THE DYNAMIC DEPOSITIONAL LAYER

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. Since the layer of previously detached material is DYNAMIC, it is referred to as the DYNAMIC DEPOSITIONAL LAYER or DDL.

The figures below show the result of a simulation of how the value of H_R for a material with single p varies over a 240mm long by 100mm wide surface covered by a flow with uniform depth and velocity after the spatially random impacts of 72,000 drops for 3 levels of k_M/k_DDL when flow velocity was 20mm/s (left) and 3 levels of flow velocity when k_M/k_DDL was 0.2 (right). Since the flow entering the upstream end brings in no sediment, H_R=0 at the upstream end but increases in the downstream direction. The rate of increase varies with k_M/k_DDL reaching a value near 0.95 over much of the surface when k_M/k_DDL = 0.6 in the 20mm/s flow. Because particle travel distance increases with flow velocity, the value of H_R at any position along the surface downstream of the upstream end falls as flow velocity increases.

As noted earlier, when materials contain particles varying in size and density, the sediment discharged contains a greater proportion of particles of small size and low density than on the surface. If x_pDd is greater than the distance from the point of detachment to downstream boundary, then RD-FT operates and particles associated with that x_pDd are not present in the DDL. Particles moving by RIS have times of concentration that vary with the length of the eroding area and inversely with their x_pDd values. This results in the sediment discharge coarsening over time from the start of a RD-RIS erosion event. Once steady state conditions develop, the composition of the DDL stabilizes and tends be coaser at the downstream end than near the upstream end (Details)

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

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

Powerpoint presentation on detachment and transport systems