SS-NFR-P-002: Technical flood risk guidance for stakeholders

Technical Flood Risk Guidance for Stakeholders (Reference: SS-NFR-P-002)

Version 9.1 (June 2015) page 1 of 34

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CONTENTS

1.0 INTRODUCTION

2.0 GUIDANCE ON USING THE SEPA FLOOD MAPS

3.0 STRATEGIC FLOOD RISK ASSESSMENT

4.0 REPORTING REQUIREMENTS FOR FLOOD RISK ASSESSMENTS

4.1 Introduction

4.2 Flood Risk Assessments - Guidance for Applicants

4.3 Reporting Requirements

4.3.1 Background Site Data

4.3.2 Existing / Historic Data

4.3.3 Methodologies

4.3.4 Required elements of a FRA

4.3.5 Conclusions

4.4 Guidance for undertaking Hydrological Modelling - FLUVIAL

4.4.1 FEH Statistical Method – FLUVIAL

4.4.2 FEH Deterministic Method - FLUVIAL

4.5 Guidance for undertaking Hydraulic Modelling - FLUVIAL

4.5.1 1D Hydraulic modelling

4.5.2 2D Hydraulic modelling

4.6 Guidance for undertaking Hydrological and Hydraulic modelling - COASTAL

4.7 Guidance for undertaking Hydrological & Hydraulic modelling – PLUVIAL

4.8 Groundwater Investigations

4.9 Land-raising and Compensatory Storage

5.0 FRA CHECKLIST

Appendix 1: Potential Sources of Flood Risk Information

Appendix 2: Flood Probability



Appendix 3: Types of Flooding



1.0 INTRODUCTION

A Flood Risk Assessment (FRA) is supplied in support of an application for development to Planning

Authorities. Where the Planning Authority considers that there might be a risk of flooding to the development

site it has a statutory duty to consult SEPA for advice and guidance on flood risk. The protocol between both

SEPA and Planning Authorities which is available on SEPA’s website at the following link and sets out

respective roles and responsibilities.

SEPA will review FRA reports and provide advice and comment to the Planning Authority on the

appropriateness of the study, its conclusions and the acceptability of the proposed development in line with

Scottish Planning Policy, (February 2014) found here and the Flood Risk Management (Scotland) Act 2009,

which can be found here.

The Flood Risk Management (Scotland) Act came into force on 26 November 2009 and prescribes a new

responsibility on the Scottish Government, SEPA, Scottish Water and local authorities to exercise their flood

risk related functions with a view to reducing overall flood risk. Whereas Current policy focuses on river and

coastal flooding, the Act covers all sources of flooding, including surface water (or pluvial), overloaded sewers

and groundwater. The premise of a more integrated and sustainable approach to flood risk management

underpins the Act. The cornerstone of sustainable flood management is the avoidance of flood risk in the first

instance. Planning has a crucial role to play in ensuring that, wherever possible, unnecessary risks are

avoided.

The technical requirements of a FRA for any site can range from the provision of detailed topographic

information to demonstrate the relative level of the development site in relation to the river bed, river banks and

floodplain, to technically detailed hydrological and 1 or 2-dimensional hydraulic modelling to investigate the

risk to the development or its impact elsewhere.

Clearly, the time taken to review FRAs is linked to the complexity of the site, including the technical complexity

of the study, however, it is also dependent on the quality of the information presented. Given that consultants

are employed by developers to undertake FRAs there is clearly scope for different corporate styles and formats

although there are core data and information that should be clearly presented to facilitate the rapid

review/audit of reports with the subsequent provision of advice to the Planning Authority. To further facilitate a

timely review of FRAs SEPA has developed a Consultant Flood Risk Assessment checklist to be completed

as an aide memoir to ensure key aspects have been considered and included in the report.

This technical guidance document is intended to outline methodologies that may be appropriate for

hydrological and hydraulic modelling and set out what information SEPA requires to be submitted as part of a

FRA. The clear presentation of key data will enable an improved review and response time to Planning

http://www.sepa.org.uk/media/136143/sepa-planning-authority-protocol-41.pdf

http://www.gov.scot/Publications/2014/06/5823

http://www.scotland.gov.uk/Topics/Environment/Water/Flooding/FRMAct

http://www.sepa.org.uk/media/159170/flood-risk-assessment-checklist.xls



Authorities thus positively affecting the planning process. SEPA therefore recommends that this technical

guidance document is reviewed and the guidance followed to ensure that appropriate techniques are applied

and that suitable information is clearly supplied to allow for an improved response time to Planning

consultations.

2.0 GUIDANCE ON USING THE SEPA FLOOD MAPS

It is imperative to realise that the SEPA Flood Maps (published in January 2014) are indicative in nature (not

absolute) and as such the perceived flood risk (i.e. the flood outlines shown on the map) could be over-

predicting or under-predicting the actual flood risk. It is also important to note that the SEPA flood map does

not account for river and coastal flooding occurring simultaneously1. As such, the flood map does not

necessarily present the ‘worst case scenario’ for locations where both river and coastal flooding can occur. It is

also important to note that the flood extent does not take account of structures such as culverts, bridges and

flood defences. The following standard caveat applies to the new SEPA Flood Maps (2014).

“The Flood Maps are indicative and of a strategic nature. Whilst all reasonable effort has been made to

ensure that the Flood Maps are accurate for their intended purpose, no warranty is given by SEPA in this

regard. Within any modelling technique there is inherent uncertainty. SEPA has assessed the confidence it

has in the maps and has shaded areas where data is not appropriate for use or where no data is available.

It is inappropriate for these Flood Maps to be used to assess flood risk to an individual property.

Whilst all reasonable effort has been made to ensure that the Flood Maps are up to date, complete and

accurate, no guarantee is given in this regard and ultimate responsibility lies with you to validate any

information given. SEPA will not be responsible if the information contained in the Flood Maps are

misinterpreted or misused by you.

SEPA cannot guarantee that the Flood Maps will be available from this website at all times and shall not be

liable for any system downtime or other associated issues.”

Further details on the Flood Map, and it’s use, are available via the SEPA website at the following link

3.0 STRATEGIC FLOOD RISK ASSESSMENT

A Strategic Flood Risk Assessment (SFRA) is designed for the purposes of specifically informing the

development planning process, i.e. Local Development Plans. An SFRA involves the collection, analysis and

presentation (by a local authority or its consultants) of all existing and readily available flood risk information for

the area of interest (Appendix 1 provides a list of potential data sources). An SFRA constitutes a strategic

1 NB: it should be noted that generally, the SEPA Flood Map did not model and does not allow for a combination of both fluvial and coastal

flooding, except for a few key locations. Within a more site specific Flood Risk study for any location close to both a river and the sea, it may

be prudent to execute a joint probability analysis.

http://www.sepa.org.uk/environment/water/flooding/flood-maps/



overview of flood risk, without necessarily meeting the reporting requirements of a detailed FRA (as described

below in section 4.0) and would generally be executed as a desk top study. However, in some instances,

greater detail may be required (if appropriate) to inform the relevant Local Development Plan. Part of an SFRA

could be the identification of priority areas and areas important for flood storage.

Detailed SEPA guidance on Strategic Flood Risk Assessments can be found on SEPA’s website at the

following link - SFRA guidance

4.0 REPORTING REQUIREMENTS FOR FLOOD RISK ASSESSMENTS

4.1 Introduction

This section relates to the commissioning and undertaking of FRAs in order to satisfy Planning Authorities

and SEPA that flood risk matters have been addressed in a manner consistent with SPP (2014).

The aim of the document is to ensure that FRAs contain all the relevant information in a clear and concise

manner, using appropriate methodologies which can be reviewed by the relevant Planning Authority, SEPA

and other involved parties. The document also reflects the current statutory environment and developments

within the science of flood risk estimation.

4.2 Flood Risk Assessments – Guidance for Applicants

An FRA is required to support a planning application when it appears that the site, or parts of the site, may be

at ‘medium to high risk’ of flooding (i.e. located on or immediately adjacent to the functional floodplain). That

means that there is a 0.5% Annual Probability of flooding in any given year. This probability is also sometimes

referred to as the 200 year flood.

Government policy states that Planning Authorities, developers and SEPA must take a sustainable and

precautionary approach when dealing with flood risk and seek to apply the avoidance principle.

The purpose of the FRA is to investigate, for a specific site, what the likely risk of flooding is. It should

demonstrate whether or not the development meets the requirements of planning policy, e.g. whether the site

is out with the flood plain, or, if appropriate, whether acceptable mitigation measures can be put in place. The

scale, nature and location of the proposed development will inform the scope of the FRA required.

SEPA’s role is to review the FRA submitted to ensure that it is technically robust (in relation to UK flood

design standards, hydrometric data considerations and reporting requirements as set out here) and contains

all the appropriate information required to assess the application. SEPA then provides advice to the Planning

http://www.sepa.org.uk/media/143351/lups-gu23-strategic-flood-risk-assessment-sepa-technical-guidance-to-support-development-planning.pdf



Authority on whether the application complies with these standards as well as national land-use planning

policy on flooding, e.g. SPP.

Prior to investing in an FRA, applicants should consider the potential outcome of undertaking any such

analysis. It is possible that the development will be inappropriate and be refused planning permission

irrespective of any FRA having been completed.

4.2.1 Advice on engaging a flood risk specialist:

As with all services, contact more than one specialist or company to discuss your requirements and

costs, ask for references and follow them up;

Does the specialist have experience of undertaking this type of assessment?

Has their previous work been accepted by SEPA in support of planning applications?

Do they know of and use the guidance available on the SEPA website?

Do they have a potential advantage by having local knowledge and experience?

Two key areas are outlined; the ‘Reporting Requirements for Flood Risk Assessments’ (see Section 4.2) which

discusses the background data, general methodologies and what information needs to be reported in the FRA

and, secondly, ‘Generic Guidance for Undertaking Hydrological and Hydraulic Modelling’ (see Section 4.3),

provides further information with respect to modelling which may be undertaken as part of the assessment.

4.3 Reporting Requirements

The detail and technical complexity of an FRA will be proportionate to the scale and potential significance of

the study but, in all cases, it should address or comply with the following requirements:

4.3.1 Background Site Data

• A clearly geo-referenced location plan at an appropriate scale that includes geographical features, street

names and identifies all water courses or other bodies of water in the vicinity which may have an influence on

the site. This should include drainage outfalls and overflows.

• A plan of the site showing levels related to Ordnance Datum Newlyn, both pre- and post-development.

• Photographs of the site showing areas of importance such as river channel, floodplain, culverts, other

structures, areas of erosion, trash lines and areas of woody debris accumulation etc.

• A more detailed indication, if appropriate, of flood alleviation measures already in place, of their state of

maintenance and their performance.



• Where culverts are concerned, an assessment of condition is required. In some cases this may require a

CCTV survey.

4.3.2 Existing / Historic Data

• A plan of the site showing any existing information on extent and depth of flood events. Information may be

anecdotal, photographic or from survey results. The events should be identified with date/time, source of the

data and supporting information provided on rainfall and/or return period, or probability of occurrence of the

flood or storm surge event, or combination. Recorded data are particularly valuable and, if available, should be

highlighted along with evidence of any observed trends in flood occurrence.

4.3.3 Methodologies

• In exceptional cases an assessment of risk of flooding to a proposed development site might only require

some basic information. This might include proposed development site and finished floor levels related to

nearby watercourses, appropriate photographs and/or any nearby known validated historical flood levels.

However if this information is insufficient to provide a robust assessment of the risk of flooding to a

development then a detailed flood risk assessment may need to be carried out by a suitably qualified

professional. Such an assessment would normally include the elements described in section 4.3.4.

• Best practice application of the Flood Estimation Handbook should be used to derive design river flood flows

and/or flow hydrographs to be used in FRAs. In certain circumstances alternative methods may be considered

with appropriate justification (i.e. small and/or heavily urbanised catchments).

• For the purposes of deriving coastal flood levels, application of the Coastal Flood Boundary (CFB) method

(Environment Agency/DEFRA Flood and Coastal Erosion Risk Management R & D project „SC060064

Coastal flood boundary conditions for UK mainland and islands‟) is now standard - see section 4.6.2 below.

• In order to determine design water levels, the appropriate application of hydraulic modelling will be required.

This will reflect the complexity of the flooding mechanisms involved and the scale and nature of the

development proposal. These may range from simple Manning’s calculations to the application of dynamic

hydraulic models. Any model must be supported by appropriate input data, and calibrated and validated to

observed/recorded data (gauging station levels / historic (epigraphic) flood levels) where available. Methods

exist for the determination of the suitability of historic levels for such purposes.

• It may be appropriate to undertake joint probability analysis where rivers meet the sea and estimate the worst

case combined 1 in 200-year event. Ideally, a range of various combinations could be presented.



• Appropriate sensitivity analysis should be carried out to determine the sensitivity of design water levels to key

model parameters, e.g. design flow, roughness, downstream boundary. Where culverts/ bridges exist, the risk

of potential blockage also needs to be considered.

4.3.4 Required elements of a FRA

• Identification of the source/type of potential flooding (e.g. riverine, coastal, pluvial, etc or combinations of

more than one type).

• An assessment of the appropriate design flows and levels at the site. This should provide sufficient

information on the derivation of the design flows and hydraulic modelling for auditing purposes.

• The plan extent, depth and any flood flow pathways should be indicated on a scale map of the site for

appropriate return periods. Cross-sections of the site showing finished floor levels, access routes or other

relevant levels, relative to the source of flooding and anticipated water levels for associated probabilities.

• An assessment of the likely rate or speed with which inundation might occur, the order in which various parts

of the location or site might flood and the likely duration of flood events. If appropriate, the identification of

routes of safe access/egress during the design event should also be made including the depth of flood water

which may be encountered on these routes. Confirmation that access/egress will be maintained in the event of

a flood should also be provided.

• Plans and description of any structures (culverts, screens, embankments or walls, overgrown or collapsing

channels etc) which may influence local hydraulics, and a summary of the findings of any hydraulic modelling

including how structures impact on water levels at the site.

• Where culverts provide a significant flow restriction, levels and discharge rates at which flow would overtop

the structure should be identified. Likely impacts of blocked culverts also need to be identified.

• An assessment of the hydraulics of all watercourses, drains or sewers, existing or proposed, on the site

during flood events to assess the risks of secondary flooding.

• Best estimates, based on the most up-to-date findings, should also be made of climate change impacts on

probabilities, flood depths and extents for both fluvial and coastal situations. Current fluvial guidance

(published by DEFRA) recommends that the 0.5% (200-year) peak flow estimate should be increased by

+20%. Reporting to the Scottish Executive in 2003, Price & McKenna provided an assessment of how climate

change could affect runoff regionally across Scotland. For projected increases in sea-level rise, refer to

UKCP09 findings.



• Details of flood mitigation measures / strategies employed. In the case of proposed ground raising, estimates

should be made of the volumes of water which would be displaced from the site for various flood levels

following development of the site, and details provided of how compensatory flood storage would be

implemented (see section 4.8 Land Raising and Compensatory Storage)..

• An assessment of the likely impact of any displaced water (whether or not full compensatory storage has

been provided) on neighbouring or other locations which might be affected subsequent to development. This

requirement also applies to coastal locations.

• A brief assessment of the potential impact of any development on fluvial or coastal ecology, habitat or

morphology and the likely longer term stability and sustainability. The assessment should indicate the

requirement for additional separate detailed investigation into these issues, which may be required to address

the requirements of the WEWS Act (Controlled Activities Regulations).

4.3.5 Conclusions

The report should conclude with a summary of its findings and how, in the applicants view, these comply with

the requirements of (i) flood design standards and reporting requirements; (ii) both national and local planning

policies on flooding (and their associated Planning Advice Notes); (iii) national and/ or local drainage

assessment guidance (if relevant) and (iv) and any relevant aspects of the WEWS Act (Controlled Activities

Regulations).

4.4 Guidance for undertaking Hydrological Modelling - FLUVIAL

The following generic guidance represents good practice in undertaking hydrological and hydraulic modelling.

The sophistication, cost and safety implications of any development proposal should be reflected in the

complexity, scope and precision of the models applied, the range of scenarios studied and the amount and

range of input data collected.

Estimation of the design flow is often the most significant variable in determining the risk of flooding at a site.

Initially the Flood Estimation Handbook (FEH), by the Centre of Ecology & Hydrology (CEH), should be used

to derive design flood flows and/ or design flow hydrographs to be used in an FRA. However, other methods

can also be used in parallel where justifiable and applicable, e.g. Flood Studies Report methods, and/or

reference to past observed floods and the unit discharge estimates associated with them.

The FEH provides a framework for flood estimation and requires user expertise and experience to judge the

most appropriate methods / data to use in any individual circumstance. No single methodology is considered

superior to others for all situations. It is considered inappropriate to provide detailed prescriptive guidance on



methodologies, data or parameter values to be used, however these should always be justified within the FRA

based upon the analysts ‘expert judgement’.

SEPA recommends that a climate change allowance of +20% on the estimated 200-year peak flow be made.

Alternatively, UK Climate Projections 2009 (UKCP09) provides tools to provide alternate future climate change

scenarios via the following link - UK Climate Projections. SEPA considers that such allowances should be

over and above any separate allowance for freeboard. SPP paragraph 254 recognises that a changing

climate will increase the flood risk in some parts of Scotland and planning can play an important role in

reducing vulnerability of existing and future development to flooding. Although SEPA would always

recommend it, SEPA considers that an additional allowance for climate change is for Local Authorities to

determine. However, all public bodies have a duty to take climate change into account under the Climate

Change (Scotland) Act 2009.

4.4.1 FEH Statistical Method - FLUVIAL

• Hydrometric authorities hold flood data for many gauging stations not included in the FEH/ Hi-Flows UK

database. Users of the statistical method should consider if there are more appropriate gauging station

datasets, not included in the FEH/ Hi-Flows UK database, for use as donor sites and for inclusion in pooling

group analyses.

• Users should be aware that the FEH describes six methods to estimate the index flood Qmed. In addition a

recent study by CEH (Improving the FEH Statistical Method) provides a revised method of estimating QMED

based on catchment descriptors; the report conclusions indicate that the revised method gives higher QMED

estimates in the north and west of the UK. While the choice of method(s) used in the FRA should still be

justified SEPA recommends that the revised QMED estimation method is considered and that a precautionary

approach is taken when estimating QMED and the subsequent derivation of the design flood.

• Revisions of FEH pooling groups should always be considered.

• Single-site analysis may be more appropriate than pooling group analysis in some circumstances,

particularly when long high quality gauged data is available.

• Historical data should be incorporated in flood estimates where it is available. Guidance on how to

incorporate such data is given in a study by CEH titled The use of historical data in flood frequency estimation.

• The FEH statistical method is often not suited to small catchments as there is a lack of small gauged

Scottish catchments in the original FEH database and the updated Hi-Flows UK database.

http://ukclimateprojections.defra.gov.uk/

http://www.scotland.gov.uk/Topics/Environment/climatechange/scotlands-action/climatechangeact

http://www.scotland.gov.uk/Topics/Environment/climatechange/scotlands-action/climatechangeact

http://nora.nerc.ac.uk/3545/

http://nora.nerc.ac.uk/8060/1/BaylissRepN008060CR.pdf



4.4.2 FEH/ ReFH Deterministic Methods (rainfall-runoff approach) - FLUVIAL

• The FEH rainfall-runoff approach (which is based on the Unit Hydrograph method) can be more appropriate

than the statistical method for small catchment applications, especially where there is a lack of small gauged

catchments. This approach may still have relevance for application in some instances. The successor to the

existing Unit Hydrograph (UH) approach, i.e. the Revitalised Flood Hydrograph (ReFH) has been recently

updated and calibrated for Scotland and launched as ReFH2 – this method is now accepted by SEPA for

application in Scotland and SEPA is pleased to have contributed to the development of ReFH2 and the

improvements that have been made for Scotland; these are principally:

- Use of more Scottish gauged data (overlapping rain and flow data) for model calibration.

- Inclusion of more small catchment data (recognising the scope to improve this further still).

- Calibration of the method up to and beyond the 200-year design event.

- The option to make estimates independent of pooling-group bias (i.e. alpha not invoked).

- Development of Scotland specific equations for the estimation of initial model conditions.

- Revision of equations for estimating model parameters from catchment descriptors.

- The functionality of the software to adjust default parameters values using local data.

The above renders (for the first time) the ReFH as applicable for use in Scotland. However, SEPA has always

recognised that it is always difficult to recommend one methodology in preference to another in terms of its

accuracy for estimating design flows. SEPA would always recommend using more that a single method for

comparative purposes when making design flood estimates - no single method can be regarded as being a

preferred approach for all catchment sizes, with or without loch storage. In the case of ReFH2, SEPA would

recommend the adjustment of default parameter values (where relevant) for sites where superior local data

exists. SEPA is satisfied to recommend ReFH2 as contributing to the suite of methods available for the

estimate of design floods in the Scotland.

By way of final caveats, SEPA would caution against the use of ReFH2 method for:

(i) Catchments where lochs and reservoirs exist as the gauging stations selected to develop and test

ReFHv2 have FARL‟s greater than 0.9 (NB: the derivation of a design inflow hydrograph at the top end of

such a catchment for routing purpose would of course be fine).

(ii) Estimation of Probable Maximum Flood for Reservoir design – the FSR method as re-stated in the FEH

should still be adhered to.

Reference can be made to the WHS website where access to the software and full technical report on the

ReFH methodology can be obtained here.

http://www.hydrosolutions.co.uk/products.asp?categoryID=4671



• In general, rainfall records are longer than river flow records. These can be used for input into rainfall-runoff

models as well as improving the parameter estimation of the model.

• The estimation of percentage runoff is the most uncertain part of flood estimation. A better estimate of

standard percentage runoff (SPR) is the most significant single improvement that can be made for flood

estimation. The FEH recommends a number of alternative methods for estimating SPR, both theoretically and

from observed data. Users of the rainfall-runoff method should take care in the selection of an appropriate

SPR estimate for the subject catchment.

4.5 Guidance for undertaking Hydraulic Modelling - FLUVIAL

A hydraulic model is an approximate and simplified mathematical representation of the real world hydraulic

processes which govern flooding mechanisms at or around an application site. Hydraulic modelling

applications can range from simple Manning’s calculations to complex hydraulic modelling solutions. For flood

modelling a variety of mathematical modelling methods e.g. 0D, 1D, 2D, 3D and combinations thereof are

available. Out of these, the 3D modelling method is the least used method for flood risk assessments. A

recent flood modelling method, the 0D (zero dimensional) modelling method – also known as the rapid flood

modelling method, is most suited for strategic level pluvial and coastal flood risk assessments and is not

preferred for fluvial and/ or site specific flood risk assessments. Each modelling method offers advantages

and suffers from disadvantages of their own. A final decision about which model, alone or in combinations

thereof, may be subject to many considerations.

4.5.1 One Dimensional Hydraulic Modelling (1D) - FLUVIAL

The 1D modelling method is the most used and preferred method for flood risk assessments. It is particularly

suited for representing river or watercourse systems which have well defined channels near the application

site. The 2D flood modelling method is preferred where the site topography is relatively flat and/ or where

other developments near the site (e.g. complex/ dense urban areas) dictate that 1D modelling is unlikely to be

the most suitable or cost effective approach to determining flood depths and extents. In certain situations it

would be essential, for model representation or cost effectiveness purposes, to link a 1D model with a 2D

model to produce an integrated 1D2D model. 1-D is most applicable to where channel processes dominate

and 2-D where floodplain processes dominate and detailed understanding of floodplain hydraulics is required.

SEPA would be keen to receive detailed models from consultants to support each FRA and, in particular, to

provide examples of best modelling practice in the field of flood risk. Models can be provided to SEPA for

review and audit, especially, in the event of any discrepancy or misinterpretation of information presented in



the FRA. Given the above, however, the following provides guidance on what is expected from any hydraulic

modelling studies that may be undertaken as part of an FRA.

Statement of objective - to explain clearly the situation being modelled and the objectives of the modelling

study, including details of the output required from the model. To demonstrate the modelling approach is

fit for purpose.

Justification of the model - to demonstrate that the model used is suitable for this study. This should

include evidence of previous applications in similar circumstances and a demonstration of experience in

the application of the model.

Data collection - all relevant data collection and measurement techniques should be quoted, including

expected errors and relevant quality assurance. It is expected that appropriate input data is collected to

support the objectives of the study. Surveyed cross-sections of the main channel and floodplain should be

comprehensive, i.e. not include ‘glass walls’ in the model.

Model calibration / boundaries - The model calibration coefficients and the procedures used to optimise

the calibration must be stated clearly. The choice of upstream and downstream model boundaries must

be justified.

Model validation – efforts should be made to validate the model against historical flood events/ high flow

events, or the use of gauge data where available.

Freeboard – this is often defined as the difference between the flood defence level and the design flood

level. It can also however be the difference between the design flood level and the finished floor levels of

any development. A minimum freeboard of 500 mm to 600 mm is recommended by SEPA. Freeboard is

required to account for (a) the uncertainties involved in flood design and (b) physical imponderables such

as post-construction settlement or wave action. CIRIA Report C624 also recommends a freeboard

allowance of 600 mm. Any allowance for climate change should be independent of the freeboard

allowance. Some local authorities have requirements in excess of 600mm. The applicant should make

sure that they are aware of what may be stipulated locally.

Sensitivity analysis - this analysis must be presented to demonstrate the effect on the key output

parameters resulting from variation of input data and controlling assumptions. This is particularly important

where limited data is available to validate the model or where there are some uncertainties involved.

Parameters that should most commonly be tested for sensitivity include design flow, roughness and

upstream/downstream boundary conditions. Culverts and small hydraulic structures that may be prone to

http://www.ciria.org/ItemDetail?iProductCode=C624&Category=BOOK&WebsiteKey=3f18c87a-d62b-4eca-8ef4-9b09309c1c91



blockage during floods deserve particular analysis (i.e. the model should be run with full and/or partial

blockage to better understand the impact of such processes).

Quality assurance - to demonstrate that the model has been subject to an evaluation procedure

establishing its suitability for the relevant tasks and that numerical and method checks have been carried

out by an appropriately qualified person.

Auditable - to ensure that there is a clear account of the modelling exercise for inspection by any

appropriate auditors.

Reporting - clear description of the model including the underlying principles and implicit or explicit

assumptions. Also, a clear summary of the numerical output should be supplied (preferably in tabular

format) in addition to likely errors, bias, sensitivity, implications for the objectives of the study and

conclusions. Diagrams showing the geographical extent of the model, plan view location of cross-sections,

the resultant flood profile in long-section and also at each cross-section, should be provided.

4.5.2 Two Dimensional Hydraulic Modelling (2D) - FLUVIAL

This section provides an overview on what SEPA expects in a submitted flood risk assessment which uses 2D

flood modelling method for flood risk assessment in support of a development management application. The

guidance is therefore applicable to planning applications submitted to SEPA for advice on flood risk in line

with the Scottish Planning Policy and the Flood Risk Management (Scotland) Act.

In terms of a choice of modelling approach we would normally expect 2D modelling to be undertaken where

floodplain processes dominate and detailed hydraulic information is required. We would also recommend this

approach where flow direction and pathways are unknown or complex and high resolution topographic data is

available.

Data requirement for a 2D flood model

The following data are required for a 2D model:

Model domain boundaries and active area

Topographic information

Information about structures

Boundary conditions

Roughness coefficient

Historic flood information



Model domain boundaries and active area

This should confirm the area represented by the model and should consider what to include in the model. The

areas where the governing equations (such as Shallow Water Equation) are invalid should be avoided as the

solution would not be appropriate. Ideally water should not reach the domain boundary and overlap with any

1D areas should be minimised.

Topographic information

The topographic information is most likely to be sourced from off-the-shelf airborne laser or radar survey data,

post processed to inform the levels for the model domain. Such data, pertaining to the limitations of the post

processing algorithms, may contain inaccuracies in the ground levels, for example areas of dense vegetation,

normally inundated areas or where the data resolution is such that hydraulically significant features are not

captured. Therefore, undertaking a ground truthing exercise to confirm the levels would be advantageous

prior to inclusion of the data in a flood model. In certain situations it may be essential or cost effective to carry

out a traditional survey to establish the ground levels. Where data from more than one source is used, further

processing to obtain a common data set may be required. All the data should be related to Ordnance Datum

Newlyn (or if preferred, OD Lerwick if in Shetland). The spatial extents of the data should be large enough to

accommodate a given simulated flooding scenario. It is possible that features that could influence hydraulics

(such as embankments, flood defences and buildings) may not be adequately shown within the model grid. In

such cases SEPA would expect topographic modifications to be undertaken to allow specific elevation data to

be used.

Information about structures

All hydraulically significant natural features and structures, such as weirs, bridges, walls, natural bunds or

landforms, hedges, ditches etc. falling within the model domain should be included to ensure that the model

represents the real world scenario as accurately as possible.

Boundary conditions

The initial conditions e.g. waterlogged or saturated areas, inflow hydrographs or other appropriate approaches

and downstream boundary conditions for the model domain should be specified. Explanation and justification

for the boundary conditions and the initial condition values thereof should be included. In coastal areas close

to a watercourse, it may be essential to perform a joint probability analysis of fluvial and coastal flooding to

establish the design scenario. Further, in coastal areas behind formal or informal flood defences and/or

natural landforms (where the site levels are lower than the 1 in 200 year flood level, single or joint), further

analysis, to establish the inflow hydrograph may be essential. Any such proposal will need to comply with the

flood risk policies of the SPP (paragraph 88) which would not generally support coastal development that

required new defences to protect against either erosion or flooding.



Roughness coefficient

The roughness coefficient was introduced primarily for 1D modelling. There is sufficient confidence in the

values of the roughness coefficient, Manning’s n, which can be specified for different type of surfaces in a 1D

model. However, specifying a single uniform value for the 2D model domain can be said to be overly

simplistic. Most of the 2D modelling software packages have a facility to input different roughness values for

different areas. Theoretically the roughness value for a specific surface type in a 2D model should be lower

than that in a 1D model. The roughness values should also be lower for areas where the grid or mesh

resolutions are lower. However, no systematic research to recommend particular values, as is available for the

1D modelling method, is available for reference. Therefore, the values used in the models should be justified

and supported by a calibration and verification exercise, else a sensitivity analysis if recorded data is lacking.

It is possible that the ISIS Conveyance Estimation System ‘unit roughness’ values may be useful for 2D

floodplain modelling (especially in rural areas) should an appropriate model domain exist that can utilise such

parameters.

Historic flood information

Historic flood information, relevant to the application site, if available is extremely useful in 2D model

calibration and validation. Such information, when available, should be used for model calibration and

validation prior to simulating the design scenario.

Choice of 2D computational software and scheme

A range of software products, capable of performing 2D flood modelling are available. While SEPA is aware of

the reliability of certain software products, supporting information justifying the reliability of some software

products may be requested. SEPA recommend that reference is made to the 2013 benchmarking study on

2D modelling, as published by the Environment Agency.

The 2D modelling method uses structured or unstructured grids in a model domain. If using structured grids,

the computation burden is significantly higher as it also includes computations for grids located outwith the

floodplain. This burden can be reduced by using nested grid models. The computation grids may be coarser

for areas outwith the floodplain or for areas where detailed information is not required, e.g. rural areas,

undeveloped / open areas with predominantly flat topography, etc. The grid size should be sufficiently fine to

represent the flooding mechanism within the model domain.

If 2D modelling is carried out using unstructured grids, particular care should be taken while creating the

computational grid. Ideally the meshes for the river channel and the floodplains should be discretised

separately. Discretising the mesh for the river channel separately ensures that the variations in the channel

bathymetry are captured better. However, this is only applies to situations where more accurate channel

bathymetry is available. The mesh should contain more elements near the meanders and bigger elements for

http://evidence.environment-agency.gov.uk/FCERM/Libraries/FCERM_Project_Documents/SC120002_Benchmarking_2D_hydraulic_models_Report.sflb.ashx



straight reaches. Similarly, in the floodplain, the mesh should contain more elements in complex urban areas

and settlements and fewer elements in rural or open areas. The mesh may also need more elements to

capture hydraulically significant features. To minimise the mass balance errors, it is a good practice to create

nearly equilateral elements. The mesh elements should have a smooth transition in sizes. More elements

should be created along the river channel so that maximum information is passed onto the elements located

in the floodplain. The selected mesh sizes should do justice to both: model representation and accuracy.

Size of main features to be represented, level of detail required and run time are all relevant to such

considerations.

Model time step

Too big a time step, although resulting in a lesser computational time, may lead to model instability and

increased mass balance errors. Thus, choosing an appropriate time step is vital for ensuring increased

accuracy while reducing the computational requirements. Some software packages have a facility for

specifying an adaptive time step which may be used to avoid modeller specific uncertainties. In general,

model time steps for 2D domains should be approximately half that of the grid cell size.

Model Issues - calibration, validation and sensitivity testing

Whenever possible, a model calibration and validation exercise should be carried out. A sensitivity analysis

should also be carried out to evaluate the effects of uncertainties of the model parameters. The modelling

report should contain justification of the modelling method, modelling software used, limitations of the

modelling method and of the software used as well as any assumption made for model simplification. It

should also include information on uncertainties as well as any other issues which may affect the accuracy of

the output e.g. flood extents touching model boundary. In certain situations a 2D model may be linked to a

1D model. In such situations, it is very important to establish the model domains of the individual models.

Too wide a 1D model domain will retain most of the floodwaters in the 1D model and a too narrow 1D model

domain will pass most of the floodwaters to the 2D model. Further, the distances between the cross-sections

in the 1D model also affect how a 1D model would interact with a 2D model.

Several methods of connecting a 1D model to a 2D model exist, the most common being linking of the 2D

model at the riverbanks through spills to the 1D model. The specific linking approach used should be

confirmed. In such situations, particular attention needs to be paid to establishing appropriate spill geometry.

The spill geometry can be taken from 1D model cross-section data if the cross sections are not too far apart

and there are no significant variations in the topography. Alternatively, (and this is generally the practice), the

spill geometry can be extracted from the topography data. However, particular care needs to be taken to see

that the spill geometry is as accurate as possible. As airborne survey data such as LiDAR (Light Detection

And Ranging) tends to be inaccurate for areas with dense vegetation and also for areas with steep slopes –

which is the case near riverbanks – it is always a best practice to carry out a reconnaissance survey along the

finalised spill alignment to identify any misrepresentations. The flood risk assessment should contain



information on how the models have been linked and how best the linking represents the flooding mechanism

in the model domain.

Presentation of model outputs

The 1 in 200 year flood outline, or where a joint probability analysis is used the flood outline corresponding to

the appropriate scenario, needs to be clearly marked on the site plan. The FRA should include justification on

how the development planning application complies with SPP.

We would also request that the mass balance error result is presented in the accompanying model report. In

addition, where a model is provided directly to us for review, we would request a copy of the log files for any

simulations, including an image that shows simulation convergence over time. The initial conditions used

should also be confirmed. We would also request that the cell wetting/drying depth is specified.

4.6 Guidance for undertaking Hydrological and Hydraulic modelling - COASTAL

The derivation of coastal design levels should follow UK standard approaches in the first instance, yet be

supplemented where possible by the use of good quality, local information. Observed flood levels from past

flood events should always be used as part of a design process, especially if they exceed design levels

produced by ‘standard’ approaches. The importance of using good quality detailed survey information related

to Ordnance Datum Newlyn (or local gauge datum in the case of Shetland and Outer Hebrides) cannot be

over emphasised. Wider coastal processes should always be considered when undertaking such analyses, in

particular thinking about how coastal flooding may be exacerbated in some locations due to any of the

physical factors that can occur individually or in combination with one another.

4.6.1 Physical causes of coastal flooding

The key physical components of coastal flooding are listed below and described in more detail in Appendix 3.

Predicted astronomical tide

Storm surge residual

Wave/ fetch effects

Local bathymetric effects

It should also be noted that coastal erosion or loss of land, can also lead to and/ or exacerbate coastal

flooding. This can often result due to velocity and force of wave action, which can include moving debris.

Wave overtopping (including wave spray) may present a risk to low lying coastal areas which under normal

conditions appear to be protected by natural berms, walls or any form of raised ground..

4.6.2 UK design standard for coastal flood design

For the purposes of deriving design coastal flood levels in the UK, application of the Coastal Flood Boundary

(CFB) method (Environment Agency/ DEFRA Flood and Coastal Erosion Risk Management R & D project „SC060064

Coastal flood boundary conditions for UK mainland and islands‟) is now the accepted standard. The project



supersedes the previous method for deriving extreme sea levels, i.e. POL Report 112, Spatial analyses for

the UK coast, published in 1997.

The Coastal Flood Boundary method provides an up-to-date, scientifically robust national evidence base and

practical guidance on appropriate design sea level and swell wave conditions around the country and how to

use them. The method was designed for and is most suitable to the open coast, although extension of the

method does now provide values (with uncertainty) for more topographically complex areas. The use of good

quality local data (where available) to supplement this generic method, (especially for such complex

coastlines, e.g. sea lochs and estuaries remote from Class A tide gauges) should be used where possible to

improve estimates. Please note that to estimate design levels within an inner estuary may require the use of a

hydrodynamic model and consideration of design fluvial and tidal events (e.g. joint probability analysis if

appropriate).

Key outputs from the CFB project include:

• A consistent set of design sea levels, uncertainty data and design surge curves around England,

Wales and Scotland.

• A consistent set of design swell wave conditions around England, Wales and Scotland.

• Practical Guidance on applying these datasets.

For full details on the new CFB methodology and its application, please see the Environment Agency web site

at the following link.

Estimates of extreme sea levels and associated uncertainty data are available at a 2km resolution around the

coastline of Scotland with the exception of Shetland and beyond the project defined estuarine limits. Donor

surge shapes and accompanying textual information are available for the entire Scottish coastline including

Shetland. There are a number of issues with the application of the coastal flood boundary methodology to the

Shetland coastline. It was deemed that coastal flood boundary data could not be derived for the Shetland

coastline with an acceptable level of confidence, except at Lerwick. The coastal flood boundary output for

Lerwick was shown to have good correlation with the existing SEPA data held for Lerwick; as such there is

confidence that existing SEPA data at 14 locations around Shetland remains fit for purpose. Values are

available for the 1%, 0.5% and 0.1% AP (1 in 100, 1 in 200 and 1 in 1000 year) water levels at these

additional locations. For those areas out with the scope of the Coastal Flood Boundary method, reference

should be made good quality local data (where available) to supplement this generic method, especially for

complex coastlines, as stated above.

4.6.3 Additional allowances – Climate Change and Freeboard

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/291216/scho0111btki-e-e.pdf



An additional allowance for climate change should always be made over and above the extreme ‘still water’

design level obtained from standard methods. Best practice guidance should follow advice provided in

UKCP09 on making allowances for sea-level change.

It is also important to make a freeboard allowance as part of any flood design exercise. Freeboard helps to

account for uncertainty associated with coastal processes not explicitly taken into account by standard

estimation methods such as the CFB, including.:

(i) inherent uncertainty associated with design flood estimation

(ii) uncertainty of wave and spray action

(iii) uncertainty with local bathymetric processes, (e.g. reflection and shoaling)

(iv) reduction of design level due to local changes in land, (e.g. erosion and settlement)

SEPA would recommend a minimum allowance of 600 mm be made for coastal freeboard. This may be

required to be more depending on local circumstances and/ or the provision of specific guidance on this

matter by local authority flood protection staff.

4.6.4 Historic coastal flood data

Observed/ recorded data on past coastal flooding events provides a valuable source of information for design

purposes. Such data can be used, (i) directly, to guide the choice of an appropriate design flood level at a

particular site and (ii) indirectly, in the calibration/ validation of modelled estimates. The importance of such

data cannot be overestimated and SEPA strongly recommends that any study should seek out any historical

flood information. This could include making reference to the SNIFFER FRM 10 project ‘Coastal Flooding in

Scotland: A Scoping Study’ (2008) available here.

4.7 Guidance for undertaking Hydrological and Hydraulic modelling – PLUVIAL

Given the requirement to consider all potential sources of flooding under both the Flood Risk Management

(Scotland) Act 2009 and SPP, the following generic guidance represents good practice in undertaking

hydrological and hydraulic modelling in relation to pluvial flood risk for potential development sites.

Pluvial modelling applications can range from basic topographic GIS analysis (i.e. rolling ball techniques),

direct rainfall models (so called 0-D, rapid flood spreading techniques or 2D hydraulic models) and fully

integrated approaches whereby a 1D model of the sewer network is fully coupled with a direct rainfall model of

the above ground topography. The scale, cost and safety implications of any development proposal should be

reflected in the complexity, scope and precision of the models applied, the range of scenarios studied and the

amount and range of input data collected.

4.7.1 Digital Terrain Model - PLUVIAL

http://ukclimateprojections.metoffice.gov.uk/23771

http://www.sniffer.org.uk/files/7013/4183/7993/FRM10_final_030908_with_security.pdf



The topography of a site is a key controlling factor determining the overland flow pathways along which rainfall

runoff will flow and or accumulate. The most widely available DTM is NEXTMap which has a relatively poor

resolution of 5 x 5 m and a limited vertical accuracy of +/- 0.7 to 1.0 m; its use introduces one of the largest

sources of uncertainty in pluvial flood models based on this underlying data. Therefore, it is suggested that

LiDAR data should be used whenever it is available and the decision on topographic data to be used will be

proportionate to the scale and nature of the development proposal. Also due to the prominent influence that

buildings and roads have on pluvial flow pathways, it may also be appropriate to represent these features

within the DTM. However, the appropriateness of this is again extremely site specific and may not be

appropriate for a one or two house development but may be necessary for large scale developments. In cases

where this is deemed to be appropriate, buildings may be represented by either 'stamping' them onto the DTM

or by using appropriate Manning's roughness values to represent buildings as part of the model grid. Road

heights should be lowered by 100 mm when NextMap DTM is used, but may not require any additional

processing when LiDAR is used. It is acknowledged that a number of other building representation methods

are also possible.

In the event that any model uses a combination of DTM datasets it is anticipated that some interpolation and

smoothing between the two DTM tiles has been carried out within a specified buffer zone. A step join between

datasets can be used where appropriate but this will be on a site specific basis and influenced by the type of

modelling approach proposed, i.e. a Rapid Flood Spreading Method would not be able to route water up a

smooth join but a 2D model may be able to route water up a slight incline formed by a smooth join.

It is also expected that Quality Assurance work should be carried out to check the accuracy of the DTM. The

data should also be checked for any potential ‘false blockages’ which could create artificial barriers to

flowpaths which exist such as in the instances of culverts or an underpass for example.

Numerical pluvial flood models need to be driven by rainfall volumes which are representative of events

leading to pluvial flooding and are one of the main sources of model uncertainty. It is suggested that

representative rainfall applied over a model domain is constructed based on the Depth-Duration-Frequency

(DDF) model of Version 3 of the Flood Estimation Handbook (FEH). The FEH CD-ROM provides estimates of

DDF parameters at a catchment level and point values on a 1km grid covering the United Kingdom (UK).

These values can be used to construct a design rainfall for any duration and return period for any location

throughout the UK.

SEPA would also recommend that climate change is considered in line with latest technical guidance such as

the DEFRA 2006 guidance on rainfall uplift over specific time horizons as reproduced in the Table 1 below.



SEPA’s advice for several years now is that a +20% allowance be applied to both peak flows and peak rainfall

intensity. The following provides guidance on what is expected from any numerical pluvial modelling studies

that may be undertaken as part of a flood risk assessment.

4.7.2 Hydrological Parameters – PLUVIAL

Rainfall Profile ~ Of the two typical recommended storm profiles in the UK, 75% winter and 50% summer,

it is suggested that the 50% summer profile should applied across both urban and rural areas in pluvial

flood modelling studies. This profile has a more pronounced peak to represent intense convective

summer storms which have a greater probability of overwhelming drainage networks and causing pluvial

flooding.

Storm Duration ~ As a result of the highly impermeable nature of urban catchments and the presence of

an engineered drainage system, it is important that storm durations in urban areas are treated differently

from those in rural catchments. Previous studies (undertaken by Halcrow and JBA Consulting) have

identified that in rural areas with higher infiltration rates, relatively longer storm durations of 3 hours or

more should be applied based on the critical storm duration approach used in FEH. In urban areas with a

well maintained drainage network these studies suggest that shorter storm durations of approximately 1

hour result in a greater extent of flooding and should be applied to built-up regions. Alternatively a range

of modelled scenarios approach can be used to help indicate model sensitivity.

Design Rainfall (Frequency) ~ In terms of rainfall return periods SEPA would be looking for the FEH

guidance (Volume 4, Section 3.2.2) to be followed. The primary rainfall event adopted should be either

240 or 200 year based on the urban extent of the modelled catchment.

Drainage Allowance ~ In urban catchments it may be appropriate to incorporate a realistic drainage value

to remove a proportion of the rainfall input based on any local/detailed information from drainage studies

for that catchment (in consultation with Scottish Water). Research conducted by the Environment Agency

during the creation of a Surface Water Flood Map advocates the application of drainage loss rates of

12mm/hr following random sampling analysis of sewer capacities across England and Wales. However,



this has now been updated and a 5-year rainfall event has now been adopted to reflect loss rates. The 5-

year rainfall event is also the value used to represent drainage losses during the creation of SEPA's

national pluvial flooding project and the River Agency's Second Generation Surface Water Flood Map.

Therefore, if any drainage allowance has been applied to the model it is suggested that a figure similar to

the 5-year rainfall event is used unless any site specific data from Scottish Water for example suggests

otherwise. Nil sewer allowance should be applied throughout rural environments.

Infiltration Rates ~ These should again vary between urban and rural areas to account for the effect of

extensive impermeable surfaces in built-up regions. Studies completed by Akan and Houghtalen (2003)

and Young and Black (2009) advocate the application of a percentage runoff (PR) of 70% in urban areas.

This was the figure used for urban PR during the construction of SEPA's national pluvial flooding project.

SEPA recommended the application of a PR of 55% throughout rural areas for the purposes of this work.

4.7.3 Hydraulic Model considerations

Model verification is also recommended - this could entail targeted site investigations/inspections to

essentially ‘ground truth’ models and identify site specific factors such as underpasses (i.e. false

blockage) and property thresholds, consideration of anecdotal evidence, calibration and validation with

observed flows and depths. SEPA would also accept that the dataset could be modified locally if needed

with more accurate survey or other data.

In some circumstances there may be a need to combine fluvial and pluvial flood risk, particularly where

out of bank flows and overland surface water flows are likely to mix.

4.8 Groundwater Investigations

Currently there are few confirmed instances of groundwater flooding in Scotland and a recent scoping project

suggests it is not a widespread problem as maybe the case in certain parts of the UK. Groundwater flooding

is possibly under represented in Scotland because of the difficulty of differentiating it from other types of

flooding. There are no known regional aquifers in Scotland which have caused significant delayed onset of

flooding as experienced in the Chalk aquifers of SE England. It is considered that groundwater flooding in

Scotland is likely to be a flooding mechanism which contributes to other sources of flooding e.g. fluvial or

pluvial on a local scale during heavy rainfall as opposed to separate distinctive events. Groundwater has the

potential to extend the duration or extent of flooding in low lying areas and may be important to consider in any

flood mitigation strategies. Flooding associated with valley superficial deposits such as gravels (known as

alluvial flooding) can occur prior to a river overtopping its banks and has been seen close to some of the large

rivers in Scotland. Also, rising groundwater, including changes in the water table due to the cessation of mine

pumping has caused some issues on a localised scale in Scotland.



Groundwater may emerge at ground surface or within man made structures such as basements or subsurface

infrastructure (e.g. cabling, pipelines, sewers). There are several mechanisms of groundwater flooding which

could include:

- Clearwater flooding: after prolonged exceptional rainfall, groundwater may emerge in low lying areas or

previously dry springs / river beds. Depending on aquifer properties, flooding may be delayed for a period

of time after the rainfall event.

- Alluvial and coastal flooding: commonly seen in low valley floors as a result of high groundwater levels

in superficial deposits isolated from fluvial flood areas or in areas adjacent to high river or sea levels

behind informal embankments prior to overtopping.

- Groundwater / minewater rebound: caused as a result of stopping the abstraction of groundwater

(normally via pumping).

- Ground subsidence: this can create low topographic areas where water can pond.

- Artificial recharge: this includes leaking supply or drainage networks

- Obstructions: either within natural groundwater flow paths or reduction of aquifer storage capacity such

as large scale cut off walls or sheet piling.

- Artificial groundwater conduits: one example is uncapped artesian boreholes.

- A combination of the above mechanisms.

If there is a perceived risk of flooding, groundwater can be investigated as part of a flood risk assessment

through desk studies or on site ground investigations and groundwater level monitoring in conjunction with

other hydrological data. An investigation may be scaled to the type of development and complexity of the

flooding risks under consideration.. Monitoring of groundwater, particularly through a flood event, is the ideal

scenario to understand the flooding mechanisms. Consideration should be given to how groundwater may

interact with proposed mitigation for other sources of flooding e.g. flood alleviation schemes or how release of

confined groundwater may occur during excavations to provide storage for surface water detention.

Additional volumes of groundwater may also need to be accounted for in the design of drainage schemes on

top of that required to mitigate against surface water flooding. Source control of rainfall is important in these

situations and storage elements below ground level (e.g. detention basins, wetlands) may require to be lined.

Groundwater can be difficult to manage if not properly considered. It is particularly important within any flood

mitigation measures to identify groundwater flow paths and how they may be altered by a development

especially those requiring substantial ground engineering works. Examples include sheet piling or cut off

walls associated with flood prevention schemes or site preparation, changes in catchment characteristics

through mine water pumping schemes or restoration of land following quarrying or mining.

Mitigation of groundwater flooding can be a difficult and limited therefore new development should be avoided

in areas at risk. Pumping localised areas or waterproofing floors of buildings / basements may be installed on



a small scale but may have variable or temporary effect. Installing physical barriers to alter groundwater flow

may have detrimental environmental impacts outside flood events.

4.9 Land Raising and Compensatory Storage 2

New development must not affect the ability of the floodplain (fluvial or coastal) to store and/or convey

floodwater. Many floodplains will do both during a flood event depending on which stage of the flood

hydrograph is dominant at any given time. Areas of floodplain can vary in scale and dominant function within

and between catchments and even between different flood events. However it is clear that the removal of

floodplain by land raising will displace floodwater and have an unacceptable impact unless it is linked to the

provision of compensatory storage (Figure 1).

If there is an overriding and exceptional need for development to be located on an area that is floodplain then

there may be a case for land raising. Land raising should always however be linked to the provision of

“compensatory storage” to replace the lost conveyance capacity/ storage volume of the functional floodplain

and counteract the displacement of flood water.

It is the view of SEPA that all land liable to flooding during a flood event up to and including the 200-year

flood, even if caused by the blockage of a structure (e.g. culvert), should be considered as ‘functional

floodplain’. Furthermore, for any site where land-raising is proposed, compensatory storage must be provided

up to the 200-year design level. This should help towards providing a neutral impact on the risk of flooding to

adjacent sites in accordance with SPP guidelines.

2 Further guidance on the calculation of compensatory flood storage requirements and details of direct and indirect replacement of flood

storage are available in CIRIA Report C624 “Development and Flood Risk – guidance for the construction industry”.



Figure 1: Schematic illustrating how, for a given flow, the flood level can vary across the floodplain as a result in

alterations of cross-sectional area / morphology.

In general, compensatory storage should be provided close to the point of lost floodplain, provide the same

volume and be at the same level relative to the design flood level as that lost. In other words, in order to

replicate the existing situation for a particular flood, an equal volume of floodplain must be created to that

taken up by the development. This is to ensure that flood water is not displaced elsewhere with potential

adverse impacts. This equal volume must apply at all levels between the lowest point on the site and the

design flood level. In many cases it is likely that there will be no suitable areas of land within a development

boundary to provide compensatory storage. The location of compensation works on (or off site), should relate

hydraulically and hydrologically to the location of the development.

Compensatory storage could be provided either by the direct or indirect replacement of floodplain volume.

Direct replacement is the preferred means however the feasibility of providing direct replacement is largely

dependent on there being available land at an appropriate level. Even where there is available land, direct

replacement of flood volume may not be acceptable if it has a detrimental impact on the environment,

landscape or cultural heritage or where there are long term issues relating to land ownership. There may

however be instances where direct replacement of flood volume provides an opportunity for the creation of new

valuable habitats, for example wetlands.



Hydraulic models, while satisfactory for investigating the impacts of land raising on floodwater conveyance and

flood levels, may not be appropriate for assessing the impact of losing floodwater storage capacity. This is

because steady state 1D models cannot accurately represent the loss of upstream storage and fully dynamic

1D models cannot fully represent the complex hydraulics associated with floodplain processes. Therefore

modelling the removal of flood storage is likely to introduce further uncertainty, especially as the majority of the

models are 'un-calibrated' due to a lack of on site flood/water level data. The application of complex 2D models

(when rarely applied) with additional survey data may reduce the level of uncertainty, however uncertainty

associated with parameterisation and the lack of calibration data may remain.

SEPA recommends that proposals for land raising are linked to the provision of compensatory storage by

replacing the lost capacity of the floodplain. To determine the volume of compensatory storage required,

SEPA recommends the area of raised ground is divided into 5-10 ‘slices’ and the volume of each slice

calculated, e.g. as shown in Table 2. Compensatory storage can then be designed so that a volume, at least

equal to that of each slice of raised ground, will be provided at the same level as that it is replacing.

Table 2: Indicative approach to calculating compensatory storage volume

Proposals will be considered on their own merits, however in general the works listed below would not be

acceptable:

raising land within an area that has not been previously developed i.e. in a Greenfield location.

excavating a hollow in the floodplain below the level of the development. In some cases, this void may

already be full of water prior to the occurrence of the design flood level and thus offer no storage

potential at all;

excavating a landlocked area isolated from the floodplain or linked by a narrow access, such as

culverts. These are prone to accidental blockage or infilling, especially if they are only used every few

years;

providing low level compensation to match high level development or vice versa. This affects the

timing of operation of the compensation storage relative to the pre-development condition;



works that will damage sensitive habitats, heritage sites etc.

works that may place surrounding properties at risk e.g. lowering ground levels close to other

properties that may already be at risk, and therefore increasing their flood risk.

an engineering solution that is dependent on frequent maintenance to maintain its design capacity

and efficient operation. Such a solution is unlikely to be sustainable over the lifetime of the

development and beyond.

Issues related to a modelling approach have been outlined and the preferred method of like-for-like

replacement storage should be followed as standard although there may be exceptions. For example, large-

scale, Brownfield, development-plan led proposals for which it has been clearly demonstrated that like-for-like

compensatory storage cannot be fully achieved may be progressed with information based on the detailed and

robust application of acceptable modelling practices in consultation with SEPA. In such cases modelling

uncertainty must be reduced as far as possible so models should be calibrated where data is available and

sufficient information supplied to demonstrate model stability and sensitivity to changes in key model

parameters – including discharge coefficients, boundary conditions and those already stated above. In

addition, information should be supplied on model run/simulation parameters to provide a full breakdown of

the modelling process (SEPA may request the provision of model files for further review). On the basis of a

satisfactorily robust model it should be clearly demonstrated that there would be no increase in flood risk

upstream or downstream of the development/compensatory storage area.

SEPA emphasises that taking a modelling approach immediately, instead of the preferred method, should not

be considered as an alternative; the Applicant/ Consultant must seek to achieve a level-for-level replacement

for storage lost to land raising. However, if it cannot be achieved, suitable and sufficient information must be

supplied to demonstrate why level-for-level replacement storage (even partially) is not possible (e.g. site

restrictions, etc) and provide suitable and acceptable justification for a modelling approach.

5.0 FRA Checklist

SEPA now supplies a FRA Checklist for consultants prior to the submission of a report in support of a

planning application. SEPA recommends that this is completed as far as possible, printed and attached to the

front cover of the FRA. In addition to providing a summary of key information in relation to the FRA it is also

intended to enable an improved response to statutory consultations. A copy of the Checklist can be found

and downloaded from SEPA’s website here FRA checklist

http://www.sepa.org.uk/media/159170/flood-risk-assessment-checklist.xls



Appendix 1 - Sources and types of information

Source Sub-source Type Comments

Local Authority Flood Prevention

Authority

Planning Authority

1) Biennial Flood Reports / flood

photos.

2) Flood Prevention Scheme

studies

3) Strategic Flood Risk

Assessment

Flood Risk Assessments for

planning

1) Often available on Council website.

2) Feasibility studies are often undertaken for

areas where no formal flood prevention

measures currently exist.

Many councils have an e-planning website.

Scottish Water Flood incident reports.

SEPA Flood Risk

Hydrology.

National Flood Risk

Assessment

Flood photos, post-flood survey

data.

Digitised records of past flooding

from multiple sources

SEPA flood risk hold information on past flood

events in Scotland in various formats

Available via SEPA website here

Scottish

Government

SEPA SEPA Flood Map (2014). Available on-line here

British

Hydrological

Society

University of Dundee Chronology of British

Hydrological Events

Available on-line at

http://www.dundee.ac.uk/geography/cbhe/

Media Television and

Newspaper

Flood reports/ photographs. Material may be found on-line.

SNIFFER Coastal Flooding in

Scotland: A Scoping

Study 2008

Final report and GIS data can be

found here

Information on past coastal flood events in

Scotland as well as the dominant coastal

processes.

Academia Academic staff

and/or students.

Flood studies for specific areas.

Local Flood

Groups

Local residents Anecdotal accounts of flooding

and/or flood photos.

Library/Archives Books, journals,

magazines,

newspapers, church

records.

Historical flood information &

photos.

Internet Web search Accounts of flooding and photos. Numerous data sources exist on-line.

Buildings/bridges Can be on a plaque Epigraphic flood data Often levels of past extreme floods are marked

on buildings and bridges.

http://www.sepa.org.uk/flooding/flood_risk_management/national_flood_risk_assessment.aspx

http://www.sepa.org.uk/environment/water/flooding/flood-maps/

http://www.dundee.ac.uk/geography/cbhe/

http://www.sniffer.org.uk/files/7013/4183/7993/FRM10_final_030908_with_security.pdf



Appendix 2: Flood Probability

The annual probability of flooding is the statistical chance (or risk) that a location will flood in any given year

and relates to a particular size or magnitude of flood, e.g. the 0.5% AP event is smaller in size than the 0.1%

AP event (although a 1% AP event will occur more frequently than a 0.1% AP event).

For any given location, the 0.5% AP flood should (in theory) affect a smaller spatial area, or, will inundate the

same area to a lesser depth (if the floodplain is constricted by topography), than the larger 0.1% AP flood. The

chance of experiencing the larger 0.1% AP flood, however, is smaller as explained below:

For the same location, the 0.5% AP flood can be expressed as ’the flood which has a 0.5% chance of

occurring in any given year’ (i.e. there is a 1 in 200 chance of experiencing a flood of that size, at that

location); also referred to as the 200-year flood or the flood with a return period of 200 years.

However, it does not follow that if a location suffers the 0.5% AP flood this year, it will not be flooded again to

this extent for 199-years. Statistically, the chance or probability of experiencing the 0.5% AP flood remains the

same in any given year. Furthermore, it also does not follow that over any 200-year period, the 0.5% AP (200-

year) flood will definitely be experienced, i.e. statistically, the chance of experiencing the 200-year flood within

a 200-year period is only 63% (see Table 1 below).

FLOOD EVENT

DESIGN LIFE

(years)

50 (2% AP) 100 (1% AP) 200 (0.5% AP) 1000 (0.1% AP)

1 2 1 0.5 0.1

10 18 10 5 1

20 33 18 10 2

50 64 39 22 5

70 76 50 30 7

100 87 63 39 10

200 98 87 63 18

Table 1: Probability of experiencing a range of flood events over different time periods (design life)



Appendix 3 - Types of Flooding3

Fluvial – flooding originating from a watercourse either natural or culverted. Normally caused when the river

channel capacity (or culvert capacity) is exceeded and water flows out-of-bank onto the floodplain, which could

either be natural floodplain or developed. A floodplain is the area(s) of land adjacent to a watercourse (of any

scale, e.g. small burns) where floodwaters naturally flow and/or are stored during times of flood.

Coastal – flooding originating from the sea (open coast or estuary) where water levels exceed the normal tidal

range and flood onto the low-lying areas that define the coast line. This coastal plain could be either natural or

developed. Coastal flooding can occur due to four physical elements (as below) either acting on their own or in

combination with each other.

Predicted astronomical tide: expected sea water level due to the gravitational effects of the sun and

the moon.

Storm surge residual: elevated sea level caused by the combined effect of low pressure and

persistent, strong wind (for every millibar drop in pressure, a 10mm rise in the sea surface elevation

occurs).

Wave effects: a function of both wind strength and open water ‘fetch’ length. As a result of high

winds, waves can also be associated with low pressure systems which cause storm surge effects as

described above.

Local bathymetric effects: topographic funnelling due to the forcing of a large volume of open sea

water into a restricted coastal embayment, e.g. estuary (Firth of Forth), tidal basin (Montrose Basin) or

sea loch (Loch Fyne), which will elevate water levels locally.

Pluvial – urban or rural flooding which results from rainfall-generated overland flow before the runoff enters

any watercourse, drainage system or sewer.

Drainage - flooding as a result of surcharging of man-made drainage systems including combined sewers

where the capacity of the system to discharge runoff has been exceeded.

Infrastructure Failure – flooding due to collapse/failure of man made infrastructure including hydro-dams,

water supply reservoirs (private or public), canals, flood defence structures, underground conduits (e.g.

sewers), water treatment tanks.

3 Information on types of flooding are also provided in many other sources including SPP, PAN 69 and CIRIA Report C624. It should be

noted that flooding may also occur due to a combination of more than one type of flood process, e.g. fluvial f looding and coastal flooding can

occur at the same time.



Groundwater - flooding due to a significant rise in the water table, normally as a result of prolonged and

heavy rainfall over a sustained period of time (can affect cellars and drainage systems). Normally associated

with catchments where porous substrate and/or aquifers exist. This type of flooding can last for a considerable

period of time, i.e. weeks, months. Currently there are few confirmed instances of groundwater flooding in

Scotland and it is not a widespread problem as maybe the case in certain parts of the UK. It may be that

groundwater flooding is under represented in Scotland because of the difficulty of differentiating it form other

types of flooding. There are no known regional aquifers in Scotland which have caused significant delayed

onset of flooding as in the Chalk aquifers of SE England. It is considered that groundwater flooding in

Scotland is likely to be a flooding mechanism which contributes to other sources of flooding e.g. fluvial or

pluvial on a local scale. These groundwater mechanisms have the potential to extend the duration or extent of

flooding in low lying areas and may be important to consider in any flood mitigation strategies. Groundwater

can be investigated as part of a flood risk assessment through desk studies or on site ground investigations

and groundwater level monitoring in conjunction with other hydrological data. Consideration should be given

to how groundwater may interact with proposed mitigation for other sources of flooding e.g. release of confined

groundwater by excavations to provide storage for surface water detention. It is particularly important to

consider groundwater flow paths and how they may be altered by substantial ground engineering works e.g.

sheet piling associated with flood prevention schemes.

Examples:

1. Basement Flooding - example is Perth, where there is frequent basement flooding and surcharging of

drainage network because of the strong groundwater/sub-surface links between the tidal reach of the river and

the lower parts of the town. Even with formal flood defences in place, this is still a problem.

2. Mine Water Pumping – example is Blindwells in East Lothian, where pumping of groundwater is required

to stop groundwater from breaking out at the surface and flooding properties. To deal with this treated

pumped groundwater is discharged to a nearby watercourse. This in itself raises base-flows in the receiving

watercourse and needs to be managed to prevent an increase in flood risk from the watercourse. Difficulties

in knowing how much water needs to be pumped is the result of long term mine workings that will draw water

in from a large area - beyond the bounds of what would normally be considered a catchment area or drainage

basin. Another consequence of restored mine workings is that sub-surface flows can be significantly altered

through stratification dependent on how the land restoration has been carried out.

