Steve Taylor Flood Forecasting Team Leader Anglian Region Environment Agency

Historical Context Ministerial Directive in 1996 Easter Floods 1998, Bye Report Boscastle 2004, Carlisle 2005 Summer 2007 floods, Pitt Report Quick run through the main political drivers that result in the current look of the flood forecasting service. Reference to Pitt to highlight the key outcomes for flood forecasting being recommended by Pitt. s.166 of the Water Resources Act 1991 gives the Agency specific powers to carry out works for the purposes of providing a flood warning system. In March 1996, the Secretary of State gave direction that the NRA would become the lead agency in disseminating warnings of the danger of flooding. Pitt rec 1 – monitoring g/water levels Pitt rec 2 - id areas at high risk of SW flooding Pitt IC 3 - develop forecasting tools Pitt IC 7 – assess possibility of forecasting where SW flooding will occur Pitt IC37 – EA/ MO assess options for issuing warnings for lower probablility events.

Flood Forecasting in Anglian Region 300,000 homes & businesses at risk Forecasting, Detection and Monitoring carried out centrally Issuing flood warnings, response and recovery carried out locally Brief run through the at risk population, tidal fluvial split and how the overall service is delivered.

Setting Flood Warning Trigger levels How bad will it be?/ When will it start to improve? When will flooding begin? How long will the flooding last? Severe Flood Warning trigger Flood Warning Trigger Explain how the workload is split between the Warning and forecasting teams. Explain the move from Action Thresholds to Result Thresholds. Severe Flood Warning threshold Building/ defence etc. Flood Warning threshold River Upstream outstation/ raingauge/ radar Downstream risk area “X” hours Areas define minimum lead time required e.g. time required for full dissemination/ response

Detection Weather radar network New radar planned for East Anglia Provide higher resolution data for East Anglia Highlight that in the absence of the Anglian radar quality of radar data is not reliable for use in flood forecasting © met office

Monitoring - SWANTEL Telemetry June/ July 2007 370,000 calls to outstations 31,000 alarms generated Monitoring undertaken using the Swantel telemetry system. Approximately 900 outstations, 15,000 potential alarms to be managed. Highlight Summer 2007 figures

Flood Forecasting in Anglian Region Model coverage based on risk Move to less complex, more “useful” models At least 2 methods for each location Prefer schedule to ad hoc Highlight that coverage is not total and focussed on areas with greatest impact. All locations must have contingency arrangements.

Forecasting – AFFS National forecasting shell used within Anglian. Will host any model type as long as an adapter is developed for new model types. Consists of operator client and customer webservice for viewing results

Forecasting - AFFS Forecasts potentially making use of radar forecasts out to 36 hours. Data quality not sufficient and lead times allow use of shorter lead time but higher quality radar data may be available in the near future if a new weather radar is built at Old Buckenham.

Forecasting - Rainfall Hierarchy Observed Forecast Raingauge Forecasting models will start from the rainfall input. This will be derived from a variety of sources to generate an input sequence. Inherent uncertainty in each input. Is the forecasting model poor or has the rainfall input been inadequate? Radar actuals Radar based forecast Numerical Weather Prediction

The move to Probabilistic Forecasts Warning message links forecast to response Little used in flood warning around the world Graphical backed up with text preferred Requires cultural shift by users Uncertainty has to be dealt with. Deterministic forecasts do not represent this uncertainty

Extreme Rainfall Alert Service Pilot Forecast for low probability high impact rainfall events 6 month free trial from July 2008 Open to Cat 1 & 2 responders Up to 24 hours in advance of rainfall Advisory issued at 1400 local time for the 24 hours from midnight. Early Alert issued 8-11 hours in advance and Imminent Alert issued 1 – 3 hours in advance. Available via e-mail, fax, sms and voicemail.

Extreme Rainfall Alert Service Pilot Greater than 10% probability of an extreme event occurring across a large geographical area. 30mm per hour, 40mm in 3 hours, 50mm in 6 hours.

Probabilistic Rainfall Forecasts Met Office increasingly making available ensemble forecasts. Environment Agency trialling how these forecasts could be used

Probabilistic Wind Forecasts Wind speed probabilistic forecasts used during major operation engineering works near Ipswich.

Probabilistic Surge Forecasts Surge ensembles give feel for uncertainty. Even large scatter only matters if warning thresholds are crossed. i.e. range could be a metre but only matters if the next highest threshold is crossed.

Probabilistic Flood Forecasting Each solution is equally valid, so which one is “right”?

Probabilistic Flood Forecasting Graphically represent “most likely” range.

Probabilistic Flood Forecasting Major flooding Start to map uncertainty to impact Moderate flooding Minor flooding

Deterministic Flood Warnings 25 mm in 3 hours 15 mm in 3 hours Very definite with deterministic system. Reduced lead time.

Probabilistic Flood Warnings 60% probability of > 20 mm/hr 20% probability of > 20 mm/hr 40% probability of > 20 mm/hr Greater lead time but increased uncertainty 30% Probability of threshold exceedance 70% Probability of threshold exceedance

Probabilistic Flood Warning Probability of a “Severe Flood Warning” Place greater emphasis of decision making on to local responders and public 9% 30% 55% 85% 47% Day 1 Day 2 Day 3 Day 4 Day 5

Probabilistic Flood Warning Information can be displayed in a whole range of ways

Probabilistic Flood Warning Updated map based forecasts preferred but potentially technically most difficult

Summary Forecasts are never ‘right’ Shift to probabilistic forecasts Desire to forecast surface water flooding Future decisions made according to risk