Systems reliability of flow control in dam safety

Abstract

The reliable performance of a spillway system depends on the many environmental and operational demand functions placed upon it by basin hydrology, the hydraulic conditions at reservoirs and dams, operating rules for the cascade of reservoirs in the basin, and the vagaries of human and natural factors such as operator interventions or natural disturbances such as ice and floating debris. These systems interact to control floods, condition flows, and filter high frequencies in the river discharge. Their function is to retain water volumes and to pass flows in a controlled way. A systems simulation approach is presented for grappling with these varied influences on flow-control systems in hydropower installations. The river system studied is a series of four power stations in northern Ontario. At the head of the cascade is a seasonally-varying inflow. The remaining three dams downstream have little storage capacity. Each has two vertical lift gates and all four structures have approximately the same spillway capacity. The problem is to conceptualize a systems engineering model for the operation of the dams, spillways, and other components; then to employ the model through stochastic simulation to investigate protocols for the safe operation of the spillway and flow control system. The reliable performance of dams and their appurtenant systems depends on the complex interactions of a large number of natural, engineered, and human systems. Historically, such systems have been designed for extreme loads, e.g., the largest flood or earthquake that might occur during some service life. But accidents and failures seldom derive from such loads: They more often occur through uncommon combinations of mishaps that are difficult or impossible to identify during design. The challenge is to develop analysis approaches that capture these systems effects. Emergent behaviors are patterns or regularities that arise through the interactions among system elements which themselves do not exhibit such properties. Adverse performance typically arises from unanticipated combinations of loadings and responses within the normal operating ranges of a dam’s performance. 1. SYSTEMS APPROACH TO DAM SAFETY The function of a flow-control system is to retain water volumes and to pass flows in a controlled way. Their reliability involves structural, mechanical, electrical, control systems and subsystems reliability, as well as human interactions, organization issues, policies and procedures, all of which, occurring in a broad spectrum of environmental conditions. 12 International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP12 Vancouver, Canada, July 12-15, 2015 2 Adverse performance typically arises from unanticipated combinations of loadings and responses within the normal operating ranges of a dam’s performance. That is, failures often do not occur under the extreme loadings considered in design, but under the uncommon combination of relatively common things. For example, instrumentation mis-performs so pool levels rise, simultaneously ice has formed on spillway gates, which then cannot be opened to accommodate discharges, and finally the operator misperceives the danger and an accident follows. A new approach is required to risk analysis for dam safety, as current engineering practices do not address the character of the most probable causes of accidents and failures. Combinations of unforeseen yet unfortunate events cannot be predicted easily—or maybe at all—ahead of time. They are emergent behaviors. In a review of the performance of spillways and operating equipment, McCann (2013) reports that exclusive of the failure of spillway structures themselves, the performance of hydraulic systems is important even during normal operations. These systems are affected by mal-operation, control system errors, and a host of interactions among factors. Figure 1. Figure 1: Causes of dam flow-control accidents