Forecasting Flood Risk at the Flood Forecasting Centre, UK. Delft-FEWS User Days David Price

Transcription

1 Forecasting Flood Risk at the Flood Forecasting Centre, UK Delft-FEWS User Days 2012 David Price

2 Overview of the Flood Forecasting Centre (FFC)

3 What is the FFC? Partnership between the Met Office and Environment Agency Created following the Pitt Review Operational since April 2009, delivering 24/7 services to government and emergency responders Forecasts for all sources of flooding: river, tidal & coastal, surface water and groundwater Combining expertise in meteorology and hydrology Providing best possible intelligence and early warning of potential flood impacts

4 Our Customers Environment Agency National Government Emergency Responders Blue Light Services (Police, Medical and Fire) Local Authorities Utility Companies Public

5 Our Services For the Environment Agency Forecasts and warnings UK Coastal Monitoring & Forecasting Service Consultancy For Emergency Responders Flood Guidance Statement Advice and guidance Flood Advisory Service For the Public Three Day Flood Risk Forecast

6 Communicating Flood Risk

7 Communicating Flood Risk Our flagship product for communicating flood risk is the Flood Guidance Statement (FGS) Covers flooding from rivers, surface water, tidal and coastal and groundwater Issued routinely at 10:30 daily to over 2200 recipients The FGS draws on a number of sources to arrive at a level of flood risk for each county across England and Wales, to provide the best possible intelligence and early warning of potential flood impacts

8 Communicating Flood Risk - The Flood Guidance Statement (FGS) Flood Guidance Statement - provides an overview of flood risk across England and Wales over 5 days and identifies developing situations Level of flood risk assigned according to a matrix Flood Risk = likelihood x impact

9 Communicating Flood Risk assigning flood risk Flood risk determined using a probabilistic approach Hydrometeorologist employs a subjective analysis of: NWP models (deterministic and ensemble) Guidance from the Met Office chief forecaster Raingauge & radar data FFC's in house grid based rainfall/runoff - routing model (G2G) EA regional flood forecasting team's catchment rainfall/runoff - routing models Pre-determined rainfall depth-duration thresholds (esp rapid response / surface water impacts) Close liaison with EA regional flood forecasting teams (catchment sensitivity etc)

10 Communicating Flood Risk assigning flood risk Hydrometeorologist assigns a level of FLOOD RISK, on a county by county basis: High Likelihood Very Low Impact VERY LOW OVERALL FLOOD RISK A x = B Low Likelihood x High Impact = MEDIUM OVERALL FLOOD RISK A B Customer s response Customer s response

11 Examples from Summer 2012

12 Summer 2012 not just a typical British summer! An exceptionally wet summer across England and Wales (2nd wettest in a series from 1910). Wettest June since 1766 (shared with June 1860). July was also wet, with many areas seeing over 150% of normal rainfall. This resulted from frequent areas of low pressure over or near the UK, driven by a jet stream which was further south than usual. Fire Crew rescue bus driver from flood at Gayton, Staffordshire. Image: BBC news.

13 Summer 2012 not just a typical British summer! Will focus on 2 events approximately one week apart. The first was driven by persistent, heavy rainfall NW England and Yorkshire 22 and 23 June The second was driven by short term, intense rainfall Midlands and NE England 28 June

14 NW & Yorkshire 22 and 23 June Slow moving occlusions wrapped around a slow moving low pressure system brought prolonged, heavy rain to northern England. Large scale system with good forcing, little convective component. Little confidence about location and persistence early in the event, but confidence that heavy rain will affect parts of England and Wales. 24 hours rainfall totals typically 50 80mm, with 100mm+ in places over 48 hours. Over 200mm of rain recorded in 48 hours at Honister raingauge, Cumbria.

15 NW & Yorkshire 22 and 23 June 00Z 22 nd 06Z 22nd 12Z 22 nd 18Z 22 nd 00Z 23 rd 06Z 23 rd

16 NW & Yorkshire 22 and 23 June Observed flow Sat 23 June Fri 22 June Thu 21 June Wed 20 June Tue19 June Henthorn gauging station, River Ribble.

17 NW & Yorkshire 22 and 23 June Observed flow Sat 23 June Fri 22 June Thu 21 June Wed 20 June Tue 19 June Samlesbury gauging station, River Ribble.

18 NW & Yorkshire 22 and 23 June Forecast rainfall

19 NW & Yorkshire 22 and 23 June

20 NW & Yorkshire 22 and 23 June FGS Performance 1217 properties reported flooded from river and surface water Flooding affecting communities Disruption to the transport network

21 Flood Events Midlands and Newcastle 28 June Spanish Plume type system Very warm, moist and unstable air lots of potential for intense rainfall. Entirely convective. Required a number of elements of forcing to come together at once Very similar situations can lead to little or no rainfall.

22 Flood Events Midlands and Newcastle 28 June Widespread 20-30mm in an hour across Midlands and northern England 58mm in 4 hours recorded in the Newcastle area (NE England) Hailstones in Leicestershire

23 Flood Events Midlands and Newcastle 28 June Thu 28 June Observed flow Tue 26 June Wed 27 June Harwood gauging station, Harwood Beck

24 Flood Events Midlands and Newcastle 28 June Thu 28 June Observed flow Wed 27 June Tue 26 June Woolsington gauging station, River Ouseburn

25 Flood Events Midlands and Newcastle 28 June Gridded flow data as an output from G2G Gridded flow data, 17:00 28 June 2012 Tends to over-estimate in upper catchments Can be used to signal location of response to rainfall Useful for convective situations where location of heaviest downpours cannot be forecast accurately

26 Flood Events Midlands and Newcastle 28 June

27 Midlands and NE England, FGS leading up to the event June 28

28 Flood Events Midlands and Newcastle 28 June FGS Performance 603 properties reported flooded from river and/or surface water Disruption to the transport network - A1 road closed / flooding of Newcastle train station and metro

29 Current Developments

30 Current Developments - Probabilistic forecasts Deterministic forecasts unable to represent the uncertainty in the rainfall (and consequent hydrological output) Capturing uncertainty in the rainfall forecast is essential to an effective risk based approach This is currently captured through a subjective analysis of rainfall inputs, catchments states etc Current development in the NFFS-FFC implementation of short range high resolution ensemble data (12 members): 2km, 15 minute resolution to t+7 hours 2km, hourly resolution to t+24 hours This will become a 24 member ensemble in December, time lagged to 48. In 2013 a 5 day ensemble forecast will be configured, with a resolution of 4km.

31 Current Developments Probabilistic forecasts (an example) Effective early warning for extreme rainfall events. Usually convective in nature.

32 Current Developments Probabilistic forecasts (an example) Rainfall ensembles currently under test for rapid response catchment application early warning Ensembles generate probability of exceeding 10mm/1 hour and 30mm/6 hour thresholds - SW England Colour coded according to likelihood of exceeding thresholds Feed this information back to the Environment Agency flood forecasting teams Currently CAT avg, will configure for CAT max Thresholds to be revised

33 Summary and Key Messages

34 Summary Hydrometeorologist analyses a number of data sources to derive a level of flood risk on a county by county basis For the most part this is currently a subjective analysis Now configuring ensemble rainfall data sets to provide more robust techniques for forecasting probabilities These are unlikely to used in isolation The customers will have to interpret these probabilities in their risk based decision making processes

35 Thank you