Topographic threshold of gully erosion on the Badlands ridges in Lower Chambal Valley, India

Abstract

Badlands along the lower Chambal valley has been recognised as one of India's worst forms of soil erosion. Theories of their origin vary from surface warping because of Himalayan orogeny to climate change (an increased rainfall in the past). Despite the wide recognition of the Chambal badlands, studies on gully morphology in the badlands have been scarce. The objectives of this study are to investigate the erosion processes governing gully erosion using the topographic threshold concept of the SA (Slope and contributing Area) relationship. As per this concept, the combination of S and A at the gully head expresses critical flow conditions that lead to the formation of the gully head, and mathematically it can be expressed as S = aAb. To study the same, 29 gullies were measured within the badlands in the Lower Chambal Valley. Considering the whole data, the relationship was found to be S = 0.041A−0.086 with a wide scatter. Interestingly, on classifying the data based on the width-depth ratio (WDR), two different and contrasting relationships were found, i.e., S = 0.032A-0.125 (WDR > 1), and S = 0.18A0.048 (WDR < 1). This may indicate that classification based on WDR can differentiate between overland and seepage flow. The weak negative SA relationship further indicates a dominant role played by soil piping and mass movement in the formation and evolution of gully within the badlands in the LCV. Moreover, strong positive relationships were found between average width and average depth, indicating the same processes were involved in shaping the depth and width of studied gullies.