Flanders, Belgium

Co-analysis for the Mariakerke (Belgium) nourishment site Sebastian Dan1 and Daphné Thoon2 1Flanders Hydraulics Research, Berchemlei 113-115, 2140 Antwerp, Belgium 2Agency for Maritime and Coastal Services, Coastal Division, Vrijhavenstraat 3, 8400 Ostend, Belgium 1 Conceptversie A1 Table of Contents Table of Contents .............................................................................................................................................. 2 1. Introduction ............................................................................................................................................... 3 2. Description of coastal system and local area ............................................................................................ 3 2.1 General morphological and hydrodynamic characteristics ............................................................... 4 2.2 Coastal infrastructure and earlier nourishments .............................................................................. 5 3. Nourishment description ........................................................................................................................... 6 3.1 Nourishment goals ............................................................................................................................ 6 3.2 Nourishment design .......................................................................................................................... 6 3.3 Placement .......................................................................................................................................... 7 3.4 Available survey data ......................................................................................................................... 9 3.4.1 Transects .................................................................................................................................... 9 3.4.2 Surveys ..................................................................................................................................... 12 3.4.3 Additional data ........................................................................................................................ 13 3.4.4 Accuracy of the data ................................................................................................................ 15 4. Nourishment development ..................................................................................................................... 15 4.1 Qualitative Morphological development ........................................................................................ 15 4.1.1 Shoreface ................................................................................................................................. 22 4.1.2 Beach ....................................................................................................................................... 22 4.1.3 Dunes/Cliff ............................................................................................................................... 24 4.1.4 Overall...................................................................................................................................... 24 4.2 Coastal state indicators ................................................................................................................... 24 4.2.1 Volume changes 1D ................................................................................................................. 24 4.2.2 Volume changes 2D ........................................................................................................................ 28 5.2.3 Bar development ..................................................................................................................... 33 5. Local Characteristics (during lifespan nourishment) ............................................................................... 34 5.1. Tides .................................................................................................................................................. 34 5.2. Storm surges ..................................................................................................................................... 34 6. Conclusions .............................................................................................................................................. 38 7. Bibliography ............................................................................................................................................. 40 2 Conceptversie A2 1. Introduction Safety against storm induced flooding at the Belgian coast is one of the most important topics for the local coastal management. Historically, hard constructions were buildt for coastal protection, but in the present decades soft methods such as nourishments are preferred. The large majority of the nourishments were carried out on the intertidal and dry beach. This method is very efficient to reduce wave overtopping but is more costly than shoreface nourishments and has a greater impact on the use of the beach while being carried out. Therefore shoreface nourishment was suggested as possible alternative. This method would still preserve the safety function by assuring a sufficient volume of sand in the nearshore area, but at significantly lower costs and less disturbance at the coast. At the area Mariakerke – Raversijde 1(just SW from Ostend harbour, Figure 1) a nourishment experiment was designed to observe the behaviour of the coast when nourished just on the dry and intertidal beach (Section 100 , Raversijde, see figure below) compared to the coast nourished on both beach and shoreface (Section 104 , Mariakerke). Figure 1 - Pilot site for shoreface nourishment experiment, Mariakerke coastal area, and the reference sections 100 and 104. NORTH SEA 104 Underwater nourishment Beach nourishment Reference sections for monitoring OOSTENDE (Belgium) 100 500 m 2. Description of coastal system and local area The Belgian coast, stretching 67 km between the borders with France and the Netherlands, was historically characterized by sandy beaches and wide sand dunes, backed by mudflats and tidal marshes. The coast is oriented SW-NE and consists of a nearly continuous dune belt which constitutes the primary natural defence of the low-lying hinterland polder areas against flooding. The polder areas extent over a width ranging between 10 km and 18 km, with an elevation between 0 and 5 m above the mean spring low water level. The 1 See figure 2 for the exact locations of the different coastal areas 3 Conceptversie A3 Belgian continental shelf is shallow and dominated by sand banks (Van Lancker, 1999). Coastal banks are the most important since they merge to the nearshore area at several locations alongshore. The dune barrier has two arcs, one stretching from Dunkirk to Wenduine and a second arc from Wenduine to Breskens (The Netherlands). Its continuity is interrupted by the estuaries of the river IJzer in the west, het Zwin in the east and the harbours of Ostend, Blankenberge and Zeebrugge. The coastal dunes range in height from + 5 m TAW2 (local reference level for water surface elevation) to + 30 m TAW, with the majority between + 7 m TAW and + 15 m TAW. At the western Belgian coastal plain (west of the IJzer estuary) the dunes are approximately 2 km wide. East of the IJzer estuary the dune belt is only a few hundred meters wide, except for three places (Westende, De Haan, and Knokke) where the maximum width of dunes is about 2 km (Lebbe et al., 2008). The beaches are in general wide and very gently sloping, characterized by spilling breakers and classified as dissipative. The slope of Belgian beaches increases from west (1.3%) to east (2.4%), having associated the narrowing of the intertidal beach width, from 500–600 m to 200–300 m (in Knokke), respectively (Deronde et al., 2006). For about one century both waterfront and polder-lowlands knew an intensive urban and resort development, combined with hard coastal defence structures and protruding sea dikes, turning it into a “squeezed coast”, with little space left for any ecological, economical or residential developments anymore. A large part of all 255 coastal sections3 are subject to structural erosions of beach and dune systems, making the plain vulnerable to extreme storm-events. The protection of the Belgian coastline is nowadays a combination of natural and artificial defence. 60% of the coast has hard coastal protection with seawalls, revetments and groynes, but for the last two decades, emphasis has shifted to soft protection schemes, especially along the most erosive coastal stretch of Ostend – De Haan, such as beach and shoreface nourishment. (Lebbe et al., 2008). Coastal erosion and accretion along the Belgian coastline is described in coastline charts and long term trends, reporting erosion and accretion rates that variate in a significant way depending on the location of the coast. The tide along the coastline is semi-diurnal with a small asymmetry. All beaches are situated in a macro-tidal regime and the tidal range is typically between 3.5 m at neap tide and 5 m at spring tide. This important tidal range is linked to quite significant tidal currents, of which the peaks generally slightly exceeds 1 m/s in the nearshore (Haerens et al., 2012). The mean tidal amplitude decreases by 0.5 m from west to east (Van Lancker, 1999). Offshore waves are mainly driven by westerly winds. Because of the shallow waters and

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