Geomorphological mapping

Geomorphological mapping is a map design technique that defines delimits and locates landforms. It combines a description of surface relief and its origin relative age and the environmental condition in which it formed. This type of mapping is used to locate and differentiate among relief forms related to geologic structure, internal dynamics of the lithosphere and landforms shaped by external processes governed by the bio-climate environment.

The analysis of the spatial relationship for structural and modeled forms and the relief shaped by climate influences distinguishes between processes governed by internal dynamics and those conditioned by climate factors that have shaped the topography in the study area. It also determines the relative chronology of tectonic climate or volcanic events and of morphogenetic systems and their cycles. Geomorphic maps will be drawn using digital mapping, aerial photographs and satellite image. Record information on surface form, materials surface fundamental units that compose the landscape.

Purpose of terrain mapping:-

  • Provide information on distribution of landforms, soil and rocks.
  • Indentify features created by surface processes.
  • Enhances engineering planning and land management projects.
  • Major types of products or maps.

Slope analysis:- The slope is the measure of the steepness of a line.

Origins of primary slopes

  • Tectonic (fault scarps)
  • Depositional (volcanoes, glacial moraines, dunes, delta etc.)
  • Erosional (glacial and river valleys etc)
  • Human activity(blasted rock slope, hydraulic mining etc)

Terminology of slope

Angle Description
00-00 Plain
00-30’ Slightly sloping
20-50 Gently inclined
50-150 Strongly inclined
150-250 Steep
250-350 Very steep
350-550 Precipitous
550 and greater vertical


Calculation of overall slope of the watershed

Relief is shown in a toposheet with the help of contour lines. Contour lines are imaginary lines joining points of same elevation. Hence no two contour lines can ever intersect. The value of a contour line shows the elevation of all point lying on it with respect to a benchmark. The contour value is usually expressed as meters above mean sea level (MSL). For a toposheet of a specific scale contour lines are drawn at specific vertical intervals. This means that each contour line is drawn at a drop or rise of 100 meters and 20 meters on a 1:250000 and 1:50000 toposheet respectively. Because the vertical interval is kept constant contour lines will come closer together on the toposheet when the slope increases conversely the contour lines will move apart when slope decrease therefore by looking at a toposheet we can make out steeply sloping hilly areas gently sloping valley and flat land we can also use a toposheet to quickly calculate the overall slope of a watershed or its section.

For this we calculate the difference between the values of two lines. This is the vertical interval. Then we calculate horizontal distance by multiplying the map distance by the equivalent ground distance derived from the scale of the map. As we already know dividing the vertical interval by distance we get the slope:-

    Vertical interval / Horizontal interval ×100 = slope (%)

Drainage basin analysis:-  A drainage basin is the topographic region from which a stream receives runoff, through flow and groundwater flow. Drainage basins are divided from each other by topographic barriers called a watershed. A watershed represents all of the stream tributaries that flow to same location along the stream channel.

Drainage   pattern:- Drainage pattern refers to spatial relationships among streams or rivers which may be influenced in their erosion by inequalities of slope rock resistance structure and geologic history of a region.

Bifurcation ratio:- It is defined as the ratio of number of streams of a  given order to the number of streams of the next higher order.

Rb= Nu/ Nu+1

Where as

  • Rb = bifurcation ratio
  • N = number of stream of given order
  • Nu+1= number of stream of the next higher order


Drainage texture:- Horton defined drainage density on the basis of stream frequency and gave formula.

Rt = Nu/P

Where as

  • Rt= drainage texture
  • Nu= number of stream
  • P =perimeter

Drainage texture affects infiltration capacity which affects the drainage density and stream frequency. This infiltration capacity in turn depends on permeability of underlying rock climate and vegetation etc.

Drainage density:- Drainage density is simply the ratio of total channel segment length accumulated for all order within a basin to the  area of the basin and its unit is usually expressed as miles per square miles or km per km2.

Elongation ratio:- Another parameter was used by schumn called the elongation ratio and defined as the ratio diameter of a circle of same area as the basin to the maximum basin length.

Re = 2(A/Pi/Lb)

Where as

  • Re = elongation ratio
  • A = area of the basin (square km)
  • Pi = 3.14
  • Lb = basin length

Basin shape:-basin shape or the outline form that covers the total area of the basin and may strongly affect the stream discharge quantitatively it can be represented by form factor Rf ; which the ratio of basin area A  and the square of the basin length Lb2. It is a dimensionless property and smaller the valve of form factor more elongated will be the basin. On the basis of attained values all of the selected basins seem to be more of elongated shape. Miller used another dimensionless parameter, circulatory ratio for analyzing the basin shape. It is defined as the ratio of basin area to the area of a circle having same perimeter as that of the basin.  The formula is given as

RC = 4 R A/P2

Where as

  • A = area of the basin
  • P = perimeter of the basin
  • R = 3.14
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