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Flood Risk Management Act www.sepa.org.uk/flooding

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SNIFFER Flood Risk Management Conference 2013

Pluvial Flood Hazard Mapping

06 February 2013 Session 6: Surface Water Management

Dr Rick Haynes


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Regional Pluvial - Purpose

• Regional Pluvial: “Based on the recommendations of the [National Pluvial]

study, SEPA now wishes to gain an improved understanding of pluvial flooding by obtaining 2D model runs based on more accurate ground model data and a wider range of rainfall return periods, model loss scenarios and storm durations.”


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Introduction

• Provide overview of Regional Pluvial Hazard contract

• Indicate purpose of outputs • Overview of methodologies used • Indicate verification process


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Purpose of Flood Hazard Maps

• Improve knowledge / understanding of Flood Hazard • Support the appraisal process • Definition of objectives & measures • 14 x Flood Risk Management strategies • Local Flood Risk Management Plans


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Collaborative Advisory Groups

SAIFF Modelling & Appraisal Strategy Group Steer Principles

Consistency

LONG TERM – focus on & support of long term implementation

SHORT TERM - Requirements of the FRM Act

SAIFF Modelling & Appraisal Working Group Technical Guidance

Methodology


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Regional Pluvial - Methodology • Software: JFlow+ (2D

hydraulic model) • Model Components:

– Composite LiDAR – NEXTMap DTM (2m and 5m resolutions)

– Buildings represented as stubby buildings (+0.3m) as derived from MasterMap dataset

• Scenarios: – 14 modelling scenarios – Varying return periods and

blanket storm durations

Return Period

Loss Blanket Storm Duration (hours)

10 1 in 5yr 1&3

30 1 in 5yr 1&3

30CC 1 in 5yr 1&3

50 1 in 5yr 1&3

100 1 in 5yr 1&3

200 1 in 5yr 1&3

200CC 1 in 5yr 1&3


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Regional Pluvial – Key Deliverables • Merged LiDAR & NextMap DTM

• To maintain high resolution in urban areas (LiDAR only)

• To allow use of lower resolution in rural areas (NextMap or merged)

• Raster grid of flood depths (pre / post 0.1m threshold)

• Inundation extent outline for two depths contours (0.1m and 0.3m)

– Pre / post removal of small ponded areas (<200m2)

• Flow velocities / direction (Raster / vector)

• Hazard band rating (Raster / vector)

• False blockages • Pluvial flood model • Rainfall inputs • Model reports


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Regional Pluvial – Limitations

• Extreme data volumes • DTM uncertainties • Pre-defined roughness parameters • National parametric value for losses • Hydrology based on defined durations as opposed

to catchment specific ‘critical durations’ • Strategic only application


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Review Process • SEPA:

– Have obtained the LA modelled outputs – Have undertaken detailed QA of all data received

• LAs: – Will undertake checks of water extent matches, a local

understanding of flooding & flood risk – Will provide expert, local knowledge

• Contractors: – Will implement agreed modifications – Will produce final model outputs for Mapping


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Reporting Requirements • EC / SG Reporting:

– 1:10yr / 1:100yr / 1:200yr – Discussed through MAST

• Stakeholders: – Support for Land Use

Planning – Support for Category 1

Responders


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Reporting Requirements • SEPA:

– Support BaU (Ops / LUP)

• General Public: – Web viewer (but sanitised) – Data / information requests


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Status and Next Steps …

• We now have: – Improved understanding / assessment of pluvial inundation – Internal (& Stakeholder) reviewed outputs

• We will have:

– Mapped Hazards for key urban centres in Scotland – Ability to report to Europe as required in the Act


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Questions

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