design flow for
each node
BC
D
E
A
200 m
+ 4,00 m
+ 3,00 m
400 m300 m500 m
+ 5,95 m
+ 4,95 m
+ 4,10 m+ 4,50 m
E BCD A
Surface water
50 m
50 m
50 m
17CIE4491 Lecture Rational Method
Determine contributing area
200 m400 m500 m300 m
50 m
50 m
Surface
water
E BCD A
50 m
Area A B C D E
Area per node (ha) 0.5 2 4 5 3
Impervious area (%) 50 60 40 30 30
Impervious area per node (ha)
Cum. Area per node (ha)
18CIE4491 Lecture Rational Method
Determine contributing area
200 m400 m500 m300 m
50 m
50 m
Surface
water
E BCD A
50 m
Area A B C D E
Area per node (ha) 0.5 2 4 5 3
Impervious area (%) 50 60 40 30 30
Impervious area per node (ha) 0.25 1.2 1.6 1.5 0.9
Cum. Area per node (ha) 5.45 5.2 4.5 2.4 0.9
19CIE4491 Lecture Rational Method
Time of concentration
Assume flow velocity 1 m/s
200 m400 m500 m300 m
50 m
50 m
Surface
water
E BCD A
50 m
Area A B C D E
Time of concentration (min)
20CIE4491 Lecture Rational Method
Time of concentration
Assume flow velocity 1 m/s
200 m400 m500 m300 m
50 m
50 m
Surface
water
E BCD A
50 m
Area A B C D E
Time of concentration (min) 29 28 25 18.3 10
21CIE4491 Lecture Rational Method
Critical rainfall intensities from IDF curve
50
10 403020 8070600 10090500
100
150
duration (min)
60
inte
nsity (
l/s.h
a)
Area A B C D E
Time of concentration (min) 29 28 25 18.3 10
Rainfall intensity (l/s/ha) 40 40 45 50 70
22CIE4491 Lecture Rational Method
Design flow for each node Area A B C D E
Time of concentration (min) 29 28 25 18.3 10
Rainfall intensity (l/s/ha) 40 40 45 50 70
Cumulative area (ha) 5.45 5.2 4.5 2.4 0.9
Design flow (l/s) 218 208 202 120 63
200 m400 m500 m300 m
50 m
50 m
Surface
water
E BCD A
23CIE4491 Lecture Rational Method
Rainwater flow, remember:
• Highly variable (0 – extreme):
no system can cope with all
possible rainfall intensities
• High rainfall intensity is critical
• Stormwater flow is large
compared to wastewater flow
• Translation into design values :
statistics and assumptions
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lag
[l/
s.h
a]
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