Coastal Defence Structures Research Articles

Vulnerability of coastal defences to climate change

Climate change will alter wave and water level conditions and hence affect the vulnerability of coastal defence structures. The likely changes in vulnerability are important to the stakeholders in new and existing coastal defence structures, as decision-makers may reasonably be expected to include the probable effects of climate change in their planning processes. Time series of present and future waves and water levels were generated using a climate model. These were used to calculate the changes in overtopping rates and in the longshore drift of sediment in front of sea defences. The results indicate that there will be considerable increases in overtopping rates if present-day defences and beaches are unchanged by 2075. However, the inclusion of sea level rise predictions in design calculations should account for the majority of the predicted change in overtopping. The work also indicates that future changes in longshore transport are unlikely to be greater than current levels of uncertainty, and these should be considered in the normal course of sensitivity testing. Qualitative and quantitative differences in future changes in vulnerability were found between the five sites examined, as they had different tidal ranges, wave climates, surge levels and joint probabilities.

Vulnerability of coastal defences to climate change

Climate change will alter wave and water level conditions and hence affect the vulnerability of coastal defence structures. The likely changes in vulnerability are important to the stakeholders in new and existing coastal defence structures, as decision-makers may reasonably be expected to include the probable effects of climate change in their planning processes. Time series of present and future waves and water levels were generated using a climate model. These were used to calculate the changes in overtopping rates and in the longshore drift of sediment in front of sea defences. The results indicate that there will be considerable increases in overtopping rates if present-day defences and beaches are unchanged by 2075. However, the inclusion of sea level rise predictions in design calculations should account for the majority of the predicted change in overtopping. The work also indicates that future changes in longshore transport are unlikely to be greater than current levels of uncertainty, and these should be considered in the normal course of sensitivity testing. Qualitative and quantitative differences in future changes in vulnerability were found between the five sites examined, as they had different tidal ranges, wave climates, surge levels and joint probabilities.

Vulnerability of coastal defences to climate change

Climate change will alter wave and water level conditions and hence affect the vulnerability of coastal defence structures. The likely changes in vulnerability are important to the stakeholders in new and existing coastal defence structures, as decision-makers may reasonably be expected to include the probable effects of climate change in their planning processes. Time series of present and future waves and water levels were generated using a climate model. These were used to calculate the changes in overtopping rates and in the longshore drift of sediment in front of sea defences. The results indicate that there will be considerable increases in overtopping rates if present-day defences and beaches are unchanged by 2075. However, the inclusion of sea level rise predictions in design calculations should account for the majority of the predicted change in overtopping. The work also indicates that future changes in longshore transport are unlikely to be greater than current levels of uncertainty, and these should be considered in the normal course of sensitivity testing. Qualitative and quantitative differences in future changes in vulnerability were found between the five sites examined, as they had different tidal ranges, wave climates, surge levels and joint probabilities.

Reflection and transmission of tsunami waves by coastal structures

Abstract This paper presents a linear solution for the interaction of tsunamis with coastal defence structures. The solitary wave is considered a valid representation of the tsunami; using the Fourier analysis, the harmonic components of the solitary wave are obtained. These harmonic components are propagated through (a) a permeable breakwater of rectangular cross section, using the wave plane criteria of Dalrymple et al. [Reflection and transmission from porous structures under oblique wave attack. J Fluid Mech 1991;224:625–44] and (b) permeable and impermeable submerged breakwaters, using the mild slope equation of Losada et al. [Interaction of non-breaking directional random waves with submerged breakwaters Coastal Engng 1996;28:249–66]. The reflection and transmission coefficients are evaluated using the inverse Fourier transform. The theoretical results show a good agreement with experimental data, while the limitations of the model are its linear character and the fact that is does not take into consideration wave breaking.

Rock quality in coastal engineering

Abstract The majority of coastal and shoreline defence structures use large quantities of rock in their construction which will be subjected to environmental conditions quite different from rock used for other purposes. Such materials are required to conform to specific size, shape and grading criteria by the designers of coastal defence structures and rock properties such as density will also be required for the design equations. The severity of the marine environmental conditions will also necessitate adequate levels of strength, abrasion resistance and durability. A quality assurance scheme for marine structures must take account of all these requirements. Test evaluations must be carried out at an early stage in the design process when alternative materials are being compared and again at the quality control stage of extraction as well as during the construction. Tests can be divided into four groups: those concerned with the geometric properties such as size, shape and grading; those concerned with intact rock strength and resistance to abrasion; with the intrinsic properties of the rock; and those related to durability characteristics such as weathering degradation. Many of the tests used to evaluate these properties were originally designed for aggregate materials, but recently two tests have been developed specifically for rock to be used in coastal defence structures. These procedures attempt to test those characteristics of the rock materials most affected by the degradation processes found in the marine environment and to offer a method of predicting the in-service performance of the rock with time.

INLET CHANGES OF THE EASTFRISIAN ISLANDS

The seven sandy islands of the Eastfrisian group would appear to be initially formed and now continually supplied with sand from the Westfrisian group and the mainland to the west. The inlets between these islands are in dynamic equilibrium with the strong tidal currents of the near 2,5 m range in the area. Hydrographic information dating back to 1650 permits the development of a hypothetical model which explains the historic changes and might predict future trends. The installation of coastal defense structures on the eroding western extremities of some islands in the mid 19th century has greatly influenced the bars by which sand is transported from island to island in an easterly direction.