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Nearshore!Currents!In!South/ Eastern!Scania!

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Nearshore!Currents!In!South/ Eastern!Scania! Master!Thesis!! TVVR!16/5015 ! Nearshore!Currents!in!South/ eastern!Scania! ________________________________________________ Emanuel!Schmidt! ! ! ! Division!of!Water!Resources!Engineering! Department!of!Building!and!Environmental!Technology! Lund!University! Nearshore Currents in Southeastern Scania Master Thesis Supervisors: Author: Magnus Larson Emanuel Schmidt Caroline Fredriksson Division of Water Resources Engineering Lund University Box 118 221 00 Lund, Sweden ii Water Resources Engineering TVVR-16/5015 ISSN 1101-9824 Lund 2016 www.tvrl.lth.se iii Master Thesis Division of Water Resources Engineering Lund University English Title: Nearshore Currents in Southeastern Scania Author: Emanuel Schmidt Supervisors: Magnus Larson, Caroline Fredriksson Examiner: Hans Hanson Language: English Year: 2016 Keywords: Nearshore currents; drifters; rip currents; beach morphology; swimming safety iv Acknowledgements First of all I would like to thank my supervisors Magnus Larson and Caroline Fredriksson for great enthusiasm regarding my project. You have been very kind, helpful and inspiring throughout my work with this thesis. I have had many good days of field work combined with surfing. I would also like to thank Max Radermacher from TU Delft for valuable input, useful tools and field work assistance in cold Baltic water. Further, I would like to thank the institution of TVRL for housing me, inviting me to floor ball and for many cups of co↵ee. It has been a great time working with the institution and I hope to see all of you again. Last but not least, I would like to thank my girlfriend N´ına for enduring all my endless hours of explaining rip currents and numerical models instead of discussing baby names. v Abstract The South East Coast of Scania is a popular destination for both Swedish and foreign tourists. The long and wide sandy beaches are well visited by swimmers and beachgoers, but the waters have shown to be treacherous at times. In Au- gust 2015, two German tourists drowned after getting caught in a probable rip current in the proximity of Hav¨ang beach. Six months after the accident, this thesis study, which aims to investigate the nearshore currents in the region was initiated. The main objective was to map the currents prevalent in the area, with special attention to the o↵shore directed rip currents. In order to accomplish this, GPS-equipped drifters were developed for in-field measurements. The field work includes both drifter measurements and grain size sampling. Data of the local bathymetry from the project Sk˚anestrand was studied and a bathy- metric survey was conducted. The study also includes o↵shore wave hindcasting and modeling of the wave transformation as waves approach shore. Drifter measurements were conducted on four beaches at varying wave condi- tions. On 17-18 September 2016, rip currents were observed at Vitem¨olla beach. The o↵shore velocity of the current was found to be 0.4-1 m/s, which resembles velocities found in earlier studies (Winter et al. (2011), Dalrymple et al. (2011), Short (1985)). The other field measurements did not register any significant rip currents but rather strong longshore currents. The results from the wave trans- formation EBED model (with hindcasted waves as input) showed that a wave height variation along Vitem¨olla beach of more than 0.5 m can occur under the similar wave conditions. It was concluded that this could generate enough wave setup variation to drive rip currents. Furthermore, it can be concluded that most of the beaches within the study area are very dynamic, and in order to predict the nearshore currents with models up-to-date bathymetry and more reliable wave data is needed. With field measurements, literature studies and documentation of historical ac- cidents as a basis, a current map for the study area was developed. This study showed that the public should be attentive when swimming in the ocean when the beaches are subject to waves. A simple measure for preventing further accidents would be to red-flag all beaches under such conditions. vi Contents 1 Introduction 1 1.1 Background . 1 1.2 ResearchQuestionsandObjectives . 1 1.3 Methodology . 2 1.3.1 Literature Study . 2 1.3.2 Mapping of Rip Current Conditions . 2 1.3.3 Construction of Drifters . 2 1.3.4 FieldMeasurements . 3 1.3.5 Data Analysis . 3 1.4 ThesisStructure ............................ 3 2 Theoretical Background 4 2.1 Ocean Waves and Nearshore Transformation . 4 2.1.1 Wave Generation . 4 2.1.2 Refraction, Shoaling and Di↵raction . 5 2.2 WaveSetup............................... 6 2.3 Flux of Momentum Under Oblique Waves . 8 2.4 InfragravityWaves ........................... 8 2.5 Nearshore Currents . 9 2.5.1 Longshore Currents . 9 2.5.2 Undertow ............................ 10 2.5.3 RipCurrents .......................... 10 2.5.4 Backwash . 13 2.6 Beach Morphology and Storm Response . 13 2.6.1 The Beach State Classification . 14 2.7 SwimmerSafety............................. 17 3 Description of Study Area 19 3.1 StudySite................................ 19 3.2 Bathymetry............................... 21 3.3 WindandWaveClimate........................ 24 3.3.1 WindClimate.......................... 24 3.3.2 WaveClimate.......................... 25 3.4 History of Accidents . 28 3.5 InterviewswithLocalFishermen . 28 4 Drifter Development 30 4.1 DrifterDesign.............................. 30 4.2 DrifterConstruction .......................... 31 4.3 DrifterVelocityCalculations . 31 vii 5 Field Measurements 33 5.1 Complementary Bathymetry Measurements . 33 5.2 GrainSizeSampling .......................... 33 5.3 DrifterMeasurements ......................... 34 6 Field Measurement Results 35 6.1 GrainSizeSampling .......................... 35 6.2 Complementary Bathymetry Measurements . 36 6.3 Summary of Beach Properties . 38 6.4 CurrentTrackingwithGPS-Drifters . 38 6.4.1 Sandhammaren April 7 . 38 6.4.2 Vitem¨olla April 26 . 40 6.4.3 Kn¨ab¨ackshusen May 31 . 42 6.4.4 Vitem¨olla September 17-18 . 44 7 Numerical Modelling of Nearshore Wave Transformation 48 7.1 EBEDSimulation............................ 48 7.2 EBEDSimulationResults . 50 7.2.1 Vitem¨olla - 17 September 2016 - 19:00 . 51 7.2.2 Hav¨ang - 17 August 2015 - 13:00 . 53 7.3 Longshore Current Modelling . 55 8 Discussion 57 8.1 DrifterExperiments .......................... 57 8.2 Morphodynamics ............................ 58 8.3 Numerical Modelling of Nearshore Wave Transformation . 59 8.4 Safety Recommendations . 60 8.4.1 WarningSystem ........................ 62 8.5 FurtherResearch ............................ 63 9 Conclusions 64 10 Bibliography 65 11 Appendix 68 11.1 Wind Comparision . 68 11.2SPM-MF ................................ 69 11.3EBED.................................. 70 viii 1 Introduction 1.1 Background In August 2015 a tragic drowning accident occurred at Hav¨ang beach in eastern Scania, where two German tourists were pulled o↵shore, caught by a probable rip current (Helsingborgs Dagblad, 2015). The existence of strong o↵shore directed currents on the southeast coast of Scania is widely known among local residents and warning signs have been placed on the beaches to notify swimmers about the strong currents. However, the processes generating the currents are poorly understood and has not previously been studied in the area. This master thesis focus on studying the nearshore currents along the southeast coast of Scania and their basic characteristics. Understanding the nearshore hydro- and morphodynamic processes can help to predict where and when currents are formed. Identifying the conditions under which currents may occur is valuable to educate the public and may form the basis for a warning system to prevent further accidents. 1.2 Research Questions and Objectives The main objectives of this study is (1) to map the nearshore currents along the southeast coast of Scania, (2) to determine the underlying mechanisms and (3) to quantify their basic characteristics. This thesis aims to answer the following research questions: • What types of currents occur along the coastline? • Along which parts of the coastline in the study area do rip currents occur? • Under which wave conditions do rip currents occur? • How is beach morphology coupled to currents? • How can knowledge about nearshore currents be used in coastal- and safety management in southeast Scania? Based on an initial investigation, some beaches will be chosen as field study sites for which the following research questions will be investigated: • What is the rip cell morphology? • How strong is the rip current? 1 1.3 Methodology The study is divided into five main parts: 1) a literature study, 2) mapping of nearshore current conditions, 3) construction of drifters 4) field measurements, and 5) data analysis. 1.3.1 Literature Study International literature on nearshore currents is abundant, but most studies have been carried out at swell dominated coastlines. However, a few studies have been carried out at coastlines dominated by wind waves (e.g., Winter, 2011). The fo- cus of the literature study was to gain understanding of the physical processes of nearshore currents (especially rip currents) and other o↵shore directed currents, how they interact with beach morphology, and under which wave and morpholog- ical conditions they are likely to occur. The literature review also encompassed previous studies where field work has been performed as a preparation for the measurements to be carried out in the present study. 1.3.2 Mapping of Rip Current Conditions Based on the conceptual understanding
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