Geomorphic Threshold and SCS-CN-Based Runoff and Sediment Yield Modelling in the Gullies of Dwarka–Brahmani Interfluve, West Bengal, India

Abstract

Gully erosion signifies instability in the landscape, and it is regarded as a threshold phenomenon under certain conditions in the landscape, relating to flow erosivity and surface resistance. The main cause of gully formation is too much water at a certain location of slope—a threshold condition that may be brought about by external factors or internal factors. Intense rainfall is the primary trigger, but the local conditions such as slope morphometry, land-use and soil characteristics control the triggering of gully erosion. The catchment size above a stream or gully head and the land-use characteristics determine the volume of overland flow but it is different from the concentrated-flow area by its position on the slope. In the present study, the models of geomorphic threshold and the M–D Envelope have emphasized on the role of overland flow (as surface runoff) for initiation of gullies in the laterite slope of Dwarka–Brahmani Interfluve, West Bengal (Neogene–Early Pleistocene lateritic terrain located in between the Rajmahal basalt traps and Bengal basin). The upstream laterite slopes above gully heads are negatively correlated (r = −0.55) with upstream drainage areas which are used as surrogate for the volume of runoff yield in the gullies of the study area. The calculated empirical trend line (S = 17.419 A−0.2517, with R2 of 0.52) represents an approximation to critical slope–area threshold relationship for gully incision in this region. Then, the Soil Conservation Service-Curve Number (SCS-CN; now called the National Resource Conservation Service, NRCS-CN) method is used for quick and accurate estimation of surface runoff in any storm event in the ungauged watersheds of gullies. Experimenting in three sample watersheds of gullies, it is found that on the basis of rainfall range of 42–137.2 mm the gullies can yield runoff of 40.02–118.0 mm in excess moisture condition of monsoon. The differences of runoff in the catchments for the same rainfall event are the direct effects of land use–land cover derived from CN values. In the gully catchments, the estimated runoff is increased above 22% from Antecedent Moisture Condition (AMC) II to AMC III and in most gullies it ranges from 22.33% to 85.73%. It is understood that if the rainfall amount is increased day by day, the runoff coefficient (Rc) is also increased consecutively and it will be high runoff event which is the sign of high vulnerability of flow erosion on the bare laterite slope. The SCS-CN analyses reflect that in prolong rainfall event of tropical depression more than 86% of rainfall can be transformed into direct runoff, as Rc is reaching up to 0.86. Alongside, it gives more hydraulic energy to gully initiation and gully head migration in AMC III condition. Based on the daily rainfall-runoff modelling and annual potential erosion rate, the estimated sediment yield varies from 5.0 to 13.45 t ha−1 in the gullies. It is learnt that in prolong rainstorm event or in cyclonic rainfall, with increase of rainfall and moisture content of surface, the transport of eroded materials is increased and substantially, the sediment yield of gully catchment is also escalated.