新潟県西蒲原地域における洪水の変化に関する一考察

抄録

Frequently innundated Nishikanbara Area in Niigata Prefecture changed into fertile rice fields as a result of many flood control works. The main works are Ohkohzu Floodway of the Shinano River, improvement works of other small rivers and national irrigation and drainage improvement works in Nishikanbara Area.
In June, 1978 when these works were almost completed, a large-scale inundation caused by poor drainage occurred in Nishikanbara Area. At that time the drainage system consisting of many pumps controlled by Nishikanbara Land Reclamation Union was almost completed. Total amount of precipitation was over the maximum estimated by the drainage system. But damage of the flood that inundated only rice fields was much slighter than that caused by the past flood of the Shinano River.
In addition to low and swampy parts that have been frequently inundated, the author found a few new types of irregular inundation spots.
The first type is caused by land subsidence resulting from pumping up ground water including natural gas. This type of inundation was found on the left bank side of the Nakanokuchi River, and total amount of subsidence from 1951 to 1975 was from 100 centimeters to 220 centimeters. Annual amount of subsidence became much less but at present subsidence continues.
The second type is found on natural levees and comparatively high altitude spots. This type of flooding spots are located at terminations of small-sized irrigation waterways.
The third type is found at back swamps. This type of spots are located along small-sized drainage waterways or at terminations of them. These spots are also adjacent to collective tableware factories removed from central part of Tsubame City on account of pollution and enlargement of business.
The reasons of the second and third type of floods are presumed as follows.
1. Maintenance works of small-sized irrigation or drainage waterways became slack because of increasing part-time farmers and decreasing full-time farmers. Most of part-time farmers work for factories or companies located in near cities where they can commute.
Waterways covering large or medium-sized area are maintained directly by Nishikanbara Land Reclamation Union. However small-sized waterways which benefit the area below 20 hectare have to be maintained by landowners.
Futhermore farmers comming from other areas to cultivate their farm land in Nishikanbara Area have increased in number. They sold their farm land near their residential site in rapidly urbanizing area for example in Niigata City, and purchased substitutive farm land in Nishikanbara where they can come to work easily by car in a short time. They tend to neglect maintenance of irrigation or drainage waterways, since they are not habitants.
2. Diversion of agricultural land have progressed. This diversion area from 1967 to 1977 was 1133 hectare which came under 5 percent of Nishikanbara Area. Agricultural lands have been diverted into residential or factory sites. Especially in southern part of Nishikanbara Area, many rice fields were diverted into factories, because it is adjacent to Tsubame and Sanjo Cities noted for tableware industry. Drainage from these factories have increased and flowed into drainage waterways managed by farmers or Nishikanbara Land Reclamation Union.
Nishikanbara Land Reclamation Union began to allot the expense for drainage to factories and municipal corporations in 1973. It was one of the earliest attempt in Japan. Also all the member of Nishikanbara Land Reclamation Union must bear the running cost of drainage and irrigation pumps. The expense in 1978 increased by 6.91 times in comparison with that in 1968.
Countermeasure for flood is determined by a scale and feature of past big floods. But flexible countermeasures corresponding with changes of the area are needed.