Mixing of salinity by ship traffic in canals

Abstract

Salt intrusion poses a threat to the fresh water supply function of inland waterways. How far inland salt intrusion reaches is dependent on a balance of buoyancy forcing, water depth, discharge of the canal and the amount of mixing. Wind, bottom roughness, and sailing ships contribute to mixing. Currently, little is known about the contribution aof the latter to mixing. It is important to better understand the role of ship traffic in order to make more accurate models and take the right measures to protect the fresh water supply function of inland waterways. The flow field in an unstratified canal has been well studied, however, little is known about the effects that ships have in a stratified canal. Ships move water when sailing through a waterway. This induces three water movements: primary and secondary waves and the propeller jet flow. These processes can contribute to mixing by shear instabilities and internal wave breaking.A sailing ship in a stratified canal has been modelled using a moving grid approach in the 3D non-hydrostatic finite element model FinLab to study these effects in detail. The propeller has been neglected to simplify the model. A parameter study has been performed to observe the influence of several parameters (such as the canal blockage, internal Froude number, and canal bank slope) on the flow pattern and the amount of mixing. The amount of mixing is found to be in the order of magnitude of 1 percent for one vessel over 600m canal length a representative range of parameters of canal geometry, density profile, vessel draught and vessel speed. The actual effect will be larger since the internal wave field is still present at the outflow boundary and since the propeller has been omitted from the model set-up. Processes around the vessel and the internal waves contribute about equally to the amount of mixing in the model domain. The internal waves are estimated to have a contribution of about twice as large if the modelled domain was longer. As the interface comes up behind the vessel, it is likely that the propeller jet can have a large impact on the density field directly behind the vessel. The vessel speed, density profile, relative density difference, layer distribution in a two-layer flow, canal blockage, draught to top layer height ratio, and the slope of canal banks are important parameters in the amount of mixing generated. Due to the cumulative effect of the ship traffic, mixing by ship traffic is estimated to be of large importance on the density distribution in a canal. More research is needed to include this effect in large-scale numerical models and find a good parameterisation. How mixing by ship traffic could finally be implemented will be dependent on the numerical model that is used, and the amount of detail that is needed.