Modeling tidal sand wave recovery after dredging
G.H.P. (Geert) Campmans
1, Pieter C. Roos
1, Suzanne J.M.H. Hulscher
11
Water Engineering & Management, University of Twente,
g.h.p.campmans@utwente.nl
Tidal sand waves are large-scale seabed patterns that are observed in many tidally dominated coastal seas (Terwindt, 1971). The dynamical behavior of sand waves, i.e. they can grow and migrate, may pose a risk to navigation in shipping channels and to offshore structures, such as cables, pipelines and wind farms. Shipping channels are frequently surveyed and dredged, when necessary, to maintain a least navigable depth. The goal of our research is to better understand the behavior of sand waves after dredging. Questions that arise are: How fast do sand waves recover after dredging? Which is more efficient: frequently dredging little amounts or less frequently dredging large amounts? Can modelled sand wave recovery be used to reduce the bathymetric survey frequency, while guaranteeing save navigation? To address these questions a nonlinear sand wave model (Campmans et al., 2017) is used to investigate sand wave recovery. The model is capable to simulate the morphodynamic evolution of sand waves in their small amplitude stage of formation as well the finite amplitude regime towards equilibrium sand wave profiles. For given environmental conditions, such as tidal flow and mean water depth, a model run results in an equilibrium sand wave profile. The obtained sand wave profile is then disturbed by various dredging interventions. The response of the sea bed topography is monitored to analyzed the effect of the various dredging interventions. In the Figure below the equilibrium sand wave profile, and various – in this example sand conserved – interventions (a) and their crest (b) and trough (c) elevation response. The black line is the sand wave profile in equilibrium.
These results show for each of the interventions the sand wave recovers to its equilibrium profile. Removing larger volumes of sand from the crest and dumping it in the trough results in longer recovery times. Per volume of displaced sand larger dredging interventions are more efficient. This is because the amplitude of the dominant wavelength is reduced most effectively. Therefore to slow down recovery of sand waves it is recommended to leave the seabed as flat as possible after dredging.
Acknowledgment
This work is part of the research programme SMARTSEA with project number 13275, which is (partly) financed by the Netherlands Organisation for Scientific Research (NWO).
References
Campmans, G.H.P., P.C. Roos, H.J. de Vriend & S.J.M.H. Hulscher (2017), The influence of storms on finite amplitude sand wave dynamics: an idealized nonlinear model, ISSF2017, Eindhoven, Netherlands, 26-28 June 2017.
