Analysis of the Material Source Conditions for the 7.24 Debris Flow in Luanchuan County

Lian-fen Shao Huanghuai University Zhu Madian 463000, China e-mail: [email protected]

ABSTRACT Luanchuan County lies in the mountains of Western Henan Province. With complicated geomorphy and topography, this area is characterized by great gully slope, complex structures, intense human engineering activities and heavy and concentrated rainfall, which have exposed the area to frequent debris flow events. On July 24, 2010, due to a widespread rainstorm, most of the county was attacked by catastrophic debris flow, causing heavy human casualties and property damages. This paper is designed to analyze material sources for the 7.24 debris flow event with a view to providing reference for the post-disaster reconstruction and prevention. Our analysis results indicate that the debris flow sources inside Luanchuan County are concentratedly distributed and fault activities as well as human activities provide abundant material sources for the formation of debris flow; human activities, including cultivation and mining, have an important share of contribution to the increased debris flow sources in the county. On these grounds we also put forward some proposes of how to improve the debris flow source conditions of the county. KEYWORDS: Debris flow; Material sources; Fault structure; Human activities.

INTRODUCTION

Debris flows are peculiar phenomena in mountainous regions, which often cause serious disasters in a short duration with sudden and fast moving mixtures of large amounts of water and loose material [1-4]. It has been widely recognized that the occurring of debris flows is the result of interactions among many factors related to topography, geology and climate [5-7]. Always, these factors are summarized into three aspects: topography, material and a water source. The main factors related to topography are average slope of the riverbed and slope gradient which determine the energy condition of debris flows. Water sources usually refer to the rainfall which is the trigger factor of debris flows while material sources refer to the amount of stored unconsolidated material which is the basic condition for debris flows to occur.

Early studies on debris flow were mostly focused on the analysis of rainfall conditions, especially on debris flow prediction or forecast [8-9]. However, as debris flow is a product of the combined action of rainfall, topography and material sources, given that rainfall conditions are the key to predicting or forecasting a debris flow, material source conditions should be the key to preventing any such events. The increasingly serious debris flow disasters are not only resulted

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Vol. 19 [2014], Bund. Y 8762 from the increased extreme weather, but have a greater relation to human economic activities that have destructed the eco environment. Human activities such as deforestation for farmland expansion, steep sloping plantation and mine exploitation all add to the debris flow sources and boost the development of debris flow. On this basis, investigating the regional debris flow source conditions and its relationship with human activities will be of great significance for predicting and evaluating debris flow disasters as well as preventing and controlling.

On July 24, 2010, a widespread heavy rainstorm across Luanchuan, with locally severe rainstorm, exposed 14 townships to rare-in-history flooding and triggered over 100 geological disaster/hazard points. Debris flow broke out in a number of places in the area and led to catastrophic human casualties and property damages. 68 people died, 21 disappeared and the direct economic damage amounted to nearly RMB 1.98 billion [10]. According to the Emergency Investigation Report on the Geological Disaster Induced by the “7.24” Severe Rainstorm in Luanchuan County, Henan Province (2010), 29 debris flow disaster points were triggered including 2 large points and 5 medium points. This paper is designed to analyze the material source conditions for the 7.24 debris flow in Luanchuan County with a view to offering instructions for the post-disaster reconstruction and the prevention. OVERVIEW OF THE STUDY AREA

Luanchuan County is located in the mountains of Western Henan Province (Figure 1). This area is characterized by complicated topography, including crisscrossed mountains, precipitous landform, high gully slope and developed unstable slopes. Rock formations within the area are complex and diverse, strongly weathered with well-developed joints and thick loose slope washes. In the area, mining and other human engineering activities are intense and frequent, and rainfall is heavy and concentrated. These factors aforementioned have resulted in physical geological events, present in the form of debris flow, landslide, collapse as well as subsidence, unstable sloping and weathering.

Geomorphy and topogaphy

Luanchuan County lies in the mountains. The northern part is Xiong’er mountain range. Eyu Ridge, a branch of Funiu Mountain, traverses the center of the county and divides it into two gully-stream zones. The south gully-stream is the Yi River basin and the north is the Xiaohe basin. The entire territory is higher in the southwest than the northeast with the highest altitude 2212.5m at the peak of Jijiaojian Mountain and the lowest altitude of 450m at the exit of the Yi River in Tangying Village. The relative relief is 1762.5m. According to the Report on the Geological Disaster Survey and Zonation of Luanchuan County, Henan Province (Mar 2002), the county comprises five geomorphic regions: viz. middle mountain, middle-low mountain, low mountain, low mountain and hill, and plain and terrace regions (Figure 2). Debris flow breaks out mostly around the Luanchuan–Shimiao–Taowan region (Figure 1). This is a middle-low to middle mountain transition characterized by complicated topography, numerous steep slopes and deep valleys, and great riverbed gradients and slope grades, which make up good topographic conditions for the formation of debris flow. Vol. 19 [2014], Bund. Y 8763

Meteorology and hydrology

Luanchuan County belongs to warm temperate zone, continental monsoon climate. The annual average rainfall of the county is 870.6mm with significant inter-annual variation. The maximum annual rainfall is 1386.6mm (1964) and the minimum is 403.4mm (1987). Normally, the annual rainfall ranges between 700~1000mm. However, the rainfall throughout the whole year is uneven, mostly in June, July, August and September, which account for 64.3% of the year, with July and August contributing 40.6% of the rainfall of the year. Such concentrated rainfall tends to give rise to flooding in spring and autumn and drought in winter and spring. Besides, the complicated topography also renders marked difference in the regional rainfall distribution: the rainfall is heavier in Nanchuan (south of Funiu Mountain) than in Beichuan (north of Funiu Mountain) and more frequent in deeper mountains than in shallower ones. The annual average rainfall in the shallow mountains in Beichuan is less than 750mm while that in the deep mountains in Nanchuan is in the order of 900mm, which is a cause for a flooded south and dry north. Furthermore, the high peaks and deep valleys in the county, distinct sunny and shady sides and wide disparity in light, temperature and rainfall, which result in a highly variable microclimate.

Figure 1: Location and debris flow distribution of the study area

Geological setting

Luanchuan has complex geological tectonics. The regional fault associated with numerous secondary faults (Figure 3).is called Jiaohe-Taowan Fault (part of the Gushi-Luanchuan-Queshan Fault), seperating the Qinling Fold Belt and the North China Platform, Also, as the area is in an uplift region where the mountains are high, valleys are deep, structural fissures, and unloaded fissures and weathering fissures are developed. Moreover, rock and soil masses consisting Vol. 19 [2014], Bund. Y 8764 typically of weathering-prone schist and granite etc. in the county have also resulted in physical geological events and numerous loose deposits.

Figure 2: Geomorphic zonation of Luanchuan County

Figure 3: Tectonic outline of Luanchuan County

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SOURCE CONDITIONS FOR THE 7.24 DEBRIS FLOW

The 7.24 debris flow disaster in Luanchuan was the product of the combined action of favorable topography, material source conditions and unusual rainstorm. Material source conditions as the basic for the formation of debris flow have close relation to human activities as well as the key to preventing or mitigating debris flow events. Therefore, analyzing the source conditions for the 7.24 debris flow could provide certain reference for the post-disaster reconstruction and prevention of the county.

Source distribution

Luanchuan has a wide territory, which makes it impractical to give a full and correct understanding of the distribution of debris flow sources simply by field investigation. To achieve more direct presentation of the material sources distribution, an “apparent amount of material sources” index is proposed in our study. The apparent amount of material sources is to outline the water catchment areas bearing noticeable material sources using google images and zone them qualitatively according to the density distribution. In our study, the entire Luanchuan County was divided into a well material source-supplied region, a fairly material source-supplied region, a moderately material source-supplied region and a poorly material source-supplied region. Figure 4 shows the image and Figure 5 shows the exact zonation of these regions.

According to Figure 4 and Figure 5, the apparent material sources of the county are mainly found in Section A and Section B in the middle and upper reaches of the Yi River, typically in and on both sides of gullies. The large white domain in Figure 4 represents a large amount of mineral waste residues stacked on both sides of gullies. Figure 1 shows the 7.24 rainstorm- induced debris flow disaster points also lie in the middle and upper reaches of the Yi River. It is obvious that the numerous loose materials in the upper reaches of the Yi River provided abundant material basis for the outbreak of this debris flow.

Our analysis has convinced us that source accumulation in Section A as presented in Figure 5 can be explained by two reasons:

1. The regional fault called Jiaohe-Taowan Fault (part of the Gushi-Luanchuan-Queshan Fault) and numerous secondary faults (Figure 3) occur in Section A. Constant fault activities have left behind them a number of joint systems of different natures; the crisscrossed joints have cut rock masses into blocks of different sizes and shapes, which consequently destroyed the integrity of rock masses, deteriorated their strength and stability and strengthened the rock weathering, thereby creating favorable conditions for collapse. All the gigantic blocks accumulated in the branch gully beds are the product of collapse action. The collapsed loose materials make up abundant sources for debris flow.

2. The well material source-supplied Section A is in an intense human activity area in the middle and upper reaches of the Yi River. As Luanchuan county lies in the remote mountains in the hinterland of Western Henan province that lack arable lands, farmers have to benefit from the increased land areas by reclaiming steep slopes. Besides, the beds of small-medium gullies (seasonal streams), especially in the middle and lower reaches, have been built into terraced fields. The county is rich in mineral resources and owns a long history of mining operations with Vol. 19 [2014], Bund. Y 8766 many mining enterprises and widespread mining wells. In addition to the destruction of vegetation and micro-landform, more serious potential geological hazards have been produced by human activities: mining waste and tailings, together with a number of unstable artificial slopes. Waste dumps are mostly located in the middle and upper reaches of gullies while tailing ponds are mostly built in open places of the gullies. All these provided abundant material sources for the 7.24 debris flow.

Figure 4: Regional google image of the apparent material source zonation (1:200000) (Top left: well material source-supplied region; top right: fairly material source-supplied region; bottom left: moderately material source- supplied region; bottom right: poorly material source-supplied region)

Xionger Mountain Note: Area D A Well source-supplied region B fairly source-supplied region Area B C moderately source-supplied region Xiao River Area C D poorly source-supplied region

Baodu Mountain Eyu Ridge Area D Area D Yu River Area B Area A Area D

Area C Yi River River Mingbai Area B Yang Mountain Area D Area D Funiu Mountain

Figure 5: Zonation map of the apparent material source areas of Luanchuan County

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Material sources analysis of typical gully

According to the analysis above, the debris flow sources inside Luanchuan County are concentratedly distributed and fault activities as well as human activities provide abundant material sources for the formation of debris flow. The following is the material sources analysis of some typical gullies. Washiyan Gully in Jiaohe Township

Washiyan Gully in Jiaohe Township extends about 7.5km long with average vertical slope grade of 67.3‰ and a relatively gentle channel. The gully lies in the alpine erosion landform region of Luanchuan as a V shape and is a tectonic uplift. Rock formations inside the area are dominated by the Lower Proterozoic Taowan Formation schist, along with a approximately 1km long interbed belt composed of Tertiary mudstone, sandstone and conglomerate at the gully estuary, called Jiaohe-Taowan Fault Belt. Therefore, the river basin presents noticeable tectonic compression and extremely broken rock formations. The gully has a large number of branches along both sides, spaced at an average of 1~2 per 100m and mostly hanging gullies. The specific stratigraphic and tectonic conditions have given rise to frequent collapses, and slides, forming loose materials, which provide abundant sources for the formation of debris flow (Figure 6). On July 24, 2010, a number of slumped masses appeared along the sides of this gully at high levels and debris flow broke out inside the gully.

Figure 6: Typical photos of the water-rock flow of Washiyan Gully

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Shanggeda Gully in Group 15, Guanxing Village, Shimiao Town

Luanchuan County is rich in mountains but poor in arable lands. Local residents often build fields inside the gullies where there is less water throughout the year and cultivate them into terrace-like plantations from the gully estuary upward (Figure 7). The sediment retention by simple retaining walls provides abundant material sources for the formation of debris flow. The most typical example is Shanggeda Gully in Group 15, Guanxing Village, Shimiao Town. This gully extends approximately 1km long with average vertical slope grade of 215‰. The downstream gully is approximately 15m wide at the widest point. Along a less than 300m length of the gully from the estuary upwards, 7-stage terraced fields of varied sizes have been built. On July 24, 2010, debris flow broke out in this gully, which swept down almost all the terraced field materials in the gully and exposed the bedrocks inside it (Figure 8). The debris flow washed off the Goukou road and killed two Goukou residents.

Simple retaining wall

Gully bed Stream Corn etc. Existing gully Farmland Corn etc. Farmland Natural channel Figure 7: Vertical and horizontal sections of Shanggeda Gully

Figure 8: Typical photos of the debris flow of Shanggeda Gully

Zhazi Gully in Shibao Village, Shimiao Town

Zhazi Gully in Shibao Village, Shimiao Town extends approximately 2.9km long with steeper vertical slope in the upper part than in the lower part. The vertical gradient of the middle and upper parts is 144‰ and that of the lower part is 71.5‰. This gully lies in the alpine erosion landform region where the mountains are high and valleys deep. The average valley slope grade is more than 30°. The river basin is well covered with vegetation. The sources consist mainly of randomly stacked mining waste (Figure 9).

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Figure 9: Source and deposit characteristics of Zhazi Gully, Shibao Village

The 7.24 severe rainstorm produced a high-velocity laminar flow in the valley slope region of Zhazi Gully, which brought along the mining waste when running down the steep slope on the banks into the gully in the form of sheet erosion and liner erosion. The waste particles so brought into the gully were reactivated by the high-velocity water currents from the upstream and mixed with the water currents into sediment-laden flows, which destructed the self-protection course of the downstream gully, swept down the mining waste sediments in the downstream gully and created a sizable water-rock flow. The debris flow outbreak washed off 2km of the coastal road and damaged 120mu of farmland. Ganjiang Gully

Ganjiang Gully flows through Lengshui and Shimao Towns and extends 2.7km long with average vertical slope grade at 259.3‰. The gully comprises granite in the middle and upper reaches and micacite in the lower reaches. The tailing pond is located in the granite region. The heavily weathered granite and developed unloaded fissures around the banks of the tailing pond, coupled with the poor blasting workmanship, have given rise to many slumped masses on the bank slopes and caused gigantic granite masses to slip directly into the waste bodies in the tailing pond (Figure 10).

The severe rainstorm on the very day of July 24 rendered a significant increase in the incoming water into the tailing pond and consequently a rapid rise in the pond level as well as a sharp rise in the rear pressure of the tailing dam. The dam finally broke down when it reached its ultimate. Spoils inside the pond facility rushed down together with the storm water like colts and mixed with each other into debris flow bodies. Flowing debris flow slurries lifted up and Vol. 19 [2014], Bund. Y 8770 conveyed large granite blocks while carrying along the alluvial deposits in Ganjiang Gully, and deposited them at the gully estuary. As recalled by Goukou residents, the debris flow in Ganjiang Gully left behind it several over 10m long granite blocks in the deposits of the gully estuary.

Figure 10: Typical photos of waste-type debris flow of Ganjiang Gully

CONCLUSIONS AND RECOMMENDATIONS

From our analysis of the formation conditions and source characteristics for the 7.24 rainstorm-induced debris flow, we can draw the following conclusions:

1. Luanchan County lies in the mountains of Western Henan Province which is characterized by favorable topography, rich rainfall, and abundant material sources provided by complex fault tectonics and human activities. The 7.24 rainstorm-induced debris flow disaster was the product of the combined action of favorable topography, material source conditions and high-intensity rainstorm.

2. The debris flow sources of Luanchuan County are concentratedly distributed and typically present in the vicinity of regional faults and human activity regions. Fault activities and human activities provide abundant material sources for the 7.24 rainstorm-induced debris flow disaster that hit the county. Vol. 19 [2014], Bund. Y 8771

3. Human activities have a share of contribution to the increased debris flow sources in Luanchuan County. These are mainly represented by:

1) Unplanned occupation of gullies for farmland expansion and construction of retaining embankments which accumulated material sources for debris flow outbreak;

2) The mining operations, simple tailing dams or random stacking of tailing waste along the gullies form the most important material sources for mine waste-type debris flow.

With the aforementioned understandings and findings, we recommend that the following remedies be taken to improve the debris flow conditions of Luanchuan County in a more effective manner:

1. Examine the existing terraced dams and tailing dams, reinforce unhealthy or hazardous dam sections and officially issue guidelines on the design and construction of terrace dams and tailing dams in Luanchuan County;

2. Officially issue a land use program of Luanchuan County to guide residents, especially those in mountains, to use the limited land resources and make scientific arrangement of building land and farmer land.

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