Li Changzhi, Liu Xingnian, Cao Shuyou, He Wenshe,
Zhang Zhixiang
State Key Hydraulics Laboratory of High Speed Flows, Sichuan University
24, Yihuan Road, Nanyiduan, Chengdu, 610065, Sichuan Province, China,
Tel:
86-28-5401148; Fax: 86-28-5405148; E-mail: caosy@mail.sc.chinfo.net
Abstract:
In accordance with the observed data series, a
research and analysis concerning the temporal features of sediment yield of
debris flow per year in the Jiangjia Gully watershed has been discussed. Then a
preliminarily conclusion is drawn that the peak value of debris flow sediment
yield per year experiences periodicity of 6 years. Following this, here come
some qualitative reasons and historical cases to identify this conclusion. A
sketchy predication, based on this trend, and attempting to describe the trend
of sediment yield of debris flow in the watershed in the coming decades, has
been presented.
Keywords: jiangjia gully watershed, debris flow, sediment yield, quasi-periodicity, predication
The
watershed is located in the northeast ranges of Yunnan Province, consisting of
Menqian Gully, Duozhao Gully and Jiangjia Gully (Fig.1). The conditions in the
Jiangjia Gully watershed are remarkably propitious for the occurrence of debris
flow due to its abundant loose debris, precipitous gradient and plentiful and
intensive precipitation. Firstly, the watershed lies just at the right bank of
the Xiaojiang River, and within the fracture zone of the Kangdian Axis, where
the surface rock is quite broken, the neo-tectonic movement is considerably
active, and earthquake is very intensive. Landslides and collapses in the
watershed are remarkably brisk. Abundant debris, therefore, is accumulated in
the watershed. Secondly, the elevation of the range varies obviously from 3,264m
at eastern end, and down to 1,045m at the western limit by the form of terraces.
The length from north to south is approximately 7km in the west part and only
about 2km in the east part, which looks like a calabash. This landform not only
provides powerful potential energy for loosen debris, but also is favorable to
produce water flow and debris flow. Thirdly, the watershed undergoes a climate
of subtropics in High Mountain area, which is characterized by abundant
precipitation, distinct reasons of wet and dry, mountain vertical climate, and a
pronounced rainfall peak in rain reason and in areas with elevation over 2,200m
(Wu Jishan, et, al, 1990). Therefore the plentiful and intensive rainfall offers
powerful energy for debris flows. Moreover, the over use of local land and the
deterioration of vegetation, play a quite important role in the frequent
occurrence of debris flow. As a result, debris flows are very serious in the
Jiangjia Gully watershed.
The observation and research of debris flow in the Jiangjia Gully Watershed, started in 1960s, experiences a history of more than 30 years, and accumulated data of a certain time series. Based on the time series data (Zhang Jun, Xiong Gang. 1998; Deng Bo. 1995), Figure 2 presents the curve of annual sediment yields of debris flow, in which the x-axis is the time (year), and the y-axis is the sediment yields (104 m3/a). Figure 2 indicates clear the tendency that the curve experiences a periodicity of 6 years. In this figure, annual sediment yields appeared 6 wave crests and 5 troughs. The 6th trough is going to form. Moreover, the time interval between wave crests is approximately equal since 1974, keeping on the period of about 6 years. The periods cover years of 74-79, 80-85, 86-91 and 92-97. Generally speaking, the annual sediment yields decreased swiftly to trough after the wave crest appeared, and then increased up to wave crest by two or three years. An approximate periodicity was within 6 years.
The writer had collected and preceded the data of sediment yields for each time of debris flow from 1987 to 1998. Figure 3 shows the column of sediment yield per time, which is accordant with the data mentioned above. The x-axis is time (the former two digits instead of the year and the latter two is the order of debris flow occurred in that year), and the y-axis is the sediment yield (104m3/a) for each time of debris flow.
Several characteristics can be found from figure 3: (i) it is corresponding to the trend of 6-year period. (ii) It is basically synchronism between peak value of transported sediment per time and the year of peak value. This figure shows that 1991 and 1997 are the years of peak value and their major transported sediments per time (9105, 9106, 9108; 9706, 9708 and 9713) are also of large scale. It is therefore quite clear that the transported sediment per time experiences a serious period while those years with serious debris flow. On the contrary, transported sediment per time goes through a shrink time while debris flow through a sag year. A case in point is, the maximum sediments transported in 1988a and 1993 a (8808 and 9301) were only 22.2×104m3 and 16.8×104m3. (ⅲ) Debris flow with large scale transported sediment occurs possibly in the year of non-peak value. For instance, 1987a is a year of non-peak value, but debris flow with sediment of 78.7 ×104m3 occurred on July 7th (8707). Similarly, 1995a is also a non-peak year and two debris flows with transported sediment of 95.5×104m3 (9503) and 110.4×104m3 (9512) occurred on July 7-8th and August 22nd, respectively.
The data of annual sediment yields from 1965a to 1998a can be supposed as a stationary time series if it is regarded as time series increasing with step of one year. Analysis on time series therefore is also utilized to understand the periodicity and tendency of sediments transported by debris flow. As far as a particular time series is concerned, the tendency indicates the variation of long period of the original series excepting the random component, and displays generally as gentle curve. The method of sliding average is simple and practical to separate tendency in case of equal time interval. Figure 4 is the smoothed curves of 3 points (a), 5 points (b), 7 points (c) and 9 points (d) sliding respectively. The trend of periodicity of 6 years is considerably obvious from the four smoothed curves.
Self-correlating function is a quantitative approach utilized to determine the interior linear dependency of a certain digital time series, which provides the intensity of linear dependency. This method is also applied to analyze and to explore the periodicity mended above (Di Junjing, 1994). Figure 5 presents the calculated result and smoothed curve of self-correlating function, in which the abscissa K denotes the correlating delay and the ordinate rk is the value of self-correlating function. Figure 5 shows clear that rk, reaching wave crest at 6 and 12 and falling to trough at 3 and 9 respectively, and experiences a periodicity of 6 years.
The watershed lies in the deep and large fault zone of the Kangdian axis, consisting of south-north, north-east and north-west three fracture zones, characterized by crossing and remarkably broken rock stratums, considerably active neo-tectonic movement, and quite intensive earthquake. Hillsides with gradient over 25%and 35% in the watershed cover the area of 55% and 16% to the total area respectively. In addition to this, there are about 200 branches and 154 dissected gullies, where landslides and collapses area covers 61% of the total watershed. About 1.98 billion cubic meters of debris flow (Yang Renwen. 1997) is therefore accumulated in the watershed. The debris per square kilometer reaches 400 million cubic meters. Moreover, the elevation is 3,269m at the eastern limit and 1,042m at the western ends, which leads the relative height up to 2,227m. As a result, the debris flows in this watershed are influenced in feature and frequency by the geomorphology and high density storage of debris. The sediments transported by debris flow depend mainly on the active capability of rainfall due to the very small variation of gradient of trunk gully in a certain period.
Depended on the data both of precipitation from June to August within 1989-1997 and of transported sediments from 1989 to 1999, Figure 6 presents the excellent correspondence between precipitation and transported sediments yearly. The watershed covers the longitudinal range of north 26°13ˊ~17ˊand east 103°06ˊ~13ˊ, where belongs to transitional zone of Tibet plateau and zone of East-Asia monsoon. Hence the atmospheric circulation in summer and autumn dominates the precipitation of rain season. During rain season, the synoptic system is considerably complicated in this region consisting chiefly of cold temperature in west-south, and subtropical high temperature of pacific, plateau trough and shear line (Cheng Jingwu, 1996; Du Ronghuan, et al. 1996). More over, the Tibet plateau plays an important role in the local atmospheric circulation. Various climatic factors compose and pile up together in this region. The writer believe that just because of the comprehensive effect of various remarkably complicate synoptic factors, the plentiful and intensive precipitation experiences a period of 6 years in the case of special geography, which results in the periodicity of 6 years of debris flow in the Jiangjia Gully watershed.
It is necessary to verify the periodicity of sediment yields of debris flow in the watershed before making a prediction. The coincident result should be attained when we make a predication if the above discussion is correct. According to the known data, 1997,1991,1985,1979,1973 are years of peak value, and 1967, 1961, 1955, 1949, 1943, 1937, 1931, 1925 and 1919 should also be years of peak value when we deduce forward. Table 1 is the historical cases for debris flows blocking the Xiaojiang River, from which one can know that cases blocking the river took place in 1919, 1937, 1949 and 1961 (the four years in peak value year deduced above). As far as 1954 and 1968, it is very closing to 1955 and 1968, which are years of peak value deduced. This just provides the information for 1955 and 1968 as peak value year from another aspect if the delay between sediment moment and floods and debris flow of certain large scale may occur near the year of peak value are taken into account. The two cases reach 6/7 of the total events of blocking the Xiaojiang River. The historical cases, therefore, offer in some degree proof that sediment yields by debris flow in the Jiangjia Gully watershed experiences a periodicity of 6 years.
On the assumption that the physical conditions of this watershed will not change much in a certain year period in the future, one can predicate the sediments transported by debris flow as figure 7, which is interpolated 10,000 points based on figure 1. Some researchers hold that the climax of earthquake in the Xiaojiang River basin would only last till 1999 (Tian Lianquan, et al. 1993). However, the writer considers that the vibration amplitude of the period will not decrease abruptly in a certain long time in the future. The reason for this viewpoint is that the detail conditions in the watershed, such as down-cut of gully source, global warming, temporal delay between earthquake and landslides, activation of landslides and collapses, increase of population and deterioration of environment, etc, must be taken into account. Therefore in figure 7, the part of real line shows the law of temporal and quantitative variation of sediments transported by debris flow in the Jiangjia Gully watershed in the past 34 years. And the part of imaginary line, deduced from the part of real line and periodicity of 6 years, presents the developing tendency of sediments transported by debris flow in the coming three decades years.
One can read from figure 7 that the years of 2003, 2009, 2015, 2021 and 2027 will be peak value year. Then it can be predicated that debris flows of large scale will occur in these years in accordance with the synchronism between peak values of transported sediment per time and the year of peak value. Therefore, in these years and near years, special attention should be paid to prevent and control hazards of debris flow and to variation of controlling factors of debris for the sake of comparison research.
Acknowledgements
Financial help received from the National Natural Science Foundation of China (No. 59890200). Thanks are also due to Prof. WANG, Prof. OU and Prof. ZHANG and other staff of Dongchuan Debris Flow Observation and Research Station, Chinese Academy of Sciences, for providing data of debris flow.
References
Wu Jishan, Kang Zhicheng, Tian Lianquan. 1990. Debris Flow Observation and Research in Jiangjia Gully Watershed, Yunnan. Science Press.
Yang Renwen. 1997. The Debris Supply for Debris Flow in the Jiangjia Gully, Yunnan. Mountain Research, 15(4)，305~307.
Deng Bo. 1995. Statistical Process Method for Data of Analysis and Test, Tsinghua University Press.
Zhang Jun, Xiong Gang. 1998. Observed Data Collection of Dynamics of Debris Flow in the Jiangjia Gully, Science Press.
Di Junjing. 1994. Study on Mid-short-term Recycle of Geological Disasters, in Study on Control against Landslides and Debris Flow in Yunnan (Vol.8), 63~82.
Tian Lianquan, Wu Jishan, Zhang Jun, et al. 1993. Erosion, Transportation and Deposition of Debris Flow, Chengdu Map Press.
Cheng Jingwu, Wang Kai, Zhu Pingyi. 1996. Characteristics of Rainfall and Debris Flow in the Upper Reach of the Yangtze River. Observation and Study on Debris Flow, Science Press, 106~115.
Du Ronghuan, Kuang Mingsheng, Li Jijun, Zhu Junjie. 1996. Study on the Temporal and Spatial Process of the Formation and Development of Quantery Debris Flow in the Xiaojiang River Basin, Yunnan. Observation and Study on Debris Flow, Science Press,
Fig. 1 Survey of the Jiangjia Gully Fig. 2 Sediments yield of debris flow from
1964 to 1998
Fig. 3 Transported sediment per time from 1987 to 1998
Fig. 4 The smoothed curve of sediments yield of debris flow in the Jiangjia Gully watershed
Fig. 5 The curve of auto correlating function Fig. 6 The relationship between precipitation
and Transported sediments
Fig. 7 Predication for the annual transported sediment of debris flows at Jiangjia Gully