Jinfeng Liu

Effect assessment of debris flow mitigation works based on numerical simulation by using Kanako 2D

Mitigation works are very essential for mitigation of debris-flow hazards in mountainous areas. Usually, it is difficult to assess the effectiveness of existing mitigation works in a catchment. This paper presented a method for quantitative assessment of debris flow mitigation measures by using Kanako system, a user-friendly GUI-equipped debris flow simulator that allows good visualization and easy interpretation. Kanako 2D (Ver. 2.04) was applied to a case study at Caijia Gully, Sichuan Province, China. Mitigation works including check dams, drainage channel, and deposition basin were constructed in the gully in 2001 and 2006. Kanako 2D can simulate debris flow from steep area to alluvial fan. 1D simulation was applied for assessing the effect of the check dams at the lower part of the gully, and 2D simulation was applied for the effect of the drainage channel and deposition basin on the alluvial fan. The simulation results indicate that debris flow will cause great damage to residential area on the alluvial fan if mitigation measures were not implemented in the gully. For old dams which have been filled up with deposits of previous debris flows, the results show that they still have the function for controlling debris flow due to the gradient reduction of the channel bed in front of the dams by the trapped debris flow deposition. After the comprehensive control of debris flow including trapping, drainage, and deposition in the gully, the simulation results indicate that the risk on the alluvial fan can be reduced to an acceptable level.

Hazard mitigation planning for debris flow based on numerical simulation using Kanako simulator

Debris flow often causes enormous loss to life and property, especially on alluvial fans. Engineering structures such as retention check dams are essential to reduce the damage. In hazard mitigation evaluation and planning it is of significance to determine the location, size and type of dam and the effects of damage mitigation. We present a numerical simulation method using Kanako simulator for hazard mitigation planning of debris flow disaster in Tanjutani Gully, Kyoto City, Japan. The simulations were carried out for three situations: 1) the simulations of erosion, deposition, hydrograph changing and inundation when there were no mitigation measures; 2) the simulations of check dams in four locations (470 m, 810 m, 1,210 m and 1,610 m from the upstream end) to identify the best location; 3) the simulations of check dams of three types (closed, slit and grid) to analyze their effects on sediment trapping and discharge reduction. Based on the simulations, it was concluded that two closed check dams (located at 470 m and 1,610 m from the upstream end) in the channel and a drainage channel on the alluvial fan can reduce the risk on the alluvial fan to an acceptable level.

Debris flow formation conditions and optimal characteristics of drainage canal following Wenchuan earthquake

The debris flow formation conditions changed after the Wenchuan earthquake. For studying the optimal characteristics of the drainage canal under the changed formation conditions, the Zhangjia gully in Beichuan County, a typical debris flow gully, was selected as the study area. Two debris flow events occurred on September 24, 2008 and July 17, 2009, respectively, which threatened the safety of the inhabitants and destroyed the properties and transportation routes on the alluvial fan. Huge amounts of solid loose materials triggered by the earthquake and the effective antecedent precipitation were responsible for the debris flow occurrence. For eliminating the damage caused by debris flows, the construction of a drainage canal on the alluvial fan became necessary. A design method for the optimal cross-section characteristics of a debris flow drainage canal is discussed in this paper. Using the standard measurements for the depth (h) and the width (b), the cross-section size parameter (F) and the cross-section configuration parameter (M) under optimal hydraulic conditions are defined and deduced. Subsequently, formulas for calculating the cross-section measurements are deduced under optimal hydraulic conditions when peak discharge (Q), grain composition (D50 and D10), and longitudinal slope (I) are known. Finally, this method is applied for the cross-section design of the debris flow drainage canal in Zhangjia gully.

Weights-of-evidence method based on GIS for assessing susceptibility to debris flows in Kangding County, Sichuan Province, China

AbstractAssessing susceptibility to debris flows is of great importance for hazard prevention and mitigation. Kangding County, a region with a high occurrence of destructive geological events, was selected as the study area, to evaluate the area’s susceptibility to debris flows using the weights-of-evidence method based on ArcGIS. First, the determination method of training points was revised. The unit cells covered by the debris flow catchments were set as the training points, rather than using one point to represent the debris flow catchment. Then, the landforms, lithology, faults, earthquakes, and river systems were selected as evidential themes for the analysis. The optimum buffer zones cutoff distance of the faults and rivers was determined by the maximum studentized value of the contrast. Finally, the map of the susceptibility to debris flow in Kangding County was obtained by combining the evidential themes. The susceptibility was categorized as low, moderate, and high susceptibility. The low-susceptibility zones cover an area of 6253xa0km2, accounting for 53.93xa0% of the county area. The moderate-susceptibility zones cover an area of 3323xa0km2, accounting for 28.67xa0% of the county area. The high-susceptibility zones cover an area of 2018xa0km2, accounting for 17.40xa0% of the county area. The moderate-susceptibility zones surround the high-susceptibility zones. The assessment results agree well with field data, demonstrating that the revised method is more suitable for debris flow assessment than the previous method. The assessment results can be used as guidelines for policy makers regarding prevention and mitigation of debris flow hazards.n

Mitigation planning based on the prediction of river blocking by a typical large-scale debris flow in the Wenchuan earthquake area

Due to scale amplification resulting from the blocking-bursting process associated with landslide dams in debris flow catchments, many of large-scale debris flows that occurred as a result of the Wenchuan earthquake blocked the main rivers and resulted in catastrophic dam-breaking floods. To decrease the damage caused by dam-breaking floods, mitigation works should be applied in debris flow gullies. Currently, few studies focus on how to determine the key parameters concerning the scale of debris flows (i.e., peak discharge and total volume) that need to be controlled for mitigation planning considering downstream objects affected by the flow. The Xiaojia debris flow is described and analyzed as a typical large-scale case from the Yingxiu area. A back-calculation method and numerical simulation were proposed for mitigation planning in Xiaojia gully based on predictions of river blocking. First, the maximum peak flood discharge that did not endanger the town of Yingxiu was calculated. Then, the permissible blockage from a debris flow was determined based on the back-calculation of a dam-breaking flood. Finally, the scale of the largest permissible debris flow was obtained for use in mitigation plans based on numerical simulations. The calculations showed that the peak discharge of a flood that would not endanger Yingxiu should be <1496.48xa0m3/s. Accordingly, based on the 1/3 and 1/2 breach modes, the permissible blocking height of a debris flow barrier dam should not exceed 43.09 and 31.64xa0m, respectively. The total volume and peak discharge of a single debris flow event should be controlled to not exceed 70.59u2009×u2009104xa0m3 and 784.59xa0m3/s for the 1/3 breach mode and 45.21u2009×u2009104xa0m3 and 551.77xa0m3/s for the 1/2 breach mode. Based on these determinations of the key debris flow parameters, the simulation results indicate that debris flow damage can be decreased to an acceptable level to ensure the safety of Yingxiu downstream by the implementation of two check dams in the downstream channel and a deposition works on the debris fan.

Debris Flow Hazards and its Mitigation Works in Xianbuleng Gully, Jinchuan County, Sichuan Province, China

This paper presented a case analysis of debris flow hazards and its mitigation works. The Xianbuleng Gully which is located in Jinchuan County, Sichuan Province was selected as study area. This gully is an old debris flow gully which once burst out many debris flow disasters in history. If debris flows occur again in this gully, the township government, the center school and 13 village houses nearly 300 persons on the alluvial fan will be exposed to great risk.The environment settings and the hazard characteristics of the Xianbuleng debris flow were introduced first in this paper. Then, mitigation works especially the drainage canal under the optimal hydraulic condition were planned and designed in the gully for decreasing the debris flow hazards.

The Calculating Method of Erosion Depth of Viscous Debris Flow

Gully erosion depth is one of the most important parameters for the assessment and prevention of debris flows. So far, there is no good method for calculating the gully bed erosion depth of viscous debris flow. This paper discussed the calculation method of erosion depth of viscous debris flow based on theoretical deduction. The formula for calculating the erosion depth was constructed based on deduction from the viewpoint of theoretical mechanics. Then, the affecting factors of gully erosion were discussed. This method can be used to calculate the erosion depth for viscous debris flow under different frequencies and provide a better evaluation and prevention tool for mitigating debris flow disaster.