Merten Et Al.: Sediment Load and Soil Detachment in Rills

mass. The energy for these processes is provided, basically, by the weight of the mixture of water and sediment According to theory, the rate of detachment of soil particles in and the downslope gradient of the flow. Studies related rills is reduced as a first-order function of the amount of sediment to the mechanics of rill erosion have shown that rates load in the flow. The first objective of this study was to determine if experimental results confirmed current detachment-transport couof soil detachment are inversely dependent upon the pling theory. The second objective was to investigate two hypothesized magnitude of the sediment load at a given time and mechanisms responsible for any coupling effect observed: The first location on the soil surface (Meyer and Monke, 1965; mechanism was that since turbulence is known to be a critical factor Rice and Wilson, 1990; and Cochrane and Flanagan, in detachment by flow, and since it is also known that sediment in 1996). The theoretical basis for this effect has been diswater reduces turbulent intensity, it was suggested that sediment in cussed by Foster and Meyer (1972) and Hairsine and flow reduces detachment via a correspondent reduction in turbulent Rose (1992a, 1992b). intensities. This hypothesis was tested indirectly by adding a sediment Foster and Meyer (1972) (later presented in more load that was carried entirely in the suspended state. The second detail by Foster, 1982) support the hypothesis that the mechanism was that sediment covering the soil bed during the erosion flow possesses finite energy, which may be expended process shields the soil from the forces of flow, thus reducing detachment. This hypothesis was tested by introducing bed-load sediment. either to detach soil particles from the bulk soil mass Sediment loads exiting the rill and detachment and deposition along or to transport previously detached sediments. Within the rill were measured. Detachment was reduced and deposition inthis framework, it might be considered that the energy creased as a linear function of the amount of sediment introduced into required to sustain movement of the sediment in transit, the flow. Results indicated that, in general, detachment did decrease as well as to initiate movement of previously detached according to current theory, but discrepancies in the erosional patterns sediment particles resting on the bottom of the bed, is were observed, which none of the current models explain. Both hyless than the energy necessary to detach new sediments pothesized mechanisms of reduction in detachment rates were apparfrom the soil mass. In this way, the energy is preferenently active in reducing detachment rates, though the shielding mechatially used for those processes related to the continuanism appeared to have a greater impact than did the mechanism tion of movement of the sediments. Any excess energy associated with a reduction in turbulent intensity. could then be available for detachment. In the conceptual model of Foster and Meyer (1972), C surface water flow is capable of the flow energy available for detachment is calculated detaching and transporting sediments from the soil as the difference between sediment transport capacity minus the energy used for transport, represented by the G.H. Merten and A.L.O. Borges, Hydraulic Research Institute, Fedsediment load in transit. Thus to estimate the rates of eral Univ. of Rio Grande do Sul, Box 15029, CEP 91501, Porto Alegre detachment it is essential to determine transport caRS, Brazil; M.A. Nearing, USDA-ARS National Soil Erosion Research Lab., Soil Bldg., Purdue Univ., West Lafayette, IN 47907-1196. pacity. Received 18 Jan. 2000. *Corresponding author (mnearing@purdue. A second theoretical model for the utilization of flow edu). energy is that of Hairsine and Rose (1992a, 1992b). In this model, Hairsine and Rose propose that flow energy, Published in Soil Sci. Soc. Am. J. 65:861–868 (2001). 862 SOIL SCI. SOC. AM. J., VOL. 65, MAY–JUNE 2001 the rill slope and width, and the impact of the changing bed morphology on hydraulics of flow (Nearing et al., 1997). The results of this model were tested using the same soil (in a different experimental setup) as is used in the current study. The results of Lei et al.’s (1998) model indicated that the erosion process in the rill can be somewhat more complex than described by the earlier models. Turbulence is a critical component of soil erosion in rills. While typical soil tensile strength are of the order of kilopascals, even for unconsolidated soils (Nearing et al., 1991), typical average shear stresses of flow are only of the order of pascals. Soil detachment occurs only because localized events of high-intensity turbulence known as “bursting” produce large fluctuations in Fig. 1. Schematic diagram of theoretical results for the case of a firststresses on the more weakly bound areas at the soil order relationship between sediment load and local detachment rate in a rill. This example is for the case of uniform bed slope, surface with enough energy to dislodge sediment from constant and uniform flow rate, and no introduction of sediment the soil mass (Nearing, 1991). It has been empirically from the upper end or sides of the rills. shown that without turbulence, detachment of soil by flow does not occur (Nearing and Parker, 1994). The represented by the stream power of the flow (V), is presence of a high concentration of sediment in runoff used by four processes (i) to overcome the threshold has a considerable effect on the velocity profile and of entrainment of the cohesive medium to initiate the turbulence structure. The presence of fine sediment, process of detachment, (ii) entrainment (detachment) which is primarily moved in suspension, reduces the of soil from the bed, (iii) entrainment of previously intensity of the turbulence (Einstein and Ning Chien, detached sediments that are found on the bottom of the 1955; Vanoni and Namicos, 1960; Wang and Larson, stream bed, and (iv) dissipation of energy as heat and 1994). Given this, it would be reasonable to hypothesize noise. Hairsine and Rose’s model also puts forth the that a reason for reduction in the rate of detachment proposition that continuous deposition causes sediwith increased sediment load may be due to a reduction ments to be deposited over the stream bed, which crein turbulent intensities imparted to the soil bed when ates a layer that protects the bottom of the bed from sediment is in the flow. erosive forces. The objectives of this study were twofold. The first Both models for soil erosion by flow (Foster and objective was to measure the effect of increasing sediMeyer, 1972; Hairsine and Rose, 1992b) produce results ment load in the flow of a confined rill on the spatial, that are somewhat similar in terms of soil detachment downslope distribution of detachment and deposition and sediment load as a function of downslope distance under conditions of constant flow rate of water and in a rill. Both are essentially first-order models. Given constant slope. This enabled us to evaluate the utility the simple case of constant slope and discharge with of the first-order relationships suggested in the models downslope distance, the models will predict an exponenof Foster and Meyer (1972) and of Hairsine and Rose tially decaying rate of detachment with distance as sedi(1992a, 1992b). The second objective was to investigate ment load increases. Sediment load will approach an the mechanism responsible for the process whereby soil equilibrium concentration representing a transport limdetachment rate decreases when sediment load is presiting state (Fig. 1). In the case of the Foster and Meyer ent. This was done by introducing bed load–size sedimodel, this state is interpreted as the condition when all ment in one case and suspended load–size sediment available flow energy is being used to transport sediment in the second case, and again investigating the spatial, and no energy remains to detach new sediment particles downslope distribution of detachment and deposition from the soil mass. In the case of the Hairsine and under conditions of constant flow rate of water and conRose scenario, the soil bed has reached a state of high stant slope. sediment cover and is well-protected, and the instantaneous rate of sediment deposition equals the instantaMATERIALS AND METHODS neous rate of sediment entrainment. Both the Foster A series of aluminum boxes were mounted on a variableand Meyer model (1972) and the Hairsine and Rose slope flume. The dimensions were as follows: four boxes of model (1992a, 1992b) have been incorporated in modi0.10 by 0.10 by 0.25 m; six boxes of 0.10 by 0.10 by 0.50 m; fied forms into practical, field-scale models of erosion and four boxes of 0.10 by 0.10 by 1.00 m. The boxes were (Foster et al., 1981; Nearing et al., 1989; and Rose et al., connected to form a small, rectangular canal 8 m long. Known 1998). quantities of dry, sieved (2.5 mm) soil were placed in the Lei et al. (1998) developed a more sophisticated, fiboxes. The soil used in the experiments was a Cecil sandy loam nite-element model for rill erosion that took into ac(fine, kaolinitic, thermic Typic Kanhapludults) from Georgia, count the morphological development of the rill during containing 714 g kg2 of sand, 174 g kg2 of silt, 113 g kg2 the erosion process, especially the circular feedback of clay. Dry aggregate size of the material after screening was loop between flow hydraulics, which drives the erosion distributed according to D5 5 140 mm, D10 5 170 mm, D50 5 650 mm, and D90 5 1830 mm. process, erosion which causes morphological changes in MERTEN ET AL.: SEDIMENT LOAD AND SOIL DETACHMENT IN RILLS 863 Table 1. Sediment input rates, flow rates, and times for the experiments. Exp. Input material Repetition flow rate Total time Steady time