Effective design of managed realignment schemes can reduce coastal flood risks

Abstract

Managed realignment (MR) constitutes a form of nature-based adaptation to coastal hazards, including sea level rise and storm surges. The implementation of MR aims at the (re)creation of intertidal habitats, such as saltmarshes, for mitigating flood and erosion risks and for creating more natural shorelines. However, some evidence suggests that the desired coastal protection function of MR schemes (in terms of high water level (HWL) attenuation) may be limited and it was hypothesized that this was due to the configuration of the remaining seawalls, which we refer to as scheme design. The effects of scheme design on within-site HWL attenuation are 2 analysed for six scheme designs that differ in terms of breach characteristics and water storage capacity. The scenarios are established by manipulating the digital elevation model of the site topography to vary the configuration of the old defence line and the breaches.Our results show that changes in scheme design, particularly storage area and number and width of breaches, had significant effects on the site´s HWL attenuation capacity. Decreasing the tidal prism by changing the number and size of breaches, with the site area kept constant, leads to increased modelled HWL attenuation rates. However, average HWL attenuation rates of > 10 cm km -1 are only achieved when site size increases. The mean high water depth of each scenario, calculated by dividing tidal prism by MR area, explains most of the variation in average HWL attenuation between all scenarios. Attention to potential within-site hydrodynamics at the design stage will aid the construction of more effective MR schemes with respect to coastal protection in the future.

Can Managed Realignment Buffer Extreme Surges? The Relationship Between Marsh Width, Vegetation Cover and Surge Attenuation

“…In a previous study, the model was calibrated and validated against field measurements of the highest spring tides of the year 2017. Root mean square errors between modelled and measured water depths ranged between 0.05 and 0.06 m and mean absolute errors of peak water levels between 0.02 and 0.04 m. Model calibration included a range of Manning's n coefficients for saltmarsh surfaces (0.035-0.09) and revealed that the model was not very sensitive to these variations (Kiesel et al 2020). We note that the effects of bottom friction are also dependent on water depth (Delft Hydraulics 2003;Loder et al 2009;Wamsley et al 2010;Temmerman et al 2012), and the calibration of the model is representative of spring tide conditions.…”

“…Results from studies based on hydrodynamic modelling suggest that the coastal protection function of MR schemes can be improved by applying a scheme design (i.e. configuration of breaches and dikes) that aims at keeping internal water depths low, but attenuation rates > 10 cm km −1 are only achieved by establishing vast and wide shallow water areas (Kiesel et al 2020). Problematically, MR sites are typically smaller than Freiston Shore (66 ha).…”

“…We additionally study how the largest MR width alters the relationship between extreme water level, vegetation scenarios and HWL attenuation. For this purpose, we use a hydrodynamic model of the Freiston Shore MR site, setup in Delft3D-Flow (Delft Hydraulics 2003), which has been calibrated and validated against field measurements of the highest spring tides for the year 2017 (Kiesel et al 2020).…”

Shared-use of flood defences

“…Finally, systems where the first dike is already breached before a storm are classified as a type V system. While the first dike can no longer prevent flooding by itself, it can still aid in reducing water levels at the second dike, as was studied for the Freiston Shore Managed Realigment site (UK) (Kiesel et al, 2020).…”

“…Water is not retained by the first dike and only wave and water level attenuation effects can be expected in these systems. Rather than an erosionovertopping model used in this study, a hydrodynamic wave and flow model is needed to assess the loads propagating towards the second dike in these systems (Kiesel et al, 2020). These examples show that the selection of models needed in the transmission step of the framework greatly depends on the type of system and its context.…”