RILL EROSION PROCESS AND RILL FLOW HYDRAULIC PARAMETERS

RILL EROSION PROCESS ANDRILL FLOW HYDRAULIC PARAMETERSFen-li ZHENG', Pei-qing XIAO2 and Xue-tian GAO'ABSTRACTIn the rill erosion process, run-on water and sediment from upslope areas, and rill flow hydraulicparameters have significant effects on sediment detachment and transport. However, there is a lackof data to quantify the effects of run-on water and sediment and rill flow hydraulic parameters on rillerosion process at steep hillslopes, especially in the Loess Plateau of China. A dual-box system,consisting of a 2-m-long feeder box and a 5-m-long test box with 26.8% slope gradient was used toquantify the effects of upslope runoff and sediment, and of rill flow hydraulic parameters on the rillerosion process. The results showed that detachment-transport was dominated in ril erosionprocesses; upslope runoff always caused the net rill detachment at the downslope rill flow channel,and the net rill detachment caused by upslope runoff increased with a decrease of runoff sedimentconcentration from the feeder box or an increase of rainfall intensity. Upslope runoff discharginginto the rill flow channel or an increase of rainfall intensity caused the rill flow to shift from astratum flow into a turbulent flow. Upslope runoff had an important effect on rill flow hydraulicparameters, such as rill flow velocity, hydraulic radius, Reynolds number, Froude number and theDarcy-Weisbach resistance coefficient. The net rill detachment caused by upslope runoff increasedas the relative increments of rill flow velocity, Reynolds number and Froude number caused byupslope runoff increased. In contrast, the net rill detachment decreased with an increase of therelative decrement of the Darcy- Weisbach resistance coefficient caused by upslope runoff. Thesefindings will help to improve the understanding of the effects of run-on water and sediment on theerosion process and to find control strategies to minimize the impact of run-on water.Key Words: Rill erosion process, Rill flow hydraulic parameters, Run-on water effects, The LoessPlateau of China1 INTRODUCTIONIn the rill erosion process, run- on water and sediment from the upslope area, and rill flow hydraulicparameters have significant effects on rill detachment and transport. However, there was few dataavailable to quantify the effects of run-on water and sediment as well as rill flow hydraulic parameters onthe rill erosion process at steep hillslopes, especially in the Loess Plateau of China. Therefore, it isnecessary to study the effects of upslope runoff and sediment and rill flow hydraulic parameters ondetachment and transport by rill flows.As compared to water flows in rivers and streams, the bed gradient of a rill flow is steeper, the flowdepth is shallower, and the channel shape is more irregular. It is necessary to determine whichparameters can be used to describe rill flow hydraulics and hydrodynamics. Moss et al.(1979) proposedthat the rill flow's basic mechanisms of erosion, transport, and deposition were similar to that of flows inriver channels. Therefore, the theories and methods for streams and rivers may be employed in studyingrill flows. The corresponding variables used to characterize flow dynamics, such as flow velocity,hydraulic radius, Reynolds number, Froude number, as well as the resistance coefficient, can be used tocharacterize rill flows.Several studies have been conducted to investigate the resistance coefficient of rill flows. Foster et al.1984) reported that the resistance cofficient was not greater than 0.5 for slope gradients of 1.7°0 to 5.16*,indicating that the resistance coefficient of rill flows did not vary greatly. However, Gilley et al. (GilleyResearch Prof, 2 Ph.D. Student and 3 Associate Prof, State KexI aharatnry nf Soil Frnein _and Dryland Farmingon Loess Plateaus, Institute of Soil and Water Conservation中国煤化工ces and Ministry ofWater Resources, Yangling, Shaanxi 712100, China), E-ml-rthwesterm Sci-TechCNM H G'University of Agriculture and Forestry, 26 Xinong Rd, YanglilNote: The original manuscript of this paper was received in May 2003. The revised version was received inMarch 2004. Discussion open until June 2005.-130-Intermational Journal of Sediment Research, Vol. 19, No. 2, 2004, pp. 130-141et al, 1990) proposed that when the Reynolds number was between 300 to 10000, the resistancecoefficient of rill flows varied greatly with runoff discharge and bed gradient.Subsequently, Abrahamset al. (1996) reported that for slopes of 0.74° to 3.2", the Darcy-Weisbach resistance cofficient in wideand shallow rills range from 0.2 to 2.84 in rill flows. Recently, Nearing and co-workers (Nearing, 1997)concluded that the Reynolds number was not a consistent predictor of hydraulic friction in rills. On thesteep hillslopes of the Loess Plateau of China, lttle information is currently available on the resistancecofficient of rill flows. Therefore, it is necessary to quantitatively study the resistance coefficient of rillflows, and the relationship between rill detachment and a rill flow's resistance coefficient.An early model concept of the effect of sediment in the eroding water on the erosion processes wasproposed more than 50 years ago by Ellison (1947) and Ellison and Ellison (1947a, 1947b). 'This modelconsiders sediment particles in the water as abrading agents and the detachment capacity of the flowingwater increases as the sediment content is increased. On the other hand, the flow also has a limitedsediment carrying capacity. Therefore, clear water has a high transporting capacity, a low detachingcapacity and causes very litte erosion. When the water is full of sediment, it has a high detachingcapacity, a low transporting capacity and again very lttle erosion. Maximum erosion occurs when theflow contains just enough abrasive sediment to detach as much soil as the flow will carry.For over so years, Ellison hypothesized how sediment in runoff water would affect erosion processes,but lttle work has been done to test the validity of these concepts, especially on its effect on detachmentand transport. Nevertheless, recent field and laboratory studies have been conducted on the gentle andsteep hillslopes on the Loess Plateau of China and in the United States of America.Chen (1992, 1993) studied the effect of runoff from upper slopes on erosion and the sediment transportprocesses at a downslope section and pointed out that an increase in runoff sediment concentration fromupper slopes caused a decrease of erosion downslope. Zheng and Kang (1998) established differentsizes of runoff plots based on the vertical distribution of sheet, rill and shallow gully erosion zones onloessial hillslopes. Their results showed that an increased runoff from upslope areas triggered anincrease of downslope sediment delivery. But, with an increase of upslope sediment concentration inrunoff, sediment delivery from the downslope segment decreased.Since conditions in the hilly-gully region of the Loess Plateau are complex, it is difficult toquantitatively identify how runoff and sediment from upslope areas affect the downslope erosion processunder different rainfall, runoff, slope, and surface conditions. In addition, slopes at the loessialhillslopes are steep, ranging from 3-12° at the interrill erosion dominant zone, to 12-24 at the rill erosiondominant zone. It has been a great challenge for soil erosion scientists working on the Loess Plateau ofChina to quantify processes contributing to the severe erosion at the steep loess landscape.Recently, the development of a dual-box system, consisting of an upslope feeder box located at theupslope, and a downslope test box located at the downslope, has provided new opportunities to study theeffects of upslope runoff and sediment on the downslope erosion process under different surfaceconditions. Huang et al. (1999) and Zheng et al. (2000) used the dual-box system to study the effects ofrun-on water and sediment on erosion processes and sediment regimes at the down- slope section. Theyreported that at a 5% slope and with low rainfall intensity, upslope runoff did not cause additionaldetachment at a downslope segment. In fact, deposition occurred at the downslope section, and thesediment regime was detachment-limiting. When either rainfall intensity or slope gradient wereincreased or the near-surface hydrological conditions were changed from free drainage to artesianseepage, the runoff from the feeder box caused additional sediment delivery in the test box, resulting in atransport- dominated sediment regime. Zheng and Gao (2001a, 2001b) used a dual-box system toevaluate the effects of upslope runoff and sediment on the erosion process at a downslope section withand without ephemeral gully on steep slopes. They found that upslope runoff always caused additionaldetachment at the downslope section, and the sediment regime dominated detachment -transport.There was few data available to quantify the effects of run-on water and sediment and changes of rillflow hydraulic parameters on the rill erosion process for the steen hillclnnes sf the Iess Plateau. .Wedesigned a laboratory study to quantify the effect of the upsl中国煤化工the rill erosionprocess at the downslope section under different rainfall intel|HC N M H Gionship betweenrill flow hydraulic parameters and rill erosion. During une experimenl, une runoff sedimentconcentration from the feeder box was varied, while a relatively constant level of runoff was maintainedunder the same rainfall intensity, to create a range of upslope boundary conditions for the test box. TheIntermational Journal of Sediment Research, Vol. 19, No. 2, 2004, pp. 130-141-131-.subsequent results can be used to evaluate the effects of run-on water and sediment on the rill erosionprocess. Results of this study will further the understanding of the rill erosion process and of the impactof rill hydraulic parameters on the rill erosion process, and will provide data for the development of amore accurate process-based erosion model at the steep hillslopes.2 MATERIALS AND METHODS2.1 Soil Sample CollectionThe clayey loess collected from Yangling town, Shaanxi Province of China, was used in this study.This soil had about 8.3% sand, 67.4% silt, and 24.3% clay.. The soil was sampled from'a very deep soillayer (6m deep) of farmland, which would be the C-Horizon layer. Soil samples collected from the fieldwere passed through a 10-mm sieve.2.2 Experimental SetupThe simulated rainfall experiments were done in the simulation hall of the Institute of Soil and WaterConservation, Chinese Academy of Sciences and Ministry of Water Resources, China. The study wasconducted on a dual-box system consisting of a 5-m-long test box and a 2-m-long feeder box on a 26.8 %slope. Both boxes were 2m wide. These two boxes could be connected by the connecting piece to feedthe runoff from the feeder box into the upper end of the test box. When these two boxes weredisconnected, runof samples could be collected separately from each box. The connection anddisconnection could be done quickly without stopping the rain.For both soil boxes, the depth of the soil was approximately 30 cm with a 15-cm layer of sand at thebottom. These two boxes were placed under two simulators with a side nozzle (Chen, 1984). Theheight of raindrops falling was 16 m; the designed rainfall intensities were 50, 90 and 130 mm h*'.2.3 Soil Box PreparationA 15-cm layer of sand was packed at the bottom of the soil box and a 30 cm layer of sieved soil(<10mm) was packed over the sand layer. A soil pan box 30 cm deep was packed in 5 cm layers toensure uniform density. The soil bulk density was 1.20 g cm*. Two concentrated flow channels or rillflow channels 0.2 m wide were formed by inserting a polyvinyl chloride (PVC) border into the ground. Inorder to minimize the boundary effects from smooth PVC sheets, and to simulate a rill flow channelborder, a special glue material was brushed onto the PVC sheets, and then sieved soil was pasted to thePVC sheet. Runoff collector troughs were fabricated and placed at each outlet of rill flow channels.The end of each outlet was 5 cm lower than the adjacent sides.2.4 Pre-rainOnce the boxes were prepared, 20 minutes of rainfall of 30 mm h^' was simulated 24 h prior to theerosion experiment, and no runoff occurred at the soil box surface. The pre-rain was to reduce surfacevariability from preparation.2.5 Experimental ProceduresBoth the test box and the feeder box were set to the selected rainfall intensity (Table 1). The runoffsediment concentration released by the feeder box was varied by progressively covering portions of thesurface with fabric sheets that prevented direct raindrop impact to create the same runoff with differentsediment concentrations during each run.Before each run, one rill flow channel was covered by two layers of fabric sheets, and another rill flowchannel was uncovered. The uncovered rill flow channel was used to run the experiment first. Afterall runoff samples were collected from the uncovered rill flow channel, the uncovered rill flow channelwas then covered by two layers of fabric streets, and then the first covered rill flow channel was used todo the experiment.中国煤化工THCNMHG-132-International Joumal of Sediment Research, Vol. 19, No.2, 2004, pp. 130-141Table 1 List of experimental treatments2-m Feeder Box5-m Test BoxTreatmentSlopeRainfallCovermm/h%26.850; 50; 75; 1009901300: 50; 75; 100本Each run was replications four timesThe run started with 0% cover on the feeder box, thus providing the highest level of sedimentproduction.Runof samples from the feeder box and the uncovered rill flow channel were collected in a3-liter plastic bucket every minute. After 8 runoff samples were collected from each box separately, thetwo boxes were connected to let the runoff from the feeder box discharge to the upper end of the rill flowchannel. After 2-3 minutes of equilibration time, four runof samples were collcted from the rill flowchannel that was receiving runoff input from the feeder box. After runoff samples were collected fromthe rill flow channel with the feeder input, the connecting piece was removed and two additional runoffsamples were collected from each box separately. These two final runoff samples were used to accountfor the temporal change of sediment delivery as the soil surface eroded.After all runoff samples werecollected with 0% cover for the feeder box, 50% of the feeder box surface was covered by fabric sheets.The sequence of collecting runoff samples was repeated: four samples from both boxes separately, foursamples from the rill flow channel with feeder input, and again two samples from each box separately.The same sampling procedure was repeated for 75% and 100% coverage of the feeder box.After all runoff samples were collected from the first uncovered rill flow channel, the rainfallsimulators were closed. Two layers of fabric sheets were quickly placed on the first uncovered rill flowchannel, and the cover on the first covered rill flow channel was removed. Then the rain started, and thesame sequence of collcting runoff samples was repeated for this run. During each run, the volume forevery sample was measured. The entire run lasted about 2 h.As each run progressed, the rill flow depth in the rill flow channel with and without feeder input wasmeasured, and the rill flow velocity in the rill flow channel either with and without feed input wasmeasured by using the method of a coloring agent for each cover on the feeder box. Each run wasreplicated four times.For each amount of coverage on the feeder box during the run, runoff and sediment rates wereaveraged from 6 samples, 4 before conection and 2 after disconnection, for both test and feeder boxesseparately, and from 4 samples when the two boxes were connected. These average runoff and sedimentresults as well as the deviation range from four replication runs are presented in Table 2. The averagerill flow velocity with and without feed input was calculated separately, i.e, the four values of the rillflow velocity for four degrees of coverage on the feeder box with and without feed input were averaged,separately (Table 4).Table 2 Average runoff and sediment delivery dataFeeder boxRill channelWithout feeder inputWith feeder inputRuRr RsRuS。RsSdRudSudL/ming/minSlope: 26.8%, rain: 50mm/h1.6(0.05)*14.2(1.2)0.3(0.03)8.8(0.2)2.0(0.12)43.3(3.6)0.101.8(0.07)10.5(1.2)0.5(0.03)11.8(1.6)2.2(0.16)56.8(4.2)752.0(0.10)4.5(0.3)0.5(0.02)8.6(0.2)2.0(0.08)122.6(4.2)-0.51001.8(0)0.5(0.01)9.6(1.6)2.3(0.06)239.3(10.6)-0.2.8%, rain: 90mm/h6.0(0.14)69.6(6.2)1.10.02)PCi33.0(4.8)7.1(0.56)739.9(18.1)6.2(0.22)55.7(4.9)0.9(0.01)31.0(0.5)6.9(0.45)1335.633.8)0.26.5(0.16) .35.8(1.3)1.0(0.01)36.801.7)7.1(0.57) 1359.7(40.2)-0.46.7(0.15)5.1(0.5)1.0(0.02)35.2(2.6)Slope: 26.8%, rain: 130mm中国煤化工8.3(0.20)120.1(10.8)1.4(0.03)184.2(20.6)CNMHG-0.38.6(0.22)83.1 (7.2)1.5(0.02)198.4(8.6)YH8.5(0.19)67.5 (5.4)1.6(0.02)200.910.7)9.7(0.80) ”3078.1(120.1)8.60.20)6.8(0.8)1.70.04)306.5(22.1)9.90.664131.4(230.2)_0.4* Values in parentheses are deviation range from four replication runs.Intermnational Journal of Sediment Research, Vol. 19, No. 2, 2004, pp. 130-141-133-.2.6 Data AnalysisLet Su and Ss be the sediment delivery from the feeder and rill flow channel separately, and Sud thesediment delivery from the rill flow channel with the feeder sediment input. Depending on themagnitude of Sud relative to Su and Sd, there are two possible process scenarios on the rill flow channel(Huang et al, 1999):Sud= Su + Sd, equilibrium, and there are no effects from upslope runoff;Sud> Su+ Sa, and there is net detachment in the rill flow channel caused by the upslope runoff.The value ofB (B= Sud- Su- Ss) is the net rill detachment caused by upslope runof. It can indicatehow the upslope runoff affects rill detachment and transport at the down slope section. The values ofBhave been tabulated in Table 3.Table 3 Sediment concentration, sediment delivery and upslope runoffand sediment effects in the rill flow channelFeeder boxTest boxBWithout feeder inputWith feeder input(Suw-Su- Sa .CoverSCa.BgCud,%g/cm3g/min_g/ming/cmSlope: 26.84%, rain: 50mm/h08.9(1.2)*14.229.3(3.3)8.821.6(5.2)43.320.3505.8(0.8)10.523.6(6.9)11.825.8(6.2)56.834.5752.3(0.4)4.517.2(3.8)8.661 3(8.8)122.6109.51000.9(0.2)1.19.2(4.9)9.6104.0(9.6)239.6228.2Slope: 26.8%, rain: 90mm/h11.6(2.2)69.630.0(6.2)33.0104.2(5.6)739.9637.39.0(0.7)55.734.4(1.6) .31.0193.6(16.4)1335.61248.95.5(0.9)35.836.8(2.6)36.8191 .5(21.2)1359.71287.10.8(0.2)5.135.2(4.3)35.2214.5(23.5)1608.41568.1Slope: 26.8%, rain: 130mm/h14.5(2.3)120.1131.6(13.6)184.2246.3(16.7) 2315.62011.39.7(1.2)83.1132.3(4.8)198.4258.9(1 8.2)2537.12255.67.9(0.9)67.5125.6(7.2)200.931 7.3(20.6)3078.12809.70.8(0.3)6.8180.3(8.7)306.5417.3(28.2)4131.43718.1* Values in parentheses are deviation range from four replication runs.Let Vu and Vud be the rill flow velocity with and without feeder input, separately; let Rs and Rud be thehydraulic radius of the rill flow with and without feeder input, separately. Let Res and Reud be theReynolds number of the rill flow with and without feeder input, separately, and let Fra and Frud be theFroude number of the rill flow with and without feeder input, separately. And let, fa and fud be theDarcy-Weisbach resistance coefficient of the rill flow with and without feeder input, separately; then thedifferences between Vud and Vu (Vud- Vu)，Rud and Rs( Rud -RJ), Reud and Ru (Rewd- Res), Frud andFrs Frud -Fr)，fud and fa (fud -f) express the relative increment of the rill flow velocity, the relativeincrement of the hydraulic radius, the relative increment of the Reynolds number, the relative incrementof the Froude number, and the relative decrement of the Darcy-Weisbach resistance coefficient caused byupslope runoff, respectively.3 RESULTS AND DISCUSSIONS3.1 Sediment Detachment and Regime and Upslope Runoff and Sediment EffectsRunoff data shown in Table 2 indicated a reasonable mass balance between total runoff from the feederbox and rill flow channel separately (Ru + Rd) and the runoff from the rill flow channel with feeder input(Ru) under experimental conditions. On the other handnm the rill flow channelwith feeder input (Sux) was always greater than the value中国煤化工runoff always causednet rill detachment in the rill flow channel (B) (TableC N M H Gediment regime wasdetachment-transport dominated in rill flow channels on steep niulslopes.- 134-Intermnational Jourmal of Sediment Research, Vol. 19, No.2, 2004, pp. 130-141.Upslope runoff discharging into the rill flow channel triggered an increase of rill detachment andtransport. The results in Table 3 and Fig. 1 showed that the net rill detachment (B) caused by upsloperunoff greatly increased with a decrease of sediment concentration in upslope runoff at a similar upsloperunoff rate. Meanwhile, the net rill detachment (B) increased as rainfall intensity increased from 50 to90mm h"', and then to 130 mm h' with a 26.8 % slope degree.3002000-+90mmih1500一+.50mmh200中1000100500重401015Sediment concentration in upsope runoff, g/cmpSediment concentation in upslope runoff,. g/cmP4000. + 130 mm/h300010002(Sediment concentration in upslope runoff, g/cm’Fig. 1 The relation between tne net rll detachment In the rll tlow channel causedby upslope runoff and sediment concentration from the feeder boxThe experimental data showed that sediment deliveries from a 5m-long and 20cm-wide rill flowchannel were somewhat constant without feeder input under rainfall intensities of 50 and 90 mm h"'This indicates that the sediment regime has reached a dynamic equilibrium for detachment, transport anddeposition processes in the 5m-long and 20cm-wide rill flow channel (i.e., 9.6 g min' at 50 mmh' vs. 34g min^' at 90 mm h*), i.e., sediment delivery reached the sediment transport capacity. These resultsappear to support the WEPP model concept of sediment transport capacity To which sets the upper limitof sediment delivery (Nearing et al, 1989). On the other hand, sediment delivery from the 5m-long and20cm-wide rill flow channel always increased when the sediment concentration from the feeder boxdecreased under experimental conditions. This phenomenon indicated the sediment regime did notreach a dynamic equilibrium, and the sediment regime was detachment-transport dominated. Theseresults showed the importance of understanding how run-on water and sediment affects the erosionprocess.中国煤化工3.2 Rill Flow Hydraulic Parameters and Upslope RunoffMoss et al. (1979) proposed that the basic mechanisms ofHC N M H Gosition were forrill flows similar to those for flows in river channels. The varnables usea to aescride flow hydrauliccharacteristics can be employed to describe rill flows. .Table 4 lists the rill flow hydraulic parameters,including, flow velocity, hydraulic radius, Reynolds number, Froude number, and resistance coefficient.Intermational Jourmal of Sediment Research, Vol. 19, No.2, 2004, pp. 130-141.135-.口一暑置暑暑冒剧|3首&屈x2? 昌=首召召台路品品品s县。只欠8， 。。只raThe data in Table 4 demonstrated that the Reynolds number was less than 500 at 50 mm h"' of rainfallintensity without feeder input, indicating that the rill flAn increase of rainfallintensity or upslope runoff discharging into the rill flow中国煤化工:gment caused the rillflow to shift from a stratum flow into a turbulent flow|Y片C N M H Glemonstrated that theFroude number of the rill flow at a 26.8% slope with or without teeder input was greater than 1,indicating that the rill flow was a torrential flow..136-International Joumal of Sediment Research, Vol. 19, No.2, 2004, pp. 130-141.g吉晏房管Table 5 Upslope runof efcts on rll flow velocity, bhydraulic radius, Reynolds oumber,Froude number and Darcy-Weisbach resistance cfficicatFeeder boxRill channelWithout feeder inputWith federe input母CoverRVReu FraVud VvJV。 Rs R/Rs Rew RewvRes Fusa Frw/Frs &’fLL/ninxm/s_ mm_cm/sm古系Slope: 26.8%, rain: 50mm/h乏.61290.70389 1.5530.27721.9 1.70 1.79 2.5617004.371.6531.060.244 0.88.815.70.66 450 2.0420.177 27.4 1.75 1.42 2.1516833.7422191.090.124 0.7075.016.80.54 390 2.314 0.125 28.1 1.67 1.19 2.215483.932.6051.130.099 0.79 .10020.00.50 430 2.861 0.082 36.3 1.82 1.16 2.3218204.23 3.1050.058 0.71Slope: 26.8%, rain: 90mm/h5.024.91.06 1141 2.443 0.112 42.2 1.69 2.97 2.8054154.74 2.6711.09 0.099 0.88 .号1.011149 2.645 0.096 46.4 1.86 2.57 2.5451644.49 2.922 1.100.078 0.81.527.50.791041 3.1300.068 49.1 1.78 2.21 2.8046894.503.4421.10).060 0.8E29.60.69 889 3.591 0.052 53.6 1.81 1.76 2.55 40644.57 3.9151.09 0.040 0.77Slope: 26.8% rain: 130mm/h10|8 8.328.61.51 1865 2.3480.121 49.0 1.71 3.79 2.5180244.302.6401.120.104 0.8s乙团8.630.61.491971 2.672 0.094 52.8 1.73 3.12 2.0981104.113.0210.073 0.773疵8.532.11562 3.050 0.072 55.2 1.72 3.01 2.6671744.59 3.41:0.065 0.90工35.10.881337 3.7840.047 58.5 1.67 2.02 2.3061074.564.155100.039 0.83邑吉官营長管承喜The Froude number was determined by the formula: Fr = VG'R". Upslope runoff discharging intothe rill flow channels caused the increase of rill flow velocity and the hydraulic radius of the rill flow. Incomparison with experimental runs without feeder input, the rill flow velocity with feeder input was 1.67to 1.86 times greater, and the square root of the rill flow hydraulic radius was 1 .45 to 1.67 times greater,which caused the Froude number to increase by 9% to 13%. Meanwhile, the Froude number alsoincreased as sediment concentration from the feeder box decreased because the rill flow velocityincreased, and the hydraulic radius of the rill flow decreased with a decrease of sediment concentration inthe upslope runoff.The Darcy-Weisbach resistance coefficient of rill flows was calculated by the formula: f = 8gRJ/V*.Under the same hydraulic slope (JD, the Darcy-Weisbach resistance coefficient was determined by the rillflow velocity (N), and the hydraulic radius of the rill flow (R). Our data showed that the Darcy-Weisbachresistance coefficient decreased with an increase of rainfall intensity from 50 to 90 mmh"' or as the runprogressed. On the other hand, when rainfall intensity increased from 90 to 130 mm h", theDarcy-Weisbach resistance coefficient decreased with or without feeder input as the run progressed, but itappeared to be constant with or without feeder input. These phenomena were attributed to variations inrill flow velocity and hydraulic radius.The experimental data demonstrated that an increase of rainfall intensity from 50 to 90 mm h"' causedan increasing value in the square of rill flow velocity (V"), which was greater than the increment of therill flow's hydraulic radius with or without feeder input under the same percentage of coverage on thefeeder box. This caused a decrease of the rill flow's Darcy-Weisbach resistance cefficient with anincrease of rainfall intensity from 50 to 90 mm h'. On the other hand, an increase of rainfall intensityfrom 90 to 130 mm h"' caused the increasing value of V2 to be similar to the increment of the rill flow'shydraulic radius with or without feeder input under the same percentage of coverage on the feeder box.Consequently, the rill flow's Darcy- Weisbach resistance coefficient with or without feeder input wasalmost constant. Meanwhile, under the same rainfall intensity, an increase of the rill flow's velocity anda decrease of the rill flow's hydraulic radius with a decrease of sediment concentration ftom the feederbox caused a decrease of the rill flow's Darcy-Weisbach resistance coefficient.One interesting feature of the data that we would like to point out, although we could not find anyexplanation for it, is the apparent nature of the run-on water effect on rill flow hydraulic parameters underthe experimental conditions. The experimental data in Table 5 demonstrated a similar increase in rillflow velocity from the run-on water under three rainfall intensities, i.e, rill flow velocity was 1.7 timesgreater, compared to the data observed without feeder input. Likewise, at three rainfall intensities,similar increases in hydraulic radius, Reynolds number and Froude number from the run-on water wereobserved when upslope runoff discharged into the downslope rill flow channel. Compared to the datawithout feeder input, the hydraulic radius, Reynolds number and Froude number with feeder input were2.4, 4.3 and 1.11 times greater, respectively. In contrast, a similar decrease of the Darcy-Weisbachresistance cofficient was also observed when the upslope runoff discharged into the downslope rill flowchannel. The Darcy-Weisbach resistance cofficient with feeder input decreased 20% at three rainfallintensities, in comparison to the coefficient without feeder input. Therefore, additional studies will benecessary to verify whether the run-on water effect on the rill flow's hydraulic parameters are indeedadditive.3.3 Relation between Rill Flow Darey-weisbach Resistance Cofficient and Reynolds NumberFoster et al. (1984) and Savat et al. (1980) proposed that the rill flow resistance coefficient decreased (as the Reynolds number (Re) increased. Nearing et al. (1997) found that the relationship between theDarcy-Weisbach resistance cofficient and the Reynolds number was not consistent, and depended onexperimental conditions. Our results were in agreement with the findings of Nearing.Fig.2 shows a plot of the relationship between the Reynolds number and the Darcy-Weisbachresistance coefficient. The data showed that when a rillor 'vas lower than 2000,the Darcy-Weisbach resistance coefficient greatly varied,中国煤化工.05 to 0.28. Whenthe Reynolds number was greater than 4000, the Darcy-wMHC N M H Gent varied from 0.04to 0.1. These results indicated that the variation of the Darcy-WeIsDacn resisuance coefficient was notonly dependent on the Reynolds number, but was also on dependent experimental conditions..138-Intermational Jourmal of Sediment Research, Vol. 19, No.2, 2004, pp. 130-141.0.3&◆宅0.20.10200040006000800010000Reynolds number of rill flowFig. 2 The relationship between Reynolds number and Darcy-Weisbach resistance ceofficient3.4 Rill Flow Hydraulic Parameters and Rill DetachmentUpslope runoff discharging into rill flow channels caused an increase of the rill flow's velocity,Reynolds number and Froude number, and a decrease of the rill flow's resistance cofficient.Consequently, run-on water triggered an increase of rill detachment. Fig. 3 shows a plot of therelationships between the net rill detachment and the relative increments in rill flow velocity, Reynoldsnumber and Froude number caused by upslope runoff, as well as the relative decrements of theDarcy-Weisbach resistance coefficient.3000y= 0002x10008名y= 2E-062420R2= 0.8261R?=0.94091001000g2030器The relative increment of ReynoldsThe relative increment of rll fownumber from run-on watervelocity from run-on water, cm/s5000y= 147268*4151y= 0.0364x2366R2=0.7594R'= 0.6802◆◆-◆0.4 :中国煤化工0.04 .0.06Arcy-weis bachThe incrementofFroude nubmerofMYHCNMH(om run-on waterrill fow fom run-on waterfig.3 Relationship between the net rill detachment and rill flow parametersIntermational Journal of Sediment Research, Vol. 19, No.2, 2004, pp. 130-141.139-.In Fig. 3, the equations for the net rill detachment caused by upslope runoff and the relative incrementof rill flow velocity (Vw- Va), the relative increment of the Reynolds number (Reud- Red), and the relativeincrement of the Froude number Frud- Fra) were described as power functions. This indicated that theincrements of rill flow velocity, Reynolds number, and Froude number, caused by upslope runoff,triggered an increase of rill detachment and transport. However, the net rill detachment decreased withan increase of the relative decrements of Darcy-Weisbach resistance coefficient (fud-fx), caused by upsloperunoff. Consequently, upslope runoff discharging into rill flow channels triggered an increase of rilldetachment and transport.4 CONCLUSIONSThis paper presented an experimental study of the effect of upslope runoff and sediment on downsloperill flow hydraulic parameters and the rill erosion process for steep slope landscapes. A dual-box system,consisting of a 2m-long feeder box and a 5m-long rill flow channel with a 26.8% slope gradient was usedto quantify how run-on water and sediment as well as rill flow hydraulics parameters affect the rillerosion process. In this study, different levels of sediment concentrations from a feeder box werecontrolled by covering varying portions of the feeder box surface. Experimental variables in this studyincluded three rainfall intensities of 50, 90 and 130 mm h"', and four levels of sediment concentrationfrom the feeder box under the same rainfall intensity. The following conclusions were derived.Under experimental conditions, sediment regimes in the rill erosion process were alwaysdetachment-transport dominated. The upslope runoff always caused the net rill detachment in the rillflow channels. The net rill detachment caused by upslope runoff increased with a decrease of sedimentconcentration from the feeder box or an increase of rainfall intensity.Run-on water and sediment had an important effect on rill flow hydraulic parameters. Upslope runoffdischarging into the rill flow channel at the down slope section or an increase of rainfall intensity causedthe rill flow to shift from a stratum flow into a turbulent flow. Meanwhile, the upslope runoffdischarging into the rill flow channel greatly increased enhanced the rill flow's velocity, hydraulic radius,Reynolds number, and Froude number, and reduced the Darcy-Weisbach resistance coefficient.Consequently, rill detachment greatly increased with an increase in rill flow velocity, Reynolds numberand Froude number or a decrease in the Darcy- Weisbach resistance coefficient.These results showed the dramatic effect of run-on water and sediment on the rill erosion process and arill flow's hydraulic parameters. These findings will help to improve the understanding of how run-onwater and sediment affects the on erosion process and will also help in finding control strategies tominimize the impact of run-on water.ACKNOWLEDGEMENTSThis study was funded by Signifcant Orientation Program of Knowledge Innovation Project for theChinese Academy of Sciences (Grant No. KZCX-SW-422) and National Natural Science Foundation ofChina (Grant No.40335050, 50239080).REFERENCESAbrahams, A. D., Gang, L. and Parsons, A. J. 1996, Rill hydraulics on a semiarid hllslope, southem Arizona. EarthSurface Processes and Landforms, Vol. 21, pp. 35~47.Chen, H. 1993, A study on soil erosion and yield sediment of hllslope and gullies in a watershed. BejjingMeteorological Press (in Chinese).Chen, H. 1992, Effets of rainfall characteristics and runoff from upslope on erosion and yield sediment. Chinese J. .Soil and Water Conservation, Vol. 6, No.2, pp. 17~23 (in Chinese).Chen, w.1984, Combined rainfall simulator with side -spray. Blletin of Soil and Water Conservation, Vol. 4, No.5,pp. 36- 41 (in Chinese). .Ellison, w.D. and Ellison, 0. T. 1947a, Soil erosion studies - Part VI: Soil detachment by surface flow. Agric. Eng,Ellison, W. D. and Ellison, O. T.1947b, Soil erosion studies -Vol. 28, pp.402- 408.中国煤化工， surface flow. Agric.Eng., Vol. 28, pp. 442~ 444, p. 450.MHCNMHGEllison, W. D. Soil erosion studies - Part I, 1947, Agric. Eng., Vol. 28, pp. 145~146.- 140-Intermational Joumal of Sediment Research, Vol. 19, No. 2, 2004, pp. 130-141Foster, G. R, Huggins, L. F. and Meyer, L. D.1984, A laboratory study of rill hydraulics: I. velocity relationship.Trans of ASAE, Vol. 27, pp. 790~796.Gilley, J. E., Kottwitz, E. R. and Simanton, J. R.1990, Hydraulic characteristics of rlls.s Trans of ASAE, Vol. 33, pp.1900~1906.Huang, C, Wells, L. K. and Norton, L. D. 1999, Sediment transport capacity and erosion processes: Concepts andreality. Earth Surface Processes and Landforms, Vol. 24, pp. 503~516.Moss, A. J, Walker, P. H. and Hutka, J. 1979, Raindrop-simulated transportation in shallow water flows: Anexperimental study. Sediment Geol, Vol. 22, pp. 165~184.Nearing, M. A, Forster, G. R. and Lane, L. J. et al. 1989, A Process-based soil erosion model for USDA- watererosion prediction project technology. Trans of ASAE, Vol. 32, No. 5, pp. 1587~1593.Nearing, M. A., Norton, L. D. and Bulgakov, D. A. et al. 1997, Hydraulics and erosion in eroding rlls. WaterResources Research. Vol. 33, No. 4, pp. 865~876.Savat, J. 1980, Resistance to flow in rough supercritical sheet flow. Earth Surface Processes and Landforms, Vol. 5,pp.103~122.Zheng, F. and Kang, S. 1998, Erosion and sediment yield in different erosion zone of the Loess Plateau. ACTAGeographica Sinica, Vol. 53, No. 5, pp. 422-428 (in Chinese).Zheng, F. and Gao, X. 2001a, Effects of upslope runoff and sediment on erosion process at downslope area.Proceedings of International Symposium on Soil Erosion Research for 21st Century, Honolulu, Hawai, U.S., Jan.3-5, pp.163~166.Zheng, F. and Gao, X.2001b, Effects of up-slope runoff on erosion processes at down- -slope shallow gully areas. InD.E. Stott (eds). Sustaining the Global Farm. Selected Papers from the 10th International Soil ConservationOrganization held May 24 -29, 1999, at Purdue University and the USDA-ARS National Soil Erosion ResearchLaboratory. pp. 737-741 (CD ROM).Zheng, F, Huang, C. and Norton, L. D.2000, Vertical hydraulic gradient and run-on water and sediment efects onerosion processes and sediment regimes. Soil Sci. Soc. Am. J,, Vol. 64, No. I, pp. 4~11. .中国煤化工MYHCNMHGInternational Journal of Sediment Research, Vol. 19, No, 2, 2004, pp.130-141.