Exploratory modelling of barrier coast dynamics
Due to the COVID-19 crisis the PhD defence of Koen Reef will take place (partly) online.
The PhD defence can be followed by a live stream.
Koen Reef is a PhD student in the research group Marine and Fluvial Systems (MFS). His supervisor is prof.dr. S.J.M.H. Hulscher from the Faculty of Engineering Technology (ET).
Barrier coasts are a distinct type of coast characterized by a chain of barrier islands and tidal inlets separated from the mainland coast by a backbarrier basin or lagoon. They cover around 10% of the world’s coastline and they display a variety of shapes and sizes. Barrier coasts possess outstanding ecological values and are economically important for activities such as navigation, resources extraction, and tourism. Furthermore, they mitigate the effect of storms on the outer sea by shielding the mainland coast. Besides their functionality, barrier islands are prone to flooding, due to their low elevation and closeness to sea. This can have significant consequences, as in 2012 when Hurricane Sandy caused three breaches in the barrier islands off the coast of Long Island.
To maintain barrier coast functionalities and to curb the natural dynamics, barrier coasts are often subject to human interventions. More recent examples include the jettification of inlets to improve the inlet navigability, dredging of shipping channels, sand nourishments to counteract erosion, and the artificial opening and closing of tidal inlets. Notwithstanding the abundance of human interventions, the knowledge necessary to effectively apply long-term management of these systems is lacking, leading to less efficient and more expensive short-term management decisions. In this thesis we support the shift from short-term management to more cost-efficient long-term management by expanding our knowledge on the long-term evolution of barrier coasts and answer the following research questions:
1. What is the influence of back-barrier basin planview geometry, and changes therein, on multiple inlet systems?
2. What is the impact of storm-induced breaches on multiple inlet systems, and how is this affected by climate change?
3. How do variations in back-barrier basin geometry and storm surge characteristics affect storm surges in the Western Dutch Wadden Sea?
To answer the first research question, we extend an existing barrier coast model, to allow for the inclusion of arbitrary backbarrier basin geometries. Using this extended model, we studied the response of multiple inlet systems to variations in basin size and study the role of tidal resonance and bottom friction on this. We find that the total tidal prism through all inlets is predominantly determined by the (cross-shore) width of the basin and identify three regimes for this. Firstly, a linear regime for the shortest basins, followed by a resonant regime, and finally a regime in which the tidal wave dissipates due to bottom friction.
To answer the second research question, we again extend an existing barrier coast model to include the formation of storm-induced breaches. Using this model, we perform Monte Carlo simulations to find how multiple inlet systems respond to storm-induced breaches. We found that if a barrier coast is in equilibrium, a new open inlet can cause the existing inlets to shrink. Furthermore, we find that the distance between a storm-induced breach and the existing neighbouring inlets is the most important predictor for whether an inlet remains open or not. Climate change driven changes in storm climate will result in changes in the time-scale in which these changes happen.
To answer the third research question, we extend the historical model used to study the impact of a large closure dam in the Netherlands on the remaining barrier coast system. We found that the maximum surge height strongly depends on the length of the basin, as a longer basin results in a longer wind-fetch length.