Workshop "QUEST4D and Beyond" A discussion about erosion and sedimentation trends in the BCP Delgado, R.; Janssens, J.; Van Lancker, V.; .Monbaliu, J.;Verwaest, T.; Mostaert, F. December 2010 WL2010814-xxR_X_Quest4D FORM: F-WL-PP10-2 Version 02 VALID AS FROM: 05/01/2009 This publication must be cited as follows: Delgado, R.; Janssens, J.; Van Lancker, V.; .Monbaliu, J.;Verwaest, T.; Mostaert, F. (2010). Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP. WL Rapporten, Projectnr. Flanders Hydraulics Research & andere: Antwerp, Belgium Waterbouwkundig Laboratorium Flanders Hydraulics Research Berchemlei 115 B-2140 Antwerp Tel. +32 (0)3 224 60 35 Fax +32 (0)3 224 60 36 E-mail: [email protected] www.watlab.be Nothing from this publication may be duplicated and/or published by means of print, photocopy, microfilm or otherwise, without the written consent of the publisher. FORM: F-WL-PP10-2 Version 02 VALID AS FROM: 05/01/2009 Document identification Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation Title: trends in the BCP Customer: BELSPO Ref.: WL2010814-xxR_X_Quest4D Keywords (3-5): Text (p.): Tables (p.): Appendices (p.): Figures (p.): Yes Customer Exceptions: Internal Confidentiality: Flemish government Released as from No Approval Author Reviser Project leader Division Head Revisions Nr. Datum Omschrijving Auteur 1_0 14-12-2010 Concept version Rosalia Delgado 1_1 Substantive revision 1_2 Revision customer 2_0 Final version Abstract FORM: F-WL-PP10-2 Version 02 VALID AS FROM: 05/01/2009 Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP 1 Introduction On 6 December 2010 the QUEST4D partners, together with end-users, met at Flanders Hydraulics Research in order to discuss the last results of the integral analysis of 10 years of bathymetric data and the resulting morphological trends at the Belgian Coast. This discussion was the basis to further define the approach to be taken for the finalisation of the QUEST4D project and to put together some ideas about possible after-QUEST4D activities. Concept WL2010814-xxR_X_Quest4D 1 FORM: F-WL-PP10-2 Version 02 VALID AS FROM: 05/01/2009 Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP 2 Ten years morphological trend analysis for the Belgian Coast 2.1 Overview The morphological trend analysis was performed for the beach and nearshore using bathymetric data obtained at different moments and zones of the Belgian coast with the help of ArcGIS software. 2.2 Data availability Beach data: • Beach data: from the dune feet until the low water line. • Shoreface data: from low waterline till 1,5 km in the sea. • Offshore data: from 1,5 km to 10 km off the coast. Here we can indentify three zones: Zuydcoote-Westende. Wielingen-Scheur. Westende-De Haan. Table 1 – Overview of data used for the morphological trend analysis Year STRAND SHOREFACE OFF SHOREFACE 1997-1 9 Westende-De Haan 1997-2 9 Zuydcoote-Westende 1998-1 9 Wielingen-Scheur 1998-2 9 1999-1 9 9 1999-2 9 9 2000-1 9 9 Wielingen-Scheur Westende-De Haan 2000-2 9 2001-1 9 Wielingen-Scheur 2001-2 9 2002-1 Zuydcoote-Westende Concept WL2010814-xxR_X_Quest4D 2 FORM: F-WL-PP10-2 Version 02 VALID AS FROM: 05/01/2009 Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP 2002-2 9 2003-1 9 Zuydcoote-Westende 2003-2 2004-1 9 9 (summer) Wielingen-Scheur 2004-2 2005-1 Westende-De Haan 9 (summer) 2005-2 2006-1 9 Zuydcoote-Westende 2006-2 2007-1 9 9 Wielingen-Scheur 2007-2 2008-1 9 9 Westende-De Haan 2008-2 2009-1 9 Westende-De Haan Zuydcoote-Westende 2009-2 2010-1 9 2010-2 2.3 Methodology The study consist of a GIS morphological analysis based on yearly topographic and bathymetric measurements spanning a 10 year period as shown in the summary about data availability presetned in the previous section. Data are interpolated to a rectangular 2mx2m grid at the beach and shoreface and to a 5mx5m grid at deeper areas. Once all bathymetric grids are ready for every year a comparison of the datasets is carried out. Areas for which no overlap between datasets is found are left out of the analysis. Concept WL2010814-xxR_X_Quest4D 3 FORM: F-WL-PP10-2 Version 02 VALID AS FROM: 05/01/2009 Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP For each pixel (5m x 5m) the erosion/sedimentation trend was calculated base don a least-squares linear regression method. The volumes of sediment were calculated above an arbitrary ground level. Due to the lack of a continuous dataset for the complete area, the analysis is performed separately for the beach, for the foreshore and for the areas beyond this last. Results: − Rasters in ArcGIS. − Erosion velocity (rate) − Correlation Coefficient R2 that is considered as relevant only if R2>0.5. − Standard error of erosion rate. The confidence level for erosion and sedimentation is set at 95% Concept WL2010814-xxR_X_Quest4D 4 FORM: F-WL-PP10-2 Version 02 VALID AS FROM: 05/01/2009 Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP 2.4 Results: general and local trends 2.4.1 Erosion and accretion patterns at the Belgian Coast The general trend indicates that erosion dominates over accretion when considering the complete study area as a unit. However, due to the complexity of the Belgian coast and the different uncertainty levels on the trends found for different subzones it is more meaningful to discuss the results for the different areas and features indentified. There AI/ Check reliability of 2001 and 2002 measurements (error on average is much smaller than error in the measurements). AI/ In order to know more about the error associated to the methodology used it is advisable to make a sensitivity analysis for a limited area and for a shorter period of time and see changes in the trends. 2.4.2 Zone analysis (must be completed with details of the trend analysis for every zone) Bank van Heist Formation and grow of the sand is apparent. A clear erosion trend is found directly downstream of this feature. De Haan An accretion line is identified and linked to the nourishment of the foreshore bar (feeder berm) done around year 95 foreshore bar (feeder berm) was done in front of Vlissegem, around 600 m from the HWL. AI/ Collect information and compare with results of the trend analysis. Zeebrugge In general, the erosion trend is found to be dominant for this area which includes the harbour of Zeebrugge and the surrounding area with navigation channels. There are two important erosion zones clearly indentified after the analysis: 1. Area directly east of the harbour next to the breakwaters and to the area of formation of the Bank van Heist. 2. North the navigation channel East of the Pas van het Zand a zone with unexplained strong erosion trend is found. 3. The limits of the navigation channel present some variations that can be related to the widening of the channel. Possible error or shift in position of the navigation channel as explanation for these changes is in principle discarded. The most likely hypothesis is that it has something to do with the dredging of the navigation channels. As expected there is also a strong accretion trend found for the Bank van Heist. FIGURE+description+comments There is a remarkable erosion are North-East the navigation channel and directly North of this last. AI/ Are there different datasets that are included in both studies for this area? AI/ Find report on morphological evolution of the Western Schelde Concept WL2010814-xxR_X_Quest4D 5 FORM: F-WL-PP10-2 Version 02 VALID AS FROM: 05/01/2009 Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP Wenduine to Blankenberge This is an area with groins where the natural evolution is expected to be influenced by the different nourishments done during the study period. Midelkerke to Wenduine Indicators of a possible migration of the crest and east part of the Stroombank. Between Oostende and Wenduine there is a 9 km erosion area on the foreshore (feederline). The Stroombank seems to be affected by some changes that need to be further investigated. The area between Oostende and Nieuwpoort is not so influenced by human action as other zones. AI/ Check the migration hypothesis for the Stroombank by adding contour lines (bathymetry). In general, it is advisable to examine in detail the evolution of the Stroombank and also fo the Niuewpoort bank. Lombarzijde The evolution of the longitudinal bars south Lombarzijde becomes extremely apparent in the trends map. FIGURE+description+comments. uKoksijde A clear erosion trend is found for the narrow channel adjacent to the bank west of Koksijde. FIGURE+description+comments 2.4.3 Some comments on quality of the results and method accuracy One of the main issues is the determination of the accuracy of these results. Also, the link between the shoreface and the beach is not as good as desirable due to the different times and extension of the different surveys originating the data used for the analysis. To improve this it would be advisable to: Homogenisation of the datasets used with focus on the edge areas linking beach and shoreface: For this purpose the new bed model developed at the BMM for the study area could be very useful. Calibration of the datas ets and of the methodology itself: One of the possibilities to do that is using a Concept WL2010814-xxR_X_Quest4D 6 FORM: F-WL-PP10-2 Version 02 VALID AS FROM: 05/01/2009 Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP specific dredge material disposal place as calibration point.