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Reducing Uncertainty in River Flood Conveyance Roughness Review Project W5A-057 July 2003 DEFRA / Environment Agency Flood and Coastal Defence R&D Programme Roughness Review By KarenFisher andHugh Dawson W5A-057 DEFRA Flood Management Division and Science Directorate Ergon House Cromwell House 17 Smith Square Dean Stanley Street London SW1P 3JR London SW1P 3JH Environment Agency Rio House Waterside Drive Aztec West Almondsbury Bristol BS32 4UD July 2003 R&D ROUGHNESS REVIEW i Publishing organisation DEFRA c/oRCEG CeresHouse,2SearbyRoad, LINCOLN,LN24DW PeterAllenWilliams, R&DCoordinator,FloodManagementDivision, Tel:01522528297(GTN6209224) Email: [email protected] ©CrownCopyright July2003 All rights reserved. No part of this document may be produced, stored ina retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying,recordingor otherwise without the prior permissionof the DEFRA and theEnvironmentAgency. Statement of use This document provides informationfor DEFRA andEnvironment Agency Staff about consistent standards for flood defence and constitutes an R&D output from the Joint DEFRA/Environment Agency FloodandCoastalDefenceR&DProgramme. Contract Statement R&D ROUGHNESS REVIEW ii CONTENTS Summary vii 1. Introduction 1 2. Review of literature - methods 2 2.1 Methodsconsideredforroughness evaluation 2 2.2 Methodstraditionallyusedforvegetatedchannels 8 2.3 Othervegetationroughnessrelationships 12 2.4 Useofroughnesscoefficientsforvegetation 15 3. Review of literature - data 16 3.1 Channelsubstrate 16 3.2 Channelfeaturesandobstructions 16 3.3 Alluvialroughness 20 3.4 Channelirregularities 24 3.5 Tidalreaches 25 3.6 Vegetativefeatures 25 3.7 Vegetationgrowthandmaintenance 27 3.8 Woodydebris 29 4. Review of field data 31 4.1 Dataavailabilityandusefulness 31 Data gaps 33 4.2 Furtherdatacollectionandresearch 33 4.3 Concurrentresearchprojects 34 5. Identification of river types and vegetation morpho-types 36 5.1 RiverHabitatSurvey -hydromorphologicalfeatures 36 5.2 RiverHabitatSurvey -vegetationmorphotypes 37 5.3 Conclusions 44 6. Roughness advisor 45 6.1 Methodsanddatausedintheroughnessadvisor 45 6.2 Limitationsofthe proposedmethod 47 6.3 Definitionoftypes 48 6.4 Vegetationgrowthandmaintenance 63 6.5 Derivationofroughness values 63 R&D ROUGHNESS REVIEW iii 6.6 UpperandLower Uncertainty bands 68 6.7 Justificationofapproach 68 6.8 Summaryoftestingofsummationmethod 68 6.9 UseoftheRoughnessAdvisorintheConveyance EstimationSystem 70 7. River maintenance procedures 71 7.1 Basic principlesof plant biologyandmanagement 71 7.2 Theflowing-waterhabitat 72 7.3 Considerationoftraditionalandalternativetechniques 76 7.4 Weedcutting 78 7.5 Indirectcontrol 82 7.6 Techniquesused 85 8. Uncertainty 87 9. Bibliography 88 Tables Table1 Typicalexamplesofthe effectsofvegetationonwaterflow 9 Table2 Summaryof equationsforfloating/submergedvegetation 10 Table3 Outlineofmethodstodealwiththehydrauliceffectduetogroynes 20 Table4 Grasscoverretardance classes(USSoilConservationService,1954) 26 Table5 Listofavailablefielddatasitesandsources 31 Table6 ListofavailablefielddatasitesfromtheRHS 32 Table7 Typesofchannelandlikelyvegetationinthosechannels 38 Table8 DescriptionofRHSvegetationmorphotypes 39 Table9 Generalisedcriticallowervelocity for plant growth,typicalmeanvelocity and upperwashoutmeaninchannelvelocitiesforRHSvegetationmorphotypes44 Table10 Valuesofroughnessfor differenttypesof bedmaterial 49 Table11 Valuesofunitroughness forirregularitiessuchas bouldersandpoolsand riffles 51 Table12 Valuesofunitroughness forirregularitiessuchas urbantrashandgroyne fields 52 Table13 Tableofroughnessvaluesforaquaticvegetationby RHSmorphotype 53 Table14 Valuesoftotalchannelresistancefor NorthAmericanchannels 54 Table15 Valuesofunitroughness formanmade bankmaterials 55 Table16 Valuesofroughnessfor banksidevegetation 56 Table17 Valuesofroughnessforfloodplaingroundmaterial 57 R&D ROUGHNESS REVIEW iv Table18 Valuesofroughnessforfloodplainirregularitiesandobstructions 58 Table19 Valuesofroughnessfor grassatdifferentretardanceclasses 59 Table20 Alternativevaluesofroughness for grass 60 Table21 Valuesofroughnessfor arangeofdifferentcrops 60 Table22 Valuesofroughnessfor cleananddirtyhedgerowsonthefloodplain 61 Table23 Valuesofroughnessfortreesandshrubs 62 Table24 CurrentControltechniquesforaquaticandmarginalvegetationbyRHS vegetationmorphotype,withcommentsonconsequencesandrecovery 80 Figures Figure1 ThevariationinManning’snvaluesfor bankfull dischargesinEnglishand NorthernIrelandrivers 7 1/2 Figure2 At-a-sitevariationsof(8/ f) withrelativesubmergenceifd/D 84 forfour combinationsofchannel slopeandstandarddeviationof bedmaterialsize distributionfor boulder-bedrivers(Bathurst,1994) 18 Figure3 Bedforms,ascategorisedbySimonandRichardson(1960) 23 Figure4 DistributionofRHSchannelvegetationtypesandspecies 43 Figure5 Exampleofasandbank 46 Figure6 Variationsinroughness valueswithdepthforthe RiverBlackwaterin NorthernIreland 47 Figure7 Exampleofatypicalweedgrowthcurve withandwithoutweedcutting 79 Appendices ReferenceTablesofdata used 134 Explanationofvariationofroughnesswithdepth 198 FullAbstractsRiver Flow2002 200 R&D ROUGHNESS REVIEW v LIST OF SYMBOLS A Channelcrosssectionalarea(m 2) C Chezy coefficient(m 1/2 /s) D Pipediameter(m) d Waterdepth(m) d16 Sedimentsize bywhich16% by weightisfinerthan(mm) d35 Sedimentsize bywhich35% by weightisfinerthan(mm) d50 Sedimentsize bywhich50% by weightisfinerthan(mm) d84 Sedimentsize bywhich84% by weightisfinerthan(mm) d90 Sedimentsize bywhich90% by weightisfinerthan(mm) f Frictionfactor g Gravitationalacceleration(m/s 2) k ks Roughnessheight(mm) Kva Vegetationcoveragecoefficient N Manning’s roughnesscoefficient nb Roughboundary coefficient Q Discharge R Hydraulicradius Re Reynoldsnumber Sf Energyslope Sw Watersurfaceslope V Meanflowvelocity(m/s) U* Shearvelocity µ Dynamicviscosity ρ Densityof water(kg/m 3) R&D ROUGHNESS REVIEW vi SUMMARY Under their joint R&D programme for Flood and Coastal Defence, DEFRA/Environment Agency are fundinga TargetedProgramme of research aimedat obtaining better predictions of floodwater levels. Inorder toachieve this,advances in knowledge andunderstanding made over the past three or four decades inthe estimation ofriverconveyancewill needtobeintroducedintoengineering practise. The project relates particularly towater level estimation,leading toa reductioninthe uncertainty in the prediction of flood levels and hence in the flood risk, and consequently facilitating better targeting of expenditure.The project will equally benefit the targeting of maintenance by providing better estimates of the effects of vegetation and its management. It is expected that the application of this knowledge from UK engineering research will have aninternational impact throughimproving the methods availabletoconsultants. The above objectives will be achieved through two core components of the new Conveyance Estimation System (CES) that will be developed: the Conveyance Generator and the Roughness Advisor. The CES is to be designed so that new knowledge from a parallel Strategic Programme of researchcanbe integratedintothe CESinduecourse. This report is the designatedoutput for TaskT4of the TargetedProgramme of research. The Task objective is to gather, validate and catalogue knowledge on the boundary roughness of UK rivers. The information obtained will then be incorporated in the CES, comprising the Roughness Advisor component. The techniques and underlying assumptions usedby the Roughness Advisor indetermining the boundary roughness of UKrivers aredescribedinthisreport. A review of the informationidentifiedinthe expert paper inthe Scoping Study of this project is provided. Inadditiona wider range of papers are reviewedincluding papers thathave beenpublishedorsourcedsincetheexpert paperwascompleted. There are a number of key references where the hydraulic data available is extensive. These papers have beenidentifiedandthe data withinthe papers forms the basis of the roughness review. Some of the data sets alsohave associated vegetationdata andthis is used alongside the hydraulic data to determine the roughness of vegetated and non vegetatedchannels. The report also incorporates a large body of knowledge related to vegetation in and aroundUK waterways andthe effect of this vegetationonwater flow. The various river types and vegetation morphotypes have been identified along with any information providedonsubstratecharacteristics. This roughness review document reviews the various methods and techniques of estimating roughness. The approach selected for use within the roughness advisor is basedonManning’s nroughness.A unit roughness is appliedfor different types across the channel andfloodplainandcombines them in a suitable manner withina type.The combinedroughness value is thenfedintothe conveyance generator. The processes R&D ROUGHNESS REVIEW vii and justifications for selection of this technique and the data used in the roughness advisorare given.Data gaps andfurtherresearchneedsaresuggested. In addition to the review of channel and vegetative roughness, information is also incorporated on river maintenance procedures. This includes the basic principles of plant biology andmanagement; traditional andalternative techniques of plant control, alongwithadviceonweedcutting. Overall this roughness review report provides the technical background informationfor theRoughnessAdvisor. R&D ROUGHNESS REVIEW viii 1. INTRODUCTION This document gives the background tothe papers andinformationaccessed,collated
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