Control And Optimization Of Open Water Systems

Open diversion canal and urban sewer network are two important systems of open water systems. Open diversion canal systems plays a significant role in the irrigation, interbasin water transfer and river navigation. Developing boundary control of open diversion canals to keep the water depth in each canal reach at a stable level can improve the efficiency for water usage, ensuring appropriate water supply in time to avoid wasting water. However, open diversion canals not only have very complex structures but also have such uncertainties as rainfalls and variations of users'withdrawals, which will have negative effects on the stable operations of canals. How to design proper boundary control to guarantee the water depth to be stable has become the key point. Urban sewer network is responsible for improving the sanitary-hygienic conditions of a city with the development of economy and society. Research on the problem of combined sewer overflows (CSO) to eliminate or decrease CSO is significant for preventing water resources from pollution. Moreover, minimizing the electricity consumption of pumping stations in urban sewer networks is the effective way for energy savings. It decreases the system operating costs. Due to the properties of strong non-linearity and long time delay, it is difficulty to use traditional control approaches to achieve good control performance. Thus the control and optimization problems of open canals and urban sewer networks have attracted high attention of researchers in engineering as well as in theoretical fields.Open channel flow is well described by Saint-Venant equations. This dissertation investigates the boundary feedback control of open channels based on Saint-Venant equations. By means of Lyapunov approach and Riemann invariants, a unified design method of boundary feedback control are proposed for three types of open channels including single reach, multi-reach and star-configuration canals to guarantee their asymptotic stability. For large-scale urban sewer networks, a predictive control and decomposition-coordination algorithms to solve the problems of CSO and pump energy consumption are proposed, respectively. The main contents and contributions of this dissertation are summarized as follows.For an open-channel system made up of two reaches in cascade with rectangular cross section, the boundary feedback control laws with parameters are introduced. By use of the weighted Lyapunov function, the method for selecting these parameters is presented to guarantee the system to be asymptotically stable. Then the stabilizing boundary feedback control is obtained with the boundary water depths as feedback. This design method is generalized to multi-reach canals composed of N reaches in cascade. The weights in the Lyapunov function and the parameters in the control laws are designed independently so that the boundary feedback control laws are proposed.For a canal system composed by a single horizontal reach with any configuration cross section, by introducing Riemann invariants, the original system described by Saint-Venant equations is transformed to its corresponding equivalent system in terms of Riemann invariants. The boundary conditions are linearized or linearized secondly, based on which the boundary feedback control is derived. The advantage of this method lies in that the boundary conditions could be expressed by elementary functions in the boundary water depths. Furthermore, the idea of boundary feedback control design for a single reach is generalized to open-channel systems consisting of multi reaches in cascade and the stabilizing boundary feedback control laws are achieved. Finally, The parameters in the control laws are estimated.The star-configuration canals meeting at one multiple node are further investigated on the basis of the boundary feedback control design for single reach and multi-reach canals. A stability theorem for star-configuration canals is proposed. According to this theorem, combined with the boundary feedback control design for single reach and multi-reach canals, the boundary feedback control laws are achieved that only take the water depths at the gate boundaries as feedback. The control parameters are also estimated.The problem of combined sewer overflow of urban sewer networks is investigated. The element models are introduced so that the network model can be established and the optimization for minimizing overflows are formulated. A gradient-based solution algorithm based on the discrete maximum principle is developed. Predictive control with aggregation scheme is presented to decrease the computational complexity and impacts of the uncertainties of rainfall-runoff. The simulation results for a large-scale sewer network show that the aggregation-based predictive control algorithm is very promising for reducing the computation time and robust to tackle the uncertainties as well.In order to solve the problem of pump energy consumption in urban sewer networks, a steady-state flow model is established. The optimization formulation of the pump energy consumption is constructed. Considering the structural complexity of large-scale sewer networks, a decomposition-coordination algorithm and corresponding hierarchical structure are proposed based on the results of decomposed sub-networks by using the network community division approach. The simulation results indicate that the proposed algorithm is a useful approach for large-scale urban sewer networks.