Separation of fresh and brackish water in polder water canals

Abstract

Polders water canals are sometimes exposed to brackish seepage from the nearby saline sea (salinization). This is a problem because the canal water is used to irrigate crops. A characteristic of water is that less dense (fresh) water floats on top of dense (saline) water. In lakes and oceans, it is common that water layers with various densities flow on top of each other (stratification). However, this has not been observed yet on a small scale, such as in polder water canals. It is assumed that, with a traditional weir, fresh water is discharged, and saline water stays behind. To encounter this, a modified weir is developed. It uses an underflow gate in front of the traditional weir to discharge the bottom water out of the canal. This research aimed to investigate if stratification happens in polder water canals and to explore how a modified weir can be implemented. The Negenboerenpolder in Groningen (the Netherlands) served as an example. Multiple methods are used because of the broad and practical nature of the problem. First, field measurements in the Negenboerenpolder demonstrated stratification in polder water canals. At multiple locations in the polder, the Electrical Conductivity (EC) is measured over the depth or continuously measured at two depths. Secondly, flume experiments gave information about the experimental procedure and implementation of a modified weir. Thirdly, 2D simulations of a modified weir are done in ANSYS Fluent, a Computation Fluid Dynamics (CFD) software. The simulations suggest that a modified weir works as expected, but the model is not completely representative. Observations are that such a weir is more effective in discharging brackish water with a low flow velocity and a larger distance between the underflow gate and the weir. Implementation advice for a modified weir is also included in this research. To conclude, this research implies that a modified weir can be used to discharge brackish water, but the design and implementation need to be done carefully. Recommendations for future research are to experiment with a modified weir in a polder and to build a 3D CFD model to investigate the flow around the underflow gate.