Evolution of beach extensions: Numerical modelling of large scale nourishments using UNIBEST & Delft3D

Recently, nourishments have been carried out on a large scale to counteract the on-going erosion along the Dutch Coast. The main advantage of these large beach extensions is that large stretches of coastline are protected for a long time scale (e.g. 20 years), which decreases the frequency of nourishing significantly. This is not only cost effective, but also positive for preserving local ecology. In this research two types of large scale nourishments can be distinguished: The ‘permanent type’ which is applied locally and has the form of a land reclamation which needs to be maintained in time (Hondsbossche Sea Defence). The other one being the ‘temporary type’, which is expected to diffuse along the coast in order to strengthen a larger stretch of coastline (Sand Motor). One of the most challenging issues in the design of large scale nourishments is estimating the erosion rates in time and consequently the lifespan of such nourishments. This is relevant because it can lead to a more efficient design or provides more control over maintenance. The problem definition reads: “Currently it is not known how the erosion rates of large scale nourishments are related to their size, shape and sediment characteristics”. The final research goal is to develop design graphs for the erosion rates and lifespans of beach extensions at the Dutch coast. For this research use has been made of two numerical models; the equilibrium based UNIBEST model and the process based Delft3D model. Various nourishments are implemented in both models in which variations are made in the seaward extent (=width of nourishment), L/W ratio and the net annual alongshore transport (indicates the wave climate’ intensity in this research). Both models are validated by using measurements at the Sand Motor. The main conclusion of this research is that the wave-induced alongshore transport is considered the most important driving force for the diffusion of nourishments. Tidal forcing does also play a role but the effect on the alongshore transport is a factor 10 less compared to the sediment transports for waves and tide combined. This conclusion is reinforced by the large resemblance between the model results of Delft3D (wave + tidal forcing) and UNIBEST (tide only). From using UNIBEST in combination with a time series wave climate (2 years) applied at the Sand Motor, it can be concluded that the sediment loss at large scale nourishments is event driven (i.e. storm events) just as can be observed in reality. The time series wave climate provides a near perfect fit of model results on measurement with respect to volume decrease in time. This research shows that the dynamic boundary within UNIBEST has a very large effect on the alongshore sediment transports. The dynamic boundary defines which part of the coast rotates in the same way as the coastline and can therefore have a significant effect of refraction. By using the Delft3D offshore wave climate while keeping the dynamic boundary close to shore, similar results can be obtained with UNIBEST as with Delft3D. Because of the presence of a dynamic boundary UNIBEST can be considered a more advanced coastline model. Furthermore, it can be concluded that when the alongshore length is increased, the erosion rates in the first years remain approximately the same. Extending a nourishment in alongshore direction simply protects an additional stretch of coast equal to the length of the additional nourishment length itself. The amount of seaward extent appears to have much more influence. The erosion rates rapidly increase when the nourishment is extended further into sea. With the UNIBEST results the half-life (amount of time it takes for the nourishment to reduce to 50% of its initial volume) is compared to the initial volume of each nourishment. The relation seems to be very linear in which each L/W ratio shows a different slope.