RD-RIFT, RD-FT AND SIDE-SLOPE EROSION

As noted previously, for a soil s and rain with a drop size distribution r, the sediment discharge is given by

q_sR(s,r) = (H k_DDL + (1 - H)k_M) I_r u f[h,r]

where k_DDL is a coefficient depending on particle size and density characteristics of the layer of previously detached material when H_R=1, k_M is a coefficient depending characteristics of the surface of the soil matrix when H_R=0, I_r is the intensity of rain, u is flow velocity and f[h,r] is the function that varies with flow depth.

When flow discharge (q_w) increases, flow velocity and depth increase. The increase in flow depth with discharge tends to be at a power of flow discharge less than 1.0. Conversely, the increase in flow velocity tends to be power of flow discharge greater than 1.0. As can be seen from the figure above, when flows are very shallow f[h,d] increases with flow depth. As a result, when flows are very shallow, q_sR(s,r) increases with q_w. Consequently, an approximation of q_sR(s,r) is given by

q_sR(s,r) = (H k_DDL + (1 - H)k_M) q_w I_r

In the USDA Water Erosion Prediction Project ( WEPP), the interrill erosion model is directed at erosion on the side-slopes associated ridge tillage systems. Shallow flows are common on these side-slopes so that this approxmation applies. Originally, the WEPP interrill model was represented by

q_sR(s) = k_i I²

but following Kinnell (1993. Interrill erodibilities based on the rainfall - flow discahrge erosivity factor. Aust. J. Soil Res. 31, 319-332), the 1995 version adopted the equation

q_sR(s) = k_i I q_w SDR_RR F_nozzle.

where k_i is the interrill soil erodibility of soil s, I is rainfall intensity, SDR_RR is a delivery ratio that varies with the random roughness of the surface, and F_nozzle is an adjustment factor to account for irrigation sprinkler energy variations. The approximation

q_sR(s,r) = (H k_DDL + (1 - H)k_M) q_w I_r

only applies when rainfall energy remains constant. Variations in rainfall energy have little effect on the transport of previously detached particles but can have a major impact on the amount detached. Consequently k_M will vary if rainfall energy varies. The WEPP model does not distinguish between the soil surface and the DDL in determining k_i and the F_nozzle factor provides a mechanism for accounting for the effect of rainfall energy on k_M. Also, k_M may vary considerably during a rainfall event, particularly in soils that develop a surface crust. This, together with the fact that values of H_R are essentially unknow in most cases, is why values for k_i are difficult to obtain except through experiment. Also, some soils develop microrills as slope gradient increases. Under these conditions, RD-RIFT, RD-FT and FD-FT systems all operate on the eroding surface. Consequently, the effect slope gradient on sediment discharge will vary between soils rather than follow a common equation.

Powerpoint presentation on detachment and transport systems

NEXT: Back to research history

Detachment and transport systems

Erosion by rain impacted flow

Effect of previously detached particles (intro)

Rainfall intensity, flow and fall velocities

Flow depth

More about previously detached particles

RD-RIFT, RD-FT and side-slope erosion