Abstract. 1. Introduction

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1 Flow Characteristics and Flushing Processes in Marinas and Coastal Embayments J.Yin Cranfield University, Cranfield, Bedfordshire MK43 OAL, UK E Mail: J. Yin@Cranfield. ac. ukl R A Falconer Cardiff School of Engineering, Cardiff University, Cardiff CF2 STB, UK E.Mail: FalconerRA@Cardiff.ac.uk K Pipilis, A.I. Stamou Dept. of Water Resources, National Technical University Athens, Greece. Stamou@Central.ntua.badhost Abstract The paper gives details of the flow characteristics and concentration distributions in harbours and marinas, and around coastal structures such as breakwaters or headlands etc. Five harbour and four breakwater models were studied using a Sontek-ADV, which measures the three-components of velocity, and fluorometers to measure concentration distributions. The influence of the harbour size and the geometry of breakwaters on the flushing processes was studied. The width of the harbour entrance was found to play a significant role in the flushing processes. The 2-D DIVAST model was also used to calculate the flows and the solute transportation processes. The predicted eddy structures and the average concentration levels were found to compare favourably with the experimental results. 1. Introduction Yachting activity and waterside recreation are major tourist activities in both Greece and the UK. Marinas and coastal embayments provide sheltered waters for small boats, together with other recreational facilities. In recent years there has been a continuing demand across Europe for additional marinas, although it has only recently been appreciated that such enclosed bodies of water can exhibit major pollution and hydroecological problems. As public concern regarding the quality of coastal waters continues to increase and legislative standards and guidelines become increasingly more widespread and significant, then developers and planners will continue to come under increasing pressure to undertake more rigorous and comprehensive environmental impact assessment studies of proposed marina and waterside development designs.

The present study forms part of a joint project entitled: Modelling of Flow, Water Quality and Flushing Processes in Marinas and Coastal Embayments, funded by the British Council and the General

2 The present study forms part of a joint project entitled: Modelling of Flow, Water Quality and Flushing Processes in Marinas and Coastal Embayments, funded by the British Council and the General Secretariat of Research and Technology, Greece. The purpose of this study was to measure the flow characteristics and concentration distributions for different geometries and to compare these results with 2-D computer model predictions, having calibrated the numerical model. The experiments were conducted in the Hydraulics Laboratory at the University of Bradford. A new version of the 2-D DIVAST numerical model was used to compute the flow and solute transport processes, with the model being originally developed by Falconer (1) and refined by Hakimzadeh (2) and more recently the lead author. The model solves the shallow-water flow equations, as well as the standard k - e turbulence model and a simple mixing length model (i.e. v, = const x 7* x H). 2. Experimental Set-up The experimental work was undertaken in a large coastal basin, which consisted of a steel frame glass clad tank, with plan working dimensions of 4.8 m x 3.6 m and a constant working water depth of 0.5 m over the planform section, see Figure 1. This modified facility was designed to study tide and current flows. The tide was generated by a vertically oscillating weir and the current flow was driven by a 15 hp pump, with several control valves to generate the required direction of flow. In this experimental study, however, only steady flow was considered. The pump flow rate was set at approximately 0.06 cumec, giving a free stream velocity of approximately 0.01 m/s. The flow entry and exit conditions to the tank were lined with aluminium mesh to straighten out the flow. The water depth was controlled by an independent HP85 computer linked vertical sluice gate at the downstream end. The tank could be drained from either end via two large pipes, with holes being drilled through the invert and located in such a manner as to produce near uniform flow across the tank. A Sontek-ADV was used to measure the three-dimensional velocity components at the sampling rate of 25 Hz, giving about sample points. Rhodamine was used as the tracking dye and two Turner design fluorometers were sited at mid-depth. At the commencement of each test the harbour entrance was first sealed and an initial concentration set everywhere to about 100 ppb. The entrance seal was then removed and the concentration measured in the harbour models. For the breakwater models, Rhodamine was injected uniformly over the depth using a wingshape pipe, with several constantly spaced holes.

3 3. Experimental Models Five breakwater models were studied in the tests, with the models being illustrated in Figure 2 and listed below:- 1. Regular breakwater, length 0.2 m, width 0.05 m. 2. T junction breakwater, length 0.2 m, width 0.05 m, tip length 0.10 m and width 0.018m. 3. T junction breakwater, length 0.2 m, width 0.05 m, tip length 0.15 m and width 0.018m. 4. T junction breakwater, length 0.2 m, width 0.15 m, tip length 0.20 m and width 0.018m. 5. Half cone headland model with 1:2 sidewall slope. For breakwaters 1 to 4 the dye injection point was located at x = -0.5 m, y = 0.4 m, with the injected concentration and flow rate being 365 ppb and 2.07 ml/s respectively. For case 5 the injection point was at the same site, but the concentration and flow rate were 5,760 ppb and 2.07 ml/s, respectively. The origin of the co-ordinate system is as shown in Figure 1 and the water depth for all tests was set at 0.15 m. Likewise, five harbour models of different length, width and entrance dimensions were considered, as shown in Figure 3 and highlighted below:- 1. Length 0.7 m and width 0.7 m. 2. Length 0.7 m and width 0.35 m. 3. Length 0.35 m and width 0.7 m. 4. Length 0.7 m, width 0.7 m and entrance of width 0.2 m. 5. Length 0.7 m, width 0.7 m and two entrances of width 0.15 m. The water depth was m in cases 1 to 4 and 0.15 m in case 5. The origin of the co-ordinate system was at the central point of the main harbour width (see Figure 1), with the x-axis being downstream. 4. Governing Differential Equations The numerical calculations were based on the solution of the time dependant depth-integrated equations of motion:

4 d(uh) dt de dx dx d_ dy d(uvh) dy Jdu dv v.h\ + ' I dx dy dx (2) d(vh) dt d(uvh) ~dx ( du dv dy d I dv 2 \v,h dy\ dy ';* (3) where = water surface elevation above datum, U, V = depth averaged velocities in x, y directions, H = total depth of flow (=h+c,, where h - depth of bed below datum), /? = momentum correction factor for nonuniform vertical velocity profile, f - Coriolis parameter, g = gravity, p - fluid density, Txb^yb ~ bed shear stress components in x, y directions, and v, = depth averaged eddy viscosity. The bed stress was represented in the form of a quadratic friction law, as given by: (4) where Vs = depth averaged fluid speed ( = Vu + V ). The Chezy value was determined from the Colebrook-White equation. In determining the value of v, for the turbulent stresses, this coefficient was based on assuming that bed generated turbulence dominated over free shear layer turbulence, and for a logarithmic velocity profile was assumed to be (Elder v, = Coed x U+ x H (5) where Coed = and U* = fluid shear velocity. However, Fischer [4] established that this value of Coed was too low and that the value of Coed should typically be in the range 0.15 to Another method used was to use the standard k-e turbulence model and then solve for k and 8 from transport equations, with v, then being evaluated accordingly :- Further details of this approach are given by Falconer and Li [5].

5 The depth integrated advective-diffusion equation was linked to the hydrodynamic model, thereby enabling the transport of a conservative tracer S to be evaluated accordingly: where D^, D^ D^, Dyy = depth mean longitudinal dispersion and turbulent diffusion tensor coefficients in x, y directions. Further details on these coefficients are given in Fischer [4]. 5. Experimental and Numerical Model Results 5.1. Harbour Models The velocity vectors and turbulent kinetic energy distributions for the five harbour models were measured. For the 0.7 m x 0.7 m harbour, the vortex centre was sited at x = 0.1 m, y = m, i.e. slightly to the right side of the harbour centre, with a large velocity existing near the right hand side wall. The maximum velocity was located close to the wall opposite the entrance. Outside of the harbour, the velocity vectors were parallel to the x-axis. Around the entrance of the harbour, there was evidence of high turbulence, due to the large velocity gradient in the Y direction. This region of relatively high turbulence grew along the harbour entrance, thereby enhancing mixing with the exterior fresh water. The larger the length of this high turbulence region, then the greater the degree of mixing. This region of high turbulence was found to play an important role in carrying dye from within the harbours. For the 0.7 m x 0.35 m harbour, the depth was reduced to half the size. The strong flow near the right hand side quickly flowed around the harbour and back to the entrance. The vortex centre was located at x = 0 12 m, y = m and again closer to the right hand side of the harbour centre. For the 0.35 m x 0.7 m harbour, the harbour entrance width was reduced to half size. The influence of mixing was weak due to the reduced mixing zone along the harbour entrance. For the 0.7 m x 0.7 m harbour, with one 0.2m entrance, the flow inside the harbour was extremely weak in comparison with the above three harbour geometries. Finally, for the 0.7 m x 0.7 m harbour with two 0.15 m entrances, the flow was likewise towards the right hand side, as for the 0.7 m x 0.7 m harbour with one 0.2 m entrance! At the left-hand side of the entrance, the velocitv vectors were

6 92 Maritime Engineering and Ports large towards the harbour, which accounted for a relatively large volume of fresh water flowing into the harbour Breakwater Models Velocity vectors and turbulence kinetic energy distributions were similarly measured for the four breakwater models. The re-attachment points were generally at around x = 2.1 m. With the growth of the T junction head size, the eddy centre moved upwards and the maximum turbulence kinetic energy level increased. A typical example of the measured velocity and turbulence kinetic energy is given in Figure Concentration Distributions Figure 5 illustrates the time history for the average concentration distribution in the harbours. For the same length of harbour, the concentration decreased faster than for the harbour with double the width. For the same width harbour, the concentration decreased more slowly (i.e. by about 50%) than if the harbour length was doubled. For the twoentrance harbour, the concentration decreased faster than that for the one entrance harbour. Measured concentration distribution maps were also produced for the breakwater models, with the injection point being located at the upstream end of the models. The dye propagated around the breakwaters and mixed quickly. There was a region of high turbulence before and after the breakwater and this induced strong mixing. Figure 6 shows a typical concentration distribution for the 0.7 m x 0.7 m harbour at t = 600 s. The highest concentration was observed at x = 0.15m, y = -0.4 m Comparison with Calculation Comparisons of the predicted and measured average concentration distributions in the harbours generally showed close agreement. For the harbours 0.7 m x 0.7 m, 0.7 m x 0.35 m and 0.7 m x 0.7 m with two entrances, the predictions were in good agreement with the experimental results. For the 0.35 m x 0.7 m harbour, the results agreed well for t < 400 s. For the 0.7 m x 0.7 m harbour with one entrance, the concentration predictions were smaller than the experimental results for t < 1000 s, and when t > 1000 s. For the last two cases, the velocities inside the harbours were very small.

7 Figures 7 and 8 show typical calculated velocity vectors and concentration distributions for a breakwater and harbour model. The predicted velocity vectors were in close agreement with the experimental results, although the predicted re-attachment point was slightly too far downstream for some breakwaters. For the harbours, all of the configurations showed good agreement between both sets of results. Summary The flow characteristics and concentration distributions in harbours and around breakwaters or headlands were measured using a Sontek-ADV and fluorometers. The 2-D DIVAST model was used to predict the velocity and concentration distributions and comparisons were made between both sets of results. The main conclusions were: 1) The harbour entrance width and its mixing processes played an important role on the flushing characteristics within the basins. 2) For the same length of harbour, the concentration decreased faster when the harbour width was doubled. For the same width of harbour, the concentration decreased more slowly (i.e. by about 50%) when the harbour length was doubled. For the double-entranced harbour, the concentration decreased faster than that for a single entranced harbour. 3) The high turbulence around the breakwater tip played an important role in defining the downstream solute concentration distribution. 4) For breakwater models, the reattachment length was about 10 times the breakwater length. 5) The 2-D DIVAST model accurately predicted the time history of the average concentration inside the harbours for the present models, except for where extremely weak flows existed 6) The 2-D DIVAST model also accurately predicted the flow characteristics and the solute concentration distributions around all of the breakwater and headland shapes considered. Acknowledgements The authors would like to acknowledge the support of the British Council, the General Secretariat of Research and Technology (Greece) and the Engineering and Physical Sciences Research Council for their financial support for this project. The authors also wish to thank Mr. Tony Daron for his invaluable assistance with the experiments and Mr C Kaldis, British Council (Greece) for his support and encouragement.

8 94 Maritime Engineering and Ports References 1. Falconer, R.A., Flow and Water Quality Modelling in Coastal and Inland Waters, Journal of Hydraulic Research, IAHR Vol.30, No , pp Hakimzadeh, H, Turbulence Modelling of Tidal Currents in Rectangular Harbours, Ph.D. Thesis, University of Bradford, 1997, pp Fisher, HB, On the Tensor Form of Bulk Dispersion Coefficient in a Bounded Skewed Shear Flow, Journal of Geophysical Research, Vol.83, No.C5, 1978, pp Elder, J.W, The Dispersion of Marked Fluid in Turbulent Shear Flow, Journal of Fluid Mechanics, Vol.5, 1959, pp Falconer, R A and Li, G, Modelling Tidal Flows in an Island's Wake Using a Two-Equation Turbulence Model, Proceedings of the Institution of Civil Engineers, Water, Maritime and Energy, Vol.96, No. 1, 1992, pp

9 Maritime Engineering and Ports 95 i Tide Weir ^^^ t^^ww^^ > 3.8m 1 1 ^ ; o ; Harbour ; i I 1 C Overflow 4.8m y- Wate Supply Manifold Tide Weir Water Supply Manifold Figure 1. Schematic illustration of laboratory tidal basin and harbour model. L= 0 10m > \ 0.05 L=0.15m 05 "V -7 ~ L= 0.20m \ '?^ T 7^T /\ 0.2m 0.2m 0.018m s ^%v^%^ W//y////y. ^^ Figure 2. Schematic illustration of breakwater models.

96 Maritime Engineering and Ports 0.7m 0.7m 0.2m ).7m 0.35m 0.35m 0.7m 0.7m Figure 3. Schematic illustration of harbour models. 0.75- T junction Breakwater Model 0.

10 96 Maritime Engineering and Ports 0.7m 0.7m 0.2m ).7m 0.35m 0.35m 0.7m 0.7m Figure 3. Schematic illustration of harbour models T junction Breakwater Model 0.05mX0.2m+0.15mX0.018m 0.0 I 1 I I I I * I I I I I I» ' OS 10 1 S Figure 4. Velocity vectors for T junction breakwater.

11 Maritime Engineering and Ports 97 Average Concentration iri Harbours...o o Harbour 0.70mX0.70m Harbour 0.35mX0.70m "... o -. - Harbour 0.70mX0.35m...*...*... Harbour 0.7mX0.7m with one Entrance Harbour 0.7mX0.7m with two Entrances - ' Figure 5. Time history for average concentrations in harbours. t(s) = 600 Figure 6. Concentration contour map for harbour of dimensions 0.7 m x 0.7, and with two entrances.

98 Maritime Engineering and Ports Vector = 0.2(m/3)T Junction Breakwater, T =0(S), Time(s) =1200-1 Figure 7.

12 98 Maritime Engineering and Ports Vector = 0.2(m/3)T Junction Breakwater, T =0(S), Time(s) = Figure 7. Calculated velocity vectors and concentration distribution for T junction breakwater. Vector = 0.2^m/a> H arbou ro. 7 m X 0.7 m, T =O(S), Time(a) =3OO Figure 8. Calculated velocity vectors and concentration distribution for harbour with dimensions of 0.7 m x 0.7 m.

Geomorphological Modelling in Coastal Waters

Abstract Geomorphological Modelling in Coastal Waters Morteza Kolahdoozan 1, Roger A. Falconer 2 (Fellow), Yiping Chen 3 Details are given herein of the development and application of a three dimensional