Nasrin Hassanpour

Coastal regions have always been alluring places for settlements due to their proximity to the ocean, abundant natural resources, and the high quality of life they provide. Nevertheless, these areas face various vulnerabilities, including the effects of climate change like rising sea levels, storm surges, and more frequent extreme weather events. Rubble-mound breakwaters are commonly used as protective structures along the coast, but recent years have brought significant challenges due to climate change and growing environmental concerns. Coastal communities and managers are now seeking solutions that are more sustainable and have fewer impacts on the landscape and ecology. These demands are reshaping the conventional approach to designing these structures. To address these challenges, it is necessary to adapt breakwaters to meet present, future, social, and environmental expectations.

Coastal erosion, extreme climate events, and the loss of biodiversity are important consequences of climate change that directly impact our society. The needs to develop effective engineering practices using nature as model and mentor are now emerging aimed to develop high-performance coastal infrastructures integrating and sustaining local marine ecosystems. In this scenario, the present article describes the concept development and preliminary experimentation of simplified bioinspired models to evaluate their hydraulic performances. This study is part of a future vision to develop submerged barriers, printed with eco-friendly materials, characterized by high hydraulic performances and cable of supporting local biodiversity. Following a top-down approach, the functional features of key organisms were abstracted and transferred to create three bioinspired models. The hydraulic performance of these models is analyzed in terms of wave transmission, reflection, and dissipation under various wave conditions. Under certain circumstances, the bioinspired barriers demonstrate wave attenuation comparable to traditional submerged breakwaters. A method based on skin friction and drag-related processes provides a simple heuristic explanation of how the shape of the models affect the transmission coefficient. The results achieved offer valuable insights for the design of future coastal defense systems that are inspired by, and integrated with, natural ecosystems.

Coastal zones have consistently been among the most appealing settlement areas due to their proximity to the sea, rich natural resources, and the high quality of life they offer (Lamberti et al., 2005). However, these regions are affected by climate change impacts, such as sea-level rise, storm surges, and an increased intensity of extreme weather events (Burcharth et al., 2014). Traditional low-crested rubble mound breakwaters are commonly used to protect coastal areas from wave damage. However, it is expected that the variability of climate conditions will induce a loss of functionality and structural integrity in the coming decades. Coastal communities and coastal managers, within the framework of sustainable development, are demanding new approaches that include not only the preservation of the hydraulic performance of the breakwaters, but also other factors, including social and environmental impacts. These requests significantly affect the traditional way to conceive those structures to be integrated into the coastal landscape. To cope with the variation of climate drivers, existing low-crested breakwaters must be adapted to accommodate social demands and environmental issues. Therefore, upgrading and maintaining the existing rubble-mound breakwater is a hot topic in coastal engineering, and deserves special attention due to the possible intensification of external loads resulting from the impacts of climate change (Stagnitti et al., 2023). Upgrading can be done by modifying the structure profile and/or adding structure elements (Burcharth et al., 2014). In 2011, Cappietti provided curves for the functional design of submerged breakwaters to be used in place of preexisting emergent breakwaters. Burcharth et al. (2014) explained that the best way to improve the structure is to put an additional layer of protection on the front slope, as long as the foreshore has a mild slope of around 1:100. But if the foreshore becomes steeper because of erosion, then a front berm will also be needed. Stagnitti et al. (2022) introduced a novel methodology based on the calculation of the failure probability during a lifetime due to independent failure modes. Their method was employed to evaluate the performance of upgraded breakwaters in response to climate change. Estimating the wave overtopping of both existing and upgraded breakwaters is essential for designing upgrade options that can ensure the safety of port operations. Stagnitti et al. (2023) applied the numerical model IH2VOF, which was calibrated using experimental data, to study the wave overtopping of damaged and upgraded rubble-mound breakwaters. The present work examines the feasibility of converting emerged rubble-mound breakwaters into submerged breakwaters to minimize their detrimental environmental effects. To achieve this goal, an investigation was conducted to evaluate the hydraulic performance of submerged breakwaters that are created by lowering the crest of existing emerged breakwaters. Additionally, two-dimensional numerical simulations were performed using IH2VOF to investigate wave interactions with the structures.

Low-crested and submerged breakwaters are frequently employed as coastal defence structures. Their efficiency is governed by wave energy dissipation, and the wave transmission coefficient can evaluate this parameter. The current study conducts experimental investigations on both low-crested and submerged breakwaters exposed to different wave conditions to compare their performance with that of emerged breakwaters. The current study provides a comprehensive review of existing formulae and highlights the impact of design variables. To evaluate the reliability of each existing formula, four “reference” configurations are used. Having these structures at the same overall volume, the results also provide a useful tool for engineers involved in the lowering operation of existing breakwaters. Nature and magnitude of governing parameters are investigated, and some points of criticism are outlined. The comparison results show that few of the existing equations give reliable estimates of the transmission coefficient for all the models tested in this study. Higher values of root mean square error are related to the emerged breakwater rather than the submerged ones. To obtain information about the transmitted wave energy, spectral analysis is applied as well. Different behaviours of the transmitted spectrum, n terms of shape and peak frequency, are highlighted. The results improve the overall knowledge on formulae that are in the literature, in order to make the user more aware.

The hydraulic behavior of a hydraulic jump can be challenging to estimate in order to design gradually expanding stilling basins with roughness elements on the bed. In this study, five dependent variables were identified that comprise: (i) the sequent depth ratio; (ii) the relative length of the jump; (iii) the relative roller length of the jump; (iv) the relative energy dissipation; and (v) the water surface profile. This study undertook a set of formulations based on the regression analysis and Sugeno Fuzzy Logic (SFL) to predict these variables based upon experimental data. Results demonstrate that the trained SFLs predicted the behavior of the dependent variables with the Nash–Sutcliffe Coefficient (NSC) greater than 0.96 in the testing phase. In contrast, the NSC values for the regression models are greater than 0.79. The higher accuracy of SFL is attributed to its capability in managing uncertainty and imprecise data owing to water surface profile oscillations of the hydraulic jump. Also, the results indicate that the prediction residuals for SFL are homoscedastic for all hydraulic parameters investigated except for the water surface profile, the prediction residuals of which for the regression equations are heteroscedastic.

The paper presents the results of an experimental study carried out to investigate the effect of geometric and hydraulic parameters on energy dissipation and location of the hydraulic jump, with a change in the height of roughness elements and the divergence of walls in different discharges. Experiments were conducted in a horizontal rectangular basin with gradual expansion 0.5 m wide and 10 m long. Four physical models were fixed in the flume. The measured characteristics of the hydraulic jump with different divergences ratio (B = b₁/b₂ = 0.4, 0.6, 0.8, 1) and the inflow Froude numbers (6 < Fr₁ < 12) were compared with each other and with the corresponding values measured for the classical hydraulic jump. The results showed that the tailwater depth required to form a hydraulic jump and also the roller length of the hydraulic jump and the length of the hydraulic jump on a gradual expansion basin with the rough bed were appreciably smaller than that of the corresponding hydraulic jumps in a rectangular basin with smooth and rough bed. With the experimental data, empirical formulae were developed to express the hydraulic jump characteristics relating to roughness elements height and divergence ratio of wall. Also, the applicability of some empirical relationships for estimating the roller length was tested.