Channel morphology | Geomorphology | Principle Of Geography

Geomorphology

Principle Of Geography

Geography Complete Study Material

(Paper - I)

Channel Morphology

            Channel morphology is also called river channel morphology (or river morphology), it is a complete study of the channel from geography aspect to channel fluids dynamics aspect. The study of channel-related aspects like channel pattern, channel geometry and the factors controlling these forms is called as Channel Morphology. The factors that govern and affect the channel are the processes by which a channel is modified. The channel morphology also includes studying the network of tributaries that join to the main river channel within the drainage basin. Channel morphology or river channel morphology includes the consideration of (i) Channel geometry, (ii) Hydraulic geometry, (iii) Channel bed  topography (iv) Channel Patterns etc.

Channel Geometry 

                The term channel is used in the context of defining the path of a water body that is narrow such as a river or a strait. Thus, in Physical Geography, Channel can be defined as the path of a river or a stream outlined by its bed and banks.The whole system of river channels with its joining branches in the form of tributaries that dissect the earth’s surface is in proportion to the valley size. Channels are occupied by permanent streams, those that flow throughout the year, some have intermittent streams and some have ephemeral streams, which are active only during and after the rains. Although the water of the river flows in its channel but during floods, the water flow exceeds the capacity of the channel and the water spills out of the channel over the floodplain causing the flood.The term channel is often taken as synonymous for the term strait that is defined as a relatively narrow body of water that connects two larger bodies of water. In this nautical context, the terms strait, channel, and passage are used interchangeably. For example, in an archipelago, the water between islands is typically called a channel or passage. For example, The English Channel is the strait between England and France.

Hydraulic Geometry

              The analysis of the relationships among stream discharge, velocity, channel shape, sediment load, channel width, channel depth, channel slope etc. is called hydraulic geometry of a river channel. The cross-sectional area of the channel is measured by multiplying channel width by channel depth while stream discharge by multiplying cross-sectional area at the gauging station by average velocity of water current.

Channel Bed Topography

                The bed topography of river channel refers to configuration of the river beds in terms of positive and negative features e.g. presence or absence of riffles and pools, sand bars and sand islands, shoals, sand dunes etc. Such depositional and erosional (pools) features developed on channel beds are the result of interactions of channel beds are the result of interactions of channel flow and transport of sediment load both as suspended sediment load and bed-material load. Generally, a long profile of a river is characterized by irregular course having a series of alternating steep and gentle segments where in steep segment denotes riffles while gentle segment is called pool. 'A pool is characterized by a water surface profile less than the mean stream (channel) gradient and by finer bed material, whereas a riffle has a water surface slope steeper than the mean  stream gradient and is composed of coarser bed material' (M. Moriswa, 1985). It may be pointed out that if the volume and discharge of water increase resulting in high gauge level, the slope of water surface over riffles and pools is neutralized. According to Petts and Foster (1985) 'a riffle is a topographic high area produced by the lobate accumulation of relatively coarse sediment, and a pool is a topographic low, usually characterized by finer material'. Spatially, riffles are wider and shallow while pools are narrow but relatively deep.

Channel Patterns

             On the basis of lithological characteristics of the region through which the river has developed its course, the river channels are divided into two broad categories viz. (i) bedrock channels and (ii) alluvial channels.

(i) Bedrock channels

                Bedrock channels are also called erosional channels and simply rock channel as they have been developed on well consolidated rocks, popularly called as bedrocks. 'They occur wherever potential rates of removal exceed sediment supply (i.e. erosive power or erosion rates are very high, streams are underloaded as sediment supply through erosion of banks and beds falls short of transporting capacity of the streams); in high mountain areas with steep slopes, glaciated hardrock regions, and in areas undergoing active tectonic uplift' (Petts and Foster, 1985). There is a wide range of variation in the size of bed rock rivers as they may be as long as several hundred meters. There are many such long rivers of the world which are partly bedrock channels and partly alluvial channels. For example, the upper reaches of the Indus, the Satlej, the  Ganga, the Yamuna, the Gandak, the Kali, the Tista, the Brahmputara etc. located in the Himalayas are bedrock streams whereas their middle and lower reaches are alluvial channels. The Colarado river (USA) presents a very fine example of bedrock channels, which has cut a 1380 m deep and 12 km wide valley representing the best example of a canyon known as Grand Canyon between Kaibab and Coconino plateaus in Arizona state (USA).

(ii) Alluvial Channel 

                    Alluvial channel develop  in the regions of sedimentation or alluviation  i.e. where thick deposits of sediments of mostly fluvial origin have taken place i.e. the Ganga-Yamuna plain of north India. They also develop in the broad coastal plains having fluvial (subaerial) as well as marine deposits. It may be pointed out that alluvial channels are characterized by degradation (erosion of beds and banks), aggradation and again degradation. This means the deposited materials are reworked by the channel during coming wet season. Alluvial channels are also characterized by sinuous to meandering and braided channel patterns, depositional and erosional bed topographic features such as pools and riffles, shoals, sand bars, point bars, sand islands, sand dunes etc.