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Flood estimation calculation record for single sites

Introduction

This document is a supporting document to the Environment Agency’s flood estimation guidelines. It provides a record of the calculations and decisions made during flood estimation. It will often be complemented by more general hydrological information given in a project report. The information given here should enable the work to be reproduced in the future. This version of the record is for studies where flood estimates are needed at a single location.

Contents

Page

1 METHOD STATEMENT -------------------------------------------------------------------------------------------- 3

2 STATISTICAL METHOD ------------------------------------------------------------------------------------------- 7

3 REVITALISED FLOOD HYDROGRAPH (REFH) METHOD -------------------------------------------- 10

4 DISCUSSION AND SUMMARY OF RESULTS ------------------------------------------------------------ 11

5 ANNEX - SUPPORTING INFORMATION ------------------------------------------------------------------ 14

Approval

Signature Name and qualifications For Environment Agency staff: Competence level (see below)

Calculations prepared by:

E. Hillcoat Liz Hillcoat BSc MSc CSci C.WEM MCIWEM

Calculations checked by:

H. Stockham Helen Stockham BSc MSc DIC CSci C.WEM MCIWEM

Calculations approved by:

T. Rouse Tom Rouse BSc MSc CSci C.WEM MCIWEM

Environment Agency competence levels are covered in Section 2.1 of the flood estimation guidelines:

Level 1 – Hydrologist with minimum approved experience in flood estimation

Level 2 – Senior Hydrologist

Level 3 – Senior Hydrologist with extensive experience of flood estimation

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Abbreviations

AEP Annual Exceedance Probability AM Annual Maximum AREA Catchment area (km2) BFI Base Flow Index BFIHOST Base Flow Index derived using the HOST soil classification CFMP Catchment Flood Management Plan CPRE Council for the Protection of Rural England FARL FEH index of flood attenuation due to reservoirs and lakes FEH Flood Estimation Handbook FRM Flood Risk Management FSR Flood Studies Report HOST Hydrology of Soil Types NRFA National River Flow Archive POT Peaks Over a Threshold QMED Median Annual Flood (with return period 2 years) ReFH Revitalised Flood Hydrograph method SAAR Standard Average Annual Rainfall (mm) SPR Standard percentage runoff SPRHOST Standard percentage runoff derived using the HOST soil classification Tp(0) Time to peak of the instantaneous unit hydrograph URBAN Flood Studies Report index of fractional urban extent URBEXT1990 FEH index of fractional urban extent URBEXT2000 Revised index of urban extent, measured differently from URBEXT1990 WINFAP-FEH Windows Frequency Analysis Package – used for FEH statistical method


1 Method statement

1.1 Overview of requirements for flood estimate

Item Comments

Give an overview which includes:

Purpose of study

Peak flow or hydrograph?

Range of return periods

Approx. time available

This assessment has been completed as part of the Corby Flood Risk Management (FRM) Study. The Corby FRM Study is assessing flood risk at key priority areas within the Corby Borough with an aim to review flood risk mitigation. The area along Gainsborough Road is one of these key priority areas. A hydraulic model has been made available for the Corby FRM Study for the watercourse along Gainsborough Road, however owing to the use of updated outfalls from the urban area and how the model has been taken forward, the hydrology within the model needs altering. This proforma outlines the estimation of inflows to the watercourses from the rural upstream catchment.

Peak flows and hydrographs are required as part of this study.

A range of return periods are to be estimated ranging from the QMED to the 1% Annual Exceedance Probability (AEP) with an allowance for climate change.

The results are required within a period of one week.

1.2 Overview of catchment

Item Comments

Brief description of catchment, or reference to section in accompanying report

The catchment is located to west of Corby, in the rural area to the west of Uppingham Road. Downstream of this catchment the watercourse is directed under Uppingham Road and enters a primarily culverted reach, but with some open sections. A location plan of the catchment is included in section 5.

The catchment is entirely rural, which is reflected through the URBEXT values (both 1990 and 2000) of 0. The catchment is not considered highly permeable with a SPRHOST of 41.69.

1.3 Source of flood peak data

Was the HiFlows UK dataset used? If so, which version? Record any changes made.

Yes – Version 3.1.2, December 2011

1.4 Gauging stations (flow or level)

(at the site of the flood estimate or nearby at potential donor sites)

Water-course

Station name

Gauging authority number

NRFA number (used in

FEH)

Grid reference

Catch-ment area (km²)

Type (rated /

ultrasonic / level…)

Start and end of flow

record

N/A

1.5 Data available at each flow gauging station

Station name

Start and end of data in

HiFlows-UK

Update for this study?

Suitable for

QMED?

Suitable for

pooling?

Data quality check

needed?

Other comments on station and flow data quality – e.g.

information from HiFlows-UK, trends

in flood peaks, outliers.

N/A


Station name


HiFlows-UK


Suitable for

QMED?

Suitable for

pooling?

Data quality check

needed?




Give link/reference to any further data quality checks carried out

1.6 Rating equations

Station name

Type of rating e.g. theoretical,

empirical; degree of extrapolation

Rating review

needed?

Reasons – e.g. availability of recent flow gaugings,

amount of scatter in the rating.

N/A

Give link/reference to any rating reviews carried out

1.7 Other data available and how it has been obtained

Type of data Data relevant to this study?

Data available

?

Source of data and licence

reference if from EA

Date obtained

Details

Check flow gaugings (if planned to review ratings)

N/A

Historic flood data – give

link to historic review if carried out.

The inundated areas as shown in the model (for which the flows estimated in this proforma will inform) will be checked against known flooding incidents along Gainsborough Road. There are no flow measurements which can be compared with the flows calculated in this proforma.

Flow data for events

N/A

Rainfall data for events

N/A

Potential evaporation data

N/A

Results from previous studies (e.g. CFMPs,

Strategies)

N/A

Other data or information (e.g.

groundwater, tides)

N/A

1.8 Initial choice of approach

Is FEH appropriate? (it may not be for very small, heavily urbanised or complex catchments) If not, describe other methods to be used.

Yes FEH is appropriate. The catchment is not considered to be small (1.74km2), urbanised (UREBEXT2000 is 0) or considered complex.

Outline the conceptual model, addressing questions such as:

What is likely to cause flooding at the site (peak flows, flood volumes, combinations of peaks, groundwater, snowmelt, tides…)

Flooding downstream of the catchment assessed in this proforma is reported to be as a result of trash screen blockage and potentially high water levels preventing surface water discharge into the receiving watercourse. Therefore flooding is considered to be resulting from peak


Might the site flood from runoff generated on part of the catchment only, e.g. downstream of a reservoir?

Is there a need to consider temporary debris dams that could collapse?

flows, owing to reduced culvert capacity due to blockage and due to volumes as water backs up from blocked culverts.

There is no reservoir influence on flows at this location.

There is no requirement to consider a temporary dam collapse.

Any unusual catchment features to take into account?

e.g.

highly permeable – avoid ReFH if BFIHOST>0.65, consider permeable catchment adjustment for statistical method if SPRHOST<20%

highly urbanised – avoid standard ReFH if URBEXT1990>0.125; consider FEH Statistical or other alternatives; consider method that can account for differing sewer and topographic catchments

pumped watercourse – consider lowland catchment version of rainfall-runoff method

major reservoir influence (FARL<0.90) – consider flood routing

extensive floodplain storage – consider choice of method carefully

There are no unusual catchment features. It is not highly permeable (SPRHOST is 41.69).

The catchment is not heavily urbanised (URBEXT2000 is 0).

It is not a pumped watercourse.

There is no major reservoir influence (FARL is 1).

There is no extensive floodplain.

Initial choice of method(s) and reasons

The choice of method is to complete the FEH Statistical method and ReFH. Hydrographs using peak flows from both methods will be run in the model. Whilst the return period of the flood event is unknown, a rainfall severity analysis indicates that the rainfall event was not considered extreme, occurring more frequently than a 50% AEP (1 in 2) event. The resultant flood extents and outlines will be compared against each other and with the known event as this may help inform the decision as to which flows are taken forward.

Software to be used (with version numbers) FEH CD-ROM v3.01

WINFAP-FEH v3.0.32 / ReFH spreadsheet / ReFH node in ISIS Free

1.9 Site details

Watercourse Site Easting Northing AREA on FEH CD-ROM

(km2)

Revised AREA if altered

Unnamed watercourse

Corby, upstream of Uppingham Road

485950 287700 1.83 1.74

1.10 Catchment descriptors (incorporating any changes made)

FARL PROPWET BFIHOST DPLBAR (km)

DPSBAR (m/km)

SAAR (mm)

SPRHOST URBEXT2000 FPEXT

1 0.3 0.326 1.35 21.6 634 41.63 0.005 0.130

1 FEH CD-ROM v3.0 © NERC (CEH). © Crown copyright. © AA. 2009. All rights reserved.

2 WINFAP-FEH v3 © Wallingford HydroSolutions Limited and NERC (CEH) 2009.

http://intranet/ams_document_library/2008/151_200/197_08.doc#FOUR_FIVE


1.11 Checking catchment descriptors

Record how catchment boundary was checked and describe any changes (refer to maps if needed)

The catchment boundary was checked against Ordnance Survey (OS) mapping and LiDAR. The catchment boundary taken from the FEH CD-ROM is broadly consistent with both OS and LiDAR, however alterations in some areas were required (as shown in the location plan in section 5.1). The most significant of these was at the downstream extent where a proportion of the catchment (as identified through LiDAR and OS mapping) was not included with the FEH CD-ROM catchment. The catchment was therefore extended to include this area, although the Uppingham Road was taken to be a boundary to flow, with features which appear to be minor drainage ditches located adjacent to the road. Other minor alterations were made to bring the FEH CD-ROM catchment in line with the LiDAR and OS mapping.

Record how other catchment descriptors (especially soils) were checked and describe any changes. Include before/after table if necessary.

Through a review of geology mapping there was no reason to alter any catchment parameters.

Source of URBEXT URBEXT1990 and URBEXT2000 for the different methods as stated within Flood Estimation Guidelines (Environment Agency, 2012)3.

Method for updating of URBEXT

CPRE formula from 2006 CEH report on URBEXT2000

3 Environment Agency. (2012). Flood estimation guidelines, operational instruction 197_08.


2 Statistical method

2.1 Search for donor sites for QMED

Comment on potential donor sites

Mention:

Number of potential donor sites available

Distances from subject site

Similarity in terms of AREA, URBEXT, FARL and other catchment descriptors

Quality of flood peak data

Include a map if necessary. Note that donor catchments should usually be rural.

The potential donors were found through the FEH CD-ROM, the hydrometric register (2008)4 and through creating pooling

groups using HiFlows data sets classed as ‘OK for pooling’ and ‘OK for QMED’. This resulted in 12 potential donors. Most of these donors were of a similar size to the subject site and also had similar catchment descriptors such as urban coverage and FARL. There were individual donors which were too different from the subject site and hence were discounted as outlined in section 2.2. Six of the donors were located more than 100km from the subject site and these were also discounted. The majority of the potential donors had relatively long record periods of records of over 30 years in length.

2.2 Donor sites chosen and QMED adjustment factors

NRFA no.

Reasons for choosing or rejecting

Method (AM or POT)

Adjust-ment for climatic variation?

QMED from flow data (A)

QMED from catchment descriptors (B)

Adjust-ment ratio (A/B)

31021 Rejected – classed in HiFlows as not ok for pooling or QMED

N/A N/A N/A N/A N/A

32003 Rejected – although considered potentially a suitable donor, 31023 was considered a better donor in relation to catchment descriptors.

AM Not needed – 71 years

10.16 8.99 1.05

31010 Rejected – although considered potentially a suitable donor, 31023 was considered a better donor in relation to catchment descriptors.


10.20 6.81 1.15

32004 Rejected – The catchment is slightly too urban, the FARL is slightly too low and the area is slightly too high in comparison to the subject site.


14.90 14.23 1.02

31026 Rejected – Other donors were more similar to the subject site in relation to permeability and urban extent. There are also three other donors at a shorter distance from the subject site.


1.08 0.31 1.48

31023 Chosen – most similar to the subject catchment in relation to permeability, urban coverage, size and SAAR in comparison to other possible donors. It is also located on the same geology as the subject catchment.


1.91 1.13 1.12

4 Centre for Ecology and Hydrology. UK Hydrometric Register, 2008


NRFA no.

Reasons for choosing or rejecting

Method (AM or POT)





27038 Rejected – Too permeable and too far from the subject site.


1.33 0.55 1.01

205999 Rejected – SAAR is too high, FARL is too low and the catchment is too far from the subject site.


0.12 3.13 0.96

76011 Rejected – The SAAR is too high and the catchment is too far from the subject site.


1.82 2.02 1.00

45817 Rejected – The SAAR is too high and the catchment is too far from the subject site.


1.34 0.92 1.00

32029 Rejected – Short record period and the catchment is in Ireland and hence too far from the subject site.

AM Would be needed – 5 years

2.54 1.42 1.15

27073 Rejected – Too permeable and too far from the subject site.


0.74 0.32 1.01

Which version of the urban adjustment was used for QMED at donor sites, and why?

Note: The guidelines recommend great caution in urban adjustment of QMED on catchments that are also highly permeable (BFIHOST>0.8).

Urban Expansion, equation 5.5 Environment Agency 20065.

2.3 Overview of estimation of QMED

Method

Initial estimate of QMED (m3/s)

Data transfer

Final estimate of QMED (m3/s)

NRFA numbers for donor sites used (see 3.3)

Distance between centroids dij (km)

Power term, a

Moderated QMED adjustment factor, (A/B)a

If more than one donor

Weig

ht

Weig

hte

d

ave

rag

e

ad

justm

en

t fa

cto

r

DT 0.50 31023 39km 0.21 1.12 - - 0.56

Which version of the urban adjustment was used for QMED, and why?

N/A – catchment is rural

Notes

Methods: AM – Annual maxima; POT – Peaks over threshold; DT – Data transfer; CD – Catchment descriptors alone. When QMED is estimated from POT data, it should also be adjusted for climatic variation. Details should be added. When QMED is estimated from catchment descriptors, the revised 2008 equation from Science Report

SC050050Error! Bookmark not defined. should be used. If the original FEH equation has been used, say so and

give the reason why. The guidelines recommend great caution in urban adjustment of QMED on catchments that are also highly permeable (BFIHOST>0.8). The adjustment method used in WINFAP-FEH v3.0.003 is likely to overestimate adjustment factors for such catchments. In this case the only reliable flood estimates are likely to be derived from local flow data. The data transfer procedure is the revised one from Science Report SC050050. The QMED adjustment factor A/B for each donor site is given in Table 3.3. This is moderated using the power term, a, which is a function of the distance between the centroids of the subject catchment and the donor catchment. The final estimate of QMED is (A/B)a times the initial estimate from catchment descriptors.

5 Environment Agency. URBEXT2000 – A new FEH catchment descriptor. Calculation, dissemination and application. R&D

Technical Report FD1919/TR, 2006.


If more than one donor has been used, use multiple rows for the site and give the weights used in the averaging. Record the weighted average adjustment factor in the penultimate column.

2.4 Derivation of pooling group

The composition of the pooling group is given in the Annex.

Subject site treated as gauged? (enhanced single site analysis)

Changes made to default pooling group, with reasons Note also any sites that were investigated but retained in the group.

Weighted average L-moments, L-CV and L-skew, (before urban adjustment)

No Flore 32029 removed as there was not enough years of data

L-CV is 0.231 L-Skew is 0.237

Notes The pooling group was derived using the revised procedures from Science Report SC050050 (2008).

2.5 Derivation of flood growth curves

Method (SS, P, ESS, J)

Distribution used and reason

for choice

Note any urban adjustment or

permeable adjustment

Parameters of distribution (location, scale and shape) after

adjustments

Growth factor for 100-year return

period

P A generalised logistic distribution for the flood frequency curve has been chosen as it is recommended in FEH

Not required No adjustments 2.914

Notes

Methods: SS – Single site; P – Pooled; ESS – Enhanced single site; J – Joint analysis

Growth curves were derived using the revised procedures from Science Report SC050050 (2008).

2.6 Flood estimates from the statistical method

Flood peak (m3/s) for the following return periods (in AEP)

50 5 1 1+CC

0.56 1.11 1.63 1.96


3 Revitalised flood hydrograph (ReFH) method

3.1 Parameters for ReFH model

Note: If parameters are estimated from catchment descriptors, they are easily reproducible so it is not essential to enter them in the table.

Method: OPT: Optimisation BR: Baseflow recession fitting CD: Catchment descriptors DT: Data transfer (give details)

Tp (hours)

Time to peak

Cmax (mm) Maximum storage capacity

BL (hours)

Baseflow lag

BR

Baseflow recharge

CD 2.93 275 30 0.725

Brief description of any flood event analysis carried out (further details should be given below or in a project report)

none

3.2 Design events for ReFH method

Urban or rural

Season of design event (summer or winter)

Storm duration (hours) Storm area for ARF (if not catchment area)

Rural Winter 4.7 0.973

3.3 Flood estimates from the ReFH method


50 5 1 1+CC

0.73 1.41 2.08 2.49


4 Discussion and summary of results

4.1 Comparison of results from different methods

This table compares peak flows from various methods with those from the FEH Statistical method for two key return periods. Blank cells indicate that results were not calculated using that method.

Ratio of peak flow to FEH Statistical peak

Return period 2 years Return period 100 years

ReFH ReFH

1.3 1.3

4.2 Final choice of method

Choice of method and reasons –

include reference to type of study, nature of catchment and type of data available.

At this stage it is proposed that the flows estimated using the FEH Statistical method will be taken forward. These results are based on observed data, rather than relying on catchment descriptors alone.

However by way of checking, both sets of results flows will be run in the model and resulting flood extents and depths compared to flooding which has been experienced in this area. Further information on these checks is provided in section 4.7.

4.3 Assumptions, limitations and uncertainty

List the main assumptions made (specific to this study)

The study assumes that the catchment has been accurately delimited (with minor manual alteration) and the catchment descriptors for the catchment are accurate.

The statistical method is heavily reliant on the choice of QMED. The calculated QMED has been altered based on a donor site; it is assumed that the donor site is suitable.

The ReFH method relies on catchment descriptors alone and thus there is a heavy reliance that the catchment descriptors are accurate.

Discuss any particular limitations, e.g. applying methods outside the range of catchment types or return periods for which they were developed

Both the ReFH and FEH Statistical methods have been applied within the recommended ranges (i.e. catchment properties and required flow events).

Give what information you can on uncertainty in the results – e.g.

confidence limits for the QMED estimates using FEH 3 12.5 or the

factorial standard error from Science Report SC050050 (2008).

The uncertainty in the estimates of QMED calculated using the QMED model in the statistical method is generally much larger than the uncertainty of estimates obtained directly from flood data.

The flows estimated use the best available data for this catchment, so will be appropriate for the current study.

Comment on the suitability of the results for future studies, e.g. at nearby locations or for different purposes.

The flow estimates would be considered acceptable for information in future studies. However new estimates would be necessary if flows were required at locations either upstream or downstream from the point of interest.

Give any other comments on the study, for example suggestions for additional work.

Nothing considered appropriate for this project.

4.4 Checks

What do the results imply regarding the return periods of floods during the period of record?

There are no records of flow rates during the previous flooding events.


What is the 100-year growth factor? Is this realistic? (The guidance

suggests a typical range of 2.1 to 4.0)

The 100 year growth curve factors are 2.914 and 2.856 for the FEH Statistical and ReFH methods respectively, which are within the typical range.

If 1000-year flows have been derived, what is the ratio for 1000-year flow over 100-year flow?

N/A

What specific runoff (l/s/ha) does the design flow equate to?

N/A

How do the results compare with those of other studies? Explain any

differences and conclude which results should be preferred.

Significant alterations required for the urban area and other such modifications have been made to the hydraulic model for this study. It is therefore not considered appropriate to make a direct comparison between the results from this study to the previous modelling study. However hydrographs from the most upstream location of open channel (approximately 2km downstream of the study catchment), as available from the original model, were broadly compared to the results from this study.

There have been no other hydrological studies for this catchment.

Are the results compatible with the longer-term flood history?

There are no records of flow rates in relation to previous flood events. However the resultant flood extents and depths, through the use of the hydraulic model, will be compared to flood records.

Describe any other checks on the results

None.

4.5 Final results that will be run in the model for checking purposes


50 5 1 1+CC

FEH Statistical 0.56 1.11 1.63 1.96

If flood hydrographs are needed for the next stage of the study, where are they provided? (e.g. give filename of spreadsheet, name of ISIS model, or reference to table below)

Hydrographs are required for the next stage.

Hydrographs have been taken from the ReFH node in ISIS. These have been directly abstracted for the flows estimated using the ReFH method.

Using the ReFH node, the hydrographs have been also been scaled to fit the peak flow estimated using the FEH Statistical method.

4.6 Additional results that will be run in the model for checking purposes


50 5 1 1+CC

ReFH 0.73 1.41 2.08 2.49

4.7 Checking following the completion of the above study

Comments As outlined in the above sections, following the completion of the hydrological assessment both sets of results were run in the hydraulic model and the results (flood depths and extents) compared to known historical flooding. This section (4.7) outlines the conclusions made from this checking process.

However through further interrogation of the model and the nature of the catchment, it was identified that the culvert under Uppingham Road, the downstream point of the study catchment, acts as a significant constriction on flow. During flood events (and using hydrographs calculated using both methods) water backs up at this location causing ponding to the west of Uppingham Road. In relation to the model, this results in a maximum peak flow from the rural catchment, passing into the developed area of Corby (east of Uppingham Road) irrespective of the flow estimation method used. The


total event volume passing through the culvert is approximately 21-23% higher when using hydrographs with peak flows estimated using ReFH in comparison to hydrographs with peak flows estimated using the FEH statistical method.

The purpose of the overall study is to review the potential feasibility of flood risk mitigation measures and therefore it is considered appropriate to use the more conservative flows (i.e. hydrographs using peak flows calculated through ReFH) to ensure that potential storage or capacity options are not under sized.

Furthermore, the flows calculated using the FEH Statistical peak were run through the model in a 50% culvert blockage scenario along Gainsborough Road. In this scenario, event volume can become more important than peak flow because the watercourse is more reliant on available storage capacity. However even when the 1% AEP flows were run for the 50% blockage scenario, the flood depths experienced during the 24th December 2013 event were not replicated. Through rainfall severity analysis it was estimated that the event on 24th December 2013 was considered ‘common place’. This suggests that the FEH Statistical method may underestimate flows within this catchment or that there are unknown inflows, providing additional support for the use of the higher, ReFH flows.


5 Annex - supporting information

5.1 Location plan

Contains Ordnance Survey data (c) Crown copyright and database right. 2014

5.2 Pooling group used (final)

Station Distance Years of data QMED AM L-CV L-SKEW Discordancy

76011 (Coal Burn @ Coalburn) 1.23 32 1.822 0.178 0.347 0.866

45817 (Rhb Trib to Haddeo @ Upton (trib)) 1.76 16 1.339 0.292 0.304 0.36

27073 (Brompton Beck @ Snainton Ings) 2.409 29 0.743 0.205 0.011 0.994

44009 (Wey @ Broadwey) 2.506 32 1.688 0.34 0.241 1.318

27051 (Crimple @ Burn Bridge) 2.525 37 4.514 0.22 0.133 0.479

45816 (Haddeo @ Upton) 2.578 16 3.539 0.331 0.427 0.914

28033 (Dove @ Hollinsclough) 2.852 30 4.654 0.257 0.403 0.438

54091 (Severn @ Hafren Flume) 3.071 34 5.915 0.184 0.27 2.84

44006 (Sydling Water @ Sydling st Nicholas) 3.088 35 0.853 0.227 0.087 0.591

54092 (Severn @ Hore Flume) 3.107 34 6.353 0.116 -0.052 2.366

26802 (Gypsey Race @ Kirby Grindalythe) 3.232 10 0.127 0.233 0.25 0.1


25019 (Leven @ Easby) 3.235 31 6.088 0.355 0.396 1.238

25011 (Langdon Beck @ Langdon) 3.413 23 15.362 0.247 0.399 1.188

25003 (Trout Beck @ Moor House) 3.459 36 15.105 0.173 0.328 0.791

91802 (Allt Leachdach @ Intake) 3.5 34 6.35 0.153 0.257 1.05

206006 (Annalong @ Recorder 1895) 3.599 48 15.33 0.189 0.052 1.373

27010 (Hodge Beck @ Bransdale Weir) 3.625 41 9.42 0.224 0.293 0.091

Total 518

Weighted means 518 0.231 0.237

5.3 Hydrographs

0.0

0.5

1.0

1.5

2.0

2.5

0 2 4 6 8 10 12 14 16 18

Flo

w (

m3/s

)

Time (hr)

FEH Statistical Hydrographs

5% AEP

1% AEP

1% + CC

50% AEP


0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 2 4 6 8 10 12 14 16 18

Flo

w (

m3 /

s)

Time (hr)

ReFH Model Hydrographs

5% AEP

1% AEP

1% + CC

50% AEP


Introduction


Contents

Page


2 REVITALISED FLOOD HYDROGRAPH (REFH) METHOD ---------------------------------------------- 7

3 DISCUSSION AND SUMMARY OF RESULTS -------------------------------------------------------------- 8


Approval














Abbreviations

AEP Annual Exceedance Probability AM Annual Maximum AREA Catchment area (km

2)

BFI Base Flow Index BFIHOST Base Flow Index derived using the HOST soil classification CFMP Catchment Flood Management Plan CPRE Council for the Protection of Rural England FARL FEH index of flood attenuation due to reservoirs and lakes FEH Flood Estimation Handbook FRM Flood Risk Management FSR Flood Studies Report HOST Hydrology of Soil Types NRFA National River Flow Archive POT Peaks Over a Threshold QMED Median Annual Flood (with return period 2 years) ReFH Revitalised Flood Hydrograph method SAAR Standard Average Annual Rainfall (mm) SPR Standard percentage runoff SPRHOST Standard percentage runoff derived using the HOST soil classification Tp(0) Time to peak of the instantaneous unit hydrograph URBAN Flood Studies Report index of fractional urban extent URBEXT1990 FEH index of fractional urban extent URBEXT2000 Revised index of urban extent, measured differently from URBEXT1990 WINFAP-FEH Windows Frequency Analysis Package – used for FEH statistical method

1 Method statement


Item Comments


Purpose of study




This assessment has been completed as part of the Corby Flood Risk Management (FRM) Study. The Corby FRM Study is assessing flood risk at key priority areas within the Corby Borough with an aim to review flood risk mitigation. The area along Cottingham Road is one of these key priority areas. A hydraulic model has been made available for the Corby FRM Study for the watercourse along Cottingham Road, however owing to the use of updated outfalls from the urban area and how the model has been taken forward, the hydrology within the model needs altering. This hydrology proforma outlines the estimation of inflows to the watercourses from the rural upstream catchment.


A range of return periods are to be estimated from the QMED to 1% Annual Exceedance Probability (AEP) with an allowance for climate change.

The results are required within a period of one week.


Item Comments


Two watercourses join upstream of the Cottingham Road priority area: hydrographs and peak flows have been estimated for both these watercourses. The point at which flows are required is at the downstream extent of the rural area prior to the watercourses entering culverts through the developed area of Corby. These catchments are located to west of Corby, in the rural area to the west of Uppingham Road. Downstream of these catchments the watercourses are directed under Uppingham Road and enter primarily culverted reaches, but with some open sections. A location plan of the catchments is included in section 4. The catchments have been termed Cottingham North and Cottingham South.

The catchments are essential rural, with URBEXT1990 values of 0.002 and 0 for the Cottingham North and South catchments respectively. The catchments are not considered highly permeable with SPRHOST values of greater than 42.



No – not used for ReFH



Water-course

Station name



FEH)

Grid reference


Type (rated /



record

N/A


Station name


HiFlows-UK


Suitable for

QMED?

Suitable for

pooling?

Data quality check

needed?




N/A



Station name



Rating review

needed?



N/A




Data available

?



Date obtained

Details


N/A



There are no flow measurements which can be compared with the flows calculated in this proforma.


N/A


N/A


N/A


Strategies)

For the purposes of the Corby FRM study a hydrological assessment has been completed for the Gainsborough Road catchment, which is located adjacent to the Cottingham South catchment. The proximity and context between these catchments is shown in section 4.2. Flows from all three catchments (the Cottingham North and South catchments and the Gainsborough Road catchment) will be included within the same model. The results from this previous hydrological assessment have informed this assessment as discussed in section 1.8.


groundwater, tides)

N/A



Yes FEH is appropriate. The catchments are not considered small (1.15 and 0.99km

2), they are not

urbanised (UREBEXT1990 are 0.002 and 0) and they are not considered complex.





Downstream of the catchments assessed in this proforma flooding is reported in areas of constriction on flow (i.e. in areas upstream of culvert inlets). Therefore flooding is considered to be resulting from peak flows, owing to reduced culvert capacity and due to volumes as water potentially backs up from culverts.




e.g.






There are no unusual catchment features. The catchments are not highly permeable (SPRHOST for both is over 42).

The catchments are not heavily urbanised (URBEXT11990 is 0.002 and 0).

The catchments are not pumped watercourses.

There is no major reservoir influence (FARL for both is 1).

There is no extensive floodplain.


The previous hydrological assessment of Gainsborough Road involved the estimation of flows and hydrographs through the FEH Statistical method and ReFH. The hydraulic model was run with both sets of results and it was considered that the ReFH results are most appropriate for the overall study. As the flows for all three catchments are being included within the same model, it was considered appropriate to use the same method for flood estimation for all three catchments. Therefore the ReFH method has been used to determine peak flows and hydrographs for the two catchments which are the focus of this proforma.


ReFH spreadsheet / ReFH node in ISIS Free

1.9 Site details

Site code Watercourse Site Easting Northing AREA on FEH CD-

ROM (km2)


Cottingham North

Unnamed watercourse

Corby, near the junction between Uppingham Road and the A427

486200 289150 1.42 1.19

Cottingham South

Unnamed watercourse

Adjacent to the southern boundary of the above catchment

486000 288350 0.71 1.00




Site code FARL PROPWET BFIHOST DPLBAR (km)

DPSBAR (m/km)

SAAR (mm)

SPRHOST URBEXT

2000 FPEXT

Cottingham North

1 0.3 0.345 1.10 10.3 629 42.11 0.002 0.192

Cottingham South

1 0.3 0.321 1.00 13.5 632 42.03 0.000 0.129



The catchment boundaries were checked against Ordnance Survey (OS) mapping and LiDAR. The catchment boundaries taken from the FEH CD-ROM are broadly consistent with both OS and LiDAR, however alterations in some areas were required (as shown in the location plan in section 4.1). The most significant of these was at the downstream extent where a proportion of the catchments (as identified through LiDAR and OS mapping) was not included with the FEH CD-ROM catchments. The catchments were therefore extended in this area, with the Uppingham Road and the A427 assumed to be boundaries to flow. Other minor alterations were made to bring the FEH CD-ROM catchments in line with the LiDAR and OS mapping.



Source of URBEXT URBEXT1990


CPRE formula from FEH Volume 4




Site code


Tp (hours) Time to

peak


BL (hours) Baseflow

lag

BR Baseflow recharge

Cottingham North CD 3.14 289.9 29.6 0.77

Cottingham South CD 2.79 270.7 28.3 0.71


None


Site code

Urban or rural

Season of design event (summer or

winter)

Storm duration (hours)

Storm area for ARF (if not catchment area)

Cottingham North


Cottingham South




Site code 50 5 1 1+CC

Cottingham North 0.46 0.89 1.31 1.57

Cottingham South 0.43 0.84 1.24 1.48






ReFH ReFH




As outlined in section 1.8, the flows for these two catchments are to be included within a wider hydraulic model, which includes a total of three rural catchments. A hydrological assessment was previously completed for the third catchment (Gainsborough Road). The hydrology for the Gainsborough Road catchment was included within the hydraulic model and the resultant flood extents from flows estimated using both the FEH Statistical and ReFH methods were determined. It was concluded that the ReFH flows provided more appropriate flows for the wider study. Therefore flows for all three catchments determined through the use of the ReFH method have been taken forward.



The study assumes that the catchments have been accurately delimited (with minor manual alteration) and the catchment descriptors are accurate.

The ReFH method relies on catchment descriptors alone and thus there is a heavy reliance that the catchment descriptors are accurate.


The ReFH method has been applied within the recommended ranges (i.e. catchment properties and required flow events).

The method does not make use of observed flow data.




The flows estimated use the best available data for these catchments, so will be appropriate for the current study.


The flow estimates would be considered acceptable for information in future studies. However new estimates would be necessary if flows were required at locations either upstream or downstream from the point of interest.


Nothing considered appropriate for this project.

3.4 Checks


There are no records of flow rates during the previous flooding events.

What is the 100-year growth factor? The 100 year growth curve factors are 2.83 and 2.89 for the Cottingham North and South catchments respectively, which are

Is this realistic? (The guidance

suggests a typical range of 2.1 to 4.0) within the typical range.


N/A


N/A



The 100 year growth factors are similar to that estimated for the Gainsborough Road catchment (2.86).

It is not possible to compare the results from this study to the previous model owing to the significant alterations required for the urban area and other such modifications. It is therefore not considered appropriate to make this comparison.

There have been no other hydrological studies for this catchment.


There are no records of flow rates in relation to previous flood events.


None.

3.5 Final results


Site code 50 5 1 1+CC

Cottingham North

0.46 0.89 1.31 1.57

Cottingham South

0.43 0.84 1.24 1.48


Hydrographs are required for the next stage.

Hydrographs have been taken from the ReFH node in ISIS.


4.1 Location plan

©©CCrroowwnn CCooppyyrriigghhtt.. AAllll rriigghhttss rreesseerrvveedd.. CCoorrbbyy BBoorroouugghh CCoouunncciill,, 110000001188779911,, 22001144..

4.2 Context of the three catchments

CCoonnttaaiinnss OOrrddnnaannccee SSuurrvveeyy ddaattaa ((cc)) CCrroowwnn ccooppyyrriigghhtt aanndd ddaattaabbaassee rriigghhtt.. 22001144

4.3 Hydrographs

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 2 4 6 8 10 12 14 16 18

Flo

w (

m3 /

s)

Time (hr)

Cottingham North Hydrographs

5% AEP

1% AEP

1% + CC

50% AEP

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 2 4 6 8 10 12 14 16

Flo

w (

m3 /

s)

Time (hr)

Cottingham South Hydrographs

5% AEP

1% AEP

1% + CC

50% AEP


Introduction


Contents

Page


2 REVITALISED FLOOD HYDROGRAPH (REFH) METHOD ---------------------------------------------- 7

3 DISCUSSION AND SUMMARY OF RESULTS -------------------------------------------------------------- 8

4 ANNEX - SUPPORTING INFORMATION ------------------------------------------------------------------- 10

Approval














Abbreviations

AEP Annual Exeedence Probability AM Annual Maximum AREA Catchment area (km

2)

BFI Base Flow Index BFIHOST Base Flow Index derived using the HOST soil classification CFMP Catchment Flood Management Plan CPRE Council for the Protection of Rural England FARL FEH index of flood attenuation due to reservoirs and lakes FEH Flood Estimation Handbook FRM Flood Risk Management FSR Flood Studies Report HOST Hydrology of Soil Types NRFA National River Flow Archive OS Ordnance Survey POT Peaks Over a Threshold QMED Median Annual Flood (with return period 2 years) ReFH Revitalised Flood Hydrograph method SAAR Standard Average Annual Rainfall (mm) SPR Standard percentage runoff SPRHOST Standard percentage runoff derived using the HOST soil classification Tp(0) Time to peak of the instantaneous unit hydrograph URBAN Flood Studies Report index of fractional urban extent URBEXT1990 FEH index of fractional urban extent URBEXT2000 Revised index of urban extent, measured differently from URBEXT1990 WINFAP-FEH Windows Frequency Analysis Package – used for FEH statistical method

1 Method statement


Item Comments


Purpose of study




This assessment has been completed as part of the Corby Flood Risk Management (FRM) Study. The Corby FRM Study is assessing flood risk at key priority areas within the Corby Borough with an aim to review flood risk mitigation. The area surrounding School Hill in Middleton is one of these key priority areas. This proforma outlines the hydrology completed as an initial estimate of runoff/flow to the location where flooding has been recorded. Owing to various unknowns in relation to flooding at this location, documented in the Corby FRM Study report (Atkins ref: 5120150/71/DG/011) to which this proforma is included as an appendix, it is considered appropriate to provide a relatively high level estimation of flow at this location at this time.


A full range of return periods will be estimated from the QMED to 1% Annual Exceedence Probability (AEP) with an allowance for climate change.

The results are required within a period of two weeks.


Item Comments


The catchment is located in Middleton which is to the north west of Corby. This catchment is also located within the River Welland valley. A location plan is included in Section 4.1.

The location at which flows are required is on School Lane at the southern extent of the developed area. The catchment to this location (termed study catchment) has been manually defined (as detailed in Section 1.8). This catchment is rural (URBEXT1990 of 0) and is not considered highly permeable (SPRHOST of 25).



No – only ReFH calculations completed.



Water-course

Station name



FEH)

Grid reference


Type (rated /



record

N/A


Station name


HiFlows-UK


Suitable for

QMED?

Suitable for

pooling?

Data quality check

needed?




N/A



Station name



Rating review

needed?



N/A




Data available

?



Date obtained

Details


N/A



N/A


N/A


N/A


N/A


Strategies)

N/A


groundwater, tides)

N/A



FEH is considered appropriate and a donor catchment has been used for this assessment. The donor catchment is 0.5km

2 and is slightly urbanised

(URBEXT1990 is 0.0395). The study catchment is essentially rural (manually defined URBEXT1990 is 0.00). Although the study catchment is small (<0.5km

2), the

method used in this assessment is considered to be appropriate and is in line with current guidelines1. These

catchments are not considered complex.





The purpose of this assessment is to assist with the identification of causes of flooding at this location. It is considered that catchment runoff is a potential cause of flooding therefore a combination of peak flows and volumes are likely to be factors.




e.g.






The donor catchment is not considered highly permeable (SPRHOST is 25).

The donor catchment is considered to be rural (URBEXT1990 is 0.0395)

The watercourse in this area is not pumped, there is no major reservoir influence (FARL is 1) and there is no extensive floodplain storage.


The study catchment is small <0.5km2 and therefore the

current recommended approach in the flood estimation guidelines

1 for such catchments is to use the FEH

methods (FEH statistical or ReFH) for a donor catchment greater than 0.5km

2 and then scale flows

based on area.

Therefore the proposed approach is to determine flows for the wider catchment (donor catchment), in which the study catchment is located and then scale flows down based on area.

At this stage it is considered appropriate to undertake the ReFH method only. This is a suitable method for this assessment given the wider study that these flow estimates will be used for.


ReFH spreadsheet

ISIS Free version 3.6.0.156 Mode 4

1 Environment Agency (2012). Flood Estimation Guidelines, operational instruction 197_08



1.9 Site details – Donor catchment


(km2)


Unnamed Middleton, Corby 483950 289700 0.54 N/A

1.10 Catchment descriptors (incorporating any changes made) – Donor catchment


DPSBAR (m/km)

SAAR (mm)

SPRHOST URBEXT

1990 FPEXT

1 0.3 0.639 0.68 36.1 625 25.27 0.0395 0



The catchment boundaries were checked against Ordnance Survey (OS) mapping and LiDAR. The donor catchment boundary taken from the FEH CD-ROM is broadly consistent with both OS and LiDAR. It may have been possible to make minor alterations to the catchment boundary through a detailed review (such as at the location of the study catchment), however this was not considered necessary because the flows will be scaled down for the study catchment based on area and hence any impact on flow was considered minimal.

The study catchment was manually determined through a review of OS mapping, LiDAR and flows paths as determined using the flow path tool in Tuflow.



Source of URBEXT URBEXT1990


CPRE formula from FEH Volume 4





Tp (hours)

Time to peak


BL (hours)

Baseflow lag

BR

Baseflow recharge

CD 1.47 521 32 1.5



Urban or rural





Flood peak (m3/s) for the following AEP

Catchment 50% 10% 5% 1% 1% +CC

Donor 0.130 0.222 0.267 0.408 0.490

Study 0.006 0.010 0.012 0.019 0.023






ReFH ReFH

N/A N/A




Owing to the study catchment being small in size and in line with the recommendations in the current flood estimation guidelines

1 flows were estimated for

a larger donor catchment and then scaled down based on area. The donor catchment used for this assessment includes the study catchment.

As outlined in Section 1, one method, ReFH, was used to estimate flows for the donor catchment. This is considered appropriate for this high level assessment.



It has been assumed that the catchment descriptors for the donor catchment represent the characteristics of the study catchment, and are therefore suitable for the scaling of flows used for this assessment.


The donor catchment is classed as slightly more urban (URBEXT1990 is 0.0395) than the study catchment (with no urban coverage which would result in an URBEXT1990 of 0). The study catchment is considered essential rural (0<URBEXT1990<0.025), but the donor catchment is categorised as slightly urbanised (0.025<URBEXT1990<0.05). However this is considered to have a minimal impact on flow rates for the study catchment, based on a quick sensitivity test of URBEXT1990 values.




The approach makes use of catchment descriptors (as taken from the FEH CD-ROM) only and therefore the assessment does not make use of observed and measured data.


These results are only considered suitable for the catchments assessed as part of this study. However if flows are required for other areas within the donor catchment, then the flows estimated for the donor catchment could be used, and scaled down based on area.


Nothing suggested at this stage, however if mitigation at this location is considered feasible during the next stage of the project then it is recommended that a more detailed hydrological study is completed.

3.4 Checks


There is insufficient information in relation to flooding at this location to make this assessment.

197_08.doc#ASSUMPTIONS



The growth factor is 3.1 and hence considered realistic.


N/A


N/A



No other studies were available for comparison.


Limited anecdotal information is available for the area; however this does not include measured data and hence cannot be compared to the results from this assessment.


No other checks.

3.5 Final results – Study catchment

Flood peak (m3/s) for the following AEPs

50% 10% 5% 1% 1% +CC

0.006 0.010 0.012 0.019 0.023


Hydrographs have been created in ISIS Free version 3.6.0.156 Mode 4.

4.2 Donor catchment hydrograph

4.3 Study catchment hydrograph

Doc no. 197_08_SD02 Version 2 Last printed 05/08/2014 Page 1 of 15


Introduction


Contents

Page


2 STATISTICAL METHOD ------------------------------------------------------------------------------------------- 7

3 REVITALISED FLOOD HYDROGRAPH (REFH) METHOD -------------------------------------------- 11

4 DISCUSSION AND SUMMARY OF RESULTS ------------------------------------------------------------ 12


Approval















Abbreviations

AEP Annual Exeedence Probability AM Annual Maximum AREA Catchment area (km

2)

BFI Base Flow Index BFIHOST Base Flow Index derived using the HOST soil classification CFMP Catchment Flood Management Plan CPRE Council for the Protection of Rural England FARL FEH index of flood attenuation due to reservoirs and lakes FEH Flood Estimation Handbook FRM Flood Risk Management FSR Flood Studies Report HOST Hydrology of Soil Types NRFA National River Flow Archive OS Ordnance Survey POT Peaks Over a Threshold QMED Median Annual Flood (with return period 2 years) ReFH Revitalised Flood Hydrograph method SAAR Standard Average Annual Rainfall (mm) SPR Standard percentage runoff SPRHOST Standard percentage runoff derived using the HOST soil classification Tp(0) Time to peak of the instantaneous unit hydrograph URBAN Flood Studies Report index of fractional urban extent URBEXT1990 FEH index of fractional urban extent URBEXT2000 Revised index of urban extent, measured differently from URBEXT1990 WINFAP-FEH Windows Frequency Analysis Package – used for FEH statistical method


1 Method statement


Item Comments


Purpose of study




This assessment has been completed as part of the Corby Flood Risk Management (FRM) Study. The Corby FRM Study is assessing flood risk at key priority areas within the Corby borough with an aim to review flood risk mitigation. Rockingham is one of these key areas owing to flooding at the Gretton Road and Cottingham Road junction. At this location a tributary of the River Welland is culverted under a small number of properties and Gretton Road. As part of the wider Corby FRM Study a culvert assessment is required for this culverted watercourse and the flows estimated in this proforma will be used in this culvert assessment.

Only peak flows are required.

A full range of return periods will be estimated from the QMED to 1% Annual Exceedence Probability (AEP) with an allowance for climate change.

The results are required within a period of two weeks.


Item Comments


The site of interest is located in Rockingham to the north of Corby, grid reference 486720, 291870. The watercourse of interest is an unnamed tributary of the River Welland which is culverted under a small number of properties and Gretton Road. A location plan is provided in Section 5.1.

The catchment is considered essentially rural, (URBEXT2000 of 0.0025) and the catchment is not considered highly permeable (SPRHOST of 47).

The geology underlying the catchment comprises Upper/Middle/Inferior Lias overlain by Boulder Clay.



Yes – Version 3.1.2, December 2011



Water-course

Station name



FEH)

Grid reference


Type (rated /



record

N/A



Station name


HiFlows-UK


Suitable for

QMED?

Suitable for

pooling?

Data quality check

needed?




N/A



Station name



Rating review

needed?



N/A




Data available

?



Date obtained

Details


N/A



N/A


N/A


N/A


N/A


Strategies)

N/A


groundwater, tides)

N/A



Although the adjusted catchment is very slightly smaller than the ideal minimum size of 0.5km

2 (being 0.46km

2)

FEH is considered appropriate. The catchment is considered essentially rural (URBEXT2000 of 0.0025) and is not complex.






The purpose of this assessment is to assist in the identification of causes of flooding at this location, through the completion of a culvert assessment. It is considered that catchment runoff and resultant flow in the unnamed tributary are potential causes of flooding therefore a combination of peak flows and volumes are likely to be factors.




e.g.






The catchment is not considered highly permeable (SPRHOST is 47).

The catchment is considered rural (URBEXT2000 is 0.0025)

The watercourse in this area is not pumped, there is no major reservoir influence (FARL is 1) and there is no extensive floodplain storage.


The flow estimates are required for a culvert assessment, which includes a review of the capacity in relation to flood events. Therefore it has been deemed appropriate to undertake two methods of flow estimation and to compare the results. The two methods that have been completed are the FEH Statistical method and ReFH. These methods have been applied in the standard way and in line with current flood estimation guidelines1.


WINFAP-FEH v3.0.33

ReFH spreadsheet

ISIS Free version 3.6.0.156 Mode 4

1.9 Site details


(km2)


Unnamed tributary of the River Welland

Culverted watercourse under Gretton Road in Rockingham

486650 292000 0.50 0.46

1 Environment Agency (2012). Flood estimation guidelines, operational instruction 197_08.


3 WINFAP-FEH v3 © Wallingford HydroSolutions Limited and NERC (CEH) 2009.





DPSBAR (m/km)

SAAR (mm)

SPRHOST URBEXT

2000 FPEXT

1 0.3 0.34 0.65 93.4 611 47.41 0.0025 0.02



The catchment boundary as taken from the FEH CD-ROM was checked against Ordnance Survey and LiDAR data. In general the catchment boundary is considered suitable, although a few minor alterations were required. These minor alterations were made in three locations. One was at the downstream end of the catchment, where the downstream point was moved upstream to the location of interest (i.e. where the watercourse enters the culvert) rather than at the downstream end of the culvert as represented in the FEH CD-ROM. The second minor alteration was made in the upstream reach to take account of the water bodies present (the ‘Top Moat’ to the south east of Rockingham Castle) and the likely impact of the sports ground and road network at the junction between Uppingham Road (A6003) and the A6116. The third alteration was in the eastern area of the catchment where a slight modification was made based on LiDAR at this location. The FEH catchment and the updated (modified) catchment are shown in Section 5.2. It was not considered necessary to make any further alterations to the catchment boundary.


Owing to the minor alteration to the catchment boundary, the DPLBAR was updated based on equation 7.1 in FEH volume 5.

The catchment boundary was compared to geology mapping and it was not considered necessary to make any further alterations to the catchment descriptors.

Source of URBEXT URBEXT1990 for the ReFH method and URBEXT2000 for the FEH Statistical method.


CPRE formula from FEH Volume 4 for URBEXT1990 and CPRE formula from 2006 CEH report for URBEXT2000.


2 Statistical method

2.1 Search for donor sites for QMED

Comment on potential donor sites

Mention:

Number of potential donor sites available

Distances from subject site

Similarity in terms of AREA, URBEXT, FARL and other catchment descriptors

Quality of flood peak data

Include a map if necessary. Note that donor catchments should usually be rural.

Potential donors were sought through reviewing those in close proximity to the study catchment using the FEH CD-ROM and the hydrometric register. Other potential donors were sought through creating pooling groups from the HiFlows data set for flow gauges defined as ‘OK for pooling’ and ‘OK for QMED’. These searches provided 12 potential donors. One donor was immediately discounted as it was categorised as ‘Not OK for pooling or QMED’ in relation to the HiFlows data set. Of the 11 remaining donors four were located within 20km, another was located 35km away and the other six located greater than 180km from the study catchment. Based on a comparison of catchment descriptors between the study catchment and the donor catchments six catchments were discounted because the descriptors were too different compared to the study catchment. These differences were in particular relation to permeability, FARL, SAAR and distance from the study catchment. Another potential donor was discounted because the catchment was too different in relation to permeability and urban coverage in comparison to the study catchment. Four potential donors were considered almost equally suitable in relation to similarity to the study catchment. Therefore the impact on the resultant study catchment QMED of using each of these donors was determined which resulted in a range of QMED from 0.13 to 0.18m

3/s.

The arithmetic average of the resultant QMED estimates was calculated to be 0.153m

3/s. Further inspection of the

catchment descriptors concluded in utilising 31023 as the donor, The resultant study catchment QMED is 0.160m

3/s.

2.2 Donor sites chosen and QMED adjustment factors

NRFA no. Reasons for choosing or rejecting

Method (AM or POT)





32003 Rejected – although considered a suitable donor and hence used to determine QMED, it was considered preferable to take another donor forward. See further description in Section 2.1.

AM Not required –69 years of data

10.16 8.99 1.0504

31021 Rejected – considered ‘not OK for pooling or QMED’ as defined within the HiFlows data set.

AM N/A - 33.98 N/A


AM Not required – 47 years of data

37.60 43.46 0.9457


NRFA no. Reasons for choosing or rejecting

Method (AM or POT)







10.85 5.52 1.2530

76011 Rejected – BFIHOST too low and SAAR slightly too high in comparison to the study catchment. The distance from the study catchment is also too large.


1.82 2.02 0.9999

45817 Rejected – BFIHOST too low and SAAR slightly too high in comparison to the study catchment. The distance from the study catchment is also too large.


1.34 0.92 1.0012

54092 Rejected – the SAAR is too high in comparison to the study catchment and the distance from the study catchment was too large.


6.35 7.67 0.9985

54091 Rejected – the SAAR is too high in comparison to the study catchment and the distance from the study catchment was too large.


5.92 8.53 0.9972

31026 Rejected – the BFIHOST and URBEXT2000 are slightly too high in comparison to the study catchment.


1.08 0.31 1.5260

31023 Chosen – the catchment descriptors were similar to the study catchment. See further description in Section 2.1.


1.91 1.13 1.1266

205999 Rejected – FARL was too low in comparison to the study catchment and the distance from the study catchment was too large.


0.12 3.11 0.9602

27038 Rejected – the catchment is too permeable and the URBEXT2000 is too high in comparison to the study catchment. The distance to the study catchment is also too large.


1.33 0.55 1.0082

Which version of the urban adjustment was used for QMED at donor sites, and why?

Note: The guidelines recommend great caution in urban adjustment of QMED on catchments that are also highly permeable (BFIHOST>0.8).

Urban adjustment was not applied - the study catchment and donor catchments are considered essentially rural.


2.3 Overview of estimation of QMED

Method

Initial estimate of QMED (m

3/s)

Data transfer

Final estimate of QMED (m

3/s)

NRFA numbers for donor sites used (see 3.3)

Distance between centroids dij (km)

Power term, a

Moderated QMED adjustment factor, (A/B)

a

If more than one donor

Weig

ht

Weig

hte

d

ave

rag

e

ad

justm

en

t fa

cto

r

DT 0.14 31023 35 0.229 1.127 - - 0.16

Which version of the urban adjustment was used for QMED, and why?

Urban adjustment was not applied – the study catchment is considered essentially rural.

Notes

Methods: AM – Annual maxima; POT – Peaks over threshold; DT – Data transfer; CD – Catchment descriptors alone. When QMED is estimated from POT data, it should also be adjusted for climatic variation. Details should be added. When QMED is estimated from catchment descriptors, the revised 2008 equation from Science Report SC050050

Error!

ookmark not defined. should be used. If the original FEH equation has been used, say so and give the reason why.

The guidelines recommend great caution in urban adjustment of QMED on catchments that are also highly permeable (BFIHOST>0.8). The adjustment method used in WINFAP-FEH v3.0.003 is likely to overestimate adjustment factors for such catchments. In this case the only reliable flood estimates are likely to be derived from local flow data. The data transfer procedure is the revised one from Science Report SC050050. The QMED adjustment factor A/B for each donor site is given in Table 3.3. This is moderated using the power term, a, which is a function of the distance between the centroids of the subject catchment and the donor catchment. The final estimate of QMED is (A/B)

a times

the initial estimate from catchment descriptors. If more than one donor has been used, use multiple rows for the site and give the weights used in the averaging. Record the weighted average adjustment factor in the penultimate column.

2.4 Derivation of pooling group

The composition of the pooling group is given in the Annex.

Subject site treated as gauged? (enhanced single site analysis)

Changes made to default pooling group, with reasons Note also any sites that were investigated but retained in the group.

Weighted average L-moments, L-CV and L-skew, (before urban adjustment)

No 32029 (Flore @ experimental catchment) was removed because it did not have enough years of data. No other changes were made to the pooling group. Final pooling group is shown in Section 5.3.

L-CV = 0.226 L-Skew = 0.229

Notes The pooling group was derived using the revised procedures from Science Report SC050050 (2008). The weighted average L-moments, before urban adjustment, can be found at the bottom of the Pooling-group details window in WINFAP-FEH.

2.5 Derivation of flood growth curves



for choice




adjustments


period

P Generalised logistic – as recommended in FEH.

No urban or permeable adjustment required.

- 2.841




for choice




adjustments


period

Notes

Methods: SS – Single site; P – Pooled; ESS – Enhanced single site; J – Joint analysis

Urban adjustments to growth curves should use the version 3 option in WINFAP-FEH: Kjeldsen (2010).

Growth curves were derived using the revised procedures from Science Report SC050050 (2008).

2.6 Flood estimates from the statistical method

Flood peak (m3/s) for the following return periods (in years)

50% 10% 5% 1% 1% +CC

0.160 0.263 0.312 0.455 0.546






Tp (hours)

Time to peak


BL (hours)

Baseflow lag

BR

Baseflow recharge

CD 1.26 285.9 26.5 0.758


N/A


Urban or rural






50% 10% 5% 1% 1% +CC

0.275 0.468 0.561 0.845 1.014






50% AEP 1% AEP

ReFH ReFH

1.72 1.86




The catchment is considered suitable for the application of both the FEH Statistical method and ReFH. It is recommended that the results taken forward to the next stage of assessment, i.e. the culvert assessment, are those estimated using the FEH Statistical method. This is because the FEH Statistical method makes use of observed and measured data rather than solely relying on catchment descriptors as taken from the FEH CD-ROM. However as it is known that there are significant uncertainties with regards to estimating flows for small catchments and owing to the difference between the two sets of results, a sensitivity assessment will be undertaken as part of the culvert assessment, which is outlined in Section 4.4.



The catchment descriptors as provided in the FEH CD-ROM are considered reasonable in terms of representation. Minor alterations have been made although the majority of the catchment descriptors have been left unaltered.

The calculated QMED has been altered based on a donor site; it is assumed that the donor site is suitable.


Whilst the FEH CD-ROM defined catchment at the study location is 0.5km

2, manual alteration required in line with LiDAR and OS

mapping slightly reduced the catchment size. Even though the approach taken for this hydrological study is considered suitable, the study catchment size is very slightly smaller than the general size for which the FEH methods were derived.




The donor catchment which has been utilised to determine QMED for the study catchment does not significantly alter the QMED. Whilst the approach and use of the donor catchment is considered suitability justified, other potential donors may have resulted in an increased or decreased QMED estimation.


The flow estimates would be considered acceptable for information in future studies. However new estimates would be necessary if flows were required at locations downstream from the point of interest. There is potential that flows estimated for this catchment could be scaled down, based on area, for smaller areas within this study catchment.


No additional work considered necessary at this stage.

4.4 Checks

What do the results imply regarding the return periods of floods during

No flood events to compare to.


the period of record?



Using ReFH the 100 year growth factor is 3.07 and using the FEH Statistical method the 100 year growth factor is 2.84. Therefore the growth factors are considered realistic.


N/A


N/A



No other studies are available for comparison.


Limited anecdotal information is available for the area, however this does not include measured data and hence cannot be compared to the results from this assessment.


It is proposed at this stage that the culvert assessment will be run with the ReFH flows as a sensitivity assessment. If the use of the ReFH flows has a major impact on the overall conclusion of the culvert assessment then the flows will be further considered.

4.5 Final results (FEH Statistical)


50% 10% 5% 1% 1% +CC

0.160 0.263 0.312 0.455 0.546


Hydrographs are not required at this stage. However if at a later stage of the Corby FRM Study hydrographs are required for this area, then these will be produced through scaling the ReFH hydrographs based on the FEH Statistical peak flow estimates.

4.6 Results for sensitivity testing (ReFH)

The results in the table below will be utilised for sensitivity testing. Further discussion in relation to this sensitivity testing is included within the main report for the Corby FRM Study


50% 10% 5% 1% 1% +CC

0.275 0.468 0.561 0.845 1.014



5.1 Location plan


5.2 Updated catchment boundary



5.3 Final Pooling Group

Station Distance Years of data

QMED AM L-CV L-SKEW

Discordancy

76011 (Coal Burn @ Coalburn) 2.082 32 1.822 0.178 0.347 0.823

45817 (Rhb Trib to Haddeo @ Upton (trib)) 2.175 16 1.339 0.292 0.304 0.375

54092 (Severn @ Hore Flume) 3.759 34 6.353 0.116 -0.052 2.352

54091 (Severn @ Hafren Flume) 3.818 34 5.915 0.184 0.27 2.473

45816 (Haddeo @ Upton) 3.877 16 3.539 0.331 0.427 0.929

44009 (Wey @ Broadwey) 3.934 32 1.688 0.34 0.241 1.302

27051 (Crimple @ Burn Bridge) 3.954 37 4.514 0.22 0.133 0.492

28033 (Dove @ Hollinsclough) 4.149 30 4.654 0.257 0.403 0.479

91802 (Allt Leachdach @ Intake) 4.516 34 6.35 0.153 0.257 1.075

44006 (Sydling Water @ Sydling st Nicholas) 4.573 35 0.853 0.227 0.087 0.562

27073 (Brompton Beck @ Snainton Ings) 4.593 29 0.743 0.205 0.011 0.972

25019 (Leven @ Easby) 4.796 31 6.088 0.355 0.396 1.24

54022 (Severn @ Plynlimon Flume) 4.808 38 14.988 0.156 0.171 0.634

25011 (Langdon Beck @ Langdon) 4.829 23 15.362 0.247 0.399 1.251

26802 (Gypsey Race @ Kirby Grindalythe) 4.849 10 0.127 0.233 0.25 0.075

25003 (Trout Beck @ Moor House) 4.892 36 15.105 0.173 0.328 0.759

206006 (Annalong @ Recorder 1895) 5.03 48 15.33 0.189 0.052 1.207

Total

515 Weighted means

0.226 0.229

197 08 sd02 flood estimation calculation record for single ... c hydrology... · flood estimation...

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