Bell, Vicky

Cole, Steve

Howard, Phil

Moore, Bob

Robson, Alice

English

NERC Open Research Archive

Probabilistic Flood Forecasting for Small Catchments using the G2G ModelSteve Cole, Alice Robson, Phil Howard,  Vicky Bell and Bob Moore Centre for Ecology & Hydrology, WallingfordDistributed Model (G2G)Lumped ModelLumped and Distributed hydrological modellingGauging Hydrological modelling• One model for each gauging station• Many parameters calibrated to observed flow location• Flow estimates for one location only• Uses catchment average rainfallstation• One model for large regions (UK)• Small set of regional parameters, strong support from digital datasets• Flow estimates in each grid (1km2)• Uses gridded rainfall estimatesNeed for distributed models of flood response355000 360000 365000 370000 375000415000420000425000430000435000Storm total4300004350001Flow m3s-1Lumped ModelDistributed (G2G) Model― Catchment-wide stormImpact of spatial extent and location of storm on flood response?355000 360000 365000 370000 37500041500042000042500043355000 360000 365000 370000 375000415000420000425000430000435000mm hr-1 mmFlow m3s-1Time (days)Hyetograph― Lower catchment storm― Upper catchment stormMoore et al. (2006), IAHS Pub. 305Darwen at Blue Br. (135km2)• Distributed hydrological models offer a natural approach to area-wide flood forecasting that includes small catchmentsBUT:• What rainfall estimates and forecasts should be Science QuestionsMotivationused?• How to formulate area-wide distributed modelsfor operational use in flood forecasting?• How do these area-wide models perform at small gauged and ungauged locations?Gridded rainfall estimators: examples• Using HameldonHill radar in North-West England• Two relatively River Kent Case StudyRiver Kent at Sedgwick (212km2)steep uplandcatchments • Strong topographic control on flow responseCole and Moore (2009), AWRDarwen at Blue Br. (135km2)Gridded rainfall estimators: examples• River Kent catchment, orographic event, 3 Feb 20042km Nimrod Raingauge-onlyGauge-adjustedRaingaugeRiver Kent Case Study: Gridded rainfall estimators15 min totals15 min totals• River Darwen catchment, convective event, 14 June 20021km raw radar Raingauge-onlyGauge-adjustedRiver gaugeGrid-to-Grid distributed model (G2G)Surface flow-routingPrecipitation EvaporationRiver Kent Case Study: G2G model• Uses digital spatial datasets (e.g. terrain)• Responds to spatial variation of rainfall input• Grid-to-Grid routing using Kinematic Wave schemeSaturation-excess surface runoffDrainageRiverSubsurface flow-routingReturn flowRiver flowRunoff- producing soil columnG2G routing: use of terrain data1. Flow directions:apply automated method to 50m DTM to infer 1km flow-paths2. Catchment boundary delineation: inferred Elevation m945 0River Kent Case Study: G2G model!Sedgwickfrom flow-path directions3. Land/river designation: drainage area + river length thresholdVictoria BridgeMint BridgeSprint MillBowston4. Select forecast locations: gauged or ungaugedRaingauge-onlyGauge-adjusted radarGridded rainfall estimators2km radar dataImproved rainfall estimatesValidated by hydrological modellingNovel multiquadric surface fitting method (Cole & Moore, 2008)G2G model assessment of rainfall estimatorsDistributed model (G2G) Observed Rating curve maximum15 min totals(70 km2) (35 km2)(66 km2) (185 km2)-1Flow m3s-1River Kent Case StudyElevation m945 0Victoria BridgeMint BridgeSprint MillBowstonG2G model assessment at ‘ungauged’ sites(212 km2)Flow m3s-Flow m3s-1Time (days)Observed flow Model flowModel baseflowUpper limit of rating equation!Sedgwick• G2G model calibrated at Sedgwick only• 15-min raingauge data used• Comparable results at ungauged sitesHydrological flood forecasts using NWPCollaboration with the Joint Centre for Mesoscale Meteorology, EA  and CEH using the Carlisle 2005 Floods (uses the PDM model).Q: Can new 1 or 4 km NWP rainfalls provide reliable flood forecasts?A: Yes, for the Carlisle floods (orographically enhanced frontal rain)Flood forecasting using high-resolution NWP1 km4 km12 km (2005)4 km (20 8)1 1112 km Caldew (246km2)Time (hours)Flow m3s-1RaingaugeFlood Warning LevelRoberts, Cole et al., 2009, Met. Apps12 km 4 km NIMROD radarFlood forecasting using high-resolution NWPBoscastle 2004 case study20 km radius from BoscastleForecasts from 03 UTCCourtesy Nigel Roberts, JCMM (Met Office)12 km 1 km NIMROD radarFlood forecasting using high-resolution NWPBoscastle 2004 case study• 1 or 4km NWP major improvement over 12km product• Still uncertainty in NWP rainfall intensities and location20 km radius from BoscastlePeak accumulationsup to 50mmCourtesy Nigel Roberts, JCMM (Met Office)Forecasts from 03 UTCEnsemble Flood Forecasting using G2G1km NWP pseudo-ensembleG2G Model 1kmriver flow ensembleComparison with river flow observations• Simple psuedo-ensemble method developed to capture NWP uncertainties. Genuine ensembles will be available in 2012(?) Flood forecasting using high-resolution NWPAcknowledgements:Collaboration with JCMM (Met Office)Probability of exceeding a given flow threshold, for a given forecast horizonRisk Map of flood exceedance using G2G ensembles and Q(T) flow return period gridsThis example employs:Flood forecasting using high-resolution NWPAcknowledgements:Collaboration with JCMM (Met Office)920 km2• NWP 1km rainfall pseudo-ensemble• 10 year return period flow thresholds• 24 hour forecast horizonPotential to identify flood risk hotspotsSTEPS 6-hour spatial rainfall forecast0900 to 1500 20 July 200720 ensemblesAvon & Tame (Midlands) catchmentsEnsemble 1 Ensemble 2Ensemble 3 Ensemble AverageMidlands Case Study – 20 July 2007HyradK raingaugeSTEPS6 hrsZero rainfall (padding out)130 km2Radar Composite HyradK raingaugeEnsemble average rainfall is less than raingauge rainfall but higher than radarEnsemble hydrographsobserved flowmodelled flow using raingauge rainfall20 flow ensembles using STEPSzero rainfall used beyond 6hr STEPST+0 T+620 6-hour STEPS ensemble rainfall forecasts in G2G 20 July 2007ObservedModelled20 STEPS ensemble members G2G ensembles using STEPS forecastsMidlands Case Study – 20 July 200791 km293 km2185 km293 km220 July 2007Traditional ensemble outputs at gauged locations130 km274 km275-100%10 year return period flood threshold6-hr STEPS forecasts then zero rainfall20 STEPS Members09:00 20 July 2007 originAvon & Tame (Midlands) catchments T+3 hours T+6 hoursProbability of exceedance flood mapsMidlands Case Study – 20 July 2007Key indicates probabilities of (number of members) exceeding the 10-year flood.During early part of storm, highest exceedance probabilities are on the very small rivers.As time progresses the main exceedance hotspots are on the larger rivers and can be tracked moving downstream and meeting at confluences.50-75%25-50%10-25%2-10%T+12 hours T+18 hours• Several EA/Defra R&D projects recommended nationwide operational trial of G2G for flood forecasting– 2004-06: Extreme Event Recognition Phase 2 (FD2208)– 2005-07: Rainfall-runoff and other modelling for ungauged/low-benefit locations (SC030227)– 2007-10: Hydrological modelling using convective scale rainfall National application of G2GNational application of G2Gmodelling (SC060087)• Pitt Review of the Summer 2007 floods– Environment Agency/Met Office Flood Forecasting Centre (FFC) for England & Wales, opened April 2009– Scottish Flood Forecasting Service (SFFS) between SEPA/Met Office opened 2010– G2G now undergoing operational trials in FFC and SFFSRunoff production key element – needs to reflect heterogeneous soil propertiesUse of Soil Survey data (HOST, Seismic, other…) to obtain 1km grids of:G2G runoff production:use of soil property associationsNational application of G2G• water content at field capacity• residual soil water content• porosity• saturation hydraulic conductivity• horizon depthIssues:ScaleEffective valuesLateral propertiesAssociation table links29 HOST soil classes to soil propertiesBell et al. (2009), JoHMoore et al. (2006), IAHS Pub. 305G2G national applicationNational application of G2G• G2G runs nationally within NFFS/FEWS using a 15 min time-step and models river flow and soil moisture on a 1km grid• Ongoing operational trial and assessmentRaingauge-adjusted radar River flow Soil moistureExamples of catchments with generally good G2G performanceJanuary & February 2008ObservedModelled93 km253 km269 km2 357 km2Demonstrates modelling of different flow regimes and catchment sizes with the G2G Model352 km2191 km29,962 km2G2G Operational Use: Data AssimilationNational application of G2G• State Correction– slow adjustment that improves long-term baseflow modelling– good quality flow data needed– adjusts upstream soil storages Simulation+ State Correction• Flow Insertion– observed flow fed in at each gauged location with good data– permits  ARMA forecast correction• Flow Insertion and Local Parameter Calibration – Flow insertion allows nested catchments to be calibrated independently of upstream modelling (e.g. use lake outflows)– River routing speed can be calibrated for each sub-catchment+ State Correction + Flow InsertionNational application of G2GG2G Model performance by area050100150200250300350<100 100 - 250 250-1000 >1000FrequencyCatchment Area (km2)Gauged catchments in NFFS • About 45% of the gauged catchments in NFFS have an area <100km2• Results for spring 2008 with state 00.20.40.60.81<100 100 - 250 250-1000 >1000R2model efficiency (pooled)Catchment Area (km2)G2G Model PerformanceR2model efficiency (pooled)R2model efficiency (pooled) correction on No insertion + global paramsLarger catchments tend to perform slightly better Flow insertion + global paramsMost benefit for large catchments Flow insertion + local paramsMost benefit for small catchmentsNational application of G2GG2G Model performance by area• National G2G performance maps for spring 2008Area <100km2All stationsR2 Summary and conclusions• G2G Model:– sensitive to spatio-temporal structure of storms– shapes flood hydrograph from storm and landscape properties – Q(T) grids allow mapping between G2G flows and flood return periods – indication of severity• National application of G2G for flood forecasting:– results show utility for small catchments and performance improves with catchment size– high-resolution (4 or 1km) NWP provides better rainfall and flood forecasts and indicative flood warnings for the next few days– data assimilation greatly improves forecast performance– can produce real-time flood risk maps, if used with ensemble rainfall forecasts: important for small catchments

Probabilistic flood forecasting for small catchments using the G2G model

http://nora.nerc.ac.uk/14480/1/Cole_et_al_BHS_ICE_SW_17_June_2011.pdf

Probabilistic flood forecasting for small catchments using the G2G model

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