www.cerf-jcr.org www.JCRonline.org A Coastline Perturbation caused by Natural Feeding from a Shoreface-connected Ridge (Headland Sint-André, Belgium) Toon Verwaest †* , Rik Houthuys ‡ , Bart Roest § , Sebastian Dan † , and Anne-Lise Montreuil †† †Flanders Hydraulics Research Antwerpen, Belgium ‡Independent Consultant Halle, Belgium §Faculty of Engineering Technology, KU Leuven Bruges, Belgium ††Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Belgium ABSTRACT 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 off- shore 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. ADDITIONAL INDEX WORDS: Littoral drift, active zone, coastal resilience, climate change. Journal of Coastal Research SI 95 701–705 Coconut Creek, Florida 2020 DOI: 10.2112/SI95-136.1 received 31 March 2019; accepted in revision 13 February 2020. *Corresponding author: toon.verwaest@mow.vlaanderen.be © Coastal Education and Research Foundation, Inc. 2020 INTRODUCTION The Belgian coast is a quasi-straight sandy coast, which is situated in the southern North Sea area (Figure 1). Despite its uniform appearance, large variations in morphological behavior are observed. Based on intensive morphological monitoring of the beaches and the shorefaces during the past 30 years it is demonstrated that at the western Belgian coast a natural feeding of sediments has occurred from off-shore. This sediment feeding has heightened the morphological active zone at a rate of 10 mm/year, which is signifcantly higher than the local sea level rise rate of 2 to 3 mm/year (Montreuil et al, 2019). One can conclude that this coast has shown a morphological resilience to climate change in the past 30 years. However, much is still unknown about the transport pathways and physical processes that explain the observed natural feeding. State of the art morphological models are often unable to simulate cross-shore feeding. More knowledge on the physical processes behind the observed feeding is needed to develop a predictive model for the coastline response. Many more questions rise in case of accelerated sea level rise. Will the natural feeding accelerate as well? Is there a threshold sea level rise rate for natural feeding rate to be suffcient to let the coast grow along? In this paper a mechanism is revealed, namely sand transport towards the coastline via a shoreface-connected ridge called “Trapegeer-Broersbank”. Study Site The Belgian sandy coast is macrotidal with a tidal range of ca. 4 m. Sediment transport in the active zone is driven by the waves as well as the tides (Brand, 2019). Numerous coastal sand banks are present on the shallow sea bottom. The study site, the western Belgian coast, is a quasi-straight san- dy coast of 14 km in length, from the French border to Nieuwpoort. The orientation of the coastline is 31° (~ from WSW to ENE). Figure 1. Location of the study site (red) in the larger North Sea area.