Channel Morphology and Hydraulic Geometry of River Kolong, Nagaon District, Assam, India:A Study from the Standpoint of River Restoration

River Kolong of Nagaon district, Assam, India has been facing serious degradation since 1964 as a result of building of an embankment across the river at its take-off point from River Brahmaputra. The river Kolong, once a thriving anabranch (distributary) of the Brahmaputra marked by its active navigability, rich biodiversity and high productivity, is presently in a moribund state. Under these circumstances, the issue of eco-hydrological restoration of the river gained added urgency among the people living in its valley. The 'natural-channel design approach' for river restoration based on the estimates of natural channel geometry and discharge is presently being applied in rivers across the globe. Adopting a similar approach in this study, based on field measurements, we determine the baseline channel dimensions (bankfull width, mean bankfull depth, bankfull cross-section, floodprone width and entrenchment ratio) across the river at four different sites, so that these parameters can be utilized in natural channel design process for restoration purpose. The study reveals that, the river is significantly entrenched with entrenchment ratio varying from 1.19 to 1.79. Moreover, bankfull discharge has also been determined and values are found to be varying from 13.85 to 918.36 cumec in the downstream direction, with an average return period of 1.7 years. The average values of 'at-a-station' hydraulic geometry exponents b, f and m are 0.22, 0.38 and 0.4 respectively, while the average values of downstream direction exponents b, f and m are found to be 0.32, 0.23 and 0.44 respectively. These values compare well with those arrived at by pioneering workers in this field and are considered useful in river restoration programmes.