The evolution of gully headcut morphology: a case study using terrestrial laser scanning and hydrological monitoring

We present observations and analysis of gully headcut erosion, which differ from previous headcut studies in both spatial and temporal detail. Using ten terrestrial laser scanning (TLS) surveys conducted over a period of 3 years, we mapped headcut erosion with centimeter‐scale detail on a sub‐annual basis. Erosional change is observed through point cloud differencing, which expands on previous studies of headcut retreat rate by revealing the evolution of the headcut morphology. Headcut retreat observations are combined with hydrological measurements to explore the controlling factors of erosional retreat. We find that (i) mass failure due to wetting, (ii) saturation weakening of shale bedrock in plunge pools, and (iii) direct wash over the headcut face all appear to contribute to headcut retreat; however, mass failure via wetting appears to be the dominant process. Soil moisture was monitored near the study headcut at 0.4 m depth, and time‐lapse photos show that soil wetting tends to be concentrated along the headcut apex after rainfall and snowmelt runoff events. We find that moisture concentration at the headcut apex leads to more rapid erosion at that location than along the headcut sidewalls, resulting in a semi‐ellipsoidal plan view morphology that is maintained as the headcut migrates up‐valley. Copyright © 2015 John Wiley & Sons, Ltd.     

切沟中土壤水分的空间变化特征

切沟侵蚀不仅破坏土地资源，而且影响下游地区环境。目前的土壤侵蚀预报模型没有包括切沟侵蚀，因此研究切沟中土壤水分空间变化是建立切沟侵蚀模型的基础，也是恢复植被的基础。在陕西省安塞大南沟流域选择一个切沟，从1998-2000年连续 3年在 4～10月间对切沟不同部位土壤水分状况进行了观测。分析结果表明，切沟中浅层土壤水分空间分异规律明显，沟顶土壤水分状况较好，土壤容积湿度达到10．3％。沟坡则较差，但沿沟坡向下到沟底，土壤水分不断增加，分别为 6．3％、6．4％和10．4％。沟坡陡崖的土壤水分条件最差，接近凋萎湿度，容积含水量仅为4.6％～5.9％。沟底土壤湿度有所好转，为 7．7％。从雨季开始到结束，整体上土壤水分呈下降趋势，但随降雨事件发生波动。季节变化比较明显的是土壤水分条件好的部位，如沟顶、沟坡底部和沟底。尤以沟底变化最大。沟坡陡崖土壤湿度随季节没有任何变化。从1998-2000年，随降水量减少，切沟所有部位土壤湿度持续下降，即使在水分条件较好的沟顶和沟坡下部，土壤湿度也仅为 8．1％～8．8％，其它部位则都接近于凋萎湿度。土壤水分亏缺相当严重。切沟不同部位土壤水分的季节和年际变化主要受降雨入渗补给影响，而沟底则与径流的产生有关。因此应进一步研究不同降雨类型与径流产生、以及土壤湿度变化的关系。     

Predicting gully rejuvenation after wildfire using remotely sensed burn severity data

The loss of surface vegetation and reduced infiltration caused by wildfires can trigger gully rejuvenation, resulting in damage to downstream aquatic resources and risk to human life and property. We developed a spatially explicit metric of burn severity — the Burn Severity Distribution Index (BSDI) — and tested its ability to predict post-fire gully rejuvenation in 1st and 2nd order basins burned in the 2000 Valley Complex fires in the Sapphire Mountains of western Montana. The BSDI was derived from burn severity data interpreted from Landsat 7 satellite imagery using the Normalized Burn Ratio (NBR) method, and ranged from 0.0 for completely unburned basins to 4.0 for basins burned entirely at high severity. In July 2001 rainstorms with peak 30-minute intensities of up to 17 mm h^− 1 triggered gully rejuvenation in 66 of the 171 basins examined. The frequency of gully rejuvenation was higher in basins with higher BSDI values, increasing from zero for basins with a BSDI less than 1.3 to 67% for basins with a BSDI greater than 3.0. Binary logistic regression indicated that BSDI was a more significant predictor of gully rejuvenation than basin morphometric variables. The absence of gully rejuvenation in several basins with a high BSDI was attributed to low gradient, dense riparian vegetation, or concentration of high burn severity at lower elevations in the basin. The presence of gully rejuvenation in several basins with a low BSDI was associated with false negative NBR classification errors in northwest aspects, and concentration of severe burn impacts in the drainage headslopes. BSDI is a useful metric for predicting gully rejuvenation after wildfire. The use of the BSDI in Burned Area Emergency Response team assessments could improve the planning, implementation, and monitoring of burned area recovery treatments.     

地震后泥石流临界雨量变化特征——以汶川地震区小岗剑沟为例

在地震的影响下,泥石流暴发所需的临界降雨条件降低.通过现场调查分析,首先得出小岗剑沟泥石流沟床起动的形成机理及其“地震-滑坡-碎屑堆积-降雨-泥石流”的形成特征,然后在建立了泥石流发生的临界雨量模式基础上,以小岗剑沟泥石流临界雨量为研究对象,采用将其地震前后临界雨量相比较的方法,得到了临界雨量的变化特征,并探索变化的原因.小岗剑沟2010年暴发泥石流的临界雨量约为地震前的21％,2011年暴发泥石流的临界雨量约为地震前的23％,地震后临界雨量大幅度下降.但随着泥石流活动的不断发生,临界雨量有逐渐恢复的趋势.     

Modelling of piping collapses and gully headcut landforms:Evaluating topographic variables from different types of DEM

The geomorphic studies are extremely dependent on the quality and spatial resolution of digital elevation model(DEM)data.The unique terrain characteristics of a particular landscape are derived from DEM,which are responsible for initiation and development of ephemeral gullies.As the topographic features of an area significantly influences on the erosive power of the water flow,it is an important task the extraction of terrain features from DEM to properly research gully erosion.Alongside,topography is highly correlated with other geo-environmental factors i.e.geology,climate,soil types,vegetation density and floristic composition,runoff generation,which ultimately influences on gully occurrences.Therefore,terrain morphometric attributes derived from DEM data are used in spatial prediction of gully erosion susceptibility(GES)mapping.In this study,remote sensing-Geographic information system(GIS)tech-niques coupled with machine learning(ML)methods has been used for GES mapping in the parts of Semnan province,Iran.Current research focuses on the comparison of predicted GES result by using three types of DEM i.e.Advanced Land Observation satellite(ALOS),ALOS World 3D-30 m(AW3D30)and Advanced Space borne Thermal Emission and Reflection Radiometer(ASTER)in different resolutions.For further progress of our research work,here we have used thirteen suitable geo-environmental gully erosion conditioning factors(GECFs)based on the multi-collinearity analysis.ML methods of conditional inference forests(Cforest),Cubist model and Elastic net model have been chosen for modelling GES accordingly.Variable's importance of GECFs was measured through sensitivity analysis and result show that elevation is the most important factor for occurrences of gullies in the three aforementioned ML methods(Cforest=21.4,Cubist=19.65 and Elastic net=17.08),followed by lithology and slope.Validation of the model's result was performed through area under curve(AUC)and other statistical indices.The validation result of AUC has shown that Cforest is the most appropriate model for predicting the GES assessment in three different DEMs(AUC value of Cforest in ALOS DEM is 0.994,AW3D30 DEM is 0.989 and ASTER DEM is 0.982)used in this study,followed by elastic net and cubist model.The output result of GES maps will be used by decision-makers for sustainable development of degraded land in this study area.     

青海省格拉沟形成演化机制及其主要工程地质问题

格拉沟位于青海省内陆滩地，其形成演化反映了西北地区典型的水土流失过程。文章在收集大量野外资料的基础上，研究了格拉沟的形成演化机制，并对该沟道主要工程地质问题进行了探讨。分析认为，格拉沟形成演化是该地区特殊的内因外因共同作用的结果，基于此对其发育演化阶段进行了划分与预测。该沟主要工程地质问题为边坡稳定性、渗透稳定性、河流侵蚀等。其中边坡稳定性主要包括天然岸坡整体稳定性及崩塌、坠溜、滑塌等；渗透稳定性主要包括潜蚀与管涌；河流下切与溯源侵蚀则成为岸坡失稳破坏的重要诱发因素。探讨格拉沟形成演化机制及其主要工程地质问题，对于研究西北地区水土流失机理及其产生的环境问题具有一定的借鉴作用。     

四川省金川县城区红桥沟、蔡家沟泥石流及其防治

红桥沟、蔡家沟为大渡河上游（大金川）右岸的一级支沟,位于四川省金川县城区,曾多次发生过严重的泥石流灾害,直接威胁金川县城的安全。这两条沟的泥石流均为粘性泥石流,重度为2．10—2．25t／m^3,具有松散固体物质以表层滑塌及崩塌补给为主,成灾快,危害严重,活动频率较高,暴雨是泥石流的激发因素等特征。在分析红桥沟、蔡家沟泥石流性质及活动特征的基础上,提出拦、停、排的防治工程设计方案,工程实施后取得了很好的减灾及生态、环境效益。     

Potential of weight of evidence modelling for gully erosion hazard assessment in Mbire District – Zimbabwe

Gully erosion is an environmental concern particularly in areas where landcover has been modified by human activities. This study assessed the extent to which the potential of gully erosion could be successfully modelled as a function of seven environmental factors (landcover, soil type, distance from river, distance from road, Sediment Transport Index (STI), Stream Power Index (SPI) and Wetness Index (WI)) using a GIS-based Weight of Evidence Modelling (WEM) in the Mbire District of Zimbabwe. Results show that out of the studied seven factors affecting gully erosion, five were significantly correlated (p < 0.05) to gully occurrence, namely; landcover, soil type, distance from river, STI and SPI. Two factors; WI and distance from road were not significantly correlated to gully occurrence (p > 0.05). A gully erosion hazard map showed that 78% of the very high hazard class area is within a distance of 250 m from rivers. Model validation indicated that 70% of the validation set of gullies were in the high hazard and very high hazard class. The resulting map of areas susceptible to gully erosion has a prediction accuracy of 67.8%. The predictive capability of the weight of evidence model in this study suggests that landcover, soil type, distance from river, STI and SPI are useful in creating a gully erosion hazard map but may not be sufficient to produce a valid map of gully erosion hazard.     

INITIATION AND DEVELOPMENT PROCESSES OF TUNNEL SYSTEMS IN THE HILLY LOESS REGION OF NORTHERN CHINA

1 INTRODUCTION Tunnel erosion or piping is very common in semi-arid areas. It is often associated with gully system development (e.g., Parker, 1965; Heede, 1971; Harvey, 1982; Crouch, 1983; Gutierrez et al., 1997). It may also become a severe soil conserv…     

Characteristics, controlling factors and importance of deep gullies under cropland on loess-derived soils

Several studies describe the formation and importance of shallow ephemeral gullies under cropland (depth<0.8 m). Some of these gullies may develop into channels with a depth of more than 0.8 m (up to 4 m). Despite their spectacular nature, these deep gullies have not yet been studied in detail. Therefore, the objectives of this study are to analyze the characteristics and the controlling factors of these deep gullies, as well as their importance in terms of sediment production.Comparison of a dataset with 28 deep gullies, formed in the period 1985–2003 and 123 shallow ephemeral gullies formed in the period 1994–1999 in central Belgium indicates that the deep gullies have clearly different morphological characteristics compared to shallow ephemeral gullies. Several factors were analyzed to understand the formation of deep gullies. Plotting runoff contributing area versus slope of the soil surface at the gully head indicates that the topographical threshold for deep gully formation is significantly larger compared to ephemeral gully formation. Deep gullies form on short, steep valley sides and their position is strongly affected by the presence of linear landscape elements. All deep gullies incised on landscape positions with a very erodible soil horizon at shallow depth. Analysis of causative rainfall showed no significant differences in incision thresholds between rills and shallow ephemeral gullies on the one hand and deep gullies on the other hand.The relation between area-specific sediment yield in central Belgium and drainage area, indicates that the development of deep gullies contributes significantly to the sediment output of small rural catchments and causes peaks in the mean area-specific sediment yield that are up to a factor of three higher compared with catchments where shallow ephemeral gullying occurs.