DETACHMENT and TRANSPORT SYSTEMS
Erosion is a process that involves the detachment (plucking) of soil particles from within the soil surface FOLLOWED by the transport of these detached particles away from the site of detachment. Both raindrop impact ad flow can cause detachment and transport to occur. Consequently, there are a number detachment and transport systems:
Interrill Erosion and Sheet Erosion occur in areas where Raindrop Detachment dominates detachment .
1. Raindrop Detachment with transport by Raindrop Splash
When erosion is driven by the energy derived from raindrops impacting the soil surface, raindrop energy is used to overcome the bonds that hold particles in the soil surface and may also be used in the transport of the detached particles away from the site of drop impact. One commonly reported transport mechanism is raindrop splash. Raindrop splash moves detached soil particles radially away from the site of detachment. The raindrop detachment - splash transport (RD-ST) system often operates at the onset of a storm when little or no surface water flow occurs. However, splash transport (ST) is a highly inefficient transport system. If the soil has no slope, material splashed away from the point of impact of one drop is replaced by material splashed by other drops in the surrounding area. If the soil surface has a slope, then material splashed downslope travels further than material splashed upslope resulting in the net downslope migration of detached material. That downslope migration increases as the slope gradient increases but it takes many drop impacts to cause much material to move down slope in most cases. Rainfall erosion is either limited by the detachment or transport capacities associated with raindrop impact or surface water flow. RD-ST is a transport limiting process .
2. Raindrop Detachment with transport by Raindrops interacting with Flow
When water flows develop on the soil surface, raindrops penetrate through the flow to detach soil particles which may then be splashed as a result of the breakup of the drop or, alternatively, may be lifted into the flow where they move downstream as they fall back to the surface. Subsequent drop impacts lift the particles into the flow again and again and they move downstream on each occasion. The resulting transport process involves both raindrop impact and flowing water and, because of this, has been called Raindrop - Induced Flow Transport (RIFT) (Kinnell, 1990). With coarse material, raindrop impact in flowing water may stimulate particles to roll rather than saltate. Consequently RIFT comprises of Raindrop-Induced Saltation (RIS) and Raindrop-Induced Rolling (RIR). RIFT provides more efficient transport than ST. RD-RIFT plays a major role in moving soil material from interrill areas to rills. Splash can also move material from areas not covered by flow to areas where RIFT operates to give RD-ST-RIFT systems. While RIFT is more efficient than ST, it still requires numerous drop impacts to move material downstream and RD-RIFT systems are transport limiting .
VIDEO: Demonstration of Raindrop Induced Saltation and Rolling
3. Raindrop detachment with transport by unassisted Flow
Flowing water moves fine material suspended in the flow (FS) by raindrop impacts without any assistance from raindrop impact once detachment has occurred. In many cases, thin surface water flows have a capacity to cause loose material sitting on the surface to roll or saltate but may not have the capacity to detach material from the underlying surface. However, raindrops penetrating the flow may be able to do this and, as a result, silt, sand and even gravel sized particles detached by drop impacts may be transported downstream without the need for raindrops to be involved in the transport process. This raindrop detachment – unassisted flow transport (RD-FT) detachment-transport system causes particles to move down stream by flow driven saltation (FDS) and rolling (FDR) and is more efficient than RD-RIFT. Both RD-RIFT and RD-FT can occur simultaneously in the same flow, coarse material being transported by RIFT, finer material by FT.
The figure below provides a schematic representation of how the detachment and transport forms vary with raindrop energy (E) and flow shear stress when raindrop energy and flow shear stress are used as measures of erosive forces associated with impacting raindrops and flowing water respectively. In the figure, the critical energy required for raindrops to detach soil particles held in the soil surface by cohesion and inter-particle friction is designated Ec. It is indicated to increase (basically with time) when there is no runoff (A). This signifies the increase in resistance to detachment that results as surface crusts develop. The increase in Ec with flow shear stress signify the greater absorption of raindrop energy in the surface water as flow depth increases
Raindrop detachment (RD) does not occur unless E is greater than Ec.
This PDF file describes how the various transport mechanisms operate in rain-impacted flows where detachment occurs as the result of drop impact: This video also demonstrates erosion by rain-impacted flows.
4. Flow detachment with transport by Flow.
Flow detachment (FD) does not occur unless the flow has a capacity to overcome the cohesive forces and inter-particle preventing particles from being detached. Critical values of hydraulic parameters such a stream power, a parameter that varies with flow discharge and slope gradient, or flow shear stress have been used to indicate when that happens but the choice of parameter is academic. Consideration of a critical flow condition for FD remains the same irrespective of what parameter is used. Rill erosion occurs where FD occurs
Powerpoint presentation on detachment and transport systems Publications on erosion by rain-impacted flow
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