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 greater scour 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 the outer edgeof bedshas produced mechanicaldamage. 7) Fishermenstill cut swathesthrough the remainingmarginal beds of emergentriver plantsand stepsin the banks. 8) Wild fowl grazea higher proportion of the bedsthan when the beds weremore extensive. Sectionaldiagrams of a typical river bank before high motor boat usage and also of presenterosion are shownin Figure l. The boat washerosion mechanismis as follows: Soil is washedfrom the roots of any emergentplants in shallow bedsclose to the banksand from the roots of marginalbank vegetation.In areas where the banks contain clay the rate of attrition is very slow and herbage may recover the receedingupper bank but at water level the bank will re- main generallysteep, bare, smooth and wave reflective exceptunder trees whereroots may form matlike protections;trees may be outflankedby erosion and fall into the river. In areaswhere clay is absentfrom the sub- soil then erosionis rapid oncethe vegetativecover disappears. A wavecut platform extendsinto the bank until a spendingbeach bay developsat a slopeand width adequateto dissipateboat washwave energy. Coarser sedimenttends to remainon the bank and finer sedimentis removedto a shoalor offshorebar. Flood dischargesremove or redistributethese shoals and subsequentlythe beacherosion will continue.The erodedbanks tend to collapsewith the rapid drawdown of river level causedby flood reces- sion or weir regulation. If the presentlevel of boat wash continuesinto the future there are several long term managementtechniques available as follows: l) Allow the processof bank erosionto continueuntil the river becomesmuch wider and shallowerwith wide beachesas on many navigatedrivers such as the Rhone and Rhine(6).Extensive perma- nent shoalswill developwhich may eventuallybe colonisedand stabilizedby river plants, such as sweetflag, when the channelhas developedand changedsufficiently to produce a suitable habitat. 2) Sheetpile the river bankson their presentline. This will eliminate the rich bank habitat. 3) Develop a techniqueto protect and reinforce the boat wash absorb- ing beds of emergentriver plants as detailed in Section5. 4) Impose and enforce a speedlimit as detailed in Section3. Broadland rivers The Norfolk Broadland rivers and particularly the Rivers Bure, Thurne and Ant near Horning were selectedfor study. The numerousold photographso)available show that before the onset of heavy holiday motor cruiser traffic the rivers were much narrower, were flanked by con- tinuous thick standsof emergentriver plants and containedextensive areas of floating leavedrooted plants. SeePlates I to 4.
The diurnal tidal influence is small in the rivers near Horning but river levelsare influenced by surgelevels in the North Sea, fluvial flood dischargesand fluvial seasonaldischarges; the typical range of normal summerlevels is 300 or 400mm.
The bank soil structure consistsof soft peat often overlaid by a post glacial deposition of clay. The clay may be deepon the bed and the banks of the larger rivers downstreambut generallydiminishes inland and on smaller streams.The clay may peter out towards the landward edgeof the marshes,where the peat subsoil may be soft enough for a man to sink in if the grassroot mat is disturbed.
The land has been slowly sinking in relatien to the sealevel for many cen- turies.Ancient peatworkings became flooded in medievaltimes and lakes known as broadswere formed. In order to make agriculturaluse of the marshesit has beennecessary to raise clay flood walls and clay is ex- cavatedfrom the marsh drainagechannels for this purpose.These chan- nelsare known as sokedykes in Norfolk, delphsin Suffolk and rhynesin Somerset.The purposeof the dyke systemis to drain the marshand to lower the water table on the marsh as required by the farmers. The weight of the clay wall gradually consolidatesthe underlying peat and clay strata and the wall continuesto sink. Before equilibrium is reachedit will generallybe necessaryto raise the wall again after a few yearsto higher levels.It is an expensivecontinuing process. The bank betweenthe river edgeand the clay wall is called the rond. Ero- sion of the rond is now generallytaking place,often rapidly. When the rond becomesvery narrow the stability of the clay wall becomesen- dangeredand it becomesnecessary to closepile the bank using steelsheet piling or a compositepiling systemusing steelking piles and closetimber sheetpiling or someother systemsuch as asbestossheet piling or all timber piling. The piling systemsmay require to be tied back and an- chored into the firm clay behind the wall. It is generallynot economicto rebuild the clay wall back on a new retirement line becauseof the very considerabletime required to consolidateand thereby increasethe struc- tural strength of the peat sub strata or the great cost of removing it. Reed bedsseem to prosper behind very old timber piling which has rotted down some100 to 400mmbelow low water level,and coloniseto the edgeof the protection.
The rapid erosion of the unprotectedrond has severalcauses shown diagramaticallyin Figure 2. Generallythe causesof bank erosion are as follows:-
l) Deteriorationin the vigour of Phragmitesaustralis, Norfolk reed,in Norfolk. George(8)considers that the Norfolk reedmay be senescentin someplaces, that thereis significantattack by the fen wainscotmoth, and that there is dieback which may be causedby changesin water quality. The writer has noticed that some Phragmitesaustralis in Norfolk is much lessvigorous than somein Australia, Africa, North America, Somerset and the River Thames. 2) Overgrazing.George also considersthat grazingof the outer edgeof the reedsmay now be excessivefrom swans,coot, greylagsand Canada geese.
3 3) Undermining. Payne(9)has examinedthe wash of boats, the effect of burrowing into the clay on the edgeof the rond by voles, water rats and shrimps, etc. and the boat wash pumping action under the reed rhizome and root mat of fine sedimentin fluid suspensionwhich is suckedout with drawdown.The reedmats soonbreak off and float away. 4) Turbulence. Payne and Garrard are also examiningthe erosion by propeller turbulenceof the soft peat river banks and Garrard is also look- ing at the effect of turbulencecaused by propellerson the river bed. 5) End attack. The endsof marginal bedsare under attack from behind where boat wash is reflected from smooth steepclay banks and from pil- ing. 6) Mechanicaleffects. The mooring of boatsagainst the outer edgeof the rond producesmechanical damage. 7) Fishing. Fishermencut swathesthrough the remaining marginal bedsof emergentwater plants.
If the presentlevel of boat wash continuesinto the future there are several long term managementtechniques available as follows: l) Allow the processof bank erosionto continueuntil the river becomesmuch wider, the flood walls becomeendangered, and even- tually sheetpile continuouslyon both banks. 2) Develop techniquesto protect and reinforce and replant the boat wash absorbingbeds of emergentriver plants as detailed in Section
3) i-oor. and enforcea speedlimit as detailedin Section3.
BOAT WASH ON THE The washof motor boatswas investigated on a straightreach of the River RMR THAMES AT Thamesupstream of Wallingford Bridge on the left bank at a location on WALLINGFORD 3 the river frontageof the HydraulicsResearch Station. The seasonaluse of recreationalmotor boatsis at its peak during August and for severaldays the boats and boat wash were monitored at the HRS boathouseduring wind free periodsof very low flow.
Boat speedwas obtained from the speedof the boat's wave train which triggeredtwo capacitancesensors located 20 metresapart and mounted on heavy planks floating parallel to and closeto the river bank. The sensors were elevated50mm above mean water level to be clear of the more fre- quent wind waves.Boats producing wash below this levelwere ignored.
Boat length and sailing line were obtained from polaroid photographs taken from a high fixed position and using a transparentoverlay calibrated for waterline length and sailing line. A continuousrecord of waveheight was obtained from a twin wire probe(Io)set one metreout from the bank in deepwater, seeFig 12. Some banks near the wave recorder were devoid of vegetationat the waterline, were cohesive,smooth and steepand were therefore reflective.
Boats sailing in the wake of other boats for instancein convoy from lock to lock wereignored because of lack of independenceof data. Boatssail- ing in periods of significant stream flow or wind waveswere also ignored. Five distinctcategories of boat hull form wererecognised (Plates 5, 6 and 7) as follows: A) Narrow boats.Traditional bluff bargeshaped narrow boatswith beams2060 to 2l85mm, designedto sail through narrow canals. B) Wide boats. Bluff barge shapedboats similar to narrow boats but with beamsin the range of 3000to 4000mm. C) Modern chine. Modern displacementmulti chine hull forms with fine entry and flaring capableof use in a seaway.
4 D) Planing hull. Modern planing forms but operatedat sub-planing speedsin the Thames. E) Full curved. Streamlinedfull curved low frictions forms as in displacementyachts and some fishing boats. F) Miscellaneous.
Additional data was obtained separatelyusing a small boat which fell roughly into categoryD above.
Comprehensivedata was obtained for some 75 boats which represented about half of the boats which sailed by during the limited times the in- vestigationswere taking placein August 1980. Threetypes of boat washwere identified from the record; l) surgewaves, 2) ship wavesand 3) a wake of transverseand reflectedwaves. l) Surgewaves. Twelve boats produced surge waves resulting from draw down effects,nine of which wereheavy displacement wide boatssail- ing on a line closeinto shore.The river width is 36m, with a crosssec- tional areaof 62.2m2.The blockagefactor for thesecraft was approx- imately0.048. Only two boatsout of the 75 produceda surgeheight ex- ceedingl00mm and the meanperiod of surgewaves was 6.2 seconds.
2) A vee shapedtrain of ship waves.The train was propagatedat an angle of approximately20 degreesfrom the sailing line. The train usually consistedof five or six curved short crestedwaves but sometimesas few as one or as many as l0 waveswere recorded. The wave period was in the rangeof one to two secondswith a meanperiod of 1.3 seconds.The wave angle of propagation of individual ship wavesvaried between20" and 70" from the sailingline.
3) A wake of transversewaves and reflectedwaves. After a boat producing ship waveshad passeda gradually decayingtrain of following wavesthen ensued.
The three types of wavesdescribed above were generallygenerated at dif- fering anglesto the bank and theseangles also changedcontinuously as the boat sailedby.
Figures4 to 7 show for eachvessel a plot of the mean of the five highest ship wavesand the highestship wavesagainst boat speed.Parameters which vary from boat to boat and contribute to the wide scatterof the resultsinclude sailing line distancefrom the probe, displacement,hull form, boat length,beam and attitude.
Although no speedlimit existson the Thames,boats are required to sail with an acceptablelow wash. It was observedthat some of the worst ex- amplesof high boat wash were generatedby boats proceedingin convoy from lock to lock with an elementof competitivejockeying for position to enterthe next lock.
Boats may find it difficult to comply with a speedlimit sincefew are equippedwith reliablespeedometer instrumentation. However, a speed limit of two metresper secondif successfullyimposed would substantially eliminate boat wash that exceededwind wavesin height on this reach of the River Thames.
BOAT WASH ENERGY REDUCTION THROUGH BEDS OF EMERGENT RIVER PLANTS 4
Tests on the River Thames The river near Wallingford was chosenfor study. The marginal beds of at Wallingford emergentriver plants have been under severestress in recent times. Only the fittest to survive have survivedthese stresses that have been brought ) about by a valiant attempt to managethe river to satisfy many usesand users.The recent stressesplaced on beds of emergentriver plants are listed in detail in Section2. The outer edgeof the beds erode becauseof flood scour, dredgingand mechanicaldamage from boats. The stemsand rhizomesare scouredby boat wash, both directly and reflected from the banks at the ends of the beds. Fishermencut swathesand wild fowl graze the edgesof the bedsof emergentriver plants. The remnant marginal bedsof emergentriver plants are therefore very capablesurviving communitiesworthy of performanceevaluation. They are essentiallylow cost low maintenancespending beaches capable of ab- sorbing boat wash wave energyand protecting the river bank. At the same time they are a scarcenatural wild life habitat and refuge.
Thereare four dominantsurviving species as follows:- l) Phragmitesaustralis (syn. P. communis),common or Norfolk reed 2) Shoenoplectuslacustris (syn. Scirpuslacustris), common clubrush, greater rush or bulrush 3) Typha angustifolia, lesseror narrow leavedreed mace 4) Acorus calamus,sweet flag, a medievalintroduction. Theseare the only emergentplants in the reachin bedsof singleor domi- nant species.However, smaller mixed bedsalso occur with a small propor- tion of Carexsp., sedges:Phalaris, reed canary grass; and Sparganium erectum,bur-reed(ll).
The four specieslisted above are of specialinterest because of wide geographicaldistribution.
Figure9 containsdetails of the four plant speciesdrawn on a substrate slopeof one in four and showingdetails of the lower stems,rhizomes and roots viewedas structureswhich haveproved capable of: l) absorbingboat washwave energy 2) maintaining vigour and stability 3) preventingsubstrate scour 4) encouragingsediment deposition. Phragmitesaustralis, common or Norfolk reed, forms denseintertwining mats of long rhizomeson the bed with deeplyanchored thin root stems and tough thin stemswhich are flexible and able to withstand wave action. The outer edgeof the bed is subjectto scour.
Shoenoplectuslacustris, common clubrush or bulrushforms short rhizomesbelow the bed with very closeintertwining mats of fine roots aboveand below the rhizomes,very resistantto waveaction. The outer edgeof bedsis also subjectto currentand propellerscour. The plant den- sity is high.
Typha angustifolia, lesseror narrow leavedreed mace, forms mats of rhizomesand systemsof fine roots eminating from the baseof every stem. The stiff thick stemsare subjectto high stressat the baseand looseningor uprooting may occur after high levelsof boat wash; neverthelessa well survivingspecies. Acorus calamus,sweet flag and a medievalintroduction. Sweetflag has rhizomeswhich can float above the bed anchoreddown by long thin, tough flexible root systems.Sweet flag is capableof withstanding breaking wavesfrom boat wash in shallow water. The foliage is of low form on the River Thamesand will withstandfloods. Sweetflag is successfullycolonis- ing stableshoals of sedimentin the River Thames. Four beds, one of each species,were selectedfor study and a fifth beach of similar bank slopeand with no vegetationas a control. The four beds were selectedfor the strengthand vigour of the plant communitiesand the
6 test sectionwas located in the bed well away from end effects, and in a part of the bed of most uniform width and most uniform plant density. The sectionsare shown in Figure 8. The most common bank slope was one in four. Figure 8 and Plate 8 also showsan unvegetatedcontrol beach sectionwhich has an offshore bank slope of one in four and also has a sandy inshore spendingbeach at a slope of one in ten.
A scaffolding was set up on the sectionto be testedsupporting two twin wire probes, one over deep water beyond the vegetationand one inside the massof vegetationbut outside the zone of breaking waves.It was observ- ed that the vegetationinhibited or even preventedbreaking waves. When a boat sails by the boat wash wavesare propagatedinto the beds firstly as surgewaves, followed by vee shapedtrains of curved short crestedship wavesfollowed by a gradual decayingwake of transverseand reflectedwaves. Wave crestsrange in angle to the sailing line from 20" to 70o and generallythe three types of wavesare generatedat quite different and changingangles which vary from boat to boat. Consequentlythe inner wave record was sometimesmarkedly different in form from the outer record becausesome wave energymight be propagatingin a direction at 20" to the bank thrdugh as much as l0 metreswidth of vegetation, whereassome other wave energymight be propagatingat 70" to the bank through only 3 metreswidth of vegetation. tloteson parametersaffecting In the River Thamesthe ship wavesare almostdeep-water waves. It was shoalingwave energy observedthat it is the ship wavesthat produce the most significant plant stemmovement and any breakingwaves that shouldoccur. In deepwater the total averagewave energy, E pet unit of surfacearea for a small amplitude wave componentis = qgH2 (r2) "= 8 Once the wavesreach shallow water in the reed bed the multidirectional boat washwave components come under the influenceof refraction,fric- tion and vegetationform drag. The vegetationform drag slows up the wave crest and inhibits wave breaking. The wave heightsare also reduced by the reed bed as comparedwith a similar wash reachingthe unvegetated control beach. On a shallowunvegetated beach Das(13) concluded that for ship wavesthe energydensity per unit surfacearea in a waveprofile is E" = VrpgC2. This indicatesthe contribution at each frequencyf given by n/T to the total energydensity or energyper unit of surfacearea, and this quantity is also proportionalto the meansquare height H2. In shallowwater Das plotted (H^*)'z as a suitable function of energydensity. Das also found lit- tle contribution to energydensity at the low frequencyrange using Fourier analysis.Sorenson(I4) also used(H^")2 as a plotting parameter.Denny(ls) concludedthat for breaking wavesconsidering them to be representedby the highestsolitary wave, the forward momentum per unit of wave crest is U, = 4.5eH2Cwhere
C = VEH(1-TTre':78) and shock impulse I is lrpghHs/2 gavl(16)in laboratorystudies for beachesof l:3, l:10 and l:30 and a 30' bulkheadon a beachof l:10 slopefound that the momentumof a solitary breakingwave is lJo = 55.5Hu5/2. I ano.H, : H-" 33-l 3VmzL Kajura(I7)found the rate of energydissipation per unit areaof unvegetatedbottom per unit of time is rkpgHz 4L rnsh'A"h/L) The most commonly observedparameter of energypropagation in shallow water, as also in deepwater is the squareof waveheight.
7 Boat wash energyreduction The progressof boat wash was observedthrough the four beds of emergentriver plants fringing the River Thamesnear Wallingford, and a continuous synchronousplot of wave height againsttime was obtained for the inner and outer waveprobes. For the bed of Shoenoplectuslacustris, common clubrush or bulrush, records were also obtained for wave probes in middle and outer locationsas shownon Figure4.
Wash from all boats sailing by during the period of observationwas in- cluded and supplementarywash was made using a small motor boat. Figure l0 showsthe meanof five highestship wavesat the outer probe and the ratio of the squareof the mean of the five highest ship wavesat inner and outer probes,which is the energyreduction. Figure I I showsthe maximumof highestship wavesat the outer probe and the ratio of the squareof the highestship wavesat the inner and outer probes,which is the energyreduction. Vegetationdensity surveysmade on the sectionlines and mean energy reductionsare shownin Table l. The vegetationdensity varied con- siderablythrough the beds of emergentriver plants from very small clear pools to very denseclumps. No doubt the ratio of energydissipation will vary for alternativesections, but neverthelessthe resultswere consistent. 6090of the shipwaveenergy was dissipated by a standof emergent Phragmitestwo metreswide; 7090by clubiush or bulrush2.5 metreswide; 66t/oby lesserreed mace2.3 metreswide and 75s/oby sweetflag 2.4 metreswide. Therewas no evidenceof any reflectedwave from the bank and no evidenceof bank damagebehind any of the four species.Wave breakingwas inhibitedby the four speciesand sweetflag is able to withs- tand an environmentof breakingwaves in shallowwater and may evenex- tend its areaof colonisationover shallowshoals. Very roughly, under suitableconditions of depth, vegetaitondensity, and a bed slopeof one in four, two metresof width of bed of any of thesespecies will dissipate almosttwo thirds of the boat washship waveenergy.
PROPOSEDPROTEC- FiguresI and 2 showthe proposedbank protectionusing marginal beds of TION AGAINST emergentriver plantsat a bank slopeof one in four or less.The least BOAT WASH 5 slopegives the bestenergy dissipation. In the Netherlandsthe bed is sometimesgraded 1u1(18). However a slopeof one in four probablyuses leastwidth for maximumbank protection.
Submergedtoe protection is required generallyin rivers with high motor boat usageif only to provide a fend off againstmechanical damage from boats.However, there are locationsof stronglateral accretion as for ex- ample below point bars or in rivers without flood scour where piling may be omitted; it may be constructedlater if necessary.Submerged toe piling will be requiredeverywhere generally in riverswith peat banks.
The use of old tyres will give adequatefend off effects to prevent naviga- tion hazard. Twenty three million old tyres are discardedfrom vehicles eachyear in Britain(I8).Indications are that the garageindustry will helpfully assistwith the supplyof old unwantedtyres for schemesintended to improvethe natural environment.
Tyresmust be organisedinto setsof uniform wheelsize and may be threadedover wire meshcylinders. The tyre sizegoverns pile spacing.Piles are driven to a level some 500mm above the waling level to support tem- porary distantpaling and the cylindersof tyresare then placed.A tradi- tional tie and anchorsystem will be requiredin Broadlandrivers for one pile in everythree or four. The submergedwaling should be at a depth of from 100to 400mm below low summer water level in Broadland rivers near to Ant mouth and Thurne mouth. The preciselevel will dependon the summerrange of levels,the hazardsand costsof working below water level,navigation re- quirementsand rond width. The bank, or rond is then backfilled with selectedfill and regradedto a slope not exceedingone in four. A filter membranemust be usedif the backfill is of low strength or if the tyres tend to be openly spacedto pre- vent loss of backfill by pumping due to boat wash. A filter membraneis essentialin Broadlandrivers.
Emergentriver plants must be planted either by heelingin rhizomesas proposedby Brooks(Ig),Eaton and Pearce(2o),George(8), Haslam(2l), Kite(22),Raven(23) and Seibert(2+:),or by spreadingdrain dredgingsas pro- posedby Brooks(I9),Pearce(20) and Robson(ll),or by spreadingreed 'feathers' in shallow water for May germination as proposedby Ellis(2s) and Kite(22)or reed turf may have to be placed as suggestedby Haslam(2l).Mixed speciesare generallyproposed, perhaps on a detailed planting plan as proposedby George(8).A very successfulplanting of mix- ed speciesbehind chestnutpaling was made by Pearce(2o)at a slope of one in four in an area of high motor boat usagein the River Dee above Chester.The planting shouldtake placein early spring.The palingsmay be removedand pilescut down to waling levelwhen the bed is established.
Someadvantages of the proposedtoe protectionsystems are as follows:
1) The piling and chestnutpalings will make excellentprotection against boat washfor a long time if the reedbed is slow to get established. 2) All timberswill finally be almostpermanently submerged and will thereforebe resistantto rot. In the brackishBroadland rivers the timber from nearby aldersmay be used for piles with long life expec- tancy. 3) Deep level wave reflection will be dispersedby the rough profile of the tyres. 4) The hydraulicroughness of the tyreswill inhibit toe scours. 5) The toe structureis flexible. 6) Additional fend off tyres may be fixed to the walings to further reducethe hazard to navigation. 7) A complexhabitat and refugeis providedin the vicinity of the wal- ings and tyres. 8) When the reed bed is fully grown the visual effect will be natural rather than man made. The proposalsshown in Figures I and,2 refer to Broadland rivers in the vicinity of Ant Mouth and Thurne Mouth and certain banks of the River Thames.The locations under considerationhave flood level variations, seasonallevel regulation and seasonalmotor boat usagepatterns which suit the systemsof toe protectionproposed in this report. Thereare many other rivers and canalssubject to heavy motor boat usagebut with dif- ferent patterns of level variation for which thesemodels of toe protection are unsuitable.For theseother channels,the methodologyproposed in this report shouldbe usedbut alternativemodels of toe protectionmay haveto be devised.
ACKNOWLEDGEMENTS 6 This report was written by Mr Alan John Bonham, senior lecturer of the University of New South Wales and formerly of the East Suffolk and Nor- folk River Authority and the Devon River Board. Thanks are extendedby the author to the Council of the University of New South Wales for approval of a specialstudies program and financial assistance.
Thanks are due to ProfessorT G Chapman and colleaguesin the Civil EngineeringDepartment at Duntroon for undertaking additional teaching. Thanksare due to the many waterwayexperts of many disciplineswho gave advice, and especiallyDr D Robson of the Weed ResearchOrganisa- tion at Yarnton who identified plant speciesin the River Thames. 9 Thanksare due to Mr M A Owen-Jones,Mr M J Cameron,Mr D K Fryer and Mr W E Biggs for technicalassistance.
Specialthanks are due to Mr F G Charlton and Mr A J M Harrison, divi- sional head, for great help and encouragement.
REFERENCES 7
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2 DOBBIE C H and PARTNERS. "Report on Bank Protectionin Rivers and Canals", HydraulicsResearch Station, Wallingford, July 1980.
3 TAUNT H et al. Unpublishedphotographs of the River Thamesbetween Bensonlock and Clevelock 1870-1900,Westgate Library, Oxford.
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5 DAWSON F H and KERN-HANSEN N. "The effect of Natural and Ar- tificial Shadeon Macroplytesof lowland streamsand the useof shadeas a managementtechnique"Iat. Perue ges. Hydrobiol, 64, 4, 1979pp 437-455.
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7 MIDDLETON C S. "The BroadlandPhotographers - J PayneJennings, P H Emersonand GeorgeC Davies", Wensum.
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ll ROBSONDR D. Personalcommunication.
12 "Shore ProtectionManual". CERC Fort Belvoir, Virginia, 1975.
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l7 KAJURA K. "On the bottom friction in an oscillatorycurrent". Bull, Earth Res.Inst. Universityof Tokyo 42, pp 147-174.
18 ADDYMAN O T and VAN BEESTENC. Personalcommunication.
19 BROOKSA. "Waterwaysand Wetlands", British Trust for Conservation VolunteersLtd., London 1976. 20 EATON DR J W, PEARCE H and MURPHY K. Personalcommunica- tion. 2l HASLAM DR S M. Personalcommunication.
22 KITE D J. "Ecologicalstudy of land drainageworks - River Stort Navigation". UniversityCollege, London, 1980,unpublished. l0 23 RAVEN P. "Ecological study of land drainageworks - River Roding", UniversityCollege, London, 1980,unpublished.
24 SEIBERT P. "Importance of natural vegetationfor the protection of the banksof streams,rivers and canals", Council of Europe,Nature and En- vironmentseries, Munich, 1968.
25 ELLIS DR E A. Personalcommunication.
BIBLIOGRAPHY 8 BAILEY D. "Boat wash", SeminarPaper, Thames Concern 1978. The River ThamesSociety, Marlow, 1978.
BELL M G W. "Protection of water for recreationalpurposes - the Nor- folk Broads". Proc. Inst. Civ. Eng. part I, 1980.
BONHAM A J. "A role for flood retarding vegetationin urban storm- water managementand creekpreservation" Hydrology Symp. 1978,I.E. Aust. Canberra,September 1978.
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DAWSON F H. "Aquatic Plant Managementin Semi-NaturalStreams: the Role of Marginal Vegetation" Journal of EnvironmentalManagement, 1978,6 pp 213-221.
FORTESW E. "River bank ersoion- The Thameswater approach", SeminarPapers, Thames Concern 1978, The River ThamesSociety, Marlow, 1978. ..BTitiSh HASLAM S M, SINKER C A and WOLSELEY P A. WATCT Plants", Field studies,Field StudiesCouncil, 4 No. 2 September1975, 243-35r.
HOGBEN N. "Automated recordingand analysisof wavepatterns behind towedmodels" RINA, 1971.
HOLLIS G E. "Mans impact on the hydrologicalcycle in the UK", Geo Abstracts,uEA, 1979.
HORIKAWA K and KUO C T. "A study of wavetransformation in the surf zone" Proc lfth Conf on CoastalEng., 1966,pp 217-233. KEOWIN M P, OSTWALT N R, PERRY E and DARDEAU E A' "Literature surveyand preliminary evaluation of streambankprotection methods", Tech. ReportHTT9US WaterwaysExperiment Station, Vicksburg,May,1979.
KOUWEN N and LI R M. "Biomechanicsof vegetativechannel linings", J Hydr. Div. ASCE HY6, June1980, pp 1085-1103.
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DDB Dd 650'449 12/81 t2 TABLE
) Cl i Ci \O r- oo s. cO \.1 eO q ta l.i-lt.c" € r9 + 9= tr o OO g-=tr = oo t\o $ o\ rl €.= o = Hco m l3 ne tnOrt'- o -': "': \q q \o q esa* e efmdOi C.l C.l et c'l q gEEE tiCr) ar E = E =H- g c.) k C|. a.l EEl9n; co \o ca € x O 6.sj'E I E .d cj c'; -j r H'E :r >EEaA E " F H el € v {) L Eq 6oF;c,-. Es. E \o <) (-l oo \OO oo roO s €f;e9.H CtHitf, C.l 9t cogq) o ()NQN Ebo - r-r ic . slisk P5 S,g? HeEE.*.dv E eE99 sGr.=entgF CAEA z v o tr a 9.9 o ' t R 3 o= € .ji = Ea o.4= c) g 'F \'n * E^5-oau E €E b 9 .^x='tro(l v-E iiclx^dd-o uP '= tr trc 3E; ctd €e€ €i. -d€ i5 o o .= EEg rHe =()-XpeP =FA i-€ EEEoaa. trtr.5.5 Figures Flood ploin Bonk with noturql Emergentrushes Riverwith meodow noturol vcgetotion rushes ond reeds rooted plonts Typicol ro nge of summerlevels Grqvel qnd/or sond ond cloy 1.Typicot section of river bqnk '- befqre high motor boot usoge //l mud Bonk collopse Erosion trom (Note: reedbeds lollows drqwdown boqt wqves until ore olreodysubstontiolly spending beqches destroyed) form in boys Sond ( Note : sometimescolonisqlion Scourof shool here by qcorus cqlomus) in winterfloods 2.Present erosion Reploni strong Chestnutpoling to protect replonled reed rhizomes morginuntil estoblishedthen sow off piles ot woling Submergedtimber woling l:1 Excovoteto this tine Otdtyres ( Note:lend off ettect) Selectbockf ill 3. Proposed protection Timber pile Bonk protection ogoinst boqt wosh for grovel qnd mixed bed rivers with high motorboot usoge Fig 1 Wqll with Rond with reeds Bonk with emergent River with rooled mown grqss€s ond sedges reeds ond rushes plonts. Typicol ronge ol summer levels - /(\ I 1.Typicol section of river bonk _,(,il_t before high motor boot usclge /y'[\ Reeds die bqck Grozing by witdfowt Pumping + by boot woves couses reed mqts to f loot owoy Erosionby onimot burrowing ond boqt wove lurbulence Erosionby propellerturbulence 2.Present erosion Tie qnd qnchor system Replontstrong hestnutpoling to protectreplonted qs for composite piling strongrhizomes morgin until estoblished then ssw otl piles ot woling ubmergedtimber woting ..-----=7- ( Note : fend off effect ) Select bocklill Old tyres of unilorm wheel size lhreoded over wire Membrone il needed mesh cylinder ond ploced over pile 3.Proposed Protection Bonk protection clgqanstboqt wosh for Fen qnd Broodlond rivers with high motor boot usoge Fig 2 -+ Benson Lock Crowmqrsh gB S,T P1 c,T,s I Hydroulics ReseorchStqtion Woltingford CrowmorshGifford fi!--- Distqnce trom Benson Lock A to Cleeve Lock is l0'5 km. c,s Not to scole s2 North Stoke Moulsford P Phrogmitesoustrolis commonor Norfolkreed. South Stoke S Schoenoplectuslqcustris commonclubrush or bulrush. T Typhoongustifolio lesser reed moce. A Acorus colomus sweet f log. c Pholoris, reedconqry gross, corex, sedgesetc. Cleeve Lock I Cross sections. S enotogrophs. Goring Plon of River Thomes of Wotlingford Fig 3 I o Itr o o .f o x x x o a x o a x X+ o 3 ./ xx x x o o- x .7 (Y) xx l. E oo o o o o x o x x o xx U) o x ^o o o o -t -c Eol Ox ];E *x o x {- OEcJ o x c o o o ;stE EE 5o (,U) -cE !:!sg54 OO x * . :l |Jl o rn o @ (o o qJ (\I T o o o o 6 o 6 o t.5 Fig 4 ro o rn o x x o f o x A o x \/ o o x tx o \-xx x o , T rtt o |r| @ o q'l GI o ro o 5 I 9 o o o o o x o? E5 E Fig 5 I o lfl xo o xx xx x r*o f o xx c\ l- -x XX {o(xx x o o x x o x x O( x.\ o .1fo e 3 x'x o o- -S.t' ,.o = *;a'xx o ax o o x -c (tl = x GI o o o o c o o Er€5 Eo !iise;r OOx*'r a \t (!l .{f (! o 9 I o o o c) o o o 9 9 o o o o r.li Fig 6 (0 o I x xa o !(X + 1X (\ f o o o x I xo x x o {.x nxxO 3 xx 19( x .Eo. Ox xx o-rlo (vt o o rF x a--?*ix xOl o xio aO Ox x Q .c. '6gl ox .C, x E K= o (\I E o x o o Otr OyCi 8 3€e L !sisg;i x * . oo + o r o \t (\l \t (\I o o o o o 9 o o o o o I o o o 6 E.lj Fig 7 C.S.1Phrqgmites oustrolis C.S.4Acorus cqlomus commonor Norfolk reed sweet f lqg (reed) PI C.S.2Schoenoplectus locustris C.S.5 Controlsection commonclubrush or bulrush with no vegetotion C = stem ond leof count zone I = inner Q= outer f: outer edge of bed p = probe M= mlddle C.S.3 Typho ongustifolio lesser or norrow leoved reed mqce Crosssections of morginol beds of river plonts Fig I 1. Phrogmites oustrqlis 2. Schoenoplectuslocustris common or Norfolk reed cornmonclubrush or bulrush 00 mm 3. Typho ongustifolio 4.Acorus colomus lesser reed moce sweet f tog (reed) Detoilsof river plonts Fig I x Outer probe lo anner probe A Outer probe to mrddle probe no x \ x (m) x (m) x xa a x xx x xx x A x XX x ,," , x f;x XX x x A x 0.06 0.06 A 0l 1.0 2 0 0 I 1.0 2.0 ( hr/ho)' ( hr/ho)' r0.1 10.2 025 0.20 x x h; 6o x (m) (m) x xx "x x x x x x xx x tx 006 'f 0.06 0.1 .0 2.o 0.1 t.0 2.0 (\ (hr/ho)' 10.3 /ho )' 10.4 * Breoking woves x Non breqking woves l0 l Phrogmites oustrolis ( syn. P communis) common or Norlolk reed r (syn. t* 10.2 Schoenoplecius locuslris Scirpus l.) * * common clubrush, greoler rush or bulrush 60 10.3 Typho ongustifolio, (m) * lesser or norrow leoved reed moce 10.4 Acorus colomus,sweet llog (reed) 10.5 Control beoch, without vegetotion 0.06 0.1 r0.5 tFlrFo)2 Energy reduction,meon of five highest ship woves Fig l0 0.25 o.25 x outer probe to inner probe A outer probe to middle probe o20 020 xx xx AA vxx xx A xx.. A xxr xx no hro' (m) x (m) AA A 0.r0 A A 0.06 0.06 0.1 t.0 2.0 0.1 1.0 2.0 " 'll.t { nio'rnlo')' ( hl"'/ h["' ) 11.2 uza ozl 0.20 0.20 x x x xx x x ho rb ,t, (m) (m) x 0.r0 xx xx x xx 006 0.06 0.'l r.0 2.0 0.1 1.0 2.O 11.3 { nfo'l rr[o')2 11'1 'II hmoxrhmor '0 !2 0.25 * Breoking woves x Non breoking woves 0.20 ll.1 Phmgmites oustrolis ( syn. P communis ) common or Nortolk reed ll.2 Schoenoplectus locustris (syn. Scirpus l.) common clubrush, greoter rush or bulrush h6 11.3Typho qngustifotio, (m) lesser or norrow lmved reed moce 11./'Acorus colomus,sweel tlog ( reed) 0.10 x 1'1.5Control beoch, without vegetotion xx 0.06L 0l r r.5 t ni*rni"') Energy reduction,moximum heightof ship woves Fig 1l .100mm MWL - l00mm 010 20 30 Boot speed = 2'71 mls Boot woterline length : 10 5 m Foro'wide'boot hull torm, similor to Plote5 A. Boot wosh ot HRS .100mm MWL - 100mm Outerwove .l00mm MWL -100mm 0r0203040 50secs Foro modernmultichine hull lorm Phrogmites oustrolis, common or Norfolk reed B. Boot wosh ot C.S.1 Wove profile of boot wosh Fig12 PLATES SeeFig 3 for locationson River Thames PLATE I Right bank below Wallingford Bridge in 1910,showing emergentand floating leaved river plants PLATE 2 Right bank below Wallingford Bridge in 1980showing extensivebank erosion PLATE 3 Right bank below Wallingford Bridge in 1890showing thick standsof emergenlriver plants PLATE 4 Right bank below wallingford Bridge in 1980without river plants PLATE 5 'Wide' recreationalmolor boal on the River Thames passingthe Hydraulics ResearchStation wave recorder PLATE 6 'Narrow' and '.wide'recreational motor boals al Seclion 2 showing wave probes and bed of Schoenoplectuslacuslris, common clubrushor bulrush PLATE 7 Recreationalmolor boat al Section4 showing wave probesand bed of Acorus calamus,sweet flag PLATE 8 Seclion 5, eroded unvegetatedspending beach wilh wave probes and breaking boal wash