Effects of riparian plant roots on the unconsolidated bank stability of meandering channels in the Tarim River, China

Geomorphology 351 (2020) 106958

Contents lists available at ScienceDirect

Geomorphology journal homepage: www.elsevier.com/locate/geomorph

Effects of riparian plant roots on the unconsolidated bank stability of meandering channels in the Tarim River, China Guo-An Yu a, Zhiwei Li b,c,⁎, Hanyuan Yang b, Jianying Lu a,d, He Qing Huang a, Yujun Yi e a

Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China Key Laboratory of Water-Sediment Sciences and Water Disaster Prevention of Hunan Province, Changsha University of Science & Technology, Changsha 410114, China d University of Chinese Academy of Sciences, Beijing 100049, China e State Key Laboratory of Water Environment Simulation and Pollution Control, School of Environment, Beijing Normal University, Beijing 100875, China b c

a r t i c l e

i n f o

Article history: Received 12 August 2019 Received in revised form 8 November 2019 Accepted 9 November 2019 Available online 11 November 2019 Keywords: Meandering channel Riparian vegetation Sand-root composite Bank collapse

a b s t r a c t Meandering channels with highly sinuous and distorted bends have developed along the middle and lower reaches of the Tarim River in northwestern China. Riverbanks composed of unconsolidated materials have much lower bank strengths and are susceptible to fluvial erosion and bank collapse, particularly under high flow conditions. Thus, the formation and development of meander bends in such environments appear unintuitive. It is logical to speculate that riparian plant roots in unconsolidated banks have significant effects on bank stability and bend development in the absence of cohesive sediments, but this has not been verified. To determine the influence of root cohesion on the bank stability of meandering channels in the Tarim River, in situ measurements and sampling of roots from locally dominant riparian vegetation, i.e., Populus euphratica (PE), Tamarix ramosissima (TR), and Phragmites australis (PA), were conducted on the meandering bends to obtain root parameters, including diameter and root area ratio. The radius of curvature of local bends corresponding to sampling sites was also calculated using remote sensing imagery. We used the Bank Stability and Toe Erosion Model (BSTEM) to quantitatively estimate the effects of different root conditions on channel bank strength. The results confirmed that roots (and sand–root composites) provide effective reinforcement to unconsolidated banks, control the bank erosion, and hence enhance bank stability. However, the effects were highly variable under different root conditions. PA roots generally improved bank strength more than those of PE or TR, with PA, PE, and TR improving bank strength by 88.2%, 73.2%, and 63.6%, respectively. Riparian vegetation in this extremely arid and unconsolidated sediment environment therefore appears to be a prerequisite for sustaining the meandering channel. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Rivers are complex natural feedback systems in which the planform morphology in alluvial reaches reflects three fundamental ingredients: flow energy, sediment size and supply, and riparian vegetation (e.g., Twidale, 2004; Gurnell and Petts, 2006; Gurnell et al., 2012; Kleinhans, 2010; Vargas-Luna et al., 2019). While flow energy and sedimentary controls were emphasized in early theoretical fluvial morphodynamic studies, the influence of riparian vegetation on inhibiting erosion, stabilizing riverbanks, and maintaining channel meandering has now been widely recognized (e.g., Corenblit et al., 2007, 2011; Gurnell et al., 2012; Li et al., 2016). Nonetheless, the ⁎ Corresponding author at: Key Laboratory of Water-Sediment Sciences and Water Disaster Prevention of Hunan Province, Changsha University of Science & Technology, Changsha 410114, China. E-mail address: [email protected] (Z. Li).

https://doi.org/10.1016/j.geomorph.2019.106958 0169-555X/© 2019 Elsevier B.V. All rights reserved.

quantification of the reinforcing effects of riparian plant roots on the inception and migration processes of meandering channels in arid environments has been limited because of the complex interplay of soil and root systems (Billi et al., 2018). Riparian vegetation (with roots) has pronounced effects on channel morphology that impacts the total channel width, braiding intensity, and relative mobility of channels (Perucca et al., 2007; Motta et al., 2014), although some studies have proposed that meandering channels can develop in the absence of vegetation (e.g., Matsubara et al., 2015). Many studies have demonstrated that riparian vegetation can increase bank strength and the stability of channel margins through several mechanisms (i.e., mechanical, hydrologic, and hydraulic effects) (Thorne, 1990; Simon and Collison, 2010; Hopkinson and Wynn, 2009). Thus, planting or protecting vegetation is considered to be an effective means of controlling soil erosion in vulnerable areas (Baets et al., 2008). The mechanical effect of vegetation relates mainly to root reinforcement, particularly by increasing both flow resistance and bank

2

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

strength (Thorne, 1990; Abernethy and Rutherfurd, 2000; Pollen, 2007; Tal and Paola, 2010), which affect sediment delivery to and sediment transport within channels. The hydrologic effects include the reduction of soil moisture content and pore pressure caused by canopy interception and transpiration, compaction of bank soils by root fibers, and the protection of near-bank soils from fluvial erosion (Krzeminska et al., 2019). Furthermore, the presence of riparian and intra-channel vegetation can increase hydraulic roughness and decrease bank toe erosion (Hupp and Osterkamp, 1996; Tooth and Nanson, 2000; Micheli et al., 2010). The intermingled roots of plants tend to bind soils together into a monolithic mass and contribute to bank strength by providing additional apparent cohesion (Hickin, 1984). Soil without plant roots has high compressional strength, but little tensile strength. Roots anchor themselves into the soil to form a high-strength sand–root composite, and support the aboveground biomass (Waldron, 1977). Root fibers increase the tensile strength and elasticity of soils by transferring shear stress in the soil matrix to tensile resistance in the root fibers (Baets et al., 2008), helping to distribute stresses and thereby enhancing the bulk shear strength of the soil (Thorne, 1990; Simon and Collison, 2010; Hopkinson and Wynn, 2009; Pollen-Bankhead and Simon, 2010). The degree of reinforcement varies with the temporal and spatial characteristics of the roots (e.g., root density, distribution with depth, and diameter), root tensile strength, and soil moisture. However, plant roots also have several negative impacts on bank stability. Negative mechanical effects are related to the forces exerted by the vegetation. The surcharge provided by the weight of vegetation (especially mature trees) increases the driving forces in the downslope direction and reduces soil stability (Pollen et al., 2004; Pollen, 2007). Meanwhile, the negative hydrologic impacts are related to the altered infiltration characteristics at the surface and in deeper soils. Canopy interception and stem flow on soil surfaces tend to concentrate rainfall around plant stems, resulting in locally higher pore water pressures (Durocher, 1990). Finally, roots and stems at the soil surface can increase the surface moisture content and pore water pressure, improving the infiltration capacity and rate of water flow during and after rainfall events. These roots and stems may also accelerate the transport of water flow deeper into the slope and reduce the matrix suction and stability of the riverbank (Simon and Collison, 2010). Furthermore, riparian vegetation (e.g., alfalfa sprouts and Tamarix) aids in sustaining channel meandering and morphology in non-cohesive experimental flumes and alluvial rivers (Graf, 1988; Braudrick et al., 2009; Tal and Paola, 2010), and may drive river pattern transition between braided and meandering channels (c.f., Graf, 1988; Birken and Cooper, 2006; Camporeale et al., 2013; Gran and Wartman, 2015). Unique fluvial morphology has developed along the Tarim River, the longest allogenic and perennial dryland river in an extremely arid region of northwestern China (Yu et al., 2016, 2017; Li et al., 2017). Previous studies have shown that planting vegetation can influence braided rivers in flume experiments (e.g., Gran and Paola, 2001; Tal and Paola, 2010) because of its role in forming well-defined banks and stabilizing bars. Although the channel planform of the upper reach of the Tarim River is dominantly braided, a meandering channel with a large number of distorted bends has developed in the middle and lower reaches. The mean sinuosity of the meandering channel ranges from 1.6–2.2, and exhibits high spatial variability and lateral bend migration rates (Li et al., 2017). Interestingly, the sediment composition of the bank is quite uniform and dominantly composed of coarse silt and fine sand, with almost no clay. Consequently, the shear strength of the channel bank is very low and prone to scouring or even collapse under high flow conditions, particularly where there is a lack of protection from riparian vegetation. The development and evolution of meander bends in the Tarim River environment appear unintuitive when compared with the lower Mississippi River, Amazon River, middle Yangtze River, and many other meandering rivers in the source region of the Yellow River on the Qinghai–Tibet Plateau (Constantine and Dunne, 2008; Constantine

et al., 2009; Constantine et al., 2014; Xia et al., 2014, 2016; Billi et al., 2018; Zhu et al., 2018; Li and Gao, 2019). To date, little is known of the causes underlying the development of meander bends in the Tarim River. Based on field surveys from 2014 to 2018, we found that riparian vegetation (namely, the tree species, Populus euphratica; the shrub species, Tamarix ramosissima; and the reed species, Phragmites australis) along the meandering reaches protects the unconsolidated bank to some extent. We therefore hypothesize that riparian plant roots may effectively reinforce the riverbanks to sustain channel meandering. However, it is unclear how these roots enhance the stability of channel banks composed of silt/sand or how they may help to synchronously balance the scouring rate of the concave bank and the depositional rate on the convex bank over inter-annual time scales. Consequently, the aim of this study is to improve understanding of the role of riparian plant roots in the non-cohesive reinforcement and stabilization of unconsolidated channel banks with uniformly graded coarse silts and fine sands based on our previous research (Yu et al., 2016, 2017; Li et al., 2017). Furthermore, we aim to assess the effects of plant roots on the development and sustaining of meander bends by calculating the additional cohesion (ΔS) and bank stability coefficient (FS). Although this field-based research focused on meandering channels with dominantly non-cohesive sediment material in the very arid Tarim Basin, the external causes of channel meandering may be of importance to other geographic regions with similar geological and climatic conditions. This is especially important when evaluating the effects of riparian plants on channel morphology and forecasting the potential evolution of channel morphology in response to changes in riparian vegetation conditions (e.g., forest clearance or reforestation). 2. Study area and methods 2.1. Study area The Tarim River is a dryland river that flows within the Tarim Basin, the largest inland river basin in China (Fig. 1). The climate of this continental basin is extremely dry. Although annual precipitation in the mountainous headwater region can reach 200–500 mm, precipitation within the basin ranges from 50 to 80 mm, with only 10 mm in the central desert area (Chen et al., 2009). The Tarim River can be regarded as an allogenic (exotic) river, because its surface runoff is predominantly derived from snowmelt and glacial melt in the surrounding high mountains, rather than from local precipitation (Yu et al., 2016). The channel planform of the upper reach (the reach from the confluence of the tributaries of the Aksu, Yarkand and Hotan rivers to site 2, Fig. 1) of the river is braided (Fig. 2a and c). However, in the middle reaches (QMGJ– site 5, Fig. 1) a meandering channel with distorted bends has developed (Fig. 2b and d). The basic features of flow, sediment, and channel morphology in the upper and middle reaches of the Tarim River are shown in Table 1. As a common feature of dryland rivers, the flow discharge decreases from upstream to downstream owing to transmission losses and water diversions. The flood season primarily occurs from June to September, with the main flooding occurring in late July and early September; annual flooding typically peaks in August (Fig. 3). The mean annual discharge at Yinbazha hydrological station (YBZ; Qm = 71.9 m3/s) is much higher than the median flow discharge (Q50 = 15.6 m3/s), indicating that the distribution is asymmetric and that low-flow events occur more frequently than moderate- and high-flow events (c.f., Yu et al., 2016, 2017). The suspended sediment concentrations (SSC) in the two reaches are not high and differ only slightly. Most of the sediment load is transported during the flood season (Table 1). The values of the macro-valley slope of the two reaches are similar, but the channel gradient in the Alar (AL) reach is obviously higher than that in the YBZ reach because of the high degree of meandering in the middle reach. Sediments from the banks and riverbed are mainly composed of coarse silt and very fine sand (Fig. 4). The size distributions of channel

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

3

Fig. 1. The location of the Tarim River basin in China, the meandering reach (blue dashed polygon), and root sampling sites (pink symbols). The abbreviation of site names on the figure are: AL – Alar; XQM – Xinquman; YBZ – Yinbazha; WSM – Wusiman; QL – Qiala; and QMGJ – Qumaogejin. The red arrows indicate the direction of the flow of the rivers, where solid arrows indicate the rivers are generally perennial and dashed arrows indicate the rivers are seasonal. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

bank sediment of the upper (AL) and middle (YBZ) reaches are similar, though particle size analysis results showed that the median grain size (D50) in the AL reach (~0.08 mm) is slightly larger than that in the YBZ reach (~0.07 mm) (c.f., Yu et al., 2016; Li et al., 2017). The sediment composition is relatively homogeneous, with a sorting coefficient (φ = D80/D20) typically ranging from 1.4 to 1.6; the clay content is low. In general, the sediment composition of the riverbed is slightly coarser than that of the banks, and the clay content of the riverbed is even lower. Hence, the main channel of the Tarim River is unstable and prone to frequent lateral migration and even meander cutoffs as well as occasional avulsions owing to the low strength of the riverbank where the protection provided by riparian plant roots is lacking. 2.2. Field sampling and measurements Three typical riparian plants, Populus euphratica (PE), Tamarix ramosissima (TR), and Phragmites australis (PA), occur along the main stem of the Tarim River, especially in the middle reaches where the meandering channel has formed (c.f., Fig. 3 in Yu et al., 2016). Our field investigations showed that PE is the dominant species in the middle reaches, that TR is prone to occur on newly formed channel banks or point bars, and that PA frequently occurs on banks prone to inundation during flooding. In situ measurements of channel morphology (including mean channel width, radius of curvature, and bank height) and root sampling were conducted in December of 2016 and May of 2018 where riparian plants grow on the meandering channel along the middle Tarim River. To effectively compare the effects of the three plant species in terms of enhancing bank stability and sustaining channel meandering, sampling sites were chosen on channel bends with a single plant species (or, one dominant species): sites I and II were dominated by PA, sites III and V by TR, and sites IV and VI by PE (Fig. 5; Table 2).

Root samples were collected at sites I–III by horizontally digging square or rectangular holes into vertical banks; at sites IV–VI, exposed roots were directly sampled, as the bank soils (silt and sand) had been eroded by near-bank flow. Sampled roots were brought to the laboratory to determine root parameters (root length, diameter, and root area ratio (RAR)). RAR was computed as Ar/A, where Ar is the total crosssectional area of all roots and A is the area of soil in the sample. To measure the local bend curvature radius, R, corresponding to each sampling site, a series of inscribed circles were drawn on the channel midline in Google Earth Pro V. 7.3.2.5776 to determine best-fit lines (Nicoll and Hickin, 2010; Li et al., 2017). Sediment deposits were also sampled from the banks and riverbed at root sampling points (sites I–VI). The grain size distributions of sediment samples were obtained with a laser-scattering particle analyzer (Mastersizer 2000, Malvern Panalytical Ltd., UK). 2.3. Numerical simulations A physically based model, the Bank Stability and Toe Erosion Model (BSTEM), developed by the National Sediment Laboratory (Agricultural Research Service, U.S. Department of Agriculture) takes into account the effects of soil composition and stratification, vegetation cover, and bank protection on bank stability to predict the bank retreat caused by fluvial erosion and collapse. This model has been widely used to analyze the mechanisms of bank erosion and collapse (Lindow et al., 2010), the impact of riparian vegetation on bank stability (Simon et al., 2006), and sediment transport rates caused by bank collapse (Simon et al., 2010). Midgley et al. (2012) comprehensively evaluated the predictive capability of the BSTEM to determine bank collapse and found that the model could not effectively determine the distribution of shear stress and pore water pressure. More recently, Zong et al. (2017) considered the

4

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

Fig. 2. Landsat satellite imagery and photographs taken by unmanned aerial vehicle (DJI Phantom 4 Advanced) in April 2018 showing contrasting river planform patterns in the upper and middle reaches of the Tarim River. (a) Braided reach near Alar (AL, site 1, Fig. 1), which is almost completely surrounded by farmlands; (b) meandering reach with distorted bends downstream of Qumaogejin (QMGJ, Fig. 1), which is the studied reach; (c) photograph of a section of strongly braided channel near AL; (d) photograph of a typical meandering bend, point bar, and Tugai forest (with Populus euphratica as dominant species) along river channel near Wusiman (WSM, site 4, Fig. 1).

near-bank fluvial erosion, mechanical properties of soil composites, and changes in the water table by using the BSTEM to simulate the bank collapse processes for the Jingjiang reach of the middle Yangtze River during different periods. We calculated the additional cohesion, ΔS, and bank stability coefficient, FS, provided by different plant roots under the initial bank slope morphology in the study area. Three plant root scenarios and one ‘no vegetation’ scenario were modeled, i.e., tree (PE), shrub (TA), reed (PA), and no roots. According to the measured hydrological data, the mean water depth at the YBZ hydrological station (Fig. 1) during the flood season (July–September) was 2.91 m and the water depth during the main flood season (August 1 to September 15) was 3.46 m. The mean water depth during the non-flood period (October–June) was 0.84 m. The BSTEM model was used to calculate the number of collapses, erosional events, and FS of the riverbank covered by different

types of vegetation in a calendar year (January–December). The time step was set to 30 d, and we used the mean water depth as input data based on the real hydrological data from both non-flood and flood seasons (1.0 m and 3.5 m, respectively). By combining field samples and channel topography measured from the stations along the meandering reaches, we obtained the bank height and bank slope. The channel slope in the model was set at 0.0001 based on the measured channel gradient of 0.00005–0.0002. Because the bank along the Tarim River is mainly composed of relatively uniform coarse silts and fine sands, the bank was divided into five layers, the fifth layer of which was set below the constant water level. The stratification and mechanical properties of the bank soils are shown in Table 3. Because of the high permeability of silt, it was assumed that there was no lag in the water table, i.e., that the elevation of the water table at different flow velocities and moments was consistent with that of

Table 1 Flow, sediment, and morphological characteristics of the braided and meandering channel pattern in the upper (Alar, AL) and middle (Yinbazha, YBZ) reaches of the Tarim River. Site

AL YBZ

Q (m3/s)a

SSC (kg/m3)b

Range

Qm

Q50

Qf

Qnf

SSCm

SSCf

SSCnf

0.42–2130 0–927

148.7 71.9

60.7 15.6

500.7 222.9

79.7 19.8

6.7 6.7

7.0 7.1

3.2 2.8

W (m)

Sv (10−4 m/m)

Sc (10−4 m/m)

Ω

~950 ~130

2.91 2.7

1.74 1.3

1.46 2.07

Q: flow discharge; SSC: suspended sediment concentration; W: channel width; Sv: mean valley slope; Sc: mean channel gradient; Ω: mean channel sinuosity. Range: Minimum–maximum daily flow discharge. Qm, Q50, and SSCm: mean annual and median flow discharge, and mean annual suspended sediment concentration over the studied period; Qf, Qnf and SSCf, SSCnf: mean flow discharge and mean suspended sediment concentration in flood (June–September) and non-flood seasons (October–May) over the study period, respectively. Data of channel geomorphical features, namely W, Sv and Sc, for AL and YBZ are mean values for the reach AL to XQM (Xinquman) (site 1 to 2, Fig. 1) and reach QMGJ (Qumaogejin) to WSM (Wusiman) (site 4, Fig. 1), respectively. a Flow data series: 1960–2011 (AL); 1992–2007 (YBZ, no data for 2004 and 2005). b Suspended sediment data series: 1960–1966, 1978–1989 and 2001–2011 (AL); No data were available for suspended sediment dynamics at YBZ; data from a gauge point ~30 km downstream of YBZ is used for reference (data series range: 1964–1971).

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

5

Fig. 3. Daily flow discharge (Q) at the Yinbazha (YBZ) hydrological station during 1992–2007; no data were available for 2004–2005. The six highest daily flow discharges and the corresponding dates during this period are shown on the plot.

the water level. The default value of the BSTEM was 18 kN/m3 for soil mass in the first to fourth layers and 8.2 kN/m3 in the fifth layer. Generally, the critical shear stress, τc, of unconsolidated soil materials was calculated using the Shields criterion, as a function of weight and particle size, if the critical dimensionless shear stress, θc is 0.06. The erodibility coefficient, k, was estimated using the default Eq. (1) of the BSTEM, which was developed by Arulanandan et al. (1980): k ¼ 0:1  τc−0:5

ð1Þ

2.4. Calculation of additional cohesion of riparian plant roots Roots have little influence on the internal friction angle of soils. However, they do strengthen the soil shear strength, mainly by increasing the cohesive force (Operstein and Frydman, 2000). In the BSTEM, the RipRoot sub-model was used to calculate the additional cohesion of plant roots. In previous studies, the relationship between root fibers and soil mechanical properties has been established to improve the model equations. For instance, Waldron (1977) corrected the Coulomb equation by considering the root fibers of plant: S ¼ ΔS þ C r þ σ n tan∅

internal friction (°). Additionally, ΔS may be calculated as: ΔS ¼ T r ðAr =AÞð sinθ þ cosθtan∅Þ

ð3Þ

where Tr is the tensile strength per unit area of the roots (MPa), θ is the relative vertical deviation angle of roots subjected to shear deformation (°), Ar is the total cross-sectional area of all roots, and A is the area of soil in the sample. Wu et al. (1979) found that the calculated results of the last term in Eq. (3) were not sensitive to changes in θ or φ (40–90° and 25–40°, respectively), and their values were 1.0–1.3, with a mean of 1.2. The RAR reflects the root density at a certain location, which is largely affected by local soil and climate conditions, and by the distribution of the plant community. The RAR generally decreases with increased distance from the ground surface and the plant trunk (Bischetti et al., 2005). The root tensile strength is influenced by plant species, root diameter, and local environmental conditions. Many experimental results have shown that Tr decreases with increasing root diameter, d (mm) (Nilaweera and Nutalaya, 1999; Mattia et al., 2005; Norris, 2005; Tosi, 2007) (Eq. (4)), where b is generally a negative number. Because the maximum tensile strength of the roots increases linearly with d, but Tr is calculated per unit area, smaller d values correspond to greater tensile strength (Simon, 2005):

ð2Þ b

T r ¼ ad where S is the soil shear strength (kPa), ΔS is the additional cohesion (cohesive force) provided by the roots (kPa), σn is the positive stress on the shear plane (Pa), cr is soil cohesion (kPa), and φ is the angle of

ð4Þ

where a and b are empirical constants that depend upon the plant species.

Fig. 4. (a) Sediment composition of channel beds (Bd) and banks (Bk) under different vegetation conditions (reeds, shrubs, trees, and no vegetation) along the studied reach; (b) photograph showing the typical uniform sediment composition along a vertical bank profile (near YBZ).

6

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

Fig. 5. Measurement and sampling of roots of typical riparian vegetation along river banks of the meandering channel in the middle Tarim River: (a–c) reeds (Phragmites australis, PA); (d– f) shrubs (Tamarix ramosissima, TR); and (g–i) trees (Populus euphratica, PE).

Abernethy and Rutherfurd (2000, 2001) found that the reinforcement of riparian plant roots decreased exponentially with the increase in the lateral and vertical distance from the plant trunk; this relationship has also been reported in other studies (c.f., Canadell et al., 1996; Jackson et al., 1996; Schenk and Jackson, 2002). Furthermore, it has been found that N50% of riparian plant roots exist within the first 0.3 m of top soil layers. The RipRoot sub-model considers the nonlinear decreasing root distribution with changes in depth. Accordingly, Jackson et al. (1996) noted that Eq. (4), proposed by Gale and Grigal (1987), best fit the vertical distribution of roots: γ ¼ 1−ηh

ð5Þ

where γ represents the cumulative root ratio (between 0 and 1) from the surface to the soil depth h (cm), and η is a fitting parameter. The Manning roughness, n, corresponding to the samples of the plant species in this study was selected based on the results of Chow (1959) (Table 4). In the BSTEM, the tensile strength of roots can be determined by choosing the values of a and b or by choosing the measured data of the 22 plant species provided in the model database. In this study, the root tensile strength parameters were set based on previous measurements (Table 3). The relationship between root shear strength and diameter is shown in Fig. 6. If d was smaller than 2.5 mm, Tr (PA N TR N PE) was primarily affected by coefficient a. Furthermore, Tr decreased rapidly with increasing d. The Tr of TR decreased at the fastest rate for higher b indices. If d was N2.5 mm, the Tr of PE exceeded that of TR. Meanwhile, when d reached 8 mm, the Tr of the three root types were all b7 MPa. The RipRoot sub-model can optionally input the number of roots for each diameter. If the value was not measured, the root density could be estimated using the empirical equation and

parameters provided in the model database according to the plant species, growth year, and approximate coverage (PollenBankhead et al., 2009). The Chapman–Richards Equation (Eq. (6)) describes the general trend of plant growth (PollenBankhead and Simon, 2009) as follows: (1) The growth rate gradually accelerates, though the rate is low in early development, (2) a relatively stable growth rate is maintained in the mature stage, and (3) growth gradually slows in the declining stage (as plants near senescence).

y ¼ α 1−e−βx


θ

ð6Þ

where y is the parameter reflecting the degree of growth (such as stem diameter or height), x is the number of growth years, and α, β, and θ are the regression parameters. Pollen (2007) improved the RipRoot sub-model to enhance the determination of two types of possible states when shear failure occurs in soil–root complexes: (1) pullout (root–soil surface destruction and slip out) and (2) breakage (tensile failure). This causes root reinforcement to change with the mechanical properties and moisture content of soils over time and space. Ennos (1990) developed a pull-out force equation: F p ¼ πdSL

ð7Þ

where F p is the pullout force (N), S is the soil shear strength (kPa), as in Eq. (2), and L is the root length (m).

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

7

Table 2 Root samples of three typical plant species at six sampling sites in the middle Tarim River. Vegetation species

Phragmites australis (PA)

Sampling site

I

III V

Populus euphratica (PE)

Sampling size (m)

41°03′34.5″ N 86°05′44.9″ E 40°58′27.8″ N 85°28′30.3″ E 41°09′20.4″ N 84°57′03.4″ E 40°56′52.0″ N 82°53′12.4″ E

II Tamarix ramosissima (TR)

Location

IV

41°12′12.3″ N 84°21′43.3″ E 41°12′08.5″ N 84°21′44.4″ E 41°12′04.5″ N 84°21′48.7″ E 40°56′52.9″ N 82°52′43.9″ E

VI

Root diameter (mm)

Root number

R (m)

W (m)

0.78

83

471.3

88.9

2.96

0.71

135

111.1

101.6

0.27–4.47

1.6

0.68

26

34.4

69.33

2.0 × 2.0 2.0 × 1.5 2.2 × 1.5 1.0 × 0.8 1.0 × 1.0

5–12 1–18 1–10 1–6 1.3–12.4

/ 2.17 2.13 ~2 3.21

/ 1.30 0.97 / 0.90

30 105 157 90 113

1322.9

217.2

194.7

173.7

1.0 × 1.0

1.2–23.6

3.89

1.23

62

2.0 × 3.0

4.6–58.6

22.7

0.63

42

2.0 × 1.2 1.8 × 1.1

1–25 6–55

3.68 15.35

0.95 0.50

434 55

1322.9

217.2

Range

Mean

Cv

0.3 × 0.4 × (0.15)

0.47–11.5

3.47

0.3 × 0.3 × (0.2)

0.24–5.57

0.3 × 0.3 × (0.2)

Cv: Coefficient of variation of the analyzed samples; R: Curvature radius; W: channel width.

3. Results and discussion 3.1. Effects of plant roots on initial bank stability The RARs of riverbanks with PE and TR roots were 0.17–0.64 and 0.03–0.08, with mean values of 0.34 and 0.05, and Cv (coefficient of variation) values of 0.56 and 0.4, respectively (Table 5). The RAR of PE is therefore generally five times higher than that of TR, and the RAR range of PE is wider than that of TR. Few PA root samples were collected, but their RAR was nonetheless higher than those of PE and TR, and their distribution spanned a larger range (0.35–0.95). The additional cohesion of plant roots (ΔS) and the bank stability coefficient (Fs) under

the initial bank slope corresponding to different root conditions (plant species and RAR) were also simulated (Table 5). The ΔS values supplied by the different root types were ordered in the sequence of PA N PE N TR, with mean values of 31.16, 5.44, and 2.46, and Cv values of 0.81, 0.48, and 0.88, respectively. The PA roots had the largest reinforcing effect on the riverbanks, followed by PE and TR roots. The Fs values of PA, PE, and TR roots were 4.29, 1.53, and 1.41, with Cv values of 0.52, 0.07, and 0.06, respectively. Our field investigations from 2016 to 2018 revealed that channel banks with PA roots were lower in height (2–3 m), than the banks with PE and TR roots were (3–4 m high). Because of the high density of PA roots and their higher tensile strength for the same root diameter, the shear strength of PA

Table 3 Parameters of configuration and mechanical properties for each soil layer in Tarim River bank. Sampling point

Soil layer

Bank height (m)

Root length (m)

Layer thickness (m)

Unit weight (kN·m−3)

Internal friction angle (°)

Cohesion (kN·m−2)

Critical shear stress (Pa)

Erodibility coefficient (cm3/N·s)

I

1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

2.5

0.15

2.5

0.15

3.5

3.00

3.5

3.00

3.5

2.00

3.5

2.00

0.3 0.4 0.4 0.4 1.0 0.3 0.4 0.4 0.4 1.0 0.3 0.8 0.7 0.7 1.0 0.7 0.6 0.6 0.6 1.0 0.4 0.7 0.7 0.7 1.0 0.2 0.3 0.2 1.8 1.0

18.0 18.0 18.0 18.0 8.2 18.0 18.0 18.0 18.0 8.2 18.0 18.0 18.0 18.0 8.2 18.0 18.0 18.0 18.0 8.2 18.0 18.0 18.0 18.0 8.2 18.0 18.0 18.0 18.0 8.2

30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0

3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

0.059 0.087 0.087 0.087 0.087 0.051 0.051 0.051 0.051 0.051 0.051 0.027 0.027 0.027 0.027 0.024 0.024 0.024 0.024 0.024 0.035 0.088 0.088 0.088 0.088 0.068 0.008 0.065 0.014 0.014

0.412 0.339 0.339 0.339 0.339 0.443 0.443 0.443 0.443 0.443 0.444 0.612 0.612 0.612 0.612 0.652 0.652 0.652 0.652 0.652 0.537 0.336 0.336 0.336 0.336 0.384 1.087 0.391 0.853 0.853

II

III

IV

V

VI

8

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

Table 4 Parameters of tensile strength (a and b), correlation coefficient (r2), and selected Manning's coefficients (n) from previous studies. r2

b

n

Data source

Vegetation species

a

Phragmites australis, PA Tamarix ramosissima, TR Populus euphratica, PE No vegetation

34.29 −0.78 0.92 0.035 Baets et al., 2008 23.60 −0.90 –

0.033 Pollen-Bankhead and Simon, 2009 18.90 −0.64 0.29 0.045 Simon, 2005 –

–

–

0.022 Chow, 1959

Notably, although the ΔS and then Fs generally increased with increasing RAR, the variation in these two parameters was not wellmatched. For example, the range of RAR of TR was smaller (0.03–0.08), but that of ΔS was greater (0.69–4.95). Moreover, the RAR value at sampling site III was roughly double that at site V-3, but the corresponding ratio of ΔS at the two sites was 5.7. The RAR value at site IV-3 (0.39) was five times greater than that at site V-4 (0.08), but the Fs values of the two sites were similar (1.51 and 1.50, respectively). The main reason for these variations was that the root-size distributions (i.e., the compositions of roots with different diameters) differed, and the tensile strength decreased with increasing root diameter. The root diameters at site III mainly ranged from 0 to 3 mm, whereas those at site V-3 were all N1 mm, and approximately 1/3 of the roots with diameters of 2–20 mm. The Tr values decreased rapidly when d ranged from 0 to 2 mm, and when d exceeded 2 mm, Tr was b1/10 of that when d was 0.5 mm (Fig. 6), which caused a low ΔS value at site V-3. The diameters of PE roots at site IV-3 was 5–40 mm, whereas those of TR roots at site V-4 had a narrower range of 2–3 mm. The Tr, as shown in Fig. 6, decreased with increasing root diameter. Hence, the results in Table 5 indicate that in addition to the RAR, other factors, such as the range of root diameters and root tensile strength, also had important effects on the stability of channel banks. 3.2. Effect of plant roots on bank collapse

Fig. 6. Relationship between root diameter (d) and tensile strength (Tr) of three plant species (Phragmites australis, PA; Populus euphratica, PE; and Tamarix ramosissima, TR).

roots could be more effectively converted to ΔS; hence, banks with PA roots had the highest Fs values. As shown in Fig. 7, the ΔS of the three species of riparian plants increased with increasing RAR (r2 = 0.68), and they exhibited a banded distribution, indicating that the ΔS of different species changes with RAR within a certain range. The ΔS of TR affected by the tensile strength parameters (a and b) increased at the fastest rate with increasing RAR, and those of PE increased with the slowest rate, causing quite different ΔS values for the same RAR. Additionally, as proposed by Pollen and Simon (2005), in the RipRoot sub-model, the endpoint is reached either when (1) all the roots have broken or (2) the driving force acting on the bank is supported by some or all of the roots. For the lower bank (with height of ~2 m), the driving forces can be completely supported by part of the roots, and the maximum load that can be withstood by all of the roots was not attained.

A total of 18 model runs were conducted to simulate the bank collapse process of a 100-m long reach at six sampling sites in one year with and without plant roots (Table 6). The last run at each site (Runs 2, 4, 6, 11, 15, and 18) was a non-vegetated scenario. The other conditions (curvature radius, channel width, and soil mechanical properties) of these runs corresponded to the original riverbank. The FS values in the non-vegetated runs were lower than those in scenarios with vegetation cover and generally lower than the critical value of 1.3 (Fig. 8). The mean FS values for Runs 15 and 18 were 0.64 and 1.01, respectively, and were among the lowest stability runs with the highest frequency of bank collapse (11 and 12 times, respectively); this was mainly owing to the large bank heights and intense bank erosion in these simulations. Most of the FS values for the banks with roots of all three studied plant species were higher than the critical value of 1.3. The mean FS values of the bank with PA roots were 1.89 and 2.67 at site I for Runs 1 and 3, respectively, which were among the highest values for all runs. The mean FS values of the PE root-bearing bank were slightly higher than those of the TR root-bearing banks. The simulated occurrence of bank collapse with roots was significantly lower than that without roots. The mean occurrences of collapse

Table 5 Root features and calculated additional cohesion (ΔS) and bank stability coefficients (FS) using the Bank Stability and Toe Erosion Model (BSTEM). Species

PA TR

PE

Sampling point

I II III V–1 V–2 V–3 V–4 IV–1 IV–2 IV–3 VI–1 VI–2

Root number in different diameter ranges (mm)⁎ 0–1

1–2

2–3

3–5

5–10

10–20

20–40

158 253 41 – – – – – – – – –

183 110 205 – 25 33 – 80 36 – – –

183 441 41 – 4 2 113 – 4 – 167 –

100 625 – – 2 6 – 15 9 – 1 –

67 63 – 8 4 6 – 18 6 2 7 6

– – – – 1 – – – 7 2 5 18

– – – – – – – – 1 4 2 4

RAR (%)

ΔS# (kPa)

Fs With root

No root

0.35 0.95 0.06 0.04 0.04 0.03 0.08 0.17 0.18 0.39 0.32 0.64

13.38 48.94 4.95 1.05 0.75 0.99 4.74 2.74 3.42 4.74 7.38 8.92

2.70 5.87 1.51 1.35 1.33 1.34 1.50 1.42 1.45 1.51 1.61 1.68

1.45 1.45 1.31 1.31 1.31 1.31 1.31 1.31 1.31 1.31 1.31 1.31

PA: Phragmites australis; TR: Tamarix ramosissima; PE: Populus euphratica; RAR: root area ratio. Notes: An asterisk (⁎) indicates the number of roots per unit area corresponding to each diameter range. The root number per unit area should be input into the BSTEM when it is measured at the sampling sites listed in Table 2; the #ΔS is the additional cohesion per unit length output in the BSTEM, which was converted into actual root length data.

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

Fig. 7. Relationship between root area ratio (RAR) and additional cohesion (ΔS) for Phragmites australis (PA), Populus euphratica (PE), and Tamarix ramosissima (TR).

in PA, TR, and PE root-bearing banks were 0.5, 2.8, and 3.2, respectively, whereas that of corresponding banks without roots increased to 8.5, 9, and 11.5, respectively. One of the main reasons is that riparian plant roots increase the roughness of near-bank flow and prevent fluvial erosion (c.f., Pollen-Bankhead and Simon, 2009). Additionally, under the same flow conditions, riparian plant roots can support larger areas of the upper bank layer, and the mass of these blocks at the bank toe may further reduce the possibility and frequency of bank collapse (Zhu et al., 2018). Our field observations revealed that banks with PA roots generally did not collapse. The lateral erosion and retreat of these banks were also low, which is consistent with the results of other studies (c.f., Van Dijk et al., 2013; Li et al., 2016; Krzeminska et al., 2019; Li and Gao, 2019). Some failing blocks along the reach covered by PE and TR roots, with some roots exposed at the bank edge, indicated that bank collapse had previously occurred (Fig. 5d, e, and g–i). These findings agree with the BSTEM simulation results (Table 6). The volumes of fluvial erosion (bank toe erosion, Ef), bank collapse (Ec), and total erosion (Et) simulated by the BSTEM also exhibited large differences (Table 6; Fig. 9). The differing intensities of bank erosion and collapse rates (in volume) arose from the mixed output of multiple factors, such as vegetation conditions (PA, TR, PE, and no vegetation), the curvature radius, R, and the channel width, W.

9

In general, smaller R values indicated lower W values and serious bank erosion. The fluvial erosion (bank toe erosion, Ef) of banks with roots was less than that without roots. The reductions in erosion (% Ef) of the banks with PA, PE, and TR roots were 77.2–79.5, 67.3–77.5, and 42.9–69%, respectively. The volumes of bank collapse (Ec) exhibited similar trends to those of the fluvial erosion (Ef), and generally there was a greater reduction ratio in the erosion volume. Among the different plant species, the PA-reinforced bank had the lowest bank collapse volume and the highest reduction ratio for erosion. Under different runs, Ec was 4–5 times greater than Ef (Table 6), indicating that sediment transport and bank morphological changes were mainly caused by bank collapse (Ec) rather than by fluvial erosion (Ef). Collapse widths indirectly reflect the lateral migration rates, Mr (m/ yr), of riverbanks in a year. The Mr is significantly constrained along banks with riparian vegetation, with the mean Mr value of the PAreinforced bank being 0.61 m/yr, and those of the TR- and PEreinforced banks being 3.90 and 4.49 m/yr, respectively. These values were 91.1, 58.9, and 69.8% lower than those of the bare bank, respectively (Table 6). The Mr value of the PA-reinforced bank at site I was 1.21 m/yr, which was among the lowest lateral migration rates in this study, and it was 1/5.6 that of the bare bank. The Mr value of the TRreinforced bank at site V-1 was 2.91 m/yr, which was 1/2.6 that of the bare bank. Because of the reinforcement of riparian vegetation, the lateral migration rates of the outer banks of meander bends in the middle Tarim River were significantly reduced. The relatively slow rate of outer bank erosion has synchronously adapted to the low depositional rate on the point bar of the inner bank at an inter-annual scale, thereby maintaining the channel width within a relatively stable range and slowing the process of bend evolution (i.e., low-sinuosity bend, Ω-shape bend, and meander cutoff) (Engle and Rhoads, 2012; Li et al., 2017; Billi et al., 2018). Riverbank soils and riparian vegetation can form high-strength soil– root composites despite the development of the river channel with noncohesive sediment material in an arid environment. The flow velocity and fluvial erosion rate are slowed by increased roughness, n, owing to the presence of near-bank plant roots. Moreover, the riparian plant roots and the RAR provided additional cohesion to differing degrees. In general, the modeling results and our field investigations confirmed that riparian plant roots lowered the frequency of bank collapse, greatly reducing the total volume of bank erosion. Plant roots are therefore of great importance for maintaining riverbank stability and channel meandering along the middle Tarim River. Evidence from other studies supports our results in the Tarim River. For example, the planform

Table 6 Bank collapse width and amount of eroded bank slope in each model run. Vegetation species

PA

Sampling point

I II

TR

III V

PE

IV

VI

Run no.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Occurrence of bank failure

1 7 0 10 6 12 2 2 2 2 6 4 3 3 11 3 3 12

Collapse width (m)

1.21 6.78 0 9.79 7.87 14.51 2.91 2.89 2.90 2.93 7.69 5.40 4.13 4.19 14.94 4.32 4.43 14.80

Erosion volume in a 100 m–long channel reach Ef (m3)

Erosion reduced (%)

Ec (m3)

Erosion reduced (%)

Et (m3)

Total erosion reduced (%)

87 382 109 532 453 794 131 132 131 117 378 243 226 225 998 258 239 788

77.2 – 79.5 – 42.9 – 65.3 65.1 65.3 69.0 – 75.7 77.4 77.5 – 67.3 69.7 –

225 1270 0 1830 2157 4047 646 640 643 667 2051 1379 1019 1041 4208 1074 1090 4074

82.3 – 100.0 – 46.7 – 68.5 68.8 68.6 67.4 – 67.2 75.8 75.2 – 73.6 73.2 –

312 1652 109 2362 2610 4841 777 772 774 784 2429 1622 1245 1266 5206 1332 1329 4862

81.1 – 95.4 – 46.1 – 68.0 68.2 68.1 67.7 – 68.8 76.1 75.7 – 72.6 72.7 –

PA: Phragmites australis; TR: Tamarix ramosissima; PE: Populus euphratica; Ef: fluvial erosion (bank toe erosion); Ec: erosion by bank collapse; Et: total erosion volume.

10

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

Fig. 8. Box-plot of distribution of bank stability coefficient (Fs) under different root conditions simulated using the BSTEM. (a) with roots of Phragmites australis (PA), Tamarix ramosissima (TR), and Populus euphratica (PE); and (b) no roots.

pattern of the San Juan River, a dryland river in Utah, USA, changed from braided to meandering/anabranching over 50 yr (1928 to 1982) because of human introduction of Tamarix (Graf, 1988). Similarly, the braided channel of the Green River, also in Utah, changed to anabranching pattern because of the invasion of Tamarix (Birken and Cooper, 2006). In West Bijou Creek, Colorado, USA, the channel pattern changed from braided to meandering in fewer than 30 yr (1969–1993) because of the development of riparian vegetation (Camporeale et al., 2013). In contrast, the clearance of natural vegetation for land reclamation in late nineteenth century in the Sacramento River, USA, resulted in a sharp decrease in channel sinuosity over two decades (from 2.2 in 1874 to 1.4 in 1898) (Lazarus and Constantine, 2013). The case of the Sacramento River was quite consistent with what happened in the Tarim River. The Tarim Basin has undergone intense and increasing human intervention over recent decades (particularly since the late 1950s, during which large-scale land reclamation took place in the basin). Natural vegetation in the Tarim Basin declined mainly as a result of the reclamation-induced clearance. The area of the Tugai forest (dominated by PE) in this basin has decreased sharply. This is particularly true for the upper reaches, where previously natural vegetation has been modified to predominantly agricultural land (Fig. 2a; c.f., Fig. 3 in Yu et al., 2016). Satellite images of the upper reaches of the Tarim River show that, many old river channels, which have been abandoned (at least since before the large-scale reclamation in late 1950s), existed near the current river channel. These abandoned channels were likely once the flow paths of the Tarim River or its tributaries. Such images indicate that the abandoned channels have much higher sinuosity and smaller channel widths than the current channel (c.f., Yu et al., 2016). Therefore, it can be deduced that the clearance of natural vegetation along the upper reaches of the Tarim River is an important reason for the

development of the braided channel pattern, and, more specifically, the changing of the previous meandering channel to the current braided one. The Tugai forest in the middle reaches of the Tarim River has not been as seriously cleared for reclamation as that in the upper reaches has. The forest near the Tarim River channel is basically intact, which is a positive factor in the maintenance of the meandering channels in the middle reaches of the river. 4. Conclusions The roots of riparian vegetation in the middle reaches of the Tarim River provide additional cohesive reinforcement for the unconsolidated soils of the riverbank and increase the bank stability coefficient of the meandering channel. The occurrence and volume of bank collapse, bank-toe erosion, and lateral migration rate were thus greatly reduced via reinforcement with plant roots. This is conducive to the formation and development of meander bends in the extremely dry and unconsolidated sediment environment of the Tarim Basin. The mechanical effects of the three plant species on riverbank stability were quantified via a combination of field measurements and the BSTEM. PA (a reed), TR (a shrub), and PE (a tree) were found to enhance riverbank stability by 88.2, 63.6, and 73.2%, respectively. Consequently, riparian plant roots verifiably play a role in developing and sustaining meander bends in this uniformly silt- and sand-dominated, highly erodible channel in the Tarim River. However, the effects were varied between the studied plant species and root conditions. Future studies should thus explore the impacts of such heterogeneity on bank stability based on in situ measurements of root properties and monitoring of bank collapses.

Fig. 9. Eroded sediment volume (V) through bank collapse and fluvial scouring at bank toe of six sites with and without roots based on BSTEM simulations. PA: Phragmites australis; TR: Tamarix ramosissima; PE: Populus euphratica.

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

Declaration of competing interest All authors declare that we have no conflict of interest. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (41571009, 41971010, 51979012, 51709020) and Project of Hunan Science & Technology Plan (2018SK1010). We thank the constructive comments and suggestions provided by anonymous reviewers and Editor-in-Chief (Prof. Scott A. Lecce) to improve the quality of this manuscript. References Abernethy, B., Rutherfurd, I.D., 2000. The effect of riparian tree roots on the mass-stability of riverbanks. Earth. Surf. Proc. Land. 25, 921–937. Abernethy, B., Rutherfurd, I.D., 2001. The distribution and strength of riparian tree roots in relation to riverbank reinforcement. Hydrol. Process. 15, 63–79. Arulanandan, K., Gillogley, E., Tully, R., 1980. Development of a quantitative method to predict critical shear stress and rate of erosion of natural undisturbed cohesive soils. Waterway Experiment Station Technical Report GL-80-5. University of California at Davis, Davis, CA. Baets, S.D., Poesen, J., Reubensm, B., Wemans, J., Baerdemaeker, D., Muys, B., 2008. Root tensile strength and root distribution of typical Mediterranean plant species and their contribution to soil shear strength. Plant Soil 305, 207–226. Billi, P., Demissie, B., Nyssen, J., Moges, G., Fazzini, M., 2018. Meander hydromorphology of ephemeral streams: Similarities and differences with perennial rivers. Geomorphology 319, 35–46. Birken, A.S., Cooper, D.J., 2006. Processes of Tamarix invasion and floodplain development along the lower Green River. Utah. Ecol. Appl. 16, 1103–1120. Bischetti, G.B., Chiaradia, E.A., Simonato, T., Speziali, B., Vitali, B., Vullo, P., Zocco, A., 2005. Root strength and root area ratio of forest species in Lombardy (Northern Italy). Plant Soil 278, 11–22. Braudrick, C.A., Dietrich, W.E., Leverich, G.T., Sklar, L.S., 2009. Experimental evidence for the conditions necessary to sustain meandering in coarse-bedded rivers. Proc. Nat. Acad. Sci. 106, 16936–16941. Camporeale, C., Perucca, E., Ridolfi, L., Gurnell, A.M., 2013. Modeling the interactions between river morphodynamics and riparian vegetation. Rev. Geophys. 51, 379–414. Canadell, J., Jackson, R.B., Ehleringer, J.R., Mooney, H.A., Sala, O.E., Schulze, E.D., 1996. Maximum rooting depth of vegetation types at the global scale. Oecologia 108, 583–595. Chen, Y.N., Xu, C.C., Hao, X.M., Li, W.H., Chen, Y.P., Zhu, C.G., Ye, Z.X., 2009. Fifty-year climate change and its effect on annual runoff in the Tarim River Basin. China. Quatern. Int. 208, 53–61. Chow, V.T., 1959. Open Channel Hydraulics. McGraw-Hill, New York, pp. 112–113. Constantine, J.A., Dunne, T., 2008. Meander cutoff and the controls on the production of oxbow lakes. Geology 36, 23–26. Constantine, J.A., McLean, S.R., Dunne, T., 2009. A mechanism of chute cutoff along large meandering rivers with uniform floodplain topography. Geol. Soc. Am. Bull. 122, 855–869. Constantine, J.A., Dunne, T., Ahmed, J., Legleiter, C., Lazarus, E.D., 2014. Sediment supply as a driver of river meandering and floodplain evolution in the Amazon Basin. Nat. Geosci. 7, 899–903. Corenblit, D., Tabacchi, E., Steiger, J., Gurnell, A.M., 2007. Reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors: a review of complementary approaches. Earth-Sci. Rev. 84, 56–86. Corenblit, D., Baas, A.C.W., Bornette, G., Darrozes, J., Delmotte, S., Francis, R.A., Gurnell, A.M., Julien, F., Naiman, R.J., Steiger, J., 2011. Feedbacks between geomorphology and biota controlling earth surface processes and landforms: a review of foundation concepts and current understandings. Earth-Sci. Rev. 106, 307–331. Durocher, M.G., 1990. Monitoring spatial variability of forest interception. Hydrol. Process. 4, 215–229. Engle, F.L., Rhoads, B.L., 2012. Interaction among mean flow, turbulence, bed morphology, bank failures and channel planform in an evolving compound meander loop. Geomorphology 163-164, 70–83. Ennos, A.R., 1990. The Anchorage of leek seedlings: the effect of root length and soil strength. Ann. Bot-London. 65, 409–416. Gale, M.R., Grigal, D.F., 1987. Vertical root distributions of northern tree species in relation to successional status. Can. J. For. Res. 17, 829–834. Graf, W.L., 1988. Fluvial Processes in Dryland Rivers. Springer-Verlag, Berlin, Germany. Gran, K., Paola, C., 2001. Riparian vegetation controls on braided stream dynamics. Water Resour. Res. 37, 3275–3283. Gran, K., Wartman, E.D., 2015. Co-evolution of riparian vegetation and channel dynamics in an aggrading braided river system, Mount Pinatubo. Philippines. Earth Surf. Proc. Landf. 40, 1101–1115. Gurnell, A.M., Petts, G.E., 2006. Trees as riparian engineers: Tagliamento River. Italy. Earth. Surf. Proc. Landf. 31, 1558–1574. Gurnell, A.M., Bertoldi, W., Corenblit, D., 2012. Changing river channels: the roles of hydrological processes, plants and pioneer fluvial landforms in humid temperate, mixed load, gravel bed rivers. Earth-Sci. Rev. 111, 129–141. Hickin, E.J., 1984. Vegetation and river channel dynamics. Can. Geogr. 28, 111–126.

11

Hopkinson, L., Wynn, T., 2009. Vegetation impacts on near bank flow. Ecohydrology 2, 404–418. Hupp, C.R., Osterkamp, W.R., 1996. Riparian vegetation and fluvial geomorphic processes. Geomorphology 14, 277–295. Jackson, R.B., Canadell, J., Ehleringer, J.R., Mooney, H.A., Sala, O.E., Schulze, E.D., 1996. A global analysis of root distributions for terrestrial biomes. Oecologia 108, 389–411. Kleinhans, M.G., 2010. Sorting out river channel patterns. Prog. Phys. Geogr. 34, 287–326. Krzeminska, D., Kerkhof, T., Skaalsveen, K., Stolte, J., 2019. Effect of riparian vegetation on stream bank stability in small agricultural catchments. Catena 172, 87–96. Lazarus, E.D., Constantine, J.A., 2013. Generic theory for channel sinuosity. PNAS 110 (21), 8447–8452. Li, Z.W., Gao, P., 2019. Channel adjustment after artificial neck cutoffs in a meandering river of the Zoige basin within the Qinghai-Tibet Plateau, China. Catena 172, 255–265. Li, Z.W., Yu, G.A., Brierley, G.J., Wang, Z.Y., 2016. Vegetative impacts upon bedload transport capacity and channel stability for differing alluvial planforms in the Yellow River Source Zone. Hydrol. Earth Syst. Sci. 20, 3013–3025. Li, Z.W., Yu, G.A., Brierley, G.J., Wang, Z.Y., Jia, Y.H., 2017. Migration and cutoff of meanders in the hyperarid environment of the middle Tarim River, northwestern China. Geomorphology 276, 116–124. Lindow, N., Fox, G.A., Evans, R.O., 2010. Seepage erosion in layered stream bank material. Earth. Surf. Proc. Land. 34, 1693–1701. Matsubara, Y., Howard, A.D., Burr, D.M., Williams, R.M.E., Dietrich, W.E., Moore, J.M., 2015. River meandering on Earth and Mars: a comparative study of Aeolis Dorsa meanders, Mars and possible terrestrial analogs of the Usuktuk River, AK, and the Quinn River, NV. Geomorphology 240, 102–120. Mattia, C., Bischetti, G.B., Gentile, F., 2005. Biotechnical characteristics of root systems of typical Mediterranean species. Plant Soil 278, 23–32. Micheli, E.R., Kirchner, J.W., Larsen, E.W., 2010. Quantifying the effect of riparian forest versus agricultural vegetation on river meander migration rates, central Sacramento river, California, USA. River Res. Appl. 20, 537–548. Midgley, T.L., Fox, G.A., Heeren, D.M., 2012. Evaluation of the bank stability and toe erosion model (BSTEM) for predicting lateral retreat on composite streambanks. Geomorphology 145-146, 107–114. Motta, D., Langendoen, E.J., Abad, J.D., García, M.H., 2014. Modification of meander migration by bank failures. J. Geophys. Res-Earth Surf. 119, 1026–1042. Nicoll, T.J., Hickin, E.J., 2010. Planform geometry and channel migration of confined meandering rivers on the Canadian prairies. Geomorphology 116, 37–47. Nilaweera, N.S., Nutalaya, P., 1999. Role of tree roots in slope stabilisation. B. Eng. Geol. Environ. 57, 337–342. Norris, J.E., 2005. Root reinforcement by Hawthorn and oak roots on a highway cut-slope in Southern England. Plant Soil 278, 43–53. Operstein, V., Frydman, S., 2000. The influence of vegetation on soil strength. Ground Improv. 4, 81–89. Perucca, E., Camporeale, C., Ridolfi, L., 2007. Significance of the riparian vegetation dynamics on meandering river morphodynamics. Water Resour. Res. 43, W03430. Pollen, N., 2007. Temporal and spatial variability in root reinforcement of streambanks: Accounting for soil shear strength and moisture. Catena 69, 197–205. Pollen, N., Simon, A., Collision, A.J.C., 2004. Advances in assessing the mechanical and hydrologic effects of riparian vegetation on streambank stability. In Riparian Vegetation and Fluvial Geomorphology, Washington, D.C., pp. 125–139. Pollen-Bankhead, N., Simon, A., 2009. Enhanced application of root-reinforcement algorithms for bank-stability modeling. Earth. Surf. Proc. Land. 34 (4), 471–480. Pollen-Bankhead, N., Simon, A., 2010. Hydrological and hydraulic effects of riparian root networks on streambank stability: is mechanical root-reinforcement the whole story? Geomorphology 116, 353–362. Pollen-Bankhead, N., Simon, A., Jaeger, K., Wohl, E., 2009. Destabilization of streambanks by removal of invasive species in Canyon de Chelly National Monument, Arizona. Geomorphology 103, 363–374. Schenk, H.J., Jackson, R.B., 2002. The global biogeography of roots. Ecol. Monogr. 72, 311–328. Simon, A., 2005. Estimating the mechanical effects of riparian vegetation on stream bank stability using a fiber bundle mode. Water Resour. Res. 41, 226–244. Simon, A., Collison, A.J.C., 2010. Quantifying the mechanical and hydrologic effects of riparian vegetation on streambank stability. Earth. Surf. Proc. Landf. 27, 527–546. Simon, A., Pollen, N., Langendoen, E., 2006. Influence of two woody riparian species on critical conditions for streambank stability: upper Truckee River. California. J. Am. Water. Resour. Assoc. 42, 99–113. Simon, A., Pollen-Bankhead, N., Mahacek, V., Langendoen, E., 2010. Quantifying reductions of mass-failure frequency and sediment loadings from streambanks using toe protection and other means: Lake Tahoe, United States. J. Am. Water Resour. Assoc. 45, 170–186. Tal, M., Paola, C., 2010. Effects of vegetation on channel morphodynamics: results and insights from laboratory experiments. Earth. Surf. Proc. Landf. 35, 1014–1028. Thorne C.R. 1990. Effects of vegetation on riverbank erosion and stability. In Vegetation and Erosion, Thorne J.B. (Ed.). John Wiley and Sons: Chichester, 125– 144. Tooth, S., Nanson, G.C., 2000. The role of vegetation in the formation of anabranching channels in an ephemeral river, Northern Plains, arid central Australia. Hydrol. Process. 14, 3099–3117. Tosi, M., 2007. Root tensile strength relationships and their slope stability implications of three shrub species in the Northern Apennines (Italy). Geomorphology 87, 268–283. Twidale, C., 2004. River patterns and their meaning. Earth-Sci. Rev. 67, 159–218. Van Dijk, W.M., Teske, R., van de Lageweg, W.I., Kleinhans, M.G., 2013. Effects of vegetation distribution on experimental river channel dynamics. Water. Resour. Res. 49, 7558–7574.

12

G.-A. Yu et al. / Geomorphology 351 (2020) 106958

Vargas-Luna, A., Duró, G., Crosato, A., Uijttewaal, W., 2019. Morphological adaptation of river channels to vegetation establishment: a laboratory study. J. Geophys. ResEarth Surf. https://doi.org/10.1029/2018jf004878. Waldron, L.J., 1977. The shear resistance of root permeated homogeneous and stratified soil. Soil Sci. Soc. Am. J. 41, 843–849. Wu, T.H., McKinnell-III, W.P., Swanston, D.N., 1979. Strength of tree roots and landslides on Prince of Wales Island. Alaska. Can. Geotech. J. 16, 19–33. Xia, J.Q., Zong, Q.L., Deng, S.S., Xu, Q.X., Lu, J.Y., 2014. Seasonal variations in composite riverbank stability in the Lower Jingjiang Reach, China. J. Hydrol. 519, 3664–3673. Xia, J.Q., Deng, S.S., Lu, J.Y., Xu, Q.X., Zong, Q.L., Tan, G.M., 2016. Dynamic channel adjustments in the Jingjiang Reach of the Middle Yangtze River. Sci. Rep. 6, 22802.

Yu, G.A., Disse, M., Huang, H.Q., Yu, Y., Li, Z.W., 2016. River network evolution and fluvial process responses to human activity in a hyper-arid environment - case of the Tarim River in Northwest China. Catena 147, 96–109. Yu, G.A., Li, Z.W., Disse, M., Huang, H.Q., 2017. Sediment dynamics of an allogenic river channel in a very arid environment. Hydrol. Process. 31, 2050–2061. Zhu, H.L., Hu, X.S., Li, Z.W., Song, L., Li, K., Li, X.L., Li, G.R., 2018. The influences of riparian vegetation on bank failures of a small meadow-type meandering river. Water 10, 6. https://doi.org/10.3390/w10060692. Zong, Q.L., Xia, J.Q., Zhou, M.R., Deng, S.S., Zhang, Y., 2017. Modelling of the retreat process of composite riverbank in the Jingjiang Reach using the improved BSTEM. Hydrol. Process. 31, 4669–4681.