Reinforcement measures are applied by water boards during critical conditions in an ultimate attempt to prevent flooding. However, the effect of these measures on the reduction in flood risk or failure probability of the flood defence is often unknown. This research focusses on the effectiveness of grass revetment reinforcement measures applied during critical conditions at the outer slope of river dikes. Reinforcement measures applied during critical conditions are characterised by time constraints and human errors. The effect of the reinforcement measure on the reliability of the flood defence is studied. A probabilistic model is developed to assess the failure probability of the reinforcement measure and the time needed to install the measure. A model named OPSCHEP-model is used to quantify the failure probability due to human action. The effect of the measures on the failure probability of the flood defence is calculated by a crude Monte Carlo simulation. It is found that the reinforcement measures are most effective for known weak spots in the flood defence. Weak spots that are to be found during emergency dike inspection result in a much lower reduction in the failure probability of the flood defence. The level of preparedness of the water board organization largely determines how effective the measures are in reducing the failure probability.

As population levels protected by large dams have risen in the last decades, accurately assessing flood hazard and risk will become increasingly important for developing and sustaining flood mitigation policies. Recent research has shown that the degree of control and protection offered by retaining structures largely depend on their operation strategies. As such, now, more than ever, it is crucial to be prepared to operate a reservoir under extreme hydrological circumstances, when the consequences of an operational error could be very serious. However, the operation schedules currently use in the engineering practice lack on mechanisms for evaluating and balancing the potential risks associated with storage and release decisions. This results in reservoir flood control strategies that, from the perspective of flood risk mitigation, are distant from the optimal. This graduation project aims to improve existing methods for developing emergency operation schedules by including the concept of risk into the optimization of flood control operations. To address this topic, the research is divided into: (1) development of a methodology for including reservoir operation effects in the risk analysis of dam-reservoir systems; (2) combination of the risk analysis procedure with an optimization algorithm to devise optimal risk-based emergency operation schedules.

The thesis culminates with a case study in which the emergency operations are optimized for the Barker Reservoir system in Houston, Texas. Susceptible to Hurricanes like Hurricane Harvey (2017) and intense precipitation events such as Tax Day (2016), the Barker system presents an operational dilemma requiring trade-offs between released flows and stored volumes. Using the methods developed in this thesis, the flood risk analysis shows that a change in the operational strategy would contribute greatly to reducing the total risk of the system. Under extreme hydrological events, an operation strategy with releases starting at the first stages of the flood event display a reduction of almost a 32% on the total risk of the system as compared with the current operation strategy, including a 40% decrease on the risk associated with the structural failure of the dam. The new operating policy, however, increases the frequency of downstream damages during non-structural low frequency failure scenarios. Therefore, an increase of the downstream channel capacity along Buffalo Bayou and adequate measures to strengthen the dam are further recommended to reduce downstream damaging flooding and diminish the failure probabilities of the structure.