Design of a groundwater-based Model Predictive Control algorithm for the operation of water table control systems

Abstract

Rapid discharge of excess water has always been the core of Dutch water management. Due to climate change, a change in strategy is required. A future-proof drainage strategy consists of three steps: 1) retention, 2) storage, and 3) controlled removal. This thesis comprises the design and evaluation of an algorithm for groundwater-based Model Predictive Control (MPC) of water table control systems in polder areas. The objective is to maximize retention of precipitation, thereby contributing to the first component of the new drainage strategy. The control algorithm is developed for water table control on a single plot surrounded by a ditch, and three variants of water table control systems are considered: 1) water table control without drains, 2) water table control with submerged drains, controlled by ditch water level, and 3) water table control with submerged drains, controlled by sump water level. As a first approach, a relatively high, fixed setpoint is applied to maximize retention of precipitation. The controller adjusts the crest level of a weir that manages the ditch or sump water level. The algorithm performance is assessed through model simulations for two case study areas with distinct geohydrological response times within the management area of Hoogheemraadschap van Rijnland: 1) Polder Het Langeveld, a sandy polder located in the Flower Bulb Region, and 2) Polder Vierambacht, a clayey polder located in the Green Heart. Groundwater-based MPC anticipates precipitation events. Since the forecast horizon for which accurate precipitation forecasts are available is restricted, the feasibility of groundwater-based MPC is dependent on the response time of the groundwater system. Systems with a fast groundwater response are not suitable for water retention, while proper groundwater table control is possible. Control of systems with long system response times is hard, but these systems are applicable for water storage. A trade-off between controllability and water storage should thus be made. For the applied weir settings and objective function, a system response time lower than 10 days is required for proper control. Direct control of groundwater levels could induce a certain groundwater storage or water retention, depending on the applied setpoint.