Hill Slopes

Why Geomorphology in Geoinformatics?

Reason  Most geomorphic processes operate at a slow rate  So geological hazards, such as volcanic eruptions, earthquakes, tsunamis and landslides, can be forecasted by using GIS mapping.

GIST OF PRESENTAION  Introduction to slope  Slope Elements  Classification of Slope  Based on Slope Elements  Based on mode of Genesis  Based on Slope Angles (Quantitative)  Process – form Approach  Slope Development Theories  Slope decline theory  Slope replacement theory  Parallel retreat theory

What is Slope? Slope is, upward or downward inclination of surface between hills and valleys and form most significant aspect of landscape assemblages.

Slope in Geomorphology The Study of slope in geomorphology is given more importance because, ‘ not only slopes do comprise the greater part of the landscape, but as an integral part of the drainage system they provide water and sediment to streams ’ - R.J Chorley et. al 1985

Slope Elements  Summital Convexity  Free Face  Rectilinear Element  Concave Element

Classification Based on Slope Elements  Convexo-concave slope  Convexo-rectilinear-concave slope  Free face-rectilinear-concave slope  Convex-rectilinear-free face-rectilinear-concave slope  Convex-rectilinear-free face slope

Eg. Convexo Concave Slope

Genetic Classification  Tectonic Slope  Erosional Slope  Slope of accumulation

Quantitative Classification  Level to gentle slope (0° - 2°)  Level slope (0° - 0.5°)  Almost level slope (0.5° - 1°)  Very gentle slope (1° - 2°)  Gentle slope (2° - 5°)  Moderate slope (5° - 10°)  Moderately steep slope (10° - 18°)  Steep slope (18° - 30°)  Very steep slope (30° - 45°)  Precipitous to vertical slope (45° - 90°)  Precipitous slope (45° - 70°)  Wall-like slope (70° - 90°)

Process – form Approach This Concept is based on that there is direct relationship between slope formation and denudational processes. Types:  Mono - Process Concept  Poly – Process Concept

Slope Development Theories  Slope decline theory  Slope replacement theory  Parallel retreat theory

Slope Decline Theory William Morris Davis  American Geomorphologist  “Father of American Geomorphology”  Study Area: US  Humid Temperate Climate  Time Dependent Model

Continue..  Davis hillslope undergo the process of cyclic development with respect to time.  Erosion Starts after Upliftment Stops.  The Sequential changes were, Youth (Convex) Mature (Rectilinear) Old (Concave)

Continue..  In Youth Stage there will be steep Convex slopes; active downcutting resulting into Valley deepening.  In Mature Stage Lateral erosion dominates over Vertical erosion and Water divides gets eroded.  In Old Stage due to continued Lateral erosion there is marked slope decline as much as more than 5°.

Aspects of Davision Model  Rounded Convexity  Graded Waste Sheet  Graded Valley Sides

Gradation  Weathered materials existing on the slope profile are called ‘ Waste Sheet ’  The gradation of Slopes starts from the base and gradually reaches upslope.  At old stage all the slopes are graded.  When the available energy for transportation of debris is equal to work to be done then the slope profile is called ‘Graded Waste Sheet’.  Whatever denudated from slope is taken off by the removal agencies  This is same in case of Valley Sides.

Slope Replacement Theory Walther Penck  German Geomorphologist  Study Area: Germany, Argentina, Turkey  Time Independent Model  Most Misunderstood Geomorphologist

Continue..  Hillslopes or Landscape is the function of Exogenetic and Endogenetic Forces.  Upliftment and Erosion are Co- existantance.  This is also called as forward looking Model.  The Mode of Operation of exogenetic Processes depends on the Characteristics of regoliths on hillslopes.

Basic Premises  Form of hill slopes depend on rate of vertical erosion and denudation by the streams  Retreat of slope depends on gradient of slope  Slope retreat also occurs in Parallel manner  Rate of Weathering of rocks in Slopes is uniform  Rate of denudation matches with the rate of weathering

 The Slope face undergoes parallel retreat due to uniform rate of weathering and instantaneous removal of weathered Materials from Slope segments.  The lowest segment does not experience Parallel retreat because of its low slope angle.  Parallel retreat can be occupied with the rectilinear slope.  Ultimately Concave Slope is formed.  If there is Successive Parallel retreat on both sides of summit then there is Successive lowering of Altitude.

Parallel Retreat Theory Lester Charles King  English Geologist  Study Area: Africa  Arid and Sub Tropical Climate  Cycle of Erosion

Continue..  King did not survey slopes in the field but his model is based on field observations only.  He was concerned to present a broad view, based upon reconaissance observations on a world scale.  King said that ideal hillslope consists of slope elements (Convexity, free face, rectilinearity and convexity)  Full development of slope elements depends on local conditions i.e. resistant and strong bedrocks.  Follows the concept of Pediplanation cycle of King.

Continue..  Scarp face experiences parallel retreat due to backwasting.  There is the balance between the supply of debris from upslope and removal of debris downslope.  This results in the formation of pediment of concave plan at the base of the hillslope.  As a result of extensive erosion surface pediment extend upto hill crest.  This is called as pediplain.

Question arises as to why no such common theory could be postulated which can be acceptable to majority of geomorphologists? Moment to Think!

“there may be no definite theory or geomorphic system that can fit all Landscapes” - C.G. Higgins