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. Hydraulics Research & andere: Antwerp,

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.

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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

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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.

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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%

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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 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

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Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP

Wenduine to

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

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 .

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

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Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP specific dredge material disposal place as calibration point. The selected place should have being in use for a number of years (as long as possible) during the study period. After collecting all information concerning disposed volumes and dates the correlation between the disposed volumes and the trends detected for different periods should be found and evaluated.

Comparison with the results of the trend analysis performed previously for a 100 years period.

It is also important to take into account the criteria used by the hydrographical service when processing bathymetric data. In principle, highest points will be always selected also when measuring on the crest of a sandbank. This can lead to an overestimation of some of the trends. In this line, it is also important to take into account that not only the processing methodology but the different instruments and settings used for the different campaigns are also a source of inaccuracy and uncertainty for the results of the trend analysis.

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Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP

3 Identification of possible natural and anthropogenic trends

3.1 General

AI/ Compare the trends West and East of Oostende (more influenced by human actions). AI/ Study of the mean and long term evolution of the beach profiles and banks.

AI/ comparison with results obtained for the 100 years trend analysis performed previously in the framework fo Quest4D.

AI/ Attempt validation for Zeebrugge and Oostende (dumping sites, artificial bar)

3.2 Possible links to climate change

General erosion trend.

Specially West of Zeebrugge, the general trend shows a steeping of the shoreface in place of inland migration of the shoreline and banks apparently moving to the East: this is consistent with a fast variation of the water depth if there is not time enough for the system to adapt at the new conditions.

AI/ Possible effects of sea level rise in 10 years from now: identification of possible natural cycles that can affect sea level rise. Define or fine related indicators that can be used at the Belgian Coast.

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Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP

4 Comments on results of the trend analysis

A relevant element that arises when looking at the results of the morphological trends analysis is how to tackle the uncertainty. One option would be focusing only on very clear features as the shoreface nourishment close to Vlissegem or the changes in the Stroombank.

It is remarkable that the trend areas on the beach are, in general, relatively easy to explain when comparing with the different options for the shoreface.

There is a gap area West fo Zeebrugge that should be filled with some existing data. AI/ Add existing data to West of Zeebrugge.

AI/ Use available measurements to study the Wandelaar and surrounding area. Is it ebb dominated?

AI/ Modelling with and without Zeebrugge.

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Workshop "QUEST4D and Beyond": A discussion about erosion and sedimentation trends in the BCP

5 Discussion about future research based on coastal morphological trends found for the Belgian Coast

The QUEST4D project would set the scientific basis for further studies. In this framework, different elements related tot eh results of the project are searched for:

-Links with sediment changes.

-Link with seabed types

-Link with macrobentos One of the main topics of this discussion is the approach to be taken to make the final project report. In general, it will be necessary to orient to the delivery of end products like, for example, the definition of a protocol or a series of recommendations for dredging procedures. It is also advisable to clearly identify the application of the results:

It would be still necessary to review in detail the methodology used for different tasks.

AI/ Organize meeting. In terms of data, the first complete mapping of the Belgian Coast occurred only in 1983. until now there is a lack of digitized data from a few tenths of years ago that can be used to compare or match with other, more recent, datasets.

AI/ Digitize Eurosense data till back to the 70’s.

AI/ It would be also very useful to put together all the existing information regarding nourishments, dredging and dumping including volumes and dates.

AI/ In order to know more about the anthropogenic activities it would be necessary first, to delineate the zones of interest showing clear trends and perform a study base don morphological maps.

There is a lot of uncertainty about what the spatial extension of the impact of the presence of a dumping site can be. After some of the participants in the meeting, this effects are extending beyond the local area.

There is also great uncertainty around the sediment budget but at the same time, it is necessary information for the numerical modelling.

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Waterbouwkundig Laboratorium

Flanders Hydraulics Research

Berchemlei 115 B-2140 Antwerpen Tel. +32 (0)3 224 60 35 Fax +32 (0)3 224 60 36 E-mail: [email protected] www.watlab.be