Recently, mega feeder nourishments have been proposed as a new strategy to nourish sediment-starving beaches. This strategy involves the placement of a large, concentrated sediment volume at a single location along the coast. Wind, waves and currents act as the natural agents to spread the sediment alongshore over the course of years to decades. This article presents the morphological development of the first full-scale implementation of this strategy, examining the 20 × 106 m3“Sand Engine” feeder nourishment and its impact on adjacent coastal sections. The analysis is based on 37 high-resolution topographical surveys, executed in a 17 km coastal cell. These unique data describe the alongshore spreading in the first five years and the response at different elevations of the coastal profile. The analysis shows rapid transformation of the nourishment’s planform shape, changing rapidly into a smooth (Gaussian-like) shape which is gradually extending alongshore over time. Within five years, sediment has been distributed to a 5.8 km stretch of coast from the initial 2.2 km peninsula footprint. Changes in cross-shore and alongshore extent varied strongly over depth, with the strongest morphological response at the mean sea level (MSL) isobath and limited morphodynamic activity at deeper water, below −8 m MSL. This depth-dependent response has resulted in decreasing subtidal slopes in eroding areas, accretive areas contrastingly show a slope increment. These results yield important insights in nourished sediment mobility at different depths near the coast and distribution over a larger coastal cell. However, this single-design assessment cannot address the wide range of mega nourishment design parameters essential for morphological development of its coastal cell. This work suggests limiting cross-shore extent, since it is uncertain whether nourished sediment at deeper water will become active in the coastal system. A continuation of the current monitoring and future research might shed more light on this.@en

Verwaest, T.; Houthuys, R.; Roest, B.; Dan, S., and Montreuil, A.-L., 2020. A coastline perturbation caused by natural feeding from a shoreface-connected ridge (headland Sint-André, Belgium). In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 701-705. Coconut Creek (Florida), ISSN 0749-0208. This paper explains how a local seaward perturbation, headland Sint-André, in the straight sandy coastline of Belgium can persist for centuries. The diffusive longshore transport gradients on the beach are counterbalanced by a sediment supply from off-shore via a shoreface-connected ridge. A UNIBEST-CL+ 1D coastline model is set up to describe the genesis of this coastline feature. From the size and shape of the perturbation and taking into account the diffusivity caused by the longshore transport gradients, the average sand supply from offshore is estimated to be 55,000 m3/year and it is hindcasted that the connection of the ridge to the coastline occurred between 300 and 400 years ago.

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The Sand Engine is an example of a feeder nourishment that is intended to nourish coastal systems. This strategy is based on placing sediments highly concentrated at one location, from which it is expected to spread alongshore over large distances on decadal timescales. Here the morphological development of the Sand Engine mega feeder nourishment and the adjacent coastal sections is presented. This study is based on 37 high-resolution topographical surveys, spanning a coastal cell of 17 km alongshore. These data are explored to examine the alongshore spreading in the first five years after construction in 2011, as well as the response at different depth contours in the coastal profile. The analysis shows that the highly concentrated nourishment supplies sediment to a stretch of coast that is several times the initial length of the nourishment, as the size of the Sand Engine peninsula increased from 2.2 to 5.8km alongshore. The plan-form shape of the peninsula is found to gradually extend alongshore, while reducing in cross-shore extent. This behaviour is found to vary strongly with depth contours. The strongest response was found around the mean sea level iso-bath in contrast to the deeper parts and Aeolian parts of the Sand Engine. This variability in response over depth results in different profile slope development in accretive and erosive areas. In coastal sections which are eroding the sub-tidal slope decreases, while accretive profiles experience a profile slope increment over time. The cross shore extent of the morphologic response shows limited morphodynamic activity below the -8m NAP depth contour and confirms earlier assessments of closure depth at this coast. The current findings at the Sand Engine imply that mega feeder nourishments can be beneficial to the sediment budget of a larger coastal cell. However, volumes that are deposited around or below the depth of closure (around 15 % for the Sand Engine) may react on much longer time-scales than intended. Therefore, the feeding characteristics of mega feeder nourishments on time-scales of years should be assessed using the nourished volumes above the depth of closure rather than the total volume.@en

The Volta delta is a very dynamic environment, forming the interface between the Volta river and the Atlantic ocean. The delta is a home for many communities, settled both at the shorelines and more inland. Furthermore the delta provides great natural values and a habitat for many species. The coastline of the Volta delta suffers from erosion, threatening these communities and natural habitats. Furthermore the delta largely consists of low-lying lands which are prone to flooding. The large scale geology and morphology of the delta are explored and the natural forcing of the coastal system is determined in terms of waves, currents and human impact. Lastly flood protection and mitigation strategies are explored for the area. It must be noted that in ultimo hard protection measures will not provide a definitive solution. Maintenance is key in any kind of coastal protection strategy. Hard structures are very costly to construct and maintain, although nourishments may not be suitable in every location. More dynamic solutions and retreating from the most hazardous locations would be a better and more sustainable way forward. This report is written for the Delft Deltas Infrastructure and Mobility Initiative (DIMI) Volta delta special case. In extension on the workshop in the Volta delta, early October 2017, a literature review is produced to bundle the existing knowledge of the coastal system of the Volta delta and to process this knowledge in practical handles for design.@en

Feeder nourishments, where sand quantities of O (10 million m³) are placed locally to feed adjacent coastal stretches, are suggested nowadays as an alternative for local, smaller-scale nourishments (< 1 million m³). These feeder nourishments rely on natural forces to spread the sediment. While processes that govern this spreading such as tidal flows, waves and wind are well known, the quantification of associated sediment transport processes remains a scientific challenge. Due to the lack of knowledge with respect to sediment spreading, no tools exist to optimize the design of feeder nourishments. The Sand Engine project that is implemented in the Netherlands in 2011 consists of 21.5 million m³ of nourished sediment, and is the largest existing feeder nourishment (Stive et al., 2013). In this paper the morphological development of the Sand Engine mega feeder nourishment and the adjacent coastal sections is presented. The alongshore extent of the analysis is 17 km and spans a coastal cell between 2 harbor entrances. @en