How Much Runoff? Coefficients for Urban land Use in South West WA and a comparison between Rational Method and volumetric runoff coefficients

Transcription

1 How Much Runoff? Coefficients for Urban land Use in South West WA and a comparison between Rational Method and volumetric runoff coefficients Written by: Jim Davies (PhD, FIEAust, Member IPWEA, SIA WA Member) JDA Consultant Hydrologists

2 Reproduced from Ladson 2008

3 When is Runoff Generated? When rainfall rate exceeds soil infiltration rate, (called infiltration excess runoff). OR When rainfall lands on saturated surface (called saturation excess runoff).

4 Runoff Coefficient Definition Runoff coefficient is a term used widely in Hydrology and has different meanings in different contexts. It is a ratio and therefore dimensionless. The fraction of rainfall which results in surface runoff: can be for an individual storm or for longer eg a year. More precisely called the volumetric runoff coefficient. In the Rational Method, the ratio of rainfall intensity to peak runoff per unit area.

5

6 Reproduced from Ladson 2008

7 Runoff Coefficient Definition Runoff coefficient is a term used widely in Hydrology and has different meanings in different contexts. It is a ratio and therefore dimensionless. The fraction of rainfall which results in surface runoff: can be for an individual storm or for longer eg a year. More precisely called the volumetric runoff coefficient. In the Rational Method, the ratio of rainfall intensity to peak runoff per unit area.

8 Rational Method Peak flow estimate: Q = F.C.I.A. Where Q = peak flow (m³/s) F = conversion factor = C = Rational Method runoff coefficient I = average rainfall intensity over time of concentration (mm/hr) A = catchment area (ha)

9 Reproduced from Argue (1986)

10 Reproduced from Argue (1986)

11 Rational Method Peak flow estimate: Q = F.C.I.A. Where Q = peak flow (m³/s) F = conversion factor = C = Rational Method runoff coefficient I = average rainfall intensity over time of concentration (mm/hr) A = catchment area (ha)

12 Reproduced from Argue (1986)

13 AR&R (EA, 2000) Section 1.16 Methods of flood estimation should be based on observed data either from the catchment itself or representative catchments. Section Rational Method Runoff coefficients must be derived from observed flood data (not arbitrary values based on experience or judgement).

14 AR&R (EA, 2000) Section Rational Method runoff coefficient.. Figure should be used in preference to the runoff coefficient relationships given in previous additions of this publication.

15 Reproduced from AR&R Pervious area runoff coefficient (f = 0) range from 0.1 to 0.7. Perth 10 I1 is 28mm/hr, just above the lower line for F = 0.5 this corresponds to approx 0.5 C10

16 Reproduced from AR&R

17 Example: Fully Impervious Catchment C R = 1.0 Q, flow (m 3 /s) Q p Catchment area = 0.1 ha tc = 20 mins I = 48 mm/hr If tb = 2tc: Vol runoff Qp = CIA/0.36 = 1.0 x 48 x 0.1/3.6 = m 3 /s = Qp x tc = x 20 x 60 = m3 Vol rainfall = x 0.33 x 1000 = m 3 C v = 15.6/15.84 = 1.0 tc tb = 2tc tb = 3tc tb = 4tc t, time (s) (if tb > 2tc, C v > 1.0, impossible. That is fully impervious catchment has time to peak = time recede (tb = 2tc)

18 Example: Partially Impervious Catchment C R = 0.5 Q, flow (m 3 /s) Q p Catchment area = 0.1 ha tc = 20 mins I = 48 mm/hr If tb = 2tc: Vol runoff Qp = CIA/0.36 = 0.5 x 48 x 0.1/0.36 = m 3 /s = Qp x tc = x 20 x 60 = 7.8 m3 Vol rainfall = x 0.33 x 1000 = m 3 C v = 7.8/15.84 = 0.5 tc tb = 2tc tb = 3tc tb = 4tc t, time (s) If tb = 4tc: Vol runoff = 2Qp x tc = 15.6 m 3 Cv = 1.0 (Partially impervious catchment likely to have longer hydrograph tail, tb = 4tc say, so that C v = 2C R )

19 To Match Peak Flows Rational Method Runoff Coefficient C v Volumetric Runoff Coefficient C v tb = 2tc Tb = 3tc Tb = 4tc N/A N/A N/A N/A Note: N/A Not Applicable (runoff volume will exceed rainfall) Need C v C R in all cases. If engineer uses C R = 0.5 in Rational Method, then applies hydrograph method (eg XP-STORM) will need to use C v 0.5 (eg C v = 1.0) to match peak flow from Rational Method. If use C v = C R, will get lower peak in hydrograph method than Rational Method. If using AR&R C R graph they need to apply C v C R for hydrograph method.

20 Conclusions IPWEA Subdivision Guidelines silent on runoff coefficient values. Ladson (2008) shows E. States fraction impervious up to 0.4. AR&R recommends Figure for Rational Method runoff coefficient. AR&R Figure indicates for f = 0.4, approx Rational Method C R = 0.5 (10yr). Need for more consistency in SW WA on values of runoff coefficients. To match flood peaks C v volumetric runoff coefficient must be greater than C R (Rational Method runoff coefficient). For example if used C R = 0.5, need to use C V say 1.0 to match flood peaks. Need for better understanding of different definitions of Rational Method runoff coefficient C R compared with volumetric runoff coefficient C v.