Original Research
Study of water renewal and sedimentation of a square harbor encapsulated in a coastal front with seawalls due to wind-induced hydrodynamic circulation

https://doi.org/10.1016/j.ijsrc.2020.11.001Get rights and content

Abstract

In the current study in the middle of a coastal zone, a harbor basin in the form of a square area recessed to the front is considered. A breakwater of variable length and position offers protection from waves. The water renewal time and the self-purification capacity of the harbor under the influence of alongshore currents due to wind-induced water circulation of varying intensities were examined. Furthermore, the transport of particulate matter released from the upstream side of the coastal zone and the possible influence (via sedimentation) on the lateral basin was investigated. The current study was based on the use of a two-dimensional depth-averaged hydrodynamic model and a quasi-three-, dimensional transport model while the effect of partial closing of the harbor's entrance was examined. The (1) adjustment and (2) validation of the reliability of the numerical model at a laboratory level were done using a recent research based on the use of a particle image velocimetry (PIV) measurements ensuring very good agreement between numerical and experimental results. The current study showed that the partial closing of the opening between the harbor basin and the extended coastal zone, with a breakwater of length equal to 1/3 of the opening, seems to be the most acceptable technical solution.

Introduction

Long straight seawalls along the coastal front are quite common, especially for coastal cities. Also, marinas and fishing harbors often are designed with an orthogonal or square geometry. Furthermore, the environmental study of harbor design is becoming more and more necessary during recent decades. In the context of the best possible environmental design of a harbor the parameters of water renewal and matter transport with possible siltation are competitive and conflicting factors. Thus, if a harbor is connected with the open sea through large openings it is obvious that the renewal of the waters of the harbour basin will be better (e.g., the waters renewal time will be small, which is certainly an advantage). However, in this case the possibility of siltation or of generally suspended matter entering the basin with consequent sedimentation and deposition is much higher (which certainly constitutes a major disadvantage). In the opposite case of small openings between a harbor and the open sea, inverse situations are expected. Thus, the scientists and engineers are called upon to face and address these issues and resolve the relevant Gordian Knots.

In recent studies (Pechlivanidis et al., 2016; Savvidis et al., 2018) the hydrodynamic circulation in a long extended area with a lateral small basin was investigated analytically both experimentally with the help of a laboratory channel and numerically with mathematical simulations. In these studies a successful combination of physical and mathematical modelling work was achieved enhancing thus the validity and importance of the research.

Several researchers (Jamieson & Gaskin, 2007; Tuna et al., 2013) had previously experimentally studied the connection between the flow of a main stream and a lateral cavity as well as transfers to the cavity. The previous research showed that turbulence structures were developed and advected along the mixing-layer constituting the governing process for these transfers.

The optimum design of the opening through which a fish refuge pond connects laterally to the main flow of a river was examined by Savvidis et al. (2017). The study was based on physical and mathematical modeling. The physical modeling was done via laboratory experiments that included an open channel with a lateral reservoir. The flow characteristics measurements were done using particle image velocimetry (PIV) method. The mathematical simulations were based on the development of a two-dimensional (2D) horizontal hydrodynamic model and a quasi-three-dimensional (3D) sediment transport model. The final results of the mathematical models and the findings of the physical model were compared, and the hydrodynamic interaction and coupling between the main flow of the channel and the lateral fish pond were successfully simulated leading to the optimum technical design of the system.

Problems similar to the aforementioned issues but for the case of a harbor lateral to a main river stream were studied by Savvidis and Keramaris (2017). Thus, in this study particle transport in a navigable river and its lateral harbor basin was investigated. This study focused on possible deposition issues that might be dangerous in the harbor's normal operations.

Matter transport processes constitute one of the most important research issues during the recent decades (Macdonald et al., 2006; Karageorgis et al., 2008; Kombiadou & Krestenitis, 2012, 2013). Concerning the renewal of the waters of a harbor basin Fountoulis and Memos (2004) proposed a methodology for the optimization of gap layouts in a harbor. The first solution was with two gaps, one at the exposed area and the other near the root of the breakwater and the second one with one gap at the corner of the breakwater. The results showed that the solution with two gaps is advisable in the case where an associated disturbance is acceptable, and, if not, then the second solution is preferable.

Langedoen et al. (1994) did a very interesting study where they studied the hydrodynamic circulation of a tidal harbor with the help of a physical model as well as the measurement of current velocities and temperature of the waters through instruments based on electromagnetic methods. Among other findings, the research showed that the flow in the entrance and the exchange of matter between harbor and river depend markedly on the phase of the tide. Additionally, the formation and decay of gyres in the harbor was shown to correspond with variations in advective and turbulent transport of heat through the harbor entrance.

In the study of Bartolic et al. (2018) field measurements were done inside and in the vicinity of a marina where eight parallel flushing culverts were embedded in the primary breakwater. The main suggestion of this study was to position the flushing culvert axis at the mean sea level following the main direction of the incident wave propagation to achieve more efficient seawater exchange in the marina basin.

Finally, Huguet et al. (2019) did a number of simulations with a 3D hydrodynamic model to describe water renewal mechanism in a highly populated marina. The results showed that there is a significant influence of the tide and the wind on the water renewal of the marina. Also the transition from spring to neap tides significantly decreases the water renewal.

The current study aims to examine the important environmental parameters for a number of alternative technical solutions, which are focused on the coastal marine environment and more specifically on the connection between a long coastal zone and a square harbor lateral to this coastal area with a seawall along its front. Here, different environmental issues of the harbor, such as renewal of the waters due to wind generated sea currents, problems of sedimentation, as well as biological demand oxygen (BOD) levels in the water column, are examined so that the most efficient operation and best environmental situation of the harbor can be achieved. The appropriate balance between the processes of waters renewal and matter transfer and siltation is the key for the success of the current specific studies. It should be noted that the current study considers only the wind induced circulation. Tides are not taken into account. So, the study concerns coastal basins with neglected tides (i.e., very small tidal signals) like Mediterranean Sea and its sub-basins.

Section snippets

Methodology and laboratory experiments

An experimental study, the subsequent adjustment and validation of the corresponding numerical models have been recently described (Savvidis et al., 2017). That modeling work constitutes the basis of the structure of the mathematical model applied for the simulation of a similar problem described in this paper. The successful validation of the aforementioned work ensures the operational value of the numerical model used in the current study. In more detail, the experiments were done in a

Hydrodynamics

The current study concerns the study of a long coastal zone of 800 m in length and 150 m in width with a square harbor lateral to the coastal area with harbor dimensions 150 m × 150 m (Fig. 3a). The depth of the water in the elongated coastal area as well as in the lateral basin of the harbor was taken as 3 m. The study was based on the development and application of a hydrodynamic model. For the numerical solution of the model, the geometry of the study area was discretized horizontally with

Results and discussion

Characteristic environmental parameters of the waters of a harbor lateral to a long coastal zone were examined through the application of a 2D depth-averaged hydrodynamic model. These parameters included the renewal time of the waters of the lateral basin, the concentrations of BOD for a maximum number of 1,000 people on the vessels in the harbor, as well as the transport of particulate matter and the trapping of a part of this matter in the lateral basin. A maximum value of the BOD

Conclusions

The current study has highlighted some very important issues that generally concern many natural or technical basin configurations similar to the cases studied in this paper. Consequently, the conclusions of the current study apply to harbors of specific geometrical characteristics, i.e., square harbors lateral to long coastal zones with sea walls. Furthermore, it should be clarified that the harbor was encapsulated in a coastal front with seawalls. This clarification implies that there is no

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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