Now the conservation project of groundwater resources is very important one. How to conserve and maintain the groundwater resources is attained its o b ject by several ways. The way to let permeate rain water into the aquifer shallow-seated through by a big diameter well is the most general and reliable one. However, it is easy to be bring out a fatal trouble in clogging of void spaces underground, when instilation of rain water would be continued for a long time. The leading cause is invasion of air or gas and all kinds of dust or colloidal materials. So we had developed special filtration system which is able to prevent such obstructions. We have newly developed filtration system by our patent to let flow out through the filtration bed from the lower horizon to the upper horizon. Invasion of air or gas into underground could be almost p e rfectly prevent by a trifline contrivance. The River Shinkashi near Tokyo Metropolitan Districts have a troublesome hydrological weak point on its drainage capacities. Town Hole System is a series of advantage o us permeating facilities to instil and enclose superabundant rain water into the sandgravel formation underground. Town Hole System consists of newly developed receiving pit in Kangaroo-type, permeating well including the above-mentioned system inside and so on. We believe that Town Hole System could be use f u l of preventing inundation in the Shinkashi River Basin which is to be had by heavy rain following by Typhoon and at the same time to cultivate directly groundwater resources in Kanto Basin.
The purpose of the sprincling tests is to understand the infiltration mechanism by rainfall in thye western plateau of Mt. Aso. We had tests result that stationary infiltration capacity is 20-40 mm/hr. In analyzing these results, we understood that groundwater recharge amount is about 38%-45% (700 mm/hr - 800 mm/hr) of annual rainfall.
An experimental study of infiltration by means of pond has been done since June in 1984 in Rokugo-alluvial fan, Akita Prefecture. Groundwater of 83 lit. per minute was intermittently supplied to a pond (area of 1.8 × 1.8 m and 1.8 m in depth). Water level in the pond attained to a height of 146 cm in late April in 1986. At this time infiltration amount is ca. 36 cm per hour. The outline is seen by reference to Figs.1-4, Table 1 and Photo 1.
Land development changes runoff characteristics, and it usually makes floods larger. To deal with larger flood, flood control facilities must be constructed, when the river don't have capacity for larger discharge. One of flood control facilities is a storage facility of rainfall, retarding basin, and other is a infiltration facility of rainfall, drainaging to underground by infiltration. The former reduces pea k discharge, but cannot reduce runoff volume. And covering large area isn't effective from the developing point of view. The latter doesn't only reduce peak discharge for in f i ltration rate, but also reduces runoff volume and recharges groundwater. It makes effective land use possible because it is constructed under ground. But it isn't sure quantitative effect and its life. Yatabe experimental field was established in Tsukuba Science City to study runoff changes due to development and to evaluate infiltration facilities. This is a report on evaluation of infiltration facilities.
In Japan, urbanization on river basins is very rapid, especially in the three megalopolitan areas (Tokyo, Osaka, Nagoya). Along with the urgent river improvement works, run-off control facilities are needed in such basins. Run-off control facilities are classified into two types, detention type and infiltration one, according to its form and function. The history of run-off control faciliti es started from the storm water detention pond. Recently application of infiltration type facilities is increasing mainly in and around the Tokyo Megalopolitan area. The study on storm water infiltration facility has p r ogressed since the infiltration ability of the Kanto loam layer was proved larger than expected. It is expected that infiltration facilities such as infiltration pit, trench and permeable pavement are useful for onsite treatment of storm water. Comparing the detention-typ e facility with the infiltration-type one from the viewpoint of run-off control, the former makes the runoff peak delayed and flat, and the latter has a basic function to cut the bottom part of the run-off. Infiltration-type facility is thought to be useful not only for run-off control but also for recharging groundwater. Thus it has a role to restore the hydrological circulation weakened by urbanization. From the viewpoint of flood control plan, techn i c a l subjects on the storm water infiltration facility, which are described below still remain. (1) Simple survey method for defining the infiltration ability of the ground (2) Estimation method of the infiltration ability of infiltration facilities (3) The influence of the facility on surrounding environment (4) Estimation method of the run-off control effect Infiltration type run-off control facilities a re used on a small scale and dispersively.
Experiments of rainfall infiltration were carried out using water from storage tank. Water was sluiced into the infiltration measuring box (2 m × 3 m × 2 m) at the rate of 80l/min without interruption on the supposition that rainfall was received at the intensity of about 50 mm/hr on the roof with the area of 96.6 m2. On finishing the artificial supply, ponding depth was measured every 5 minutes until the water table came down to the bottom level of the infiltration measuring box. Using the experimental data of total infiltration depth and the time required, the average velocity of infiltration was calculated to be about 80 cm /m2 (1,920l/hr). Furthermore, the terminal infiltration capacity was determined to be 79.1 cm/hr/m2(1,898l/hr) for the first attempt and 72.6 cm/hr/m2 (1,742l/hr) for the sec ond. When the average velocity of infiltration is assumed to be constant, rainfall d uration needed to fill the infiltration measuring box with water is estimated to be ∞,∞,5.348,2.574 and 1.696 (hours) at the rainfall intensity of 10,20,30,40 and 50 (mm/hr)respectively and it takes 0-1.415 hours for the water to be fully drained. On the other hand, when the average velocity of infiltration is assumed to be variable, the computa tion gives such values for rainfall duration as 5.764,3.390,2.488,1.998 and 1.685 (hours)at each intensity and 1.5-6.0 hours are needed for draining the water. Comparison of these two cases reveals that infiltration effect becomes larger as rainfall intensity grows down. We can see the groundwater recharge effect in terms of groundwater rise in the observation well. There are some peaks of groundwater rise between 3 and 5 hours after starting the artificial rainfall supply. This, however, is based' on calculations using too large permeabilities (100-10-1 cm/sec) that further study is required.
This paper describes the experimental facilities for the quantitative analysis on the infiltration, which is constructed in the baseball field, University of Tokyo. Th is facilities consist of 10 blocks, each of 12 m × 21.5 m, with permeable pavement. These blocks have some arrangements of depth of permeable strata, number of drain pipes, with or without water tight sheets and so on. On the construction stage, many sensors and measurement devices have been installed in the basement and strata, which are rainfall gauge, lysimeters for the measurement of evaporation, water level sensors in the strata, tensiometers temperature sensors, pipes for drainage from the permeable strata and its measurement facilities. These hydrological informations have been collected every 4minutes, for 3 years, with self-made soft wares, controlled by the most popular personal computer and have been also used for the theoritical analysis. This paper introduces the results of the summary of the observation records and several remarks and also proposed new simulation models which are derived from the tremendous volume of data.