The influence of the Sand Engine on the sediment transports and budgets of the Delfland coast

Abstract

The Sand Engine is a new innovation in coastal protection, a mega feeder nourishment. This pilot project was constructed in 2011 along the Delfland coast, which is historically prone to erosion. Since its construction, the Sand Engine is intensively being monitored to track the morphological development.

The objective of this thesis is therefore to assess how the morphology of the Sand Engine is evolving over time and how this evolution contributes to the sediment budgets of the Delfland coast. In this thesis the first five years of morphological evolution of Sand Engine and its surroundings are investigated.

A data analysis is performed on bi-monthly bathymetry measurements, covering the first five years of morphodynamic evolution of the Sand Engine and the surrounding coastal cell. The high temporal and spatial resolution in both alongshore and cross-shore direction, provided the opportunity to perform research in great detail and precision.

Volume changes and sediment transports are successfully derived. Sediments are redistributed in both alongshore and cross-shore direction over the coastal cell. The accretive areas slowly extend alongshore while the erosive part of the Sand Engine remains in the same position. The Sand Engine has a positive net contribution to the sediment budget of 5.8km of coast after 5 years, which is an extension of 3.2km. 4.2Mm^3 of the initially nourished volume of 17.5Mm^3, has been redistributed after 5 years.

A Gaussian curve is fitted to several iso-lines of constant altitude to describe the plan-form adjustment of the Sand Engine at different altitudes. The Gaussian parameters are useful in accurately describing the spatio-temporal behaviour. Yet large differences in adjustment rates are found over the altitude. The decrease in cross-shore extent is fastest around 0m+NAP, and decreases to near zero at -8m+NAP.

Cross-shore profile behaviour is investigated by inspection of the temporal evolution of characteristic cross-sections in accretive and erosive locations along the Sand Engine. Erosive and accretive profiles behave considerably different. The profiles are adjusting rapidly and bed-level activity varies considerably over the altitude, with a maximum morphological activity in the intertidal zone, rapidly decreasing with increasing depth.

The results show that the Sand Engine spreads alongshore and feeds the adjacent coastal sections in the five years after construction. A mega feeder nourishment is therefore capable of supplying sediments to the adjacent coast.
The first year morphodynamic response was much stronger than in any subsequent year. The current results therefore give an improved view on the long-term development of the Sand Engine.