Ravine Erosion in India

Story Padmini Pani, S N Mohapatra
Photo Prasad
 

The ravines of Chambal have probably originated from tectonic activity and have till date shown no obvious relation to climate, but continued deforestation exposes the nutrient deficient soil, which exacerbates ravine expansion. Extreme climatic events in such a scenario can speed up erosion and prompt a disaster.

Chambal ravine formation significantly increases soil loss from agricultural lands and severely impacts agricultural productivity. A review of ephemeral gully erosion and spreading rates of the ravenous tracks of Lower Chambal Valley using geospatial tools shows that both the ravenous and the marginal lands have increased during the last 15 years. A slow natural disaster- ravine erosion is an obvious threat to the inhabitants of the region.

Conventionally, ravine formation, classified as soil erosion, does not feature as a natural disaster, and yet it is a plague-like disease which slowly engulfs valuable agricultural land each year. In its totality, however, it exerts an impact similar to disasters in terms of destroying the socio-economic fabric of a region. A livelihood threat, ravine formation should thus be addressed in much the same way as one would treat a disaster and minimise damage. 

Defining Ravines

H S Sharma in his 1968 paper ‘Genesis of Ravines of the Lower Chambal Valley, India’ presented in the 21st International Geographical Congress, defined ravines as a channel of ephemeral flow, denuded and guided essentially by the process of rejuvenated streams, and having steep sides and head scarps with a width and depth always greater than a gully. Geomorphologists draw a distinction between gully and ravine, based on their morphological and genetic characteristics. Ravine formation begins along river sides and encroaches upon the catchment area by headward growth. Active gully systems commonly develop in unconsolidated materials due to changing patterns of land use and associated change in catchments hydrology. Monitoring gully development has provided important information on processes, rates and geomorphologic controls of gully initiation and its growth.

Gully erosion due to river channel trenching is a problem that threatens vast tracts of the world’s agricultural land. The damage is greatest in the alluvial plains of the semi arid and arid zones and most serious where it threatens precarious subsistence agricultural systems.

Formation and Distribution of Ravines

Scant attention has been paid to the morphological study of ravines and gullies in India. Geoscientists point out that most of India’s ravine lands are found on the margins on the Gangetic Plains. R Ahmed in his paper ‘Soil Erosion by the Indus and its Tributaries,’ published in Pakistan Geographical Review in 1973, proposes a peripheral uplift of the peninsular shield, pressed against the Himalayas, and suggests that the discontinuous pattern of incision is due to differential rates of disturbance. In 1980’s, H S Sharma extends the argument by pointing out that there is no simple correlation between intensity of human occupancies or deforestation and intensity of ravine erosion along the margins of the Deccan. He cites a number of geomorphological studies which illustrate the polycyclic character of river valley in peninsular India. Western theories which relate ravine and gully erosion to climate find little support within the Indian scientific community. In India the main zones of ravine erosion have no obvious relation to climate. In Gujarat and Rajasthan rainfall is only 500 to 750 mm per year, while in the Yamuna-Chambal ravine zone it ranges upward from 750 to 1330 mm per year and in the western sub-Himalayan Zone the annual rainfall is 1125 to 1225 mm per year. Certainly the enormity of the Chambal and Yamuna ravines, which achieve depths of 60 to 80 metres, inclines arguments towards the geological explanation. Neo tectonics may have paved the way for ravine erosion, but it is most definitely exacerbated by human activities. Although, there is field evidence that ephemeral gully erosion is responsible for significant soil losses, little is known about the contributing factors. In addition, the relative contribution of ephemeral gully erosion to total sediment production in agricultural catchments has not been assessed - despite the fact that ephemeral gully erosion in agricultural land is an important source of sedimentation. 

Rate of Ravine Expansion

Globally, an estimated 1.965 million hectare of land are subjected to degradation. Of this, 1.094 million hectare are subject to soil erosion by water, and 549 million hectare of land are affected by salinity or sodicity or both. (UNEP/ISRIC, 1991). The problem of ravines with consequent loss of agricultural land is being reviewed with interest in tropical and mid latitude countries in recent decades. There are no historical records to trace the beginning of ravine formation in India. Within the Indian subcontinent studies point towards a critical magnitude of ravine erosion over the hill slopes of the Himalayas, Siwaliks, Hazaribagh and Chotanagpur Plateau, and along the Yamuna and its major tributaries like the Chambal. Planning Commission (1965) estimates show that about 3 million hectares of agricultural land are affected by the ravines in India - out of which 0.5 million hectares are found along the Chambal. Ravines line the Yamuna River for nearly 250 km and in the Agra and Etawah districts of Uttar Pradesh attain depths of more than 80 metres. Chasms flank the Chambal in a 10 km wide belt, which extends southward from the Chambal-Yamuna confluence, some 480 km, to the town of Kota in Rajasthan, through Madhya Pradesh.

The Chambal Ravine

Chambal, the largest, voluminous and turbulent tributary of Yamuna which runs almost parallel to the northern boundary of Madhya Pradesh has carved out a deep valley for itself. The base level of its smaller streams has turned discordant, which is responsible for active gullying and accelerated soil erosion in the basin. These ravines have adversely affected the socio-economic aspects of life as also communication such that the region has acquired its infamous reputation of being a land of dacoits.

Soil characteristics, upliftment of land and ecological factors have played an important role in the genesis of these ravines. Additionally, the region is semi-arid, marked by extremes of temperature and great uncertainty of rainfall. The climatic conditions with cold winters and hot and dry summers may be attributed to the inland location, lack of vegetative cover, nature of soil and bare rock. It is of interest to note that the total 16, 05,300 hectares of Chambal region predominantly bears a rural character with its activities such as overgrazing and unsustainable agricultural activities further adding to soil erosion. The Chambal Valley, particularly its lower reaches, the focus of the geo-spatial analysis here, has been attracting the attention of scientists, planners and engineers.

Role of Geospatial Technology

Ground efforts can now be actively supported by modern geospatial tools and GIS applications. multi temporal Landsat TM, SPOT and IRS images have been successfully used for the mapping of eroded lands, salt affected and waterlogged soils, and areas of shifting cultivation. Ravine mapping and monitoring can be done by measuring the health of vegetation, an index which helps in monitoring the photo synthetically active vegetation; and normalised deference vegetation index (NDVI), a function of green leaf area and biomass, can be used to find out the depths of ravine formation.

For monitoring ravine erosion, it is pertinent to understand genesis, ravine type and morphological parameters to subsequently categorise them into well defined classes. U-shaped ravines with steep side slopes and scarps are comparatively more stable than V-shaped ravines, which usually develop rapidly. The ravine types classified by their average depth of <5m, 5m to 20m and >20m respectively, into shallow, moderately deep and deep ravines, all qualitative, may be discriminated to some extent from the false colour composites (FCC). But the delineation of areas under each of these three categories is best possible from fused image products of IRS-1C LISS III and PAN data. The FCC generated from first three principal components of LISS-III data has great potential in identifying ravine lands.

End Note

It is important to understand that hazards are often converted to disaster by man, especially in the context of land degradation. We need to be aware of possible short and long term impacts on terrain capacity and potential. While gully erosion due to river channel trenching and formation of ravines are slow processes its effects are severe. To mitigate such a disaster we need to stabilise the spread of ravines by intimately studying the physical, sociological and climatic aspects of each region affected. Efficient reclamation schemes - contour bunding, afforestation, sustainable agricultural practises, limits to grazing etc., deployed in the area could arrest the ravine expansion rates and partially stabilise the region.  

Padmini Pani - Centre for the study of Regional Development, Jawaharlal Nehru University, New Delhi
S N Mohapatra – School of Studies in Earth Science, Jiwaji University, Gwalior

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