Some aspects of the form & origin of hillslopes in western New Territories, Hong Kong

by Luk, Shiu-hung

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An appraisal of research on hillslopes suggests that a morpholo-

gical approach is conductive to the explanation of spatial variations in

slope form and the making of inferences on slope development.

A systems approach is adopted in the study of hillslope forms in

four selected drainage basins in the western part of the New Territories,

Hong Kong. Map data and other relevant background information have been

used to justify the feasibility of the project design. A modified tape-

and-abney method has been employed to measure 100 hillslope profiles

selected at random in three of the four drainage basins. Specific

techniques have been developed to identify and describe quantitatively the slope units of the measured profiles.

Attempts at classification of the profiles reveal a dominance of

convex-straight-concave slope series in the sample basins and the

existence of a relationship between profile groupings and specific slope

units in a slope-angle mean-variance scatter. Analysis of variations in

decli vi ties downslope and across slope units suggests that significant

differences exist both at a within-basin and a within-profile level. An

examination of the collected data supports the contention that 'characteristic slope angles' at 380, 33? 260, and 210 occur in the selected

basins. A morphological model has been erected to represent tpe slope

uni ts in these basins. Incl uded in the model are an upper straight

slope unit, an upper convex slope unit, an upper rock face unit, a midstraight slope unit, a lower concave slope unit, a lower convex slope

unit, and a basal rock face unit.

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The pattern of spatial variations in slope declivity has been explained at three different levels of aggregation. Basin orientation and structure are s1gni?cant factors at an inter-basin level; lithology, relief, and channel charac teristic s at a 'tn thin-basin level,; and vegetation cover at a within-profile level. In addition, differences in the amount of explained variation between basins are considerable. Under condi tions of dynamic equilibri urn, mean valleyside slope angle is well adjusted to other variables of the landsurface set but a lack of adjustment occurs when the system is dominated by disequilibrium forms.

Slope development is studied within both 'partial cyclic' and 'graded' time spans. In partial cyclic time, a decline in slope angle

is accorrpanied by an increase in the importance of the upper convex slope expressed as a percentage of the total slope distance. Slope retreat occurs locally where rapid down cutting as a result of ,tructural control is eminent. A model of slope development based on the input-output budget of the t colluvial sub-system' at slope-foot locations is tested against the collected data. The notion of 'threshold slope angles' in slope development in graded time is found to be applicable and the model is thus partially substantiated.

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