AN INTEGRATED MASTER PLAN FOR FLANDERS FUTURE COASTAL SAFETY

Transcription

1 AN INTEGRATED MASTER PLAN FOR FLANDERS FUTURE COASTAL SAFETY Mertens, T. 1, DeWolf P. 1, Verwaest T. 2, Trouw K. 3, De Nocker L. 4, Couderé K. 5 This paper reflects the realisation of an Integrated Master Plan to protect the Flemish coastline against erosion and flooding on a short and long term basis, looking ahead at the year Different measures and alternatives to prevent present and future flooding are being worked out on the basis of safety checks and flood risk calculations along the entire coast. The different solutions will be subjected to a social cost - benefit analysis and an environmental impact assessment. The final master plan is expected to be ready in 2010 and will detail the priorities and the needs for coastal protection investments along the coastline. THE BELGIAN COASTLINE The Belgian coast is situated at the southern part of the North Sea. The coastline is 67 km long consisting mostly of sandy beaches with sea walls in front of the cities and dunes in between. There are 4 harbours at Nieuwpoort, Oostende, Blankenberge and Zeebrugge and the Zwin (tidal inlet) (Fig. 1). In the flood prone area live about people. Although Belgium has a small coast, every kilometre is intensively used. Residential neighbourhoods, ports, industries and important nature reserves are present. The pressure from tourism and recreation is immense. To balance the needs of all these interests, at present and in the future, an integrated approach is necessary. Nonetheless, special attention has to be given to coastal safety. Due to climate changes (e.g. sea level rise, more severe storms with increased wave energy) and continuing development of the coastal zone, protection against coastal erosion and flooding will become increasingly difficult and costly to guarantee. To counter this problem, good spatial planning, cooperation between different governmental organisations and collaboration with neighbouring countries will be essential. 1 Belgian Coastal Division, Vrijhavenstraat 3, 8400 Oostende, Belgium, tina.mertens@mow.vlaanderen.be 2 Flanders Hydraulics Research, Berchemlei 115, 2140 Antwerp, Belgium 3 International Marine and Dredging Consultants, Coveliersstraat 15, 2600 Antwerp, Belgium 4 Flemish Institute for Technological Research (VITO), Boerentang 200, 2400 Mol, Belgium 5 Resource Analysis, Coveliersstraat 15, 2600 Antwerp, Belgium 1

2 2 Figure 1. The Belgian coast is 67 km long and has 4 harbours at Nieuwpoort, Oostende, Blankenberge and Zeebrugge. At the borders two important nature reserves (Westhoek and Zwin) are located. SET-UP OF A MASTER PLAN In Belgium coastal protection is a regional responsibility. Up to now, the Flemish government (Flemish region) has defined the minimum safety level of the coastal protection at once in 1000 year. However, this safety standard is not implemented in any law or decree. Every 5 years the safety of the entire coastline is checked and yearly monitoring enables to update the achieved safety level. Awaiting for the master plan to establish the desired safety level, every year smaller beach nourishments are carried out. For several years no new sea walls have been built, because these hard safety measures intervene with the natural dynamic of the coastline whereas soft measures, like nourishments work together with the accretion and erosion processes. A lot of coastal communities however do not achieve the safety standard. So far, a minimum safety level of once in 100 year is guaranteed along the entire coastline. The yearly budget does not add up to meet the standard. There is a need for long-term planning Hence, for the first time, the Coastal Division of the Flemish region started up a study to work out an integrated master plan for Flanders future coastal safety. The aim of this study is to protect the Flemish coast against erosion and flooding on a short and long term basis, looking ahead at the year 2050, based on the principles of ICZM. Therefore the time aspects of

3 investments, sea level rise, beach erosion, are also taken into account. This integrated master plan must in particular define the measures needed to develop and guarantee a safe coastline. The study started up March 1st 2007 and will last 40 months. A total budget of 1 million is extracted to set up the master plan. The infrastructure works that will follow still need to be budgeted. Within the preparation and the execution of the coastal policy in general and the policy concerning coastal safety in particular there are four different important angles to take into account: Policy angle: strategic vision development and foundation of it leading to policy choices how to handle coastal safety, leading to vision documents, policy plans and management plans. Operational angle: the implementation of the actual management and maintenance (including matters such as monitoring, testing/inspection, warning) and the execution of specific measures for improvement of the coastal safety. Legal angle: the legal anchoring of the coastal safety policy in terms of safety norms, touristic demands for broader sea walls and the tasks, roles and responsibilities of the parties involved. Financial angle: the regulation of financing and the financial statement and flows of funds between involved parties. According to the expectations the term master plan for coastal safety is in the first place related to the policy angle. In the master plan the policy visions should be developed and specifically translated in a policy plan for the further development of the coastal area, the required protection of the hinterland and the maintenance and/or the improvement of the natural and artificial sea walls that are located in the coastal zone. The master plan is a plan on a higher abstraction level and should offer the framework for the more specific management and execution plans, which form the basis for the operational execution of the coastal policy. On the other hand there are important relations between the master plan and the legal and financial frameworks. The different topics of the study are summarized in Fig. 2 and will be addressed in the outline of this article. 3

4 4 safety check flood risk calculations measures/alternatives SCBA EIA legal framework EU - projects risk management finalised ongoing MASTERPLAN flood risk reduction COMMUNICATION Figure 2. Different topics of the Integrated Master Plan. SAFETY CHECK AND FLOOD RISK CALCULATIONS To highlight weak links in the coastline a safety assessment of the existing coastal protection systems (dikes, dunes, beaches, harbours) and flood risk calculations are performed. Thus, not only the impact of extreme storm conditions on the local infrastructure, but also the consequences for the hinterland caused by flooding is being looked at. The return period of extreme storm events resulting in serious flooding is of the order of magnitude of years or more. In the framework of this master plan the impact of a 1000 and 4000 year storm event is envisaged and even worse credible storm events are incorporated in the flood risk calculations. To perform these calculations, seventy-five years of measurements of water levels and twenty-five years of deep water wave measurements at the Belgian coast are available. Important parameters are wind, wave and storm surge statistics. The storm surge level of an extreme storm event is the most determining storm characteristic with respect to the associated flooding. Wave characteristics are also important, but found to be relatively well correlated with the storm surge level. Fig. 3 details the different steps of the safety assessment. Statistics on water levels, wind velocity, wave heights and periods were established at deep water measurement locations. These wave statistics have been transformed to near shore wave characteristics using a calibrated numerical wave model (SWAN). The results of this consist of wave parameters at a line along the coast, with a water depth at about 5 m below low water, a position at which the bathymetry will not change considerably during storms (depth of closure).

5 During a storm the beach in front of the dike will erode. Due to the lowering of the bed level, waves will transfer more easily towards the toe of the dike, hence to know the wave height at the toe of the dike, it is important to calculate the erosion of the beach. The erosion of the beach during the storm was determined with DUROSTA (Steetzel 1993). In principle, the wave height must be determined at the toe of the dike. However, most wave models (Swan, Endec, etc.) produce less reliable wave heights at very shallow water depths. Therefore the wave height at a distance of 5 times the significant wave height at deep water from the toe of the dike is used. Only for very steep slopes (<1:30) the waves are determined closer to the dike. As a final phase the failure mechanisms of the sea defence (dunes, dikes, quay walls and sluices in harbours) are tested. To estimate the erosion risk of the dunes, the Vellinga approach was used (Vellinga 1986). A breach is assumed to occur if the dune volume above the maximum water level is smaller than a critical volume. For dikes the overtopping discharge must not exceed a certain limit (i.e. 1 liter/s/m) and the stability of the structure must be guaranteed. The same goes for harbour infrastructure, bearing in mind that the crest of quay walls must be higher than the storm surge level and sluices must withstand storm conditions. 5 Figure 3. Different steps of a safety assessment. Statistics on water levels, wind velocity, wave heights and periods are established at deep water locations (1) and transformed to near shore wave characteristics (2) using a calibrated numerical wave model (SWAN). The erosion of the beach during the storm is determined with DUROSTA (3). As a final step the failure mechanisms of the sea defence (dunes, dikes, quay walls and sluices in harbours) are tested (4). The safety assessment concluded all harbours to be weak links and in total almost 30% of the entire coastline doesn t meet the safety standard of long-term protection against a 1000 year storm event (sea level rise included). The overtopping discharges of dikes are too high and quay walls are too low, thus forming key factors to counter in the set-up of protection measures.

6 6 Once a coastal protection system fails and a breach occurs, it s important to look at the consequences for the hinterland. As one of the partners in the study to prepare an integrated master plan for Flanders future coastal safety Flanders Hydraulics Research performs flood risk calculations for the actual situation at the Belgian coast as well as for alternative designs of measures to improve the coastal safety. Flemish water management today no longer chooses to prevent floods at all costs, but instead seeks to limit the damage, or more general to limit the negative consequences of floods. Therefore, Flanders Hydraulics Research developed a customer-tailed flood risk methodology. Depending on available data and customer needs this approach allows to determine the expected damage, the possible human casualties and the associated flood risk. As a result flood maps and flood risk maps are made up for several worst credible storm events. Detailed information can be found in Verwaest et al. (2008). SOFT AND HARD PROTECTION MEASURES Different measures and alternatives to prevent present and future flooding are being worked out on the basis of these safety checks and flood risk calculations. Protection against 1000 and 4000 year storm events as well as the possibility of a differentiated protection level is being looked at. Soft and hard protection measures will be examined. Soft solutions (shore face, beach or dune nourishments) have the advantage of flexibility with regard to sea level rise, positive impact on recreation and their overall impact is relatively small, but the disadvantage of maintenance costs (local seaward movement of the coastline). Hard constructions (increasing the height of existing dikes, storm return walls, ) have a greater impact on the human side. The possible measures can be classified as follows: for sea walls in front of cities beach nourishments beach nourishments + storm return walls on the sea walls beach nourishments + stilling wave basin (= storm return wall on the slope of the sea wall) increased roughness of slopes for dunes dune nourishments beach nourishments for harbours storm flood barrier storm return walls around the harbour increased roughness of slopes

7 7 floatable breakwater at entrance of harbour Besides these permanent measures, maintenance costs (of nourishments) can also be lowered by for example the construction of groins, the planting of marram grass, the use of wind shields or shore face nourishments. The design of the beach nourishment takes into account the available sand at borrow areas, since the grain size determines the equilibrium beach slope and the beach erosion. In the long shore direction a gentle transition of the beach extension to the neighbouring coastline is necessary. The use of groins and breakwaters is evaluated to decrease long shore sand losses. Defence strategies as retreat, holding the line and seaward extension are regarded as an option for mid-term and long-term strategy. Whereas the current policy is based on a holding-the-line approach, retreat is a reasonable option in broad dune areas and to increase the biodiversity. In the first stage possible measures are worked out technically for one location. All possible combinations of measures are examined and at the end discussed in a workshop with coastal specialists. In a second workshop the possible solutions are presented at specialists who will carry out the social cost benefit analysis and who will be involved in the Environmental Impact Assessment studies. Both workshops aim to obtain an optimal set of possible measures to be examined more in detail for the whole coast. For all measures both the initial as the maintenance costs have to be examined as an input for the social cost benefit analysis. EVALUATION OF MEASURES The different solutions will be subjected to a social cost - benefit analysis (SCBA) and an environmental impact assessment (EIA). Also the risk reduction is considered, especially regarding human casualties. Social cost benefit analysis The social cost benefit analysis compares construction and maintenance costs with the benefits. However, not only the technical costs and benefits are important, also social, ecological and economic impacts and especially impacts on recreation and tourism have to be looked at. The overall cost-benefit framework follows the recommendations of the OEEI guidelines, developed for the cost benefits analysis of transportation plans (Eijgenraam et al. 2000). It is adapted to take account of the specificity s of flood protection analysis and nature development. Special attention is given to take account of long time horizons and economic growth and discounting. The approach is in line with current thinking on flood risk modelling and evaluation (see e.g. guidelines from Ramsar (Barbier et al. 1977), guidelines from Defra

8 8 (MAFF 2000)). The approach however goes a step further in the detail of the analysis in different fields (both technical and economic parameters), in the scope of the analysis (esp. time horizon), sensitivity and uncertainty analysis. The optimal scheme will be the result of the best balance between the extra costs necessary to materialize the higher protection level, and the extra benefits resulting from these investments as compared to the zero option. On the cost side, the necessary investment costs for the flood protection works have to be considered, and all necessary maintenance and operation costs during project life time (considered 50 years). If the analyzed scheme implies relocation of agricultural areas or forests, the corresponding costs are also considered. On the benefit side, the avoided flood risks (i.e. safety benefits) are considered by comparing the remaining flood risks, after implementing different measures, to the zero option (do nothing). Where necessary, avoided costs are added. Where applicable, benefits resulting from nature development and recreation have also been considered and translated in monetary values. An important aspect in the SCBA is handling uncertainties in both the flood risk modelling as in the normal future economical development (interest rates ). The social cost-benefit analysis results in an overview of all the costs and all the benefits of a project. With this overview a ranking of all projects can be made and it will be proven that the project has an added value for the society. It will be input, together with the EIA, for decision makers to decide on the best alternative. Environmental impact assessment The environmental impact assessment (EIA) is executed in parallel with the SCBA since exchange of information between these two parts of the study is necessary. In the EIA all possible effects on the environment are considered both of soft and hard measures. Amongst others, results of a recent study on the ecological effects of beach nourishments (executed by Ghent University and commissioned by the Coastal Division) will be taken into account, but also the social effects of e.g. a storm return wall on a dike. The following are EIA operating principles of good practice and performance amended from Sadler (1996). First of all a strategic-eia is executed in which the general possible solutions are compared and their effects are studied without going in too much detail. Together with the social costbenefit analysis and other policy decisions this results in (some) most desired alternatives. The alternatives are worked out in more detail and it is examined if the strategic-eia is sufficient or if project-eia s are necessary. Fig. 4 shows the general scheme for the EIA-study.

9 9 Figure 4. General scheme for the EIA-study. The execution of project-eia s falls out of the scope of this master plan. It is expected that some coastal zones won t need a project-eia to start construction works in 2010 (after application for a building permit). Those zones that do need a project-eia will be investigated into detail afterwards, in parallel with the first constructions. Risk reduction for human casualties Casualties can only be avoided partially by evacuation plans. Reference is made to criteria for every individual in the region, as well as for groups that are applied in other sectors of external safety. However, acceptability is still a topic of debate. FROM MOST DESIRED ALTERNATIVE TO MASTER PLAN Once the most desired alternatives are known the implementation of these measures needs to be further examined. Legal framework (DeWolf and Berteloot 2006) The juridical part aims at inventorying the license pathway for the presented measures and their implementation alternatives by offering a step-by-step guide under the form of a procedure handbook. It appears that plural licence pathways have to be explored because of, among others, the nature, the scope and the location of the presented measures; several permission procedures and/or appraisals are employed. Concrete solutions are formulated, thus investigating for example how the coastal protection policy can come about (Flanders has no legally binding

10 10 document that states how coastal protection has to be carried out), and if there are spatial implementation plans (RUP s) and existing legislation that must be complied with. This part will give an overview of the legal framework of the coastal protection works and the implementation alternatives: the legal and administrative procedures (required licenses), the appraisal of the sectors, the determined sticking points and the possible solutions. Project risk management For all large infrastructure projects in Flanders the decree concerning monitoring of large infrastructure projects applies. This decree has the aim to prevent or at least restrict incidents (juridical problems, exceedings of construction budget, technical problems during realisation, ) that might jeopardize the realization of the objectives for a given project. The project risk management has the following aims: Determination of the aims for the risk management Identification of the possible risks; Analysis of the possible risks; Management of the most important possible risks; The final master plan will eventually summarize the entire evaluation process, resulting in a programme which outlines the most desired alternative and the realisation procedure (measures, finances, legislation). And last but not least, the final instrument in the set-up of this master plan is the communication part. COMMUNICATION In the course of the study special attention is given to the communication with different stakeholders and the broader public (questionnaires, presentations, brochures, digital newsletter ). Moreover, consultation of national and international governmental institutes is essential. In the framework of the European project Safecoast knowledge on climate change and coastal flood and erosion management was shared and information and ideas concerning master planning was exchanged. As a result, relevant information from existing coastal zone master plans in neighbouring countries will be integrated in Flanders master plan. Another example of stakeholder involvement is a poll that was organised at two coastal towns to evaluate the visual impact of possible protection measures. 300 surveys were performed by means of digital simulations (Fig. 5). The main results were:

11 most people are in favour of hard measures (e.g. storm return wall or stilling wave basin); young people prefer larger beaches (perfect playground for children); old people complain about the extra sand transport on the sea walls (coming from beach nourishments). The results of this poll will be imbedded in the evaluation process of the different measures (SCBA and EIA). 11 a) b) Figure 5. Digital simulation of possible protection measures: a) a storm return wall and b) a stilling wave basin on top of an existing sea wall. CONCLUSION The Integrated Master Plan for the Flemish coast is expected to be ready in 2010 and will finally detail the priorities and the needs for coastal protection investments along the coastline, so as to minimise the risk of flooding in the nearby and distant future.

12 12 REFERENCES Barbier, E., M. Acreman, and D. Knowler Economic valuation of wetlands: a guide for policy makers and planners, Ramsar, Switzerland. DeWolf, P., and M. Berteloot Strategic and project environmental impact studies for a combined harbour and coastal protection scheme at Ostend, Belgium the experience with European guidelines, Pianc-congress, Estoril. Eijgenraam, C., C. Koopmans, P. Tang, and A. Verster Evaluatie van infrastructuurprojecten en leidraad voor kosten-batenanalyse. MAFF (Ministery of Agriculture, Fisheries and Food) FCDPAG5 Flood and Coastal Defence Project Appraisal Guidance, pp. 69. Mertens, T., K. Trouw, K. Bluekens, L. De Nocker, K. Couderé, C. Sauwer, P. De Smedt, C. Lewis, and T. Verwaest SAFECoast: INTEGRATED MASTER PLAN FOR FLANDERS FUTURE COASTAL SAFETY, Coastal Division of the Flemish Community, Belgium. Sadler, B Environmental Assessment in a changing world. Evaluating Practice to Improve Performance, CEAA and IAIA. Steetzel, H.J Cross shore transport during storm surges, Thesis, Civil Engineering, Delft University of Technology, Delft, The Netherlands. Vellinga, P Beach and dune erosion during storm surges, Ph.D. thesis Delft University of Technology. Verwaest, T., K. Van der Biest, P. Vanpoucke, J. Reyns, P. Vanderkimpen, L. De Vos, J. De Rouck, and T. Mertens Coastal flooding risk calculations for the Belgian coast. Proceedings of 31st International Conference on Coastal Engineering, ASCE (this publication).