Publication of Rainwater (Internal Water) Flooding Assumption Area Map

Overview

　

　In recent years, localized heavy rainfall has increased flooding damage, and it is important to implement hard measures against rainwater, as well as to minimize damage through soft measures such as publishing and disseminating flood risk information.

　Against this background, in order to enhance the evacuation system for internal flooding during heavy rain and to improve disaster prevention awareness, we have simulated the flooding situation in the event of internal flooding occurring throughout Inagi City and created a map of the expected internal flooding areas.

Map of Areas Expected to Experience Internal Water Flooding

　This is a diagram showing the inundation area and depth of flooding that occurs when the drainage capacity of existing sewerage facilities and waterways is exceeded during heavy rainfall, resulting in internal flooding. This time, it has been created assuming a scenario of internal flooding without considering external 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.

　In the future, we will reflect the results of the internal flooding assumption area map in the formulation of the comprehensive rainwater management plan and the update of the Inagi disaster prevention map, and utilize them.

Flood Simulation Conditions

• 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 status of sewerage development and data on waterways, etc. are based on data as of March 2023.
• When conducting the simulation, it does not take into account flooding from rivers other than sewerage and waterways, overflow from rainfall exceeding the target rainfall of the simulation, or flooding from rivers. Therefore, there may be flooding even in areas not designated as internal water flooding assumption 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

Inland Flooding

The phenomenon where drainage facilities are unable to process rainwater during heavy rainfall, causing water to accumulate on the ground

Flooding from External Water

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

Comprehensive Rainwater Management Plan

A plan that summarizes the basic matters of the areas where flood control measures using sewerage should be implemented, the target level of development, and the development policy of facilities.

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 that exceeds that scale has a probability of occurring in a year of less than 1/1,000.

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