Recently, natural disasters frequently exceed design standards and thus are unpredictable. In fact, as people don’t have enough time to prepare for them, massive human and financial losses are inevitable. As rivers connected to the downstream of a reservoir may exert direct influence on the safety of people who live near the river, it may be deemed very important to get and use information for early response through an analysis of abnormal rainfall and flood that occur upstream a reservoir or downstream a river.
The study has performed an analysis to ensure that managers of facilities, specific local governments, and residents adopt standards for preparing for disasters by analyzing the direct effects on the upstream rivers exerted by the water levels that change (rise) when PMF that has occurred upstream the reservoir flows into the reservoir and overflows into spillway, instead of calculating the downstream areas damaged with flood caused by a possible collapse of a dam, figuring out how to evacuate residents, and calculating the size of the damage.
The study predicts that tapping into the relationship between the overflowing water level (the amount of discharge) from the reservoir upstream the river and the change in the downstream water level could estimate the possible effect on the downstream of the reservoir as the overflowing water level and use it as an indicator in responding to disasters.
If ‘attention level (Blue)’, the early one of emergency management stages, is defined as “from the issuance of special weather alert to the water level that comes at 10% of the planned overflowing water level from flood in the reservoir spillway”, it is deemed possible to alert residents in downstream areas even before an emergency occurs in the reservoir.
The study also shows that if ‘caution level (Yellow)’, which predicts continuous rise of overflowing water level in the spillway from continuous rainfall, is defined specifically as “a water level that comes at 10% to 100% of a planned flood level in the reservoir spillway due to a massive flood”, it can be helpful in implementing necessary disaster responses such as prompting evacuation of residents from the vulnerable downstream areas or ordering their evacuation.
The results of this study are expected to contribute to implementing a disaster countermeasure system that can be applied to both downstream rivers and reservoirs by using the overflow water depth of reservoir spillways focused on disaster countermeasures at reservoirs.