About the Dender basin

With resp. 32.830 and 18.448 inhabitants, Geraardsbergen and Lessines are two small cities along the Dender river. In 2010, both communes were severely hit by an extreme weather event, which affected larger parts of Belgium. Since Geraardsbergen is situated in the Flemish Region and Lessines in Wallonia, they form an interesting case for cross-regional comparison.

The Dender is part of the international Scheldt basin, but the catchment has its own distinctive features. The river has a length of 69 km, half of its basin lies on Flemish territory and the other half in Wallonia (CIW, 2008). Below is an image of the Dender basin.[1]


 Figure 1: Dender basin with indication of Geraardsbergen and Lessines. Source: Grenzeloze Schelde, 2014.

The main part of the basin is merely accessible for small, recreational vessels (max. 300 ton), but between Aalst and Dendermonde the river can be used for freight transport. As a consequence, the river is categorised as navigable and managed by navigable watercourse managers Waterwegen & Zeekanaal (Flanders) and DGO2 (Wallonia). Its tributary rivers, on the other hand, are controlled by several non-navigable water managers.

The Dender is known as an erratic river and is very sensitive to flooding. The river is highly influenced by precipitation; in dry periods its flow discharge can be very low but once a period of continuous rainfall occurs, it turns into Flanders’ fastest waterway (Milieuboot, 2010).

Since the floods of November 2010, actions have been taken in both parts of the basin in the form of immediate protection measures and longer-term prevention.


About the city Geraardsbergen and its flooding problems

Geraardsbergen is a small city in the south-west of the province of East-Flanders with approximately 32.800 inhabitants. It is located in the Flemish Dender basin, at the border with Wallonia. Apart from the city centre, Geraardsbergen exists of 15 submunicipalities.

In the last 20 years, Geraardsbergen has proven to be very vulnerable to floods. In November 2010, several submunicipalities of the city were flooded. Most significantly affected were Overboelare and Zandbergen, despite the fact that these submunicipalities had been protected by dike infrastructure after the floods of 2003. These were however, on the basis of past experiences, calculated on a flood with a maximum flow rate of 85 m³/s, whereas the November 2010 floods reached 130 m³/s as a result of intense rainfall and a saturated soil.

The November 2010 floods led to severe material damage. Insurance companies paid € 4.808.664,36 damage compensation for 398 flooded houses in Geraardsbergen (Assuralia, 2011).

 GeraardsbergenFigure 2: Helicopter view of the floods in November 2010 at provincial domain De Gavers (source: CIW (9 November 2011), Toetsing aandachtsgebied De Gavers (Geraardsbergen))


Changes after the 2010 floods

The calamity formed an eye-opener. Until then, the floods of 2003 had been pictured as an exceptional event, which would not occur again in the near future. The fact that not even 10 years later an even more severe flood hit the city thus came as a shock. As a result, the event led to some important changes and an increased awareness among citizens and politicians.

Where in the past for instance advises of water managers were sometimes passed by, the city of Geraardsbergen nowadays tries to be very strict in applying the water test on new building projects. The strictest advice is always followed. This however rarely leads to non-development of flood-prone areas. In most cases adaptive measures, such as a floodable cellar, are nonetheless imposed. Though the enforcement forms a barrier for the effectiveness of these measures. Respondents namely declare that a lack of staff makes it difficult or even impossible for the city to check on the implementation in the field of the conditions set forth by the water test.

The city has also invested in mobile dams and a coordinator and alderman for integrated water management were appointed. They have the responsibility to coordinate all flood related activities of the different departments of the municipal government. Furthermore, the subsidies for carrying out water preventive measures to a house and for placing a rainwater well were maintained, despite saving rounds within the administration. In terms of crisis response, an SMS warning service has been installed to warn citizens in time about approaching risks.

Nonetheless the good intentions and increased awareness, not all necessary changes have been carried out. For instance, plans for renovation of the sluice complex at the Dender exist, but their implementation is consistently delayed. The complex decision-making is hereby considered as the main cause of delay. Also the heritage of some spatial planning decisions of the past remains a problem. Solutions for the wide-scale legalised building in flood-prone areas are for instance considered as expensive and politically risky. New legislative tools, such as signal areas and land swap, could make these solutions however more affordable and acceptable in the future.




[1] Source: Integrated Water Policy Flemish Region,

Assuralia, ‘Meest getroffen gemeenten overstromingen 2010’,, 2011 (accessed at 12/11/2014).

CIW (2008). Het bekkenbeheerplan van het Denderbekken (2008-2013). Integraal waterbeleid in de praktijk. Erembodegem.

Milieuboot (2010). De Dender tussen Ath, Geraardsbergen en Aalst nader bekeken. Nieuwsbrief De Milieuboot 60. Aalst.