Fine root distribution pattern of different aged Leucaena leucocephala trees in debris flow source area in Jiangjia Gully,China

Fine root is critical to restrain soil erosion and its distribution pattern is of great influence on the restraining effects. This study studied the fine root biomass(Br) distribution of different aged Leucaena leucocephala(5, 10, 15 years) in debris flow source area in Jiangjia Gully by digging downward to the bottom at different distances to stem in three directions on slope. The results showed the Br increased dramatically by 143% from 5 years to 10 years and then rose slowly by 38% from 10 years to 15 years. The Br of 5 years was significantly asymmetric between uphill and alonghill directions, but there was little difference among directions for other ages, and a concentration trend appeared to exist in downhill and alonghill directions. Moreover, fine root(D≤1 mm) was significantly heavier than that of fine root(1mmD≤2 mm), playing a leading role in the vertical distribution of the whole fine root, with a logarithmic or an exponential function. The results presented may shed light on fine root distribution pattern and evaluation of its effect on slope stability in debris flow source area.     

遥感技术在山区土地荒漠化评价中的应用--以东川市为例

通过遥感数字图像统计分析与处理,确定了分类图,沟谷图,绿度指数图是评价山区土地荒漠化的有效图象,进行了沟谷指标,绿度指数分级,肯定了遥感技术应用于山区土地荒漠化评价的可能性。     

THE STABILITY OF ROCK-VENEERED HILLSLOPES

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Gully erosion spatial modelling: Role of machine learning algorithms in selection of the best controlling factors and modelling process

This investigation assessed the efficacy of 10 widely used machine learning algorithms (MLA) comprising the least absolute shrinkage and selection operator (LASSO), generalized linear model (GLM), stepwise generalized linear model (SGLM), elastic net (ENET), partial least square (PLS), ridge regression, support vector machine (SVM), classification and regression trees (CART), bagged CART, and random forest (RF) for gully erosion susceptibility mapping (GESM) in Iran. The location of 462 previously existing gully erosion sites were mapped through widespread field investigations, of which 70% (323) and 30% (139) of observations were arbitrarily divided for algorithm calibration and validation. Twelve controlling factors for gully erosion, namely, soil texture, annual mean rainfall, digital elevation model (DEM), drainage density, slope, lithology, topographic wetness index (TWI), distance from rivers, aspect, distance from roads, plan curvature, and profile curvature were ranked in terms of their importance using each MLA. The MLA were compared using a training dataset for gully erosion and statistical measures such as RMSE (root mean square error), MAE (mean absolute error), and R-squared. Based on the comparisons among MLA, the RF algorithm exhibited the minimum RMSE and MAE and the maximum value of R-squared, and was therefore selected as the best model. The variable importance evaluation using the RF model revealed that distance from rivers had the highest significance in influencing the occurrence of gully erosion whereas plan curvature had the least importance. According to the GESM generated using RF, most of the study area is predicted to have a low (53.72%) or moderate (29.65%) susceptibility to gully erosion, whereas only a small area is identified to have a high (12.56%) or very high (4.07%) susceptibility. The outcome generated by RF model is validated using the ROC (Receiver Operating Characteristics) curve approach, which returned an area under the curve (AUC) of 0.985, proving the excellent forecasting ability of the model. The GESM prepared using the RF algorithm can aid decision-makers in targeting remedial actions for minimizing the damage caused by gully erosion.     

NATURAL AND ANTHROPOGENIC FACTORS CONTROLLING GULLY EROSION IN THEBASALTIC UPLAND OF SOUTHERN BRAZIL

This paper describes an analysis of natural and anthropogenic factors controlling the evolution of gullies in a rural basin in the basaltic upland in the State of Rio Grande do Sul, Southern Brazil. In this region of deep ferrallitic soils with more than 60% clay, runoff and erosion are of increasing concern. In the TaboAo drainage basin （100 km^2）, gully erosion was studied in a field survey that measured rills and gullies. Eighty-four gullies were identified. They had an average length of 136 m, were 10 m wide, and 3 m deep and had a volume of 15.458 m3. Each gully was characterised in terms of factors that included slope, geological structure, presence of piping, drainage, soil use, and the presence of surface and subsurface flow. On average, the main channels had knickpoints varying from 2 m to 7 m, and their evolution in the vertical plane increased until bed-rock basalt material was reached, after which gullies increase in width and length. Gully development was also monitored from 1991 to 2003. Subsurface flow appears to be the principal agent controlling their development. Results show that both natural （slope, surface curvature, geological structure and rainfall） and anthropogenic （soil use, road construction） factors are important in gully development. The change in cultural practices throughout the drainage basin from conventional to direct seeding has led to increased subsurface flow, which was more important than surface runoff in causing erosion. However, the higher rainfall during E1 Nifio Southern Oscillation （ENSO） events and the consequently higher subsurface flow were the dominant factors. From 1991 to 2003 a total land loss of 1,013 m3 was observed in one gully, with 236 m^3 lost during the 1992 ENSO and 702 m3 during the 1997 ENSO; 95% of the total volume lost occurred during ENSO periods.     

Development of gullies and sediment production in the black soil region of northeastern China

Gully erosion is an important environmental hazard in the black soil region of northeastern China. It is a primary sediment source in the region which needs appropriate soil conservation practices. Gully incision in rolling hills typical of this region was monitored using real-time kinematic GPS to assess the rates of gully development and the resultant sediment production. From 2002 to 2005, gully heads in the study area retreated between 15.4 and 33.5 m, giving an average retreat rate of 8.4 m yr^− 1. Field measurements showed that total sediment production due to gully erosion during the three years ranged between 257 and 1854 m³ yr^− 1, which is equivalent to 326 to 2355 t yr^− 1, with gully-head retreat accounting for 0 to 21.7% (4.4% in average). The sediment delivery ratio was especially high during the summer rainy season (56% in average). Sediment production by ephemeral gullies and permanent gullies was 1.5 times greater than that from surface erosion. Gully heads retreated faster in the spring freeze–thaw period than in the summer. The stage of gully development could be identified based on short-term changes in the gully erosion rate.     

芦山“4·20”地震后宝兴县城打水沟泥石流发育趋势及防治方案

打水沟为宝兴河右岸的一级支流,位于宝兴县城城区,流域面积1.01 km2,主沟长度1.85 km,主沟比降640‰.受芦山“4·20”地震的影响,流域中下游直接引发了大量的崩滑体,加之地形陡峻,且位于青衣江-鹿头山暴雨区边缘,极容易发生泥石流.一旦发生泥石流,将会对沟口建筑和人员的安全造成巨大危害.通过对打水沟地震前后遥感影像的解译,结合野外考察,综合判定该沟为地震触发的潜在性泥石流沟.进一步分析估算,由于地震震动导致流域内发育6处崩塌体,总面积20375 m2,松散固体物质总量为15.20×l04m3.在分析泥石流形成条件与发展趋势的基础上,提出了打水沟泥石流减灾方案,并对泥石流排导槽梯形最佳过流横断面进行了设计.通过计算分析得到,在排泄设计标准的泥石流时,排导槽的最佳过流断面宽度为2.38 m,深度为2.23 m.     

“6·28”矮子沟特大泥石流灾害成因研究

通过对四川凉山矮子沟流域的勘查,结合遥感解译及矮子沟流域的地质活动历史,分析“6·28”矮子沟泥石流的影响因素,在此基础上分析了矮子沟泥石流的成因。“6·28”矮子沟泥石流是在地震、人类活动及干旱影响下,受强降雨激发,坡面及支沟先发生泥石流,支沟泥石流堵塞主沟泄洪通道,强降雨使堵塞体瞬间溃决形成较大山洪,山洪的强大揭底作用使沟床松散固体物质启动,沿途不断有岸坡坍塌补给,中下游段有大块石加入,规模不断增大而形成。采用形态调查法计算的泥石流流量为925．91m3／s,泥石流总量为57．4×104m3,一次固体物质总量为27×104m3。通过对矮子沟流域的物源、地形及流域的气候特征研究,得出初步结论：近期泥石流的规模和频率都会降低,但在极端干旱、地震和强降雨的偶然联合作用下,依然可能发生大规模的泥石流灾害。     

基于DEM不同路径算法的沟壑密度提取

沟壑密度是评价小流域沟蚀程度的重要指标,同时也是反映地表破碎程度的因子。采用合理的汇流阈值设置方法,在黄土高原县南沟流域,通过基于坡向流量分配的多流向算法(DEMON)和传统的单流向算法D8实现沟壑密度的提取。结果表明:多流向算法和单流向算法都能提取较合适的沟壑密度,其中多流向算法提取的结果更好一些。因此,本研究对地表破碎程度的描述和当地地形地貌的反映有重要的意义。     

流域演化与泥石流的系统性——以云南东川蒋家沟为例

以典型的云南东川蒋家沟为例,根据Strahler面积-高程曲线的积分将其分区,揭示了流域的演化差异.证明了泥石流发生在特定的演化区域,并具有特定的物质组成.通过概率模型,定性说明了泥石流在流域的形成和扩张,其结果就表现为间歇性的阵流.阵流的时空特征反过来证明了泥石流的系统性.统计还表明,阵流的流量分布和衰减取决于最大流量;最大流量序列很好继承了作为流域系统行为的动力学特征.最后,可以根据单沟泥石流的过程系统考察区域泥石流活动.泥石流在大尺度水系的分布,从概率过程看,完全等同于泥石流源地小分支在单个流域的分布,而且满足相似的分布特征.