Beach Nourishment Behavior Modeling of Beach Nourishment Planform Evolution: a Case Study of the Coast of North Zealand
Total Page:16
File Type:pdf, Size:1020Kb
Beach nourishment behavior Modeling of beach nourishment planform evolution: a case study of the coast of North Zealand UNIVERSITY OF COPENH AGEN FACULTY OF SCIENCE Master’s thesis Sofie Kamille Astrup and Serena Pilunnguaq Sørensen Beach nourishment behavior Modeling of beach nourishment planform evolution: a case study of the coast of North Zealand Main academic supervisor: Troels Aagaard, IGN External co-supervisor: Nils Drønen, DHI Submitted: February 8, 2016 Number of characters: 185,760 Department of Geosciences and Natural Resource Management Chapter 1 / Introduction Tak til alle 1 Chapter 1 / Introduction Abstract Beach nourishment is an important method for future protection of the Danish coastline. However, the modeling of beach nourishment behavior and estimates of predicted life-span and efficiency is associated with large error margins. This present thesis considers beach nourishment behavior, shape, symmetry in transport gradients and evolution in time. The 20-year planform evolution of beach nourishments in simplified settings is examined using the shoreline evolution model in the Littoral Processes FM framework. 15 schematized nourishment schemes were constructed in order to investigate the influence of various wave parameters on life-span and sediment diffusion pattern. This included variations in the angles at which the waves approach the shoreline. A simulation of the behavioral pattern of nourishments on a natural coastline is exemplified by a case study of potential nourishment scheme behavior at Tisvildeleje, Raageleje and Udsholt Strand, all located along the receding coastline of North Zealand. In order to evaluate qualitative assessments of potential longshore sediment transport produced using LITDRIFT, the actual erosion rates were determined using the GIS shoreline analysis tool DSAS. Recent research has shown that migration and formation of shoreline perturbations is related to the angles at which the waves approach the shoreline. Beach nourishments can be perceived as a sudden unnatural perturbation along a shoreline and therefore this study has investigated the possibility of recreating downdrift migration of beach nourishments. As the linear model has limitations with respect to modeling the morphological response to high incidence wave angles, migration of beach nourishments could not be modeled using the applied shoreline evolution model. The study of erosional patterns along the nourishments fostered the idea that a nourishment introduced on a sediment deprived beach will migrate in the downdrift direction if the littoral drift potential upstream of the nourishment is not satisfied. Since it was found that the examined coastline of North Zealand suffers from a chronic sediment deficit and a drift potential which exceeds the observed erosion rates, it is suggested by this study that migration of beach nourishment could occur on the coast of North Zealand. Keywords: Beach nourishment; Planform evolution; Shoreline undulations; Coastal erosion; Coastal protection; Soft engineering methods; Historic shoreline change; North Zealand, LITLINE; Littoal drift 2 Chapter 1 / Introduction Dansk resumé Strandfodring er en vigtig del af den fremtidige beskyttelsesstrategi for de danske kyster. Imidlertid er simuleringer af kystfodringers udvikling, herunder projektets levetid og effekt, forbundet med store fejlmargener. I denne rapport undersøges kystfodringers morfologiske udvikling over tid, form og symmetri i transport gradienter. Den 20-årige udvikling i planform for sanddepoter er undersøgt for simplificerede forhold ved brug af kystlinjeudviklingsmodellen, som indgår i modeleringsværktøjet Littoral Processes FM. Der blev opstillet 15 skematiserede kystfodringsscenarier, med henblik på at undersøge indflydelsen af forskellige bølgeparametre på strandfodringens levetid og sedimenttransportmønstre. Simuleringerne inkluderede variation af bølgeindfaldsvinklen. Simulering af en strandfodring på en naturlig kyststrækning er eksemplificeret ved et casestudie af to mulige kystfodringsscenarier for strækningerne Tisvildeleje, Rågeleje og Udsholt Strand, som er beliggende på den eroderende kyststrækning mellem Hundested og Gilleje på Sjællands Nordkyst. For at kunne vurdere den potentielle langsgående sedimenttransportrate fundet ved brug af LITDRIFT, blev de faktiske erosionsrater bestemt på baggrund af en analyse af historiske flyfotos, foretaget ved brug af GIS-værktøjet DSAS. Nyere forskning har påvist, at dannelse og migration af kystlinjeundulationer kan relateres til skævtindfaldene bølger. En strandfodring kan opfattes som en kystlinjeundulation, og derfor er det i dette studie undersøgt, om migration af en sådan undulation kan genskabes ved brug af LITLINE. Da denne lineære model er begrænset i forhold til at kunne modellere det morfologiske respons i situationer med meget skævtindfaldende bølger, kunne migration af kystlinjeundulationer ikke simuleres i dette studie. På baggrund af observerede erosionsmønstre, er der udviklet en hypotese om, at migration af kystfodringer kan forekomme, i tilfælde hvor kysten lider af et kronisk sedimentunderskud. I tilfælde hvor den potentielle langstransportrate opstrøms for et kystfodringsdepot ikke er opfyldt, vil der blive eroderet langs den opstrøms side af depotet og aflejret langs den nedstrøms side, hvilket vil resultere i at depotet forflyttes i nedstrøms retning. Da det blev konstateret, at Nordsjællands kyst lider af et generelt sedimentunderskud, fordi den potentielle langstransport overstiger den faktiske erosionsrate, foreslår denne undersøgelse derfor, at der er mulighed for, at fremtidige kystfodringer på strækningen vil vandre i den dominerende langstransportretning. Keywords: Strandfodring; Morfologisk udvikling; Kystlinjeundulationer; Kysterosion; Kystbeskyttelse; Historisk kystlinjeudvikling; Nordsjælland, LITLINE; Langstransport 3 Chapter 1 / Introduction Contents Abstract ............................................................................................................................................................. 2 Dansk resumé .................................................................................................................................................... 3 1 INTRODUCTION ..................................................................................................................................... 7 1.1 Coastal zone management ........................................................................................................................ 8 1.2 Scope of thesis ......................................................................................................................................... 9 1.3 Structure of thesis .................................................................................................................................. 10 1.4 Erosional problems at the coast of North Zealand ................................................................................... 10 2 GENERAL THEORY .............................................................................................................................. 14 2.1 Causes of beach erosion ......................................................................................................................... 14 2.2 Responses to beach erosion .................................................................................................................... 15 2.3 Wave processes ...................................................................................................................................... 17 2.3.1 Wave characteristics ........................................................................................................................ 17 2.3.2 Wave energy ................................................................................................................................... 17 2.3.3 Wave transformation ....................................................................................................................... 18 2.4 Sediment transport ................................................................................................................................. 20 2.5 The cross-shore profile ........................................................................................................................... 21 2.6 Longshore currents and littoral drift ........................................................................................................ 23 2.7 Application of numerical modeling for prediction of beach nourishment evolution ................................. 25 3 STUDY AREA ............................................................................................................................................ 27 3.1 General description ................................................................................................................................ 27 3.2 Geological setting .................................................................................................................................. 28 3.3 Wind, waves and tides ............................................................................................................................ 29 3.4 Local sediment budget ........................................................................................................................... 30 3.5 Selected beach sections .........................................................................................................................