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Article Abrasion Behavior and Anti-Wear Measures of Debris Flow Drainage Channel with Large Gradient

Dongxu Yang 1,2,3, Yong You 1,*, Wanyu Zhao 1, Hai Huang 2, Hao Sun 1,3 and Yang Liu 1,3

1 Key Laboratory of Mountain Hazards and Earth Surface Processes, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China; [email protected] (D.Y.); [email protected] (W.Z.); [email protected] (H.S.); [email protected] (Y.L.) 2 Institute of Exploration Technology, Chinese Academy of Geological Sciences, Chengdu 611734, China; [email protected] 3 University of Chinese Academy Sciences, Beijing 100049, China * Correspondence: [email protected]; Tel.: +86-028-8523-2541



Received: 25 May 2020; Accepted: 25 June 2020; Published: 29 June 2020


Abstract: Debris flow gullies have high potential energy and geomorphic characteristics including a steep longitudinal slope and abundant loose material sources. They often experience debris flow with a strong impact force and a large instantaneous flow. Drainage engineering measures are most commonly used for mitigation in these gullies. However, the abrasion of drainage channels with large gradients (DCLG) is complex and strong because of the high-speed flushing of debris. In this study, the abrasion behavior of debris flow in DCLG is analyzed based on the kinematic characteristics and the theory of composite abrasive wear. Energy dissipation and anti-wear measures are suggested, and their effects are summarized with reference to a case study and in situ observation. The results show that there are four main types of wear morphology in drainage channels. The abrasion system of drainage channels shows the characteristics of system dependency, time dependency and multidisciplinary coupling. Energy dissipation and anti-wear measures include prefabricated reinforced concrete boxes as substrate, transverse roughening belts, adding a wear-resistant admixture, etc. The flow velocity of the debris flow is reduced by 5.7–37.1% after passing through the energy dissipation section. The distribution of abrasion and the mud depth show that the variation trend of the flow velocity in the channel is ”acceleration deceleration reacceleration“. According to → → tracking observations during two flood seasons, the energy dissipation and anti-wear measures are the most effective.

Keywords: debris flow; drainage channel; large gradient; energy dissipation; scour and abrasion

1. Introduction Debris flow gullies in earthquake-affected areas and mountain areas vary, with differences in the micro-geomorphic conditions, loose material conditions and hydrological conditions [1]. High potential energy debris flow is a type of debris flow which has a steep topography in the longitudinal direction and a narrow topography in the transverse direction, and its activity characteristics and mitigation measures are distinct from others [2]. According to relevant research [3], there was a considerable amount of debris flow in the area affected by the Wenchuan MS 8.0 earthquake which occurred on 12 May 2008, which was observed in a large gradient gully with a watershed area of F 5 km2, ≤ a gully bed with an average longitudinal slope of I 300% and a basin integrity coefficient of δ 0.4 ≥ ≤ (a parameter representing the shape of a basin, which can be calculated by dividing the watershed area by the square of the length of the gully [4]). Such debris flows are defined as ”narrow-steep“ debris flows. Compared with ordinary debris flow gullies, such debris flow gullies feature a higher

Water 2020, 12, 1868; doi:10.3390/w12071868 www.mdpi.com/journal/water Water 2020, 12, 1868 2 of 17 longitudinal gradient, a larger amount of material source per square kilometer, a reduced critical rainfall for triggering debris flow and a high frequency and group occurrence of debris flow events. Typical cases are the mass debris flows which occurred on 13 August 2010 in Northwest Sichuan, China [5], the mass debris flows which occurred on 17 August 2012 in Mianzhu City, and the debris flows which occurred on 20 August 2019 in Wenchuan County, resulting in greater damage, loss and environmental impacts than those which occurred before the earthquake [6]. In terms of the mitigation strategies for debris flows with a high potential energy, the engineering measures are also different from those of common debris flows [7]. A reasonable drainage gradient of a conventional debris flow is generally 30% to 100% (dilute debris flow) or 50% to 180% (viscous debris flow) [8], while the average longitudinal slope of a high potential debris flow is generally more than 300%. Large altitude differences in the catchment area have two consequences: one is the high flow velocity of the debris flow, some of which can be as high as 8.0m/s; and the other is the strong carrying capacity, by which it can carry a large quantity of large-sized boulders (generally with a grain size 1.0 m). For example, the largest boulder found in the Sanyanyu debris flow gully was ≥ 12.9 m 8.6 m 7.2 m [7]. Therefore, the debris flow’s ability to abrade is significant for the movement × × process. In projects for debris flow prevention and control, the limit velocity of the abrasion resistance of masonry material is 8–10 m/s and that of concrete material is 10–12 m/s [8]. If the velocity of the debris flow exceeds this limit, the structure will suffer serious abrasion, which will affect its normal operation, even reducing its service life. The need to effectively reduce the abrasion damage of the debris flow to the bottom plate and side wall of the drainage channel and to ensure the set service life of the drainage channel are important issues in the design of drainage channels with large gradients (DCLG) [3,9]. From the perspective of engineering applications, the possible solutions can be summarized as follows [10]: (1) the approach of optimizing the structural design so as to dissipate energy at the channel bottom; (2) the measure of improving the construction material’s performance, such as improving the concrete grade and adding admixtures; (3) the measure of adding auxiliary structures, such as laying formed steel structures on the channel bottom. A series of experimental studies were carried out using a new type of drainage channel with an energy dissipation structure [11–14], as well as different types and energy dissipation characteristics of debris flow deceleration baffles [15]. Chen [16] proposed a debris flow drainage channel with a step-pool configuration. Zhou [17] analyzed the erosion mechanism and stress of a debris flow slurry on the drainage channel, but this study was insufficiently supported by quantitative data. In terms of the use of auxiliary materials to improve the wear resistance of concrete, the current research mainly focuses on the wear resistance of the structural concrete of hydraulic structures, such as spillways, discharge holes and ship locks. Steel fibers, epoxy resin, steel rail and steel plate are added to the concrete as auxiliary measures [18–21]. For example, silicon powder with high hardness is suitable for micro-cutting wear, while polypropylene fibers and steel fibers with high flexibility and integrity are suitable for mitigating the impact of deformation wear [22]. However, most of the current research is focused upon model experimentation and numerical simulation, which have not been applied and efficiency-tested in engineering practice. It is necessary to further study the abrasion characteristics, energy dissipation and anti-wear technical measures of the DCLG for steep gullies. The influencing factors of debris flow abrasion capacity, the composition of the abrasion system and features of abrasion morphology, the regulating effects of energy dissipation and anti-wear measures of the drainage channel and the design and construction methods should be included so as to expand the applicable range of the use of drainage channels as a mitigation measure and improve the service life and operational uses of drainage channels. Water 2020, 12, 1868 3 of 17

2. Materials and Methods

2.1. Study Site Description This study takes the Wenchuan earthquake zone, a representative area in the high mountain region of Southwest China, as the research area, and focuses on the Xiaogangjian debris flow drainage Water 2020, 12, x FOR PEER REVIEW 3 of 18 channel as an example in order to analyze the characteristics of abrasion and anti-wear measures. The Xiaogangjiandrainage channel gully as an is locatedexample inin theorder town to analyze of Qingping, the characteristics Mianzhu City,of abrasion Sichuan and Province. anti-wear It is on themeasures. right side The of Xiaogangjian the road from gully Mianzhu is located to Maoxian in the town and of on Qingping, the left bank Mianzhu of the City, Mianyuan Sichuan River. The coordinatesProvince. It is of on the the gully right fan side are of Nthe 31 road◦300 14.75”from Mianzhu and E 104 to ◦Maoxian07044.55”. and The on the horizontal left bankform of the of the watershedMianyuan is shaped River. Thelike coordinatesa salix leaf of (Figure the gully1), and fan the are areaN 31°30 of land′14.75″ sloping and E 104°07′44.55″. by more than The 25 ◦ in the gullyhorizontal accounts form forof the 85.29% watershed of the is totalshaped basin like a area. salix Moreover,leaf (Figure the1), and free the face area has of land become sloping formed by of loosemore deposits, than 25° which in the provides gully accounts favorable for conditions85.29% of the for total the rapidbasin area. collection Moreover of rainfall, the free runo faceff hasand the become formed of loose deposits, which provides favorable conditions for the rapid collection of development of adverse geological phenomena such as collapse, landslide and unstable slopes, as well rainfall runoff and the development of adverse geological phenomena such as collapse, landslide as theand transportation unstable slope ofs, aas loose well as solid the transportation source of debris of a flow.loose solid source of debris flow.

FigureFigure 1. Location 1. Location of Xiaogangjian of Xiaogangjian gully andgully its and full view. its full (a ) view Remote. (a) sensingRemote image sensing of Xiaogangjian’s image of watershedXiaogangjian in 2010’s andwatershed (b) layout in 2010 of and the downstream(b) layout of the mitigation downstream works mitigation in 2011. works in 2011.

The XiaogangjianThe Xiaogangjian gully gully is a typical is a typical debris debris flow with flow a with high a potential high pote energy,ntial energy, due to a due longitudinal to a longitudinal slope of 506‰ in the formation area. The cross-section of the gully is of a deep "V" slope of 506% in the formation area. The cross-section of the gully is of a deep “V” shape. shape. The longitudinal gradient of the outlet section of the gully is 337.8‰, and on the left bank, The longitudinal gradient of the outlet section of the gully is 337.8%, and on the left bank, there is there is a loose accumulation with a volume of 2.26 × 105 m3 and a slope of 42°~70°. There was no a loose accumulation with a volume of 2.26 105 m3 and a slope of 42 ~70 . There was no ditch ditch developed before the “5.12” earthquake.× After the earthquake,◦ the◦ topographic and developedhydrodynamic before theconditions “5.12” earthquake.of the gully were After facilitated, the earthquake, and it was the eroded topographic intensely.and Then hydrodynamic, the gully conditionsdeveloped of the a high gully susceptibility were facilitated, and high and frequency it was eroded of occurrence. intensely. From Then, 12 May the 2008 gully to developed 5 September a high susceptibility2011, 10 debris and highflows frequency with a total of volume occurrence. of more From than 125 × May 104 m 20083 occur tor 5ed, September resulting in 2011, the gully 10 debris flowsbeing with cut a total into volumea deep trench of more with than a depth 5 10 of4 30m–345occurred, m and a widt resultingh of 48 in–65 the m. gully The debris being cutflow into burst a deep × trenchout with of the a depthgully and of 30–45rushed m into and the a widthMianyuan of 48–65River. m.It not The only debris buried flow the bursthighway out several of the times gully and rushedbut into also the blocked Mianyuan the river, River. forming It not only a barrier buried lake the highwayupstream several of the accumulationtimes but also fan. blocked The the geomorphological parameters of the gully are listed in Table 1. river, forming a barrier lake upstream of the accumulation fan. The geomorphological parameters of the gully are listed in TableTable1. 1. Geomorphological parameters of Xiaogangjian gully.

Ditch Length Mean Gradient Minimum Maximum Basin Area (km2) (km) (‰) Elevation (m) Elevation (m) 1.36 2.59 412 810 1987

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Table 1. Geomorphological parameters of Xiaogangjian gully.

Mean Gradient Minimum Maximum Basin Area (km2) Ditch Length (km) (%) Elevation (m) Elevation (m) 1.36 2.59 412 810 1987

2.2. Disaster Mitigation Needs Water 2020, 12, x FOR PEER REVIEW 4 of 18 For this kind of debris flow mitigation, there is a significant disparity between the large amount of loose2.2. materialDisaster Mitigation sources andNeeds the limited retaining conditions. According to a geological survey, there is, in total,For 3.343 this kind106 ofm debris3 of loose flow materialmitigation, in there the Xiaogangjian is a significant gully, disparity and between the possible the large dynamic amount reserve × involvedof loose in the material debris sources flow activities and the limited measures retaining 1.568 conditions.106 m3. The According calculated to a loosegeological sources survey, per km2 × are 2.458there is10, in6 mtotal3/km, 3.3432, which × 106 m indicates3 of loose that material the debris in the flowXiaogang wasjian controlled gully, and by the the possible transport dynamic of material × 6 3 in thereserve 5–10 yearsinvolved following in the debris the earthquake flow activities [23 ].measures 1.568 × 10 m . The calculated loose sources 2 6 3 2 However,per km are because2.458 × 10 of m the/km narrow, which and indicates steep terrainthat the conditions,debris flow was the econtrolledffective intervention by the transport area for of material in the 5–10 years following the earthquake [23]. retaining works, such as checking dams, is limited to only 15% of the total area of the basin. Moreover, However, because of the narrow and steep terrain conditions, the effective intervention area for the width–depthretaining works, ratio such of the as ditch check ising only dams, 1.0~1.5, is limited so the todam only locations 15% of that the are total suitable area of for the construction basin. and theMoreover reservoir, the capacity width–depth are very ratio limited.of the ditch Once is only debris 1.0~ flow1.5, so occurs the dam in locations the upper that reaches, are suitable the old for loose depositsconstruction in the lower and the reaches reservoir of the capacity basin willare bevery cut limited. through Once in order debris to flow increase occurs the in supply the upper sources alongreaches, the gully the and old increase loose deposits the scale in the of the lower debris reaches flow. of the basin will be cut through in order to Fromincrease above, the supply due to sources the need along to preventthe gullythe and formation increase the of scale loose of deposits the debris at flow. the terminus of the gully and to protectFrom theabove, highway due to onthe theneed fan to andprevent change the formation the angle of loose the debris deposits flow at enteringthe terminus the of Mianyuan the River,gully it is and necessary to protect tobuild the highway a drainage on the channel fan and in chan orderge to the quickly angle of drain the debris the rush-out flow entering materials the from the middleMianyuan regions River, to it the is necessary outside of to thebuild gully a drainage (Figure 2channel). The channelin order willto quickly pass overdrain the the highwayrush-out and materials from the middle regions to the outside of the gully (Figure 2). The channel will pass over enter the desilting field. Therefore, a DCLG must be adopted. the highway and enter the desilting field. Therefore, a DCLG must be adopted.

FigureFigure 2. Analysis 2. Analysis of design of design needs needs for drainage for drainage channel channel of Xiaogangjian of Xiaogangjian debris debris flow. (The flow. continuous (The continuous arrows indicate that a drainage channel with a large gradient (DCLG) is needed for the arrows indicate that a drainage channel with a large gradient (DCLG) is needed for the mitigation and mitigation and it will encounter abrasion, and the dashed arrows show the countermeasures for it will encounter abrasion, and the dashed arrows show the countermeasures for reducing the abrasion; reducing the abrasion; the two blocks with different colors indicate key nodes in the design.). the two blocks with different colors indicate key nodes in the design.). 2.3. The Abrasion Behavior of DCLG 2.3. The Abrasion Behavior of DCLG Abrasion phenomena are caused by the local jet or circulation which is formed at the bottom of Abrasionthe structural phenomena plane by are the caused turbulent by flow the local of fluid jet or with circulation high kinetic which energy, is formed which at drives the bottom the of the structuralrepeated impact plane by and the scour turbulent of the flow block of stone fluid and with gravel. high kineticAbrasion energy, behavior which varies drives due the to repeated the impactcha andnges scour in fluid of properties, the block stonemovement and characteristics gravel. Abrasion and channel behavior characteristics. varies due to Debris the changes flows with in fluid properties,high bulk movement density characteristicsand high viscosity and have channel the characteristics.characteristics ofDebris wide gradation, flows with huge high particles, bulk density and highcomplex viscosity interactions have and the energy characteristics exchange between of wide solid gradation, and liquid huge phases. particles, As a result, complex the abrasion interactions and energymechanism exchange of a debris between flow drainage solid and channel liquid is also phases. very complex, As a result, and it theis obviously abrasion different mechanism from of a debrisother flow types drainage of abrasion channel behavior is also such very as complex,mechanical and friction, it is obviouslywind sand diflow,fferent high from sediment other flow types of and so on. Under the combined action of debris flow slurry erosion and coarse particle impact, the lining surface of the drainage channel shows the process of ”shear or impact → strength weakening → crack generation and expansion → local stress concentration producing eddy current → gravel abrasion → gradual wear expansion and deepening → bottom stripping, cutting groove or potholes“. It could eventually cause the entirety of the bottom of the channel to be damaged, maybe

Water 2020, 12, 1868 5 of 17 abrasion behavior such as mechanical friction, wind sand flow, high sediment flow and so on. Under the combined action of debris flow slurry erosion and coarse particle impact, the lining surface of the drainage channel shows the process of ”shear or impact strength weakening crack generation → → and expansion local stress concentration producing eddy current gravel abrasion gradual → → → wear expansion and deepening bottom stripping, cutting groove or potholes“. It could eventually Water 2020, 12, x FOR PEER REVIEW→ 5 of 18 cause the entirety of the bottom of the channel to be damaged, maybe even causing the side wall to overturneven or causing wear through the side the wall bottom, to overturn thus losing or wear the through drainage the function. bottom, thus losing the drainage function. 2.3.1. Characteristics of Abrasion Morphology (AM) 2.3.1. Characteristics of Abrasion Morphology (AM) According to field investigation in the study area, the AM of debris flow on structures is mainly divided intoAccording four categories to field investigation (Figure3): (a) in Relativelythe study area, uniform the AM surface of debris peeling flow (USP);on structures that is, is the mainly scraping divided into four categories (Figure 3): (a) Relatively uniform surface peeling (USP); that is, the on the surface is basically flat and creates a smooth layer by layer thinning, which appears on the scraping on the surface is basically flat and creates a smooth layer by layer thinning, which appears bottomon surface the bottom of the surface drainage of the channel drainage and channel the inner and the faces inner of the face sides of thewalls, side when walls, the when particles the in the fluidparticles are relativelyin the fluid uniform are relatively and uniform the flow and state the is flow stable; state (b) is stable; Plowing (b) Plowing groove alonggroove the along streamline the directionstreamline (PG), whichdirection generally (PG), which appears generally at the appears bottom at of the a fullybottom lined of a drainagefully lined channel, drainage which channel, is steep and long,which with is steep the and fluid long, viscous, with the the fluid bulk viscous, density the highbulk density and the high fluid and composed the fluid composed of hard and of hard angular blockand stones; angular (c) Jumping block stones; erosion (c) pits Jumping (JEP), erosion which are pits similar (JEP), towhich the “potholes”are similar developed to the "potholes" on the bed rock ofdeveloped the high on sandy the bed rivulets rock and of the rivers high in sand mountainy rivulets areas and [24 rivers], which in mountain mainly occur areas on[24 the], which bottom of a fullymainly lined occur channel on withthe bottom a large of longitudinal a fully lined slopechannel or underwith a thelarge drop-sills—the longitudinal slope flow or is under turbulent the and drop-sills—the flow is turbulent and easily forms eddy currents; (d) Surface wear of the ground sills easily forms eddy currents; (d) Surface wear of the ground sills (SWGS), mainly in the middle of the (SWGS), mainly in the middle of the rib sill, forming a transverse slope from both sides to the rib sill, forming a transverse slope from both sides to the middle. middle.

FigureFigure 3. Typical 3. Typical abrasion abrasion morphology morphology of mitigation of mitigation works works of debrisof debris flow. flow (a). uniform(a) uniform surface surface peeling (USP)peeling in Yinxingping (USP) in Yinxingping gully, Wenchuan gully, county; Wenchuan (b) plowingcounty; (b groove) plowing (PG) groove in Haermu (PG) in gully, Haermu Lixian gully county;, Lixian county; (c) jumping erosion pits (JEP) in Xiaogangjian gully, Mianzhu county; (d) surface (c) jumping erosion pits (JEP) in Xiaogangjian gully, Mianzhu county; (d) surface wear of the ground wear of the ground sills (SWGS) in Qinglinwan gully, Beichuan county. sills (SWGS) in Qinglinwan gully, Beichuan county. In field investigations, several forms of AM usually exist at the same time. With a change in working conditions, different forms of wear have different primary and secondary causes.

2.3.2. Composition and Influencing Factors of Abrasion System

Water 2020, 12, 1868 6 of 17

In field investigations, several forms of AM usually exist at the same time. With a change in working conditions, different forms of wear have different primary and secondary causes. Water 2020, 12, x FOR PEER REVIEW 6 of 18 2.3.2. Composition and Influencing Factors of Abrasion System From the perspective of tribology, the debris flow and channel surface can be regarded as a tribologicalFrom the system perspective [25] (Fig ofure tribology, 4a). The the abrasive debris process flow and has channel both sliding surface friction can be properties regarded asand a rollingtribological friction system and [ 25micro] (Figure-corrosion4a). The friction abrasive properties process has [26 both]. As sliding a multiphase friction propertiesand multi- andingredient rolling fluid,friction debris and micro-corrosion flow should be frictionconsidered properties as a structural [26]. As afluid multiphase composed and of multi-ingredient slurry and coarse fluid, particles debris [2flow7,28 should]. Special be consideredfriction pairs as a are structural formed fluid by debriscomposed flow of and slurry the and structural coarse particles surface [27 of ,28 a drainage]. Special channel,friction pairs forming are formed the macro by debris-phenomenon flow and the of scour, structural abrasion surface or of deposition a drainage channel,upon the forming structural the surface.macro-phenomenon of scour, abrasion or deposition upon the structural surface.

Figure 4. Schematic diagram diagram of of abrasion abrasion system system of of DCLG DCLG.. ( (aa)) Profile Profile of of d debrisebris flow flow in the drainage channel; ( b) e elementslements of abrasion system of DCLG.

As a a tribological tribological system, system, abrasion abrasion can can be bestudied studied from from the effects the eff ectsof system of system dependence, dependence, time dependencetime dependence and multi and multi-disciplinary-disciplinary coupling coupling [29]. (shown The abrasion in Appendix behaviorA)[ 29is ].system The abrasion dependent, behavior and theis system characteristics dependent, of its and elements the characteristics are time dependent. of its elements The are elements time dependent. of the abrasion The elements system of of the DCLGabrasion are system shown of inthe Figure DCLG 4b are. shownIn the in abrasion Figure 4 system,b. In the relative abrasion motion system, and relative interaction motion occur and betweeninteraction the occur debris between flow andthe debris the structure flow and surface, the structure and the surface, two and friction the two pairs friction have pairs their have own properties.their own properties. The attributes The of attributes debris flow of debris include flow the includephysical, the mechanical physical, mechanicaland kinematic and properties kinematic, properties,and the attributes and the attributes of a drainage of a drainage channel channel are the arestructure the structure types typesand material and materials.s. In terms In terms of the of disciplinarythe disciplinary properti properties,es, these these include include the the physical physical wear wear effects effects of of sliding, sliding, impinging impinging and and friction friction on the bottom bottom surface surface of of the the drainage drainage channel channel by theby the block block stones stone in thes in debris the debris flow that flow occurred that occurred during duringthe abrasion the abrasion process, process as well, as asthe well chemical as the corrosionchemical corrosion between the between slurry the and slu concrete.rry and Moreover, concrete. Moreoverthe abrasion, the is abrasion also closely is also related closely to the related disciplines to the disciplines of mechanics of (fluidmechanics mechanics, (fluid mechanics, solid mechanics, solid mechanicsand so on),and and structural so on) and engineering. structural Theseengineering. attributes These and attributes behaviors and are concentratedbehaviors are and concentrated coupled in andthe system,coupled and in the they system vary, with and thethey system’s vary with state the and system time.’s state and time. Analyzing in in terms terms of of energy, energy, when when the the debris debris flow flow passes passes through through the the drainage drainage channel, channel, it overcomesit overcomes the the shear shear resistance resistance of ofthe the structural structural surface, surface, and and some some of the of theenergy energy is dissipated is dissipated [30]. [30 At]. Atthe thesame same time, time, some some of ofthe the energy energy will will be be dissipated dissipated when when the the fluid fluid collides collides with with the the structure structure’s’s surface and with the particles inside the fluid.fluid. Therefore, Therefore, the the abrasion abrasion power power of of the debris flow flow is mainly related toto suchsuch factors factors as as bulk bulk density, density, velocity, velocity, particle particle composition, composition, duration, duration, mud mud depth depth and andscouring scouring angle angle [28,31 [2,328,3],1 while,32], while the abrasion the abrasion resistance resistance of the of drainage the drainage channel channel is related is related to section to sectiontype, material type, strengthmaterial (shear strength strength, (shear compressive strength, compressive strength), aggregate strength), grading aggregate and hardness. grading and hardness.Based on the influencing factors, the abrasion quantity could be expressed as follows: Based on the influencing factors, the abrasion quantity could be expressed as follows: H = f = ( γ, ν, τ0, µ, FD, d90/d10, σ, α, E) (1) H （ f， ， 0， ， FD， d 90/ d 10， ， ， E） (1) where γ, ν, τ0, μ and FD respectively represent the density, velocity, initial shear force, viscosity coefficient and drag force; d90 and d10 represent the particle size of the debris flow, which refers to the particle size of a sample when the cumulative percentage of particle size distribution reaches 90%

Water 2020, 12, 1868 7 of 17

where γ, ν, τ0, µ and FD respectively represent the density, velocity, initial shear force, viscosity coefficient and drag force; d and d represent the particle size of the debris flow, which refers to the Water 2020, 12, x FOR PEER REVIEW90 10 7 of 18 particle size of a sample when the cumulative percentage of particle size distribution reaches 90% and 10%;and 10%σ, α; andσ, α Eandrespectively E respectively represent represent the positive the positive pressure, pressure, scouring scouring angle angle and elastic and elastic modulus modulus of the channelof the channel bottom. bottom.

2.3.3. Dynamic Source of the Abrasion 2.3.3. Dynamic Source of the Abrasion The dynamic source of the abrasion is the high potential energy of the debris flow. As they The dynamic source of the abrasion is the high potential energy of the debris flow. As they originate on high and steep hillsides, the longitudinal slopes of debris gullies are far larger than those originate on high and steep hillsides, the longitudinal slopes of debris gullies are far larger than of common rivers (Figure5). With the high potential energy caused by the large height di fference, those of common rivers (Figure 5). With the high potential energy caused by the large height debris flow washes against the DCLG, and the potential energy is rapidly converted into kinetic energy, difference, debris flow washes against the DCLG, and the potential energy is rapidly converted into with the flow velocity increasing along the path. The large particles contained in the fluid also move kinetic energy, with the flow velocity increasing along the path. The large particles contained in the and jump rapidly under the action of gravity and drag force, which causes the impact, peeling and fluid also move and jump rapidly under the action of gravity and drag force, which causes the furrowing of grooves on the drainage structure. impact, peeling and furrowing of grooves on the drainage structure.

Figure 5. Source of debris flow flow kinetics in narrow narrow-steep-steep gullies.gullies.

AccordingAccording to the analysis of the debris flowflow activities in the Xiaogangjian gully from 2009 to 2013, the the formation formation process process is is shown shown in inFigure Figure 5.5 The. The gully gully was was covered covered by a by layer a layer of very of verythick thickloose looseoverburden, overburden, and a and free a surface free surface developed. developed. Once Once the debris the debris flow isflow formed is formed under under the action the action of a ofrainstorm, a rainstorm, it will it will rapidly rapidly drain drain along along the the steep steep gully, gully, which which is is easily easily cut cut and and eroded. eroded. The The most important process is the conversion ofof thethe highhigh potentialpotential energyenergy intointo highhigh kinetickinetic energy.energy.

2.3.4. Calculation Method of Debr Debrisis Flow Abrasion The abrasion behavior of debris flow flow on the concrete of a drainage channel is as follows: a bed load with a large particle size acts on the the surface surface of of the the structure structure by by rolling rolling friction, friction, jumping jumping impact, impact, sliding friction, etc.,etc., andand thethe suspendedsuspended sediment sediment scours scours the the surface surface with with the the flow flow fluctuation, fluctuation, which which is similaris similar to the to abrasion the abrasion effect effect of sand of in sand high-speed in high sand-speed flow sand on a flow hydroelectric on a hydroelectric spillway.According spillway. toAccording the theory to ofthe abrasive theory of wear abrasive energy wear in sedimentenergy in ladensediment flow laden [33], flow the function [33], the of function the bed of load the andbed load and suspended load is expressed in a unified way and the bulk density of the debris flow is considered. The simplified expression can be written as follows:

112 2 2  I =[ ( Vs sin K )  V s cos  sin( n  )](  -  w )/( s -  ),(    s ) 22   (2) 112 2 2  IVKV =[ (s sin ) s cos  ](  -  w )/( s -  ),( ＞  s ) 22 

Water 2020, 12, 1868 8 of 17 suspended load is expressed in a unified way and the bulk density of the debris flow is considered. The simplified expression can be written as follows:

1 2 1 2 2  Iα= [ (Vs sin α K) + V cos α sin(nα)](γ γw)/(γs γ), (α αs)  2ε − 2ϕ s − − ≤  1 2 1 2 2  (2) Iα= [ (Vs sin α K) + V cos α](γ γw)/(γs γ), (α>αs)  2ε − 2ϕ s − − where Iα is the concrete weight loss rate of abrasion (%); α is the scouring angle (◦); αs is the critical angle of abrasion (◦); Vs is the velocity of the large particles in the debris flow (m/s); γ, γs and γw are the bulk density of the debris flow, solid particles and water (g/cm3); K, ε, ϕ and n are the critical velocity (m/s), the energy consumption factor of the impact wear (kg m2/g s2), the energy factor of · · the micro-cutting wear (kg m2/g s2) and the horizontal resilience factor (dimensionless), which can be · · determined by the concrete wear resistance experiment.

2.4. Research Procedures Theoretical analysis, field investigation, engineering practice and ongoing status observations were addressed in this study. Firstly, based on the investigation of the landform conditions and source distribution of the Xiaogangjian debris flow, the design parameters were put forward and the design concept of the DCLG was presented. Secondly, cases of drainage channels with similar natural geographical environments in the study area were investigated in order to determine the influencing factors of the abrasion. Then, the anti-wear countermeasures and details of the structure were designed and constructed. Finally, field observation was carried out during the debris flow’s outbreak period, including the abrasion development process, wear morphology and the distribution and amount of damage to the lining structure caused by the debris flow’s scour. Moreover, the response characteristics of the anti-wear measures of the DCLG were analyzed.

3. Results

3.1. Anti-Wear Measures of DCLG Structure type and size were designed according to the discharge demand and terrain characteristics. The designed structural parameters of the drainage channels are shown in Table2. The main body of the channel was cast with C25 reinforced concrete, with a full lining structure.

Table 2. The structural design parameters of the drainage channel of the Xiaogangjian debris flow.

Length Height Mean Maximum Width Depth Sectional Form (m) (m) Gradient (%) Gradient (%) (m) (m) 170.8 45.2 264.64 349.0 6.0 3.5 compound rectangle

The drainage channel was designed in three sections along the flow direction: the inlet drainage section (A), the middle steep straight section (B) and the outlet deceleration section (C), as shown in Figure6. When the debris flow enters the drainage channel, it first decelerates from the flat gradient of section A, then turns into the steep section B to accelerate, and it finally decelerates in the outlet section C. The length of section B was 95 m and the gradient was 349%. Without energy dissipation (ED) measures, the velocity of the debris flow in the fully lined drainage channel can be calculated according to the improved Manning formula [34]:

2 1 1 3 2 Vc = (Rc Ic ) (3) nc × where VC is the average velocity of the debris flow (m/s); nC is the roughness coefficient of the gully bed value according to the bottom condition of drainage channel; RC is the hydraulic radius of the calculation section (m); and IC is the hydraulic gradient of the debris flow. Water 2020, 12, 1868 9 of 17 Water 2020, 12, x FOR PEER REVIEW 9 of 18

FigureFigure 6. 6. DesignDesign drawing drawing of of longitudinal longitudinal section section of of DCLG DCLG in in Xiaogangjian Xiaogangjian gully. gully.

WithoutAccording energy to Formula dissipation (3), the (ED) calculated measures, velocity the velocity of the debris of the flowdebris in eachflow channelin the fully section lined is drainagelisted in Tablechannel3. can be calculated according to the improved Manning formula [34]:

Table 3. Segmented design parameters1 2 3 of Xiaogangjian 1 2 drainage channel. VRIc c  c  (3) nc 3 Section Length (m) Discharge (m /s) nc Rc (m) Ic Velocity (m/s) where V C is the average velocity of the debris flow (m/s); n C is the roughness coefficient A 40.0 0.025 0.66 0.117 10.40 of the gullyB bed value 100.2 according53.33 to the bottom 0.017 condition 0.40 of drainage 0.349 channel; 19.06 RC is the hydraulicC radius 30.6of the calculation section (m); 0.017 and IC 0.54is the hydraulic 0.120 gradient 13.53 of the debris flow. According to Formula (3), the calculated velocity of the debris flow in each channel section is It can be seen from Table3 that the designed gradient and velocity of this drainage channel listed in Table 3. exceeded the design properties of the concrete materials. In addition, the bulk density of the debris flow reached 2100 kg/m3, which can cause serious abrasion at the bottom of the channel and affect Table 3. Segmented design parameters of Xiaogangjian drainage channel. the durability and safety of the structure. Therefore, appropriate energy dissipation and anti-wear measuresSection must be considered.Length (m) Discharge (m3/s) nc Rc (m) Ic velocity (m/s) Three measuresA were40.0 introduced in the channel:0.025 0.66 0.117 10.40 B 100.2 53.33 0.017 0.40 0.349 19.06 The A section of the energy dissipation (ED) structure was set in the middle of the drainage • C 30.6 0.017 0.54 0.120 13.53 channel (Figure6, Sec. B 2). The bottom of the channel was lined with prefabricated reinforced Itconcrete can be (PRC)seen from boxes, Table arranged 3 that inthe a ladderdesigned shape gradient along a thend flow velocity direction, of this so drainage as to reduce channel the exceededkinetic the energy design and properties flow velocity; of the concrete materials. In addition, the bulk density of the debris In the lower section3 of the drainage channel (Figure6, Sec. B ), a number of roughening strips flow• reached 2100 kg/m , which can cause serious abrasion at the 3bottom of the channel and affect the durabilitywere added, and which safety were of the perpendicular structure. Therefore, to the flow appropriate direction and energy protruded dissipation from the and bottom anti- ofwear the measurestrough, must so asbe toconsidered. increase the roughness of the trough bottom and change the boundary conditions Threeof the measures debris flow’s were movement; introduced in the channel: A wear-resistant layer with a 50 mm thickness, made of C30 steel fiber concrete, was laid on the • The A section of the energy dissipation (ED) structure was set in the middle of the drainage surface of the channel bottom of Sec. B1 and Sec. B3 (Figure6) to improve the wear resistance of channel (Figure 6, Sec. B2). The bottom of the channel was lined with prefabricated reinforced the structure. concrete (PRC) boxes, arranged in a ladder shape along the flow direction, so as to reduce the 3.2. EDkinetic Structure energy Lined and Withflow PRCvelocity; Boxes  In the lower section of the drainage channel (Figure 6, Sec. B3), a number of roughening strips The construction methods of the ED section lined with PRC boxes were as follows [35,36]: (1) On were added, which were perpendicular to the flow direction and protruded from the bottom of the transverse, the cavity size of a single box was set to 1.0 m 1.2 m, which could be combined into the trough, so as to increase the roughness of the trough× bottom and change the boundary different horizontal rows according to the design width of the channel bottom; (2) In the longitudinal conditions of the debris flow’s movement;

Water 2020, 12, x FOR PEER REVIEW 10 of 18

 A wear-resistant layer with a 50 mm thickness, made of C30 steel fiber concrete, was laid on the surface of the channel bottom of Sec. B1 and Sec. B3 (Figure 6) to improve the wear resistance of the structure.

3.2. ED Structure Lined With PRC Boxes

Water 2020The, 12construction, 1868 methods of the ED section lined with PRC boxes were as follows [35,3106]: of (1) 17 On the transverse, the cavity size of a single box was set to 1.0 m × 1.2 m, which could be combined into different horizontal rows according to the design width of the channel bottom; (2) In the direction,longitudinal it was direction, of stepped it was form of accordingstepped form to the according design gradient to the ofdesign the channel, gradient and of eachthe channel, row of boxes and reducedeach row step of byboxes step reduce alongd the step flow by direction, step along and the the flow height direction, difference and of adjacentthe height steps difference should be of adjustedadjacent accordingsteps should to the be gradient;adjusted (3)according A deep buriedto the ribgradient; guard was(3) A set deep every buried 6 m, with rib guard a buried was depth set ofevery 2.5 m,6 m, as awith longitudinal a buried controldepth of node, 2.5 m, to provideas a longitudinal stable support control and node, protection to provide for the stable whole support section; (4)and For protection the detailed for the design whole of section; the box (4) materials, For the C30detailed steel design fiber concrete of the box was materials, used for keyC30 anti-scoursteel fiber parts;concrete (5) was The lowerused for half key of eachanti-scour box was parts; filled (5) with The boulders,lower half and of each the upper box was half filled was leftwith with boulders, empty grooves,and the upper as a space half forwas debris left with flow empty falling, grooves, mixing and as amaterial space for exchange, debris flow in order falling, to promote mixing and the ematerialffective energyexchange, dissipation in order ofto thepromote debris the flow. effective energy dissipation of the debris flow. The whole ED section was designed with 18 rows of boxes and four rib sills, so the total length was 22 m.m. The four walls and bottom plates of thethe boxbox werewere equippedequipped withwith steelsteel mesh.mesh. DrainageDrainage holes and connection holes were left on the box wall. The box was made in the prefabrication yard and cured as required, before being transported to the site for assembly. Compared with an in-situin-situ placing channel structure, this hadhad the advantages of easiereasier manufacture, fast installation and good adaptability to the terrain conditions (Figure(Figure7 7).).

(a) (b)

Figure 7. 7. PicturesPictures of of energy energy dissipation dissipation (ED) (ED) section section lined lined with with prefabricated prefabricated reinforced reinforced concrete concrete (PRC) boxes(PRC) completed.boxes completed. (a): From (a): an From upstream an upstream perspective perspective of ED section, of ED (b section) Structural, (b) detailsStructural of PRC details boxes. of PRC boxes. After the acceleration of the debris flow through the upper section (Figure6, Sec. B 1), it enters the middleAfter of thethe EDacceleration section (Figure of the6, debris Sec. B 2flow), and through the surge the head upper firstly section falls (Fig intoure the 6, box Sec. body. B1), Throughit enters the repeated process of ”falling stirring rising and climbing falling again“, the particles in the middle of the ED section (Fig→ure 6, Sec.→ B2), and the surge head→ firstly falls into the box body. theThrough fluid collide,the repeated displace process and replaceof ”falling each → other, stirring and → they rising rub withand climbing the box walls, → falling mixing again with“, the bouldersparticles in which the fluid were collide, pre-placed displace in theand boxes. replace The each energy other, isand partially they rub dissipated with the box and walls, the speed mixing is decreased.with the boulders As a result, which the were acceleration pre-placed process in the of boxes. the debris The flow energy is interrupted, is partially anddissipated the continuous and the increasespeed is ofdecreased. the flow velocityAs a result, and abrasionthe acceleration are prevented, process with of the threatdebris of flow the debrisis interrupted, flow to the and lower the channelcontinuous section increase being of reduced. the flow velocity and abrasion are prevented, with the threat of the debris flow to the lower channel section being reduced. 3.3. Roughening Belts for Channel Bottom Turbulence 3.3. RAfteroughening being Belts affected for Channel by and controlledBottom Turbulence by the energy dissipation section, the debris flow enters into the lower section of the channel (Figure6, Sec. B ). The acceleration process starts again, which may still After being affected by and controlled by3 the energy dissipation section, the debris flow enters cause abrasion damage. In order to further control the increase in flow velocity, a series of transverse into the lower section of the channel (Figure 6, Sec. B3). The acceleration process starts again, which rougheningmay still cause belts abrasion are set up damage. at the bottom In order of to the further channel control to increase the increase the roughness. in flow Thevelocity, raised a stonesseries of in the belt cause disturbances by colliding with the particles in the fluid, so as to change the flow regime of the underlying fluid, reduce the flow velocity, and then reduce the kinetic energy of the debris flow and, hence, the erosion. The structure of the roughening belt was a strip bulge that ran horizontally through the bottom, with the top of the strip protruding from the bottom slab. The design width along the flow direction was 1.0 m, and the transverse length was the same as the width of the channel bottom. The spacing between adjacent strips was 5.0 m. Hard and wear-resistant block stones (hard sandstone, granite, etc.) Water 2020, 12, x FOR PEER REVIEW 11 of 18 transverse roughening belts are set up at the bottom of the channel to increase the roughness. The raised stones in the belt cause disturbances by colliding with the particles in the fluid, so as to change the flow regime of the underlying fluid, reduce the flow velocity, and then reduce the kinetic energy of the debris flow and, hence, the erosion. The structure of the roughening belt was a strip bulge that ran horizontally through the bottom, Waterwith 2020the ,top12, 1868of the strip protruding from the bottom slab. The design width along the flow direction11 of 17 was 1.0 m, and the transverse length was the same as the width of the channel bottom. The spacing between adjacent strips was 5.0 m. Hard and wear-resistant block stones (hard sandstone, granite, wereetc.) were selected selected to be to buried be buried in the in concrete the concrete of the of bottom, the bottom, and blocksand blocks of di ffoferent different particle particle sizes weresizes werestaggered staggered to give to the give top the of thetop wholeof the stripwhole both strip concave both concave and convex and shapes,convex withshape as, raised with heighta raised of height5 cm~20 of cm5 cm~ and20 a cm buried and deptha buried of 1depth/2~2/3 of of 1/2~ the2/3 particle of the size particle of the size block of the stone block (Figure stone8). (Figure 8).

(a) (b)

FigureFigure 8. Pictures 8. Pictures of roughening of roughening belts belts for channel for channel bottom bottom turbulence turbulence completed. completed. (a): From (a): From an upstream an perspectiveupstream of roughening perspective belts, of (roughb) Structuralening belts details, (b) of Structural roughening details belts. of roughening belts.

4. Observation and Discussion 4. Observation and Discussion After the completion of the mitigation project, the Xiaogangjian gully encountered heavy rainfall After the completion of the mitigation project, the Xiaogangjian gully encountered heavy on many occasions during the flood season of 2012–2015, which led to eight debris flows, including rainfall on many occasions during the flood season of 2012–2015, which led to eight debris flows, four flows with a scale of more than 1 104 m3, and a total scale of more than 2.2 105 m3 of including four flows with a scale of more ×than 1 × 104 m3, and a total scale of more than 2.2× × 105 m3 of deposited sediment. deposited sediment. 4.1. Observation and Analysis of ED Effect of PRC Boxes 4.1. Observation and Analysis of ED Effect of PRC Boxes From the field observation of the Xiaogangjian debris flow in 2012, the regulation effect on debris From the field observation of the Xiaogangjian debris flow in 2012, the regulation effect on flow of the PRC boxes was mainly shown in two ways: (1) the velocity of debris flow before and debris flow of the PRC boxes was mainly shown in two ways: (1) the velocity of debris flow before after passing through the section B2; and (2) the concrete abrasion on the upstream and downstream and after passing through the section B2; and (2) the concrete abrasion on the upstream and baseplate of the section B2. downstream baseplate of the section B2. 4.1.1. Influence on the Velocity of Debris Flow before and after the ED Section 4.1.1. Influence on the Velocity of Debris Flow before and after the ED Section Using the debris flow video recorded in the flood season, the time of flow through each control sectionUsing was the found. debris According flow video to recorded the distance in the and flood the season, time intervals the time betweenof flow through the adjacent each control sectionsections, was the found. average According velocity of to the the debris distance flow and on thethe plate time ofinterval this sections between was accuratelythe adjacent calculated control (Figuresections,9). the The average selected velocity control of sections the debris were flow divided on the intoplate four of this parts: section the starting was accurately point of calculated the steep (Figurechannel 9). (0 +The000 selected m), the startingcontrol sections point of thewere PRC divided box section into four (0 + parts:046 m), the the starting starting point point of of the the steep PRC channelbox section (0 + (0 000+ 066 m), m) the and starting the endpoint point of of the the PRC steep box slope section section (0 (0+ +046100.2 m), m).the Instarting total, the point flow of time the PRCdata ofbox 15 section debris flows(0 + 066 were m) given.and the endpoint of the steep slope section (0 + 100.2 m). In total, the flow Thetimechange data of trend15 debris of the flows average were velocitygiven. of each debris flow in the different channel sections showed that the process of the debris flow in the three sections was as follows: acceleration → deceleration reacceleration. Except in the case of No.1, the debris flow was gradually slowed down → by the roughening belts and lay on the bottom of section B3 due to the small scale. The other data basically conformed to this change trend. By calculating the velocity attenuation in each section, the average velocity of the debris flow was reduced by 5.7~37.1% in the ED section lined with PRC boxes

(Figure 10). The attenuation rate (Ra) of velocity in the ED section can be calculated by Equation (4):

VB1 VB2 Ra = − (4) VB1 Water 2020, 12, x FOR PEER REVIEW 12 of 18

Water 2020, 12, 1868 12 of 17

where Ra is the rate of velocity reduction in the ED section (sec. B2), and VB1 and VB2 is the velocity of Waterthe debris 2020, 12 flow, x FOR in PEER section REVIEW B1 and section B2 (m/s). 12 of 18

Figure 9. Observation of flow velocity change of debris flow in drainage channel (17 August 2012). Lengths of each section are LB1 = 46 m, LB2 = 20 m, LB3 = 34.2 m.

The change trend of the average velocity of each debris flow in the different channel sections showed that the process of the debris flow in the three sections was as follows: acceleration → deceleration → reacceleration. Except in the case of No.1, the debris flow was gradually slowed down by the roughening belts and lay on the bottom of section B3 due to the small scale. The other data basically conformed to this change trend. By calculating the velocity attenuation in each section, the average velocity of the debris flow was reduced by 5.7~37.1% in the ED section lined with PRC boxes (Figure 10). The attenuation rate (Ra) of velocity in the ED section can be calculated by Equation (4): VV R = BB12 a V (4) B1 where RFigureFigurea is the 9. 9. rateObservationObservation of velocity of of flow reduction flow velocity velocity in change changethe ED of ofsection debris debris flow(sec. flow in B in 2drainage) drainage, and V B1channel channel and V B2(17 (17 is August Augustthe velocity 2012 2012).). of the debris flow in section B1 and section B2 (m/s). LengthLengthss of of each each section section are are LB LB11 == 4646 m, m, LB LB2 2= =2020 m, m, LB LB3 =3 34.2= 34.2 m.m.

The change trend of the average velocity of each debris flow in the different channel sections showed that the process of the debris flow in the three sections was as follows: acceleration → deceleration → reacceleration. Except in the case of No.1, the debris flow was gradually slowed down by the roughening belts and lay on the bottom of section B3 due to the small scale. The other data basically conformed to this change trend. By calculating the velocity attenuation in each section, the average velocity of the debris flow was reduced by 5.7~37.1% in the ED section lined with PRC boxes (Figure 10). The attenuation rate (Ra) of velocity in the ED section can be calculated by Equation (4): VV R = BB12 a V (4) B1

where Ra is the rate of velocity reduction in the ED section (sec. B2), and VB1 and VB2 is the velocity of (a) (b) the debris flow in section B1 and section B2 (m/s). FigureFigure 10. 10.ObservationObservation data data of of velocity velocity of of debris flow. flow. (a(a)) Velocity Velocity change change before before and and after after ED section; ED section(b) attenuation; (b) attenuation rate ofrate velocity of velocity in the in ED the section. ED section.

Moreover,Moreover, the the change change trend trend of the of the debris debris flow flow’s’s velocity velocity in the in thechannel channel coul couldd also also be seen be seen from from thethe measu measurementsrements of the ofthe mud mud depth. depth. Figure Figure 11a 11 showsa shows that that the themud mud trace trace of the of thedebris debris flow flow which which occurredoccurred on on26 26July July 2012 2012 suddenly suddenly bec becameame higher higher in in the the initiating initiating terminal wallwall ofof thethe ED ED section section and andstayed stayed higher, higher, indicating indicating that that the the velocity velocity of the of debristhe debris flow flow in this in sectionthis section became bec slowerame slower and the and mud thedepth mud was depth greater. was greater. Figure 11 Figureb shows 11 thatb shows the boulders that the with boulders larger particle with larger sizes wereparticle concentrated sizes were and deposited in the ED section, which also shows that the flow velocity slowed down and the carrying capacity of the debris flow weakened. It could be seen that the flow pattern of the debris flow changed from flowing through normally to local deposition, which meant that the kinetic energy decreased.

(a) (b)

Figure 10. Observation data of velocity of debris flow. (a) Velocity change before and after ED section; (b) attenuation rate of velocity in the ED section.

Moreover, the change trend of the debris flow’s velocity in the channel could also be seen from the measurements of the mud depth. Figure 11a shows that the mud trace of the debris flow which occurred on 26 July 2012 suddenly became higher in the initiating terminal wall of the ED section and stayed higher, indicating that the velocity of the debris flow in this section became slower and the mud depth was greater. Figure 11b shows that the boulders with larger particle sizes were

Water 2020, 12, x FOR PEER REVIEW 13 of 18

concentrated and deposited in the ED section, which also shows that the flow velocity slowed down and the carrying capacity of the debris flow weakened. It could be seen that the flow pattern of the debrisWater 20 20 flow, 12, xchanged FOR PEER from REVIEW flowing through normally to local deposition, which meant that13 ofthe 18 kinetic energy decreased. concentrated and deposited in the ED section, which also shows that the flow velocity slowed down and the carrying capacity of the debris flow weakened. It could be seen that the flow pattern of the Waterdebris2020 flow, 12, 1868 changed from flowing through normally to local deposition, which meant that13 of the 17 kinetic energy decreased.

Page: 13

Figure 11. Photos of mud marks and siltation from the debris flow that occurred on 26 July 2012, in Figure 11. Photos of mud marks and siltation from the debris flow that occurred on 26 July 2012, in the the ED section. (a) The mud depth of the 26 July 2012 debris flow in sec. B1 and B2 (ED sec.); (b) ED section. (a) The mud depth of the 26 July 2012Page: debris 13 flow in sec. B and B (ED sec.); (b) boulders boulders deposited in the ED section. 1 2 deposited in the ED section.

4.1.2.4.1.2.Figure AbrasionAbrasion 11. Photos onon UpstreamUpstream of mud andmarksand DownstreamDownstream and siltation BaseplatesfromBaseplate the debriss ofof thethe flow EDED that SectionSection occurred on 26 July 2012, in the ED section. (a) The mud depth of the 26 July 2012 debris flow in sec. B1 and B2 (ED sec.); (b) According to the field investigation after the debris flow occurred, the variation in the concrete Accordingboulders deposited to the field in the investigation ED section. after the debris flow occurred, the variation in the concrete abrasionabrasion atat thethe bottombottom ofof thethe channelchannel waswas measured,measured, asas shownshown inin FigureFigure 12 12.. PhotosPhotos takentaken onon sitesite areare 1 shownshown4.1.2. Abrasion inin FigureFigure on13 13, ,Upstream whichwhich indicateindicate and Downstream thatthat thethe abrasionabrasion Baseplate ofof sectionsections of the BB1 ED (Figure(Figure Section 1313a)a ) waswas strongerstronger thanthan thatthat ofof sectionsection BB3 (Figure(Figure 1313b).b). According3 to the field investigation after the debris flow occurred, the variation in the concrete abrasion at the bottom of the channel was measured, as shown in Figure 12. Photos taken on site are shown in Figure 13, which indicate that the abrasion of section B1 (Figure 13a) was stronger than that of section B3 (Figure 13b).

Water 2020, 12, x FOR PEER REVIEW 14 of 18 FigureFigure 12.12. DevelopmentDevelopment ofof abrasionabrasion atat thethe bottombottom ofof thethe upperupper andand lowerlower sectionssections ofof thethe EDED section.section.

Figure 12. Development of abrasion at the bottom of the upper and lower sections of the ED section.

Figure 13.13. Abrasion atat the the bottom bottom of of the the upstream upstream and and downstream downstream sections sections of the of channel.the channel. (a) The (a) steelThe

barsteel at bar the at bottom the bottom of the sectionof the section B1 was B almost1 was destroyed;almost destroyed; (b) the section(b) the B section3 was only B3 was partially only exposed.partially exposed.

4.2. Observation and Analysis of Roughening Belt on the Bottom of the Channel Figure 14a shows the mud depth on the side wall and the abrasion on the bottom of the roughening belt section (sec. B3) after the debris flow of 26 July 2012, with Figure 14b showing the flow pattern of the 14 August 2012 debris flow passing through the roughening belts.

Figure 14. Mud depth and flow pattern of debris flow in the roughening belt section. (a) Mud depth on the side wall of sec. B3 after the 26 July 2012 debris flow; (b) flow pattern of the 14 August 2012 debris flow.

It could be seen from the mud trace of the 7.26 debris flow that the mud depth was 20 cm~30 cm higher than that of the upstream and downstream regions when the roughening belt was laid at the bottom of the channel, indicating that the roughening belt had changed the flow pattern of the debris flow and caused a flow pattern of the ”rise up → drop“ type in the underlying fluid. Similarly, the 8.13 debris flow also showed a disturbance in the roughening belt section, forming a flow pattern of “waves moving forward”. The bumps in the roughening belts collided and rubbed with the particles in the debris flow, consuming part of the kinetic energy, thus reducing the abrasion force. According to the investigation of the abrasion on the concrete at the bottom of the channel in the section B3, it was relatively uniform throughout the section, and abrasion at the tail end was not more serious than that at the leading end, which showed that the velocity and the abrasion force of the debris flow were not obviously increased along the section B3. That is to say, the roughening belts had restrained the development of concrete abrasion to a certain extent.

4.3. Applicability of Anti-Wear Measures (1) Analysis on the different effects of abrasion resistance. Both the PRC box and the roughening belt are designed to reduce the flow velocity and abrasion by changing the roughness coefficient of the channel bottom [37]. The roughness coefficient of a drainage channel with abrasion resistance

Water 2020, 12, x FOR PEER REVIEW 14 of 18

Figure 13. Abrasion at the bottom of the upstream and downstream sections of the channel. (a) The

steel bar at the bottom of the section B1 was almost destroyed; (b) the section B3 was only partially Water 2020, 12, 1868 14 of 17 exposed.

4.2. Observation Observation and and Analysis Analysis of R Rougheningoughening B Beltelt on the B Bottomottom of the C Channelhannel FigurFiguree 14 14aa shows shows the the mud mud depth depth on the onside the wall side and wall the and abrasion the abrasion on the bottom on the of the bottom roughening of the roughening belt section (sec. B3) after the debris flow of 26 July 2012, with Figure 14b showing the belt section (sec. B3) after the debris flow of 26 July 2012, with Figure 14b showing the flow pattern of flowthe 14 pattern August of 2012 the 14 debris August flow 2012 passing debris through flow passing the roughening through the belts. roughening belts.

Figure 14. Mud depth and flow flow pattern of debris flow flow in the roughening belt section. section. ( (aa)) Mud Mud depth

on the side wall of sec. B 3 after the 26 July 2012 debris debris flow; flow; (b)) f flowlow pattern of the 14 August 2012 debris flow. flow.

It could be seen from the m mudud trace of the 7.26 debris flow flow that the mud depth was 20 cm~30cm~30 cm higher than that of the upstream and downstream regions when the roughening belt was laid at the bottom of the channel, indicating that the roughening belt ha hadd changed the flow flow pattern of the debris flowflow and caused a flow flow pattern of the ” ”riserise up → drop“drop“ type in the underlying fluid.fluid. Similarly, the → 8.13 debris flow flow also show showeded a disturb disturbanceance in the roughening belt section, forming a flow flow pattern of “waves“waves moving forward”. The bump bumpss in the roughening belts collide collidedd and rub rubbedbed with the particles in th thee debris flow, flow, consumingconsuming part ofof thethe kinetickinetic energy,energy, thusthus reducingreducing thethe abrasionabrasion force.force. According to the investigation of of the abrasion on on the concrete at the bottom of of the channel in the section B 3,, it it was was relatively relatively uniform uniform throughout throughout the the section, section, and and abra abrasionsion at at the the tail end was not more serious thanthan thatthat atat thethe leading leading end, end, which which showed showed that that the the velocity velocity and and the the abrasion abrasion force force of the of thedebris debris flow flow were were not obviously not obviously increased increased along thealong section the section B3. That B is3. toThat say, is the to rougheningsay, the roughening belts had beltsrestrained had restrained the development the deve oflopment concrete of abrasion concrete toabrasion a certain to extent. a certain extent.

4.3. Applicability Applicability of of Anti Anti-Wear-Wear Measures (1) Analysis on the the different different effects effects of abrasion resistance. Both the PRC box and and the the roughening roughening belt are designed to reduce the flow flow velocity and abrasion by chang changinging the roughness coefficient coefficient of the channel bottom [ 3377]. The The roughness roughness coefficient coefficient of of a drainage channel with abrasion resistance structures is always larger than that of a smooth rectangular drainage channel because of the different boundary conditions at the channel baseplate. However, there are differences in the extent of the two values. The reason is that, compared with the roughened belt, the debris flow in the PRC box obtains a larger drop range and a more intensive drop frequency, resulting in a larger dissipation of kinetic energy. (2) The design method needs to be improved and the applicability of specific structure types and sizes needs to be analyzed. Limited to the strength and impact resistance of the block stone material, the raised height of the roughening belt in the current design is not high enough. Hence, it can only affect the fluid in the bottom layer, which is more suitable for a wide-shallow type channel; for a narrow-deep channel with a high mud level, its deceleration and wear resistance effect may not be significant. In order to enhance the effectiveness of the roughening belt, high-strength and impact-resistant materials (hard sandstone, steel, etc.) should be adopted for use as a bulge. The height of the bulge should be increased and its layout optimized. In addition, the width and spacing of the Water 2020, 12, 1868 15 of 17 roughening belt should be adjusted appropriately, taking into account the differences in longitudinal slope and the flow velocity. (3) The performance of the ED structure could be enhanced in the next step. During the flood season of 2012, the PRC boxes in the ED structure had been filled with debris flow sediments, and they evolved into a stepped structure. In this situation, the sediments should either be delivered by follow-up torrential floods or be removed artificially. On the other hand, the side walls of the PRC boxes were not thick or strong enough to withstand the impact force of boulders in the debris flow. As a result, the tops of the boxes were partially damaged by local impact, and the height of the boxes became uneven, which partly affected the falling range of the debris flow. In the future, the structural type of the boxes, the width of the row and the height difference between different rows should be designed more rationally in order to avoid damage by the debris flow’s deposition or impact. For example, the channel bottom of the frame formed by the cast-in-place foundation beam could be adopted.

5. Conclusions (1) Due to its high potential and kinetic energy, the DCLG experiences high velocity and strong abrasion. There are four types of abrasion morphology in regard to debris flow drainage channels: (a) uniform surface peeling (USP); (b) plowing groove along the streamline direction (PG); (c) jumping erosion pits (JEP); (d) surface wear of the ground sills (SWGS). (2) The abrasion system of a debris flow drainage channel has the characteristics of system dependency, time dependency and multidisciplinary coupling. The main influencing factors of abrasion intensity are bulk density, velocity, grain composition, duration, mud depth, scouring angle of debris flow, section shape, material strength (shear strength, compressive strength), aggregate grading and hardness of drainage channel structure. (3) The engineering measures to reduce debris flow abrasion include the ED structure lined with PRC boxes in the middle section, the roughening strip in the lower section of the drainage channel and the wear-resistant layer of steel fiber concrete on the surface of the channel. Among these, the ED structure lined with PRC boxes is most suitable for the DCLG. Through changing the flow pattern of the debris flow’s movement, the kinetic energy is partially dissipated and the impact force and abrasion force are weakened, which can achieve the effect of interrupting the acceleration process of the debris flow and preventing the increase in abrasion along the course. According to observational data from the flood season of 2012, the average velocity of a debris flow is reduced by 5.7%~37.1% in the ED section. The PRC box structure was shown to have the significant effect of slowing down and reducing the abrasion of the debris flow and could reduce the threat of the debris flow to the downstream section of the drainage channel. (4) The other two structural measures also show a certain degree of abrasion reduction. According to the observations made during the flood season, the flow pattern of the debris flow in the roughening strip zone is changed and the abrasion at the bottom of the channel is relatively uniform after the overflow, which shows that the increase in flow velocity and erosion intensity along the roughened section is not significant, and the roughened zone plays a certain role in reducing abrasion. Adding high-strength fiber wear-resistant material to the concrete at the bottom of the channel is more suitable for medium and small-scale and low velocity debris flows, but the effects on high velocity and long-term debris flows are limited.

Author Contributions: Conceptualization, D.Y. and Y.Y.; methodology, Y.Y.; software, D.Y.; validation, W.Z. and H.H.; formal analysis, D.Y.; investigation, H.S. and Y.L.; resources, W.Z.; data curation, H.H.; writing—original draft preparation, D.Y.; writing—review and editing, D.Y.; visualization, H.H.; supervision and project administration, Y.Y.; funding acquisition, D.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (Grant No. 2019QZKK0902); the National Natural Science Foundation of China (No. 41807300); and the China Geological Survey Programs (No. DD20190644 and 20190505). Water 2020, 12, 1868 16 of 17

Acknowledgments: The authors want to thank Xiaoqing Chen for his kind guidance and suggestion with the design of drainage channel. Furthermore, we would like to thank all the anonymous reviewers and editors for their comments. Conflicts of Interest: The authors declare no conflict of interest.

Appendix A Definition of System dependence, time dependence and multidisciplinary coupling [29]:

System dependence: The simple system and any more complex system constituted by the simple • system, whose behavior can no longer be independently realized by any one of its elements, but must be unified in the whole system to be realized. As to the abrasion system of DCLG, its elements are the properties of physical, mechanical, and kinematics of debris flow; and the structure type and material of drainage channel. Time dependence: In the tribological system, the changes caused by the relative movement and • interaction of the surfaces are much faster than those caused by other behaviors in the components. Therefore, a general system, which is usually treated as a time-invariant system, must be treated as a time-varying object in the tribological analysis. In the abrasion system of DCLG, the time refers to the duration of debris flow surges. Multi-disciplinary coupling: The tribology system is the study of the law of interaction between • different behaviors in the tribological environment, which is beyond the theories and methods of any other discipline to which the behavior belongs. It focuses on behavioral relationships. The disciplines of abrasion system of DCLG may contain physical wear (friction and impact), chemical corrosion, mechanics (fluid, solid and fracture mechanics), and structural engineering.

References

1. Cui, P.; Chen, X.Q.; Zhu, Y.Y.; Su, F.H.; Wei, F.Q.; Han, Y.S.; Liu, H.J.; Zhuang, J.Q. The Wenchuan Earthquake (May 12, 2008), Sichuan Province, China, and resulting geohazards. Nat. Hazards 2009, 56, 19–36. [CrossRef] 2. Cui, P.; Zhuang, J.Q.; Chen, X.Z.; Zhang, J.Q.; Zhou, X.J. Characteristics and Countermeasures of Debris Flow in Wenchuan Area After the Earthquake. Adv. Eng. Sci. 2010, 5, 10–19. [CrossRef] 3. Yang, D.X.; You, Y.; Chen, X.Q.; Zhao, W.Y.; Shi, S.W.; Xie, Z.S. Typical characteristics and mitigation of debris flow in narrow-steep gullies in the Wenchuan earthquake areas. Hydro. Eng. Geol. 2015, 42, 146–153. [CrossRef] 4. Han, Z.; Xu, L.R.; Su, Z.M.; Wang, L.; Chen, S.Y. Research on the method for calculating the bulk density of debris flow based on the integrity coeficient of watersh ed morp hology. Hydro. Eng. Geol. 2012, 39, 100–105. (In Chinese) 5. Xu, Q. The 13 August 2010 Catastrophic Debris Flow in Sichuan Province: Characteristics, Genetic Mechanism and Suggestion. J. Eng. Geol. 2010, 18, 596–608. (In Chinese) 6. Gong, X.L.; Chen, K.T.; Chen, X.Q.; You, Y.; Chen, J.G.; Zhao, W.Y.; Lang, J. Characteristics of a Debris Flow Disaster and Its Mitigation Countermeasures in Zechawa Gully, Jiuzhaigou Valley, China. Water 2020, 12, 1256. [CrossRef] 7. Chen, X.Q.; You, Y.; Cui, P.; Li, D.J.; Yang, D.X. New Control Methods for Large Debris Flows in Wenchuan Earthquake Area. Adv. Eng. Sci. 2013, 1, 14–22. [CrossRef] 8. Zhou, B.F.; Li, D.J.; Luo, D.F.; Lv, R.R.; Yang, Q.X. Guidelines For Debris Flow Prevention; Science Press: Beijing, China, 1991; pp. 125–126. ISBN 7-03-002612-8. 9. Tímea, K.; Gabriel, J.A.; Károly, F. Bank Processes and Revetment Erosion of a Large Lowland River: Case Study of the Lower Tisza River, Hungary. Water 2019, 11, 1313. [CrossRef] 10. Yang, D.X. Key Technologies of Treatment for Debris Flow in Narrow-Steep Gullies in Wenchuan Earthquake Areas: A Case Study for Xiaogangjian Debris Flow in Mianzhu Country. Master’s Thesis, Univ. of Chinese Academy of Sciences, Beijing, China, 2013. 11. Chen, J.G.; Chen, X.Q.; Li, Y.; Wang, F. An experimental study of dilute debris flow characteristics in a drainage channel with an energy dissipation structure. Eng. Geol. 2015, 193, 224–230. [CrossRef] Water 2020, 12, 1868 17 of 17

12. Chen, J.G.; Chen, X.Q.; Chen, H.Y.; Zhao, W.Y. Characteristics of viscous debris flow in a drainage channel with an energy dissipation structure. J. Mt. Sci. 2016, 13, 223–233. [CrossRef] 13. Chen, J.G.; Chen, X.Q.; Zhao, W.Y.; Yu, X.B.; Wang, X.J. Experimental study on the characteristics of a debris-flow drainage channel with an energy dissipation structure. Bull. Eng. Geol. Environ. 2017, 76, 341–351. [CrossRef] 14. Liang, C.F.; Saeed, A.; Hanif, P.; Poorya, T.; Samkele, T. Investigation of Flow, Erosion, and Sedimentation Pattern around V aried Groynes under Different Hydraulic and Geometric Conditions: A Numerical Study. Water 2019, 11, 235. [CrossRef] 15. Wang, F.; Chen, X.Q.; Chen, J.G.; You, Y. Experimental study on a debris-flow drainage channel with different types of energy dissipation baffles. Eng. Geol. 2017, 220, 43–51. [CrossRef] 16. Chen, X.Q.; Chen, J.G.; Zhao, W.Y.; Li, Y.; You, Y. Characteristics of a Debris-Flow Drainage Channel with a Step-Pool Configuration. J. Hydraul. Eng. 2017, 143, 04017038. [CrossRef] 17. Zhou, F.C.; Huang, B.S.; Yang, G.; Chen, H.K. Mechanism of Turbulent Debris Flow Destroying Drainage -canal. J. Mt. Sci. 2001, 19, 470–473. 18. Horszczaruk, E.K. Hydro-abrasive erosion of high performance fiber-reinforced concrete. Wear 2009, 267, 110–115. [CrossRef] 19. Horszczaruk, E. Abrasion resistance of high-performance hydraulic concrete with polypropylene fibers. Tribologia 2012, 1, 63–72. 20. Lim, T.Y.; Paramasivam, P.; Lee, S.L. Analytical Model for Tensile Behavior of Steel-Fiber Concrete. Mater. J. 1987, 84, 286–298. 21. Ziad, B.; Jack, Z. Properties of Polypropylene Fiber Reinforced Concrete. Mater. J. 1993, 90, 605–610. 22. Chithra, S.; Kumar, S.R.R.S.; Chinnaraju, K. The effect of colloidal nano-silica on work ability, mechanical and durability properties of high performance concrete with copper slag as partial fine aggregate. Constr. Buil. Mater. 2016, 113, 794–804. [CrossRef] 23. Bovis, M.J.; Jakob, M. The role of debris flow conditions in predicting debris flow activity. Earth Surf. Proc. Land. 1999, 24, 1039–1054. [CrossRef] 24. Zhou, S.Z. Are all Potholes Markers of Quaternary Glaciations? Quat. Sci. 2006, 26, 117–125. 25. Xie, Y.B. On the tribology design. Tribol. Int. 1999, 32, 351–358. [CrossRef] 26. Wen, S.Z. Research Progress on Wear of Materials. Tribology 2008, 28, 1–5. 27. Shu, H.P.; Ma, J.Z.; Yu, H.C.; Marcel, H.; Zhang, P.; Liu, F.; Qi, S. Effect of Density and Total Weight on Flow Depth, Velocity, and Stresses in Loess Debris Flows. Water 2018, 10, 1784. [CrossRef] 28. He, Z. Interactions between severe environment and concrete resistance to abrasion and erosion. J. Hydraul. Eng. 2015, 46, 138–146. 29. Xie, Y.B. Three Axioms in Tribology. Tribology 2001, 21, 161–166. 30. Pan, H.L.; Huang, J.C.; Ou, G.Q. Mechanism of Downcutting Erosion of Debris Flow Over a Movable Bed. J. Mt. Sci. 2015, 12, 243–250. [CrossRef] 31. Huang, X.B.; Yuan, Y.Z.; Wang, S.X. Mechanism and Prediction of Material Abrasion in High-Velocity Sediment-Laden Flow. J. Hydrod. 2006, 18, 760–764. [CrossRef] 32. Banihabib, M.E.; Iranpoor, M. Determination of the Abrasion of Aprons of Dams by Debris Flow. Int. J. Mater. Struct. Integ. 2015, 9, 262–271. [CrossRef] 33. Li, Y.J. Estimation Method of Sand Abrasion in Hydraulic Structures. J. Hydraul. Eng. 1989, 20, 60–66. 34. Hu, K.H.; Tian, M.; Li, Y. Influence of flow width on mean velocityof debris flows in wide open channel. J. Hydraul. Eng. 2013, 139, 65–69. [CrossRef] 35. Chen, X.Q.; Cui, P.; You, Y.; Li, D.J. Engineering measures for debris flow hazard mitigation in the Wenchuan earthquake area. Eng. Geol. 2015, 194, 73–85. [CrossRef] 36. Chen, J.G.; Chen, X.Q.; Zhao, W.Y.; You, Y. Debris Flow Drainage Channel with Energy Dissipation Structures: Experimental Study and Engineering Application. J. Hydraul. Eng. 2018, 144, 06018012. [CrossRef] 37. Auel, C.; Albayrak, I.; Boes, R.M. Turbulence Characteristics in Supercritical Open Channel Flows: Effects of Froude Number and Aspect Ratio. J. Hydraul. Eng. 2014, 140, 04014004. [CrossRef]

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