Laboratory experimentation for the statistical derivation of equations for soil erosion modelling and soil conservation design
Since Ellison (1947) described the process of erosion as comprising a) the detachment of soil particles from the soil mass by raindrop impact, b) detachment by runoff, c) the transport of the detached particles by raindrop impact, and d) transport by runoff, research has been directed into the mechanics of each of these four phases and how the phases might be linked together in the form of a soil erosion model, such as the Meyer-Wischmeier (1969) model. From a literature review, it became evident that in spite of this work, gaps in knowledge still exist and that i) most studies on soil erosion tend to lump the processes together; ii) whilst a considerable amount of investigation has been carried out on splash erosion, the other processes have received very little attention; iii) there is no explicit study on the effects of factor-interactions on the processes and the role of the laboratory as a place for studying interactions by controlling factors has not attracted much attention; iv) equipment and techniques for the separate evaluation of the detachment and transport of soil particles by overland flow are not available; and v) studies on the hydraulic characteristics of overland flow in relation to the detachment and transport of soil particles in such flows are scarce. This study was therefore specifically aimed at establishing a sounder research base for modelling the subprocesses and ultimate~ for soil conservation design b,y: i) evaluating separate~ each of the above subprocesses; ii) assessing the influence of the factors affecting the processes, particular~ their interactioDS; and iii) examining the hydraulics of soil particle detachment and transport by overland flow with and without rain. As a means to achieve these objectives, a factorial experiment vas set up in the laboratory to examine both the individual effects of rainfall intensity (50, 80, 110, 140 mm h- 1 ) , soil ~ (standard sand, ISIUld, clay loam and clay) and slope steep:1.8Ss (3.5, 7.0, 10.5 and 14.0 per cent) and their interactions on each of the above subprocesses. Additionally, the effects of four rates of runoff (1.0, 1.6, 2.2 and 2.8 ~min) on the hydraulic characteristics of flow such as velocity, depth, Reyuolds number, Froude number and friction factor were examined and used in characterizing the detachment and transport of soil particles in these flows. For each subprocess, these variables were replicated four times. Splash detachment and transport were determined by simulating rainfall from a nozzle simulator over a target soil placed in a rectangular soil tray (10 x 20 x 4 cm) which being set in the centre of a catching tray (90 x 80 x 30 cm) allows for the separate determination of ups lope and downslope splash. The separate measurement of the detachment and transport of soil particles by overland flow with and without rain was carried out b,y using a specially designed rainfall simulator - bed flume facility with runoff and sediment input and measuring devices. The results were analysed by analysis of variance to show the Significance of soil type, rainfall in tensi ty, flow rate t slope steepness and their first and second order interactions in influencing the processes studied. Multiple correlation techniques were used to search for the best associations between the erosion influencing variables and soil loss. RegreSSion analySis was used for establishing predictive equations for detachment and transport rates. Detachment of the test soils by splash can be placed in rank order of standard sand, sand, clay and clay loam with increasing resistance. For splash transport the order is standard sand ) clay > sand > clay loam. For each soil type there are significant increases in splash detachment and transport with increasing rain intensity and slope steepness. The most significant interactions influencing the two splash processes are soil x intensity and slope x intensity for detachment and transport respectivel,J. Significant interactions show that the factors are not independent of each other; the simple effects of a factor differ, and the magnitude of any simple effect varies according to the level of the other factors of the interaction term. The factors influencing detachment by flow without rain rank in ~ order of importance as soil type, slope steepness and discharge. The corresponding order for flow with rain is discharge, slope steepness and soil type. The order of soil detachability for both flow with and without rain is standard sand , sand ~ clay loam> clay. There are also significant increases in detachment rate as slope steepness and flow rate increase. It is further shown that the first and second order interactions of the above factors Significantly influence detachment by flow. On a relative basis, the second order interaction is small and the importance of the first order interactions can be placed in an increasing order of slope x soil, slope x discharge t and discharge x soil for flow without rain. For flow with rain, they rank as slope x soil, discharge x soil, and slope x discharge. The slope x soil interaction showed that as slope steepens the influence of each Boil on detachment rates increases with the proportionate increase being greater for sand and standard sand than for clay and clay loam. The slope x discharge interaction revealed significant increases in detachment rate for all slopes as discharge increased. The magnitude of the response is however greater at the lower than higher slopes. As slope steepness increases, detachment rates by flow with and without rain are also enhanced. The increase was proportionately more for the 1.0 and 1.6 J/min than 2.2 and 2.8 J/min flows. The Boil x discharge interactiC?n also indicated that, for flow without rain, detachability increases more for clay and clay loam than for the sand and standal'd sand as discharge increases. In the presence of rain however, the response of the soils did not differ much. Detachment by flow without rain is predominantly by rilling. In the presence of rain, detacbment rates by flow are increased about three fold and relatively even removal of soil particles from the eroding bed is characteristic. Raindrop impact thus appears to inhibit rill formation by overland flow especially on small slope steepnesses. There is a critical slope steepness at which both raindrop impact and overland flow contribute equally to total detachment. At slopes lower than the critical value, raindrop impact is the main detaching agent whilst flow predominates the detachment process at steeper slopes. The critical slope steepness is soil specific and decreases in the order of clay ~ clay loam ) sand ~ standard sand. The transport of soil particles by combined flow and rain is significantly influenced by soil type, slope steepness, flow rate and their first and second order interactions. Transport rates decreased in the order of sand > standard sand ) clay ) clay loam. Increases in discharge and slope steepness significantly increased transport capacity. For a discharge range of 1.0 - 2.8 l/min, transport capacity increased four fold. The most significant interaction that influences transport capacity is slope x soil. Where factors interact significantly, interpretation of results based solely on the main effects of the influencing factors m&1 result in loss of vital information and lead to wrong conclusions. For example, examination of the slope x soil interaction showed that at lower slopes (3.5 and 7.0 per cent) combined flow and rain has a greater transport capacity for the larger clay and clay loam aggregates than for the fine grains of sand and standard sand. This is obscured when effects are averaged over all the slopes as is the case when only main effects are considered.