TY - BOOK

T1 - Wave-induced responses of stepped revetments

AU - Kerpen, Nils

N1 - Doctoral thesis

PY - 2017

Y1 - 2017

N2 - Hard coastal structures represent a crucial element in current coastal defense strategies. The surface roughness of coastal structures, often impermeable and sloped, causes an increase in turbulence during the wave run-up. The increased turbulence results in a reduction of wave reflection, wave run-up and wave overtopping in a range of 10 − 50% compared to smooth slopes which are normally asphalt- or grass-covered. Stepped revetments naturally exhibit surface roughness which acts as an obstacle to the interacting flow. In cases where the steps are specifically designed to walk over or sit on, stepped revetments enable save and easy access to the beach or the water. Then, the dimensions of the steps will inevitably have a significant influence on the energy dissipation of the waves. For the first time, the processes of the energy dissipation over a stepped revetment are examined in this study in a comprehensive manner. Based on an in-depth literature review 60 years of research on stepped revetments are summarized. As a result of the literature review, it was found that the most significant relationship between the step height of stepped revetments and the wave height of the incoming waves on the overall energy dissipation has never been studied in a broader framework; this knowledge gap has thus been addressed in this work. Therefore, physical model tests were conducted in a 100m long wave flume. Wave impacts on a stepped revetment by plunging and surging wave breaking were evaluated for 34 varying sea states. Three slopes were tested for three different step heights. Additional tests with regular waves focused on the vorticity induced at the step edges. Further analysis concentrated on the energy dissipation of waves interacting with a stepped revetment which was thoroughly discussed in analytical terms. Previously derived methods which consider the energy dissipation at a stepped revetment result in significant scatter in their range of applicability. This is a result of the large inhomogeneity of the underlying data they are derived from. Published reduction coefficients, describing the reducing impact of stepped revetments relative to a smooth revetment, range from 0.35−0.9. For cases where the step heights are larger than the wave height, it was detected that the wave reflection and run-up over a stepped revetment can be reduced by 10−30% compared to a smooth slope. The efficiency of stepped revetments in reducing the wave run-up increases if the wave height is larger than the step height (30 − 60%). If the wave height is two times larger than the step height, the energy dissipation is most effective. If the step height is two times larger than the wave height, the revetment is highly reflective and the wave overtopping cannot be reduced significantly (2 − 10%). Due to the macro-roughness of a stepped revetment, the overtopping volume of plunging waves is reduced most effectively (40 − 60%). For surging waves, 10 − 30% reduction is detected. The importance of the still water level with respect to the step edge on the wave overtopping is derived for step heights equal to the wave height. Changes in the reduction of approximately ±20% are possible for this case. The highest energy dissipation can be achieved if the still water level is close to the step edge. The maximum pressures are measured at the still water level. All dynamic pressures decrease significantly with increasing water depth (< 20% for depths two times larger the wave height). Above the still water level the pressure distribution is comparable with a plain vertical wall. In this study, four formulae, valid for a wide range of applications, are derived empirically to predict (1) the wave reflection coefficient, (2) the reduction coefficient for wave run-up and (3) wave overtopping to be used in the context of the widely known Eurotop (2016) equations and (4) the pressure loading for stepped revetments. For the first time, a holistic prediction of the reduction coefficient correlated to the wave run-up and overtopping considers the wave steepness, the slope and the step ratio coincidently. In many places coastal protection structures are located in urban areas. Consequently, it often leads to conflicts between touristic interests and the protective function. Ideal coastal protection structures should provide access to coastal waterfronts and simultaneously provide an optimized protection level against storm surges. In this regard, stepped revetments promise an architecturally pleasing synthesis of both demands allowing the access to the waterfront easily and assuring designed safety margins for coastal protection. As a key characteristic, stepped revetments create higher surface roughness in comparison to conventional revetment surfaces, e.g. grass, asphalt or block revetments. Consequently, the principle of placing roughness elements on revetment slopes in the wave run-up area has proven to be a most effective measure to reduce the wave run-up and wave overtopping.

AB - Hard coastal structures represent a crucial element in current coastal defense strategies. The surface roughness of coastal structures, often impermeable and sloped, causes an increase in turbulence during the wave run-up. The increased turbulence results in a reduction of wave reflection, wave run-up and wave overtopping in a range of 10 − 50% compared to smooth slopes which are normally asphalt- or grass-covered. Stepped revetments naturally exhibit surface roughness which acts as an obstacle to the interacting flow. In cases where the steps are specifically designed to walk over or sit on, stepped revetments enable save and easy access to the beach or the water. Then, the dimensions of the steps will inevitably have a significant influence on the energy dissipation of the waves. For the first time, the processes of the energy dissipation over a stepped revetment are examined in this study in a comprehensive manner. Based on an in-depth literature review 60 years of research on stepped revetments are summarized. As a result of the literature review, it was found that the most significant relationship between the step height of stepped revetments and the wave height of the incoming waves on the overall energy dissipation has never been studied in a broader framework; this knowledge gap has thus been addressed in this work. Therefore, physical model tests were conducted in a 100m long wave flume. Wave impacts on a stepped revetment by plunging and surging wave breaking were evaluated for 34 varying sea states. Three slopes were tested for three different step heights. Additional tests with regular waves focused on the vorticity induced at the step edges. Further analysis concentrated on the energy dissipation of waves interacting with a stepped revetment which was thoroughly discussed in analytical terms. Previously derived methods which consider the energy dissipation at a stepped revetment result in significant scatter in their range of applicability. This is a result of the large inhomogeneity of the underlying data they are derived from. Published reduction coefficients, describing the reducing impact of stepped revetments relative to a smooth revetment, range from 0.35−0.9. For cases where the step heights are larger than the wave height, it was detected that the wave reflection and run-up over a stepped revetment can be reduced by 10−30% compared to a smooth slope. The efficiency of stepped revetments in reducing the wave run-up increases if the wave height is larger than the step height (30 − 60%). If the wave height is two times larger than the step height, the energy dissipation is most effective. If the step height is two times larger than the wave height, the revetment is highly reflective and the wave overtopping cannot be reduced significantly (2 − 10%). Due to the macro-roughness of a stepped revetment, the overtopping volume of plunging waves is reduced most effectively (40 − 60%). For surging waves, 10 − 30% reduction is detected. The importance of the still water level with respect to the step edge on the wave overtopping is derived for step heights equal to the wave height. Changes in the reduction of approximately ±20% are possible for this case. The highest energy dissipation can be achieved if the still water level is close to the step edge. The maximum pressures are measured at the still water level. All dynamic pressures decrease significantly with increasing water depth (< 20% for depths two times larger the wave height). Above the still water level the pressure distribution is comparable with a plain vertical wall. In this study, four formulae, valid for a wide range of applications, are derived empirically to predict (1) the wave reflection coefficient, (2) the reduction coefficient for wave run-up and (3) wave overtopping to be used in the context of the widely known Eurotop (2016) equations and (4) the pressure loading for stepped revetments. For the first time, a holistic prediction of the reduction coefficient correlated to the wave run-up and overtopping considers the wave steepness, the slope and the step ratio coincidently. In many places coastal protection structures are located in urban areas. Consequently, it often leads to conflicts between touristic interests and the protective function. Ideal coastal protection structures should provide access to coastal waterfronts and simultaneously provide an optimized protection level against storm surges. In this regard, stepped revetments promise an architecturally pleasing synthesis of both demands allowing the access to the waterfront easily and assuring designed safety margins for coastal protection. As a key characteristic, stepped revetments create higher surface roughness in comparison to conventional revetment surfaces, e.g. grass, asphalt or block revetments. Consequently, the principle of placing roughness elements on revetment slopes in the wave run-up area has proven to be a most effective measure to reduce the wave run-up and wave overtopping.

U2 - 10.15488/9005

DO - 10.15488/9005

M3 - Doctoral thesis

ER -