Lebbe, L.

Van Meir, N.

Open Marine Archive

Proceeding of SURVAS Expert Workshop on European Vulnerability and Adaptation to impacts of Accelerated Sea-Level Rise (ASLR)
Hamburg, Germany, 19th-21st June 2000
Belgium
80
Implications of Accelerated Sea-Level Rise (ASLR) for Belgium:
Luc Lebbe1,2 and N. Van Meir2 (1 Fund of Scientific Research-Flanders, 2 Gent University, Department of
Geology and Pedology)
General overview
The Belgian coast (65 km long) is situated at the southeastern end of the North Sea. It is subject to high physical energy
through the combined effect of sea currents, tides and wind. The Belgian coast could be defined as a mesotidal wind
dominated clastic coast with a tidal amplitude of 4 to 5 m. Predominant winds are from the west, storms are usually
accompanied by winds from the southwest, west or northwest. It is a rather linear coast with a shallow offshore band
characterised by many gullies and offshore bars. Its natural coastal defence system consists of 3 parts, namely the fore
shore, back shore and dunes. This is a mobile structure that remains in dynamic equilibrium with gulfs, climate, wind
and sediment characteristics, in this case, sand (MVG, 1993). Its economic importance is mainly a consequence of
beach tourism and agriculture in the low-lying polder areas. The combined economic activities from tourism, fishing
and harbours produce an added value of about 2.66 billion Euro (Allaert, 1996). Agriculture was not included in this
calculation. At present, the Belgian coastal plain is mainly influenced by human induced factors, about 60% of the
coastline has some form of coastal defence structure (De Wolf, 1996 in WL, 2000). The latter would inhibit the
establishment of a new dynamic equilibrium should sea level rise; there is simply no space to accommodate for a
possible transgression. The coastal plain is defined landward by the topographical contour of 5 m TAW (TAW: the
Belgian reference level), and extends seaward, up to 1,500 m from the lowest observation point on the shore (MVG,
1993; WL, 2000). Maintenance and investments of coastal defence structures as well as the monitoring of beach and
dune pollution raises costs to about 25 million Euro (pers. comm. P. De Wolf). In this discussion the sea-harbour cities
of Antwerp and Gent situated along the river Scheldt have been included in the coastal zone as their importance for
Belgium/Flanders economy can hardly be overestimated.
Factors influencing national resilience/vulnerability to impacts of sea-level rise
Future impacts of sea-level rise could include:
1. permanent inundation of the low-lying coastal plain,
2. loss of the (currently protective) beach,
3. increased inundation hazard,
4. salt water intrusion, and,
5. the breaching of protective dikes that could lead to the progression of the coastline landward up to a maximum of
20 km.
To tackle these possible impacts, the Belgian coastal plain is already largely protected by man-made structures along its
coastline. These can be subdivided in hard defence structures and soft protection measures. In the first group, there are
sea-dikes, dikes, groynes, Longard system, sluices, etc. Soft protection measures that have been applied to the Belgian
coast include beach nourishment at different points along the coast, as well as the installation of an underwater berm,
sand berms on fore shore, underwater screens, temporary or permanent wind screens, beach scrapings, planting osier
hedges etc. (MVG, 1993). On the other hand, some phenomena contribute to decreasing the current level of protection
afforded by the measures mentioned above. Tourism leads to high population pressure on the coastal zone, and can
result in the degradation of natural dune vegetation. The construction of harbours can seriously interfere with natural
dynamic equilibrium. For example around Zeebrugge, this has lead to considerable erosion on the eastern side of the
harbour. Moreover, extraction of fresh groundwater present in the dune fields, lowers the water table, and can promote
sand instability. The latter can in turn have an adverse effect on dune vegetation. Erosion along the Belgian coastal
plain has lead locally to the steepening of foreshore profiles, as it is the case in the central part (between Nieuwpoort
and Blankenberge).
Existing efforts towards assessment of vulnerability and associated problems and their results
During the past 25 years, concern about rising sea level, increasing coastal erosion and the growing significance of
coastal economy has lead to a more or less permanent monitoring of a series of coastal parameters. The possibility of a
breaching of the first natural coastal defence system (i.e., the natural dune fields) was calculated for the different
sectors of the Belgian coastal zone and an indicative figure for breaching periodicity was calculated. Return periods of
potential breaching range from 1 in 1,000 years for the most vulnerable parts to 1 in 40,000 years for the least
vulnerable parts (Verwaest et al., 1998 in WL, 2000). The overall aim of the man-made coastal defence structures is
that they should withstand a storm with a return period of 1,000 years. At present, however, the largest part of the
coastline is protected for storms with a return period of 100 to 250 years. In some cases, the existing protection
structures would only withstand storm return periods as short as once in every 50 years.
Proceeding of SURVAS Expert Workshop on European Vulnerability and Adaptation to impacts of Accelerated Sea-Level Rise (ASLR)
Hamburg, Germany, 19th-21st June 2000
Belgium
81
The nature of the predominant winds is measured at three points along the Belgian coast since 1977. A cautious
prognosis for the future indicates a possible change of 2 degrees to the north. The behaviour of gulfs is measured with 6
buoys, from which it was deduced that storms are associated with gulfs coming from the west to the north. Tidal
currents have a northeastern direction for spring tides, a southwestern direction for neap tides (MVG, 1993).
Tide observations cover in places periods from the mid-nineteenth century (1835 to 1853 and 1927 to 1998 for
Ostende, 1932 to 1998 for Nieuwpoort and 1932 to 1998 for Zeebrugge) to the present. These indicate a linear increase
of High Water (HW) level by 2 cm, Low Water (LW) by 1.5 cm and Mean Sea Level (MSL) by 1 cm per decade (Van
Cauwenberghe, 1999). No significant accelerated sea-level rise has so far been measured, although compared to earlier
centuries, a definite acceleration trend has been observed (WL, 2000). In Antwerp, tide measurements of the river
Scheldt are recorded and analysed for the period between 1891 to 1990. During this period, a 0.56 m increase in MHW,
a 0.24 m decrease in MLW and a 0.8 m increase in TA have been observed. Further, an increase in the number of storm
tides has been recorded. It should be mentioned, however, that these trends are the primarily the consequence of rising
MHW and TA-change. Changes in MLW and MHW are in great part due to dredging activities. The higher peak
discharge recorded for the river Scheldt is partly caused by growing urbanisation.
The very nature of the low-lying polders is cause for concern. Most of this area has a clay-peat composition the surface
of which can be significantly lowered with over pumping and drainage measures. Drainage also influences the flow and
the distribution of fresh, brackish and salt water in coastal aquifers. An ongoing study is focussing on the impact of sea-
level rise on fresh-salt water distribution. Sea-level rise is expected to indirectly result in diminishing fresh water
supplies. Fresh groundwater extraction works within the wide dune belt located west of Nieuwpoort are already
threatened by saltwater intrusion. Northeast of Nieuwpoort, saltwater seepage will increase in the polder areas behind
the narrower dune belt. In a very recent study (Schoeters and Vanhaecke, 1999 in WL, 2000), the effects of a doubling
of the CO2-content when no policy measures were taken, lead to the following conclusions for Belgium. The mean air
temperature will rise by 2 to 2.5 ° C, on average the minimum temperature will rise more than the maximum
temperature and the temperature will rise more in winter than in summer. Storm frequency may rise by about 30%.
Rainfall will decrease by about 3% or remain the same during summer, whereas in winter it is expected to increase by
10%. Projected regional sea-level rise is thought to be of the order of 40 to 70 cm by 2100.
The way forward to respond to sea-level rise
The Waterways and Maritime Affairs Administration presently includes a 60 cm sea-level rise by the next century
when planning future coastal protection structures. As indicated above strengthening coastal defence structures is a
very urgent matter for some cities. Ostende is such an example. There, current proposals suggest the creation of a
backshore beach, which would relieve the current inadequate sea wall of most of its protective task. A general tendency
in coastal management is to use soft protection measures rather than hard protection measures. The latter has the
advantage that adjustments to changing circumstances can be implanted more easily.
References:
Allaert, G. 1996. The BelgianCoast: Signs of recent revival? Internal report, Faculty of Applied Science, Ghent
University.
MVG (Flanders Authority), 1993. Kust 2002. Deel 1: De zeewerende functie van de kust. Stand van zaken.
Voorbereidend rapport. Departement LIN, Administratie Waterinfrastructuur en Zeewezen, Bestuur Havens. Coast
2002, Part 1:The coastal defence function of the coast. Present situation. Preparational rapport. Department of
Natural Environment and Infrastructure, Waterways and Maritime Affairs Administration, harbour council (in
Dutch).
Van Cauwenberghe, C. 1999. Relative sea-level rise along the Belgian coast: Analyses and conclusions with respect
to the high water, the mean sea and the low water levels. Infrastructuur in het Leefmilieu 3/99, p 513-537.
WL (Flanders Hydraulics), 2000. Effecten van een mogelijke klimaatverandering op het zeespiegelniveau, de
rivierafvoer en de frequentie van hoogwaters en stormen. Literatuurstudie. Effects of a possible climate change on
the sea-level, river discharge and frequency of high water and storms. Literature study.


Implications of Accelerated Sea-Level Rise (ASLR) for Belgium

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