ff'; :
Ministerie va'n Verkeer -en Waterstaat."<
Directoraat-Géneraal Rijkswaterstaat
Dienst Weg- en Waterbouwkunde
TOEPASSING VAN BEACH MANAGEMENT SYSTEM I N NEDERLAND
(STRAND DRAINAGE)
B I D O C
(bibliotheek en documentatie)
Dienst Weg- en Waterbouwkunde Postbus 5044, 2600 CA DELFT Tel. 0 1 5 - 2 5 1 8 363/364
M.J. Koster
Rijkswaterstaat, Dienst Weg- en Waterbouwkunde augustus 1994
WBA-R-94053
B I B L I O T H E E K Dienst Weg- en Waterbouwkunde
Van der Burghweq
Postbus 8044, 2600 GA Delft
TeL 015 - 699111
V{i
3-6-3fi
Toepassing van Beach Management Svstem in Nederland (strand drainage)
M.J. Koster
Rijkswaterstaat, Dienst Weg- en Waterbouwkunde augustus 1994
WBA-R-94053
B I B L I O T H E E K Dienst Weg- en Waterbouwkunde
Van der Burghweg Postbus 5044, 2600 GA Delft
Tel. 015-699111
- 8 SEP. 1994
Strand Drainage RWS-DWW, aug 1994
Toepassing van Beach Management System in Nederland.
(strand drainage) M.J.Koster
Rijkswaterstaat, Dienst Weg- en Waterbouwkunde augustus 1994
Inleiding
De laatste tijd staat een kustbeschermingsmethode in de belangstelling, welke in Nederland niet wordt toegepast. Het betreft hier het zogenaamde "Beach Management System". Het systeem is bekend onder enkele waterbouwkundigen in ons land. Tot op heden heeft geen serieuze studie/afweging plaatsgevonden voor wat betreft toepassing in Nederland. Een nieuw alternatief voor kustbescherming behoort in ons kwetsbare land serieus te worden bestudeerd.
Door de RWS-DWW is het initiatief genomen enkele relevante stukken hierover te verzamelen en te bundelen, zodat deze voor een ieder beschikbaar zijn. Tevens werd het initiatief genomen om de "Danish Coast Authority" een brief te schrijven met een aantal gerichte vragen. Eveneens is correspondentie gevoerd met de heer Per Bruun uit de V.S., welke een autoriteit is op het vakgebied.
Het bestuderen van dit alternatief behoort niet tot een bestaand projekt bij de DWW, noch bij een andere specialistische dienst van de RWS. De DWW heeft echter gemeend dat een aanzet hiertoe op z'n plaats is. Deze notitie is bedoeld ten behoeve van een meningsvorming en voorlichting.
In het hierna volgende wordt de beantwoorde vragenlijst besproken en voorzien van commen- taar.
De werkgroep RKO (Rijks Kust Onderzoek) van RWS, heeft op basis van deze notitie besloten dat vanuit Rijkswaterstaat geen verdere akties wenselijk/noodzakelijk zijn.
In de paragrafen 2 t/m # wordt de integrale tekst gegeven van een eerder concept dd novem- ber 1991. Sindsdien is er enige nieuwe informatie verschenen welke in paragraaf 7!£wordt toegelicht. Deze nieuwe informatie heeft niet geleid tot een nodige aanpassing van eerder genoemde tekst.
Conclusies.
De conclusie is dat strand drainage niet gezien kan worden als een reëel alternatief voor zand suppleties als kustbeschermingsmethode voor de structureel eroderende kusten in Nederland.
Toepassing komt (net als andere alternatieven) in aanmerking in gebieden waar sprake is van andere (voornamelijk recreatieve) belangen.
Strand drainage kan worden toegepast in gebieden van beperkte lengte langs de kust (tot enkele honderden meters) daar waar men tijdelijk een breder strand wenst. De praktijk leert dat een dergelijk strand na een storm weer snel herstelt. Dit geldt overigens eveneens voor een natuurlijk strand.
blz 1
Strand Drainage RWS-DWW, aug 1994
Wat is BMS ?Het BMS (Beach Management System) is een kustbeschermingsmethode waarbij drainagebui- zen onder het strand worden aangebracht, welke permanent water afzuigen. Het systeem wordt uitgelegd in de folder "Beach Management System", bijlage 1.
Dit systeem is min of meer bij toeval ontdekt door DGI (Danish Geotechnical Institute).
Projektcn waarbij BMS wordt toegepast (naar men beweert met succes) zijn:
a. Twee projektcn in Denemarken.
b. Florida, USA (bijlage 5)
c. Uit een folder, bijlage 5b, blijkt dat ook in Australië, Japan en Namibië ervaringen zijn opgedaan met het draineren van liet strand.
Sinds 1985 is er in Denemarken (Thorminde) een prototype proef, waarbij uitgebreide monitoring plaats vindt. Hierbij is de DCA (Danish Coast Authority) betrokken. Volgens DGI laten de resultaten zien dat een voormalige erosie van ca. 4 (m/jr) door toepassing van BMS heeft geresulteerd in een aanzanding van ca. 30-35 (m / m / j r ) (bijlage 9).
Uit bijlage 6 blijkt dat in de aangrenzende kontrole gebieden een erosie van ca. 75 (m-lmljr) is gemeten. Door het BMS wordt dan ca. 105 (m-lm I jr) "gewonnen".
Overigens zijn deze getallen: "gemeten boven zee nivo".
Het is niet aanmemelijk dat door gebruik te maken van het BMS systeem elk jaar weer van de zee wordt gewonnen. Er zal zich toch een soort evenwichtssituatie op den duur moeten instellen, waarbij een vooroever is gecreëerd welke evenwicht vormt met de (veranderde) randvoorwaarden. In het begin wordt mogelijk wat zand gewonnen. Deze winst zal tot uiting komen in een steilere vooroever. Echter na verloop van tijd zal vermoedelijk geen aanzanding meer plaatsvinden.
Antwoord op enkele gerichte vragen
Er werd een brief verstuurd aan DCA met een gerichte vragenlijst en verzoek om meer informatie (bijlage 2). De brief was niet verstuurd aan DGI, omdat DGI de uitvinder en exploiteur van BMS is. Er werd verwacht dat DCA (een soort Deense RWS) ongekleurde informatie kon verstrekken. DCA heeft echter de brief doorverstuurd aan DGI. Het antwoordt op de gerichte vragen moet dus kritisch worden bekeken ! (bijlage 9). In een veel later stadium is door DCA alsnog gereageerd op het verzoek een toelichting te geven op hun visie op het BMS systeem. De visie van DCA wordt gegeven in (par. 5.)
Is er een logische verklaring voor de werking van BMS ?
Een antwoord kan worden gevonden in de brochure en de papers.
Volgens de brochure zou BMS een energie absorberende werking hebben ten aanzien van de golven die erosie veroorzaken. Het is echter niet aannemelijk dat een strand met daaronder drainagebuizen, meer energie absorbeert dan een gewoon strand. Dergelijke uitspraken in de brochure zijn daarom misplaatst en misleidend.
Volgens de brochure berust de werking hierop: BMS zorgt ervoor dat een onverzadigde zone aanwezig is (die anders verzadigd zou zijn). Iedere golf die het strand op rolt
(run-up) brengt zand mee het strand op. Normaliter zou deze golf weer terug rollen (run-down) met het meenemen van een zekere hoeveelheid zand. Echter door BMS zal een deel van het water percoleren (het strand indringen) en daarbij wat zandkorrels
b l z 2
Strand Drainage RWS-DWW, aug 1994 achter laten. (Voor meer informatie: blz 9 van de brochure, bijlage 1).
Het systeem is dus min of meer dat onder normale omstandigheden elk golfje wat meer zandkorrels achterlaat dan voorheen. Het resulterend effekt per jaar kan kennelijk groot zijn.
Teneinde de geloofwaardigheid hiervan te toetsen kan de volgende som worden ge- maakt: Uitgaande van een werking van BMS van 100 (m-l'm j jr.) en uitgaande van permanete golven van 5 (s), kan dit worden omgerekend naar ca. 16 (cm-1 m I golf), ledere golf zou dus ongeveer 16 dobbelstenen zand meer achterlaten dan normaal. Dat lijkt niet echt verwerpelijk. Gevoelsmatig is het echter moelijk te begrijpen dat een dermate kleine ingreep zo'n groot gevolg kan hebben.
Wat zijn de kosten per meter kust ?
Een simpel antwoord was natuurlijk niet te verwachten. De kosten hangen af van vele zaken : de aannemers, de pompen, de energiekosten, de lengte van het BMS systeem etc. Door DGI wordt ingeschat dat een kosten indikatie per meter kust voor Nederland is : fl 1.500,- a fl 2.000,-.
Dat is per km : 1,5 a 2,0 miljoen. Een projekt zoals Texel (ca 3 tot 5 km) komt dan uit op : ca 4,5 a 10,0 miljoen.
Hierboven komen nog de jaarlijkse energie kosten, welke volgens DGI worden ingeschat als ca. fl 50,- (m/jr) = 50.000,- (km/jr).
De opgegeven kosten komen zeer gunstig uit, vergeleken met traditionele kustbescher- mings methoden. Uitgaande van een zandprijs van fl 7,- per (m-) , kan men voor het bedrag van fl 2.000,- slechts ca 280 (m-) zand suppleren per meter kust. Ter indikatie : voor Texel is o.a. gesuppleerd : 600 (m-lm), dit is bovendien in 3 a 5 jaar weer verdwenen.
Volgens bijlage 3 zijn de kosten zelfs nog goedkoper. Op de laatste bladzijde staat : $ 120,- per linear foot. Dat is omgerekend ca fl 650,- per (m) kust. Verder wordt in bijlage 3 gezegd :
"Over a 30-year period, beachface dewatering will cost less titan half that of competing methods"
Wat is de ervaring tot op heden met BMS ?
Volgens DGI is de ervaring tot op heden : daar waar BMS is aangebracht, werkt het zeer zeker.
Bestaat er meer achtergrond informatie ?
Zie de bijlagen. Er is tevens een rapport in voorbereiding met betrekking tot de evaluatie van de prototype proeven 1985 t/m 1991.
Zijn er bewijzen dat BMS werkt ? Dit wordt niet nader beantwoord.
6 Werkt BMS in getijde gebieden ?
Volgens DGI werkt BMS des te beter indien er sprake is van getijbeweging. De argumentatie is
blz 3
Strand Drainage RWS-DWW, aug 1994 de volgende : BMS voorkomt dat onder hoogwateromstandigheden de freatische lijn zich kan ontwikkelen. Hierdoor wordt voorkomen dat bij vallend water hel zand gemakkelijk wordt opgepikt, en zeewaarts getransporteerd door de golven.
NB : de toepassingen in Denemarken en USA,Florida zijn tevens getijde gebieden.
Werkt BMS onder stormcondities, of onder normale condities ?
De bedoeling van deze vraag was : werkt BMS zo goed dat zelfs tijdens stormen geen erosie te verwachten is ?. Dat de vraag niet echt begrepen is, blijkt uit het antwoord dat de pomp op een willekeurig moment uit te zetten is en dat de pomp ook tijdens storm werkt.
Ik denk dat de grootschalige duinafslag tijdens hevige stormen in Nederland hiermee niet tegen te houden is. Het systeem kan er echter wel voor zorgen dat na die stormen weer een snelle opbouw van het verloren strand mogelijk is.
In de folders mbt Florida wordt gezegd dat het met BMS behandelde kustvak tijdens storm veel minder erodeerde dan aangrenzende kustvakken welke niet met BMA zijn behandeld. Kennelijk help het ook wat onder stormomstandigheden.
Is BMS een goed alternatief voor zand suppleties ?
Men dient zich ervan bewust te zijn dat met zandsuppleties zand aan het systeem wordt toegevoegd. Dal is met BMS niet zo. Volgens DGI is een combinatie van beide methoden : een optimale zachte kustbescherming.
Wat is de hoeveelheid erosie die BMS kan verhelpen ?
Een eenduidig antwoord wordt niet gegeven. In Hirtshals, waar BMS werd toegepast, werd een erosie van ca. 8 (m/jr) tot nul gereduceerd.
Enkele opmerkingen n.a.v bestuderen van de bijlagen.
Enkele nog niet genoemde voordelen zijn :
a. Bij normale kustbeschermingswerken (dijken, breakwaters, duinvoetverdedigingen, strandhoofden etc) loopt men het risico dat een ernstige storm ook een behoorlijke schade teweeg brengt, voorbeeld : de stormen begin 1991. De schade bij BMS is beperkt tot : tijdelijk enige erosie, welke later door het systeem weer wordt hersteld.
b. Het is een uit rekreatie en milieu aantrekkelijke oplossing.
Enkele kritische opmerkingen :
a. Door BMS wordt geen zand toegevoegd aan het systeem. Het zand moet dus ergens vandaan komen. Ik denk dat het van de situatie afhangt waar het zand van ontrokken wordt. Is de omstandigheid zodanig dat er geen of nauwelijks langstransport aanwezig is, dan zal het zand afkomstig moeten zijn van de onderwateroever. Dat hoeft niet zo erg te zijn, want BMS blijft volgens mij geen zand hiervan te ontrekken, immers vermoedelijk treedt een cvenwichtssituatie op. Dessalnietemin moet rekening worden gehouden met mogelijk enige eenmalige versteiling van de onderwateroever.
Het is jammer dat in bijlage 6 enkel de profielen boven zee nivo worden gegeven, ik kan mij niet voorstellen dat bij dit grootschalige experiment deze onderwaterprofielen niet zijn opgemeten. Heeft men opzettelijk deze gegevens niet gepubliceerd ?.
b l z 4
Strand Drainage RWS-DWW, aug 1994 Indien er sprake is van langstransport, dan zal het door BMS gewonnen zand in ieder geval niet ten goede komen aan het benedenstrooms gelegen kustvak. Volgens mij zijn dan de effekten van BMS identiek aan de traditionele oplossingen van strandhoofden etc, welke, zoals bekend, erosie problemen kunnen veroorzaken in de benedenstrooms gelegen kustvakken.
b. BMS berust o.a. op pompen. Dus is er energie nodig. Hoe kom je aan energie in gebieden waar geen stroomvoorziening aanwezig is. Een futuristisch aandoende oplossing zou kunnen zijn dat windmolens moeten voorzien in de energiebehoefte van de pompen. Er zou bekeken kunnen worden of dit een haalbare en rendabele oplossing zou kunnen zijn. Er is, denk ik, een groot probleem bij deze oplossing. Als er geen wind is, is er ook geen energie. Dan werken de pompen dus niet. Wat is het effekt van het langdurig uitvallen van de pompen?. Immers er is kunstmatig een "te steil" strand gekreeerd. Wordt dan de "winst " onevenredig snel afgebroken?. Er is in een van de bijlagen informatie m.b.t. het uitzetten van de pompen, dat ziet er niet best uit!, De bijlage is niet erg duidelijk hierover.
c. Ik begrijp uit de informatie dat de tot heden toegepaste BMS systemen kleinschalig zijn. Men moet denken aan 200 tot 500 (m) kust wat hiermee is beschermd. Dat is dus zeer lokaal..Dat wil nog niet zeggen dat veel grootschaliger toepassingen (orde kilome- ters) verworpen moeten worden. Er is alleen nog geen ervaring mee.
d. De reklame brochures van bijlage 5 vind ik niet echt indrukwekkend.
Ie De meest essentiële informatie niet gepresenteerd. Dat is nl : hoe was de kustachteruitgang in de jaren voordat BMS werd aangebracht.
2e In een van de figuren is te zien dat de "downdrift area", (waar geen BMS is aangebracht) gedurende het eerste jaar ook aangroeit, vervolgens is er een eenmalige sterke achteruitgang (storm?), en daarna is er weer aangroei.
3e In de figuren zijn drie gebieden weergegeven, waarvan er een met BMS is uitgerust. Je kunt ook de zandbalans bekijken van de drie gebieden gezamenlijk i.p.v. afzonderlijk. Mijn konklusie is dan dat er veel minder zand wordt gewonnen door BMS dan gesuggereerd.
e. BMS werkt volgens mij zeer plaatselijk, nl dicht bij de drainagebuizen. Uit de informatie maak ik op dat gedacht moet worden aan : 20 tot 30 (m) zeewaarts van de drainagebuizen is er nog enige invloed. Bij een sterk fluktuerende kustlijn moet er dus rekening mee worden gehouden dat er tijden kunnen aanbreken waarbij BMS totaal geen effekt heeft. Dat is nl zo indien de kustlijn zich verder zeewaarts verplaatst.
Echter in dat geval heb je toch geen kustbescherming nodig?.
Konklusie en aanbeveling
Volgens de brochures en gerapporteerde toepassingen met betrekking tot het Beach Manage- ment System (BMS), is deze kustbeschermingsmethode zeer veelbelovend.
In deze notitie zijn echter redenen genoemd die nopen tot enige terughoudendheid ten anzien van toepassing in Nederland.
Er bestaat echter een groot voordeel : indien het inderdaad werkt, dan is het erg goedkoop vergeleken met traditionele oplossingen voor kustbescherming. Een manier om de vele onzekerheden te elimineren is om een prototypeproef uit te voeren. Het grote voordeel hierbij is dat in Nederland de kust over lange tijdsduur is gemeten (Jarkus raaien etc). Het moet dus
blz 5
Strand Drainage RWS-DWW, aug 1994 mogelijk zijn om na een proef aan te geven of BMS inderdaad enig effekt heeft gehad.
Indien de door Danish Geotechnical Institute opgegeven schatting van de kosten korrekt zijn, dan kost een proef voor 500 (m) kust ca 0.5 a 1 miljoen gulden. Dit is uiteraard exklusief de monitoring van de proef.
Een andere opstelling zou de volgende kunnen zijn : Alom wordt momenteel ervaring opgedaan met dit systeem. Waarom zouden wij dan ook nog een proef willen uitvoeren?.
Nederland zou een afwachtende houding kunnen aannemen en de resultaten afwachten van de gebeurtenissen op dit gebied de komende jaren.
BIJLAGEN
1. Folder : "Beach Management System"
2. Brief van Koster (RWS-DWW) aan Danish Coast Authority, met een vragenlijst.
3. Inlet & Beach - Newsletter # 4 , vol No 1- Summer 1991 4. Coastal Drain System - a new approach to coastal restoration.
Danish Geotechnical Institute.
5. 2 folders van Coastal Stabilization, Ine m.b.t Sailfish Point, Florida, USA : a. Stabeach makes the difference at Sailfish Point, july 1989
b. Stabeach stops beach erosion at Sailfish Point, aug. 1990 6. Coastal Drain System, Full Scale Test- 1985, Summary Report.
Danish Geotechnical Institute
7. 2 foto's : voor en na het aanbrengen van het systeem.
8. Pumping In and Pumping Out: Case Histories of Fluidized Sand Bypassing for Channels and Beachface Dewatering for Beaches.
Jim Parks, paper Coastal Zone '91
9. Het antwoord van Danish Geotechnical Institute op de vragenlijst ( n r 2 )
b l z 6
Strand Drainage RWS-DWW, aug 1994 Nieuwe ontwikkelingen.
Expert opinion Per Bruun
De vragenlijst gericht aan het DCA is eveneens toegestuurd aan prof. Per Bruun. Zijn reaktie op het BMS is in sommige opzichten ronduit vernietigend. In grote lijnen komt zijn reaktie hierop neer:
1. BMS wordt gepropageerd door commerciële ingestelde personen/organisaties. De voorlichting hierover in de vorm van uitgevoerde projekten, reklame, bijdragen aan congressen etc, moet argwanend worden beekeken omdat deze altijd veelbelovend zijn, terwijl de praktijk anders blijkt te zijn, informatie soms achter wordt gehouden of erger: niet correct is, relevante zaken niet genoemd worden etc etc.
Eén voorbeeld: bij de veelbelovende verhalen over de succesvolle toepassingen in Florida wordt vergeten bij de analyse rekening te houden met de natuurlijk optredende zandgolven, welke ter plekke optreden en welke het succes grotendeels verklaren.
Nog een voorbeeld: de geroemde tests in Denemarken blijken helemaal niet zo'n succes te zijn geweest, zie ook paragraaf 7.2
2. BMS is absoluut geen alternatief voor grootschalige zand suppleties in geval van een structureel eroderende kust.
3. BMS werkt enkel in geval van een mild golfklimaat. Stormen vernietigen het daarvoor eventueel behaalde voordeel.
4. BMS vormt is geen preventieve maatregel tegen erosie. Het positieve effekt van BMS is slechts van tijdelijke aard.
Reaktie Per Roed Jacobsen
P.R. Jacobsen (Danish Coast Authority, Kystinspektorated) heeft alsnog een officië"le reaktie gegeven op de naar hun gestuurde brief en vragenlijst. Een voor zich sprekend citaat hieruit:
The area of influence of the system is obviously limited to the part of the coastal prof He for which the drainage significant!)' effects the processes.
Consequently the operation cannot solve problems, which are caused by overall profile erosion, the modifying approach to this situation remains beach nourishment.
We definitely see a future for the system where its main fanction should be to guarantee a beach, where this is needed and justified on recreational and touristically grounds".
Soortgelijke maatregelen.
Recentelijk worden in Australië nieuwe technieken in praktijk getest door: "Public Works Department, Coastal Management, Coast & Rivers Branch".
Dit systeem wordt genoemd: Gravity Drainage. In wezen is dit hetzelfde als BMS, doch bij Gravity Drainage vindt drainage plaats door enkele loodrecht op de kust en onder het strand geplaatste drainage sleuven. In deze sleuven bevinden zich bijv. buizen gemaakt van filterdoek en gevuld met goed doorlatend materiaal. Deze buizen zijn voorts onder een lichte helling geplaatst. De drainage vind op natuurlijke wijze plaats (dwz: zonder pompen).
De bevindingen zullen de komende jaren zeker in de verschillende internationale symposia worden gepresenteerd. Enkele kleinschalige projekten zijn reeds gedeeltelijk geëvalueerd en de bevindingen zijn tot heden positief.
b l z 7
Strand Drainage
RWS-DWW, aug 19946.4 Toepassing in Engeland
In een artikel uit Dredging + Port Construction June 1994: "Beach Management system goes on trial with M A F F funding" wordt melding gemaakt van een zeer recente toepassing. Het betreft een test projekt bij Towan Beach, Newquay (Cornwall), als aanvulling op zandsuppleties. Dit is een populair touristisch strand. De intentie is om de 90 jaar oude standmuur te beschermen tegen de erosie als gevolg van golfaanval. Er zou door BMS aanzanding moeten plaatsvinden en zodoende zou het strandnivo moeten verhogen, waardoor de golfenergie zou moeten verminde- ren welke de standmuur in gevaar brengt. Daarnaast hoopt men op een aantrekkelijk zand- strand voor de recreatie.
Het projekt wordt gcfinancieerd door lokale instanties en gesubsidieerd door M A F F (Ministry of Agriculture, Food and Fisheries).
commentaar:
Een strandmuur zal temeer in gevaar komen bij stormen waarbij sprake is van golfaan- val van betekenis. Doorgaans zal dit eveneens gepaard gaan met een verhoging van de waterstand als gevolg van windopzet. Het lijkt twijfelachtig of onder deze omstandig- heden BMS ook voldoende bescherming biedt. Gezien de reeds geplaatste opmerkingen elders in deze notitie is het aannemelijk dat BMS onder die omstandigheden geen enkel effect heeft. Een storm zal het aangezande strand eroderen, met gevolg dat de bescher- ming van de strandmuur teniet wordt gedaan.
blz 8
I I I I I
l BIJLAGE
l
« 1
l l
I
A
I I I I I I I I I I I I I I I I I
A new efficiënt method for control- ling coastal erosion
Large. unprotected parts of our coastline are eroding away. particularly during the winter. The loss of beaches threatens buildings and other structures and has a direct impact on local. state and national economies.
BEACH MANAGEMENT SYSTEM is a new approach to coastal erosion control and beach restoration. It is an efficiënt system that offers advantages compared with traditional methods of coastal protection.
Dynamic
The BEACH MANAGE- MENT SYSTEM responds to the dynamic action of waves by absorbing the part of the wave energy that causes erosion.
The system creates an unsatu- rated zone in the beach sand.
As result water from the wave run-up percolates down- wards. depositing the suspen- ded sand particles on the beach face.
Thus. by absorbing wave energy and reducing the backwash on the active beach face. the BEACH MAN- AGEMENT SYSTEM causes more sand to be deposited on the beach and halts coastal erosion.
Effective
A full scale demonstration test of the BEACH MAN- AGEMENT SYSTEM at the severely eroded Danish North Sea coast. notorious for its ravaging winter gales, proved that the system was able to stop the erosion rate of 12 feet per year within weeks.
In only etght months. the continued accretion of sand caused bv the BEACH MANAGEMENT SYSTEM led to the formation of a stable shoreline 60-75 feet seaward of the installation.
Although erosion still can occur during periods of heavy storm activity. the system has demonstrated a unique ability to reclaim the newiy eroded area and re-establish a balance along the line of stabilization.
Since the demonstration svstem was installed in the a'utumn of 1985. the BEACH MANAGEMENT SYSTEM has proved itself capable of maintaining a stable shoreline and a wide sandy beach year after year.
Clean
A properlv installed BEACH MANAGEMENT SYSTEM causes no physical obstruc- tions along the beachfront.
Buried beneath the sand. it is not only out of sight. but it is noiseless as well. Thus. the system completely restores and maintains the idyllic charactcr of an open sandy beach.
The seawater which passes
throunh the BEACH MAN-
AGEMENT SYSTEM is
naturally filtered by the
beach sand and may be
pumped into marine aquari-
ums, seawater swimming
pools or stagnant saltwater
channels and lagoons. or may
be discharged back into the
ocean.
Before installation of the Beach p
Management System p
Serious erosion - narrow beaches - undermining.
of structures - loss of tourism revenues. m
Z_j_
Coastal protection and restoration free from adverse environ- mental effects
The BEACH MANAGE- MENT SYSTEM is indivi- dually engineered to stabilize the coastline at each specific site. It consists of a series of filter-encased drainage pipes.
a connecting well. a pumping station and a discharge pipe.
The overall dimensions of the system depend on the proper- ties of the sand. Usually the system is divided into several modules, each about 1.000 feet long.
Water from the wave run-up percolates downward through the sand to the drainage pipes. The water is filtered when it enters into the pipes where it tlows by gravity into the central collection well and from there to the pumping station. The water may then be pumped back into the ocean or. having been natu- rally filtered. it may be utilized for a variety of purposes, as mentioned bet'ore.
When the beach nas been widened and the shoreline sufficiently stabilized. pump- ing can be reduced to a maintenance level.
Dunne periods of heavy storm activity. the shoreline may temporariiy move towards the land but in- creased pumping will counte- ract such erosion. When the storm has passed. the beach
front will restore itself. the shoreline will restabilize in accordance with the range of effect of the BEACH MAN- AGEMENT SYSTEM, and pumping can again be redu- ced to maintenance level until the next storm occurs.
This ability to constamly re-establish the shoreline makes the BEACH MAN- AGEMENT SYSTEM particularly well suited for use in connection with a beach nourishment program- me. When installed concur- rentlv. the BEACH MAN- AGEMENT SYSTEM will efficiently hold the beach nourishment in place for a long period of time without need for further artificial nourishment.
Comprehensively controlled and monitored full-scale tests on the extremelv exposed west coast of Denmark and east coast of The United States of America have demonstrated that down- stream beaches do not suffer bv upstream installation of tlie BEACH MANAGE- MENT SYSTEM.
On the contrary. adjacent downsrream beaches have benefitted by volumes of sand leaked from the continuously replenished bulge of sand in front of the system.
Downstream beaches are not
concurrently starved or
eroded as the BEACH
MANAGEMENT SYSTEM
does not block or divert the
longshore drift out to an area
further offshore.
I
After restoration with the Beach m
Management System |
Erosion halted and beach widened to a stabiüzei
position.
I I I
Discharge pipe transports clean fOtered water to ocean, backwater area or other suitable location.
Pumping station
Pipeline to additional collec- tion wells. (optional)
Nearly buried concrete .structure contains submerged
pumps for pumping water out through discharge pipes.
. Single line pipe transports water by gravity flow to the pumping station.
iuried collecting well.
Beach Management drain pi- pes and filter system.
I I I
Water flows by gravity through sloped pipes to collecting well.
• r S '
&P
The area of dynamic interac- tion of the system with active beach face wave swash.I
7
«'«.
Construction method
Excavated trench with tempo- rary groundwater lowenng by wellpoints
Pipe system consisting of pre-formed filter packs on slotted collectionpipes and
unslotted transport pipes
Pumptng station on top of berm
Pumptng station discharge into
ditch leading to estuary
I I I 1 I I 1 1 1
rn^rn
!?-;v
Thorsminde beach after instal- lation of the BEACH MA- NAGEMENT SYSTEM
Aerial view ofthefull scale BEACH MANAGEMENT demonstration facility at
Thorsminde, Denmark
Operation concept
The BEACH MANAGE- MENT SYSTEM is based on a simple physical principle:
water percolates downward through an unsaturated porous medium such as the sand of an active beach face.
The BEACH MANAGE- MENT SYSTEM reinforces the natural summertime process of sand accretion and prevents the erosion that is normally caused in the winter bv the action of storm waves.
In the summer. winds are generally gentle and waves are long and shallow. The active beach face becomes oniy partially saturated. wave run-up easily percolates downward. and the net result is an accumulation of sand.
In the winter, on the other hand, frequent storms and short and steep waves satu- rate the beach face.
The turbulent surf from large breaking waves keeps sand suspended. and the higher water table produces positive seepage out of the beach face. allowing the backwash to lift sand off the beach face and into suspension. The result is that the backwash of sand is equal to or greater than the uprush. and the net effect is erosion.
By lowenng the watertable adjacent to the drain tube.
the BEACH MANAGE- MENT SYSTEM reduces the hydraulic pressure and creates an unsaturated zone in the beach face. This zone makes downward percolation of was e run-up possibie throughout the year and cuts off the subterranean flow of water to the ocean.
As a result. the volume of the backwash is less. the erosion process is reduced. and additional sand is deposited on the beach.
Long-term controlled tests in Denmark and the United States of America have proven that downstream beaches do not suffer from upstream installation of the BEACH MANAGEMENT SYSTEM. The system does not bloek or divert the long- shore drift of sand out to ar area further offshore.
BEACH MANAGEMENT SYSTEMS. DANMARK A/S, 54-64 Nymoellevej. P. O. Box 119, DK 2800 Lyngby, Denmark.
Telephone +45 42 88 44 44. Telefax +45 42 88 12 40. Telex 37230 geotec dk. Cable Dangeotec.
Worldwide paiended
BIJLAGE
2
To : Per Hoed Jakobsen Danish Coast Authority Hoajbovaj 1
p.o. box 100 DK-7620 Lemvig Denemarken
from: H.J.Koster Rijkswaterstaat p.o box 5044 2600 GA Delft tel +3115699453 Date : 11" september 1991
S u b j e c t : Beach Management System Dear S i r :
As we have never met each other, please let me introducé myself:
I work with the (dutch) Hinistry of Transport, Public Works and Water Mana- gement, Road and Hydraulic Engineering Division (Rijkswaterstaat). This must be familiar to you, for as far as I understand you know my former collegae Mr. Henk Jan Verhagen, who was head of our Advisory Department.
Mr Verhagen, who left the Rijkswaterstaat, was stongly involved in the dutch coast management, I will continue his work.
A collegae of me came back from a conference in Japan with Information on a beach management system that is used in Danmark. The principle of this erosion control is : drainage of the beach. As we understand, this system is operational in Danmark, and it is said that the system indeed works as erosion prevention.
As you know, the dutch are very concerned with their coast. Therefore we are very interested in any potentional erosion prevention measures, speci- ally if they are applied in practise and proove to be effective.
I understood that the Danish Coast Authority is probably the organisation that could answer any questions I have concerning this beach management system. It is therefore that I write this letter to you.
Can you supply us with any information that might be of importance for us with respect of mentioned beach management system ?.
Some questions I have are :
- Is there a logical clear physical explanation of how the system works ?
- What are the costs per meter coast ?
- What is the experience until now with the system ? - Are there any reports (even in Danish) ?
- Is there any proof that the system works ? - Does it work in tidal areas ?
- Does it work under storm conditions or under calm weather conditions?
- Is it a good alternative for a beach nourishment ?
- What is the rate of erosion per year that this system can prevent ? etc etc
Please let me know if you are able to answer my questions. If not, then perhaps you can inform us who , or which organisation, we can contact to get more information.
ïours sincerely, Maarten Jan Koster
1
BIJLAGE
3
u v a i riant
Jim Parks, President
813/254-6645 INLET &
Newsletter # 4 - Vol 2
BEACH
no 1 - Summer 1 9 9 1
'J
D Y N E Q S L t d . P.O. Box 2 0 4 3 Tampa, FL 3 3 6 0 1WHAT IS THE PROBLEM?
One of the fundamental problems in the Coastal Zone is that of sand management. All too often, sand is being eroded where we want It to stay (recreational and storm-protectlon beaches), and Is accumulating where we don't want It (shoals in harbors, entrances, passes and inlets). The traditional and conventional approaches to these problems — dredging, beach nourishment, jetties. groins, seawalls and breakwaters — are increasingly inadequate to the task and much too expensive and environmentally damaglng.
NEW SOLUTIONS FOR OLD PROBLEMS
The new technologies of fluidization (for channel maintenance and for effective bypassing of sand around Inlets) and of dewatering (for beach stabilization and accretion) can change the practice of sand management in the Coastal Zone:
a. These methods will be significantly less expensive overthe long run than conventional methods of dredging and beach restoration and nourishment.
b. Both of the improved methods will be more effective than previous methods.
c. Both of the improved methods are significantly less envi- ronmentally damaging than conventional methods of dredg- ing and beach nourishment.
For channel fluidization and for fluidized sand bypassing, the new improved methods of pipe burial underwater allows use of cheaper longer-iasting plastic pipe and more efficiënt configurations.
For beachface dewatering, the drain-pipe can now be placed seaward of the low-tide line, thus increasing its effectiveness and increasing efficiency by reducing pumping costs.
TECHNOLOGY TRANSFER
One of the difficulties in the transfer of new technology from university research laboratories to the marketplace is a reluctance on the part of many to try new methods. When faced with a beach erosion situation, most cities or counties will employ a reputable consulting coastal professional to study the problem and make recommendations. Most coastal engineers will recommend some variant of the "Wed and true" conventional beach nourishment procedure as a matter of prudence; and most city/county commis- sioners will not go against their own expert's advice in spending taxpayer's monies. This attitude has resulted in a very slow rate of acceptance of the new technology of beachface dewatering for stabilizing and widening beaches. In order for the public (or private) sector to adopt a concept that has been evorving so positively but is still "new", it will be necessary to conduct, document and publicize the results of one or more successful real-scale field demonstratlon projects.
DEMONSTRATIONS OF NEW METHODS
Several full-scale demonstratlons of both methods in a variety of conditions will be initiated in the coming months. All of these will be monitored in detail for at least a yearto provide new "hard data"
on results.
a. Rag Harbor Yacht Haven marina entrance - The first fluidized sand bypassing installation will be in Calvert County, Maryland, on the western shore of Chesapeake Bay, near St. Leonards.
b.
I I
IU-
ng-
1
ch- h è-
Long Beach (Calvert County) Maryland - The private commu- nity beach of this unincorporated vil lage has suffered long term beach erosion and shoreline retreat "caused" by t h | jetties of the Rag Harbor Yacht Haven channel entrance, few hundred yards updrtft to the north. A 200-foot beach- face dewatering demonstratlon will be put in the commi nlty beach in the Fall of 1 9 9 1 . The two systems operatinJ In close proximity will provide an interesting test of thé interaction of these new technologies,
c. St. Petersburg Beach (Pinellas County) Rorida - Uphanfl|
Beach, a public beach facing the Gulf of Mexico, has a Ion J | history of rapid erosion after several episodes of beach nourishment. The most recent renourishment took place ir late December and January 1 9 9 1 , with material dredgec from nearby updrift Blind Pass. A 200-feet long beachface dewatering demonstratlon system will be emplaced on the outer margin of this nourished beach in October 1 9 9 1 in a n M effortto holda portion of this beach in place forlongertharH the expected slx month lifetime after a few months for normal proflle adjustment.
d. Vero Beach (Indian River County), Rorida - In order to provide a test in a different wave climate, another short 200-foot demonstratlon of the improved beachface dewa- tering system is planned at Conn Beach Park, facing the Atlantic Ocean. This should also be emplaced in October e. Long Beach (Brunswick County), North Carolina - This
public beach faces south to the Atlantic Ocean along an 8- mile stretch of east-west coastline between Myrtle Beach and Wilmington. The 2 0 0 foot demonstration test will be monitored fora yearfollowing its installation in late 1 9 9 1 . EXPERIMENTAL NATURE OF PROPOSED PROJECTS
I I I I
These projects are experimental only in the sense that the new' improvements have not been implemented together on a full-scale project. The basic concepts were proven in the prototype, although • it did not perform as well as it should have. A lot has been leamed | from the performance of the prototype, and the subsequent improve- ments (2 U.S. Patents allowed, and other patents pending) have _
been tested in the laboratory and at pilot-scale. • The beachface dewatering at St. Petersburg Beach, FL and U , •
Beach, NC will be the first full-scale projects with all the improve- ments: The demonstration of fluidized sand bypassing at Rag Harbor • Marina, Calvert County, MD on the Chesapeake Bay will be the first • full-scale project with all the improvements to that system.
IMPROVED FLUIDIZED SAND BYPASSING
I
Ruidization is defined as the pumping in of additional water bevond the quicksand point to make a 50:50 sand-water slurry that will flow down a slight grade and is pumpable for significant • distances. In the fluidized sand bypassing version (Rgure 1), the | fluidizerperforated pipes are buried lengthwise along the axis of the channel, sloped from each distal end down to the mid-length _ position. Ajet-eductorpumplocatedlmmediatelyabovethefluidizing I pipes moves the sand through a discharge pipe to the downdrift ™ beach. After fluidization is initiated in one direction with the full flow capacity of the clear water pump, a part of that clear-water stream is • diverted to the fixed jet-eductor pump, and as the level of fluidized • sand is pumped down in a local "cone of depression", lateral flow along the length of the trench "feeds" the jet pump.
I
I
* 3;5CH>F.;E
FLUIOIZER PIPES Flgure 1
ADVANTAGES OF GHAN/NAV FLUIDIZED SAND BYPASSING 1. No Lag Time - With conventional dredging, there is an
unavoidable delay between the time that a decision is made to dredge the newly developed shoal in the channel that has become a hazard to boat navigation, and the time when the dredge is available and can be mobilized and brought to the site — and during that lag time of one to eight months, safe navigationai access is restricted.
With DYNEQS Chan/Nav System permanent- ly installed, there is no delay, as it only takes a "flick of the switch" to start the Fluldlzed Sand Bypassing process.
2. No mobilteation/demobilizatlon costs -With conventional dredging of small jobs (under 20,000 cuyds), a large part of the project costs are for bringing the dredge to the site (mobilization and demobilization expenses), and thus the per cubic yard costs for small dredge projects may be as high as $7.50 to $10 per cubic yard, whereas for larger projects the per unit costs may be on the order of $3/cuyd.
As the Chan/Nav Fluidized Sand Bypassing system is permanent, it is always available without mob/demob costs.
3. Malntenance program expenses known • When conven- tional maintenance dredging is planned for every 3 years but the inlet shoals up in 2 years, one of two things must happen — either the boaters must suffer for a year with hazardous access through the shoals in the channel, or additional funds must be found for an unscheduled early dredging, and dredging maintenance program expenses are difficult to antlctpate. This also happens when dredg- ing of an inlet is only scheduled "when needed" — it is difficult to provide for such in annual budgets.
With a regular schedule of frequent periodlc
"flushings" of the channel by Fluidized Sand Bypassing, the annual costs of the malnte- nance program are known years ahead.
4. Unantieipated storm events - When a storm event occurs shortly after completion of conventional dredging, sand can fill in most of the dredged channel almost ovemight.
When this happens with a Chan/Nav system, a 'fiick of the switch" will rectify the problem almost instantaneously, with only a small charge for an additional "flushing".
5. Increase storage capaerty - Attempts have been made to overcome this problem by "overdredglng" (Advance dredg- ing in the Corps' pariance) a deeper and widerchannel than is needed for navigationai purposes, in an attemptto make the channel "last longer" between successive dredgings.
This is nearty always afutile attempt, in that it iswell known that "Nature abhors a vacuüm", and a larger hole only fills in faster. This also cuts off natural bypassing and thus exacerbates the downdrift beach erosion problem.
The frequent (monthly?) Fluidized Sand By- passing flushings of a channel will maintain the alignment, depth and width to those dimensions adequate for navigation without oversizing.
6. Efficiency and Effect!veness- Some form of "sand bypass- ing" is currently being recommended in many INLET SAND MANAGEMENT PLANS now being prepared for most Florida inlets. The DYNEQS approach is to use the boat navigation channel itself asthe "sand trap", andto empty that trap by pumping out fluidized sand on a frequent periodlc basis.
The frequency of the pumping schedule will be site- specific-some inlets may require weekly pumping while others may only require it monthly or every-other-month.
Pumping can also be done during and immediately after each storm event. Thus adequate depths of water will be maintained at all tlmes, and the location and alignment of the boat channel will be fixed in position, allowing perma- nent channel markers for tncreased boatlng safety. The typicaily mid-length fixed jet-eductor pumps down a "cone of depression" in the fluidized sand & thus induces lateral flow along the length of the fluidized channel (Rgure 2).
HOW DOES BEACHFACE DEWATERING WORK?
Beachface dewatering works by lowering the top of the water- saturated zone (water table) immediately beneath the active beach face (see Rgure 3). Part of the wave uprush "soaks in", causing reduced backswash and thus part of the suspended sand brought onto the beach with each wave uprush is left on the beach.
Secondary effects include local "hardenlng" or densification of the sand by withdrawing supporting interstltial porewater, and reducing seepage outflow at the toe of the beach.
The DYNEQS system incorporates several patented improve- ments that increase the effectiveness of beachface dewatering and cut the costs to less than half that of previous systems. Briefly, the differences are: (1) the Sailfish Point system was installed some 60 feet Inland of the high tlde line, using heavy power machlnery and Standard construction dewatering techniques, wlth a pump lift station farther inland behind the dunes; (2) the DYNEQS system wlll be Installed between high and low tide Unes using a fluidized self- burial technique requirlng no heavy bull-dozers and power shovels on the beach, and will use a submersible pump buried in the beach near the drain-pipe to push the water to a diffuser located offshore.
Extensive testing'of simllar systems in Denmark over several years showed that a widened beach could be maintained 20-30 meters (65-100 feet) seaward ofthe buried drairvpipes. The system at Sailfish Point was installed near the limit of lts potential effectiveness.
EXPERIENCE FROM PROTOTYPE DEWATERING INSTALLATION Sailfish Point • The prototype American installation of beach- face dewatering at Sailfish Point (Hutchinson Island) near Stuart, Flonda has been operatlng for nearly 3 years. An analysis of the data from 2 1/2 years of beach profile monitoring has demonstrated that erosive retreat has been halted and seasonal beach width fluctua- tions have been significantly damped out, as compared with both updrift and downdrift control zones, without detectable downdrift bad effects (Robert G. Dean in report for Coastal Stabilization, Ine dated July 25, 1990).
COMMONLY ASKED QUESTIONS
What artarnatlvas ware consldarad? These include periodic
conventional beach nourishment with dredging from offshore sand sources and pumplng sand to the beach every few years; construc- tion of shore- parallel near-offshore breakwaters; construction of various types of shore-perpendicular grolns; and the STABEACH version of beachface dewatering. All of these altematives are considerably more costly than the proposed work. The altemative advocated by Prof. Orrin Pilkey — doing nothing — would result in loss of recreational beach, boardwalk and roads.
How 9m Mlwtgd rian bo lurtlfl«ti at Iwrt gnvlrvnrrwrrtaiiy da maging to water or wetland area»? Conventional beach nourish- ment disrupts offshore fisheries, creates considerable turbidity, and the dumping of large quantities of sand on the eroded beach disrupts shore biologie communities. Shoreparallel near-ofTshore breakwa- ter* constitute attractive nuisance hazards to swimmers and surf- ers, and create sand shoal connectlons from the shore to the breakwater that disrupt normal longshore drift of sand; shore- perpendicular grolns are now considered to be harmful to downdrift beaches, and are aesthetically unsightly to beach users; and the STABEACH version of beachface dewatering causes a drastic disruption ofthe beach during installation (very wide and deep trench dug by heavy machinery) and is comparatively ineffective in being located too far landward from the high tide-line.
How much dlsturbance of beach during Installation? The selected plan (DYNEQS version of beachface dewatering) is in- stalled near the low tide line without use of massive machinery or wide deep trenches, is potentially more effecttve in accretion, and data from 2 1/2 years of beach profile monitoring at the Sailfish Point, Florida prototype has revealed a conspicuous lack of harmful effects on downdrift beaches, and the water pumped back Into the sea is filtered seawaterthat if anything is beneficial to the receiving body of water. Protection against drawdown of a landward freshwater groundtable will be achieved with a plastic impermeable banier landward of the drain pipes.
What efforta are made to mlnlmtoe adversa Imnnef ? Plans have been developed fora method of installation (burial) ofthe drain pipes without resorting to heavy machinery (power shovels, bulldoz-
I
ers, back-hoes, etc) for digging massive trenches. These new' methods are the subject of 2 U.S. Patents that have recently been allowed and will be issued In the near future, and several more patentaa applications have been submitted. The area to be treated is a narrowl sand beach devold of any vegetatlon. No wetlands wlll be lost—the™
exlstlngsandy beach will begradually widened and stabilized against
seasonal erosion. • APPUCATION FOR DEMONSTRATION PERMIT
This application is for a permlt for a small (200 foot longjaaj temporary (one year) experimental demonstration of new technolo-l gy (improved beachface dewatering) on an eroding beach. There Is™
little likelihood that the demonstration will do any lasting harm, and a great deal will be leamed during the year of intensive and extensive •
monitoring of both primary and side effects. g | Monitoring will include:
(1) multiple monthly beach proflles from stations located wellfl above high tide line out to depths of 10 feet, in both the B treated zone and control zones updrift and downdrift;
(2) recording of groundwater levels in observation wells at m several sites on beach during entire tidal cycle, with and • without pumping;
(3) recording of volumes of water pumped under a rang;
pumping schedules (continuous to initiate maximum ef- fects, or few hours per day or week to maintain optimum effects); and
(4) observations of effects on wildlife (turtles, birds, etc).
I I
More experimentatlon of this type is needed under a variety of' wave and beach conditions before anyone can be sure that this new
approach will prove useful and safe and economical for routine • treatment of eroding beaches and receding shorelines. In the very I unlikely event that harmful side-effects will be revealed, the process is controllable (extent of both nrimary and sidc effects nan limitPri hy decreasing the pumping schedule) and reversible (at the end of the I demonstration year or before, the system can be easily removed and m the beach will quickly return to "normal").
There Is little likelihood of harmful effects in case of a mishap: M for example, the components are buried deeply enough that it would • take a major storm to exhume the buried system and the parts are • unlikely to become "battering rams". The extent of groundwater lowering toward the inland freshwater system is limited by an • impermeable plastic curtain installed landward ofthe drain syste |
The operation of this demonstration fora year will increase our understanding ofthe physical processes involved, will improve our __
ability to predict potential posltive and negative effects, and will • establish a "track record" to help avoid the need for "case-by-case" • risk assessment for each application of dewatering.
I
COASTAL SAND DUNES
Coastal sand dunes are recognized as an important "line of defence" against storm-drivenshoreline recession. Mucheffort has . gone into artificially vegetating existing dune areas, and "snow- • f e nee" methods to accrete additional dune build-ups from wind- • driven sand.
The natural source of sand for dunes is the "dry" beach width • above normal high tide. Only from this "permanently dry" area is it • possibie for wind to piek up appreciable quantities of sand and carry it inland a few tens/hundreds of feet (meters) and deposit it on dunes.
On an eroding beach, there is commonly only a very narrow width of permanently dry sand, if any. On an accreting beach there is usually an appreciable width of dry sand. A restored or nourished beach starts off, at least, with a wide dry beach zone that accomplish-
I
I
I
es some dune nourlshment from wind action.
Stablllzation of a beach (reducing seasonal beach width chang- es) would also help stabilize and accrete beach dunes by providlng year-round dry beach zones for wind to act on.
KEEPING ENVIRONMENT CLEAN
Two prime environmental concerns are (1) waterquality, and (2) potential harm to endangered wildllfe species. These new methods are environmentaily beneflcial rather than harmful. The effluent water from beachface dewatering is filtered seawater, and unless the re is a local need for such (research flsh tanks, or to "sweeten"
nearby stagnant lagoons or harbors) it will be piped back to sea. In the fluldization applicatlon, the Inputwateris local seawaterthat has been filtered, and the effluent as fluidized sand was demonstrated In the Anna Maria prototype to not create turbidity. This will continue to be true as long as appllcatlons are limited to tidal Inlets with clean well-sorted sandy bottom — extending this method to muddy bot- toms may well create unwanted turbidity (which can be controlled).
The fluidized condition is maintained only as long as water Is pumped into the fluidlzing pipes — when the pumps are turned off (90 percent or more of the time), the bottom sediments will quickly (minutes or hours) revert to normal conditions, and will not stress bottom-iivingwildlife. The effects will be much less severe than those caused by conventional dredging and normal disposal of dredged materials.
Dewatering of the active beachface can be controlled so as to not affect nearby fresh-watergroundwater, as the improved method for pipe emplacement makes it easy to incorporate an impermeable plastic sheet banier landward of the drain pipes. The effects are aimed at the active swash-zone, and the amount of draw-down is relatively minor (so asto tip the balance toward accretion rather than erosion). There should be no discemible effects on the "dampness"
needed for turtle egg-laying and hatching, but this will be closely monitored during the year.
ACTIVE vs PASSIVE
Beach erosion and channel shoaling are deleterious end prod- ucts of dynamte shoreline processes driven by waves, currents and tides. Up till now, most of the methods used to "fight" these bad effects have been passive or "one-shot" episodic remediations. The passlve methods include groins, seawalls, jetties, bulkheads and breakwaters; and the "one-shot" methods that have to be repeated from time to time include dredging of channels and beach nourlsh- ment with sand dredged from offshore "borrow areas".
Perhaps it is time to consider flghting these activa continuous pemicious processes with activa continuous beneflcial systems, such as fluidized sand bypassing for channel maintenance and alleviatlon of downdrift beach erosion; and baachface dewatering for beach stabilization and beach widening.
PAYMENT & FINANCING
One-shot remediation actions such as conventional channel dredging or beach nourlshment usually require full payment on completlonofthe project—with noguarantees, eventhough a storm event immediately afterwards mayfill in the dredged channel or erode the renourished beach.
Because the new technologies of fluidized sand bypassing and beachface dewatering each invorve the Installation of permanent equipment (pipes, pumps, valves, control systems, etc), they may qualily for long term financing in much the same manner as potable water supply or sewage systems. Many communities, counties or regions have established "special taxing districts" for long term maintenance of inlets and beaches which can serve as the vehicle for paying off long term obligations.
BOTTOM LINE • COSTS
[NOTE: Each situation is unique and will require site-speciflc design and engineering, and thus costs are not readily predictable.
The following are generallzed approximations.]
Fluidized gand Bypassing- The cost of installation of a fluidized sand bypassing system is about the cost of two orthree dredglngs.
The subsequent low costs of operation of the system will make the annualized costs over a 20-30 year period less than half that of periodic conventional dredging.
Beachface Dewatering - The DYNEQS "Beach/Advancer" sys- tem of beachface dewatering Is currently priced at $120 per linear foot of beach ($600,000 per mile). Conventional beach nourishment costs from $1 to 3 million dollars per mile, and must be repeated on an average 5 year cycle. Overa 30-year period, DYNEQS beachface dewatering will cost less than half that of competing methods (see enclosed Cost Comparison).
DYNEQS Ltd.
P.O. Box 2043 Tampa, FL 33601
H, K o W ^
Mr. K. -W,-i • - -
Bykswaterst Road/Byd Eng DJ.V Van der Burghweg,
F O. Box 5044, Delft
THE NETHERLANDS 2600 GA
BIJLAGE
4
Danish Geotechnical Institute « « « « d
F I L I A T E D T O A T V • T H E D A N I S H A C A D E M Y O F T E C H N I C A L S C I E N C E S
G O A S T - Ï V l L i D R A X N S Y S T E M - a new approach to coastal restoration
by Hans Vesterby, Danish Geotechnical Institute - DGI Lyngby, Denmark
SYNOPSIS The loss of beaches threatens recroative revenues, structures and has a direct impact on local and nationai economies. In combatting the erosive forces from wave actions soft engin- eering solutions are likely to be more effective than structional defences. The Coastal Drain System is a new effective soft approach to coastal erosion control and beach restoration which has no impact on human activity and the environment. By reducing the hydraulic pressure, the system produces gradiënt towards its drain, cut-off the natural ground water seepage and creates and un- saturated aone of deprossion under the beach face which allows downwards percoiation of water from tha up-rush. This reduces the backswash on the beach face limiting the erosion process while de- positing more sand on the beach with a changed and more stable profile. Full-scalo controlled fa- cilities in Oenmark and USA have shown that the system does not even halt the erosion but causcs substantial accretion as woll.
INTR0DUCTI0N
Many different approaches have been made in at- tompt to solve or alleviate the problems of beach erosion. Hard engineering such as revet- ments, seawalls, groins, breakwaters and jetties normally offer only a short term respitc merely serving to delay the inevitable - or even aggra- vate the ultimate losses that will occur during an infrequent evont. Most of these construc- tions introducé serious long term detrimental effocts on downstream beaches and have at the sarac time substantial adverse environmental ef- focts.
For ycars Beach Nourishment has been consider- ed the most effective response to the erosion of beaches and at the sant time a fair protectlon of the inland since a major part of the wave cnergy will be absorbed on a wide sandy beach.
A well-known problera with beach nourishment is that sometimes it does not last.
In samo cases a significat portion of tho new sand is lost during the next storm or by the natural profile adjustment if the sand is not placcd carcfully in the profile.
A now approach to the alleviation of beach erosion problems has rocently boen introduced and testcd in practico - The Coastal Drain Sys- tem (Vesterby, 1907). This system causcs arti- ficial intorforonco with natures morphology through a localized slowdown of ono natural pro- cess. and cpecd-up of anothor.
It involves permanent installation of pipes and pumps, but is not a visible eyesore or phy- sical obstruction as almost all components are buried. Extcnsive long-torm, controlled full- scale domonstrations have shown tho positive ef- fect from tho Coastal Drain System and no no- ticcablo sido cffects on downstrpam beaches (Hanscn, 1986; Tcrchunian, 1909).
THE COASTAL DRAIN SYSTEM
In Denmark in 1981 the North Sea Research Centre required large volumes of filtered seawator (-300 mVhour) for aquaria and wave tanks, but pri- marily as a sourco of heat for a heat-pump sys- tem. The author proposed and designed a "hori- zontal" well, and it was installed below sea level and parallel to the shore line just inland of the high water line at Hirtshals, Denmark
(Figure 1 ) .
Fig.i Doach at Hirtshals, bcfore installation
This novol well, using tho sand of tho beach as a filter, produccd tho required quality and quantity of seawator - for .about two weeks af ter which it was found that the amount of water was continuously tapcring off.
HANS VESTERBY
M.Sc. Civll Engineering
•anish Geotechnical Institute
Lyngby, Denmark
DGI
A site investigation showed that the slotted well pipe had not clogged, but the shore line was several meters farther frora the well line that it had been initially, thus greatly
lengthening the filter path of the intake water and thereby decreasing the yield of the well.
The water supply quantity problem was solved by installlng a second horizontal well as an exten- sion of the first.
The use of this water in the heat pump system required full-time pumping frora the filter sys- tem during the winter. It was noticed that the beach in the vicinity of the Coastal Drain Sys- tem grows wider than narrower through the win- terstorm season (Figure 2). Strong winds piced up the dry sand and blew it up to and over the sea wall into the city of Hirtshals.
Fig.2 Same as Fig.1 duning operation of the Coastal Drain System
Snow fences were installed to oontrol the blow- ing sand, and sand dunes built higher and high- er. Eventually this "excess" sand has been trucked away to nourish a nearby beach in an
mount of 25,000 m3 every year.
PRINCIPLE OF FUNCTION
Some of the scientific principles behind the ef- fect frora the Coastal Drain System have been known for a long time (Duncan, 1964; Emery and Foster, 1948; Crant, 1948; Harrison et al, 1971;
Harrison, 1972; Holland, 1972; Waddell, 1976) and small-scale tests performed (Chappel et al, 1979; Kawata and Tsuchiya, 1986; Machemehl et al, 1975; Sato 1986) but never reduced to prac- tica.
Wave action brings sand as bed load onto the beach face by the wave rup-up and the returning bach swash takes sand off the beach.
The net effect under a specific set of condi- tions (wave period, wave height, beach slope, etc) may be either negative or positiva. The balance may be tipped towards accretion by low- frequency, gentle waves during the summertime and towards erosion by higher-frequency, steeper waves from storms usually associated with win- tertime.
With gentle waves, there is sufficiënt time between each wave run-up for a substantial por- tion of the water in the upper beach face to soak down to the water table, leaving a zone of unsaturated sand which can absorb a small por- ti on of the next wave run-up. The result is that less water runs off the beach face than runs up and thus less sand is carried away than was brought to the beach face.
With steeper, high-frequency waves there is not enough time between waves for water to drain out of the upper boach face. This zone remains saturated and all the run-up from ono wave runs back off. Because the upper beach face sand is saturated, sand grains are more readily incorpo- rated into the runn-off bed load resulting in net erosion. The ground water table may actual- ly become slightly elevated, causing a seaward seepage through the beach face, further enhanc- ing erosion by the backswash.
The Coastal Drain System works by lowering the water table and creating an unsaturated zone un- der the beach face. This is accomplished by pumping water out of a buried almost horizontal slotted pipe.
COASTAL DRAIN SYSTEM
UpruthZont Wgvei
Sea BottoïnT BEFORf
Prjvjgus GreundwotBf Lev{|
Lowered Graundvater
•jquipotentiai Line
Fig.3 Cross-section lowcred water table. Principles.
