Flood and runoff estimation on small catchments

Duncan Faulkner

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

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)

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?

What is a small catchment?

6

How low can you go?

7

What about brownfield sites?

8

And after the builders move in…

9

Are these rural or urban?

10

Why talk about them?

11

Because there are so many of them…

Why talk about them?

12

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

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

Flow processes

• Upland catchment (Coalburn)

– 2.6km2

– 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

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 national-

scale gridded data on soils etc.?

16

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

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

Sources of data

HiFlows-UK

20

9% of total dataset

How representative is HiFlows-UK?

21

Graph and table

© Symonds Group

Other stream flow datasets on small catchments

• NRFA: 127 flow gauges on catchments under 25km2

• Other gauges: experimental sites, water companies etc.

• Level gauges on engineered channels in urban catchments – may be possible to derive rating

22

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

IH 124 (1994)

25

ADAS 345 (1982)

26

FEH (Statistical, rainfall-runoff, ReFH)

27

Sewer design methods for urban catchments

28

Guidance on choice of method (2005)

• “IH Report 124 should be used to calculate greenfield runoff peak flow rates”

– For catchments under 2km2

– 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

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

Possible alternative reason for dominance of IH 124

• It’s got small in its title

31

Empirical support for the methods

32

Number of catchments under 10km2

used to develop method

Empirical support for the methods

33

Number of small plots

used to develop method

Er – none (smallest catchment 0.9km2)

Maybe (large field drainage systems)

None

None

None

None

Performance tests

Catchments chosen for tests

• 73 catchments under 25km2

• 40 from HiFlows-UK

• All but 9 essentially rural

35

0

5

10

15

20

25

30

Nu

mb

er

of

Ca

tch

me

nts

SAAR (mm)

Braid Burn

Method for tests

36

• 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 345

Results: comparison of QMED

37

0.001

0.010

0.100

1.000

10.000

100.000

0.010 0.100 1.000 10.000 100.000

Est

ima

ted

QM

ED

(m

3/s

)

Observed QMED (m3/s)

FEH Statistical

ReFH

IH 124

ADAS 345

Results: bias

38

0.5 1.0 2.0

FEH Stat

ReFH

IH124

ADAS

Bias in QMED

All catchments (73)

Urban only (9)

Excluding urban and permeable catchments (54)

Excluding high-rainfall catchments (44)

Results: mean error

39

0.0 0.5 1.0 1.5 2.0

FEH Stat

ReFH

IH124

ADAS

RMSE in log QMED

All catchments (73)

Urban only (9)

Excluding urban and permeable (54)

Excluding high-rainfall catchments (44)

Comparison of growth curves

40

2

3

4

5

0 5 10 15 20 25

Q100 o

ve

r Q

ME

D

AREA (km2)

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

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

What about greenfield runoff?

• Why do we need greenfield runoff estimates?

– Design of water management measures for:

• Site level of service protection (typically to a 30-year return period)

• River water quality protection

• 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

Plot scale

Catchment

scale

Nearly all

data is at

this scale

“one of the most daunting scientific challenges in hydrology” (Wigmosta and Prasad, Encyclopedia of Hydrological Sciences, 2005)

What methods are suitable at the plot scale?

44

FEH No – based on

flow data from

catchments with

watercourses

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?

Same true for IH 124

and ADAS 345.

If methods based on

data from watercourses

can’t be used,

what can?

Scaling of peak flow with area

• Large catchments:

– FEH statistical method :QMED varies with AREA0.85.

– IH 124: QBAR varies with AREA0.89.

– Catastrophe curve based on maximum peak discharges (Acreman 1989)

shows that specific discharge increases markedly as the catchment area

reduces from 10,000 km2 to 10 km2.

– 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

What does the small catchment data say?

• Nested pairs of small catchments at Plynlimon, Wales

46

River Ratio of

area

Ratio of

QMED

Severn 2.51 2.54

Hore 2.00 1.80

Cyff/Wye 3.39 2.98

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 3.2km2

– Ratio of areas is 1: 792

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

24 Jun 07 12:00 25 Jun 07 00:00 25 Jun 07 12:00 26 Jun 07 00:00 26 Jun 07 12:00

Dis

ch

arg

e (

l/s)

Discharge from plot Flow on watercourse scaled down by area

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

17 Jan 07 12:00 18 Jan 07 00:00 18 Jan 07 12:00 19 Jan 07 00:00

Dis

ch

arg

e (

l/s)

Discharge from plot Flow on watercourse scaled down by area

18 Jan 07 25 Jun 07

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.

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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 0.5km2 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

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 post-development 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

Plans for Phase 2

• Task 1: More data

– Up to 40km2

– Down to hectares

– Checks on ratings for non-HiFlows-UK gauges

– Urban catchments including possible ratings for level gauges

– Contributions welcome!

53

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

Plans for Phase 2

• Task 3: Reporting and recommendations

– Report

– User guidance

55

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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

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

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:

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