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Andy Van Kerckvoorde* & Sophie Vermeersch – INBO*andy.vankerckvoorde@inbo.be

Constructed marginal shallow water zones as ecological restoration tool for navigable waterways: a case study along a canal in Flanders (Belgium)IntroductionBanks of navigable canals are often stabilized with “hard” materials such as concrete revetments or metal sheet-piles. As a result, riparian habitats along navigable canals are small with an artificially sharp transition between water and land. In this way, conditions for marginal riparian vegetation along navigable canals are unsuitable. In addition, hydraulic forces induced by moving vessels cause difficulties for vegetation establishment or hamper vegetation growth. Because of ecological consciousness there is, nowadays, a growing practice to use ecologically friendly bank stabilisation methods attempting to restore, rehabilitate, enhance and protect the riparian zone and its physical, chemical and biological functions.

DiscussionThe constructed marginal shallow water zones along the canal Ghent-Bruges clearly provide appropriate conditions for helophyte vegetation. This indicates that the defence dam inhibits hydraulic forces. In contrast, a helophyte vegetation is sparsely developed along the canal Ghent-Bruges in banks without shallow water zones.In plots from the defence dam side, cover by herbs had lower values compared with plots form the canal bank side. A possible explanation is a difference in construction materials. The defence dams are made by riprap covered with mastic asphalt and the canal banks by rock-filled gabion baskets. In contrast with gabion baskets, mastic asphalt as cover limits interstitial spaces. The presence of interstitial spaces helps vegetation colonization and succession may occur faster. Indeed, the observed higher competitiveness and lower ruderality in plots on the canal bank side compared with plots on the defence dam side are in agreement. The shallow water zones along the canal Ghent-Bruges are of little importance for rooted aquatic plant species. Possible reason are unfavorable abiotic conditions because a thick (>10 cm) and soft substrate was observed at the bottom of the shallow water zone.

Concluding remarksA constructed marginal shallow water zone can enhance biodiversity along navigable canals. Studies like this increase our knowledge on the effectiveness of measures to mitigate negative ecological effects associated with navigation. Such bank rehabilitation measures may be necessary to achieve the ecological goals of the European Water Framework Directive.

Further reading:Van Kerckvoorde A., Verschelde P., Vanderhaeghe F., Raman M., & Vermeersch S., (2013). Constructed marginal shallow water zones along a navigable canal: possibilities and constraints for helophyte and aquatic vegetation. Annales de Limnologie - International Journal of Limnology 49: 51–63.

ResultsIn the studied shallow water zones riparian vegetation developed on the sides (mean cover for 2013: 70 %) and in the middle (mean cover for 2013: 19 %). Only Callitriche sp. was found as rooted aquatic taxon; the taxon was scarcely observed at the sides (mean cover for 2013: 0,2 %). Differences in number of taxa, cover of herbs, Grime’s competitiveness and ruderality were observed between the canal bank and the defence dam (figure 2).

Study areaThis case study focuses on constructed (in 1998) marginal shallow water zones along the navigable canal Ghent-Bruges (figure 1). At the northern bank a shallow water zone with a length of 700 m was established; at the southern bank the shallow water zone has a length of 300 m. The shallow water zone has been constructed by building a defence dam parallel and in front of the actual canal bank. The shallow water zone has a width of 4,1 m and a depth of maximum 50 cm, both at normal water level. The shallow zones are connected to the main channel by means of 1 m wide openings in the defence dam (4 in the northern bank; 2 in the southern bank). No planting or seeding was performed.

MethodsIn order to evaluate plant vegetation development, its spatial variation and its succession, sixty 10 m x 1 m vegetation plots on the canal bank side, on the defence dam side and in the middle of the shallow water zone, were recorded in 2006, 2009 and 2013 (figure 1).

Figure 1. Location of the canal Ghent-Bruges in Belgium, the constructed marginal shallow water zones and the 4 survey locations with plots examined. Arrows show normal water flow direction. The hand indicates the place and direction of the photograph. The lines on the photograph illustrate positions of vegetation sampling at canal bank side (two left lines), defence dam side (two right lines) and at the middle (two centre lines) of the shallow water zone.

Figure 2. Boxplot for the number of taxa, cover by herbs, Grime’s competitiveness and ruderality in the canal bank or defence dam per year of investigation (2006, 2009 or 2013). # indicates significant (t-test; P < 0,05) difference between canal bank and defence dam within same year.

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