Surface Water Flooding Component for NHP HIM: Phase 1 Report

Cole, S.J.; Moore, R.J.; Mattingley, P.S.. 2015 Surface Water Flooding Component for NHP HIM: Phase 1 Report. Bristol, UK, Environment Agency, 55pp. (CEH Project no. C05501, C04881, C04880) (Unpublished)

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Abstract/Summary

Following recent UK floods (summer 2007 and Cornwall 2010) and the UK Government’s Pitt Review, forecasting and warning of surface water flooding (SWF) has received much attention. To support effective mitigation actions there is a growing demand for more robust, accurate and timely forecast and alert information on surface water flooding and its impacts at local, regional and national scales. The Natural Hazards Partnership (NHP) aims to provide co-ordinated information on natural hazards from across UK government departments, agencies, trading funds and public sector research establishments. Under NHP, a Hazard Impact Model (HIM) framework is being developed and SWF is one of three initial hazards being trialled. This report on the SWF HIM outlines the plan of work and reports on Phase 1, which developed a proof-of-concept of a new approach against a background review of existing operational capabilities. In contrast to rainfall threshold based methods in present use, the new NHP prototype SWF HIM approach builds on the dynamic gridded surface runoff estimates from the Grid-to-Grid (G2G) hydrological model already employed for operational fluvial flood forecasting across Britain. Methodologies for generating dynamic maps of the possible impact are derived using national datasets of population, infrastructure, property and transport. An initial prototype approach is explained and illustrated through a case study. This is then further developed to exploit detailed information contained in the updated Flood Map for Surface Water (uFMfSW), and derived offline using design storm “effective rainfalls” as input to a surface water inundation model configured at 2m resolution. Maximum depth, velocity and hazard on a 2m grid for the Critical Storm Duration (CSD) are available for three return periods, and these are linked to impacts using the national datasets. The link to G2G surface runoff (at 1km resolution) is established by equating this to the “effective rainfall” (at 250m resolution) which facilitates a link to inundation information (at 2m resolution) and the assessment of impact at this scale. The prototype SWF HIM has assumed a fixed CSD of 3 hours as this information was not readily available within the uFMfSW. This assumption was investigated by obtaining the CSD dataset for one tile encompassing Newcastle: this showed 1 hour to be more typical, especially for higher return periods. However, the variable nature of CSD across 1, 3 and 6 hours suggests that ideally this should be accommodated in a revision of the prototype approach. If the resulting impact calculations prove too onerous, then a compromise to consider is to employ a fixed CSD of 1 hour instead of 3 hours. Four end-to-end case studies using the prototype SWF HIM are developed, the first employing foreknowledge of radar rainfall observations as deterministic input to the G2G hydrological model. The remaining three use blended ensemble rainfall forecasts as input to G2G as an offline close emulation of operational system conditions. This allows SWF hazard and impacts to be assessed - in a probabilistic, risk-based context aligned to the Flood Risk Matrix - when used in combination with the uFMfSW information and an Impact Library developed by the Health and Safety Laboratory. A comparative assessment, although not completely “like-for-like”, with the FFC’s operational SWF Decision Support Tool served to provide initial evidence on the potential advantages of the new prototype SWF HIM approach. A perspective on future developments is given, including planning for a real-time end-to-end trial using computer platforms employed by the Flood Forecasting Centre.

Item Type:	Publication - Report
Programmes:	CEH Topics & Objectives 2009 - 2012 > Water > WA Topic 3 - Science for Water Management > WA - 3.1 - Develop next generation methods for river flow frequency estimation and forecasting
UKCEH and CEH Sections/Science Areas:	Reynard
Funders/Sponsors:	Environment Agency, Centre for Ecology & Hydrology
Additional Keywords:	forecasting, hazard, impact, Natural Hazards Partnership (NHP), pluvial, surface water flooding
NORA Subject Terms:	Meteorology and Climatology Hydrology Atmospheric Sciences
Related URLs:	Funder/Sponsor
Date made live:	24 Mar 2017 11:35 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/513835

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