For many years external flooding hazards have been recognized as significant contributors to plant risk. However, it was not until the events at Fukushima that there was a concerted effort on the part of the utilities to reassess the plant external flood design basis, identify external flood vulnerabilities and take actions to address them. For many plants, resolution of low probability high consequence floods will likely be addressed by a combination of actions involving enhancements to flood protection and hazard mitigation strategies.
Over time, as plants decide on which strategies to apply there is an expectation that the most effective way to develop and justify these strategies will involve probabilistic risk assessment (PRA) concepts. The PRA framework is well suited for performing a human reliability analysis (HRA). Within that framework, HRA evaluations focus on operator and plant staff actions taken in response to plant initiating events (e.g., loss of offsite power, etc.). For many external floods, advance warning of an impending external flood event provides the trigger for pre-emptive manual actions to potentially reconfigure the plant through temporary installation of flood barriers. Unlike the post-initiator actions which tend to be more narrowly focused, these pre-emptive actions are taken in a less controlled environment, may be ad hoc, and may potentially be in competition with site investment protection activities, site evacuation, etc.
The purpose of this paper is to define the challenges in defining an approach for treating external flood actions, identifying external flood timelines, identifying the manual actions/organizational environment during external flooding scenarios and proposing an integrated strategy for quantifying those actions. The proposed quantification process is rooted in management science concepts for evaluating project reliability. The overall methodology identifies flood significant performance shaping factors, and identifies three (3) factors, namely time available for flood mitigation, proper access to plant site following flood and environmental factors, as having an overarching impact on the performance shaping factors affecting each of the flood mitigation tasks.
Analysis of urban flood using synthetic unit hydrograph (SUH) and flood mitigation strategies along way Halim River: a case study on Seroja street, Tanjung Senang District