In this article we will discuss about:- 1. Meaning of Catchment Area 2. Slope of a Catchment Area 3. Relation of Catchment Area with Its Length 4. Watershed Leakage 5. Classification of Catchment Areas 6. Karst Topography 7. Catchment Flow Characteristics.
Meaning of Catchment Area:
Catchment area may be defined as the area from which the surface runoff is derived. It is also known as watershed area, drainage area, drainage basin or simply basin or catchment.
The unit for the catchment area is km2. If the catchment area is less than 25 km2, it is mentioned in terms of hectares. The catchment areas formed by the divide lines at A and B are shown in Fig. 2.3.
Slope of a Catchment Area:
It is noticed that the slope of a channel is normally steep at the early stage and it goes on reducing as it flows downstream. The average slope of the catchment area is calculated as in Fig. 2.4.
Draw the ‘L’ section of the main stream. Divide the length of the main channel into equal horizontal segments taking into consideration its slope.
Relation of Catchment Area with Its Length:
After analysing a number of catchment areas, the following relation is noticed:
L = 1.2736 A0.6
where L = Length in kilometre of the main channel
A = Catchment area in km2
Water infiltrates into the ground. Because of some subsurface impervious layer sloping in a different direction away from the surface slope, the groundwater from a catchment may flow into the adjoining catchment area. This is known as watershed leakage.
Classification of Catchment Areas:
The catchment areas are classified depending upon the shape as follows (2.5):
a. Fan shape.
b. Fern leaf-type or elongated.
a. Fan Shape:
The shape of a catchment may be similar to a fan, i.e. circular. This will affect the flood intensity from the catchment.
b. Fern Leaf-Type:
The shape of a catchment may be similar to a fern leaf, i.e. elongated. This will affect the flood intensity from the catchment.
Regions underlain by stable rock formations like limestone have characteristic undulating surfaces with conical found hillocks and circular sinks. Such regions are said to have Karst topography and are known as Karstic regions. The runoff from such areas is excluded from the surface runoff estimation, since the precipitation over such areas may be lost into the underground passages to far distant streams, lakes or even sea.
Catchment Flow Characteristics:
i. Overland Flow:
Precipitation falling on a ground surface, after satisfying the infiltration demand, is temporarily detained on the ground surface. When sufficient depth is built, it travels over the ground surface towards the stream. It is somewhat like a sheet flow. It is laminar flow and may not continue for more than 50 m. Then it forms small streams. This called overland flow.
The overland flow stops shortly after the precipitation stops.
ii. Bank Storage:
When the river is in flood, the water surface level in the rivers rises abruptly and is much higher than the groundwater table in the vicinity. Under such condition, water flows from the river into the banks of the river and is stopped there temporarily. The quantity of water thus stored by the banks temporarily is known as bank storage.
When the flood recedes, the water surface level in the river drops. The banks may retain substantial quantity of water, which may flow back into the river or enter the groundwater storage (Fig. 2.6).
iii. Valley Storage:
When heavy precipitation occurs, there is likelihood of flood in the river. However, this flood flow may reduce as it travels downstream because of the storage in channels and depressions. Such storage in channels and depressions is known as valley storage.