Modeling the effects of hard structures on dune erosion and overwash

Abstract

Many of the most densely populated areas are located near the coast. Climate change and population growth put more and more pressure on these coastal areas. As free space is becoming sparse, coastal disaster risk reduction plans need to be spatially efficient. In this thesis the sandy coast with hard structures, such as buildings or dune revetments, is addressed. These structures can either provide additional protection or enhance erosion. Field measurements and experimental data featuring these phenomena are scarce, but the measurements of the devastating impact of Hurricane Sandy (October 2012) on the New Jersey shore provide new model validation possibilities. Hard structures in the barrier have three effects: 1) The main effect of a structure is the impact on the sand balance (both cross-shore and longshore), by cutting of (part) of the supply of sediment (WL | Delft Hydraulics, 1987). 2) In cross-shore direction a structure may result in the development of scour at the toe as a result of higher energetic conditions at the toe. However, in the post-Sandy bathymetry at the (buried) seawall at Bay Head, NJ, no scour holes were found. XBeach (Roelvink et al., 2009) simulations have reproduced these profiles and suggests this is the result of infilling of scour after the peak of Sandy. 3) In longshore direction a hard element will result in the extra erosion at the sides of the structure as a result of exchange of sediment and locally higher short waves. XBeach simulations have shown that the presence of a condo at Camp Osborne, NJ, during Sandy resulted in 32% additional erosion in adjacent locations However, these effects are no reason to state on forehand that multifunctional use of the barrier is not possible. Its applicability needs to be addressed case-by-case. Process-based models, like XBeach, can accurately reproduce the effects noticed in the field. Calculation rules, like Deltares and Arcadis (2013), do not reflect the true complexity, but can give a rough first indication of the longshore effect.