SOES 6030 Advanced Independent Oceanography Research Project
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SOES 6030 Advanced Independent Oceanography Research Project HYDRODYNAMIC IMPACT AND SEDIMENT MOBILITY AT GRAVEL DREDGED-PITS: A CASE STUDY OF THE TROMPER WIEK AREA (BALTIC SEA). ALICE LEFEBVRE 2004-2005 UNIVERSITY OF SOUTHAMPTON SCHOOL OF OCEAN AND EARTH SCIENCES UNIVERSITÉ BORDEAUX 1 UFR DES SCIENCES DE LA TERRE ET DE LA MER Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea). Contents List of Figures iii List of Tables iv List of Plates v List of Annexes v 1. Introduction 1 1.1. General introduction 1 1.2. Marine aggregates extraction 1 1.2. Studied area 8 1.3. Objectives 12 2. Theoretical background 13 2.1. Sediment mobility 13 2.1.1. Introduction 13 2.1.2. Currents 14 a) Generalities 14 b) Current-induced bed shear-stress 15 2.1.3. Waves 16 a) Generalities 16 b) Wave-induced bed shear-stress 18 2.1.4. Combined waves and currents 20 2.1.5. Threshold of motion 21 2.2. Suspended sediment concentration 23 2.3. Grain size analysis 26 3. Materiel and Methods 28 3.1. Instrumentation 28 3.1.1. Electro-Magnetic Current Meter 28 3.1.2. Pressure sensor 29 3.1.3. Optical Backscatter Sensor 29 3.1.4. Wavelog 30 3.2 Field experiment 31 3.2.1. Deployment 31 3.2.2. Sediment samples and meteorological conditions 33 3.3. Calibration 33 3.4. Grain size analysis 35 3.4.1. Sieving and weighing 35 3.4.2. Settling tower 36 3.4.3 Coulter Counter 37 3.4.4. Total Particle Size Distribution 38 3.5. High frequency data processing 38 3.5.1. Pressure 38 3.5.2 U and V components of the flow 39 3.5.3. Turbidity 40 i Alice Lefebvre University of Southampton School of Ocean and Earth Science Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea). 4. Results 42 4.1. Sediment characteristics 42 4.1.1. Video 42 4.1.2. Seabed sediment 43 4.1.3. Bottles sediment 46 4.2. General conditions during the experiment 48 4.2.1. Meteorological conditions and waves 48 4.2.2. Water level variations 51 4.2.3. Currents 53 a) Comparison of methods 53 b) Currents description and analysis 54 4.2.4. Suspended sediment concentration 56 4.3. Comparison of data at the three stations 57 4.3.1. Currents 57 4.3.2. Waves 58 a) Wave height and period 58 b) Directional wave spectra 59 c) High frequency surface elevation 60 4.3.3. Suspended sediment concentration 62 4.4. Sediment mobility 64 4.4.1. Current-only bed shear stress 65 4.4.2. Wave orbital velocity 65 a) Amplitude of the wave orbital velocity under the crest and under the trough 67 b) Comparison of theoretical and measured wave orbital velocity 68 c) Differences between the stations 68 4.4.3. Wave-only bed shear-stress 69 4.4.4. Total bed shear-stress 70 4.4.5. Fraction of sediment on motion 72 4.5. Suspended sediment concentration 74 5. Interpretation 77 5.1. Hydrodynamics effects 77 5.2. Effects on sedimentation 78 6. Conclusions 82 Bibliography 84 Annexes 89 ii Alice Lefebvre University of Southampton School of Ocean and Earth Science Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea). List of Figures Figure 1.1: The two most commonly used methods for marine aggregates extraction. A. Anchor hopper dredging, B. Trailor suction dredging. 3 Figure 1.2: Possible biological impacts of marine aggregate extraction. 5 Figure 1.3: Chain of possible effects caused by changing the bathymetry. 6 Figure 1.4: Reduction of current and wave velocity by increasing the water depth in dredged area. 7 Figure 1.5: Zone of study. 9 Figure 1.6: Generalized sediment distribution map for the whole of Tromper Wiek Bay and sites of marine aggregate extraction. 10 Figure 1.7: Side Scan Sonar from Tromper Wiek Bay. 11 Figure 2.1: The velocity profile for steady current flow over a bed showing current shear (length of arrow proportional to velocity) in the boundary layer. 13 Figure 2.2: Types of surface waves, showing the relationship between wave frequency and period, the nature of the forces that cause them, and the relative amounts of energy in each type of wave. 17 Figure 2.3: Airy waves showing the particle orbits at various depths below the surface. (a) In deep water, the particle or orbits are circular and their radius decays exponentially with depth. (b) With a depth of L/2, the orbits, including those of the surface particles have become elliptical. 18 Figure 2.4: Schematic diagram of non linear interaction of current-only (τc) and wave-only (τw) bed shear-stresses. 20 Figure 2.5: Threshold of motion of sediments beneath waves and/ or currents. 22 Figure 2.6: Wentworth grain-size classification together with the range of various analysis techniques. 26 Figure 2.7: Ternary diagram for mixtures of clay, sand and gravel. 27 Figure 3.1: Schema of the ABLs. 28 Figure 3.2: Site of deployment of the three ABLs. The bathymetry is given in meters, coordinates in UTM system. 31 Figure 3.3: General arrangement of settling tower. 34 Figure 3.4: Calibration curve 36 Figure 3.5: a. Separation of wave and current components from the total high frequency files by applying a filter. b. Wave components which let determine wave significant and maximal orbital velocity under crest and trough and wave direction. c. Current components which let determine mean current speed and direction for each burst. 40 Figure 4.1: Frequency histogram and cumulative frequency curve representing the grain size distribution at each sampling station. The median diameter of the sediments found in the bottles is also shown. 45 Figure 4.2: Frequency histogram and cumulative frequency curve representing the grain size distribution of the sediment found in the bottles at each sampling station. 47 Figure 4.3: Pressure and wind at Cape Arkona (the direction indicates where the wind comes from); wave height, direction (where it propogates towards) and period (Tz), current (where it goes) and depth at the station V1 during the experiment (HL = High water Level; LL = Low water Level). 49 Figure 4.4: Comparison of current speed and direction given in tidestat files and calculated from puvt files; example for station V1. 54 Figure 4.5: Schematic representation of the currents at station V1 and wind iii Alice Lefebvre University of Southampton School of Ocean and Earth Science Hydrodynamic impact and sediment mobility at gravel dredge pits: a case study of the Tromper Wiek area (Baltic Sea). at Cape Arkona before, at the beginning, at the end and after the storm. The size of the arrows gives an indication of the speed of wind and currents. 55 Figure 4.6: Currents at the three stations during the experiment. 57 Figure 4.7: Wave height and period for the three stations during the experiment. 58 Figure 4.8: Frequency, direction and spectral power density estimated using EMEP method. 59 Figure 4.9: Surface elevation during the burst 73 (20/10, storm conditions) showing the waves for the three stations. 61 Figure 4.10: Suspended sediment concentration (SSC) during the experiment for the three stations. 62 Figure 4.11: Current-induced bed-shear stress (τc) and threshold bed shear-stress (τcr); example for the station V1 for a roughness length z0 of 0.0003m (sand/gravel) and 0.006m (rippled sand). 64 Figure 4.12: Significant and maximum orbital velocity under crest and trough at station V1. 65 Figure 4.13: Difference of amplitude of wave orbital velocity under the crest and under the trough at station V1. 66 Figure 4.14: Significant measured and calculated orbital velocity during the experiment at station V1. 67 Figure 4.15: Significant wave orbital velocity for the three stations during the experiment. 68 Figure 4.16: Wave bed shear-stress (τw) calculated for a roughness length (z0) of 0.0003m (sand/gravel) and 0.006m (rippled sand) at stations V1 and V2 and a roughness length of 0.006m at station V3. 69 Figure 4.17: Wave-induced bed shear stress, (τw), maximum bed shear-stress for combined flow (τmax) and threshold bed shear stress (τcr) during the experiment; example for the station V1. 70 Figure 4.18: Relative importance of the wave-induced bed shear-stress on the maximum bed shear-stress during the experiment for the three stations. 71 Figure 4.19: Wave bed shear-stress and threshold of motion for different quartile at the three stations during the experiment. 73 Figure 4.20: Suspended sediment concentration (SSC) calculated at the height (z) of each OBS for the three stations. 75 Figure 5.1: Measured SSC at the three stations during the experiment, time of sediment movement calculated from the bed shear-stress and time of sediment suspension at the height of the sensor from the calculated SSC. 79 Figure 5.2: Schematic representation of the sequence of events during the storm. 80 List of Tables Table 3.1: Presentation of puvt files. 30 Table 3.2: Presentation of Tidestat (a) and Wavestat files (b). 30 Table 4.1: Percentage in weight of the different fractions for each sample. 43 Table 4.2: Summary of statistical parameters (phi units) of the particle size distribution of the seabed sediments for the three stations and their description according to McManus (1988).