The influence of a tidal inlet system on a nearby mega-feeder nourishment

Abstract

All over the world coastal communities are at risk due to sea-level rise and intensifying weather conditions. Many sandy beaches are eroding as a result of human-induced factors. Currently, the preferred coastal protection measure in the United States are beach nourishments. In Europe, there also is a general shift from hard to soft coastal protection measures. However, beach nourishments are not a long-term solution. Recently, in the Netherlands, a new concept called a large-scale (mega-feeder) nourishment has been introduced (the Sand Engine). Numerous studies on this new concept have been conducted. However, not for a mega-feeder nourishment nearby a tidal inlet system. About 10\% of the world's beaches consist of barrier islands. Emphasizing the importance of investigating the development of a mega-feeder nourishment nearby a tidal inlet system, under various hydrodynamic conditions. Therefore the research question is as follows: “How does a nearby tidal inlet system influence the development of a mega-feeder nourishment?” The research question is answered by investigating the effects various hydrodynamic conditions have on the development of a mega-feeder nourishment nearby a tidal inlet system. This is done for fixed morphodynamic features, such as the dimensions of the tidal basin and the dimensions and orientation of the tidal inlet. The only variable morphodynamic feature is the alongshore position of the mega-feeder nourishment. Four distinct hydrodynamic scenarios are modelled to investigate their effects on a mega-feeder nourishment. The tidal range (η), significant wave height (Hs), peak wave period(Tp) and peak wave direction (Dp) are varied. This resulted in the following hydrodynamic scenarios: 
•Mild wave conditions: (η = 1.5m; Hs=1.0m and Dp=0°);
•Oblique wave conditions: (η = 1.5m; Hs=1.0m and Dp=-45°);
•Storm wave conditions: (η = 1.5m; Hs=variable and Dp=0°);
•High tidal range: (η = 3.0m; Hs=1.0m and Dp=0°).
These hydrodynamic conditions and their effect on a mega-feeder nourishment are modelled by utilizing a process-based numerical model called Delft3D. In Delft3D, two locations of the mega-feeder nourishment per hydrodynamic scenario are evaluated. A mega-feeder nourishment is placed at an alongshore distance of 2 kilometers and 5 kilometers from the tidal inlet. This to get insight in the tidal flow nearby a tidal inlet and up to what alongshore distance this tidal flow affects the development of a mega-feeder nourishment. The hydrodynamic conditions were simplified, meaning steady wave characteristics and a single M2 tidal constituent. Using real time-varying hydrodynamic conditions yields similar results compared to the simplified hydrodynamic conditions. Therefore, simplifying the hydrodynamic conditions is justified. The results show that there will be additional erosion near a tidal inlet if the mega-feeder nourishment is located inside the influence of the tidal inlet. The influence is the alongshore distance where the currents owing to the tidal inlet (residual currents) still affects the total alongshore sediment transport (larger than 50 m³/6y/m). The alongshore distance of the influence increases with an increasing tidal range (tidal prism). However, there is no shoreline retreat owing to the tidal inlet at the location (2km from the tidal inlet) of the mega-feeder nourishment over a time period of 6 years. Only the adjacent coast on the inlet-side of the mega-feeder nourishment erodes significantly more than without a tidal inlet, with an increasing magnitude in the shoreline retreat towards the tidal inlet. Hence, it is expected that if the mega-feeder nourishment is placed close to the tidal inlet (i.e. several hundreds of meters), then the influence of the currents owing to the tidal inlet will enhance the shoreline retreat at the location of a mega-feeder nourishment. To conclude, the tidal inlet does influence the development of a mega-feeder nourishment nearby a tidal inlet (order several hundreds of meters). However, this is not necessarily seen in the retreat of the shoreline but instead in deeper depth contours. The governing process in the sediment transport at a mega-feeder nourishment is the incident wave angle for small tidal ranges (η < 1.5m) and mild wave conditions (Hs > 1m). However, for a large tidal range (η > 3.0m) the residual currents owing to the tidal inlet will become the governing process.