Delft-FEWS User Days 2 nd & 3 rd November 2011

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1 Delft-FEWS User Days 2 nd & 3 rd November 2011 Recent developments in EA Midlands Flood Forecasting using FEWS NFFS Ian Clayton Flood Forecasting Technical Specialist Environment Agency Midlands Flood Forecasting Team

2 Agenda Introduction to Midlands Flood Forecasting Developments in EA Midlands 2010/2011 Lower Severn ISIS Mini-Ensemble Modelling MCRM Snowmelt Flood Forecasting Historic Forecast Performance Tool (HFPT) Further developments planned for 2012/2013 Winter/Summer ISIS Mini-Ensemble Modelling on River Soar G2G unmeasured lateral inputs into ISIS models Further developments in HFPT

3 Midlands Flood Forecasting Properties in flood risk areas = 170,000 (approx) Length of main river = 3,873 km Length of defences = 1,460 km Number of structures = 4,500

4 Midlands Flood Forecasting Models Midlands Catchment Runoff Model (MCRM) Conceptual model Uses catchment average rainfall (observed/forecast) Models reservoir storage and spill Models snow pack development and melt 142 Midlands Routing Reach Model (DODO) Uses MCRM as inputs Routes water upstream to downstream Operates on principle of conservation of mass (i.e. volume conservation is expressed as a simple water budget approach, with lag and attenuation) features) Based on Muskingham Storage Function (i.e. channel volume storage relates to the reach inflows and reach outflows to each reach) 119 Hydrodynamic ISIS Models Use MCRM and DODO as upper boundaries 6 operationally (26 forecasting nodes) 6 under development

5 Lower Severn ISIS Mini-Ensemble Modelling Bewdley (fluvial) to Avonmouth (tidal) Large inflows of Stour, Teme and Avon 130km Implemented operationally in 2007 Performed well during high flow events in 2007/ saw a number of bankfull/lower flood warning only events Performed poorly during lower events Why?

6 Lower Severn ISIS Mini-Ensemble Modelling Further investigations: Channel roughness changes over time? Generally smooth silt channel Vegetation (trees/bushes) just out-of-bank increasing roughness Bankfull/just out-of-bank is where operational decisions are needed (i.e. barrier erection) Existing model was not calibrated/verified at these lower levels, only for higher magnitude events

7 Lower Severn ISIS Mini-Ensemble Modelling Existing model above 4.6m

8 Lower Severn ISIS Mini-Ensemble Modelling New in-bank model below 4.6m

9 Lower Severn ISIS Mini-Ensemble Modelling Average absolute errors for both models

10 MCRM Snowmelt Flood Forecasting History MCRM has built-in snow paramaters used successfully in old Midlands FFS Recent winters have been very cold with large accumulations of snow at times Problems arose in MCRM editing snow parameters incorrect snow packs developing forecast temperature fixed value FFC required impact of snow melt as did Area Flood Warner s/operational staff/management Largest flood in Midlands in 1947 caused by snow melt

11 MCRM Snow Accumulation - Theory Precipitation recorded in rain gauge Snow is melted by rain gauge heaters Temperature gauges used to convert back to snow Snow pack accumulates Further snow adds to snow pack Model sets fresh snow density to 10%

12 Snow Pack Ripening - Theory Temperature gauges used to ripen pack Density moves from 10% to 35% Above 35% snow melts WE = density x (snow pack depth) i.e. approx 10cm fresh snow = 0.10 density or 10mm WE approx 10cm ripe snow = 0.35 density or 35mm WE Ripening of pack relies on correct temperatures (observed & forecast) Used in conjunction with forecast rainfall

13 Example Forecast fluvial response (snow + rain) Observed precipitation Observed snow pack Forecast snow pack melt Snow pack builds Observed temperature Is this Correct? Forecast temperature

14 Problems & Solutions How accurate is forecast temperature? Need to assess over winter Snow observation resolution Recruitment drive Use Met Office snow observation network Snow not melted by rain gauge heaters Edit snow packs manually Double counting of snow Decrease edited snow pack by WE observed snow melt Use Flood Forecasting Centre/Met Office products (i.e. G2G) EA climate station network Use Met Office observed temperature network

15 MCRM Snowmelt Flood Forecasting Summary Relatively cheap and semi-automated Takes less than 2 minutes for whole Midlands Only needed for a few months a year Better understanding of site specific flood risk Use in conjunction with: 15 minute MCRM functionality Reservoir MCRM functionality

16 Historic Forecast Performance Tool (HFPT) Used for fluvial flood forecasting Provides estimates of uncertainty of the deterministic forecast time series as a plume Uncertainty estimates derived from past performance (usually offline studies) Accounts for ALL sources of uncertainty, including rainfall forecast if all sources were included in the offline studies Can be applied to updated / error corrected forecasts

17 Generating HFPT Uncertainty Estimates Forecast error conditioned on: Magnitude of forecast Lead time Estimates error quantiles through Quantile Regression (QR) Requires long, homogenous data set of forecast and observed discharge or stage at each location In principle, need to rerun the analysis if any component of the forecasting cascade changes

18 Quantile Regression

19 Case Study: Welshbridge Real event run in hindcast February 2011 Routing reach on River Severn at Shrewsbury in Shropshire (2288km 2 ) Susceptible to heavy prolonged rainfall over high ground in headwaters of Severn/Vyrnwy in Powys 2 day travel time from headwaters to Shrewsbury Temporary/demountable barrier site

20 Lead-time = 29 hours

21 Lead-time = 17 hours

22 Lead-time = 8 hours

23 Case Study Summary The 50%ile and actual forecast gave good advice at over 24 hour lead-time on the first forecast (i.e. both were showing at least 3 phases of deployment needed) Even with the error in 2 nd forecast the barriers would have been on site and the 3 rd forecast with 8 hour lead-time backed up the need for 3 phases New technique. Currently under development: River Severn (whole length) River Dove and Tame (tributaries of River Trent) All 7 Flood Forecasting Teams developing Develop other techniques such as cost/risk ratio matrix

24 Future Development 2012/13 Winter/Summer ISIS Mini-Ensemble Modelling on River Soar Why? Channel/flood plain weed/vegetation growth (May Oct/Nov) Climate change = increase extreme rainfall during summer? Flood risk high Solution? Mini-ensemble using separate ISIS models Winter rating (Oct/Nov Apr) Summer rating (May Oct/Nov) Run at the same time Forecaster decides which one to use

25 Example Direct Flow Verses Rated Flow

26 G2G unmeasured lateral inputs into ISIS models Currently: Use large ( km) unmeasured lateral MCRMs Source of uncertainty in DODO/ISIS forecasts G2G? physical-conceptual distributed, grid-based runoff production and routing model (FFC Quarter One Report February 2011) Can provide a discharge forecast in 1km² grids across UK Use smaller G2G catchments as input into DODO/ISIS on River Soar as a trial More accurate representation???

27 Questions?