Abstract:
Since Ellison (1947) described the process of erosion as comprising
a) the detachment of soil particles from the soil mass by raindrop
impact,
b) detachment by runoff,
c) the transport of the detached particles by raindrop impact, and
d) transport by runoff, research has been directed into the mechanics
of each of these four phases and how the phases might be linked together
in the form of a soil erosion model, such as the Meyer-Wischmeier (1969)
model. From a literature review, it became evident that in spite of
this work, gaps in knowledge still exist and that
i) most studies on soil erosion tend to lump the processes together;
ii) whilst a considerable amount of investigation has been carried out
on splash erosion, the other processes have received very little
attention;
iii) there is no explicit study on the effects of factor-interactions
on the processes and the role of the laboratory as a place for
studying interactions by controlling factors has not attracted
much attention;
iv) equipment and techniques for the separate evaluation of the
detachment and transport of soil particles by overland flow are
not available; and
v) studies on the hydraulic characteristics of overland flow in
relation to the detachment and transport of soil particles in
such flows are scarce.
This study was therefore specifically aimed at establishing a sounder
research base for modelling the subprocesses and ultimate~ for soil
conservation design b,y:
i) evaluating separate~ each of the above subprocesses;
ii) assessing the influence of the factors affecting the processes,
particular~ their interactioDS; and
iii) examining the hydraulics of soil particle detachment and transport
by overland flow with and without rain.
As a means to achieve these objectives, a factorial experiment vas set
up in the laboratory to examine both the individual effects of rainfall
intensity (50, 80, 110, 140 mm h-
1
) , soil ~ (standard sand, ISIUld,
clay loam and clay) and slope steep:1.8Ss (3.5, 7.0, 10.5 and 14.0 per
cent) and their interactions on each of the above subprocesses.
Additionally, the effects of four rates of runoff (1.0, 1.6, 2.2 and
2.8 ~min) on the hydraulic characteristics of flow such as velocity,
depth, Reyuolds number, Froude number and friction factor were
examined and used in characterizing the detachment and transport
of soil particles in these flows. For each subprocess, these
variables were replicated four times.
Splash detachment and transport were determined by simulating rainfall
from a nozzle simulator over a target soil placed in a rectangular
soil tray (10 x 20 x 4 cm) which being set in the centre of a catching
tray (90 x 80 x 30 cm) allows for the separate determination of ups lope
and downslope splash.
The separate measurement of the detachment and transport of soil
particles by overland flow with and without rain was carried out b,y
using a specially designed rainfall simulator - bed flume facility
with runoff and sediment input and measuring devices.
The results were analysed by analysis of variance to show the Significance
of soil type, rainfall in tensi ty, flow rate t
slope steepness and their
first and second order interactions in influencing the processes studied.
Multiple correlation techniques were used to search for the best
associations between the erosion influencing variables and soil loss.
RegreSSion analySis was used for establishing predictive equations for
detachment and transport rates.
Detachment of the test soils by splash can be placed in rank order of
standard sand, sand, clay and clay loam with increasing resistance. For
splash transport the order is standard sand ) clay > sand > clay loam.
For each soil type there are significant increases in splash detachment
and transport with increasing rain intensity and slope steepness.
The most significant interactions influencing the two splash processes
are soil x intensity and slope x intensity for detachment and transport
respectivel,J. Significant interactions show that the factors are not
independent of each other; the simple effects of a factor differ, and
the magnitude of any simple effect varies according to the level of
the other factors of the interaction term.
The factors influencing detachment by flow without rain rank in ~ order
of importance as soil type, slope steepness and discharge. The corresponding
order for flow with rain is discharge, slope steepness and soil type. The
order of soil detachability for both flow with and without rain is standard
sand , sand ~ clay loam> clay. There are also significant increases in
detachment rate as slope steepness and flow rate increase.
It is further shown that the first and second order interactions of the
above factors Significantly influence detachment by flow. On a relative
basis, the second order interaction is small and the importance of the
first order interactions can be placed in an increasing order of slope x
soil, slope x discharge t
and discharge x soil for flow without rain. For
flow with rain, they rank as slope x soil, discharge x soil, and slope x
discharge.
The slope x soil interaction showed that as slope steepens the influence
of each Boil on detachment rates increases with the proportionate
increase being greater for sand and standard sand than for clay and clay
loam. The slope x discharge interaction revealed significant increases in
detachment rate for all slopes as discharge increased. The magnitude of
the response is however greater at the lower than higher slopes. As slope
steepness increases, detachment rates by flow with and without rain are
also enhanced. The increase was proportionately more for the 1.0 and
1.6 J/min than 2.2 and 2.8 J/min flows. The Boil x discharge interactiC?n
also indicated that, for flow without rain, detachability increases more
for clay and clay loam than for the sand and standal'd sand as discharge
increases. In the presence of rain however, the response of the soils did
not differ much.
Detachment by flow without rain is predominantly by rilling. In the
presence of rain, detacbment rates by flow are increased about three fold
and relatively even removal of soil particles from the eroding bed is
characteristic. Raindrop impact thus appears to inhibit rill formation
by overland flow especially on small slope steepnesses.
There is a critical slope steepness at which both raindrop impact and
overland flow contribute equally to total detachment. At slopes lower
than the critical value, raindrop impact is the main detaching agent whilst
flow predominates the detachment process at steeper slopes. The critical
slope steepness is soil specific and decreases in the order of clay ~
clay loam ) sand ~ standard sand.
The transport of soil particles by combined flow and rain is significantly
influenced by soil type, slope steepness, flow rate and their first and
second order interactions. Transport rates decreased in the order of
sand > standard sand ) clay ) clay loam. Increases in discharge and slope
steepness significantly increased transport capacity. For a discharge
range of 1.0 - 2.8 l/min, transport capacity increased four fold.
The most significant interaction that influences transport capacity is
slope x soil. Where factors interact significantly, interpretation of
results based solely on the main effects of the influencing factors m&1
result in loss of vital information and lead to wrong conclusions. For
example, examination of the slope x soil interaction showed that at lower
slopes (3.5 and 7.0 per cent) combined flow and rain has a greater transport
capacity for the larger clay and clay loam aggregates than for the fine
grains of sand and standard sand. This is obscured when effects are
averaged over all the slopes as is the case when only main effects are
considered.