Managing Dyke Retreat: Importance of Channel Network Evolution and Mainland Slope on Storm Surge Dissipation Over Salt Marshes

Abstract:

A type of hybrid coastal defense consisting of a salt marsh ecosystem at the seaward side and dykes on the landward side has been globally implemented to improve coastal resilience. In this hybrid approach, vegetated marshes are valuable ecosystems that help dissipate storm surges and have an intrinsic ability to keep up with sea-level rise by natural deposition of mineral and organic sediments. Dykes instead, if not overtopped, prevent the surge entirely from penetrating inland. However, it has been recently recognized that dykes can exert a blockage effect, which consists of a water setup against these structures that can lead to their overtopping. For this reason, dyke retreat is now considered as a valuable option that allows creating new marshland in front of the relocated dyke thus avoiding blockage and maximizing storm surge dissipation. Most studies investigating the effect of dyke retreat on storm dissipation do not incorporate the morphodynamic expansion of the tidal channel networks in the previously reclaimed land, which is modeled as an unchanneled vegetated marsh. In this work, we implement the morphodynamic model Delft3D together with a marsh evolution module to study the formation of tidal networks after dyke retreat and their effect on storm dissipation. We consider different combinations of vegetation density and height, storm surge height and duration, sea-level rise, dyke location, and mainland slope. Our model results suggest that new channel networks can develop on a time scale of 10-25 years. Results also indicate that storm surge dissipation rates are overestimated if the morphodynamic evolution of the tidal network after dyke relocation is neglected, since the newly formed creeks favor the landward propagation of the surge. The maximum landward extension of the channel network is dictated by the mainland slope. Minor dissipation is obtained for flat mainland regions in which the network can expand, while maximum dissipation is given by steeper slopes.