A Quick Scan Analysis of Submerged Breakwater Designs

Bruin, T.P.B. de (2016) A Quick Scan Analysis of Submerged Breakwater Designs.

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Abstract:Many studies suggest coastal structures with a core of geotextile sand elements could serve as a reasonable alternative to conventional structures. Although applications with geotextile tubes are widely used abroad, this is certainly not the case in the Netherlands. Important aspects why this technique is just rarely used in the Netherlands, is because of relatively high risk of instalment due to inexperience and lack of confidence. This research focuses on the question whether geotextile tube breakwaters can be seen as genuine alternative for rock breakwater applications. The main purpose is to elaborate on the performances, sustainability and costs of geotextile tube breakwaters in comparison with conventional rock breakwaters. To do so, a case study is done in which both types of breakwater are examined on these aspects. The case study for this research is the project “Lido of Sète” at the south coast of France. Part of this project aimed to reconstitute 70 meters wide beach and ensure protection against erosion. To accomplish this, submerged geotextile tube breakwaters are built parallel to the shore. Based on project data and the geotextile tube breakwater design, a comparable conventional rock breakwater is designed in such a way that it is practically equivalent with the geotextile tube breakwater in terms of the normative performance wave transmission. This is done by first calculating the coefficient of transmission for the already known geotextile tube breakwater. Then, the formula of the coefficient of wave transmission for the conventional rock breakwater is transformed, so that the height of the structure is expressed in the width of the structure crest and coefficient of transmission (with a fixed slope angle). From this transformed formula, the optimum geometry for the comparable rock breakwater design is derived. With both breakwaters now clear, the hydraulic performances of wave reflection and wave dissipation are discussed. The extent to which breakwaters reflect waves depend mainly on the roughness and porosity of the material and slope angle of the structure in combination with the wave steepness. The wave reflection for both breakwaters is calculated by using relatively simple, empirical formulas. Wave dissipation is determined by applying the law of conservation of energy. Wave dissipation is the amount of energy that will be absorbed by the structures. High wave dissipation is generally desirable, because this implies a low wave transmission and/or wave reflection. Furthermore, it is checked whether both breakwater designs are sufficiently stable or not. This is determined for the normative failure mechanism. The normative failure mechanism for geotextile tubes is sliding of tubes. The stability of geotextile tubes is measured by a stability number which indicates the displacement of rocks under wave attack. Both designs appear to be stable. The last part of this research involved the non-functional aspects sustainability and costs. In order to determine the sustainability of the designs, the amount of carbon dioxide released when realizing the breakwaters is calculated by using indicators per unit of material. The costs are determined from the client’s perspective and are, just like sustainability, based on indicators for costs per unit of material. Results show that, in Mediterranean conditions like the Lido of Sète project, geotextile tube breakwaters are able to provide the same normative hydraulic performance (i.e. wave transmission) as conventional rock breakwaters. However, the maximum wave height reduction for submerged geotextile breakwaters is limited. Conventional rock breakwaters are capable of weaken waves more than geotextile tube breakwaters, due to the permeability and roughness of the rock core and gentler front slope. An unwelcome side effect of geotextile tubes is high wave reflection, caused by an impermeable core and steep structure slope. Advantages of geotextile tubes over rock breakwaters are a low carbon footprint (mainly due to very low CO2 emissions from transport) and costs
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
Link to this item:http://purl.utwente.nl/essays/71037
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