Hydrodynamic modelling of habitat availability in the Common Meuse

Ecological standards for macrofauna are not met in the Common Meuse, a gravel bed river reach in the south of the Netherlands. The hypothesised cause is the lack of available suitable habitat, due to strong armouring of the riverbed, bank protection, pore blocking by mud and hydropeaking. Our aim was to estimate (1) the area extent of suitable habitat for five macrofauna target species with respect to flow velocity and water depth, (2) the amount of continuous suitable living area (including grain mobilisation) and (3) the speed with which species have to move to find new suitable habitat during a hydropeak. The Delft3D Flexible Mesh model was used to model water depth and flow velocity in the growing season (June-July) over the 36 km long river reach. The results showed that in less than 50 per cent of the main channel of the Common Meuse both suitable flow velocities and water depth occurred during the modelled growing season for four out of five targeted macrofauna species. Grain mobilisation during the 300 m s^-1 discharge conditions was limited to 4 percent of the main channel, and therefore expected to have had little effect on the amount of suitable living area during the growing season. For three out of five species, 75 percent of the time their speed was sufficient to relocate to new suitable habitat. Their speed was insufficient for the higher flow velocities. One of the species could only move fast enough in 50 percent of the cases. Ongoing work includes modelling of an entire hydrological year, grain size data analysis and inclusion of habitat connectivity, flow direction and coping mechanisms (seeking shelter, relocating) during unfavourable circumstance. Improved understanding of these aspects can help identify concrete morphological management measures that enhance biodiversity.