Flood and runoff estimation on small catchments. Duncan Faulkner

Transcription

1 Flood and runoff estimation on small catchments Duncan Faulkner

2 Flood and runoff estimation on small catchments Duncan Faulkner and Oliver Francis, Rob Lamb (JBA) Thomas Kjeldsen, Lisa Stewart, John Packman (CEH) Funding from JBA, EA and CEH

3 Flood and runoff estimation on small catchments Duncan Faulkner and Oliver Francis, Rob Lamb (JBA) Thomas Kjeldsen, Lisa Stewart, John Packman (CEH) Funding from JBA, EA and CEH Initial results published in Journal of Flood Risk Management: Faulkner et al. (2011)

4 Contents Why talk about small catchments? Sources of data Methods available Tests and results Plot-scale runoff Next phase of research 4

5 Why talk about small catchments? 5

6 What is a small catchment? 6

7 How low can you go? 7

8 What about brownfield sites? 8

9 And after the builders move in 9

10 Are these rural or urban? 10

11 Why talk about them? Because there are so many of them 11

12 Why talk about them? 12

13 Why talk about them? Because there are so many flood studies on small catchments: Flood risk assessments and SUDS design; flood mapping; flood warning; design of storm sewers; design of road drainage and culverts; design of pumping stations and other infrastructure; appraisal of options for flood alleviation; reservoir studies. 13

14 Why talk about them? Because flow processes may be different from large catchments Is there a size at which the balance of flow processes within the catchment changes? 14

15 Flow processes Upland catchment (Coalburn) 2.6km 2 Stream network Clear topographic boundary Mixture of riparian areas, hillslopes etc. Flow can be measured Standard flood estimation methods can be applied although with concern about applicability of regression equations given scope for local conditions to dominate 15

16 Flow processes Lowland catchment No watercourse No stream data May not form a complete catchment Some catchment descriptors are meaningless How much will it contribute to downstream flooding? What methods are suitable for flood estimation? Can the model parameters be reasonably estimated from nationalscale gridded data on soils etc.? 16

17 Flow processes Processes: Infiltration-excess and saturation-excess runoff Sub-surface flow paths Stream flow: concentrating and conveying What is surface runoff? Quick flow in streams may have originated as infiltration and subsurface flow Even if an area does not appear to yield local surface runoff, it may contribute to storm flow in the stream network further downstream 17

18 Flow processes Balance of processes changes with catchment size: Soil flows dominate for small plots and catchments Flow rates are cm/s In-stream processes become more important for larger catchments Flow rates are m/s So extrapolation of flood estimation across catchment scales is uncertain Need to think carefully about why we need estimates of greenfield runoff 18

19 Sources of data

20 HiFlows-UK 9% of total dataset 20

21 How representative is HiFlows-UK? Graph and table Symonds Group 21

22 Other stream flow datasets on small catchments NRFA: 127 flow gauges on catchments under 25km 2 Other gauges: experimental sites, water companies etc. Level gauges on engineered channels in urban catchments may be possible to derive rating 22

23 Runoff data at the plot scale Pontbren (Wales) North Wyke (Devon) ADAS (various) Forest of Bowland (Lancashire) Upper Teesdale (Durham) Mostly short records, problems with long gaps during winter months 23

24 Methods available 24

25 IH 124 (1994) 25

26 ADAS 345 (1982) 26

27 FEH (Statistical, rainfall-runoff, ReFH) 27

28 Sewer design methods for urban catchments 28

29 Guidance on choice of method (2005) IH Report 124 should be used to calculate greenfield runoff peak flow rates For catchments under 2km 2 Regression formula for QBAR combined with growth curves from FSR Reasons: Stronger empirical support than ADAS 345 Meeting the pragmatic needs of the industry 29

30 Guidance on choice of method Recommendation to use IH 124 for rural runoff estimation in small catchments or for greenfield runoff is repeated in several subsequent guidance documents, including: Design Manual for Roads and Bridges (Highways Agency 2004); Exceedance in Urban Drainage (Balmforth et al. 2006); SUDS Manual (Woods-Ballard et al. 2007); Sewers for Scotland (Scottish Water 2007); Code for Sustainable Homes Technical Guide (Department for Communities and Local Government 2009); Drainage Manual (Network Rail 2010). 30

31 Possible alternative reason for dominance of IH 124 It s got small in its title 31

32 Empirical support for the methods Number of catchments under 10km 2 used to develop method 32

33 Empirical support for the methods Number of small plots used to develop method Er none (smallest catchment 0.9km 2 ) None Maybe (large field drainage systems) None None None 33

34 Performance tests

35 Number of Catchments Catchments chosen for tests 73 catchments under 25km 2 Braid Burn 40 from HiFlows-UK All but 9 essentially rural SAAR (mm) 35

36 Method for tests Compared predicted QMED with observed QMED from annual maximum flows QMED is the median of annual maximum flows (return period 2 years) Four methods: FEH Statistical ReFH (not FEH rainfall-runoff) IH 124 ADAS

37 Estimated QMED (m 3 /s) Results: comparison of QMED FEH Statistical ReFH IH 124 ADAS Observed QMED (m 3 /s) 37

38 Results: bias ADAS IH124 ReFH FEH Stat Bias in QMED All catchments (73) Urban only (9) Excluding urban and permeable catchments (54) Excluding high-rainfall catchments (44) 38

39 Results: mean error ADAS IH124 ReFH FEH Stat RMSE in log QMED All catchments (73) Urban only (9) Excluding urban and permeable (54) Excluding high-rainfall catchments (44) 39

40 Q100 over QMED Comparison of growth curves AREA (km 2 ) FEH pooled growth factors for typical small catchment (FARL=1) FSR growth factor for N Scotland FSR growth factor for S Scotland ReFH for selected small catchments 40

41 Conclusions on small rural catchments Scope to include more small catchments in HiFlows-UK and datasets used to develop FEH methods Guidance to use IH124 or ADAS 345 is related to ease of use and desire to avoid needing FEH software rather than appropriateness of the methodologies Continued use of IH124 cannot be justified especially using coarse FSR soil maps FEH Statistical works well for small catchments ReFH works well apart from on highly permeable catchments 41

42 Plot-scale runoff 42

43 What about greenfield runoff? Why do we need greenfield runoff estimates? Design of water management measures for: Plot scale Site level of service protection (typically to a 30-year return period) one of the most daunting scientific challenges in hydrology (Wigmosta and Prasad, Encyclopedia of Hydrological Sciences, 2005) River water quality protection Catchment scale Nearly all data is at this scale River regime protection (ensuring that runoff does not exceed the greenfield rate for common events, typically with a 1-year return period) River and site flood protection (typically to a 100-year return period) (Kellagher, 2004) 43

44 What methods are suitable at the plot scale? FEH No based on flow data from catchments with watercourses Same true for IH 124 and ADAS 345. If methods based on data from watercourses can t be used, what can? What about Rational? Assumes flow proportional to area so why not just scale down FEH results by area? Does peak flow scale linearly with area? 44

45 Scaling of peak flow with area Large catchments: FEH statistical method :QMED varies with AREA IH 124: QBAR varies with AREA Catastrophe curve based on maximum peak discharges (Acreman 1989) shows that specific discharge increases markedly as the catchment area reduces from 10,000 km 2 to 10 km 2. Areal reduction effect / storm intensity decreasing / attenuation in channels Small catchments: More linear? Rainfall spanning whole catchment, less attenuation, runoff dominated by hillslope processes 45

46 What does the small catchment data say? Nested pairs of small catchments at Plynlimon, Wales River Ratio of area Ratio of QMED Severn Hore Cyff/Wye

47 Discharge (l/s) Discharge (l/s) And the plot-scale data? Nested catchments at Pontbren, Wales Hillslope drained by field drain (0.36ha) and overland flow (0.44ha) Stream flow from catchment of 3.2km 2 Ratio of areas is 1: Jan Jun Jan 07 12:00 18 Jan 07 00:00 18 Jan 07 12:00 19 Jan 07 00: Jun 07 12:00 25 Jun 07 00:00 25 Jun 07 12:00 26 Jun 07 00:00 26 Jun 07 12:00 Discharge from plot Flow on watercourse scaled down by area Discharge from plot Flow on watercourse scaled down by area 47

48 Heterogeneity Gradient of hillslope is more than twice that of whole catchment Substantial variations in soil properties and runoff processes even over very small scales Features such as macropores can have a large influence on runoff difficult to account for without intensive field study Need some consideration of site characteristics otherwise a greenfield runoff rate applied as an average across a small catchment may be too high or too low for a particular development site Overestimation would result in the limiting discharge being set higher than the actual rate, hence increase in downstream flood flows. 48

49 Conclusions on greenfield runoff Advantages of methods developed at catchment -scale: Well-founded, drawing on large datasets Focus attention on the downstream impact Avoid the need to subjectively define coefficients (e.g. in Rational method) But important to consider site-specific characteristics too More investigation in Phase 2 of project 49

50 Interim recommendations from Phase 1 of research Use FEH methods Assess results against information on flood history and channel capacity For catchments smaller than 0.5km 2 and small plots, scale FEH estimates down by area The decision to translate FEH estimates from catchment scale to plot scale should be accompanied by an assessment of whether the study site is representative of the surrounding catchment area. 50

51 Post-development runoff Current approach: often urban drainage design methods at plot scale As for greenfield runoff, risks losing focus on downstream flood risk Would be preferable to use consistent method for both pre and postdevelopment runoff Methods such as FEH Statistical and rainfall-runoff are calibrated on urbanised catchments only represent net effect of urbanisation More work in Phase 2 51

52 Phase 2 of research 52

53 Plans for Phase 2 Task 1: More data Up to 40km 2 Down to hectares Checks on ratings for non-hiflows-uk gauges Urban catchments including possible ratings for level gauges Contributions welcome! 53

54 Plans for Phase 2 Task 2: development of methods High-resolution catchment descriptors Simpler method for flood growth curves on small catchments Analysis of plot-scale data to develop guidance Guidance on use of local data: soils, vegetation, channel size Short-duration design rainfall (<1 hour) 54

55 Plans for Phase 2 Task 3: Reporting and recommendations Report User guidance 55

56 56

57 ReFH in Scotland SEPA s concerns include: Shortage of Scottish calibration data Calibration to pooled growth curves - dominated by larger catchments 150 year return period limit of calibration 57

58 ReFH in Scotland: Time line Feb 2006: ReFH was released Results specific to the use of the revitalised FSR/FEH rainfall-runoff method in Scotland will be reported separately. Oct 2006: Work to validate ReFH in Scotland ongoing (ReFH Forum) 2011 and July 2012: SEPA provided rainfall and flow data to WHS to aid in improving ReFH 2012: CEH are revising ReFH want to avoid producing separate version for Scotland 58

59 ReFH in Scotland: my perspective Shortage of Scottish calibration data ReFH: 5 Scottish catchments. FSR RR: 10/11 Calibration to pooled growth curves, mostly large catchments FSR RR calibrated to single-site curves, also mostly large catchments 150 year return period limit of calibration FSR RR: limit was 10 years. ReFH is extensively used in N Ireland no calibration gauges there MSc by David Lee, Leeds, 2010: 59