Sediment transport and underwater light climate affected by flexible aquatic vegetation.

---

Friday 3 july 2015

11:30 - 11:45h at North America (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 15F. Environment - Ecohydraulic

---

The ecosystem service of underwater vegetation to benefit coastal protection is increasingly recognized. However, the reconfiguration of these usually flexible plants makes modelling hydrodynamic modelling cumbersome. Flow and wave reduction are often modelled by assuming that vegetation increases bottom roughness, but this is not realistic as sediment dynamics would likely increase under that assumption. To avoid that, the actual flow reduction by the vegetation at the water-sediment interface needs to be taken into account. By combining a plant motion model (Dynveg) with a hydrodynamic and morphological model (Delft3D) and validating it in a real life situation, we succeeded in adequately simulating flow and sediment processes under influence of aquatiq vegetation. By evaluating scenarios with vegetation types that differ in density and flexibility, the model showed that suspended sediment transport was less affected by vegetation than bed load transport. Nevertheless, in all vegetated scenarios, the sediment concentration was effectively reduced, which resulted in increased water clarity. Because light limitation is probably the limiting factor for seagrass occurence in deeper waters, we thus showed that, at a landscape scale, the presence of vegetation can extend the lower limit of occurrence. The model also shows that seagrass is more effective in preventing erosion than in reducing sediment transport. The effects of vegetation on sediment transports are likely more pronounced in shallower areas where plants occupy a larger part of the water column, or in more turbid water with higher suspended sediment concentrations.