A Study of New Alert Information on Storm Surge and Inland Flood

The Flood Control Act amended in 2015 stipulates that prefectural and municipal governments set the “inland flood danger water level” in sewer systems prone to flood risks that may cause severe damage to underground facilities such as shopping malls, and issue “inland flood danger information.” This article examines the characteristics of the “inland flood warning” from the viewpoint of emergency communication. The findings include the following.- “Inland flood danger information” is issued when the water height in a designated sewer pipe reaches the “inland flood danger water level”. Each level is set according to the expected lead time needed for the transmission of information and evacuation during water rising. The Ministry of Land, Infrastructure, Transport and Tourism (MLIT) gives examples of the lead times for local governments. These lead times are much shorter than the expected lead times set for flood forecasting for large rivers and water-level notification for mid-size rivers, as well as for newly specifying the “storm surge/flood danger water level” in each designated coastal region.- The lead times set for the “inland flood danger water level” are inevitably shorter mainly due to the high speed of water rising in sewer pipes. As sewer pipes have smaller capacity to hold water than the rivers specified for the “flood forecast,” the water levels in the pipes are prone to surge in a short amount of time at the time of heavy rain. If the lead times were set longer, the “inland flood danger water level” would become lower and local governments would have to issue the “inland flood danger information” many times.- The lead times set for the “inland flood danger information” are shorter, therefore the information is issued when, for example, an underground shopping mall faces with an imminent risk of inundation caused by the sewer overflowing. The hazardous level of this information is set between the “heavy rain (inundation) warning” and the “heavy rain (inundation) emergency warning.” This shows the urgency level of the “inland flood warning” is very high.