Geomorphology 204 (2014) 532541
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Simulated headward erosion of bank gullies in the Dry-hotValley Region of southwest China
Zhengan Su a, Donghong Xiong a,, Yifan Dong a, Jiajia Li a,b, Dan Yang a, Jianhui Zhang a, Guangxiong He c
a Key Laboratory of Mountain Hazards and Earth Surface Processes, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, Chinab Department of Environmental Engineering, Chengdu University of Information Technology, Chengdu 610225, Chinac Institute of Tropical Eco-agricultural Sciences, Yunnan Academy of Agricultural Sciences, Yuanmou 651300, China
Corresponding author. Tel.: +86 28 8559 2865.E-mail address: firstname.lastname@example.org (D. Xiong).
0169-555X/$ see front matter 2013 Elsevier B.V. All rhttp://dx.doi.org/10.1016/j.geomorph.2013.08.033
a b s t r a c ta r t i c l e i n f o
Article history:Received 30 January 2013Received in revised form 15 August 2013Accepted 31 August 2013Available online 8 September 2013
Keywords:Headward erosionLandform changeDEMFractal dimension
Although the development and migration of gully headcuts can increase soil loss and accelerate landscape deg-radation considerably, little attention has been paid to the spatiotemporal variations of the morphological char-acteristics of bank gully heads in the Dry-hot Valley Region of southwest China. This study explored the in-situvariations in soil loss rates and morphological characteristics in active bank gully heads, testing the overlandflowdischargewith a range of 30 to 120 L min1. In response to thisflow, activelymigrating headcuts developedwith retreat rates ranging from2.6 to 7.9 mm h1. All experimental runs resulted in a gradual increase in soil lossvolume, incision depth, and retreat distance over timedue to setflow rates. For the gully beds and upstreamareasof gully heads, the soil erosion rates were greatest at the beginning of each run and progressively decreased dur-ing the scouring test. Non-steady state soil erosion rates were observed in the headwall for the flow dischargelevels examined for this study. This was due to an abrupt slope collapse after long-term scouring effects. As deg-radation progressed, similar trends emerged for temporal variationswithin the fractal dimensions of topographicsurfaces, in both the gully heads and upstream areas. After the scouring was run for a period of 90 min, asymp-totic fractal dimensions of topographic surfaces were attained in the upstream areas and gully heads, suggestingthat steady state morphological characteristics had been realised. It should be noted that headwall collapses aretypically associated with a substantial increase in sediment yield where no other obvious change in morpholog-ical characteristics occurs in the headwall. Therefore, even though a significant difference in the soil erosion ratesand fractal dimensions of topographic surface values could be found between the bank gully heads and upstreamareas, the temporal variation in the morphological characteristics of bank gully heads was similar to those ob-served in upstream areas where ephemeral gullies developed.
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Gully erosion has been recognised as one of the most important pro-cesses in sediment production and land degradation in a wide range ofenvironments (Chaplot et al., 2005; Valentin et al., 2005; Zhu, 2012).Soil sediment generated by gully erosion represents a minimum of 10%and up to 94% of the total sediment yield caused by water erosion(Bennett et al., 2000; Poesen et al., 2003), suggesting that soil loss on ag-ricultural lands and hillslopes as a result of gully erosion may greatly ex-ceed losses due to sheet and rill erosions. Moreover, the formation anddevelopment of gullies can substantially increase soil loss from agricul-tural lands and severely impact farm productivity (Bennett et al., 2000).Gully erosion also plays an important geomorphic and hydrologic rolein many parts of the world (Wells et al., 2009) including Europe(Nachtergaele et al., 2002; Kirkby et al., 2003; Gimnez et al., 2009; DiStefano and Ferro, 2011), North America (Bennett et al., 2000; Bennett
and Alonso, 2006; Galang et al., 2010), Asia (Zhu, 2012), Oceania (Bettset al., 2003; Hancock and Evans, 2006), and Africa (Oostwoud Wijdenesand Bryan, 2001). While ephemeral gullies (small erosion channels onagricultural fields formed by concentrated flows within topographicswales) have been intensively studied (Poesen et al., 2003; Gordonet al., 2007a), relatively few studies have dealtwith the headward erosionof bank gullies.
Some studies have suggested that bank gullies expand primarily byheadward erosion (Oostwoud Wijdenes et al., 1999, 2000). A headcutis a sudden change in bed elevation where excessive localised erosiontakes place and is due to the jet impact of overland flow from upstreamareas (Bennett and Alonso, 2006; Dey et al., 2007). As gully headsmoveupslope (retreat), headcut erosion releases sediment into channels andexposes new channel walls to erosion (Oostwoud Wijdenes and Bryan,2001). Furthermore, headcut erosion usually takes place when the sur-rounding surfaces are protected from erosion by cohesive or chemicallyhardened top layers or grass cover (Dey et al., 2007). Once headcut ero-sion has begun, it will not naturally cease until a threshold condition hasdeveloped (Kirkby et al., 2003). Hence, depending on the rates of re-treat, gully heads can be substantial sediment sources in catchments,
Fig. 1. Location of the study area.
Fig. 2. Large-scale gully head situation at the Yuanmou Gully Erosion and Collapse Exper-imental Station operated by IMHE, CAS.
533Z. Su et al. / Geomorphology 204 (2014) 532541
with subsequent harmful effects occurring downstream. To predict fu-ture erosion and sediment release and to implement effective measureto reduce gully expansion, it is important to understand erosion pro-cesses at gully heads.
In the Dry-hot Valley Region of the upper reaches of the YangtzeRiver, themagnitude of gully erosion results from a combination of ero-sive precipitation, steep slopes, and anthropogenic influence (Zhanget al., 2003). Erosion at bank gully heads has become a major concernfor land and river management in this region because it drastically re-duces arable land and yields abundant sediment that leads to degrada-tion in downstream reaches (Zhong, 2000). Despite the severity of thisproblem, erosion processes in bank gullies have not been well docu-mented in this region.
The impact of sediment transport is often underestimated due tocoarse elevation data taken from field measurements and the coarseresolution of the digital elevation model (DEM), from which terrain at-tributes can be derived (Ramos et al., 2008). Compared to the total sta-tion and real-time kinematic global positioning system (RTK-GPS),terrestrial laser scanning (TLS) provides the most efficient method for3-dimensional measurements and 3-dimensional image documenta-tion (Milan et al., 2007; Momm et al., 2011). Additionally, many exam-ples exist in literature describing how a coarse DEM resolution cancause an overall increase in erosion predictions and an underestimationof sediment deposition (Schoorl et al., 2000; Ramos et al., 2008; Mommet al., 2011). These studies concluded that increased soil redistributionwas found in smaller cell sizes because its landscape representationwas substantially more detailed. Therefore, determining the appropri-ate DEM cell size is important for studying erosion and deposition.
Despite the importance of gully headward erosion, there is limiteddata regarding the rates of soil loss and morphological changes at ac-tively gully heads. This study, therefore, investigates gully heads in thefield to (1) evaluate the accuracy and resolution of the DEMs used and(2) examine the temporal variations in gully head morphology andsoil erosion rates in response to a range of overland flows.
2. Material and methods
2.1. Study area
Experiments were carried out at the Yuanmou Gully Erosion andCollapse Experimental Station, a field station operated by the ChengduInstitute of Mountain Hazards and Environment (IMHE), Chinese Acad-emy of Sciences (CAS). The station is situated within Yuanmou County(lat 2523 N to 2606 N, long 10135 E to 10206 E). It is representa-tive of the Dry-hot Valley Region of southwest China, in particular theJinsha River Valley (Fig. 1). This mountainous region of southwestChina is an ecologically fragile zone, considered as one of themost diffi-cult areas to re-vegetate in the upper reaches of the Yangtze River(Zhang et al., 2003). It is characterised by a dry and hot climate, with amean annual precipitation of 634.0 mm, a mean annual temperatureof 21.8 C, and an average annual potential evaporation of 3847.8 mm
image of Fig.2
Table 1Distribution of data points for each gully position.ntot = total number of data points. ngen = number of data used for DEMgeneration.nval = number of data used for validation.D = datasampling density.
Pre-scouring operations Post-scouring operations
Number of observations D Number of observations D
ntot ngen nval (Point m2) ntot ngen nval (Point m2)
Gully 1 Headwall 1.03 31,847 30,255 1592 30,919 117,299 111,434 5865 113,883Gully bed 10.61 246,218 233,907 12,311 23,206 643,988 611,789 32,199 60,696Upstream area 23.13 27