Transcript Slide 1

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
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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
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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)
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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
FEH CD-ROM
 DDF parameters
NextMap SAR
Rainfall events: 30yr,
200yr, 200yr+20%
Storm durations
Process: roads, buildings
Rainfall depths
Catchment-based models
Deduct urban/rural infiltration/sewer losses
ISIS FAST – spreads effective rainfall over the
Digital Terrain Model
Post-process – remove small ponds,
threshold, aggregate
National pluvial flood mapping data
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Validation
Digital Terrain Model
Digital Terrain Model
NextMap SAR
• NextMap SAR (synthetic aperture radar) DTM – most
accurate readily available national terrain data
Process: roads, buildings
• ‘bare earth’ at 5m x 5m grid
Catchment-based models
• OS Vector Map – add building thresholds (+0.3m)
• OS Vector Map – lower roads (-0.1m)
• Catchment or tile-based models?
• About 4000 catchment models
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Rainfall
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 depth-duration-frequency (DDF) parameters
• 30yr, 200yr, 200yr+20% for climate change scenario
• Storm durations: 1hr urban, 3hr rural
• Effective rainfall (at 100m grid):
5km grid of rainfall DDF
• Urban: 70% runoff, 12mm/hr sewer loss
• Rural: 55% runoff, no sewer loss
100m grid of
effective rainfall
5m grid of flood depths
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Flood spreading - ISIS FAST
ISIS FAST – spreads effective
rainfall over the DTM
• 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
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Results processing
Post-process – remove small ponds,
threshold, aggregate
• 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
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Validation
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
ISIS FAST
ISIS FAST
Reported 2002
flooding
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InfoWorks ICM
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
National Pluvial Flood Map for Scotland - Pascal Lardet
Aberdeen
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Alloa
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Coatbridge
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Dumfries
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Edinburgh West
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Forfar (Angus)
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Elgin (Moray)
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Glasgow
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Inverness
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Perth
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Stirling
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National pluvial flood mapping data set
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
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Questions / Comments? ([email protected])
Rainfall
Digital Terrain Model
FEH CD-ROM
 DDF parameters
NextMap SAR
Rainfall events: 30yr,
200yr, 200yr+20%
Storm durations
Process: roads, buildings
Rainfall depths
Catchment-based models
Deduct urban/rural infiltration/sewer losses
ISIS FAST – spreads effective rainfall over the
Digital Terrain Model
Post-process – remove small ponds,
threshold, aggregate
Validation
National pluvial flood mapping data
Thanks to Nick Burke, Mark McLaughlin & Beth Mennie of SEPA
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