Slide 1

Surface water flood risk mapping in the UK - approaches and challenges

Slide 2

Surface water flood risk mapping in the UK

• Chris Elms, Department for Environment, Food and Rural Affairs

• Selena Peters, Environment Agency (England)

• Kirsten Thorburn, Scottish Environment Protection Agency

• Linda MacHugh, Northern Ireland Department for Infrastructure

Working Group F – Pluvial flood risk, Berlin, October 2016 2

Flood risk management is a devolved matter in the UK. Each of the four nations – England, Scotland, Wales and Northern Ireland - is responsible for risk assessment, mapping and plans.

We have many players involved in the Floods Directive in the United Kingdom and flood risk management is a devolved matter. Each of the four nations- England, Scotland, Wales and Northern Ireland is responsible for the risk assessment, mapping and plans Defra co-ordinates reporting and delivery of the requirements of the Directives such as PFRAs and FRMPs. The UK government delegates responsibility for preparation and publication of the PFRAs, FRMPs etc in England to the Environment Agency, as its competent authority. Similar arrangements apply in Scotland, Wales and Northern Ireland.

Local authorities are responsible for the management of pluvial flood risk as they are best placed to deal with the impact of surface water flooding in local areas My colleagues will outline our respective approaches to understanding and managing the risks from pluvial flooding. It is important to note that though we have common needs and there are common theme, each nation has adapted its approach to deal most effectively with the issues for them, and this flexibility provided by the Directive is important. For example, in England and Wales we mapped for surface water flooding on a national basis before the Directive came into force following extensive flooding in 2007 which led to new legal responsibilities for local authorities in respect of assessing, mapping and planning for flood risk and has informed our approach. Slide 3

Common needs

• Develop understanding of pluvial flooding

• Inform FRMP and support decision making

• investment in structural defences

• improve emergency response

• land use planning

• Raise public awareness

• Understand future impacts

• adapting to climate change

Working Group F – Pluvial flood risk, Berlin, October 2016 3

The common needs which we all share are: Firstly the need to develop our understanding of pluvial flooding We need this to inform our Flood Risk Management Plans and support our decision making in respect of our investment in structural defences, to improve our emergency response and for the purposes of land use planning We also need to raise public awareness and understand future impacts, in particular how we adapt to climate change.

Slide 4

Water

National legislation definition

Current day scenario

Urban/rural adjustments

3 storm durations (1,3,6hr)

3 annual frequencies

(30, 100, 1000)

50% summer rainfall profile

Surface

68% LiDAR

2m resolution

Buildings raised (0.3m)

Roads lowered (0.125m)

91,000 manual

structure edits

No defences

Manning’s n (0.02,

0.03, 0.1)

Local validation

Communication

Local modelling

Cleaning

Outputs

Extent

Depth

VelocityHazard

Suitability

Approach to modelling and mapping surface water flood risk in England (and Wales)

Surface Water Mapping history in England (and Wales until April 2013): • 2008: Areas Susceptible to surface water flooding (AStSWF) – The Government’s Pitt Review

following flooding of 2007 which focused on surface water flooding, previously not such an issue. The map was produced independently by Jeremy Benn Associates and we licenced its use.

• 2010: Our first Flood Map for Surface Water (FMfSW) – a step forward from AStSWF, however still a national scale model with no local input. We used this map to determine our Flood Risk Areas in the PFRA 1st cycle.

• 2012: updated Flood Map for Surface Water (uFMfSW) – improvements to both data and method. We published it in December 2013, now called Risk of Flooding from Surface Water (RoFW).

Water (Hydrology) • National legislation defines surface water flooding as surface runoff from rainwater (Flood

and Water Management Act, 2010: “flooding that takes place from the 'surface runoff' generated by rainwater (including snow and other precipitation) which: (a) is on the surface

of the ground (whether or not it is moving), and (b) has not yet entered a watercourse, drainage system or public sewer“)

• Current day scenario only • Urban vs rural adjustments for effective rainfall. Urban assumptions of 70% rainfall

infiltration and drainage rate 12mm/hr. • 3 storm durations – 1 hr, 3 hrs and 6 hrs – results merged together to form worst case and to

give a better chance that the frequency of the flood is appropriate. • 3 annual frequencies – 1 in 30 chance of happening in any given year, 1 in 100… and 1 in

1000… (annual exceedance probabilities) Surface (DTM) • Ground surface made up of LiDAR downgraded to a 2m resolution for national consistency • Maintained flowpaths where possible:

• 2m resolution to pick up small structures • Raised buildings and lowered roads • Things that weren’t picked up automatically, we edited manually (eg tunnels through

rail embankments) • No defences included • Roughness values varied according to land use Ran using a 2D model

Then • included results of any local modelling from local authorities – better quality • outputs produced in line with EU Directive requirements (Article 6) – plus an additional

output we called suitability which is a measure of confidence in the outputs and the reliability of the outputs if used at various scales

• cleaning – remove isolated flooding (>100m3), filled in dry areas <50m3, remove anything below defined hazard threshold (0.575) to create an outline

• local validation – we shared the outputs online with local authorities for their comment and confidence

• communication – published on our website in December 2013 (next slide for progress), available to non-commercial and commercial companies with a licence (data free of charge since April)

Documents available describing our Risk of Flooding from Surface Water (RoFSW) information:

1. https://www.gov.uk/government/publications/flood-maps-for-surface-water-how-they-were-produced

2. https://ea.sharefile.com/d-s9f758a3d19c4f908 (Using the RoFSW Information – dataset documentation)

Slide 5

How we communicate national SW flood risk to the public (ANIMATED SLIDE): • Published our Flood Risk Maps. Showing the adverse impacts of flooding. The slide shows

the page and map from the Thames Basin District where the Flood Risk Area is London. The image on the slide shows Risk to People. Others available are Economic Activity, Natural and Historic Environment. https://www.gov.uk/government/collections/river-basin-districts-

flood-risk-maps

• Previously map based as only method of communication (currently running in parallel to

new method) http://watermaps.environment-agency.gov.uk/wiyby/wiyby.aspx?lang=_e&topic=ufmfsw&layer=default&scale=2&x=357683&y=355134#x=357683&y=355134&scale=2

• Now we have the Long Term Flood Risk Information: postcode search for flood risk

information focussed on action before risk (R&D telling us what people are interested in) and simpler map visuals with a slider from basic to more detailed information

New approach to communicating risk, now live but running in parallel to the above (Long Term Flood Risk Information): https://flood-warning-information.service.gov.uk/long-term-flood-risk Research report into communicating flood risk (Sciencewise): http://www.sciencewise-erc.org.uk/cms/public-communication-and-engagement-on-flood-risk/

The actions that are listed in the

report change order and/or

content according to the relative

risk of the sources of flooding

Slide 6

National

pluvial

rapid flood

spreading

modelling

All Scotland

Data in

• Digital Terrain Model (NextMap)

• Topographic map

• Rainfall

• urban and rural areas

Assumptions

• Runoff

• Drainage

• Rainfall (profile, duration, return period, climate change)

Outputs • Extent

• Depth

Regional

pluvial

2D

hydraulic

modelling

In high risk areas

Data in

• Digital Terrain Model (LiDAR)

• Topographic map

• Rainfall

• Land cover map

Assumptions

• Runoff

• Drainage

• mannings

• Rainfall (profile, duration, return period, climate change)

Outputs

• Extent

• Depth

• Velocity

• Hazard rating

Water company flood hazard from

sewer system

1D Sewer model and 2D spreading

Locations of flooding, extents and

depths

Scotland pluvial flood modelling Before the floods directive and our national legislation in Scotland we had no national

understanding of pluvial flood risk

2011 National pluvial modelling. For the preliminary flood risk assessment in 2011 we developed a national pluvial flood model covering the whole of Scotland – this was based on relatively simple modelling software - rapid flood spreading model – giving higher level and strategic outputs to enable us to consider pluvial flood risk in our pfra.

We did this because we had experienced some significant pluvial flood events (e.g. Glasgow 2002 and the pluvial floods in England in 2007)

The data we needed to input to this modelling was:

o DTM (we had an existing national Digital Terrain Model based on NEXT map not as accurate as LiDAR)

o Detailed topographic map to add surface features to DTM (e.g. buildings, roads, remove false blockages)

o Rainfall data (from Centre of Ecology and Hydrology (CEH))

o Map showing urban and rural areas for applying different model assumptions

What it was used for

o The 2011 pluvial modelling gave us an understanding of the scale of pluvial flood risk in Scotland (based on economic damages to homes and businesses pluvial flooding accounted for 38% of damages, fluvial flooding 45%, coastal flooding 17%)

o It provided information on pluvial flood risk in each ASFR

o It was used to identify those areas with higher pluvial flood risk where more detailed pluvial modelling was carried out.

By 2013 we then carried out more detailed pluvial modelling in the highest risk areas (covers 11% of Scotland), this informed our hazard and risk mapping for areas of significant flood risk and the flood risk management plans. This used more detailed modelling software – 2D hydraulic modelling – giving more outputs, as well as extent and depth we also have velocity and hazard rating. It also gives us more confidence in the outputs. The water company also provided some modelling of the drainage network.

The key new data needed to do this more detailed modelling was o improving the accuracy of the DTM using LiDAR, and o land use cover map to determine mannings and more detailed identification of

urban and rural areas.

What it was used for

o Provided an updated understanding of the scale of pluvial flood risk in Scotland (based on economic damages pluvial flooding accounted for 23% of damages, fluvial flooding 56%, coastal flooding 21%)

o Informed the FRMP in consultation with local authorities (was used to identify those towns and cities with greatest pluvial flood risk where local authorities would lead on the management of this risk)

o Was provided to local authorities to inform management of pluvial flood risk (local authorities will validate in detail and do more detailed modelling where required)

o Was provided to land use planning authorities to inform new development. Scotland has policy for pluvial flooding and new developments - new developments must be designed to ensure that buildings are not at risk of pluvial flooding and that it does not increase the risk of flooding out with the development. This is different to planning policy for fluvial and coastal flooding where new development is not allowed in the flood extent http://www.gov.scot/Topics/Built-Environment/planning/Policy

Pluvial flood hazard maps made publically available online. https://www.sepa.org.uk/environment/water/flooding/flood-maps/

Slide 7

7

Scotland National pluvial modelling Scotland Regional pluvial modelling

Coverage All Scotland In high risk areas

Modelling software Rapid flood spreading model (ISIS Fast) 2D hydraulic modelling (Jflow+)

Data inputs required Digital Terrain Model (Next Map)

0.7m vertical accuracy, 5m resolution

(Based on topographic map) - Buildings raised

0.3m, roads lowered 0.1m, false blockages

removed

No flood defences

Digital Terrain Model (LiDAR)

0.15m vertical accuracy, 1-2m resolution

(Based on topographic map) - Buildings raised

0.3m, false blockages removed

No flood defences

Map showing urban and rural areas Land Cover Map

Detailed topographic map Detailed topographic map

Rainfall data Rainfall data

Assumptions

Urban Rural Urban Rural

Model resolution 5m 5m 2m 5m

Runoff 70% 55% 70% 55%

Mannings Does not take into

account roughness

Does not take into

account roughness

Based on land cover

map

Based on land cover map

Drainage capacity 12mm/hr 0mm/hr 1in5yr rainfall return 0mm/hr

Rainfall Profile 50% summer profile 50% summer profile 50% summer profile 50% summer profile

Rainfall duration 1hr 3hr 1hr, 3hr 1hr, 3hr

Rainfall return

period

10yr,30yr,30yrCC,

50yr, 100yr, 200yr,

200yrCC

10yr,30yr,30yrCC,

50yr, 100yr, 200yr,

200yrCC

10yr,30yr,30yrCC,

50yr, 100yr, 200yr,

200yrCC

10yr,30yr,30yrCC,

50yr, 100yr, 200yr,

200yrCC

Climate change

uplifts

20% uplift in rainfall intensity based on UK Climate Projections 2009 (UKCP09) and review of other

studies in the UK.

Post processing Applying depth thresholds, removing small ponded areas

Outputs Extents, depths Extents, depths, velocity, hazard rating

Working Group F – Pluvial flood risk, Berlin, October 2016

This slide shows more information about the data used for the modelling and assumptions made.

Slide 8

8

Flood Hazard Research Centre

Adverse consequences of flooding

Example of adverse consequences of pluvial flooding risk

(1in200yr shown here)

£ damage (based on depth) forhomes, businesses and roads

Working Group F – Pluvial flood risk, Berlin, October 2016

GIS

analysi

s

Scotland, how we assessed the adverse consequences of flooding (Animated Slide) Using our flood hazard maps we then assessed the adverse consequences of flooding

(higher level assessment done for pFRA in 2011, more detailed assessment done for maps of ASFR in 2013)

Data needed for this:

o Hazard outputs from modelling (extent, depth)

o Dataset on a national scale with information on receptors of flooding (e.g. properties, cultural heritage, roads, rail etc and information on these e.g. property type (home, business, community facility), floor area, areas more socially vulnerable to flooding,

The adverse consequences assed included (done for all return periods):

o Number of residential properties at risk and £damage to residential properties

o Number of residential properties at risk in areas that are more socially vulnerable to flooding

o Number of non-residential properties at risk and £damage

o Number and type of community facilities at risk

Number and type of cultural heritage buildings and sites at risk

o Infrastructure at risk (roads, rail, airports, energy distribution infrastructure, water and waste water infrastructure)

o Impact on environmental designated sites was not done for pluvial flooding as risk not significant more detailed local modelling needed to understand environmental risks for pluvial flooding.

Methods for identifying adverse consequences based on manual from flood hazard research centre at Middlesex University http://www.mcm-online.co.uk/ including economic damage based on depth of flooding in £.

We have found the numbers of properties at risk from pluvial flooding is very sensitive to how buildings are represented in the Digital Terrain Model and the method used to identify what properties are within the flood extent.

Slide 9

9

Approach to modelling and mapping surface water flooding in

Northern Ireland

• NI methodology for creating Surface Water Flood maps similar to England & Wales

• NI focus is on Flood Hazard only

• Maps for all of NI communicated via ‘Flood Maps (NI)’ website

• NI Viewer presents interactive map with linked PDF maps showing depth information for high, medium, low probability flood scenarios

• Information is presented for present day & climate change epochs

Working Group F – Pluvial flood risk, Berlin, October 2016

•NI methodology for creating the Surface Water Flood maps is very similar to England & Wales (as illustrated on other slides of the presentation). •NI Surface Water maps focus on flood hazard only. •The maps are communicated via the Flood Maps (NI) website - https://www.infrastructure-ni.gov.uk/articles/what-flood-maps-ni •The map viewer presents an interactive map for all of NI, along with preformatted PDF maps showing depth information for all 3 flood scenarios (high, medium, low probability). Information is presented for both present day and climate change epochs. •The maps are used within the planning process to trigger drainage assessments where appropriate. •Stats are produced at a NI (and parliamentary boundary) scale to indicate the number of properties that are impacted by surface water flooding.

Slide 10

Creating

•DTM

•Calibration and verification

•Drainage and runoff rates

Using

•Counting properties

•Receptor datasets

•Suitability

Sharing

•Licencing

•Large dataset size

Understanding

•Strengths and limitations

•Fluvial vs pluvial

•Language for risk

•Precision vs accuracy

Improving

•Amendments

•Keeping up to date with developments

What we’re working on in surface

water modelling and mapping

Creating • DTM – LiDAR coverage challenges. LiDAR vs NEXTMAP - cost vs benefit of having a more

accurate DTM where there is lower risk (rural areas) • Calibration/Verification – limited history, unreliable records, how/when to record, mixed

sources, quick onset and recession • Drainage and runoff rates – various possible approaches:

• blanket drainage capacity based on defined rainfall rate • regionally variable capacity based on a local rainfall capacity (eg SEPA: 5yr) • locally derived drainage rates based on local knowledge (Environment Agency

Research & Development) • Runoff rates urban vs rural variations

Using • Counting properties – this is very sensitive to the methods used

• modelling includes buildings, so using a property point counting method significantly underestimates property counts.

• Alternative method: perimeter method – buffering issues/internal boundaries eg. Terraced houses, what counts as being at risk?

• Receptor datasets – availability of and confidence in national receptor/property datasets is important to get consistent and comparable impact assessments

• Suitability – what level of scale is it appropriate to use the data? A key question to enable practitioners to understand the reliability of decisions based on the information.

• Not property level. Recommended for use for surface water management plans (strategic level)

• England – country-county-town level only except where local model outputs included. Advise to always check with local authorities for detail

• Scotland – our more detailed regional hazard and risk data may be appropriate to identify streets and neighbourhoods at higher risk when combined with local authority knowledge and verification at a more local level.

Sharing • Licencing restricting our ability to make the information Open Data (available unrestricted to

all) • England – includes local models which need local authorities permission for us to

share as Open Data – progress being made • Scotland – challenges for local authorities sharing the data with consultants as part

of further investigation of surface water flooding as well as where water companies’ local modelling has been included.

• Large dataset size means difficultly sharing and storing. (England total size = 550GB – too big for sensible download times)

Understanding • appreciating the strengths and limitations of surface water modelling and mapping is key to

effective sharing and use – we produce documentation to support local authorities in using the information in an informed way

• Fluvial vs pluvial – pluvial flooding is a different kind of flooding all together • ground variations at a very local level can have a significant impact, therefore

modelling is a lot more unreliable. This is not always appreciated by users (and the public) who assume reliability is the same as for fluvial modelling.

• rainfall inputs may be more accurate than fluvial flow inputs due to the longer record history, but this may not outweigh the ground variation challenge

• Pluvial mapping may produce more reliable outputs for very small watercourses where fluvial modelling is uncertain – dangerous to ‘silo’ sources

• Language for risk – it’s vital that we can communicate the size of a flood / the amount of risk. Need to be taken seriously for people to act! An area that we are doing lots of work on at the moment.

• Probabilities – Using technical terms “1 in 100” or “1%” are not effective. Some possible strategies are to compare to previous floods, or in terms of relative time frames eg. X times in the duration of a house mortgage (about 30yrs) or lifetime etc

• Uncertainty – how can we easily communicate uncertainty?

• England – Suitability/Reliability table (linked to the above) • Precision vs accuracy – resolution 2m doesn’t mean it’s accurate to that level Improving • Amendments – how can we update the maps if it needs changing in particular locations?

• England – not very easily. The local authorities must (re)model the local area and submit the outputs for us to update.

• Modelling must be compatible (guidance issued). • Keeping up to date with developments:

• A challenge to keep current as knowledge, information and datasets develop • Climate change – UK Climate Projections 2009 has limitations but it currently being

updated for 2018. • National Rainfall Data

• Scotland - The changes to the Depth-Duration-Flow grids that support the rainfall input to national models is likely to have a significant impact on pluvial maps. This will require model re-runs and, where changes are significant, represents a challenge of communication as well as re-evaluation of pluvial impacts.

Forecasting (notes only) • Scotland - pilot of the Surface Water Alert scheme in 2014 in Glasgow. Brings together flood

hazard and risk modelling for pluvial flooding in an operational arena. Further work needed before a national system is available.

• England • Flood Forecasting Centre (joint MetOffice – Environment Agency) issue high level

extreme rainfall alerts on a Flood Guidance Statement • Current Research and Development include impact forecasting using the surface

water maps – now in operational test phase