Land Deformation through the Development of Large-scale Housing Estates in the Hiroshima Area

Abstract

The purpose of this paper is to examine the extent of land deformation through the development of large-scale housing estates and the disasters that will be brought about by this land deformation.
In the Hiroshima area most new land needed as residential, agricultural or industrial land has been obtained by reclamation of the shallow bay in front of the Hiroshima delta after Hiroshima castle wae built in 1591. But since about 1960 there has been a change in these land uses, that is, large-scale housing estates have been developed in the granitic dissected pediments and hills around the Hiroshima delta.
The longitudinal lengths of pediments in this area are about 300-2000 meters, and inclinations are about 7-8 degrees in the lower-middle sections of the pediments and 12-15 degrees in the upper section. The inclinations of escarpments behind the pediments are about 26-30 degrees, occasionally 45 degrees. The height of the lower ends of these pediments is about 50 meters above sealevel. These pediments have been dissected by valleys to depths of 50-100 meters. As a result most of the original pediment surfaces have disappeared. It is supposed that the hill lands are dissected pediplains. Most of these dissected pediments and hill lands have been deeply weathered and are readily cut into by bulldozers.
The large-scale housing estates are reclaimed by the method of cutting spurs and filling valleys, taking into account a balance between the quantities of cut and fill. The mean cut depth of the spurs is about 20 meters.
The number and area of large-scale housing estates in this area are shown in Table 1. Large-scale housing estate development has been carried out here since 1960, but since the oil crisis development has slowed down. Data was obtained on the area and movement of earth in 15 housing estates in order to calculate the quantities of earth moved artificially for the development of large-scale housing estates in this area (Table 2). The data from which these volumes have been calculated are as follows : 1) The mean depth of cut earth is 10.02 meters; 2) The ratios of cut to fill are almost even both in area and in volume; 3) The area of large-scale housing estates developed or planned to be developed in this area is 3203.7 hectares. Thus the amount of earth moved or planned to be moved in this area is about 160, 000, 000 m³ (1, 600 ha × 10 m). The volume of moved earth per person living in this type of housing estate is estimated at about 500 m³ from a density rate of 100 persons per hectare. The population of Hiroshima area in 1979 was 1, 144, 455. The population growth in this area from 1960-1979 was 489, 250 and about 46% of this consists of people living in this type of housing estate.
In the Hiroshima area mudflows caused by heavy rainfall have occurred ten times since the beginning of this century. The total number of people killed by these mudflows is 2, 032. All these mudflows occurred in the escarpments behind the pediments, and most of the mudflows ran along the valleys that dissected the pediments. The valleys that have dissected the pediments in the large-scale housing estates are now filled up with artificially moved earth. So if mudflows occur in the escarpments behind the pediments in which large-scale housing estates have been developed the filled parts of the housing estates will suffer heavy damage.