Coastal Dynamics of the Danube Delta

Coastal Dynamics

of the

Cover photography: sand dunes at Sf. Gheorghe beach (seaward view) Cover photography and design: Sebastian Dan

Coastal Dynamics of the Danube Delta

Kust Dynamiek van de Donau Delta

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorziter van het College voor Promoties,

in het openbaar te verdedigen op maandag 27 mei 2013 om 12.30 uur

door

Sebastian DAN

Master of Science in Geology, University of Bucharest geboren te Campina, Romania

Prof. dr. ir. M.J.F. Stive

Samenstelling promotiecomissie Rector Magnificus

Prof. dr. ir. M.J.F. Stive

Prof. emer. dr. S.B. Kroonenberg Prof. dr. ir. Z.B. Wang

Prof. dr. ir. C. Dinu Ir. D.J.R. Walstra Dr. ir. N. Panin

Dr. A.J.F. Van der Spek Prof. dr. ir. W.S.J. Uijttewaal

voorzitter

Technische Universiteit Delft, promotor Technische Universiteit Delft

Technische Universiteit Delft University of Bucharest Technische Universiteit Delft GeoEcoMar Bucharest

DELTARES Delft

Technische Universiteit Delft, reservelid

This thesis should be referred to as: Dan, S. (2013). Coastal Dynamics of the Danube Delta. Ph.D. thesis, Delft University of Technology.

ISBN: 978-94-6191-730-0

Copyright © 2013 by Sebastian DAN

Printed by: Ipskamp Drukkers B.V., Enschede, the Netherlands.

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval sys-tem, without written permission of the author.

vii

A

BSTRACT

The retreat of deltaic coastlines is a global problem and causes large disruptions in the life of numerous and often large communities. The erosion which causes the shoreline retreat is induced by two types of factors, natural and anthropogenic. The natural factors are related to the evolution of the river system generally caus-ing a decrease of sediments reachcaus-ing the beaches, to the subsidence resultcaus-ing from tectonic (eustatic) movements and/or from the compaction of the sediments and to the shifting of the active sedimentation between different distributaries of a delta. The anthropogenic factors are related mainly to the structures in the river and its tributaries (embankments, barrages, etc.) and on the coast (sea defences and harbours). These interventions diminish the volume of sediments reaching the coasts and disturb the natural patterns of coastal sediment circulation. A special factor is accelerated climate change, believed to be a combination of natural evo-lution and man induced changes, which may cause a larger number or more ex-treme events and increasing rates of relative sea rise resulting in accelerated coastal erosion.

The Danube Delta coastal zone, the largest delta in the European Union member states and part of the Danube Delta Biosphere Reserve, is affected by all of the factors described above. Large parts of this coast are affected by erosion, some-times at staggering retreat rates of 20 m/year. Although the shoreline retreat is part of the natural evolution of this delta, the high rates recorded in the last dec-ades are due to human interventions. They started approximately one century and a half ago when the lower course of the Danube and the central distributary of its delta were subject to regularization works to improve the navigation and to limit the periodic floods. The works continued until modern times with larger barrages (to generate hydropower) and coastal structures, such as the Sulina jetties, meant to facilitate navigation.

In order to investigate and quantify the main processes governing a large part of the Danube Delta coast a systematic approach is necessary. Initially, the factors forcing the hydrodynamics are investigated by analyzing a comprehensive wind climate (eleven years). The wind data and the study zone bathymetry formed the primary input for a hindcast of the wave climate. The northern wind and wave di-rections are found to be dominant, already suggesting the direction of the sedi-ment transport. However, due to the changing of shoreline orientation and to the

sheltering against certain wind and wave directions, each beach sector has its par-ticular dynamics.

The next essential parameter for investigation of the coastal dynamics is the clo-sure depth, defined as the largest depth where the waves influence the morphology of the sea bottom over a certain period of time (i.c. engineering scale related). The lack of systematic measurements of the active beach profiles was resolved by calculating the closure depth using the largest waves occurring in more than one decade. The distribution of the closure depth values along the study zone high-lights the degree of exposure for different beach sectors to the main wave direc-tions.

To gain insight into the sediment transport along the study zone a schematic computation is performed using two bulk formulas, CERC and Kamphuis for which wave parameters were manually extracted. Based on the findings a detailed computation of the alongshore sediment transport was made using the one line numerical model UNIBEST CL+ and applying two formulas, CERC and Bijker. Two categories of data were used: existent data - the bathymetry of a large part of the Danube Delta coast and the water and sediment characteristics, and com-putationally derived data – the wave hindcast and the closure depth. The results indicate a net alongshore sediment transport generally southward oriented with steep gradients for the Sulina (the northern limit of the study zone) – Sf. Gheor-ghe area and for Sahalin spit, and northward oriented with mild gradients in sed-iment transport for the area Sahalin – Portita Inlet (the southern limit of the study zone). Assuming that the beach profile does not change when the shoreline moves, a sediment budget is derived from the gradients in the alongshore transport. The computed advance/retreat rates match well with the observed ones, except for Sahalin spit. This geomorphologic feature is controlled not just by gradients in the alongshore transport, but also by cross-shore processes and by the total input of sediments. The realistic results of the computations give credibility to the predictions made for the future position of the shoreline. The same one-line numerical model was used to simulate the position of the shoreline for the next 25 and 50 years and the results reveal that the actual trends and the ero-sion/advance rates will be similar to those of the present day. Moreover, the effect of relative sea rise (the Bruun effect) is estimated and added to the results of the simulations.

The use of multiannual averages for the wave characteristics and sediment trans-ports is a common approach when a coast is investigated, but the changes of the active beach mainly occur during extreme events (storms, floods). Therefore, the vulnerability of the Danube Delta coast to episodic water level changes induced by storms is investigated. The low dunes, confining the active beach at the landward side, the low lying inland areas and the relative energetic wave climate make this area potential by vulnerable to storm-induced floods. To investigate the effect of the short term water level changes a selection of the ten most significant storm

Abstract ix

scenarios was made. Among them one is a hypothetic extreme case: the combina-tion of the largest wind speed recorded in 11 years (40 m/s) and the direccombina-tion causing the largest water elevation (north-east). To quantify the response of the aerial active beach ten profiles placed at representative locations (except at Saha-lin spit) were selected. These profiles, extending from a water depth of -1.5 m to the dunes’ crest, were measured during three consecutive years. The results show significant water level increases, especially for storms from northern directions (in-cluding the extreme case) when the storm water level can rise over 1 m. The max-imum flooding of the beaches (50 to 100% of the aerial part) is taking place in the central part of the area Sulina – Sf. Gheorghe and it is induced by the storms from the northern directions. This area is particularly vulnerable to extreme storms which can cause breaching of the dune system with negative consequences for the freshwater inland ecosystem, the infrastructure and the local economic ac-tivities.

As the one-line numerical model did not produce reliable estimates for sediment circulation in the Sahalin spit system, a more in depth investigation was made us-ing historical data and the state-of-the-art numerical model Delft-3D. In the first stage an idealised spit was designed to analyze the wave field distributions and the relationship between along- and cross-shore components of the sediment transport. The wave fields show convergence towards the spit and the cross-shore sediment transport is found to be proportional to the water level. At an imposed water level of +0.5 m total the cross-shore sediment transport is of the same order of magnitude as that of the alongshore transport, but at a level of +1.0 m the cross-shore transport is 4-5 times larger than the alongshore transport.

In the second stage the investigation focuses on Sahalin spit. Using the findings from the idealised case the dynamics of Sahalin are investigated by simulation of the wave climate, the quantification of the sediment volumes circulating in the system and the formulation of a conceptual model for the formation and evolution of a spit in general. The wave convergence and the full exposure of the spit to the main storm directions lead to large volumes of sediments transported along- (maximum value 1.6 million m3_{/yr) and cross-shore (a total of 1 million m}3_/yr).

The hypothesis that Sahalin spit’s elongation is fed by alongshore sediment transport and its mainland migration is caused by the cross-shore sediment transport. The spit system has two main sources of sediments: the Sf. Gheorghe distributary and the alongshore current from the north. The calculations indicate that similar volumes of sediments are stored at the southern tip due to the spit’s elongation. Meantime, the sediments transported from the sea side are deposited at the landward side of the spit and they are of the same order of magnitude as the accommodation space created by relative sea level rise, explaining the relative constant depth of the bay created behind the spit. The position of Sahalin spit at a certain moment is the result of the constant interaction between cross-shore processes which roll the spit towards the mainland and alongshore processes re-storing the equilibrium, but always closer to the mainland. The conceptual model

developed in this thesis describes a four stage cycle for formation, development and merging with the mainland of a spit formed at a river mouth in this mi-crotidal environment and the spit cycle is regarded as part of the wave-dominated deltaic lobe development. The evolution of a spit strongly depends on the sedi-ment input and the wave energy level. As expected for the future, the sedisedi-ments volumes reaching the coasts will decrease and the wave energy will increase, the spit’s formation and evolution cycles can be different than in the past or even stop.

The morphological models formulated in this study explaining and quantifying the processes controlling the Danube Delta coast have a broader range of application. They can be used for similar environments in the north-western Black Sea: mi-crotidal, abundant sediment supply, wave climate dominated by one direction, and sudden changes in the coast orientation.

The general recommendations following from this study refer to mitigating the rate of shoreline retreat affecting especially the Sulina – Sf. Gheorghe sector. Since this retreat is caused largely by a human induced sand deficit, artificial nourish-ment is suggested to be the most sustainable solution and it can be done using sand dredged from the Sulina canal and sand accumulated just north of this ca-nal. Other recommendations are to avoid interventions which can diminish the volumes of sand reaching the coast and those which might disturb the sediment circulation in the Sahalin spit system.

xi

S

AMENVATTING

Het terugtrekken van de kustlijn van delta’s is een mondiaal probleem en veroor-zaakt grote verstoringen in het leven van talrijke en vaak grote gemeenschappen. De erosie, waardoor de kustlijn zich terugtrekt, wordt veroorzaakt door twee ty-pen factoren: natuurlijke en door de mens veroorzaakte factoren. De natuurlijke factoren zijn gerelateerd aan de evolutie van het riviersysteem door een verminde-ring van sediment toevoer naar de stranden, door daling van de bodem ten gevol-ge van tektonische (eustatische) bewegingevol-gen, en/of door de compressie van sedi-ment en verschuiving van actieve sedisedi-mentatie tussen de verschillende zijtakken van een delta. De door de mens veroorzaakte, ofwel antropogene, factoren zijn voornamelijk gerelateerd aan bouwwerken in rivieren en haar zijtakken (dijken, stuwdammen, enz.) en aan kustbouwwerken (zeeweringen en havens). Deze inter-venties verminderen het volume van sediment dat de kusten bereikt en verstoren de natuurlijke circulatie van kustsediment. Een bijzondere toegevoegde factor is de verandering van het klimaat die in versneld tempo plaats vindt onder invloed van een combinatie van zowel natuurlijke evolutie als door de mens veroorzaakte veranderingen. Het groot aantal extreme gebeurtenissen en de relatieve zeespiegel-stijging hebben een versnelde kusterosie tot gevolg.

Het kustgebied van de Donaudelta, de grootste delta in de Europese Unie, en een deel van de “Donau-Delta Biosphere Reserve”, is onderhevig aan alle hierboven beschreven factoren. Grote delen van deze kust zijn aangetast door erosie, soms met een schokkende terugtrekking van 20 m per jaar. Hoewel erosie van de kust-lijn deel is van de natuurlijke evolutie van deze delta, is de hoge toename in de laatste decennia te wijten aan menselijke interventies. Dit begon ongeveer ander-halve eeuw geleden toen de beneden loop van de Donau en de centrale tak van de Donaudelta gereguleerd werden voor betere navigatie, en om periodieke overstro-mingen in te perken. Deze werkzaamheden, de bouw van 3 grote stuwdammen voor het genereren van waterkracht en kustwerken zoals de Sulina steigers om na-vigatie te vergemakkelijken, zijn tot zeer recent uitgevoerd.

Om de processen in de Donaudelta te onderzoeken en te kwantificeren, is een sys-tematische aanpak nodig. In eerste instantie zijn de hydrodynamische factoren onderzocht in een analyse van het wind klimaat over elf jaar. De wind-gegevens en de bathymetrie vormden de basis gegevens voor een “hindcast” van het golfkli-maat. De noordelijke richting van de wind en de golven zijn dominant en

suggere-ren daarna de richting van het sedimenttransport. Echter, ten gevolge van een veranderde oriëntatie van de kustlijn en door afscherming van bepaalde wind- en golf richtingen, heeft ieder stuk strand een specifieke en eigen dynamiek.

Vervolgens is voor het onderzoek van de kust-dynamiek de “closure-depth”, (gede-finieerd als de grootste diepte waarin golven over een bepaalde tijdsduur de mor-fologie van de zeebodem beïnvloeden), een essentiële voorwaarde.

Het ontbreken van systematische metingen van strandprofielen heeft geleid tot de berekening van de “closure-depth” met als uitganspunt de grootste golven per de-cennium. De distributie van de waarden van de “closure- depth” in het gebied van deze studie, geven voor ieder afzonderlijk stuk strand de blootstelling aan de do-minante golf richting aan.

Om inzicht te krijgen in de dynamiek van het sedimenttransport in dit gebied, is een schematische berekening uitgevoerd met behulp van de twee formules van CERC en Kamphuis, waarvoor de gegevens over golven handmatig zijn verkregen basis van deze bevindingen is een gedetailleerde berekening gemaakt van het se-diment langstransport langs de kuststrook met het één-lijn numerieke model van UNIBEST CL + en twee formules, namelijk CERC en Bijker. Twee categorieën van gegevens zijn gebruikt: bestaande gegevens, namelijk de ‘bathymetrie’ van een groot deel van de Donau-delta kust in relatie tot de water en sediment kenmerken, en rekenkundig afgeleide gegevens,-een zogenaamde ‘hindcast’ van de golven en de ‘closure-depth’. Deze resultaten geven een netto zuidwaarts georiënteerd sediment-transport langs de kustlijn aan met een steile gradiënt voor de Sulina (de noorde-lijke grens van het studiegebied)-Sf.Gheorghe gebied en de Sahalin ‘spit’, en een noordwaarts georiënteerd sedimenttransport met een matige gradiënt voor het ge-bied van de Sahalin-Portita inham (de zuidelijke grens van het studiegege-bied). Uit-gaande van de stelling dat het strandprofiel niet verandert wanneer de kustlijn verplaatst, kan een sediment budget worden afgeleid uit de helling in het langs-transport. De berekeningen van de verhoudingen van aanwas en terugtrekking komen overeen met de observaties, met uitzondering van observaties van de Saha-lin “spit”.

Dit geomorfologische kenmerk wordt niet alleen beïnvloed door verloop van transport langs de kustlijn, maar ook door processen van dwarstransport en door de totale aanvoer van sediment. De realistische resultaten van de berekeningen ondersteunen de voorspellingen over de toekomstige positie van de kustlijn. Hetzelfde een-lijn numeriek model is gebruikt om de positie van de kustlijn voor de volgende 25 en 50 jaar te simuleren en uit deze resultaten blijkt dat de verhou-ding in trends en erosie/aanwas vergelijkbaar zal zijn met de hedendaagse situatie. Bovendien is het effect van relatieve zeespiegel stijging (de Bruun effect) inge-schat en toegevoegd aan de resultaten van de simulaties. Het gebruik van meerja-rige gemiddelden van golf eigenschappen en sediment transport is een algemeen geaccepteerde benadering bij kustonderzoek, maar de werkelijke veranderingen

Samenvatting xiii

van een actieve kust vinden plaats tijdens extreme gebeurtenissen (stormen, over-stromingen). Dit is dan ook de reden om de kwetsbaarheid van de Donaudelta kust tijdens episodische waterniveauveranderingen, vooral veroorzaakt door stor-men, te onderzoeken. De lage duinen die het actieve strand landwaarts inbedden, de laag liggende inlandse gebieden en het relatief energetische golfklimaat maken dit een potentieel kwetsbaar gebied voor storm-geïnduceerde overstroming. Om het effect te onderzoeken van deze kortstondige waterniveau veranderingen is er een selectie gemaakt uit de tien meest significante storm scenario’s, waaronder een extreem hypothetisch geval: de combinatie van de hoogst genoteerde windsnelheid in 11 jaar (40 m/s) en de (Noordoostelijke) windrichting die de hoogste water-stand veroorzaakt.

Tien profielen, op representatieve locaties (met uitzondering van Sahalin “spit”), werden geselecteerd om de reacties van de zichtbare actieve kust te kwantificeren. Deze profielen, vanaf de waterdiepte van -1,5 m. tot aan de duintop, werden tij-dens drie opeenvolgende jaren gemeten. De resultaten tonen een aanzienlijke toe-name in het water niveau, vooral voor stormen vanuit noordelijke richtingen (met inbegrip van de extreme conditie) waarbij het water niveau met meer dan 1 m kan stijgen. Het maximum aantal overstromingen van het strand (50 tot 100% van het boven water gelegen deel) vindt plaats in het centrale deel van het Sulina-Sf. Gheorghe gebied en wordt veroorzaakt door stormen vanuit noordelijke richting. Dit gebied is bijzonder kwetsbaar voor extreme stormen die kunnen leiden tot aantasting van het duin-systeem met negatieve gevolgen voor het landinwaartse zoetwater ecosysteem, de lokale infrastructuur en de lokale economische activi-teit.

Omdat het een-lijn numeriek model geen betrouwbare schattingen voorsediment transport in het Sahalin ‘spit-systeem’ kon produceren, is een diepgaander onder-zoek gedaan met historische gegevens en het ‘state-of-the-art’ numeriek model: Delft-3D. In de eerste fase is een geïdealiseerde ‘spit’ ontworpen om de distributies van het golfveld en de relatie tussen langstransport en dwarstransport van sedi-ment te analyseren. De golfvelden convergeren naar de “spit” en het dwars-transport blijkt proportioneel te zijn met het water niveau. Bij een opgelegd wa-terniveau van + 0.5 m in totaal is het dwarstransport van een zelfde orde van grootte als die van het langstransport, maar bij een niveau van + 1.0 m is het dwarstransport 4 tot 5 maal zo groot als het langstransport.

In de tweede fase richt het onderzoek zich op de Sahalin spit. Met behulp van de bevindingen van de geïdealiseerde studie, wordt de dynamiek van Sahalin onder-zocht door simulatie van het golfklimaat, kwantificering van de sediment volumes die in het systeem circuleren, en doorformulering van een conceptueel model voor de vorming en evolutie van een spit in het algemeen. De convergentie van golven en de volledige blootstelling van de “spit” aan de dominante stormrichtingen lei-den tot grote waarlei-den van sedimenttransport, zowel in langstransport (maximale waarde van 1,6 miljoen m3_{/jaar) als in dwarstransport (een totaal van 1 miljoen}

m3_{/jaar). De hypothese is dat de lang uitgerekte Sahalin spit wordt gevoed door}

langstransport en dat de landwaartse migratie wordt veroorzaakt door dwars-transport.

Het spit systeem heeft twee belangrijke bronnen van sedimenten: de Sf. Gheorghe zijrivier en de noordelijke langstransport stroom. De berekeningen geven aan dat vergelijkbare volumes van sediment door de lang uitgerekte vorm aan de zuidpunt van de spit worden opgeslagen.

Ondertussen wordt het sediment dat van de zeezijde wordt getransporteerd aan de landwaartse kant van de spit afgezet en deze afzettingen zijn van dezelfde orde van grootte als de ruimte die ontstaan is door de relatieve stijging van de zee spiegel; dit verklaart de relatief constante diepte van de baai die achter de spit ontstaat. De positie van de Sahalin spit op elk moment is het resultaat van de constante wisselwerking tussen dwarstransport processen, die de spit landwaarts “rollen”, en langstransport processen, die het evenwicht weer herstellen, maar die nog steeds dichter bij het vaste land eindigen.

Het conceptuele model in deze thesis beschrijft een cyclus van vier stadia van vorming, ontwikkeling en samenvoeging van het vasteland met een spit gelegen aan een riviermonding in een “microtidal” (=ondiep getijde) gebied; de spit cyclus wordt beschouwd als onderdeel van de ontwikkeling van het door golven gedomi-neerde delta lob. De evolutie van een spit is sterk afhankelijk van de aanvoer van sediment en het energie niveau van de golven. Volgens toekomstverwachtingen, zal het volume van sediment voor de kust afnemen en zal de golfenergie toenemen; de formatie en de evolutie cycli van de spit kunnen veranderen, of zelfs stoppen, ten opzichte van het verleden.

De morfologische modellen in deze studie die de dominante processen van de Donau deltakust beschrijven en kwantificeren hebben een breder scala van toepas-sing. Deze modellen kunnen worden gebruikt voor soortgelijke milieus in de Noordwestelijke Zwarte Zee: “microtidal”, een overvloedige sediment aanvoer, een golfklimaat gedomineerd door één richting, en plotselinge veranderingen in de kust oriëntatie.

De aanbevelingen die uit deze studie voortkomen zijn gericht op vermindering van verlies van kustlijn, in het bijzonder met betrekking tot de Sulina - het Sf Ghe-orghe gebied. Aangezien de terugtrekkende kustlijn vooral het gevolg is van een door de mens veroorzaakt zand tekort, wordt als de meest duurzame oplossing de kunstmatige aanvoer van zand, gebaggerd uit het Sulina-kanaal en met het ten noorden van dit kanaal geaccumuleerde zand voorgesteld. Tot slot de aanbeveling om ingrepen te vermijden die de hoeveelheid zand wat de kust bereikt beïnvloe-den en die het sediment transport binnen het Sahalin spit systeem kunnen ver-storen.

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EZUMAT

Retragerea liniei tarmului pentru multe delte reprezinta o problema globala si cauzeaza dificultati in viata a numeroase si, deseori, mari comunitati. Eroziunea, ce induce retragerea liniei tarmului, este cauzata de doua tipuri de factori: naturali si antropici. Factorii naturali sunt reprezentati de evolutia fluviilor ce duce la scaderea volumului de sedimente aduse pe tarm, de subsidenta indusa de catre miscarile tectonice (eustatice) si/sau de catre compactarea sedimentelor si de schimbarea sedimentarii active de la un brat deltaic catre altul. Factorii antropici sunt legati, in principal, de structurile construite de-a lungul fluviilor si afluentilor lor (diguri, baraje etc.) si al coastei (lucrari de protectie a tarmului si porturi). Aceste interventii diminueaza volumele de sedimente ce ajung pe coasta si schimba circulatia sedimentelor in zona litorala. Un factor special este reprezentat de schimbarile climatice, rezultat al combinatiei dintre schimbarile naturale si cele induse de activitatile umane, care induce o marire a numarului de evenimente extreme si rate mari de crestere ale nivelului relativ al marii, conducand astfel la eroziune costiera rapida.

Zona costiera a Deltei Dunarii, cea mai mare delta din statele membre ale Uniunii Europene si parte a Rezervatiei Biosfera Delta Dunarii, este afectata de toti factorii descrisi mai sus. Largi parti ale acestei coaste sunt afectate de eroziune, uneori la rate enorme de 20 m/an. Desi retragerea liniei tarmului face parte din evolutia naturala a acestei delte, ratele mari inregistrate in ultimele decade sunt cauzate de interventiile umane. Acestea au inceput aproximativ acum un secol si jumatate cand cursul inferior al Dunarii precum si bratul central al deltei au fost subiectul lucrarilor de regularizare pentru a imbunatati navigatia si limita inundatiile. Lucrarile au continuat pana in timpurile moderne cu mari baraje (pentru generarea de hidroenergie) si structuri costiere, cum ar fi digurile de la Sulina, pentru facilitarea navigatiei.

Pentru a investiga si cuantifica principalele procese ce guverneaza o mare parte a coastei Deltei Dunarii o abordare sistematica a fost necesara. La inceput factorii ce controleaza hidrodinamica au fost investigati prin analiza datelor de vant dintr-un interval reprezentativ (dintr-unsprezece ani). Datele de vant si batimetria zonei de studiu au constituit datele de intrare primare pentru reconstituirea climatului valurilor. Directiile nordice ale vanturilor si valurilor au fost identificate ca fiind dominante, deja sugerand directia transportului de sedimente. Cu toate acestea,

din cauza schimbarii orientarii liniei tarmului si a adapostirii impotriva anumitor directii ale vantului si valurilor, fiecare sector de plaja are dinamica sa.

Urmatorul parametru esential pentru investigarea dinamicii costiere este adancimea de inchidere, definita ca fiind cea mai mare adancime la care valurile influenteaza morfologia fundului marii intr-o anumita perioada de timp. Lipsa unor masuratori sistematice ale profilelor plajei active a condus la calcularea adancimii de inchidere folosind cele mai mari valuri ce au avut loc in mai mult de o decada. Distributia valorilor adancimii de inchidere de-a lungul zonei de studiu arata gradul de expunere pentru diferite sectoare de plaja la principalele directii de valuri.

Pentru a incepe investigarea transportului de sedimente de-a lungul zonei de studiu un calcul schematic a fost facut folosind doua formule, CERC si Kamphuis, pentru care parametrii valurilor au fost extrasi manual. Pe baza rezultatelor un calcul detaliat al transportului sedimentar de-a lungul tarmului a fost facut folosind modelul numeric unidimensional UNIBEST CL+ si doua formule, CERC si Bijker. Doua categorii de date au fost folosite: cele existente – batimetria unei mari parti a coastei Deltei Dunarii, caracteristicile apei si sedimentelor, si date derivate din calcule – climatul reconstituit al valurilor, adancimea de inchidere. Rezultatele indica transport net de sediment de-a lungul tarmului in general indreptat inspre sud cu un gradient abrupt pentru zona Sulina (limita nordica a zonei de studiu) – Sf. Gheorghe si orientat spre nord si cu un gradient moderat pentru zona Sahalin – Gura Portitei (limita sudica a zonei de studiu). Presupunand ca profilul plajei nu se schimba atunci cand linia tarmului se deplaseaza, un buget al sedimentelor a fost derivat din gradientii in transportul de sedimente de-a lungul tarmului. Ratele de avansare/retragere calculate se potrivesc bine cu cele observate, exceptand zona Sahalin. Evolutia aceastei formatiuni geomorfologice este controlata nu numai de gradienti in transportul longitudinal, dar si de procese perpendiculare pe tarm si de sedimentele ce intra in sistemul sau. Rezultatele bune ale calculelor sustin predictiile privind viitoare pozitie a liniei tarmului. Acelasi model numeric unidimensional a fost folosit pentru a simula pozitia liniei tarmului peste 25 si 50 de ani, iar rezultatele indica mentinerea actualelor tendinte si rate de retragere/avansare. Mai mult, efectul cresterii relative a nivelului marii (efectul Bruun) a fost estimat si adaugat rezultatelor simularilor.

Folosirea mediilor multianuale pentru caracteristicile valurilor si ale transportului de sedimente reprezinta o practica curenta atunci cand o zona costiera este investigata, dar schimbarile in morfologia plajei active au loc mai ales in timpul evenimentelor extreme (furtuni, inundatii). De aceea a fost investigata vulnerabilitatea coastei Deltei Dunarii la schimbarile episodice ale nivelului marii induse de catre furtuni. Dunele joase, delimitand plaja activa la partea dinspre uscat, altitudinea mica a uscatului si climatul relativ energetic al valurilor fac aceasta zona vulnerabila la inundatiile produse de catre furtuni. Pentru a

Rezumat xvii

investiga variatiile nivelului marii pe termen scurt o selectie a zece celor mai reprezentative scenarii de furtuna a fost facuta. Printre acestea se afla si un caz extrem ipotetic: combinatia intre cea mai mare viteza a vantului inregistrata in 11 ani (40 m/s) si directia ce produce cea mai mare ridicare a nivelului marii (nord-est). Pentru a cuantifica raspunsul plajei active emerse zece profile de plaja pozitionate in puncte reprezentative (cu exceptia peninsulei Sahalin) au fost selectate. Aceste profile, ce se extind de la adancimea de -1,5 m pana la creasta dunelor, au fost masurate in trei ani consecutivi. Rezultatele arata cresteri ale nivelului apei semnificative, in special pentru furtunile dinspre directiile nordice (inclusiv cazul extrem ipotetic) cand nivelul apei poate creste cu peste 1 m. Inundarea maxima a plajelor (intre 50 si 100% din partea emersa) are loc in partea centrala a zonei Sulina – Sf. Gheorghe si este produsa de catre furtunile dinspre directiile nordice. Aceasta zona este in mod particular vulnerabila la furtunile extreme ce pot cauza spargerea cordonului de dune cu consecinte negative pentru ecosistemele de apa dulce, infrastructura si activitatile economice locale.

Pentru ca modelul numeric unidimensional nu a produs rezultate realistice pentru circulatia sedimentelor in sistemul peninsulei Sahalin o investigatie detaliata a fost facuta folosind date istorice, precum si modelul numeric de ultima generatie Delft-3D. In prima etapa o peninsula idealizata a fost proiectata pentru a analiza distributia campurilor de valuri si relatia dintre componentele longitudinala si perpendiculara ale transportului de sedimente. Campurile de valuri arata convergenta catre peninsula, iar transportul perpendicular este proportional cu nivelul apei. La un nivel impus al apei de +0,5 m transportul de sedimente perpendicular total este in acelasi ordin de marime cu cel longitudinal, dar la un nivel al apei de +1,0 m transportul perpendicular este de 4-5 ori mai mare decat cel longitudinal.

In a doua etapa investigatia se concentreaza pe peninsula Sahalin. Folosind rezultatele investigatiei peninsulei idealizate dinamica Sahalinului este analizata prin simularea climatului valurilor, cuantificarea volumelor de sedimente ce circula in acest sistem si formularea unui model conceptual pentru formarea si evolutia unei peninsula de acest tip in general. Convergenta valurilor catre Sahalin, precum si expunerea completa a acestuia la principalele furtuni duc la volume mari de sedimente transportate de-a lungul (valoarea maxima 1,6 milioane m3_/an)

si perpendicular (un total de 1 milion m3_{/an) pe tarm. Aceste valori sugereaza}

ipoteza ca elongatia peninsulei Sahalin este alimentata de catre transportul longitudinal, iar migratia catre uscat de catre transportul perpendicular pe tarm. Sistemul Sahalin are doua surse principale de sedimente: bratul Sf. Gheorghe si curentul longitudinal dinspre nord. Calculele indica ca volume de sedimente similare sunt depozitate la extremitatea sudica din cauza elongatiei peninsulei. In acelasi timp, sedimentele transportate de la partea dinspre mare sunt depozitate la partea dinspre uscat a peninsulei si sunt in acelasi ordin de marime ca si spatiul de depozitare creat de cresterea relativa a nivelului marii, explicand astfel

mentinerea relativ constanta a adancimii golfului format in spatele peninsulei. Pozitia peninsulei Sahalin la un moment dat este rezultatul interactiunii dintre transportul perpendicular ce deplaseaza peninsula catre uscat si transportul longitudinal care restabileste echilibrul, dar mereu la o pozitie mai apropiata de uscat. Modelul conceptual dezvoltat in aceasta teza descrie un ciclu in patru etape pentru formarea, dezvoltarea si unirea cu uscatul a unei insule la varsarea unui fluviu intr-o mare cu maree foarte mici, iar acest ciclu este considerat ca parte a dezvoltarii dominate de valuri unui lob deltaic. Evolutia unei astfel de peninsule depinde foarte mult de alimentarea cu sedimente si de nivelul energiei valurilor. Daca, asa cum este de asteptat in viitor, volumul de sedimente ce ajunge pe coasta va descreste, iar energia valurilor va creste, formarea unei astfel de peninsule si ciclurile sale de evolutie vor fi diferite fata de cele din trecut sau chiar vor fi stopate.

Modelele morofologice formulate in acest studiu explica si cuantifica procesele ce controleaza evolutia coastei Deltei Dunarii si pot avea o aplicare mai larga. Pot fi folosite pentru medii similare cu nord-vestul Narii Negre: maree mici, fluxuri mari de sedimente, climat al valurilor dominat de o singura directie, precum si schimbari bruste in orientarea liniei tarmului.

Recomandarile generale de la sfarsitul acestui studiu se refera la diminuarea ratei de retragere a liniei tarmului ce afecteaza in special sectorul Sulina – Sf. Gheorghe. Pentru ca aceasta retragere este in mare masura cauzata de deficitul de sedimente indus de activitatile umane alimentarea artificiala este sugerata ca fiind cea mai durabila solutie, iar acest lucru poate fi facut folosind nisipul dragat din canalul Sulina, precum si cel acumulat la nord de acest canal. Alte recomandari privesc evitarea interventiilor care pot diminua volumele de nisip ce ajung pe coasta si cele care pot afecta circulatia naturala a sedimentelor in sistemul peninsulei Sahalin.

xix

C

ONTENTS

1.2 Historical evolution of the Danube Delta --- 2

1.3 Current state of the Danube Delta coastal zone --- 6

1.4 Objectives and study approach --- 9

1.5 Outline of the Thesis --- 12

Chapter 2 Wave climate, sediment and shoreline dynamics changes on the Danube Delta coast 15 2.1 Introduction --- 15

2.2 Wind climate --- 16

2.3 Wave climate --- 18

2.4 Closure depth --- 25

2.5 Alongshore sediment transport capacity --- 27

2.5.1 Schematic computation ... 28

2.5.2 Detailed computation ... 30

2.6 Sediment Budget --- 32

2.7 Predictions for the future position of the shoreline --- 35

2.8 Discussion and conclusions --- 36

Chapter 3 Episodic water level variations on the Danube Delta coast 43 3.1 Introduction --- 43

3.2 Water level variations --- 44

3.3 Aerial beach response --- 51

Chapter 4 Spit formation and development 57

4.1 Introduction --- 57

4.2 General principles of spit development --- 58

4.3 Idealised spit --- 63

4.4 Discussion and conclusions --- 71

Chapter 5 Sahalin spit 73 5.1 Introduction --- 73

5.2 Historical evolution of Sahalin spit --- 74

5.3 Sediment circulation --- 79

5.4 Sediment budget --- 86

5.5 Conceptual model --- 88

5.6 Discussion--- 91

5.7 Conclusions --- 93

Chapter 6 General conclusions and recommendations 97

6.1 Conclusions --- 97

6.1.1 The linear coast ... 97

6.1.2 The non-linear coast ... 99

6.2 Recommendations --- 101

6.2.1 Danube Delta coast ... 101

6.2.2 Spit evolution study ... 103

Bibliography 105

Appendix A Photos of the study zone shore 113

Appendix B Alongshore sediment transport 119

Appendix C Water level elevation 133

Appendix D Wave fields and sediment transport for Sahalin spit 137

List of figures 149

List of tables 155

Acknowledgements 157

1

Chapter 1

G

ENERAL INTRODUCTION

1.1 I

Coastal erosion is a worldwide problem and most affected by it are low lying areas such as deltas. This process can be part of a natural evolution or induced by hu-man intervention, but often it is a combination of both. A typical natural delta would experience sedimentation resulting in lobe development and every time when the major sediment input shifts from one distributary to another this will be reflected in the progradation of a new lobe and the retreat of the abandoned lobe. Natural changes of the climate can increase or decrease the total sediment input for a delta resulting in progradation or retreat of the entire delta. Human inter-ventions can disturb the natural patterns of sediment circulation by the construc-tion of structures along the river and its tributaries, in the delta or in the deltaic coastal zone.

An example of such a delta is the Danube Delta, one of the largest deltas in Eu-rope, formed by the Danube River discharging into the Black Sea. This delta formed at the beginning of the Holocene when sea level started to stabilize. Ini-tially, the Danube Delta was a gulf which started to be filled in with sediments supplied by the Danube River. The sedimentation was favoured by the formation of a sand bar in the mouth of the gulf, supplied with sediments by the littoral drift. After the sedimentation of the initial gulf, a few main lobes successively de-veloped leading to the present day configuration of the Danube Delta. Currently, the natural trends for the Danube Delta’s three main lobes are different: the northernmost lobe (called Kilia secondary delta) is the only one to prograde, sup-plied by the majority of sediments transported by the Danube River; the middle one, Sulina, by now a former lobe, is continuously retreating due to the shift of the active sedimentation towards the other two lobes. The third lobe, Sf. Gheor-ghe, is apparently retreating, but in fact prograding through the formation and evolution of a series of spits.

Although relatively pristine when compared with other European deltas formed under similar conditions (Ebro, Rhone, Po Deltas) the Danube Delta was and is still subject to human interventions. The first category of interventions was de-signed initially to regulate the river and its tributaries in order to prevent floods and to dry the wetlands associated with the river for agricultural purposes. The second category of interventions was made in order to improve the navigation thus connecting Central Europe to the Black Sea. These works started at the middle of the nineteenth century and mainly consisted of dredging works to maintain a min-imum depth, shortcuts in the Sulina branch and the construction of jetties at the Sulina mouth. The third category of interventions was initiated in the twentieth century and consists of the construction of a series of barrages along the Danube River and its tributaries with hydropower production as the main purpose. Due to both natural trends and human interventions a large part of the Danube Delta shoreline is eroding at different rates, sometimes with more than 20 m per year. The most intense erosion is taking place in the central part of the Sulina – Sf. Gheorghe sector where the natural erosive trend is enhanced by human inter-ventions which diminished the sediments reaching the coast and blocked the sedi-ment drift. Another sensitive area is Sahalin spit formed at the Sf. Gheorghe river mouth. The human induced decrease of the sediment discharge is expected to strongly affect the natural cycles of spit formation and evolution in the future.

1.2 H

D

D

The Danube River springs in the Schwarzwald Massif, Germany and after a long journey (2 860 km) throughout Central and Eastern Europe discharges into the sea forming the Danube Delta.

This delta is situated in the north-western part of the Black Sea where the conti-nental shelf is very large, covering about 30 % of the total area of the sea. In this part of the sea the majority of the large rivers discharge and the input of the Danube River delivers 40% of the total fresh water input.

The Black Sea, one of the largest enclosed seas in the world, has a maximum depth of 2 212 m and 80% of the total volume of water is anoxic, contaminated with H₂S (below a depth of 150-200 m). The water salinity ranges from 17‰ at the surface to 22 ‰ close to the sea floor and decreases to 10-12 ‰ in the vicinity of the Danube River mouths.

Based on the processes controlling its formation and evolution, the Danube Delta can be divided in three main geomorphologic units (Fig. 1.1). The first unit in the seaward direction is the delta plain consisting of a fluvial part (4 000 km2_{), where}

the relief is dominated by river processes, and a marine part (1 800 km2₎

1.2 Historical evolution of the Danube Delta 3

Figure 1.1: The Danube Delta major geomorphologic and depositional units: fluvial delta plain (1a); marine delta plain (1b); fossil and modern beach-ridges and littoral accumula-tive formations (1c); delta front platform (2a); relics of the “Sulina Delta” (2b); delta front slope (2c); Danube prodelta (3); continental shelf area and depth contour lines (4) (after Panin, 1989).

The second major unit is the delta front, divided into two sub-units: the delta front (1 300 km2_{) and the delta front slope (500 km}2_{). The third unit, situated}

off-shore, is the prodelta covering 6 000 km2 _{(Panin, 1989).}

Figure 1.2: Evolution of the Danube Delta during the Holocene (modified after Panin et al., 1997).

The Danube Delta is located in an area of high crustal mobility, characterized by strong subsidence and intense sediment deposition. The subjacent geologic units, the Pre-Dobrogean Depression and the Scythian Platform, display a succession of sedimentary successions consisting of: Palaeozoic limestone; Lower Triassic red continental detrital deposits and volcanic rocks (400 - 2500 m thickness); Middle-Upper Triassic marine sediments accumulated on carbonate rocks in the lower part (800 - 1000 m) and of detrital rocks (450 m thickness) in the upper part; Ju-rassic marine sediments, consisting of detrital deposits at the base (500-1700 m thickness) and carbonate deposits at the top (1000 m thickness); Lower Creta-ceous red continental deposits (500 m thickness); Sarmatian-Pliocene sediments

1.2 Historical evolution of the Danube Delta 5

consisting of alternating clay, sand and sandstone (200 - 350 m thickness) (Panin et al., 2002).

There are several theories about the formation of the Danube Delta, the majority converging to the hypothesis that the actual delta started to form approximately at the beginning of the Holocene. The most consistent and widely accepted theory is the one proposed by Panin (1974, 1989, 1996) and Panin et al. (1983) based on the combination of geomorphologic, textural, geochemical and mineralogical data, fauna analyses and by 14_{C dating. Approximately 12 000 years ago the mean sea}

level in the Black Sea was 3 to 4 m higher than today and the actual Danube Del-ta was rather a gulf, bay or estuary. In the period between 11 700 to 7 500 years BP a sand barrier (“initial spit”) formed in front of the gulf (Figs. 1.1 and 1.2), most probably related to a relative sea-level fall. The sand barrier nearly closed the gulf allowing the sediments discharged by the Danube River to be deposited in an energetically quiet environment, resulting in the infilling of the gulf. While the gulf was slowly filled in, the river breached the sand barrier in the southern part of the gulf and a first delta (St. George I) developed in the period between 9 000 to 7 200 years ago.

Over the next 5 000 years, after a lobe switching, the Sulina delta started to pro-grade reaching a maximum extension offshore among all the Danube Delta lobes. At approximately 2 000 years BP, the shoreline at Sulina mouth had prograded 10 to 15 km farther offshore than today. After this rapid progradation a lobe switch-ing occurred and the Sulina delta started to erode, while the active sedimentation moved to the St. George II delta site (Fig. 1.2). At approximately the same time the Kilia delta developed at a rather fast rate in the north, prograding 16 – 18 km into the sea. In the southern part of the Danube Delta, due to the sediment dis-charge of a former distributary a secondary delta formed and evolved (3 500 – 1 500 years ago). This delta, called Cosna-Sinoe, was eroded and the reworked ma-terial built the barriers closing the Razelm-Sinoe lagoon.

In modern times, over the last 150 years, the Danube Delta has maintained the same configuration and trends as 2 000 years ago. Due to the large quantities of sediment discharged into the sea the Kilia secondary delta is prograding, while the area between Sulina and Sf. Gheorghe is strongly eroding, both due to a natural trend and anthropogenic activities. Although the asymmetric lobe of Sf. Gheorghe is apparently retreating, at a scale of centuries it is advancing by the formation of successive spits which develop in the direction of the alongshore drift and merge with the mainland (Panin, 1974, 1989, 1996, 1997, 1998, 2005; Panin et al., 1997, 1983; Panin and Jipa, 2002). More recently, Giosan et al. (2006) suggest that parts of Danube Delta could be younger than previously found. Based on radio-carbon and optical dates it was found that the embayment where the Danube Delta formed, started to accumulate sediments approximately 5 200 years ago, the Sulina lobe started its expansion approximately 3 650 years ago and the Kilia sec-ondary delta formed no earlier than 1 200 years ago. However, Giosan et al.

(2006) confirm the mechanism of lobe formation and their succession as presented above.

1.3 C

D

D

In modern times, over the last 100 years, the Danube Delta (Fig. 1.3) has main-tained the same configuration and trends as 2 000 years ago. The Danube River discharged and still discharges into the Black Sea through three branches (from north to south): 1) Kilia, which transports approximately 58% of the water and sediment discharge, 2) Sulina, the major waterway, 19% and 3) Sfântu Gheorghe, 23%, (Bondar and Panin, 2001). While the tide regime is rather small (average amplitudes of 7 – 11 cm (Bondar et al., 1973)), a larger role in the creation of sed-iment accommodation space was and still is due to relative sea level variations. The relative sea level rise is between 2.8 - 3.1 mm/year consisting of eustatic sea level rise, estimated for the past century at about 1.3 mm/year (Malciu, 2000) and subsidence, with estimated values of 1.5 – 1.8 mm/year (Panin, 1999). Result-ing from the balance between sedimentation and increasResult-ing accommodation space, the Kilia secondary delta is still prograding, favoured by the relatively large sup-ply of sediment, viz. more than 50% from the total sediment discharge of the Danube River (Bondar and Panin, 2001). The shoreline in the Sulina area is re-treating rapidly and the process is enhanced by human intervention, as we will highlight and discuss below. Presently, the St. George (Sfântu Gheorghe) lobe is still advancing moderately, forming and reworking the Sahalin spit. The reworking of the spit by wave action makes it merges with the mainland in the direction of the alongshore current, developing the asymmetric lobe of Sf. Gheorghe.

The Danube Delta coastal zone has been extensively affected by the construction of various structures along the river and on the coast. The dams built along the Danube River and its tributaries to regulate the water discharge and to produce hydropower, have dramatically decreased the amount of sediment reaching the delta. Among these structures the most important are the barrages Iron Gates I and II (approximately 900 km upstream from the Black Sea), constructed in 1970 and 1983, respectively. These two barrages alone lead to a decrease of 35 - 50% of the discharged sediments (Panin, 1996). Locally, the Sulina jetties (Fig. 1.3) were built for navigation purposes also having a profound impact on the evolution of the coastal zone. Starting in 1858, works were undertaken to transform the Sulina branch into a navigational channel including meander cut-offs. Without anticipat-ing further consequences, the strategy was adopted to successively elongate the Sulina jetties in order to avoid or at least reduce the accumulation of sediments at the seaward opening of the channel. As a result the navigation canal nowadays ex-tends 8 km into the sea (Panin, 1999; Stanica et al., 2007), completely blocking

1.3 Current state of the Danube Delta coastal zone 7

the alongshore drift from the North resulting in clogging of Musura Bay (north of Sulina) and causing downdrift erosion (south of Sulina).

Another human intervention causing “beach starvation” in the Sulina – Sf. Gheorghe area is the dredging of the sand bar formed at the Sulina branch mouth. The sediment is periodically dredged in order to maintain a minimum 7 m depth for navigation and, then, is released out of the nearshore system by dump-ing on offshore locations.

The Romanian deltaic coast extends from the border with Ukraine (north) to Cape Midia (south) having a total length of 140 km. The study area is part of this Danube Delta littoral zone, being confined by the Sulina jetties northward and Portita inlet southward and covering a range of depths from the shoreline (west) up to 35 - 40 m (east). The total length of the shoreline from the study zone is 94 km, including only the open sea side of Sahalin spit (Fig. 1.3).

In a global sense the Sulina–Sf. Gheorghe section (34 km) is characterized by an alongshore current towards the south (Fig. 1.4) and a retreat of the shoreline. An exception is the coast close to Sulina (8 km in length), which is strongly influ-enced by the jetties, the alongshore current is northward oriented due to wave dif-fraction processes and the present day status of the coastal zone is accretive. Near Sf. Gheorghe (6 km) the coast is on average stable, periods of erosion alternating with periods of accretion. However, the section in between (20 km) is strongly eroding with rates ranging from 5 to 20 m/year (Panin 1996, 1999; Ungureanu et al., 2000; Stanica et al., 2007).

At Sf. Gheorghe mouth an arcuate mouth bar, called Sahalin, formed. The sand bar emerged in 1897, after an exceptional flood, and continuously stretched to-wards south-west reaching a current length of 17 km. Simultaneously, processes such as overwash caused a shoreward migration of the sand bar, sometimes with a rate of 70 m/year (Panin, 1996). In the late 1970s the northern part of sand bar connected to the mainland Sahalin became a typical spit. Behind the spit the shoreline is advancing due to the quiet environment and the sediment supplied by secondary distributaries of Sf. Gheorghe arm.

In the section about 20 km long south-westward from Sahalin (Fig. 1.3), in some parts, the coastline retreated more than 500 m since 1950, due to merging of the lakes from this area with the sea. Southward, down to Portita Inlet over a dis-tance of 23 km the shore is reported to be in dynamic equilibrium with episodes of alternating erosion/accretion of 5 to 10 m/year (Panin, 1996).

The sediment that forms the active beach and surfzone of the Danube Delta coastal zone is well-sorted fine sand, mostly quartzitic, in places enriched with heavy minerals. The main sedimentary source is the alluvial sediment delivered by the Danube River.

1.4 Objectives and Study Approach 9

Figure 1.4: View from the Sulina lighthouse (situated at the seaside extremity of the Sulina Jetties), eastward. The fresh water plume is directed to the right (southward) by the alongshore current.

1.4 O

S

A

The general objectives of this dissertation are to derive (1) a better understanding of recent and present, engineering scale processes governing the evolution of a large part of the Danube Delta coastal zone and (2) a quantification of the along-shore sediment transport and associated erosion/accretion rates. While the dy-namics of the major part of the studied zone are mainly dictated by the along-shore current and sediment transport gradients, Sahalin spit is a more complex system. Besides alongshore sediment transport gradients other processes play an important role as well which are taken into account. For Sahalin spit two objec-tives are set: 1) to investigate the evolution of Sahalin spit and 2) to derive gener-ic conclusions based on the evolution of Sahalin for spits in general.

To address the main processes and forcings relevant to the study zone a typical engineering approach was employed. Figure 1.5 schematically shows the steps re-quired to obtain a comprehensive morphological model of the Danube Delta coastal zone with a particular attention to Sahalin spit.

Figure 1.5: The main steps taken to investigate and quantify the processes controlling the Danube Delta coastal zone. Grey boxes indicate data and tools; half grey boxes are results which served also as input; white boxes show final results.

Starting with wind records and a bathymetric map as input for a wave generation and propagation model, the near shore wave climate was derived. Further, the wave climate and the closure depth served as the main inputs for the computation of the alongshore sediment transport capacity. The concept of closure depth was used in order to delineate the active beach and surf profile. Assuming invariance of the active profile sediment budgets are determined by alongshore sediment transport gradients alone, i.e. we have adopted the one-line coastal modelling ap-proximation.

The computation of the sediment transport was made at two levels of detail. First, a schematic computation was performed to identify trends and order of magnitudes of the sediment transport. Next, this knowledge was used to develop a

Wind Records Bathymetric Charts Water Characteristics Sediment Characteristics Historical Evolution Sahalin Spit Sediment Transports Conceptual Model

Wave Climate Idealised Spit

Sediment Budget Predictions Closure Depth Alongshore Transport Sediment Budget Schematic Computation Water Level Variations Storm Impact

1.4 Objectives and Study Approach 11

numerical model with a relative large alongshore resolution that allowed us to de-rive detailed alongshore sediment transport estimates. For all alongshore sediment transport computations the closure depth, based on local wave characteristics, was used to derive shoreline changes from gradients in alongshore transports. On del-ta-scale the resulting alongshore sediment transport gradients and associated sed-iment budgets comply very well with the observations, confirming the dominant role of wave-induced alongshore transport and giving confidence in our predictions regarding the shoreline position for the next 25 and 50 years, including the effect relative sea level rise.

The simulation of the water level variations induced by extreme storms is the last investigation referring to the entire study zone. Due to its particularities (orienta-tion, wave exposure etc.) each beach sector of the deltaic coast has a different re-sponse of the aerial active beach to flooding generated by exceptional storms. Having completed the investigation and quantification of the main processes con-trolling the dynamics of the entire study zone, the attention is focussed on the particular case of Sahalin spit.

The detailed wave climate and the alongshore sediment budget, along with the observed rates of migration are used to analyze the sediment circulation in the Sahalin domain. It turns out that the sediment budget of the Sahalin domain is not determined by the alongshore sediment transport gradients alone, so that ad-ditional processes were identified to explain the Sahalin sediment budget.

We conjecture that two main processes controlling spit evolution are the gradients in alongshore sediment transport and cross-shore sediment transport processes, especially overwash. A natural geomorphologic feature such as Sahalin spit formed and evolved under the influence of these two main factors and many others such as: extreme events (floods, storms etc.) resulting in significant sediment input and water level variations; irregularities of the bathymetry caused by spatial variation of the sediment characteristics; development of vegetation and others. In order to explore the relationship just between the main processes shaping a spit we have simplified the natural setting by designing an idealized spit. The emerged and submerged domain of this “ideal” case of a spit has quasi-parallel depth contours, one type of sediment and no sediment input from the river or up drift alongshore current. Wave field distributions and the induced sediment transport (along- and cross-shore) were investigated for wave directions varying 270° around the sea side of the ideal spit.

A next step was to simulate a representative wave climate for Sahalin spit which formed the primary input for the simulation of the wave-induced currents. The sediment transport for both alongshore and cross-shore directions was computed and analyzed. The results of the investigation on the ideal spit are confronted with those derived from the investigation of the real case.

Based on the results of the sediment transport computations and supplementary information such as the past evolution of the Sf. Gheorghe lobe, relative sea level rise and different sediment sources we constructed a sediment budget for Sahalin spit.

Finally, using the information derived from the present study as well as the find-ings of previous authors we propose a conceptual model for the evolution of a spit formed at a river mouth. This model is developed as a generic concept applicable to spits formed under similar conditions as Sahalin spit.

1.5 O

T

Throughout this dissertation a detailed investigation and quantification of the processes controlling the dynamics of a large part of the Danube Delta coastal zone is presented. Chapter 1 consists of an introduction of the Danube Delta for-mation and the current state of its coastal zone followed by study objectives and approach.

Chapter 2 presents a comprehensive investigation and quantification of the pro-cesses controlling the general dynamics of the Danube Delta coast. The first part is dedicated to wind analyses and to a hindcast of the wave climate since these are the main forces driving the sediment transport in the study zone. The second part focuses on the sediment circulation and on the consequent shoreline position changes. Using the simulated wave climate as well as supplementary information, such as sediment characteristics and historical evolution, and an one-line numeri-cal model the alongshore transport capacity and the resulting sediment budget are derived. The final step was to estimate the shoreline position for the next 25 and 50 years by simulating the current processes and taking into account the effect of the relative sea level rise.

Chapter 3 is focused on water level variations induced by storms and the response of the aerial active beach. The results indicate that each beach sector has a differ-ent level of vulnerability according mainly to the exposure to waves generated by extreme storms and to the local characteristics of the active beach.

While for the majority of the study zone the coastal dynamics is controlled by the gradients in the alongshore sediment transport, for Sahalin spit cross-shore pro-cesses play a significant role. The investigation of the complex propro-cesses dictating the evolution of this spit is the subject of the next two chapters. Chapter 4 de-scribes the exploration of the relationship between the along- and cross-shore gra-dients in sediment transport using an idealised spit. The results provide valuable insight for the study of the Sahalin spit, which is the subject of Chapter 5. The approach for Sahalin spit includes two main steps: computation of the sediment transport and budget and the construction of a generic conceptual model for

for-1.5 Outline of the Thesis 13

mation and development of sandy spits based on the evolution of Sahalin, but ap-plicable to other spits formed in similar environments.

Finally, Chapter 6 synthesizes the main findings of this work highlighting the ne-cessity to restore the natural patterns of sediment circulation in order to stop or at least diminish the severe erosion recorded in some parts of the Danube Delta coastal zone.

15

C

HAPTER

2

W

AVE CLIMATE

,

SEDIMENT AND SHORELINE DYNAMICS

CHANGES ON THE

D

ANUBE

D

ELTA COAST

2.1 I

At present many deltas globally are subject to a strong pressure due to increasing human impacts (Syvitski et al., 2005). Although the Danube Delta is relatively untouched by urbanization, human intervention such as the construction of struc-tures in the Danube catchments as well as on the coast have led to major changes in the state of the coast by decreasing the amount of sediments reaching the coast and by disturbing the natural patterns for coastal sediment circulation. The wave-induced currents transport sediments alongshore and the gradients in this transport control the active beach evolution resulting in changes of the shoreline position. For a wave dominated delta, such in the case of a large part of the Dan-ube Delta, the wave climate and the associated currents are even more important for the dynamics of its coast than for other deltas (e.g. river- or tide-dominated). However, for a more complex coastal feature, such as Sahalin spit, gradients in the alongshore transport alone do not explain their dynamics. To investigate such a feature it is necessary to take into account the cross-shore component of the coastal transport (especially overwash). Another concern regards the capability of numerical models to simulate coastal processes when a sudden change in the coast orientation occurs.

The study presented in this chapter has as main objectives the investigation of the processes dictating the evolution of a large part of the Danube Delta coastal

* Excerpts from this chapter were published as: Dan, S., Stive, M.J.F., Walstra, D.J., Panin, N., 2009. Wave climate, coastal sediment budget and shoreline changes for the Danube Delta, Marine Geology, vol. 262, issues 1-4, pp. 39 – 49.

zone, the quantitative estimation of the alongshore sediment transport and associ-ated erosion/accretion rates and the prediction of the future shoreline position. To achieve the objectives the wind climate was analyzed and used as primary in-put to obtain a complete wave climate hindcast. Along with the comin-puted closure depth, the wave climate was used to derive the alongshore sediment transport and, in a later stage, the sediment budget. Finally, predictions of the shoreline po-sitions for the next 25 and 50 years were made based on numerical simulations and considering the effect of relative sea level rise.

2.2 W

The wind climate represents one of the first steps when the dynamics of a coast is investigated. In order to obtain reliable alongshore sediment transport estimates, local wave climates are imperative as sediment transports along the Danube Delta coast are induced by wind wave generated currents. The only continuous series of wave data available in Romania have been collected near Constanta, approximate-ly 100 km southward of our study zone. The vicinity of the Constanta Harbour and coastal protection structures influence these wave records. In front of the Danube Delta the available wave data are based on visual observations and the records are not continuous, with blackouts occurring especially during extreme events. Reliable local wind measurements are available and as the wave climate at the Danube Delta is almost entirely generated by local wind it was decided to use these data.

Collected wind data consist of two sets. The first set covers a 10 years period (from January 1991 to December 2000) and is recorded at Sulina meteorological station situated at the landward end of the Sulina jetties (Fig. 1.3). The second set covers one year (2002), but the measurements were made simultaneously at Sulina and the Gloria Platform (situated offshore, approximately 30 km eastward from Portita Inlet, Fig. 1.3). For both datasets, Sulina and Gloria, records were taken four times per day containing speed (m/s) and direction (nautical conven-tion, 16 directions). At Sulina the records were made at a height of 10 m, but at Gloria Platform at 36 m height above the mean sea level. The Gloria dataset was converted to the standard elevation of 10 m above the mean sea level (U10 ≈

U₃₆/1.2) (Shore Protection Manual, 1984).

In order to find out if the first dataset (Sulina 1991 – 2000) is representative for the offshore wind conditions a comparison was made between the nearshore (Sulina 2002) data and the offshore (Gloria 2002) data. The correlation between the two locations turned out to be fairly good, almost 71% of records being iden-tical or very similar.

2.2 Wind climate 17

Categories from a total of 1454 pairs of analyzed data (Sulina 2002 – Gloria 2002) indicate as follows:

 same wind direction, less than 2 m/s wind speed difference: 18.5%;

 same wind direction, more than 2 m/s wind speed difference: 14.2%;

 consecutive wind directions, less than 2 m/s wind speed difference: 21.0%;

 consecutive wind directions, more than 2 m/s wind speed difference: 17.0%;

 different wind directions 29.2%.

However, wind directions crossing the land (from SSW to NNW clockwise) did not correlate very well. To solve this problem a land vicinity correction was ap-plied. An empirical method was used, obtained from a study of winds in the Great Lakes area, USA, (Shore Protection Manual, 1984) resulting in a dimensionless coefficient of 1.2 as a multiplication factor for the recorded wind speed. As a re-sult of the correction a better correlation was obtained between the wind speed data from the Sulina 2002 and Gloria 2002 stations. Based on this result, only the wind speeds of the wind directions crossing the land from the first set (Sulina 1991 – 2000) were corrected for the land vicinity. The final result is a set of wave data recorded close to the shore, but corrected to be representative for the off-shore conditions.

Figure 2.1: Distribution for ten years (1991 – 2000) of wind speeds and directions at Sulina meteorological station.

0 200 400 600 800 1000 1200 N _NNE NE ENE E ESE SE SSE S SSW SW WSW W WNW NW NNW below 7 m/s over 7 m/s

The average wind speed is approximately 7 m/s so that we divided the wind rec-ords into two categories: lower than the average, winds generating small waves, with low potential for coastal erosion and larger than the average, winds generat-ing larger waves which control the coastal dynamics (Fig. 2.1). The general wind climate is dominated by northern and southern directions, but the significant storms (wind speeds over 18 m/s) occurring on average 10 times per year and covering 2.2% from the total wind records are distributed as follows: 70% north-ern directions, mostly in the wintertime; 15% southnorth-ern directions and westnorth-ern and eastern directions the remaining 15%. The largest wind speeds were recorded in December 1991 when winds from north reached speeds between 28 and 40 m/s for three days.

2.3 W

A complete and representative wave climate is necessary to reach the general ob-jectives set for the present study: viz. to analyze and quantify the dynamics of the Danube Delta coastal zone on engineering scales.

The bathymetric map used as input for the wave simulation was issued by GeoE-coMar in 1995. The distance between bathymetric cross-shore profiles is approxi-mately 3 km and the measurements were made using Hi-Fix system (Sheriff, 1974). Data from navigation maps were used to improve the accuracy of the map for the nearshore waters.

Along with wind and bathymetric data another important variable for the simula-tion of a wave climate is the fetch, the distance over which the wind condisimula-tions generate waves. The fetch was chosen to be 100 km from Sahalin Spit towards the north, east and south in accordance with the spatial extent of the typical storm systems in the north-western part of the Black Sea (Ginzburg et al., 2002). SWAN (Simulating WAves Nearshore) (Booij et al., 1999), the numerical model used for simulating the wave climate, is a third-generation numerical wave model to compute random, short-crested waves in coastal regions with shallow water. It accounts for processes such as refraction, wave-wave interactions and dissipation processes due to bottom friction and depth-induced wave breaking. The model is based on an Eulerian formulation of the discrete spectral balance of action density that accounts for refractive propagation over arbitrary bathymetry and current fields and it is driven by boundary conditions and local winds. In SWAN the waves are described with the two-dimensional wave action density spectrum. The evolution of the wave spectrum is described by the spectral action balance equa-tion, which, for Cartesian coordinates, is:

     S N C N C N C y N C x N t x y                (2.1)