| Numerous experiments have been carried out on laboratory and on field plot scales in order to
assess the different sub processes and factors involved with soil erosion. For this assumptions
are made that experimentally obtained equations and/or relationships can be used to
simulate the complex erosion process at the hill slope and/or (micro) watershed
level.
Erosion occurs when the erosive forces, represented by the momentum flux of the
raindrop impact and the momentum flux of the overland flow, exceed the cohesive forces
between the soil particles and the downward component of the turbulent overland
flow.
Given the fundamental principle that rainfall erosion occurs when the acting forces
(raindrop impact and runoff) exceed the cohesion of the soil particles and the fact that
different soil types erode at different rates, many authors searched for threshold driven
detachment and transport equations. The most determining parameters in the equations are:
the critical momentum flux , the mean runoff velocity, the unit stream power or the critical
shear strength. However, the parameters as critical momentum flux and critical shear
strength are difficult to measure and good relationships with the measured soil
physical properties are lacking. And because discharges are easily measured at stream
outlets, it is better to apply relationships using the discharge instead of the velocity
parameter.
For the erosion process in rills and gullies, a simple but very accurate transport function
using the stream power Ω concept can be used: Ω = mg S q, where Ï is the density of the
water (g/cm3), g the gravitational constant (cm/s2), S the slope (m/m) and q the
discharge per unit width (cm2/s). Moreover, under wind-driven rains, the energy
fluxes of both flow (Ωflow = ÏgquS) and raindrop splash (Ωdrop=ÏgIwdd50), with
Iwdthe intensity of wind-driven rain and d50 the median raindrop diameter, are to be
considered
Sediment transport models are presented based on laboratory field rainfall simulations for
the sheet erosion process in which the sediment is delivered towards rills in an agricultural
field. A comparison is made between the sediment transport processes under interrill erosion,
typically observed during small scale laboratory simulations and the complex sediment
transport processes occurring during field rainfall simulations. Finally a comparison is made
with measurements of runoff and erosion events caused by natural rainfall occurring on a
hillslope.
Key words: stream power, runoff, unit discharge, sediment transport, rainfall simulation,
laboratory, field. |