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: email@example.com (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 23.13 276,872 263,028 13,844 11,970 369,869 351,376 18,493 15,991
Gully 2 Headwall 1.03 32,540 30,913 1627 31,592 70,675 67,141 3534 68,617Gully bed 12.25 209,570 199,092 10,479 17,108 689,805 655,315 34,490 56,311Upstream area 22.84 239,731 227,744 11,987 10,496 254,746 242,009 12,737 11,154
Gully3 Headwall 1.05 48,832 46,390 2442 46,507 95,294 90,529 4765 90,756Gully bed 11.19 248,657 236,224 12,433 22,221 576,126 547,320 28,806 51,486Upstream area 22.74 294,222 279,511 14,711 12,939 456,483 433,659 22,824 20,074
Gully 4 Headwall 1.06 40,147 38,140 2007 37,875 151,202 143,642 7560 142,643Gully bed 11.3 264,475 251,251 13,224 23,405 747,990 710,591 37,400 66,194Upstream area 23.14 260,853 247,810 13,043 11,273 302,381 287,262 15,119 13,067
534 Z. Su et al. / Geomorphology 204 (2014) 532541
(Zhong, 2000). Dominant soils in the region are dry, red soil andvertisols, with a mean dry bulk density of 1.4 to 1.8 g cm3. Zonal veg-etation type is tropical bushveld with a sparse distribution of trees.Dominant species are Heteropogon contortus and Dodonaea riscosa. Soilerosion is acute in the Yuanmou Dry-hot Valley, with a rate of1.64 104 t km2 a1. Gully erosion plays a dominant role in overallsoil erosion, with a gully density ranging from 3 to 5 km km2.
2.2. Experimental procedure
To assess temporal variations in gully morphology, an active bankgully head was selected as the experimental site. A 6 m long gully bedand a 12 m long upstream area were divided into four sections andframedwith 2 mwide experimental platforms to control runoff (hereaf-ter referred to as gullies 1 to 4). In October 2011, four platforms of similardimensionswere trimmed, and a straight headwall was constructed. Theupstream area had a horizontal length of 11.89 m and amean slope gra-dient of 7.5. The wall of the gully had a vertical height of 0.7 m with amean slope gradient of 90, while the bed of the gully head had a hori-zontal length of 5.6 m with a mean slope gradient of 20 (Fig. 2). Soilsampling was carried out in March 2012 (dry season). Soil particle-sizefractions were analysed using the pipette method following H2O2 treat-ment to destroy organic matter and subsequent dispersion of soil sus-pensions by Na-hexametaphosphate (Nanjing Institute of Soil Science,Chinese Academy of Sciences, 1978). Soil texture in the upstream areawas sandy loam (FAO, 1988), composed of approximately 63% sand,
Table 2RMSE values determined for each DEM at each gully position for different cell sizes.
Value Landform Cell size and pre/post-scouring
0.1 m 0.05 m 0.0
Pre-scouring Post-scouring Pre-scouring Post-scouring Pre
0.006 0.006 0.006 0.005 0.0
Headwall 0.017 0.019 0.014 0.015 0.0Gully bed 0.014 0.018 0.010 0.011 0.0
0.009 0.008 0.007 0.006 0.0
Headwall 0.023 0.027 0.015 0.023 0.0Gully bed 0.021 0.020 0.013 0.016 0.0
0.013 0.009 0.010 0.006 0.0
Headwall 0.032 0.042 0.017 0.028 0.0Gully bed 0.025 0.021 0.016 0.020 0.0
0.003 0.002 0.002 0.000 0.0
Headwall 0.007 0.011 0.001 0.006 0.0Gully bed 0.006 0.001 0.003 0.004 0.0
24% silt, and 13% clay. Soil texture in the gully heads was also sandyloam (FAO, 1988), composed of approximately 72% sand, 20% silt, and8% clay.
A series of simulated scouring tests was carried out throughout a 6-day period in August 2012, during which no precipitation fell. To main-tain a stable flow discharge, water was initially fed into a current stabi-liser that consisted of a sweep tee, equalisation pond, and flow metre.Flow discharges of 30, 60, 90, and 120 L min1 were released ontothe soil bed in the upstream area byway of gullies 1, 2, 3, and 4, respec-tively. To prevent concentrated flows in the water pipe from directlyscouring the soil bed in the upstream area, water was transported to awater storage groove above the upstream area. The simulated scouringtest lasted 150 min. A collection groove was installed at the downslopeend of the gully bed to direct the runoff to a cylindrical sediment collec-tion tank,with a radius of 0.3 m and height of 0.4 m. A one-sixth aliquotof possible overflow from the sediment collection tank was separatedby a multi-slot divider and collected in a second 1 m3 tank. Followingthe scouring event, the gully head runoff was manually collected usinga 500 ml volumetric flask at the outlet of the collection groove at 5-minute intervals. Sediment samples were precipitated with alum,decanted with water, and oven dried at 105 C prior to determiningthe soil sediment mass.
Riegl's TLS system LMS-Z420i was used to collect topographical datafor use in conjunction with a high-resolution DEM. The system is com-prised of a high-performance long-range 3-dimensional scanner andRiScan Pro, the accompanying operating and processing software. A
1 m 0.005 m 0.001 m
-scouring Postscouring Pre-scouring Post-scouring Pre-scouring Post-scoring
04 0.004 0.003 0.004 0.003 0.004
10 0.009 0.008 0.008 0.007 0.00707 0.008 0.007 0.007 0.006 0.00705 0.005 0.004 0.004 0.005 0.004
11 0.010 0.009 0.010 0.008 0.00807 0.009 0.007 0.008 0.007 0.00707 0.005 0.006 0.005 0.006 0.005
11 0.011 0.010 0.012 0.009 0.00808 0.010 0.007 0.010 0.007 0.00801 0.001 0.001 0.001 0.001 0.001
00 0.001 0.001 0.002 0.001 0...