著者

大矢 雅彦 中山 正民 高木 勇夫

出版者

The Association of Japanese Geographers

雑誌

Geographical review of Japan, Series B (ISSN:02896001)

巻号頁・発行日

vol.61, no.1, pp.35-49, 1988-05-31 (Released:2008-12-25)

参考文献数

18

被引用文献数

5 9

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(1) 平野地形をつくるもっとも重要な要因は上流から運搬されてくる砂礫の質,量である.これは流域の地形,地質と密接な関係をもっている.この地形,地質には著しい地域差がある. 一般的に日本は山地地域は地盤の隆起を,平野は沈降をくりかえしてきた.そして平野地形は河川が山地より運搬してきた砂礫によって形成されてきた. (2) 平野における基本的な地形要素の組合せは次のようになる. 扇状地+自然堤防(後背湿地)+デルタこれらの地形要素とその組合せは洪水の繰返しによって形成され,その代表例は中部日本の濃尾平野である. (3) しかし,実際には沖積平野の地形要素の組合せには著しい地域差がある.この地域差をもたらした重要な原因は上流における盆地及び遷急点をもった峡谷の有無である.盆地があると上流から供給された砂礫のうち,かなりの部分が盆地で堆積してしまい,小粒径のものだけが峡谷を経て流下する.このため扇状地はないかあっても小規模となる.これらの事を最上川など数河川の河床砂礫の粒度分析でたしかめた.盆地・峡谷の機能はその形態,規模,縦断勾配などによって異なる.これらの相違は河川の流向が島孤に対して平行であるか,交叉するかによって影響を受けている.また,海岸部では沖積面は基準面(海面)変動の影響をうけている. (4) 沖積扇状地,自然堤防,後背湿地,デルタなどの地形要素とその組合せの特色は洪水の歴史を示すものである.それ故平野の地形分類を行えば将来万一洪水氾濫があった場合の洪水の状態の予測が可能のはずである.水害地形分類図の価値は濃尾平野をおそったべラ(伊勢湾)台風によって立証された.この他地形分類図は地震の際の液状化地点の予測にも利用できる.

著者

中山 正民

出版者

The Association of Japanese Geographers

雑誌

地理学評論 (ISSN:00167444)

巻号頁・発行日

vol.27, no.12, pp.497-506, 1954-12-01 (Released:2008-12-24)

参考文献数

15

被引用文献数

1 1

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There have been made public many papers on the roundness of gravel, but no work has so far been done on various sizes of gravels over a long distance in field. However, in order to make clear the relations between roundness and transportational agency it is necessary to investigate these factors. The purpose of this paper is to clarify the change in roundness of various sizes of pebbles over a distance and the relations between roundness and size in particular stations. The investigation was made in the river-bed of the Tanla River extending over about 60km. from Hikawva to Futako-Tamagawa and 12 sampling stations were selected there. The mathod of measuring is as follows: First samples are divided into four clesses, those of 61_??_. 32_??_. 16_??_. and 8_??_mm. in diameter respectively, and next, about 150 graywacke pebbles of each class were photographed and then the roundness was measured by the method developed by Wadell. The results are as follow: (1) The number of pebbles necessary for measuring mean roundness was determined in the following way. At Ome, where the frequency of roundness seems to be most complex, the relations among number and mean roundness and confidence interval were elucidated (Tab. 1). The table shows that the smallest number with small confidence interval is approximately 150. (2) The roundness of pebbles is not always the function of transported distance as shown in fig. 2. Where the detritus from valley walls are mixed with transported gravels in the upper reaches, the roundness decreases abruptly. The location where the roundnesss suddenly decreases is different according to pebble size. For the size of detrital materials differs according to the density of crack or joint spacing which differs as the outcropping locality differs. (3) In the lower reaches there are locations where the roundess of pebbles increases abruptly. The location where the roundness increases abruptly lies in comparatively upper streamn in case of larger pebbles and in comparatively lower reaches incase of smaller pebbles. It is probable that laorger rabbles change first the mode of heir downward shifting by decrease of the gradient of river course and friction acts on them earlier than it does on smaller pebbles. (4) The relation between size and roundness is not always expressed as y=mxn, where y is roundness, x is size, n is a coefficient, and. in is a constantas shown in fig. 3. In the upper reaches where the detritus from the valley walls are mixed with river gravels, no relation like this can be found or the converse relation is observed. At each sampling station in the river-bed from Ome to Futako-Tanzagawa, roundness of pebbles of 8_??_4mm. in size is remarkably lower than that of larger pebbles. This fact suggests that pebbles of a size of 8_??_4mm. are transported by saltation, while the larger pebbles by rolling or sliding in the reaches above-mentioned.