Vaigai

The Vaigai basin is an important basin among the 12 basins lying between the Cauvery and Kanyakumari. This basin is bounded by the Varushanadu hills, the Andipatti hills, the Cardaman hills and the Palani hills on the West and by the Palk strait and Palk Bay on the East. This basin is divided into two major topographical divisions namely (i) the hilly areas and (ii) the plains. The basin is elongated in shape and drains into the Palk Bay. The Vaigai drains an area of 7,741 Sq.Km, which entirely lies in the state of Tamil Nadu.

The Vaigai river rises on the Eastern slopes of the Varushanadu hills at an elevation on 1,200 m above m.s.l near Kottaimalai in the Madurai district at North latitude 9º 32' and East longitude 77º 23' and flows in the Northerly and North-Easterly directions up to its confluence with the Varahanadhi and then takes a turn towards the East and South-East to flow through Madurai, Sivagangai and Ramanathapuram districts. After traversing for about 258 Km, the river Vaigai discharges into Ramnad big tank and some other tanks. The surplus water from the tanks finally discharges into the Palk Bay near Mandapam. On its way, the Vaigai receives two important tributaries namely, the Suruliyar and the Manjalar on its left bank, besides a large number of small streams and rivulets. The river has been dammed down stream of its confluence with the Suruliyar.

The Suruliyar and the Manjalar, the two important left bank tributaries together account for nearly 20 percent of the total catchment area of the Vaigai. The Suruliyar, the principal tributary of the Vaigai also rises in the Eastern slopes of the Varushanadu hills and flows in the North and North-Easterly direction. It receives Theniar on its left bank, just before its confluence with the Vaigai. The Manjalar another major tributary rises in the Palani hills and flows generally in the Easterly direction before joining the Vaigai below the Vaigai dam. The Manjalar is joined by the Marudhanadhi on its left bank before joining the Vaigai. The Vaigai also receives another minor tributary namely, the Varahanadhi on its left bank below the Vaigai dam. The catchment area, length and elevations at source of the important tributaries are given below.

The rainfall in this basin varies from region to region. The rainfall decreases from 1,270 mm at the Western extremity of the basin to 635 mm at the Eastern extremity and the average rainfall is about 850 mm.

As this basin is situated in the rain shadow area of the Western Ghats, it receives very little rainfall during South-West monsoon in the hilly region. At times, the basin receives heavy rainfall in the coastal regions whenever intensive depression is formed in the Bay of Bengal during North-East monsoon.

This basin is subject to a tropical climate and as such the temperature varies with the region. In the hilly Western part, variation of temperature during the year is less, whereas in the plains the mean temperature is around 25° C in January and 27.5° C to 35º C in May.

Vaigai river basin consists of hard crystalline rock masses of archaean age for the most part (74%) on the western portion and sedimentary rocks of upper gondwana, tertiary and quarternary age on the eastern portion (26%). The archaean formations are metamorphosed rocks such as cordierite, sillimanite bearing gneisses, quartzites, calc, gneisses and granulites. They are associated with basic charnockites and pink granites. Pegmatites intrude in the charnockites, pink granites and gneisses at several locations as intrusive body. Pyroxenite intrusions are also found to occur at fewer places. These archaean formations are well exposed on the western and central area of the vaigai river basin. The high rising mountains composed of the above mentioned rock types have yielded material due to weathering and erosion and they were deposited in the valley and intermontane basins as valley filled sediments and are of quaternary age. On the eastern portion from manamadurai to the bay of bengal, rocks of upper gondwana, teritiary alluvium and coastal alluvium are spread over the archaean formations unconformably. Aeolian sands are also found to occur in isolated pockets in the cumbum valley and coastal areas.