Post on 15-Dec-2015
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
15/06/20113 National Pluvial Flood Map for Scotland - Pascal Lardet
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)
15/06/20114 National Pluvial Flood Map for Scotland - Pascal Lardet
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
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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
15/06/20116 National Pluvial Flood Map for Scotland - Pascal Lardet
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
Process: roads, buildings
NextMap SAR
Catchment-based models
15/06/20117 National Pluvial Flood Map for Scotland - Pascal Lardet
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
FEH CD-ROM DDF parameters
Rainfall events: 30yr, 200yr, 200yr+20%
Storm durations
Rainfall depths
Deduct urban/rural infiltration/sewer losses
5km grid of rainfall DDF
100m grid of effective rainfall
5m grid of flood depths
15/06/20118 National Pluvial Flood Map for Scotland - Pascal Lardet
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
15/06/20119 National Pluvial Flood Map for Scotland - Pascal Lardet
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
Post-process – remove small ponds, threshold, aggregate
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
15/06/201110 National Pluvial Flood Map for Scotland - Pascal Lardet
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
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Aberdeen
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Alloa
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Coatbridge
15/06/201114 National Pluvial Flood Map for Scotland - Pascal Lardet
Dumfries
15/06/201115 National Pluvial Flood Map for Scotland - Pascal Lardet
Edinburgh West
15/06/201116 National Pluvial Flood Map for Scotland - Pascal Lardet
Forfar (Angus)
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Elgin (Moray)
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Glasgow
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Inverness
15/06/201120 National Pluvial Flood Map for Scotland - Pascal Lardet
Perth
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Stirling
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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
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Questions / Comments? (lardetP@halcrow.com)
Thanks to Nick Burke, Mark McLaughlin & Beth Mennie of SEPA
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