Channel morphology and Slope development

I. Channel Morphology

Introduction

Channel morphology is a branch of study that focuses on analyzing the shape, direction, and characteristics of a river or a specific segment of a river. Unlike the drainage system, which examines the river along with its tributaries and distributaries, channel morphology delves into the specific details of a single river or a portion of it. This analysis depends on factors such as the gradient of the river and the texture of the surface over which the water flows.

Some Related Terms

• Channel: It refers to the stream itself and represents its three-dimensional form, which is defined by its slope, cross-section, and pattern. There are two types of channels: bedrock channel and alluvial channel.
  • Bedrock Channel: This type of channel is characterized by very high erosional capacity and no deposition by the river. It is usually straight due to the presence of a hard channel that prevents the river from finding alternate paths. Bedrock channels are formed when the river flows at high speeds, resulting in significant erosion and material transport. Himalayan rivers are examples of bedrock channels.
  • Alluvial Channel: Alluvial channels are associated with old or late-mature stages of rivers. In this stage, the river begins deposition, making it more sluggish and changing its path. It follows a meandering path, and although the depth of the river reduces, the width remains unchanged. The river valley experiences extensive deposition of finer sediments in this stage.
• Cross Over: It refers to a line that joins two opposite ends or sides of the main body of water along the length of the river.
• Length of the River: The length of the river can be determined by drawing crossovers at regular intervals.
• Longitudinal Profile of River: It is the line joining the midpoints of the crossovers, representing the longitudinal variation of the river’s features.
• Width: The width of a river is not a constant factor and depends on the amount of water, the surface, and the shape of the valley.
• Thalweg: Thalweg refers to the line that connects all the points of maximum depth of water along the channel in the downstream direction.
• Sinuosity: Sinuosity refers to the deviation of the actual channel path from the expected theoretical path. It is often quantified using the sinuosity index.
  • Sinuosity Index: The sinuosity index is the ratio of the channel length to the length of the meandering axis (valley length). If the sinuosity index is less than 1.05, it indicates a straight river flowing along a bedrock channel with high velocity. If the sinuosity index is greater than 1.5, it indicates a meandering river in its late or senile stage. A sinuosity index between 1.05 and 1.5 signifies a sinuous river.
• Radius of Curvature: The radius of curvature refers to the radius of a circle drawn through the apex of a bend, connecting two crossover midpoints.
• Channel Pattern: Channel patterns are typically associated with alluvial channels and can be classified as straight, meandering, or braided.
  • Leopald and Wolman’s Classification: Leopald and Wolman classified river channels into three types based on sinuosity index. A sinuosity index greater than 1.05 indicates a straight channel, a sinuosity index between 1.05 and 1.5 signifies a sinuous channel, and a sinuosity index less than 1.5 represents a meandering channel.
  • Schumm’s Classification: Schumm divided river channels into three types based on sediment load types. A suspended sediment load channel is characterized by vertical erosion, valley deepening, and deposition along the river banks. A mixed-load channel exhibits characteristics between suspended and bedload, similar to a sinuous channel according to Leopald and Wolman. A bedload channel is characterized by fine sediments settling at the river bed, lateral erosion, and widening of the valley.
  • Morisawa Classification: Morisawa presented five major categories of channel patterns by amalgamating the classificatory schemes of Schumm and A.D. Miall. These categories include straight channel, sinuous channel, meandering channel, braided channel, and anastomosing channel. The anastomosing channel is a special type of braided pattern where multiple channels remain stable and retain their identities with changing discharge and time. There is also an additional category called anabranching pattern, where the river first braids and then rejoins the main stream through anabranches or offshoots.

Hydraulic Geometry

Hydraulic geometry refers to the relationship between the velocity of water and various channel characteristics such as width, depth, and surface.

• Definition: Hydraulic geometry quantifies the relationship between discharge, width, depth, and velocity of water in a river. Discharge is considered an independent variable, while velocity, depth, and width are dependent variables.
• Leopold and Maddock’s Study: Leopold and Maddock introduced the concepts of at-a-station condition and downstream condition. In at-a-station condition, any change in discharge directly affects the width, depth, and velocity of the river. However, the magnitude of these changes can vary. For example, a river with an irregular thalweg may not experience a significant change in velocity despite a change in discharge. Similarly, a river with a rectangular valley may not show an increase in depth even with a change in discharge. In downstream conditions, for the same change in discharge, the width increases more downstream than at the station condition. This phenomenon explains why heavy rainfall upstream often leads to flooding downstream.
• Ripple: Ripples are shallow areas in the channel where coarse sediments accumulate.
• Pools: Pools are areas of greater depth with a gentler slope than the channel gradient. They often accumulate finer sediments.

II. Slope

Introduction

Slope is a fundamental aspect of landforms and plays a crucial role in shaping the Earth’s surface. It refers to the inclination of a land surface and is characterized by segments of varying steepness. Slopes are primary attributes of landforms and can be seen as sets of inclined surfaces put together. Understanding slope characteristics and their classification provides valuable insights into landform development and processes.

Classification

Slopes can be classified based on their steepness and slope angle, as well as their genesis (origin) and location.

• On the basis of Slope and Slope Angle:
  • Summital Convex: In this type of slope, the slope angle decreases with height. It forms the uppermost portion of a landform and is often associated with soil creep.
  • Vertical Cliff/Scarp Face: These slopes have a constant slope angle of 90 degrees. They are characterized by a vertical face where sediments do not accumulate. The upper part of the slope is known as the upper wash.
  • Straight Rectilinear Slope: Also referred to as debris-controlled slopes, these slopes maintain a constant angle with height. They continuously move down sediments and are influenced by gravity.
  • Basal Concavity: Basal concavity slopes, also known as concave slopes, have an increasing slope angle with height. They are often associated with downwash or the lower wash slope.
• Genesis (Origin):
  • Tectonic Slopes (Structural Slope): These slopes are formed due to tectonic processes and are influenced by the underlying geological structures.
  • Denudational Slopes:
    • Erosional Slope: These slopes are shaped by erosion processes such as weathering, mass wasting, and fluvial action.
    • Depositional Slope (Aggradational Slope): Depositional slopes are formed by the accumulation of sediments and the process of aggradation.
• Location:
  • Epigene Slope: Epigene slopes are formed by processes that occur outside the surface, such as weathering and erosion. They are impacted by denudation processes.
  • Hypogene Slope: Hypogene slopes are associated with intrusive landforms and are formed below the surface.

Approaches of Slope Study

There are two main approaches to studying slopes: the historical approach and the process approach.

• Historical Approach:
  • This approach is time-based and uses techniques of denudation chronology to understand landform development. It requires the study of erosional surfaces and is often speculative, generalized, and descriptive, following the Davisian tradition.
  • The historical approach views landforms as the product of their age, considering the cumulative effects of various processes over time.
• Process Approach:
  • The process approach is time-independent and gives primacy to the processes that create landforms. It acknowledges the role of factors such as rock composition and texture, as well as the influence of endogenetic and exogenetic processes.
  • This approach sees landforms as a consequence of interacting processes that create and shape them. It is not purely descriptive and focuses on the understanding of processes rather than speculating about the landform’s age.
  • However, the process approach has limitations. Many geomorphic processes operate over long time scales, making it difficult to observe their immediate consequences. It is challenging to determine the precise processes that operated in the past, and landforms are often the result of multiple processes, making it complex to attribute specific processes to individual landforms.
• Consequences of Different Processes:
  • Various geomorphologists have proposed theories on the consequences of different processes:
    • Gilbert explained summital convex slopes as a result of soil creep, whereas Fanman attributed summital convexity to rainwash, polishing, and ineffective erosion.
    • Lawson observed summital convexity due to more effective erosion by rivers, while basal concavity resulted from less effective erosion.
    • Baulig suggested that concavity is caused by deposition rather than erosion.
    • Penck emphasized the importance of climate controls and identified diagnostic landforms representative of specific climatic conditions.
    • L.C. King rejected the idea of climatic geomorphology and believed in the concept of climatic uniformitarianism, stating that all types of landforms can be found in all types of climates.
    • A.N. Strahler, based on the process approach, considered slopes as elements of equilibrium controlled by factors involved in their development.