Tatsuya MARUYAMA
Fundamental study on groundwater recharge of rainwater using wells for snow melting facilities
supervisor Hirofumi TOYOTA
Recently, local severe rain, which is heavy rainfall in a short time in the limited region, has happened frequently. Although a clear definition of the local severe rain is not in the meteorology, the case to exceed 50 mm per hour rainfall in area from ten to several ten km is applied as the local severe rain. A disaster induced by the local severe rain domestically occurs every year for six years from 2003 to the present, and the number goes up to eight times.
The 7.13 flood damage (Niigata and Fukushima heavy rainfall) occurred in July 2004 is still fresh in our memory as the local severe rain of Niigata prefecture. It heavily poured from the night of 12 July in Chuetsu area of Niigata prefecture and Aizu city of Fukushima prefecture, and the total amount of rainfall exceeded 400 mm in former Tochio city (present Nagaoka city) and former Shitada village (present Sanjo city). The flood damage has frequently happened in Mitsuke city because the rainfall is too much to exclude its rainwater to the river etc. using drainage in recent urbanised town.
The personal storage tank of rainwater is also promoted by the subsidy in Mitsuke ciity besides equipping with the rainwater storage sewer. Moreover, snow melting pipes, which is an effective technique to melt snow on the roads, is widespread because Mitsuke city is a heavy snowfall area. There are 388 wells for snow melting facilities in city and prefectural roads, and the number of wells exceeds 1,000 by including private wells. It is problem that the decrease of underground water and the ground subsidence, etc. are induced by a large amount of drawing of underground water, although the snow melting pipes are effective as the snow and frozen road control.
Considering that situation, ¡Ègroundwater recharge of rain water using wells for snow melting facilities¡É is proposed as a environmental measure against heavy rainfall, which is low-cost and also effective for prevention of ground subsidence.
The results obtained from the study is shown below.
1) The wells for snow melting facilities had enough injection ability as the measure against heavy rainfall, there was not large difference in permeability coefficient among wells and the direction of infiltration.
2) When the method for pumping test (recovery method) was used, a coefficient of permeability calculated was bigger than other methods. It seems that wells, which is near the injection well, should be used for monitoring the ground water level. (It was more than 100 m away this time).
3) The good quality of injection water is necessary because the groundwater in Mitsuke city has the same revel of quality as the standard of water-supply.
4) In addition, the behaviour of the groundwater level should be observed to estimate the effectiveness of this method in the experiment where water injects into several adjacent wells.
As the results, groundwater recharge of rain water using wells for snow melting facilities is effective for the measure of heavy rainfall and ground subsidence, although the problems still remain in the quality of injection water and the method of injection.
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