Hydraulic and environmental design aspects of hard coastal structures - The example of stepped revetments

Abstract

To continue safeguarding coastal communities and infrastructure in the face of climate change requires a range of coastal protection measures. Simultaneously, the greater awareness for sustainable development and the sense of urgency to preserve and restore the coastal environment, underline the importance of designing coastal protection measures sustainably. Hard coastal structures are effective coastal protection measures against wave overtopping and flooding, as they form physical barriers against waves. However, the effects of hard structures on hydrodynamics, sediment dynamics and habitats, bring unintended negative changes to the environment. The first part of this dissertation critically discusses the use of hard coastal structures and their environmental impacts. To minimise the environmental impacts of hard coastal structures or create new ecosystem services, environmental aspects should be incorporated in standard coastal engineering practice from the earliest design stages. Based on examples and recommendations in literature, this dissertation provides guidance on environmental aspects to be considered in the design of hard coastal structures to increase their sustainability, i.e. consider future environmental, social and economic needs. The second part of this dissertation develops recommendations for the design of sustainable hard coastal structures, with the example of stepped revetments. Stepped revetments reduce wave overtopping effectively in comparison to smooth dikes, as their steps dissipate energy as waves interact with the structure. In addition to their primary function of ensuring coastal safety, stepped revetments offer opportunities for ecological enhancement and social benefits. The multi-functionality of stepped revetments make these structures especially suitable in urban and touristic settings. This dissertation improves design recommendations for stepped revetments by identifying environmental design aspects and improving design formulae for their hydraulic responses (wave reflection, wave run-up and wave overtopping). Environmental design aspects are identified based on literature and include suggested nature-based adaptations to stepped revetment designs. For instance, the vertical and horizontal step surfaces of stepped revetments provide areas here roughness and surface complexity can be maximised to increase habitat variety and promote biodiversity. Additionally, their steps could be adapted to mimic habitats for intertidal organisms, e.g. by altering revetment steps to incorporate water retaining features like rock pools. The feasibility and success of these nature-based adaptations are highly dependent on the local environmental conditions, including hydrodynamics. With knowledge of environmental design aspects, coastal engineers gain a greater interdisciplinary understanding, thereby facilitating sustainable designs. Hydraulic responses of stepped revetments are studied and analysed to improve and expand design recommendations for wave overtopping, wave run-up and wave reflection. In full-scale wave flume experiments, two stepped revetment cross-sections, each with a slope of 1:3, were studied. The first cross-section had uniformstep heights of 0.50 m, which was selected to add the secondary function of providing seating, i.e. serve as a bench. For the second cross-section, uniform step heights of 0.17 m were selected, as a typical height for walking up a staircase. Wave heights (Hm0) up to 1 m and wave periods (Tm−1,0) up to 6.5 s were generated. Based on the measurements of the physical model tests, empirical formulae were developed for estimating wave overtopping, wave run-up and wave reflection. The tested stepped revetments effectively reduced wave overtopping in comparison to smooth dikes, resulting in influence factors for roughness (γf ) between 0.43 and 0.73. Compared to smooth dikes, the energy dissipation of the revetment stairs reduces wave reflection and wave run-up. Within the tested range of boundary conditions, the stepped revetment with large steps (Sh=0.50 m) proved more effective in dissipating energy and reducing wave overtopping (0.43 ≤ γf ≤ 0.54). The higher effectiveness of the large steps is also confirmed with the measured wave reflection. Wave conditions were repeated for the large (Sh = 0.50m) and small steps (Sh = 0.17m) showing that reflection coefficients were 55 % higher at the small steps. Individual overtopping volumes at the tested stepped revetments are described by a two-parameter Weibull distribution, revealing a higher median shape factor (b=1.63) for stepped revetments compared to breakwaters, smooth dikes or vertical walls. The wave flume tests provide greater insight in the functioning of stepped revetments and enable the quantification of the hydraulic responses of stepped revetments. The experimental work presented in this dissertation provides one of the first investigations into the hydraulic responses of stepped revetments at full scale. Compared to small-scale wave run-up and overtopping measurements, this study reveals that hydraulic responses measured in small scale are likely affected by scale effects. Small-scale studies overestimate the wave overtopping reduction (γf ) by 2-31 % and underestimate relative wave run-up heights (Ru2%/Hm0) by 31-51 %. As a result, basing the designs of stepped revetments on small-scale measurements could therefore lead to unsafe designs. The gained knowledge on environmental aspects and hydraulic responses (wave reflection, run-up and overtopping) improves design recommendations for stepped revetments with regard to coastal safety and sustainability. In terms of coastal safety, the presented full-scale model tests provide reliable design recommendations that are not affected by scale. In terms of sustainability, the dissertation provides a review of environmental design aspects of coastal structures in general, and stepped revetments in particular. Hence, this dissertation contributes to recommendations for designing sustainable coastal structures.