JpGU-AGU Joint Meeting 2020

Presentation information

[J] Poster

H (Human Geosciences ) » H-QR Quaternary research

[H-QR06] Quaternary, Diachronic dynamics of human-environment interactions

convener:Mamoru Koarai(Earth Science course, College of Science, Ibaraki University), Yusuke Yokoyama(Atmosphere and Ocean Research Institute, University of Tokyo), Koji Okumura(Graduate School of Letters, Hiroshima University), Yasufumi Satoguchi(Lake Biwa Museum)

[HQR06-P03] Characteristics of gradient of valley plaine in Ibaraki Prefecture

*Mamoru Koarai1, Tomoya Sekine1, Takayuki Nakano2 (1.Earth Science course, College of Science, Ibaraki University, 2.Geospatial Information Authority of Japan)

Keywords:valley plaine, garadient, liquefaction

In the liquefaction risk assessment using landform classification, Nakano et al. (2015) divided the valley plain/flood plain into two parts with a gradient of 1/100, but the reason is not described clearly. The gradient of all valley plain/flood plain in the Land Condition Map in Ibaraki Prefecture was measured using the section function of “GSI Maps”. In addition, we sampled a valley plain with hand auger and investigated the relationship between gradient and particle size of structure materials.

As a result, the valley plain/flood plain steeper than gradient 1/100 accounted for only less than 10 percent by the extended, and its location was limited to a valley head. The gradient of the valley plain dissecting the plateau is steeper than roughly 1/500. Liquefaction caused by the 2011 off the Pacific coast of Tohoku Earthquake didn't occur at the valley plain steeper than gradient 1/500.

On the other hand, most of the flood plain along great rivers in Ibaraki Prefecture have a gentler gradient than the gradient of approximately 1/500, and no particular correlation was found between the liquefaction occurrence spots and the gradient.

For the liquefaction risk assessment based on the landform classification, the valley plain/flood plain needs to be divided into the flood plain along the great rivers and the valley plain dissecting the plateau, and the valley plain has a gradient more than 1/500.

In addition, as a result of sampling using a hand auger and particle size analysis in the valley bottom plain dissecting the plateau, coarse sand was dominant at a deeper than 3 m in the upstream core with a gradient more than 1/100. The result of analysis is supporting the supposition that a rough grain-size deposits are accumulated at steeper valley plains.


Prof. Naofumi Yamaguchi of Ibaraki University lent us bowling machinery. Miss. Kaho Hashimoto who is Ibaraki University student and her family had been very kind for an offer in land of sampling sites.