Estimation of Geomorphic Threshold in Permanent Gullies of Lateritic Terrain in Birbhum, West Bengal, India

The present geomorphic study focusses on predicting threshold conditions and vulnerable locations where gully heads might develop in the lateritic terrain, located at the eastern plateau fringe of Rajmahal Basalt Traps, Birbhum, West Bengal, India. The modern concept of geomorphic threshold is applied here on gully erosion hazard to identify the critical slope of gully head (S) and upstream drainage area (A) with a core relationship of S = aA^-b. Based on 118 gully heads we have statistically derived significant relationships between slope and drainage area (r = -0.55); overland flow (Q) and slope length (L; r = 0.694); relative shear stress (τ) and slope (r = 0.915); as well as overland flow detachment rate (H) and eroding force of overland flow (F; r = 0.980). The established S-A critical relationship, as geomorphic threshold, is expressed as S = 17.419A^-0.2517, above which gully initiation occurred on the laterites. This equation can be used as a predictive model to locate the vulnerable un-trenched slopes (i.e. potential gully erosion locations) in other lateritic areas of West Bengal. The constant b value (0.2517) and Montgomery-Dietrich envelope suggest a relative dominance of overland flow (52.51% of sample gully heads) in the erosion processes. The result of erosion model predicts an annual soil loss of 2.33-19.9 kg m^-2 year^-1 due to overland flow above the gully heads.