Tosca Kettler1*, Matthieu de Schipper1, Arjen Luijendijk1,2, Stefan Aarninkhof1
1 TU Delft; 2 Deltares
Projections of high rates of sea level rise have stimulated proposals for adaptation strategies with increasingly high nourishment volumes. A widely accepted perception is that coastal profiles respond to nourishment by rapid equilibration to a (new) equilibrium shape including the added sand volume. The validity of this viewpoint depends on the rate and extent of sediment redistribution over the nourished site. The profile shape may undergo significant deformation when high volumes of nourishment are applied. Analysis of the impact of nourishment scenarios on decadal cross-shore dynamics requires a level of detail that can typically not be obtained from (semi-)empirical models, while process-based models are too complex for robust decadal analysis. Therefore, we introduce the numerical, diffusion-type model CROCODILE (CROss-shore COastal DIffusion-type Long-term Evolution model), which combines inductive assumptions on dynamic profile response and current state-of-the-art knowledge of nourishment behavior in a predictive tool.
In this work, CROCODILE is introduced and applied to case study locations that vary in morphological setting and nourishment history. The model computes an 'instantaneous' profile response with a time- dependent profile evolution approaching a 'dynamic' equilibrium profile. Changes in the coastal system (e.g. SLR, erosion, or implementation of nourishment(s)) lead to horizontal and vertical translation of the dynamic equilibrium profile as given by a sediment volume balance. The nourishment shape is added to the 'instantaneous' profile, and the time-dependent evolution is calculated following a diffusion-type approach inspired by Stive et al. (1991). Hereby, the rate and extent of sediment dispersion are calculated as the sum of four components that depend on the scale of the nourishment relative to the static profile; cross-shore diffusion (fig 1B), background erosion (fig 1C), nourishment lateral loss (fig 1E), and aeolian loss (fig 1F). The simulated cross-shore profile shape is then translated to different coastal state indicators (e.g. profile volume (fig 1A,D), coastline position, and beach width).
Hindcast results show that CROCODILE is able to reproduce the time-varying response of different coastal state indicators to the implementation of nourishments. This approach can provide a more comprehensive understanding of the impacts of shore nourishment scenarios in different coastal settings, with special attention to sea level rise.
Figure 1 – (A,D) CROCODILE hindcast and observations of volume ΔV at Monster. Panel D is an enlargement of panel A. (B,C,E,F) The instantaneous profile of the first nourishment (black), the direction and magnitude of the red arrows indicate its time-dependent evolution.
Stive, Nicholls & De Vriend (1991): Sea-level rise and shore nourishment: a discussion, Coastal Engineering, vol. 16, pp. 147–163.