Water Infiltration During Rainy Weather Multifunctional Manhole Covers as A Tool To Analyze

(1) Water infiltration during rainy weather Most of the Tama area of Tokyo (Figure 1) is served by regional sewerage systems (Figure 2), of which about 80% are separated sewerage systems. During Typhoon Hagibis in 2019, which saw 24-hour rainfall of more than 300 mm in the entire Tama area, a large amount of rainwater flowed into the sewage pipes of the public sewerage system managed by the municipalities, causing overflow from the sewage trunk line of the regional sewerage system managed by the Tokyo Metropolitan Government. This caused major damage such as flooding and flooded roads (Photo 1). The causes of water infiltration during rainy weather are presumed to be diverse, such as misconnection of rainwater drainage system in houses to the sanitary sewer, inflows of large amounts of rainwater from outdoor sinks (Photo 2) in parks, and damaged aging pipes and catch basin. The efforts of municipalities, which are responsible for the maintenance and management of public sewerage pipes and the instruction on private drainage systems, are essential in identifying the causes and taking countermeasures. However, it takes time to identify the causes of rainwater infiltration because the area where rainwater is expected to infiltrate into sewer pipes is vast. The Tokyo Metropolitan Government has so far conducted a survey to narrow down the areas where large amounts of water infiltrate into the public sewerage system during rainy weather, and technically supported municipalities with water infiltration countermeasures. This survey requires manual installation of water level gauges and other equipment and collection of data, which is costly, and it is impossible to collect data for a long period of time. For this study, a new multifunctional manhole cover and water level gauge were installed, and an initiative to enable continuous water level observation was started.
(2) Multifunctional manhole covers The Regional Sewerage Office began installing multifunctional manhole covers in 2020 as a countermeasure against water infiltration during rainy weather. The multifunctional manhole covers were developed through joint research by Tokyo Metropolitan Sewerage Service Corporation, Meidensha Corporation, and Hinode, Ltd. A communication device (LTE) and battery are built into the manhole cover, and it is possible to measure the water level and other data in the sewerage pipe in real time by connecting it to a water level gauge or other measuring devices (Photo 3) (Photo 4) (Figure 3).　 One of the data reflects the radar rainfall information (XRAIN) provided by the Ministry of Land, Infrastructure, Transport and Tourism. The effect of rainfall on sewage inflow can be checked by simultaneously visualizing rainfall and water levels. The measurement results are stored in a cloud server and can be checked at any time, anywhere using the internet. (Figure 4). In addition, the covers have the strength, durability, and water resistance to be used on public roads, and can be installed simply by replacing existing manhole covers without the need for separate electric wiring, or service wire and communication cable work.
(3) Purpose and effect of using multifunctional manhole covers 　By installing multifunctional manhole covers and continuously observing water levels in trunk lines, more data can be obtained than with conventional surveys, and it is expected that it will be possible to verify seasonal and annual changes and the effectiveness of countermeasures. 　The Tokyo Metropolitan Government installed 37 multifunctional manhole covers at connection points between regional sewerage system trunk lines and public sewerage pipes, at the boundary points of municipalities, and at locations where overflows have occurred in the past in order to narrow down the sources of water infiltration during rainy weather (Figure 5). After a certain period of time, we compared the results of the measurements under sunny and rainy conditions, and found that there were areas where the difference in flow rates between the two conditions was large (Figure 6) and areas where the difference in flow rates was small. (Figure 7) 　From these results, we were able to conclude that the drainage areas with large flow differences have more water infiltration during rainy weather, while the drainage areas with small flow differences have less water infiltration. These results were similar to those of a previous survey. It is possible to identify areas with a high percentage of water infiltration during rainy weather and take effective countermeasures. In addition, the relationship between rainfall and water levels / flow rates was checked in the areas where water infiltration during rainfall was determined to be high, and it was found that there were cases where the water levels / flow rates were linked to rainfall (Figure 8) and cases where they were unlinked (Figure 9). This enables the estimation of whether water infiltration is caused by direct infiltration due to misconnections, or indirect infiltration due to damage to drainage pipes caused by a rise in groundwater levels, and the understanding of the characteristics of water infiltration during rainy weather, which is expected to lead to the prioritization of countermeasures. In December 2020, a meeting was held with municipalities to promote countermeasures against water infiltration during rainy weather to provide information on the results obtained from the multifunctional manhole covers, and to request prompt detailed surveys and countermeasure construction (Photo 5).
(4) Future initiatives The measurement results will be accumulated to further narrow down the source of water infiltration during rainy weather. In addition, because it is possible to verify the effectiveness of the countermeasures by comparing the situation before and after measures against water infiltration, these results will be provided to municipalities for use in countermeasure promotion. We will also consider using the ability to measure water quality as well as water levels in real time for water reclamation center operation management and water quality monitoring. In order to contribute to the safety and security of the Tama area, the Regional Sewerage Office will continue to make further use of multifunctional manhole covers to create efficient sewerage projects.