The application of an expert knowledge‐driven approach for assessing gully erosion susceptibility in the subtropical Nigerian savannah

This study assessed gully erosion susceptibility in Southern Gombe State, Nigeria. The objectives of the study were to: (1) prepare gully inventory of Southern Gombe State, (2) apply the Analytical Hierarchy Process to assess the contribution of gully erosion predisposing factors, and (3) produce a gully erosion susceptibility map of Southern Gombe State. Based on geomorphological study involving interpretation of Google Earth images and field surveys, 127 gullies were identified and 13 gully erosion predisposing factors assumed to influence gully erosion susceptibility were selected. Identified gullies were randomly split into training (89 or 70 per cent) and validation (38 or 30 per cent) datasets. The contribution of each gully erosion predisposing factor was obtained using the Analytical Hierarchy Process. The results indicated that slope (0.130), stream density (0.121), and distance from stream (0.121) play crucial roles in gully erosion susceptibility. By overlaying the gully erosion susceptibility factor maps, a gully erosion susceptibility map was created. A natural break method was then used to classify gully erosion areas into relatively safe (6.04 km²), less susceptible (3332.46 km²), moderately susceptible (1811.49 km²), highly susceptible (1146.35 km²), and extremely susceptible (1726.77 km²) categories. Field verification confirmed that the map accurately classified 92.11 per cent of the validation datasets, signifying the Analytical Hierarchy Process as a reliable method for gully erosion susceptibility assessment. The created gully erosion susceptibility map can assist land planners to identify critical gully erosion areas where prevention and mitigation actions should be implemented.     

The use of SfM-photogrammetry to quantify and understand gully degradation at the temporal scale of rainfall events: an example from the Ethiopian drylands

Abstract

With the recent technological advances offered by SfM-photogrammetry, we now have the possibility to study gully erosion at very high spatial and temporal scales from multi-temporal DEMs, and thus to enhance our understanding of both gully erosion processes and controls. Here, we examine gully degradation and aggradation at a gully headcut and at four re-incisions along a gully reach in Northern Ethiopia. Environmental controls recorded are topography rainfall, runoff, land use and cover, land management, and soil characteristics. The overall vulnerability of the catchment to erosion is low as calculated from the RUSLE (average 11.83 t ha^?1 y^?1). This reflects the successful land management of the past years. The runoff coefficient was on average 7.3% (maximum 18.2%). Runoff events caused most geomorphic change in the gully, but slumping of the gully bank also occurred on dry days. Most geomorphic change was caused by one major rainfall event of 54.8 mm d^?1, and smaller runoff events caused both degradation and aggradation, often asynchronous between studied sites. Although most research focuses on gully heads alone, re-incisions at lower locations can still cause important gully degradation, which ultimately will reach the gully head and cause instability.     

Influences of vegetation disturbance on hydrogeomorphic response following wildfire

Quantifying the linkages between vegetation disturbance by fire and the changes in hydrologic processes leading to post‐fire erosional response remains a challenge. We measured the influence of fire severity, defined as vegetation disturbance (using a satellite‐derived vegetation disturbance index, VDI), landscape features that organize hydrologic flow pathways (relief and elongation ratios), and pre‐fire vegetation type on the probability of the occurrence of post‐fire gully rejuvenation (GR). We combined field surveys across 270 burned low‐order catchments (112 occurrences of GR) and geospatial analysis to generate a probabilistic model through logistic regression. VDI alone discriminated well between catchments where GR did and did not occur (area under the curve = 0.78, model accuracy = 0.72). The strong effect of vegetation disturbance on GR suggests that vegetation exerts a primary influence on the occurrence of infiltration excess run‐off and post‐fire erosion and that major gully erosion will not occur until fire consumes aboveground biomass. Other topographic and local factors also influenced GR response, including catchment elongation, per cent pre‐fire shrub, mid‐slope riparian vegetation, armoured headwaters, firehose effects, and concentration of severe burn in source areas. These factors highlight the need to consider vegetation effects in concert with local topography and site conditions to understand the propensity for flow accumulation leading to GR. We present a process‐based conceptual hydrologic model where vegetation loss from fire decreases rainfall attenuation and surface roughness, leading to accelerated flow accumulation and erosion; these effects are also influenced by interactions between fire severity and landscape structure. The VDI metric provides a continuous measure of vegetation disturbance and, when placed in a hydrologic context, may improve quantitative analysis of burned‐area susceptibility to erosive rainfall, hazard prediction, ecological effects of fire, landform evolution, and sensitivity to climate change. Copyright © 2015 John Wiley & Sons, Ltd.     

Measuring rill erosion at plot scale by a drone‐based technology

The traditional direct method (i.e. metric ruler and rillmeter) of monitoring rill erosion at plot scale is time consuming and invasive because it modifies the surface of the rilled area. Measuring rill features using a drone‐based technology is considered a non‐invasive method allowing a fast field relief. In the experimental Sparacia area a survey by a quadricopter Microdrones md4‐200 was carried out, and this relief allowed the generation of a Digital Elevation Model (DEM), with a mesh size of 1 cm and a resolution elevation equal to 2 mm, for three plots (L, G and C) affected by rill erosion. At first for the experimental L plot, which is 44 m long, the rill features were surveyed by a ‘manual’ method which was carried out by drawing on the PC screen the rill paths obtained by a visual orthophoto interpretation. This manual method was not applicable for the plots in which rills of limited depth occurred and were not detectable. Then, for both L plot and the other experimental plots having a length ranging from 22 to 44 m, an ‘automatic’ extraction method of rills from DEM was applied. Using an appropriate calculation routine, a vector coverage of transects orthogonal to the main flow direction (i.e. the maximum slope steepness path) was generated. The intersection of each plot DEM with the transect coverage allowed to obtain both the cross sections and the main rill morphological features. For the L plot the comparison between the rill morphological features obtained by the two different methods (manual, automatic) was carried out. Finally, the length–volume relationship and a dimensionless relationship proposed in literature were tested for all studied experimental plots. Copyright © 2015 John Wiley & Sons, Ltd.     

The hydrogeomorphic influences on alluvial gully erosion along the Mitchell River fluvial megafan

Hydrogeomorphic processes influencing alluvial gully erosion were evaluated at multiple spatial and temporal scales across the Mitchell River fluvial megafan in tropical Queensland, Australia. Longitudinal changes in floodplain inundation were quantified using river gauge data, local stage recorders and HEC‐RAS modelling based on LiDAR topographic data. Intra‐ and interannual gully scarp retreat rates were measured using daily time‐lapse photographs and annual GPS surveys. Erosion was analysed in response to different water sources and associated erosion processes across the floodplain perirheic zone, including direct rainfall, infiltration‐excess runoff, soil‐water seepage, river backwater and overbank flood inundation. The frequency of river flood inundation of alluvial gullies changed longitudinally according to river incision and confinement. Near the top of the megafan, flood water was contained within the macrochannel up to the 100‐year recurrence interval, but river backwater still partially inundated adjacent gullies eroding into Pleistocene alluvium. In downstream Holocene floodplains, inundation of alluvial gullies occurred beyond the 2‐ to 5‐year recurrence interval and contributed significantly to total annual erosion. However, most gully scarp retreat at all sites was driven by direct rainfall and infiltration‐excess runoff, with the 24‐h rainfall total being the most predictive variable. The remaining variability can be explained by seasonal vegetative conditions, complex cycles of soil wetting and drying, tension crack development, near‐surface pore‐water pressure, soil block undermining from spalling and overland flow, and soil property heterogeneity. Implications for grazing management impacts on soil surface and perennial grass conditions include effects on direct rainfall erosion, water infiltration, runoff volume, water concentration along tracks, and the resistance of highly dispersible soils to gully initiation or propagation under intense tropical rainfall. Copyright © 2012 John Wiley & Sons, Ltd.     

内蒙古准格尔旗丘陵沟壑区退耕还林(草)模式

根据准格尔旗的自然环境特点,社会经济状况,以及水土流失及沙漠化灾害的特点,总结出适用上述条件下的退耕还林(草)的几种模式,恢复生态,发展经济。     

岷江上游大白杨沟泥石流特征及成因

大白杨沟流域位于岷江上游四川省茂县境内,处于叠溪镇较场山字型构造北端。该沟曾于1935年和1991年暴发过两次较大规模泥石流灾害,其中1935年暴发的泥石流受1933年叠溪7.5级地震影响,规模很大,其泥石流峰值流量达到557 m3/s。通过对大白杨沟流域两条支沟(小沟、大沟)的地形条件、水源条件及物源条件的特征分析,得出两条支沟具有形成泥石流的充分条件:(1)大沟沟道平均纵比降275‰,小沟为398‰,相对高差达2 023 m;(2)大沟拥有的物源总量为29.21万m3,小沟达到42.01万m3,物源丰富。这些条件很好地解释了小沟泥石流的强活动性。对后期泥石流的形成机制进行分析,大沟以"消防水管效应"形成泥石流为主,小沟则以"消防水管效应"和堰塞体溃决两种方式形成泥石流。最后,通过泥痕法还原了历史泥石流流量,并对后期泥石流流量做了较合理的预测,为大白杨沟泥石流灾害防治提供可靠的数据。     

矿山泥石流沟谷形态的分形分维

以2个区域的矿山泥石流为背景,利用分形理论分析了矿山泥石流沟谷形态特征,并与4个区域的自然泥石流沟谷形态进行比较研究。沟谷形态的分形维数具有相同的地学意义:维数越小,该要素对沟道及泥石流的发育和形成的影响越大。沟道比降、汇水面积和沟道长度3个沟谷形态要素中,汇水面积影响最为显著;但矿山泥石流沟谷具有汇水面积少、沟谷尺度小,沟谷比降陡的特点;在此基础上提出了自然—矿山型泥石流的概念。     

基于指数滤波法估算沟道根区土壤水分

为更加有效获取黄土高原沟道地形根区土壤水分信息,基于表层（0~20cm）土壤水分观测值,采用指数滤波法在样带和沟道尺度对根区（20~80cm）土壤水分进行估算。结果表明,指数滤波法能够较准确估算沟道根区土壤水分,但随着土层深度增加估算误差逐渐增大。当有根区土壤水分初始值时,估算精度明显提高,但主要集中在时间序列前期。指数滤波法关键参数特征时长T的最优值（T_opt）随土层深度增加而增大,呈现每增加20cm,T_opt值较20~40cm增加1倍的变化规律。对于根区某一土层,不同空间位置（样带）和尺度T_opt交换使用没有明显降低土壤水分估算精度,表明指数滤波法对T_opt的空间敏感性较弱。需注意黄土高原区域尺度土壤性质具有极强空间异质性,指数滤波法在更大尺度上对T_opt的敏感性需进一步深入研究。     

基于信息不对称的EPC项目管理

本文介绍了信息不对称及其导致＂逆向选择＂和＂道德风险＂的机会主义倾向,在EPC项目管理中控制与管理好机会主义倾向,对于业主和总承包商在双方信息不对称条件下的利益博弈中达到一定程度的双赢具有重要的探索意义。