CIE4491 Lecture. Quantifying stormwater flow Rational method 27-5-2014 Marie-claire ten Veldhuis, Watermanagement Department Delft University of Technology Challenge the future
Robust method stationary modelling IDF curves Storm event dynamic modelling Design storms Rainfall series dynamic modelling Standard rainfall series Rainfall runoff modelling Rainfall runoff modelling Rainfall runoff modelling Rational method Branched networks (few loops) Hydrodynamic model calculations Detailed branched and looped networks Hydrodynamic model calculations Simplified branched and looped networks Stationary hydraulic calculations Dynamic hydraulic calculations Dynamic hydraulic calculations Water levels in nodes Q-t diagram per node per storm event Q-t diagram per node for series of storm events 2
Rational method for stormwater drainage system design Rational method: Q t = C. I. A Q = inflow into manhole (m 3 /s) C = Runoff coefficient, representing runoff losses (-) I = rainfall intensity (mm/h or l/s/ha) A = catchment area connected to urban drainage system (m 2 ) 3
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 4
Runoff coefficient C: dependent on catchment type Flat roofs Inclined roofs Impervious area Pervious area 5
Inflow points 6
Contributing area per inflow point (manhole) 7
Catchment areas connected to sewer system Example: Bomenwijk Delft 8
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 (t e ) time of flow (t f ) t t t c e f Return period (y) Time of entry (min) 1 4-8 2 4-7 3 3-6 9
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 10
Rational method for urban drainage design Steady state stormwater flow: n n m m m 1 Q i C A Q n = 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 C m = runoff coefficient of catchment discharging to section m A m = catchment area discharging to section m (ha) 11
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 t c Steady state flow reached when t storm t c Critical rainfall intensity for at points in system at time t = t c 12
Rational method: stationary conditions Hydrograph response to continuous rainfall (graph b) 13
intensity (l/s.ha) Critical rainfall intensity 150 100 Intensity duration curve 50 i 0 0 10 20 30 40 50 60 70 80 90 100 t=t c duration (min) 14
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 (t c ) for connected catchment areas Read critical rainfall intensity from IDF curve Calculate flow rate in each pipe according to: n n m m m 1 Q i C A Adjust diameters of pipes if necessary Check flow velocity and concentration time 15
Example: calculate design flow for each node E D C B A + 5,95 m Surface water E + 4,95 m D + 4, C + 4,10 m+ 4,00 m B A + 3,00 m 500 m 300 m 400 m 200 m 16
Determine contributing area E D C B A Surface water 300 m 500 m 400 m 200 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) 17
Determine contributing area E D C B A Surface water 300 m 500 m 400 m 200 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 18
Time of concentration Surface water E D C B A 300 m 500 m 400 m 200 m Assume flow velocity 1 m/s Area A B C D E Time of concentration (min) 19
Time of concentration Surface water E D C B A 300 m 500 m 400 m 200 m Assume flow velocity 1 m/s Area A B C D E Time of concentration (min) 29 28 25 18.3 10 20
intensity (l/s.ha) Critical rainfall intensities from IDF curve 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 70 150 100 60 50 0 0 10 20 30 40 50 60 70 80 90 100 duration (min) 21
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 Surface water E D C B A 300 m 500 m 400 m 200 m 22
Rainwater flow, remember: Precipitation on 24 August 2002, Prinsenbeek 8 120 Highly variable (0 extreme): no system can cope with all possible rainfall intensities 7 6 5 4 3 2 1 0 250 100 80 60 40 20 0 High rainfall intensity is critical 200 Stormwater flow is large compared to wastewater flow Translation into design values : statistics and assumptions neerslag [l/s.ha] 150 100 50 0 10 00:00:00 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 Precipitation in mm/10min Cumulative precipitation volume (mm) 20 00:05:00 30 00:10:00 40 00:15:00 50 00:20:00 70 00:25:00 90 00:30:00 110 110 00:35:00 00:40:00 70 00:45:00 40 00:50:00 20 00:55:00 0 0 0 0 0 0 0 01:00:00 01:05:00 01:10:00 01:15:00 01:20:00 01:25:00 01:30:00 23