Unravelling Drivers of Morphological Change

Abstract

Flood defense will have to be strengthened all over the world as sea level rise poses great challenges for the safety of coastal settlements. Strengthening traditional hard flood defenses is becoming increasingly more difficult due to limited space in the coastal zone and large costs inflicted with renovation. Adaptive ‘soft solutions’ pose an alternative as they make use of natural processes that are able to adapt to changing boundary conditions. The Prins Hendrik Sand Dike is a pilot project of such a soft solution placed in front of an existing hard sea dike. The Prins Hendrik Sand Dike is the subject of this research.
This research strives to identify the role of waves, currents, and water levels on morphological development of the Prins Hendrik Sand Dike. Which requires an identification of the morphological development during the 3.5 years of service lifetime. This is followed by a characterization of the forcing climate based on measured on-site data. This knowledge is used to hindcast the full forcing climate during the PHSD lifetime and calculate morphological development. Lastly, a comparison of calculated and observed morphological development reveals suitability of engineering formulae on low-energy beaches.
Results indicate the spit head develops at a faster rate than predicted in model studies. In addition, constant erosion in the form of coastline retreat is observed in a section that serves as primary sea defense. These findings hamper successful development of habitat and possibly decrease the longevity of the sea defense if maintenance is not performed. Furthermore, waves have been determined to be the dominant forcing mechanism resulting in sediment transport and morphological development. In addition, systematic water level changes that occur under specific wave conditions are identified causing increased Northeastward transport of sediment. These findings likely explain discrepancies between the previously modelled and observed morphological development.
Implications of this research contribute to an understanding of the interconnected nature of forcing scenarios at low-energy beaches and can be used to improve modelling efforts of low-energy beaches. Furthermore, it provides a basis for Hoogheemraadschap Noorderkwartier to make decisions on maintenance activities.