Soil Erosion by Water/Transport of Soil Particles by Flow

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Transport of Soil Particles by Interrill Flow

Transporting Capacity of Flow

Once sediment has been entrained within the flow, it will be transported until actual sediment concentration in the flow exceeds it’s transporting capacity and deposition occurs. The transporting capacity of the flow $T_{f}$ varies with the fifth power of velocity (Meyer and Wishmeier, 1969):

$T_{f}\propto Q^{\frac {5}{3}}s^{\frac {5}{3}}$    (1.18)

This relationship relates to the action of overland flow on its own, whereas, in practice, flow is usually accompanied by rainfall. The interaction with raindrop impact causes a slight rise in the value of the exponents (Quansah, 1982):

$T_{f}\propto Q^{2.13}s^{2.27}$    (1.19)

While detachment capacity of flow is reduced by raindrop impact (eqn. 1.12, cf. section → Rill Erosion), transporting capacity is enhanced (Savat, 1979; Guy and Dickinson, 1990; Proffitt and Rose, 1992). The degree of enhancement depends on the resistance of the soil, the diameter of the raindrops and the depth and velocity of the flow.

Nearing et al. (1989) derives an equation for $T_{f}$ by simplifying the general transport equation to:

$T_{f}=k_{t}\tau ^{\tfrac {3}{2}}$    (1.20)

where $k_{t}$ is an empiric transport coefficient and $\tau$is hydraulic shear stress exerted flow (according to the Darcy-Weisbach equation, cf. section → Rill Erosion).

Maximum Sediment Concentration

Transport capacity of overland flow is defined as the maximum sediment concentration $C_{max}$ that can be carried (Govers, 1990):

$C_{max}=a(sv-0.4)^{b}$    (1.21)

where $a$ and $b$are empirical coefficients dependent on grain size. For particles with median grain diameters (D₅₀) of 33, and 390 μm, respectively, $b$ amounts at values from 1.5 to 3.5 (Everaert, 1991). The impact of rainfall has an negligible influence on the relationship for fine particles but reduced $b$ to 1.5, indicating that rainfall diminishes the ability of overland flow to transport coarse material.

Deposition Rate

Instead of trying to define transport capacity olny in terms of flow properties, some researchers have attempted to relate transport capacity to the maximum sediment concentration that a flow can carry when a balanced condition exists between detachment and deposition (Rose et al., 1983; Styczen and Nielsen, 1989). The rate of deposition $D_{p}$ is:

$D_{p}=v_{s}C$    (1.22)

where $v_{s}$ is the settling velocity of particles (Proffitt et al., 1991).

Given the rather shallow depths of interrill overland flow, the considerable role played by surface roughness and the generally low Reynolds and Froude numbers, it can be proposed that most of the sediment transported is derived by raindrop impact and that grain shear velocity rarely attains the level necessary to detach soil particles (except on steep slopes or smooth bare soil surfaces).

In contrary to raindrop impact, where particles between 0.063 and 0.250 mm in size are most detachable (cf. section → Rill Erosion), those most detachable by overland flow are within the 0.1-0.3 mm range, and the sediment carried in overland flow is deficient in particles larger than 1 mm and enriched in finer material.

Over time, areas of erosion on a hillside will become progressively sandier and areas of deposition will be enriched with clay particles. Most of the sediment splashed into the flow is moved only relatively short distances before being deposited. Since deposition is a particle-size selective process, with the coarser particles being deposited first, the deposited layer becomes progressively coarser (Proffitt et al., 1991) and may develop into a depositional crust (cf. section → Rill Erosion). Less of the finer material is then exposed to erosion. This mechanism can take place even within an individual storm so that detachment is highest ant the beginning of the storm and transport capacity is reached very quickly.

However, the relationship between sediment transport by interrill overland flow and discharge, as measured in the field, does not always conform with what may be expected from the research described above. The transport process may under certain conditions be dominantly one of rolling of the particles over the soil surface as bed load. The sediment component contributed to overland flow by raindrop impact is moved as bed load, whereas the component contributed through detachment by the flow itself is moved as suspended load (Kinnell, 1990). The process is extremely dynamic, so that the most relevant equation for describing sediment transport by interrill overland flow continually changes through time.

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