Physiography & Landforms

Sikkim, a part of Eastern Himalayas, is characterised by rugged topography with series of ridges & valleys

Drainage :

Two perennial rivers namely the Tista and Rangit drain through the area with numbers of tributaries. Generally six orders of streams are available and the drainage pattern are sub-parallel.

Rainfall :

The average rainfall in the State is 3000 mm.

Soil :

The soil pattern in the state varies from skeletal soils, Mountain meadow, brown red & yellow soils and lateritic soils.

Geology & Hydrogeology :

Sikkim falls in the Easten Himalayas, the terrain in general, is underlain by geologically younger rocks. Similarly metamorphic high grade rocks occur at relatively higher elevations. The various rock types of the area represent pelitic and carbonate rocks of Tso Lhamo and Lacchi series, carbonate of Everest lime stone formation and alternate pelite-psammopelite and psammites of the Everest pelite group, sandstone, slate, coal and pebbly slate of Gondwana group rocks (Late Palaeozoic), Daling group (quartzite, phyllite, dolomite), Darjeeling gneisses, Kanchanjunga gneisses, Chunthang formation (quartzite, marbel, graphitic schist, mica schist, granite gneisses) and Lingtse granite, The contact between the Dalings and Darjeeling gneiss marked by the presence of streaky gneiss and occasional mylonites is indicative of doubtful thrust.
The geological units have undergone metamorphism due to diastrophism during upheaval of the Himalayas. The Kanchanjunga gneisses, Darjeeling gneisses, Chunthang formation occur as nappes due north of the main boundary fault which actually delimitates the allochthonous mass from the autochthonous molasses succession of the Siwalik represented prominently by well established thrust plane. Quaternary deposits representing alluvial terraces etc. are developed sporadically along the streams and rivers courses.
Hydrogelogically Sikkim is divided into two divisions viz, (i) Non permafrost area and (ii) Permafrost area.
Ground water occurs largely in disconnected localized bodies under favourable geological conditions, such as jointed fractured zones in various lithological units, weathered zones in the phyllite, schist, gneisses and quartzite. The ground water is available from some perennial springs, from nalas present in all geological formations in the area and in the bored wells constructed by the Central Ground Water Board.
Due to higher relief and steep gradient of the terrain ground water comes out as seepages and springs, wherever the land surface intersects local ground water body. The area is characterized by high rainfall, the primary source of ground water. The springs examined are superficial in nature. Direct infiltration and rainfall through joints, fracture, weathered zones of the rocks and through soil covers is the principal mode of recharge of the springs. Due to steep gradient most of the precipitation in the area flows out as surface runoff through streams, kholas. Only a minor fraction of total precipitation percolates down through the thickly vegetative permeable soil cover and through highly fractured rocks. The springs discharge in different geological formations and their discharges are as under

In Sikkim most of the springs show much variation in fluctuation range in dry and wet season from 3to6 times thus categorized as good.


Achievements :

The Central Ground Water Board constructed 29 no. exploratory wells in Sikkim, out of which 27 no. are in South Sikkim and 2 no. are in East Sikkim. Out of the constructed bore wells, 8 nos show significant discharge varying from 1.5 to 15Ips, Yet another group of 8 nos bored wells yield between less than one to one 1 Ips, in general. The bored wells have encountered up to six fractures in depth range of 10 - 70 m bgl, The upper fractures down to about 46 m bgl, are ascribed to a local pattern. The lower fracture zones below the depth of about 50 m bgl. are seen to be persistent and suggestive of regionally extensive, implying a regional pattern. Transmissivity varies from 5.32 m2/day to 316 m2/day for Gondwana rocks and 16 m2/day to 200 m2/day for the Daling rocks.
Permafrost zone occurs in 90% area of North Sikkim and 10% area of North-Western Port of West Sikkim, The frozen zone occurs at the elevation above 5000m from mean sea level, Three types of the underground water occurring in the Permafrost zone, namely suprapermafrost, interpermafrost and infrapermafrost. There are many glaciers, aprons in Sikkim. As the glaciers have maintained the incipient recessional trend over the years, these contribute around 80% of total surface discharge.

Findings of CGWB in Sikkim :

Potential Fractures have been identified in the State of Sikkim down to 70m bgl which can yield 8 to15 lps.

Formation	District	Findings
Gondwana/Daling	South Sikkim.	Fractures (maximum of 6 sets) encountered in the depth span of 13-70 mbgl with discharge of 0.4-8 lps. Fractures down to 45 mbgl are not very persistent and appear to be of local pattern, while the same occurring below 50mbgl are rather persistent and probably of a regional nature.
Buxa	South Sikkim	Fractures encountered in the depth span of 4-36mbgl With insignificant discharge (0.12 lps) within the drilled depth of 101 mbgl.
Darjeeling Gneiss	East Sikkim	Fractures encountered in the depth span of 7-47mbgl Yielding as high as 15 lps.