Talk How to evaluate potential ecological effects of dredging operations in French navigated rivers

Inland waterway navigation can be considered as a sustainable transport solution because of reduced CO2 emissions and its effect on terrestrial roads traffic. Even though it offers a competitive cost, it remains largely under-exploited in Europe.

However, inland water transport has also strong environmental impacts, starting with the impacts on the river itself due to engineering, waves and noises caused by navigation.

A substancial part of inland waterways networks, especially around barrages and locks, need to be regularly dredged to avoid sediment accumulation in the channel. These operations induce various rates of sediment extraction and resuspension, depending on the volume of the intervention site and the dredging method. The sediment resuspension can affect the ecosystem through the release of polluting susbtances potentially trapped, but also through the release of nutrients, the increase of turbidity and the decrease of oxygen concentration. Moreover, the deposition of resuspended material can alter the viability of local habitats, both for the primary producers and for the fauna. Thus, dredging presents risks at the same time for organisms living in the sediment, the water column and connected benthic habitats.

However, dredging operations often occur in heavily developed waterways with concentrated human activities, where the ecosystem is strongly modified by the engineering of the river bank and the river bed, the daily wave wash caused by barges and other anthropogenic perturbations which commonly take place in these watersheds. The local habitats and organisms communities might therefore be adapted to high level of pressure and be resilient to dredging episodes.

Dredging operations in French rivers are regulated and controlled by authorities, especially in the WFD (EU Water Framework Directive) context and submitted to impact analysis. Surprisingly, the tangible effect of dredging operations on aquatic ecosystems is poorly documented. Whereas some physical and chemical parameters are clearly mentioned in monitoring programs, the “study of biological compartment” remains unclear and there is a need of precise methods and indicators to investigate this question. Moreover, the level of field survey campaign in time and space should be adapted to the dredging operation typology.

We experimented a standard protocol of macroinvertebrate community sampling upstream and downstream a bucket dredging operation on the Oise river, located in Janville (Nord-Pas-de-Calais-Picardie Region). Uptsream and downstream sites were sampled before and after this operation, and combined to physical and chemical parameters (%O2, Suspended material, turbidity, conductivity, Nitrogen, pH, dissolved Fe2+) measured during the operation.

In this study we discuss the relevance of various indicators and parameters to assess the environmental consequences of dredging operations. Our results suggest that the local benthic communities are lightly affected by dredging. We also highlight that ecological monitoring of such activities should keep having a flexible methodology to be adapted to local feature of the river and the dredging typology.

Sediment, inland water dredging, macroinvertebrate communities,water transport