Classification of rivers based on topography and flood hydrographs

Articles > Classification of rivers based on topography and flood hydrographs

Introduction:

Classification of rivers is mainly based on the topography of the river or the basis of flood hydrographs.

Classification based on topography:

Under this type of classification, rivers are subdivided into 3 types

Rivers in hills (Upper reaches): These rivers generally take off from the mountains and flow through the hilly regions before traversing the planes. They are further subdivided into-

Incised or Rocky River stage: In this type flow channel is formed by the process of erosion. The sediment transported in this reach is often different from the river bed materials, since most of it comes from the catchment denudation and soil erosion
Boulder River stage: The river bed in these reaches consists of a mixture of boulders, gravels, shingles, and alluvial sand deposits created by itself. During a flood, the boulders, shingles, and gravels are transported downstream. But as the flood subsides the materials get deposited in heaps.

Rivers in alluvial flood plains (Lower reaches): The chief characteristic of these river reaches is the “Zig-Zag'' fashion in which they flow, called meandering. They meander freely from one place to another and carry sediment which is similar to bed material. The material gets eroded from the concave bank(i.e. Outer edge) and gets deposited on the convex bank(i.e. Inner edge) of successive bends or between two successive bends to form a bar. Further subdivided into-

Aggrading type: An aggrading river is a silting river. Such a river increases its bed slope, which is called building up of slope. Silting may occur due to various reasons such as-
- Heavy sediment load.
- Construction of an obstruction across the river, such as a dam or a weir.
- The sudden intrusion of sediment from a tributary etc.
Degrading type: If the river bed is getting constantly scoured to reduce and dissipate available excess land slope, then the river is known as a degrading river. Found either above the cut-off or below the dam or weir etc.
Stable type: A river that does not change its alignment, slope, its regime significantly is known as stable river. Changes such as silting or scouring or advancement of the delta into the sea may take place, but they are negligible and fail to produce and change in the regime of the channel.
Braided type: When the river flows in two or more channels around alluvial islands, it is known as a braided river. The braided pattern develops due to the deposition of coarse materials, which cannot be transported under prevailing conditions of flow and which subsequently grow into islands consisting of coarse as well as fine materials.
Delta type: A river before it joins the sea, gets divided into branches, thus forming a delta shape. As the river approaches the sea, its velocity decreases, and consequently the channel gets silted, and the water level rises in spills and eventually the formation of a new channel.

Fig. Alluvial river classification

Courtesy: Semantics scholar

Tidal Rivers: The tail reaches of the river adjoining the ocean are affected by the tides in the ocean. During floods, the ocean water enters the river and the water level rises. But during ebb tide, the water level falls as the water flows out. Therefore, the river undergoes periodical rise and fall in water level, depending upon the nature of the tide.

Classification based on flood hydrographs:

Under this type of classification, a river is divided into two types-

Flashy Rivers: If the flood rise and the flood fall in the river are sudden, then the river is known as a flashy river. In a flashy river, the flood flow occurs suddenly and therefore, the rise and fall of water are very quick. The flood hydrograph is very steep, indicating floods all of sudden.
Virgin Rivers: In the Arid zone, a river may completely dry before it meets the river or before meeting another river, such a river is known as the virgin river. After flowing through a certain region, the water of such a river disappears due to high percolation or due to excessive evaporation.

Fig. River hydrograph showing the peak discharge during monsoon

Courtesy: wikipedia

Indian Rivers and their classification:

Apart from the above classification mentioned, Indian rivers are mainly classified based on their contact with the Himalayas. Hence Indian rivers are classified as follows-

Himalayan Rivers: These rivers take off the Himalayas and flow through alluvial plains. They mainly derive water from rain during monsoon and winter and melting of snow in summer. These rivers are therefore perennial and can give dependable yields throughout the year. Himalayan rivers carry huge sediment because of two reasons-

The Himalayan rocks are soft and friable
The Himalayan zone, particularly the North-Eastern part is susceptible to earthquake disturbances, causing landslides and increasing rock sediment.

Examples of Himalayan rivers are- Ganga, Brahmaputra, Ravi, Gomti, Sutlej, Indus, Jhelum, etc. Due to heavy rainfall in July and August, these North Indian rivers rise in high floods. These large variations in discharge and sediment load make the hydraulics of these rivers very complex and cause them to meander.

Non-Himalayan Rivers: These are non-perennial rivers that receive major water supply only in rainy seasons and for the rest of the year, they may draw water from ground water as a base flow. These rivers are much more stable than the Himalayan rivers and have lesser problems, as they flow through non-alluvial soil. The line dividing the Himalayan and the Non-Himalayan river is the boundary hatched by the left bank of river Sutlej, the right bank of Yamuna and Ganga, and the left bank of Brahmaputra. Examples of Non-Himalayan rivers are- Chambal, Cauvery, Mahanadi, Godavari, Tapti, Narmada, etc.

Conclusion:

Rivers based on their classification show different properties, possess different minerals and can be used for different work accordingly. The classification helps to understand the flow conditions of a river and most importantly for any civil engineering construction works based on rivers, the behavior of the river must be known. For example,for any civil engineering-related works,a perennial type of river is preferred and hence a proper study of the river classification is a must in this regard. Another situation where the classification of rivers plays a great role is in case of construction dams or case of any hydropower project. Hence to have complete knowledge of the behavior and the type of flow, the rivers are classified accordingly.

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