Formulation of the Comprehensive Rainwater Management Plan

Background

　To address flood damage caused by recent localized and intense rainfall as well as urbanization, it is necessary to implement measures from both hardware and software perspectives.

　In Inagi City, a Comprehensive Rainwater Management Plan has been formulated as a phased rainwater countermeasure plan that takes into account the impacts of future climate change.

What is the Comprehensive Rainwater Management Plan

　In implementing flood control measures through the sewerage system, this plan indicates the areas where flood control measures should be implemented in the short, medium, and long term, the target development standards, and the policies for facility development.

　By creating the Comprehensive Rainwater Management Plan, flood risk is assessed for each district, enabling planned sewerage development to be carried out starting with districts where the expected flood damage is greatest.

Response to the Request for Public Comments conducted in February 2024

Opinions

• There is no description regarding the flow volume burden on rivers leading to the Tama River, such as the Tama River, Misawa River, and Misawa River diversion channel, so please include it. Also, please describe the impacts of increases or decreases in flow volume burden (such as whether there is capacity or not).
• Please illustrate the route by which rainwater within Inagi City flows into the Tama River.

Response

　Thank you for your valuable opinions regarding this plan.

　The Sewerage Division's stormwater plan for Inagi City is basically designed to align with the river basin boundaries, assuming that rainwater generated by rainfall is properly drained into the rivers where it should naturally flow.

　On the other hand, in the current flat areas of the northern part of the city, there are many districts where sewerage rainwater pipes have not been developed, and there are locations where drainage is carried out through agricultural water channels and the like. For this reason, it is observed that some of the rainwater that should be discharged into the Tama River according to the plan is flowing into the Misawa River. In response to your comments, we have partially added the above current situation.

　In this comprehensive stormwater management plan, by directly discharging a portion of the stormwater currently flowing into the Misawa River into the Tama River, it contributes to reducing the flow volume of the Misawa River. We also consider that discharging stormwater from the planned drainage area into the Tama River poses no problem. This plan targets sewerage facilities under the jurisdiction of the city and organizes the basic concepts and development policies for stormwater drainage; it does not establish the flow allocation or river maintenance plans themselves for rivers managed by river administrators. Therefore, this plan does not quantitatively evaluate the increase or decrease in flow burden on the Tama River, Misawa River, or the Misawa River diversion channel.

　Regarding the evaluation of river flow and the examination of capacity, it is necessary to conduct analyses covering the entire watershed and ensure consistency with river maintenance plans. We believe that these considerations and adjustments will be carried out by the respective river managers under the jurisdiction of the national government, Tokyo Metropolitan Government, and Kanagawa Prefecture through watershed councils and other forums.

　Going forward, based on the development policies outlined in this plan, we intend to progressively advance the construction of sewerage rainwater pipes and pumping facilities while sharing information and coordinating with relevant organizations, aiming to ensure appropriate rainwater drainage across the entire watershed.

　Regarding the routes through which rainwater flows within Inagi City, for areas such as the New Town district where rainwater pipes have been installed, you can check the Sewerage Ledger on the city website or confirm it at the Sewerage Division counter (City Hall 3F). Additionally, for areas where rainwater pipes have not been installed, explanations about the routes can be provided at the Management Division counter (City Hall 3F), so we kindly ask you to visit the counter.