Feasibility study on the use of a floating breakwater to protect a new artificial beach in Balchik, Bulgaria

The coast of Bulgaria is subject to erosion. Because of this fact, around the town of Balchik, which is situated on the Black Sea coast, almost no sandy beaches are present. Last years there have been vast tourist developments in and around Balchik. Because of these developments, the province demands solutions to adapt coastal protection structures to become more attractive regarding recreation. In this study, a small pilot project on the creation of an artificial beach will be described. One possible source of nourishment sand is to dredge it from the local bottom. However, this sand is too fine to form a stable beach without a breakwater in front of it for protection. In this study is investigated whether it is technically feasible to use a floating breakwater to protect the new proposed beach in Balchik. In order to do this, the following approach is followed. A description of the current situation as well as boundary conditions regarding wind, waves, water level, bathymetry and sediment properties is given. Next, the maximum allowed wave height to form a stable beach is determined by means of sediment transport calculations in cross-shore and longshore direction and several possible nourishment sand sizes are considered. In order to lower the incoming waves (boundary conditions) to the maximum allowed wave conditions, a floating breakwater with a certain transmission coefficient is necessary. An investigation on possible types of floating breakwaters is made and the possibilities to produce the floating breakwater are described. From the types of breakwaters found, the most suitable type is selected. The last step is to determine the required dimensions and offshore distance of the floating breakwater in order to achieve the required transmission coefficient. If the required dimensions stay within reasonable limits, it can be concluded that it is technically feasible to use a floating breakwater as beach protection. It is chosen to use concrete caissons to create the floating breakwater, because of the fact that in the vicinity of the project location a company is located which can produce floating concrete structures. Regarding beach nourishment materials, the equilibrium beach profiles with mean grain diameters 0.1, 0.2 and 1.0 mm are considered. It is chosen to further elaborate the case of the locally available sand with a mean grain diameter of 0.1 mm, because for this sand a floating breakwater is necessary in order to stabilize the nourishment. The other two sediment sizes may form a stable beach without a breakwater in place. In order to determine the required dimensions of the floating breakwater, a distance of 200 m between the floating breakwater and the coastline is chosen. With the aid of a literature study on floating breakwater dimensions it is found that for this particular location and governing wave conditions, the draft of the floating breakwater is the governing parameter which determines the transmission coefficient. The breadth of the structure is determined by stability requirements. In this design, it is assumed that the breakwater is fixed in space, which is, regarding the main outline of the design, a reasonable assumption. The result of the study is a preliminary design of a beach nourishment with a volume of 91,000 m3 which is protected by a 240 m long floating breakwater consisting of three elements. The final conclusion of this study is that it is technically feasible to use a floating breakwater as beach protection measure.