Publication of Rainwater (Internal Water) Flooding Assumption Area Map

Overview

　

　In recent years, flooding damage has increased due to localized heavy rain, and while hard measures against rainwater are being implemented, it is also important to minimize damage through soft measures such as publishing and disseminating flood assumption information.

　Against this background, with the aim of enhancing evacuation systems for internal water during heavy rain and raising disaster prevention awareness, a simulation of flooding conditions in the event of internal water flooding was conducted for the entire area of Inagi City, and an internal water flooding assumption area map was created.

About the Internal Water Flooding Assumption Area Map

　This map shows the extent and depth of flooding when internal water flooding occurs, exceeding the drainage capacity of existing sewerage drainage facilities and waterways during heavy rain. This time, it was created assuming the occurrence of internal water flooding without considering external water flooding from rivers.

　This internal water flooding assumption area map has been created based on the "Internal Water Flooding Assumption Area Map Creation Manual (Draft) (July 2021)" overseen by the Ministry of Land, Infrastructure, Transport and Tourism. Please note that the internal water flooding assumption area map formulated this time does not designate the rainwater discharge flooding assumption area based on the provisions of Article 14-2, Paragraph 2 of the Flood Control Act.

　Going forward, the results of the internal water flooding assumption area map will be reflected and utilized in the formulation of the comprehensive rainwater management plan and the update of the Inagi Disaster Prevention Map.

Conditions for Flooding Simulation

• Target Area: Entire Planning Area of Public Sewerage (Rainwater) (1,746ha)
• Target Rainfall: Expected Maximum Scale Rainfall (153mm per hour)

Other prerequisites

• The internal water flooding calculation is performed by dividing the target area into 10m grids. Additionally, the ground elevation of the 10m grids uses data published by the Geospatial Information Authority of Japan as of the end of February in the 7th year of Reiwa (2025), with the update date ranging from October 1, 2016, to January 30, 2019. Therefore, there may be cases where the effects of detailed topography are not accurately represented.
• The maintenance status of the sewerage system and data on waterways, etc., use data as of March 2023.
• When conducting the simulation, flooding of rivers other than sewerage and waterways, overflow caused by rainfall exceeding the target rainfall of the simulation, and flooding caused by rivers are not taken into account; therefore, flooding may occur even in areas not designated as internal water flooding assumed areas.
• Changes in the terrain due to land development and the status of stormwater drainage facilities may result in variations in the locations and depths of flooding.

About Terms

Internal Water Flooding

A phenomenon where drainage facilities become unable to process rainwater during heavy rain, causing water to accumulate on the ground.

External Water Flooding

The phenomenon of river water levels rising, overflowing from levees, or breaching levees.

Comprehensive Rainwater Management Plan

A plan summarizing the basic matters of areas where flood control measures using sewerage should be implemented, the target development standards, and the basic policies for facility development when implementing flood control measures using sewerage.

Inagi Disaster Prevention Map

This document contains information related to disaster prevention, including evacuation center information by type of disaster, and preparations for earthquakes, wind and water disasters, aimed at improving disaster awareness and encouraging self-help and mutual assistance during emergencies.

Expected Maximum Rainfall

Rainfall with a probability of occurring less than or equal to 1/1,000 in one year

(1/1,000 means it occurs approximately once every thousand years.)