Abstract
A significant increase in surface water salinization in low-lying deltas is expected globally due to saline groundwater exfiltration driven by rising sea levels and decreasing freshwater availability. Sustaining fresh water-dependent agriculture in such areas will entail an increased demand for fresh water flushing. Unfortunately, the flushing of surface water is not operationally optimised and results in excessive use of scarce freshwater. To meet the increased demand for flushing, while minimizing the need for diverted freshwater, new operational designs are required. This paper presents a novel network model based approach that uses De Saint Venant (SV) and Advection Dispersion (AD) equations to optimize multiple objectives on water level and salinity control using a Nonlinear Model Predictive Control (NMPC). The resulting NMPC problem is solved with a receding horizon implementation, where the nonlinear program (NLP) at each iteration is solved using state-of-the-art large scale interior point solver (IPOPT). We evaluate the performance of the proposed approach and compare it to the traditional fixed flushing for a representative Dutch polder. Firstly, the approach is shown to be capable of controlling the water level and salinity level in the polder. Secondly, the results highlight that the network of canals, which were originally made for drainage, could not be made sufficiently fresh with current intake capacity. A simple design approach was used to identify appropriate new capacities for two of the gates that allow optimal flushing to guarantee the required water level and salinity constraints.
Highlights
Polders are low-lying, artificially drained embanked lands sur rounded by storage canals (Fig. 1)
This paper presents a novel network model based approach that uses De Saint Venant (SV) and Advection Dispersion (AD) equations to optimize multiple objectives on water level and salinity control using a Nonlinear Model Predictive Control (NMPC)
Water storage canals are used for providing extra freshwater during dry periods to replenish precipitation deficits, and for creating storage space for the surplus water from polders during wet periods (Schoubroeck and Kool, 2010; Agricultural activities as well as the freshwater ecosystem in the polders are threatened by surface water salinization due to saline groundwater exfiltration De Louw et al, 2011; Raats, 2015)
Summary
Polders are low-lying, artificially drained embanked lands sur rounded by storage canals (Fig. 1). Feshwater use and pumping cost should be minimized The relation between these sub-objectives may be conflicting: addi tional freshwater from the intakes is necessary to satisfy the salinity level objective, which will result in increased usage of freshwater and pump flows. This may result in violations of water levels, resulting in a com plex multi-objective control problem. We consider a NMPC strategy that is based on the receding horizon principle It can optimize the predicted future system behaviour by solving a nonlinear program (NLP) on-line at each control time step Tavernini et al (2018)) and has been used for water systems in (Nederkoorn et al, 2012; Wang et al, 2017). We investigate the results and suggest an improve ment to the performance of the controller by upgrading the fresh water intake capacities of the polder
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