Traditional rubble mound breakwaters are increasingly designed with lower crest heights, utilizing a berm to dissipate wave energy. Single-layer interlocking concrete units, such as Xblocs, are frequently applied to steep slopes. However, existing empirical models for wave overtopping are primarily calibrated for traditional rock armour on milder slopes. The specific overtopping performance, concerning both mean overtopping discharge (q) and maximum individual wave volumes (V_max), of steep-sloped, Xbloc-armoured breakwaters with a berm remains largely unknown.

To address this knowledge gap, 2D physical model tests were conducted on steep-sloped (cot(α) = 1.33) Xbloc-armoured breakwaters. The experimental program investigated the influence of berm width, berm elevation, and wave steepness under constant relative freeboards (R_c/H_m0 = 0.67 - 1.68). Two berm widths were tested, yielding dimensionless relative widths of B/H_m0 = 1.05 - 4.20 and B/L_m-1,0 = 0.06-0.12. Berm elevations were positioned at the Still Water Level (SWL) or emerged, resulting in relative levels of B_L/H_m0 = 0.40 - 1.69 (relative to crest height) and h_b/H_m0 = 0 - 0.67 (relative to water level). These configurations were subjected to low and high wave steepnesses (s_m-1,0 = 0.025 - 0.028 and 0.038 - 0.046). Furthermore, a reference case without a berm was tested to establish a baseline.

The results demonstrate that berm elevation is the dominant structural factor in reducing overtopping discharge. Berms positioned at the Still Water Level (SWL) proved inefficient at dissipating wave energy, particularly for low-steepness waves. Under these conditions, the relative wave run-up (R_u2%/H_m0) reaches its maximum due to the the very steep slope angle; since the SWL berms fail to disrupt this run-up, adding a small or wide berm yields no significant overtopping reduction compared to a straight slope. Conversely, emerged berms significantly reduced overtopping discharges compared to SWL.

A comparison with existing literature revealed that standard models cannot be directly applied to predict overtopping discharges for this specific steep-sloped, Xbloc-armoured configuration. For the mean overtopping discharge, an optimal constant friction factor of γf= 0.40 was established within the baseline framework of Van Gent et al. (2022) Additionally, the berm influence factor (γ_b) was recalibrated. In this new formulation, the berm level is incorporated using its elevation above the water level scaled by the wave height (h_b/H_m0), instead of its height below the crest scaled by the armour crest height (B_L/A_c).

Regarding individual overtopping volumes, the standard Weibull distribution already provided an accurate baseline prediction. However, it inherently lacked the capability to differentiate between geometric configurations. Specifically, for test series containing approximately 1000 waves, it systematically overestimated V_max for bermed profiles by failing to account for the berm's dampening effect, and underestimated V_max for the steep straight slope (cot(α) = 1.33) by failing to capture severe surging. To resolve this, a new empirical expression for the Weibull shape parameter (b) was formulated, which integrates the surf-similarity parameter (ξ_m-1,0) alongside the relative berm dimensions (B/L_m-1,0 and h_b/H_m0). While this recalibrated formula successfully reduces the prediction error and captures the underlying physics more accurately, the absolute gain is relatively modest. Consequently, despite this theoretical improvement, the added complexity may not be necessary for design practices.
... Read more

The completion of a fourth sea dike (Binh Minh 4) in Kim Đông, Ninh Bính, in Vietnam will reshape local hydrodynamics and will result in land reclamation. Thus, the balance between mangrove forest health, flood protection, local livelihoods and development goals will be changed. This report addresses the impacts and fills knowledge gaps regarding the completion of this fourth dike on hydrodynamics in Kim Đông, and examines hydrodynamic, ecological, governance, and social factors in relation to the mangrove ecosystems in the region.

To achieve this, a governance analysis and system analysis are combined with a hydraulic analysis and a conceptual hydrodynamic model in Delft3D. This approach is supported by literature and institutional reviews, expert interviews, a community survey, and hydraulic data, including satellite-derived wave and wind records, as well as field measurements. Together, these elements shape the model set-up, boundary conditions and interpretation. The modelling framework compares a baseline situation without a dike with two with-dike scenarios, simulating both storm and regular weather conditions. Physical changes in the model are linked to drivers of ecological change, allowing an assessment of their impact on mangrove forests and socio-economic conditions in the area.

Findings from the conceptual Delft3D model indicate that the completion of a fourth dike is likely to reduce tidal connectivity and sediment supply to the enclosed area, leading to longer inundation times and lower water levels. These change in conditions drive mangrove squeeze, reducing habitat, shelter and carbon storage potential. At the same time, land reclamation creates development opportunities for aquaculture. Interventions to mitigate the effects of the fourth dike are presented using a COCD box, and evaluated via a multi-criteria analysis. Suitable interventions are further elaborated on their trade-offs and implementation process.

Decision-making in Vietnam, and in Kim Ðông especially, is highly hierarchical, which limits involvement of local authorities and residents even though they are most affected. Conflicting priorities among sustainability, economic growth and climate adaptation further complicate this involvement in decision making. Additionally, limited data availability and experience with nature-based solutions also constrain adaptive management and long-term monitoring. With this in mind, four potential interventions are assessed: adaptive gate management, mangrove co-management with incentives, sustainable aquaculture \\certification, and a digital monitoring platform. No single measure resolves all challenges. However, co-management with incentives and sustainable aquaculture emerge as promising options that couple ecological restoration with socio-economic benefits, supported by adaptive gate operations and a digital platform to improve coordination and transparency. ... Read more