Global Assessment on the Lifetime of Mega Nourishments

Recently an innovative nourishment method has been applied as a counter measure for coastal recession in the Netherlands. This soft engineering solution, also known as the Sand Engine, consists of a shoreline perturbation of $21.5$ Mm\textsuperscript{3} sand which will be redistributed along the shore by the natural forces of waves, tides and winds over a stretch of $10$ km to prevent coastal retreat. It is expected that this multifunctional 'building with nature' solution has several economical, ecological and recreational advantages compared to the traditional nourishment approaches. The mega nourishment intervention may therefore become one of the common nourishment methods to combat beach erosion. The international potential is assessed by determining the relative influence of wave climate parameters and nourishment dimensions on the lifetime of a mega nourishment for a straight alongshore uniform sandy shore. The Mega Nourishment tool has been developed to obtain a first estimate of the lifetime for a geographical location in the world by computing the nourishment redistribution and the cross shore erosion of the shore using wave climates from two global wave models. The redistribution of the nourishment is caused by gradients in the \gls{LST} and is simulated with the Unibest-CL+ coastline model. For the coastal erosion two process are taken into account: The erosion due to a rising sea level and the landward directed sediment loss during a storm simulated with a 1D Xbeach model. A comparison has been made between the tool results and an other model simulation of the Sand Engine. All simulations estimated different redistribution rates for the Sand Engine depending on the wave climate used. The lifetime predictions for the simulations were however similar. The Sand Engine is estimated to be sufficient to protect 10 km of coast for a period of about 50 years. The tool is furthermore used to simulate many cases with different wave climates, nourishment dimensions, cross shore profile shapes and sediment sizes. Using these results several conclusions are drawn. The redistribution rate is predominantly determined by the wave energy. A larger directional spreading and wave steepness also slightly increases the redistribution rate. The spreading of the nourishment is found to be less sensitive to different shapes of the cross shore profile and grain sizes larger than 250 $\mu$m. For smaller grain sizes the redistribution however rapidly increases. The erosion rate calculated by the tool can only be used as an indicative estimate. Based on the model results several empirical relations and a design graph are derived to estimate the redistribution and the lifetime of a mega nourishment. These equations can be used for an initial design of a mega nourishment. To obtain an accurate lifetime prediction it is recommended to pay sufficient attention to the quality of the wave data and the estimation of the shoreline erosion.

Subject

Mega nourishment
Sand Engine
Lifetime

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Student theses

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