1national pluvial flood map for scotland - pascal lardet isis fast for national and catchment scale...

1 National Pluvial Flood Map for Scotland - Pascal Lardet

ISIS FAST for national and catchment scale pluvial flood modelling – Application to national pluvial flood mapping for Scotland

Pascal Lardet, Jon Wicks, Kashif Shaad

Halcrow

15/06/20112 National Pluvial Flood Map for Scotland - Pascal Lardet

Contents

• Why develop a national pluvial flood map for Scotland?

• Development of the flood mapping approach

• Digital Terrain Model

• Rainfall

• Flood spreading using ISIS FAST

• Results processing

• Results

• Conclusions


Why develop a national surface water flood map?

• 2009 Flood Risk Management (Scotland) Act

• Fluvial + Coastal + Pluvial inform preliminary Flood Risk Assessment

determine areas at significant flood risk

inform requirements for future detailed flood risk/hazard mapping

• 2009 Jacobs/HRW/Met Office scoping project for SEPA, includes definition of ‘pluvial flooding’:

• “Flooding as a result of rainfall when water ponds or flows over the ground before it enters a natural or man-made drainage system or watercourse, or when it cannot enter because the system is already full to capacity”

• Halcrow appointed in September 2010 national pluvial flood mapping data (78,000km2)


Development of approach

• Phase 1 of project was pilot – Glasgow / Clyde / Loch Lomond (3900km2)

• Assess data and method refinements

• Balance: Data, data processing, method, computational demands – accuracy requirements & programme constraints

• Best available national datasets

• Nationally consistent method


Digital Terrain Model Rainfall

ISIS FAST – spreads effective rainfall over the Digital Terrain Model

FEH CD-ROM DDF parameters

Rainfall events: 30yr, 200yr, 200yr+20%

Storm durations

Rainfall depths

Deduct urban/rural infiltration/sewer losses

Process: roads, buildings

NextMap SAR

Catchment-based models

Post-process – remove small ponds, threshold, aggregate

National pluvial flood mapping data

Validation


Digital Terrain Model

• NextMap SAR (synthetic aperture radar) DTM – most accurate readily available national terrain data

• ‘bare earth’ at 5m x 5m grid

• OS Vector Map – add building thresholds (+0.3m)

• OS Vector Map – lower roads (-0.1m)

• Catchment or tile-based models?

• About 4000 catchment models

Digital Terrain Model


NextMap SAR



Rainfall

• 5km grid of depth-duration-frequency (DDF) parameters

• 30yr, 200yr, 200yr+20% for climate change scenario

• Storm durations: 1hr urban, 3hr rural

• Effective rainfall (at 100m grid):

• Urban: 70% runoff, 12mm/hr sewer loss

• Rural: 55% runoff, no sewer loss

Rainfall



Storm durations

Rainfall depths


5km grid of rainfall DDF

100m grid of effective rainfall

5m grid of flood depths


Flood spreading - ISIS FAST

• ISIS FAST to spread effective rainfall over DTM

• ISIS FAST:

• Simplified hydraulics (‘wet’ method volume filling method)

• Identifies network of linked ‘depressions’

• Routes volumes between ‘depressions’ (conserves volume)

• Intersects water levels with 5m grid DTM 5m grid of water depths

• Focuses on final ‘ponded’ locations (rather than flow pathways)

• Fast, part of ISIS suite, robust (no instabilities)

• Under 48hrs for whole of Scotland per rainfall scenario

ISIS FAST – spreads effective rainfall over the DTM


Results processing

• Remove small ponded areas (<200m2)

• Depth thresholds of 0.1m and 0.3m

• Aggregate results:

• Polygon shapefiles for nation at 0.1m and 0.3m thresholds (30yr, 200yr and 200yr+20%)

• Raster grids at 5m x 5m of flood depths


Indicative results of broad scale pluvial mapping for illustrative purposes only and subject to change © SEPA. Ordnance Survey data © Crown copyright and database right 2011


Results

• Validation:

• Comparisons with historical observed pluvial flooding data

• Comparisons with simulated results from more detailed models

• ‘Sensibility’ checks

• Qualitative comparisons with:

• Glasgow East End flooding of 30 July 2002 (MWH report)

• InfoWorks ICM model for Glasgow Surface Water Management Strategy by Halcrow/MWH using integrated surface-drainage model & LiDAR DTM

Validation

Indicative results of broad scale pluvial mapping for illustrative purposes only and subject to change © SEPA. Ordnance Survey data © Crown copyright and database right 2010

ISIS FAST InfoWorks ICMISIS FAST

Reported 2002 flooding


Aberdeen


Alloa


Coatbridge


Dumfries


Edinburgh West


Forfar (Angus)


Elgin (Moray)


Glasgow


Inverness


Perth


Stirling


Conclusions

• Flexible method developed making best use of national available data

• Proven to be rapidly implementable at national scale ( SEPA March 2011)

• First time national pluvial map for whole Scotland

• Validation: difficult but suggests satisfactory for intended use (areas susceptible to pluvial flooding influenced by topography)

• Improvements where necessary for improving decision making:

• More accurate DTM (eg LiDAR)

• More detailed modelling

• Local historic data and local knowledge

• Selection of most appropriate modelling approach, balancing: scale, accuracy, speed/budget, decision to support - leaving sufficient time for interpretation and use

National pluvial flood mapping data set


Questions / Comments? ([email protected])

Thanks to Nick Burke, Mark McLaughlin & Beth Mennie of SEPA

Digital Terrain Model Rainfall

ISIS FAST – spreads effective rainfall over the Digital Terrain Model



Storm durations

Rainfall depths



NextMap SAR



National pluvial flood mapping data

Validation

1national pluvial flood map for scotland - pascal lardet isis fast for national and catchment scale...

Documents

scotland pascal lardet

grid of flood depths

scotland pascal lardet

significant flood risk

grid of rainfall ddf

grid of effective rainfall

result of rainfall

available national datasets