Rill Hydraulics - An Experimental Study on Gully Basin in Lateritic Upland of Paschim Medinipur, West Bengal, India

This paper reports the results of field investigations aimed to establish relationship between morphologic and hydraulic characteristics of rills. A rill network within a small Rangamati Gully Basin, on the bank of Kansai at Paschim Medinipur, in West Bengal, India of 256 m2 was mapped. Experimental basin is exposed to natural rainfall of varying intensity and characterised with an average of 25-35% slope gradient. The depth, length, gradient, width, runoff contributing areas of all the 33 rills within the basin were recorded and thoroughly mapped. Present study incorporates close monitoring of runoff and velocity along the channels during a storm on 31.08.2010 at 15 minutes interval using dye tracer technique. The velocity along each rill was linked with gradient, width-depth ratio and Manning’s roughness coefficient. Analysis shows that flow velocity is not directly controlled by rill gradient (R2=0.15), but is influenced by hydraulic roughness coefficient (R2=0.53). Finally, the relationship between Reynolds number and Froude number reveals that sub-critical turbulent flow is responsible for rilling process. No significant relationship between W/D ratio and channel roughness (n) can be established (R2=0.083) by the present study.