1National 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

15/06/2011 2National 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

15/06/2011 3National 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 (78,000km 2 )

15/06/2011 4National Pluvial Flood Map for Scotland - Pascal Lardet Development of approach Phase 1 of project was pilot – Glasgow / Clyde / Loch Lomond (3900km 2 ) Assess data and method refinements Balance: Data, data processing, method, computational demands – accuracy requirements & programme constraints Best available national datasets Nationally consistent method

15/06/2011 5National Pluvial Flood Map for Scotland - Pascal Lardet 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

15/06/2011 6National 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

15/06/2011 7National 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

15/06/2011 8National 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

15/06/2011 9National Pluvial Flood Map for Scotland - Pascal Lardet Results processing Remove small ponded areas (<200m 2 ) 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/ 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

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

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

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

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

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

15/06/ National Pluvial Flood Map for Scotland - Pascal Lardet Forfar (Angus)

15/06/ National Pluvial Flood Map for Scotland - Pascal Lardet Elgin (Moray)

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

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

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

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

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

15/06/ National Pluvial Flood Map for Scotland - Pascal Lardet Questions / Comments? 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