•ingford Water Floodplain Mapping - Model Study of the River Frorne 2- (Gloucestershire) Final Report ,.• Volume 1 Report EX 3191 August 1995 =I OM MN MO MO MO MO MN OM MI MN IMO MN MI MI MI MN a 'tea& Water Floodplain Mapping - Model Study of the River Frome (Gloucestershire) Final Report Volume 1 Report EX 3191 August 1995 EX 3191.Rob 07/09/9S MIIIMINMIONNIIIIMIIMINENNIMIMMINEMIMIIMINNa a ngford Water Contract This report descrthes the "Floodplain Mapping - Model Study of the River Frame" carried out by Wallingford Water and commissioned by the National Rivers Authority - Severn Trent Region. The NRA representative was Mr D Pettifer, and Wallingford Water was represented by Mr TE Parkinson and Mr RJ Millington. The HR Wallingford job number was RQR 1568. The work was carried out by Mr PG Hollinrake (HR), Mr RJ MHIlnglon (HR) and Miss HA Houghton-Carr (IH). Prepared by .9.4 x11644 r.I1159.cl 4-Pn •41 (name) (Job title) Approved by STRAwP Pials:W49. Date — —9S HR Wallingford Limited & Institute of Hydrology 7 September 1995 EX 3191.Rob 07/09/95 Water ttleord Summ ary Floodplain Mapping- Model Study of the River Frome(Gloucestershire) Final Report Report EX 3191 August 1995 In September 1993 the National Rivers Authority - Severn Trent Region commissioned Wallingford Water, the joint venture between the Institute of Hydrologyand HR Wallingford,to cany outthe "Roo*lain Mapping, Model Study of the River Frome (Gloucestershire)" study. This reportgivesa detaileddescription of the hydraubcmodelling components of the study, and provides a summary of the hydrological components which are presentedindetailin ReportEX3171. The objective ofthe study was to develop a computationalhydraulic model for the purposes ofidentifying flooded outlines for eventsof varbus returnperiods. The hydraulic modelwas constructed using the SALMON-Fmodelingsoftware,modifiedto cope with supercritical flows, and was calbrated using observed data from three events,as well as data available fromthe physicalmodelling study of the Ebley Mill gaugingstation, presented in Report EX3170. EX 3191.Rob 071D0/95 egford Water Contents Page Title page Contract Summary Contents Volume 1 Text, Tables, Figures & Appendices 1 Introduction 1 1.1 Background 1 12Terms of reference 1 13The River Frome 2 2 Hydrological investigation 4 2.1 Introduction and approach to hydrological study 4 2.2 Physical modelling of Ebley Mill gauging station 5 2.3 Data collection and processing 5 2.4 Derivation of model parameters 6 2.5 Derivation of calibration inflows and design inflows 7 2.6 Flood frequency analysis 7 3 The hydraulic model 8 3.1 General description 8 32 Availability of survey data 8 3.3 The River Frome Models 9 3.4 Model schematisation 10 3.5 Topographical data 13 3.6 Bridge data 14 3.7 Stnicture data 15 3.8 Hydrometric data 16 3.9 Initial conditions 17 4 Hydraulic model calibration 17 4.1 Calibration events 17 42 Tidal water levels at Framilode 18 4.3 Fluvial inflows 18 4.4 Calibration of Ebley Mill gauging station 18 4.5 January 1994 event 19 4.6 May 1979 flood event 21 4.7 December 1965 flood event 21 4.8 Calibration parameters 21 4.9 Discussion of calibration results 22 Design event tests and tloodplain mapping 25 5.1 Introduction 25 5.2 Test resutts 25 5.3 Method of floodplain mapping 26 5.4 Discussion of results 26 EX 3101.Roa 07/0W95 ittleordWater Contents continued 6 Sensitivity tests 30 6.1 Introduction 30 6.2 Sensitivity to flows 31 6.3 Sensitivity to structure coefficients 33 6.4 Discussion of results 34 7 Modelling of additional flows downstream of the Gloucester to Sharpness Canal and the effects of tide lock 35 7.1 Introduction 35 7.2 Modelling of flows and tides 36 7.3 . Results of additional modelling 36 8 Conclusions 37 9 References 39 Tables Table 1 Results of flood event analysis Table2 Estimation of unit hydrograph and losses model parameters by different methods Table3 Refinement of unit hydrograph and losses model parameters for subcatchrnents Table4 Breakdown of features of the River Frome sub-models Table5 Upper Frome sub-model structure elements Table6 Lower Frame sub-model structure elements Table7 Nailsworth Stream sub-model structure elements Table8 Upper Frome sub-model sluice gate openings Table 9 Lower Frome sub-model sluice gate openings Table 10 Nailsworth Stream sub-model sluice gate openings Table 11 Embankment and causeway structures included in the River Frome sub-models Table 12 Locations of model inflows for the River Rome Sub-models Table 13 Availability of calibration data for the River Frome model Table 14 Comparison of SALMON-F model and physical model levels at Ebley Mill Table 15 Comparison of observed and predicted peak water levels - January 1994 calibration event Table 16 Comparison of observed and predicted peak water levels - January 1979 calbration event Table 17 Comparison of observed and predicted peak water levels - January 1965 calthration event Table 18 Fmal roughness parameters for the River Rome Models Table 19 Fmal structure coefficients - Upper Frome sub-model Table20 Final structure coefficients - Lower Fmme sub-model Table21 Final structure coefficients - Nailsworth Stream sub-model Table22 Design event return period water levels Table23 Additional modelling downstream of Wheatenhurst Sluices - 4.88m Peak level tide Table 24 Additional modelling downstream of Wheatenhurst Sluices - 6.00m Peak level tide EX 31171.Rob 07AWDS Water tigiord Contents continued Table 25 Additional modelling downstream of Wheatenhurst Sluices - 8.00m Peak level tide Table 26 Additional modelling downstream of Wheatenhurst Sluices - 10.00m Peak level tide Table 27 Additional modelling downstream of Wheatenhurst Sluices - 10.50m Peak level tide Table 28 Structurecoefficientsensitivitytests - locations where upstream water level increases exceed 0.10m Figures Figure 1 Frome catchment and river system Figure 2 Frome basin showing locations of flow gauging stations and meteorologicalsites Figure 3 Frome basin showing sub-catchmentboundaries Figure 4 Comparisonof flood frequency plotsfor Ebley Mill Figure 5 Ebley Mill gauging station calibration results - May 1979 calibration event Figure 6 January 1994 calibration results - Cl (Persimon Homes) Figure 7 January 1994calibration results - C2 (Dudbridge Weir) Figure 8 January 1994calibration results - C3 (Thrupp) Figure 9 January 1994calibration results - 04 (Golden Valley) Figure 10 January 1994 calibration results - C5 (Egypt Mill) Figure 11 January 1994calibration results - stage at Ebley Mill Figure 12 January 1994calibration results - discharge at Ebley Mill Figure 13 May 1979calibration results - stageat Ebley Mill Figure 14 May 1979calibration results - dischargeat Ebley Mill Figure 15 December 1965calibration results - stage at Ebley Mill Figure 16 December 1965calibration results - discharge at Ebley Mill Figure 17 5 year return period flood event - stageat Ebley Mill Figure 18 5 year return period flood event - dischargeat Ebley Mill Figure 19 10 year retum period flood event - stage at Ebley Mill Figure 20 10 year return period flood event - discharge at Ebley Mill Figure 21 25 year return period flood event -stage at Ebley Mill Figure 22 25 year return period flood event - discharge at Ebley Mill Figure 23 50 year retum period flood event - stage at Ebley Mill Figure 24 50 year return period flood event - discharge at Ebley Mill Figure 25 100 year return period flood event- stage at Ebley Mill Figure 26 100 year return period flood event - discharge at Ebley Mill Figure 27 150 year return period flood event - stage at Ebley Mill Figure 28 150 year return period flood event - discharge at Ebley Mill Figure 29 Design event downstream boundary condition Figure 30 100 year water levels upstream of Framilode outfall Appendices Appendix 1 Design event maximum water levels Appendix 2 Propertiesat riskof flooding in the 100 year return period flood event Appendix 3 Sensitivitytest maximum water levels EX 3101.Rob 06/011/95 Water tigtord Contents continued Volume 2 Model layout drawings Drawing1 R/R/1568/RFL1 RiverFromemodelschematisation Drawing2 R/R/1568/RFL2 RiverFromemodelschematisation Drawing3 R1R11568/RFL3RiverFromemodelschematisation Drawing4 R/R/1568/RFL4 RiverFromemodelschematisation Drawing5 R/R/1568/RFL5 RiverFromemodelschematisation Drawing6 R/R/1568/RFL6 RiverFramemodelschematisation Drawing7 R/R11568/IRFL7RiverFramemodelschematisation Drawing8 R/R/1568/RFL8 RiverFrornemodelschematisation Drawing9 R/R/1568/RFL9 RiverFromemodelschematisation Drawing10 R/R11568/RFL10RiverFromemodelschematisatbn Drawing11 R/R/1568/RFL11RiverFromemodelschematisation Drawing12 R1R11568/RFL12RiverFromemodelschematisation Drawing13 R/R/1568/NSL1 NailsworthStreammodelschernatisation Drawing14 R/R/1568/NSL2 NailsworthStreammodelschematisation Drawing15 R/R/1568/NSL3 NailsworthStreammodelschematisation Volume 3 Long profile drawings Drawing1 R/R/1568/RFP1 RiverFrornelongprofile Drawing2 R/R115681RFP2RiverFramelongprofile Drawing3 R/R/1568/RFP3 RiverFromelongprofile Drawing4 R/R/1568/RFP4 RiverFromelongprofile Drawing5 R/R/1568/RFP5 RiverFromelongprofile Drawing6 R/R/15681RFP6 RiverFromelongprofile Drawing7 R/R/15681RFP7 RiverFrornelongprofile Drawing8 R/R/1568/RFP8 RiverFrometongprofile Drawing9 R/R/15681RFP9 RiverRome longprofile Drawing10 WR/1568/RFP10 RiverFromelongprofile Drawing11 R/R/1568/RFP11 RiverFromelongprofile Drawing12 WR11568/RFP12RiverFromelongprofile Drawing13 R/R/1568/NSP1 NailsworthStreamlongprofile Drawing14 R/R/1568/NSP2 NailsworthStreamlongprofile