BANK PROTECTIONUSING EMERGENTPLANTS AGATNST BOAT WASH IN RIVERS AND CANALS A J Bonham ReportNo. IT 206 December1980 Crown Copyright HydraulicsResearch Station Wallingford Oxon oxto 8BA Telephone 0491 35381 SYNOPSF Field test results are presentedof the boat wash from recreationalmotor boats in the summeron the River Thamesnear Wallingford. A speedlimit of two metresper secondif successfullyimposed would substantially eliminate boat wash that exceededwind wavesin height on this reach of the River Thames. Field test resultsare presentedof the attenuation of the ship wavesfrom motor boat wash in beds of:- Phragmitesaustralis (syn. P communis) common or Norfolk reed; Schoenoplectuslacustris (syn. Scirpuslacustris) common clubrush, greaterrush or bulrush; Typha angustifolia, lesseror narrow leavedreed mace; and Acorus calamus,sweet flag. Under suitable conditions of depth, vegetationdensity, and a bed slope of one in four, two metresof width of bed of any of thesespecies will dissipatealmost two thirds of the shipwaveenergy. Bank protection is proposedagainst boat wash for gravel and mixed bed rivers and also for fen and Broadland rivers with high motor boat usage. The submergedtoe protection utilises old tyres, with the advantageof fend off effect. The beds may then be shapedand re-establishedwith mixed speciesof emergentriver plants to give bank protection and to provide a scarcenatural habitat. This report was preparedby Mr A J Bonham during his temporary attach- ment to the Hydraulics ResearchStation. The views expressedand the conclusionsdrawn are those of the author; they do not necessarilyreflect the official views of the Station. SYMBOLS b subscript, breaking depth C wave velocity (celerity) E total energyin one wave length per unit of crest width E" energydensity per unit surfacearea in a wave profile E total averageenergy per unit of surfacearea f wave frequency g gravitational acceleration h ship wave height I- mean of five highestship waves h-* maximum ship wave height H wave height H-"' maximum wave height I shockimpulse I inner count zoneor probe k constant L wave length L" boat hull waterline length n ratio of group velocityto individual wavevelocity O outer count zoneor probe p probe T wave period U forward momentum per unit of wave crest V, boat speed Q massdensity of water CONTENTS PAGE 1 INTRODUCTION THE RIVER THAMES AND THE NORFOLK BROADLANI) RIVERS, EXAMPLES OF RIVERS WITH BANK EROSION FROM BOAT WASH I The River Thames 1 Broadland rivers 3 BOAT WASH ON THE RIVER THAMES AT WALLINGFORI) BOAT WASH ENERGY REDUCTION THROUGH BEDS OF EMERGENT RIVER PLANTS 5 Tests on the River Thamesat TVallingford 5 Notes on parametersaffecting shoalingwave energy 7 Boat wash energyreduction 8 5 PROPOSED PROTECTION AGAINST BOAT WASH 6 ACKNOWLEDGEMENTS 7 REFERENCES 10 8 BIBLIOGRAPHY 1l TABLE I Boat wash energyreduction through beds of well surviving emergentriver plants in the River Thamesnear Wallingford FIGURES I Bank protection againstboat wash for gravel and mixed bed rivers with high motor boat usage Bank protection againstboat wash for Fen and Broadland rivers with high motor boat usage Plan of River Thamesat Wallingford CONTENTS (Cont'd) 4 Mean of five highestship waves(1) 5 Maximum height of ship waves(1) 6 Mean of five highestship waves(2) 7 Maximum height of ship waves(2) 8 Cross sectionsof marginal beds of river plants 9 Details of river plants 10 Energy reduction, mean of five highestship waves 11 Energy reduction, maximum height of ship waves 12 \ilave profile of boat wash PLATES (seeFig 3 for locations on River Thames) 1 Right bank below Wallingford Bridge in 1910,showing emergentand floating leavedrooted river plants 2 Right bank below Wallingford Bridge in 19E0showing extensivebank ero- sion 3 Right bank below Wallingford Bridge in 1E90showing thick standsof emergentriver plants 4 Right bank below Wallingford Bridge in 19E0without river plants 5 'Wide' recreationalmotor boat on the River Thamespassing the Hydraulics ResearchStation wave recorder 6 'Narrow' and 'wide' recreationalmotor boatsat Section2 showingwave probes and bed of Schoenoplectuslacustris, common clubrush or bulrush 7 Recreationalmotor boat at Section4 showing wave probes and bed of Acorus calamus,sweet flag E Section5, eroded unvegetatedspending beach with wave probes and breaking boat wash INTRODUCTION 1 Rivers and canalsare a magnificent recreationalresource for peoplepro- viding opportunities for angling, boating, ornithology, bank side rambling and many other pastimes.They are also required to efficiently discharge flood water, to preservedrainage levels, to provide convey and store water supplies,to dilute effluents, to support fisheriesand sometimesto carry freight. Thesewatercourses are also a most significant and scarcepart of the natural ecologysupporting very rich plant and animal communities.A great responsibilityexists to preservethe environmentin the bed and banks of channelsas a habitat and a refuge. In recent yearsthe recreationaluse of rivers and canalshas increasedand the wash from motor boats is eroding the banks. Where the waterway systemis sufficiently extensiveto support numerousholiday motor cruisers the high traffic density and long seasonalduration has led to a significant river widening in somelocations or expensivebank protection. Examples of waterwaysunder stressare the River Thames,the Norfolk Broadland riversand someof the riversand canalscontrolled by the British Water- waysBoard. In riversand canalsthe relationshipis profound betweenmotor boat usage,river plant survival and bank protection. Where the usageof motor boats is small plant growth may be excessiveand will require weedcontrol by cutting or herbicidesor dredging. However, as the usageof motor boats increasestheir propellerscut plants and if the boat traffic is dense their wash is sufficient to sweepaway the bank vegetation(l).The effect of very frequent wash from recreationalmotor boats is the scour of soil from the roots of emergentriver plants closeto the bank and the undermining and battering of natural vegetationin the bank area. Generallyonce vegetationis lost the rate of erosion increasesand regenerationis in- hibited. Whereextensive damage has beenpermitted to occur, bank pro- tection or restoration may be carried out as detailed in the "Report on Bank Protectionon Riversand Canals"(2).It may be necessaryto usepil- ing which will have the unfortunate effect of reflecting boat wash whereas the original emergentriver plants and natural bank vegetationwould have absorbedand dissipatedthe wave energy.A chop will persistlonger in the piled channelwhich will increasethe damageto unprotectedbanks nearby and the consequenterosion may outflank the endsof the piling works themselves.A processmay be set in train resulting eventuallyin very ex- tensiveand expensivecontinuous lengths of protectionon both banksof the channelas may be seenon some Broadland rivers and canals. The purposeof this report is to showexamples, and a methodology,of how channelsmay be manageda little better for the essentialand recrea- tional purposesoutlined above and at the sametime the banks and margins may also be protectedto support a natural environmentusing engineeringtechniques which make the greatestpossible use of the intrinsic engineeringproperties of the natural plant communitiesthemselves. THE RIVER THAMES AND THE NORFOLK BROADLAND RIVERS, EXAMPLES OF RIVERS WITH BANK EROSION FROM BOAT WASH 2 The River Thames The River Thamesnear Wallingford was selectedfor study. This reach of l0.5km from Bensonlock to Cleevelock is the longestreach between locks on the River Thames,and is shownin Figure3. One hundred years ago Henry Taunt travelled through Oxfordshire often by punt on the River Thamestaking superbphotographs, many of which show extensivebeds of emergentriver plants and large areasof floating leavedrooted plants. Thirty-six old photographswere found of this reach, I many by Taunt(3),and comparativemodern photographsof this reach show a gradual loss of emergentvegetation after the secondworld war followed more recently by a rapid elimination of emergentand floating leavedrooted plants until the presentday when only a few beds exist in favoured locations. It will be shown thdt the larger beds of surviving emergentspecies are well suited to withstand some boat wash and they are located on bedsprofiled to help the plants to withstand the impact of wave energy. The marginal beds of emergentriver plants in this reach have recently been under stressfrom the following causes: l) The river was dredgedin the early 1960swhich steepenedthe bank slopesand damagedthe outer edgeof the beds. 2) River levelsare kept lower in winter by weir regulation to assistland drainageby rapid removal of flood water. The lower stagedischarge relationship resultsin greaterscour at the edgeof beds during floods and betweenfloods the rhizomesare unprotectedfrom frost. 3) The heavyseasonal load of recreationalmotor boatshas increasedin recent yearsresulting in scour from boat wash turbulencearound the stemsand rhizomesof emergentriver piants. 4) The endsof marginal bedsare under increasingattack from behind from boat washwhich is reflectedfrom smoothsteep banks. 5) Propeller action from the heavy suminer seasonalload of motor boats has decimatedthe floating leavedrooted river plants, thereby reducing channelroughness and at high flows reducing stageand in- creasingscour velocity at the outer edgeof the beds. (The drag effect of submergedriver plantsis not alwaysappreciate6(+' 5);. 6) The mooring of boatsagainst