Empirical relations for the sediment transport capacity of interrill flow

Experiments were carried out in order to measure the sediment transport capacity of interrill flow with and without rainfall and to relate the transport capacity to selected hydraulic parameters, such as effective stream power and shear velocity. Different sediments were used in order to study the effect of grain size. The proposed relationships show considerable variations with grain size and there is only a minor effect of rainfall on the transport capacity which also seems to be grain size dependent. The proposed relationships can be used to predict sediment transport capacity of interrill flow and can therefore contribute to the development of physically based erosion models.     

The relationship between throughflow generation and the solute concentration of soil and stream water

Variation in solute concentrations of soil and stream water during throughflow events was studied at Bicknoller Combe, Somerset, England. The main hydrological process acting in the catchment involves a delayed throughflow discharge pulse a day or two after the rainfall event. During the period of storm runoff, coincident with the rainfall, the solutes in the stream are diluted, but their concentration in the throughflow remains unchanged. During the delayed throughflow pulse, concentrations of both soil and stream water increase. This is due to additional leaching from the soil in hollows where saturated moisture conditions prevail. The results suggest that two distinct erosional environments may exist: on the spurs, leaching seems to be related solely to infiltration processes, whilst in the hollows, saturated throughflow also contributes to the solute removal. This contrast in erosional processes may perhaps account for the difference in slope form and development between the hollow and spur zones.     

Hillslope hydrographs by the finite element method

Simulation models may be used to explore the implications of making specific assumptions about the nature of a real world system, and then to make predictions of the behaviour of that system under a set of naturally occurring conditions. It is important that understanding generated by the former should be gained before predictive use of the system model. This paper describes and uses a finite-element model of transient, partially saturated water flow within a hillslope soil mantle overlying an impermeable bedrock, to make an investigation into the effects of parameter variations and initial conditions on the hillslope hydrograph. The results clearly demonstrate that the response of the hillslope system to rainfall is highly non-linear and that the initial conditions, particularly in the unsaturated zone, are of paramount importance in governing the timing and magnitude of the hydrograph peak. Hillslope convergence appears as the dominant topographic parameter but the non-linearity of the response and the complex interdependence between the soil and topographic parameters restrict the possibility of further definite conclusions about the relative sensitivity of the simulated hillslope hydrograph to changes in these parameters.     

On the relationship between bedrock lowering and regolith thickness

A general equation is proposed for the relationship between bedrock lowering and regolith thickness which includes, as special cases, hypotheses by Ahnert, Armstrong, Culling, Kirkby and Young, at least approximately.     

Field measurement of the velocity of overland flow using dye tracing

A new method of computing the mean velocity of overland flow using dye tracing is proposed in which a specified cross-section is divided into zones of relatively uniform flow characteristics, termed partial sections. The mean surface velocity for each partial section is determined by timing the passage of injected dye, and this figure is multiplied by 0.67, 0.70, or 0.80, depending on whether the flow is laminar, transitional, or turbulent, to give mean velocity. The mean velocity for the entire cross-section is calculated by multiplying the mean velocity of each partial section by its cross-sectional area, summing the products for all the partial sections, and dividing by the total area of the cross-section. A field test shows that mean velocity derived in this manner differs from mean velocity derived by the discharge method (i.e. by dividing discharge measured volumetrically by cross-sectional area) by an average of only 7.07 per cent. Thus the partial-section technique appears to provide a reliable method of estimating mean velocity of overland flow.     

An assessment of surface-mine reclamation based upon sheetwash erosion rates at the Glenrock coal company,Glenrock, Wyoming

Comparative erosion rates provide a rational basis for evaluating reclamation success because the work performed by erosion processes is a function of both forces and resistances operating on a surface. Sheetwash erosion data collected by the LEMI technique over several years at the Dave Johnston Mine in east-central Wyoming show that, based upon average values, there is generally little difference between the erosion rates of natural and reclaimed hillslopes at this locale. Therefore, it is permissible to infer that the reclamation programme has been successful from a geomorphic perspective. However, there is considerable variation in erosion rates about the average for individual hillslopes in both the natural and reclaimed groups. Examination of these data by hillslope element and segment show that, based upon average values, the concave elements of natural and reclaimed hillslopes experience the highest rate of sheetwash erosion. Again, however, there is considerable variation about the average and considerable erosion can occur in any element or segment of particular hillslopes. Lastly, there are seasonal oscillations of hillslope surface elevation with frost-heaving prevalent during the late autumn to early spring months and erosion prevalent during the late spring to early autumn months.     

Rilling of hillslopes reclaimed before 1977 surface mining law,Dave Johnston Mine,Wyoming

The erosion of hillslopes reclaimed following the surface-mining of coal is an important geomorphologic concern. However, progress in the design of post-mining topography and its hydrologic response has been hampered by the paucity of research focusing on the propensity of rill formation on these disturbed surfaces. This investigation is intended to partially rectify this situation through the development of regression equations for the prediction of rill frequency and magnitude based upon site characteristics. Rill width can be estimated using soil bulk density, hillslope age, and hillslope length with a standard error of 0.152 log₁₀ units at this location. Rill depth can be estimated using soil shear strength, hillslope length, and hillslope age with a standard error of 0.114 log₁₀ units. Rill length can be estimated using hillslope length, percent silt in the soil, hillslope age, Bouyoucos Clay Ratio, and soil bulk density with a standard error of 3.515 m. Rill frequency can be estimated using soil bulk density, Bouyoucos Clay Ratio, soil compaction, soil reaction (pH), and hillslope age with a standard error of 0.241 rills/m. The cross-product of rill width times depth can be estimated using soil shear strength, hillslope length, hillslope age, and soil bulk density with a standard error of 0.260 log₁₀ units. These results must be tested further under various environmental conditions. Nevertheless, prediction of rill formation seems to be a problem capable of solution.     

Processes of accelerated pluvial erosion on desert hillslopes modified by vehicular traffic

Accelerated pluvial erosion on hillslopes modified by off-road vehicles (ORVs) is analysed using results from 50 rainfall simulation experiments conducted in the Mojave Desert, California. Sediment yield from 1 m² hillslope plots subjected to intense, 20-minute rainfalls is typically increased 10 to 20-fold following ORV use. Salient effects of vehicle traffic, which reduce infiltration, increase runoff sediment transport efficiency, and enhance gully formation, are further studied by combining simple theoretical relations with experimental data. This analysis helps identify factors controlling erosion on natural desert hillslopes, as well as those used by ORVs. Erosion of natural or vehicle-used desert surfaces is heavily influenced by runoff hydraulics. Calculated Darcy-Weisbach friction factors decrease by an average of 13-fold following vehicular slope modification, whereas runoff Reynolds numbers increase by an average of 5 1/2-fold. The capacity of overland flow to transport sediment is related to runoff power and its degree of localization, which usually increase considerably following ORV activity; however, the ability of overland flow to move large grains (competency) is related to a combination of factors not always systematically influenced by ORV use. Kinematic runoff routing, which is used to extrapolate experimental results to longer slope lengths, leads to the suggestion that the hydraulic roughness of desert hillslopes strongly influences their erosional behaviour.