Hang- en puinmorfometrie van heuwels met 'n dolerietdekrots in die semi-ariede Suidelike Vrystaat

[摘要] English: An investigation of the literature on slopes revealed that some researchers found arelationship between rock fragment size and slope angle on debris slopes whilst others findno such relationship at all. This project endeavored to empirically test whether such arelationship exists.The literature reveals confusion regarding the concept of debris slopes. It was found that theterms scree slope, debris slope and boulder- controlled slope were used interchangeably.Research findings of scree slopes were used for debris slopes and vice versa.The debris slopes identified for this project conform to the requirements as stated by Young(1972) and comprises a regolith covered slope occurring beneath a free face on whichboulders display an upwards increasing tendency in size.Four hills in the southern Free State (semi-arid climate) were selected and on these tenprofiles were surveyed. The literature reveals purposive sampling as opposed to randomsampling of hills for study and this project is no exception. The choice of profiles were ledby the requirements proposed by Young (1972) as well as including only hills with a debrisslope of the Beaufort series overlain by a dolerite caprock. Control of the latter variablesuccessfully allowed for isolating the origin of debris on the debris slope (as these compriseddolerite from the caprock). Research that did not control this variable ran into troubleisolating the exact origin of debris on the slopes which impacted on results and conclusions.The profiles were surveyed using an Abney level and the profile- meter of Le Roux (1980)which is 2m in length. Profiles were surveyed down the true slope (perpendicular to thecontours) and a line sample of all debris 30 mm and larger on the profile were included forstudy. The debris sample is therefore representative of a particular measurement of slope.Differences amongst researchers exist regarding the downs lope boundary of a debris slope.This also impacts on research findings. In this project the downslope boundary wasdetermined using the two variables of slope angle and percentage debris cover as opposed toonly using slope angle (and this value was in many cases arbitrarily determined) as revealedin previous research. Thus for this study the debris slope comprises the part of slopescommencing at the foot of the free face with the lower boundary based on a low slope angle(�?5°) combined with a low percentage debris cover (�?2% > 30mm diameter).In previous research only one dimension of debris was measured as opposed tomeasurement of all three dimensions as in this project. A variety of indicators for debris sizewere calculated and correlated with slope angle and transformations of angle. Theseindicators were also correlated with distance from the bottom of the free face. Small, butstatistically significant correlations were obtained. Comparison of correlations obtained forthe different indicators indicated trends in the data which impacted on the research findings.Calculation of the indicators also allowed for controlling variation in calculatingcorrelations. This demonstrated why many researchers were led to deduce a causalrelationship between slope angle and debris size. Manipulation of data which included theisolation of outliers as well as variations in measuring length indicated increases in valuesobtained for correlations. The latter indicated the importance of the size of the measuringlength in obtaining results. This revealed a possible reason why researchers using ameasuring length of >2m found a causal relationship between slope angle and debris size.Measurement of all three dimensions of the debris allowed for an investigation into theshape of particles. This demonstrated the importance of particle shape on sorting as well asin the processes operating on slopes and hence the effect of particle shape on slopedevelopment.Slope angle frequency distributions were examined and it revealed that debris slopes in the5° to 10° and 15° to 20° categories could be considered stable for the sampling space.Slopes above 350 are rare and therefore can be regarded as unstable by using ergodicreasoning.From the research it was concluded that debris slopes in this area are mainly structurallycontrolled but that slope angle and shape comprise a complex function of structure andprocess over time. With a relatively stable base level slope development is the result ofparallel retreat.

[发布机构] University of the Free State