Macro-scale performance of a mega-feeder nourishment

Abstract

A mega-feeder nourishment is a concentrated nourishment that disperses over time due to wind, waves, and tide, and as a result ‘feeds’ its adjacent coasts. During the design phase of a nourishment, performance indicators can enable an objective comparison of a mega-feeder nourishment to alternative nourishment strategies, In the case of an existing mega-feeder nourishment, predictions of the performance enable an objective assessment of the need for additional nourishments in the proximity of a mega-feeder nourishment.
Today, engineers and policy makers rely on expert judgement when assessing alternative nourishment strategies, or the need for additional nourishments. Currently no tool exists that enables quantification of the performance of a mega-feeder nourishment. Existing models that describe and predict developments of nearshore coastal areas have been applied before to assess the developments of a mega-feeder nourishment, but have not been validated specifically to describe the performance in terms of alongshore feeding. The research question is therefore: "How to describe and predict macro-scale mega-feeder nourishment performance with behaviour-based coastal indicators?"
A tool is proposed that enables the quantification of the performance of a mega-feeder nourishment, as well as the validation of models that describe and predict the performance of a mega-feeder nourishment, The tool is specifically aimed at quantifying the performance in terms of the alongshore feeding. The tool comprises of indicators that representatively quantify the contribution of sediment, the region of feeding, and the time it does so.
The tool is applied to the Sand Engine, used as a case study to examine the applicability. This Sand Engine, constructed during the spring of 2011, has a nourishment volume of 21.5Mm3, initially spanned a width of 2.4km and extended 1km into the sea. Application of the indicators to the Sand Engine confirmed the performance developed with continuous trends, suggesting the performance can be predicted.
From the validation of the model approaches, it is concluded that the analytical model can be used to predict the initial performance of a mega-feeder nourishment, if the longshore spreading is derived from the LST equation by Kamphuis. Second, if the bathymetry is regularly monitored (e.g. semi-annually), the input of the analytical model can be calibrated, enabling long-term predictions of the performance. If more detailed descriptions of the developments of a mega-feeder nourishment are needed, a bruteforce process-based numerical modelling approach can serve as a method to describe and predict the feeding performance of a mega-feeder nourishment.
The performance of the Sand Engine is predicted with the validated models up until 2030. The volumetric growth will stagnate on the south, while on the north it is expected to grow up to 4Mm3. The width will increase to approximately 7km. From this, it is concluded that it is unlikely the Sand Engine will meet its original objective of contributing to coastal safety along the Westland coast (17.2km). Further, the half-life of the Sand Engine will be reached near 2030, indicating the actual lifetime is significantly longer than the design lifetime (20 years).