1 Yachiyo Engineering Co., Ltd., e-mail: tk-seki@yachiyo-eng.co.jp
2 Faculty of Agriculture, Niigata University, e-mail: gonda@agr.niigata-u.ac.jp
Temporal change of step-pool morphology in a mountain stream after a debris flow event
Takafumi SEKI
1
, Yutaka GONDA
2
, Hiroshi KAWABE
2
Introduction
Step-pool bed forms play an important roll in sediment transport phenomena through their production, destruction processes and faculty of sediment control in a mountan stream. In recent years, societal demand has increased for consistent synthetic river sediment management through Sabo projects in the field of river and erosion-control engineering. Therefore, it is thought to be important to identify temporal change of step-pool morphology which control sediment transport phenomena in a mountain stream. In order to identify temporal change of step-pool morphology, continuous reserches are being conducted on step-pool morphology in a mountain stream, where a debris flow event occurred. In this paper, the change in number and structure of step-pool during a three year period since the debris flow event are reported.
Study site and methods
Okochi River is a typical mountain stream which flows in Osado area of Sado Island, Niigata, Japan. A debris flow even occurred near the head waters of the Okochi River on December 23, 2010 and river bed at the upper reach of the river were heavliy disturbed. Step- pool morphology at four study reaches were surveyed 10 times since July, 2011 till November, 2013. Tempoal changes in stuructures of each step-pools were detected by compareing photographs taken during the current survey with those from previous surveys. Daily maximum discharges at four study reaches during non-snow-cover periods were estimated by using a water level records monitored in the middle of study site and those during snow-cover periods by using rainfall records monitored in the University Forest of Niigata University and a water level records monitored 3.6km downstream from the study site. Using estimated daily maximum discharges, tempoal changes in stuructures of step-pools were estimated by using hydrodynamic equations based on the dynamic conditions proposed by Ahida et al.(1984) under which step-pools are formed.
Results and discussion
Although the stream bed was heavily disturbed by the debris flow event, severe riverbed degradation and riverbank erosion ceased during the course of several months. Therefore, the mean value of river-channel and step-pool geometries over each reach remained nearly constant during the three-year period since the first survey in July 2011.The mean value of the channel and step-pool geometries over each study reach, and the total number of step pools at each reach remained nearly constant for more than three years after the debris flow event.
However, many step pools were deformed, destructed, and reconstructed repeatedly during a three-year period (Fig.1). Estimated temporal changes in step-pool structure generally coincided with detected change by using photographs, although the number of stable step- pools tended to be overestimated by the hydrodynamic equations. It was assumed that the overestimation of the number of stable step-pools occurred mainly because of errors in evaluating critical tractive force on each particle composing step (Fig.2).
References
Ashida, K., Egashira, S. and Ando, N. (1984) Generation and geometric features of step- pool bed forms (Japanese with English abstract). Bulletin of the Disaster Prevention Research Institute, Kyoto University 27(B-2), 341-53.
0 0.2 0.4 0.6 0.8 1 1.2 1.4
2011/7/9 2011/10/17 2012/1/25 2012/5/4 2012/8/12 2012/11/20 2013/2/28 2013/6/8 2013/9/16
Discharge(m3/sec)
Discharge Rational
formula
Rational formula
Rational formula A day’s
snowmelt + Rational formula
0 30 60 90 120 150 180
Number of step-pool
Total number of step-pool Period during when
discharge was estimated
0 0.2 0.4 0.6 0.8 1 1.2 1.4
2011/7/9 2011/10/17 2012/1/25 2012/5/4 2012/8/12 2012/11/20 2013/2/28 2013/6/8 2013/9/16
Discharge(m3/sec)
Discharge
0 10 20 30 40 50 60
Number of step-pool
Number of destructed step-pool Number of formated step-pools Period during when
discharge was estimated
Rational formula
Rational formula
Rational formula A day’s
snowmelt + Rational formula
(a) (b)
Fig. 1 Relationship between discharge, (a) the total number of step pools at each survey, and (b) the number of destructed and formed step pools between each survey.
Fig.2 Comparison between estimated structural changes of step pools by using hydrodynamic equations and those observed
Keywords: step-pool, debris flow, mountain stream, production, destruction
