3. Slope Instability Analysis Using Morphometric Parameters: A Sub-watersheds Scale Study

Morphometric analysis of any drainage basin has paid immense importance in hydrological investigation. Morphometric parameters describe the topology, the structure, the planform and the relief of basin which are being applied for the prioritization of watersheds. In the present study, an attempt has been made to prioritize sub-watersheds based on morphometric analysis in relation to slope instability. The base map of stream network were digitized from toposheets no. 78A/4, 78A/8, 78B/1, 78B/5 and 78B/6 (Scale 1:50,000) and then updated on Google earth. Arc GIS 9.3 software and MS excel-2007 were used to assess 13 morphometric parameters i.e. bifurcation ratio (Rb), length of overland flow (Lof), drainage density (Dd), stream frequency (Fs), texture ratio (Rt), drainage texture (Td), compactness coefficient (Cc), constant of channel maintenance (Ccm), shape factor/basin shape (Sf), form factor (Ff), circularity ratio (Rc), elongation ratio (Re) and relief ratio (Rr) of the Balason River basin of Darjeeling Himalaya and to prioritize subwatersheds (SW). The results showed that SW 4, SW 5 and SW 8 fall under very high priority class in respect of soil erosion and soil loss or simply instability having Rb of 3.532–4.002, Dd of 5.986–6.538, Fs of 14.934–29.447, Td of 9.765–23.158, Rt of 7.055–16.958, Lof of 0.069–0.084, Rr of 0.209–0.302, Cc of 1.265–1.554, Ccm of 0.139–0.167, Sb of 2.969–5.556, Ff of 0.180–0.337, Rc of 0.420–0.634 and Re of 0.479–0.655. Sub-watershed wise highest priority obtained by SW 5 followed by SW 8, SW 4, SW 3, SW 9, SW 1, SW 6, SW 16, SW 2, SW 17, SW 15, SW 13, SW 7, SW 10, SW 11, SW 12, SW 14, and SW 18.