Naresh Kazi Tamrakar and Jaya Laxmi Singh

Central Department of Geology, Tribhuvan University, Kathmandu, Nepal

The Malekhu River is one of the major rivers of the Trishuli Ganga River contributing from the south in Malekhu region in Central Nepal. Riverbank slope stability is of concern because rock mass condition and slope stability of riverbank cut slopes are important parameters which can have effects on riverbank erodibility. This study incorporates the rock mass classification and stability of riverbank slopes along the Malekhu River. Fourteen sites in the Malekhu River were selected for rock mass rating (RMR) and then slope mass rating (SMR) by a graphic method. The potentially vulnerable sites were identified after conducting detail field study in different slopes. The results indicate that there occur all the modes of failure ranging from stable (good rock mass) to partially stable (normal rock mass) in all the study sites. The unstable (bad rock mass) and completely unstable (very bad rock mass) slopes are however distributed only in some slopes. The unstable slope of plane failure mode is Ka1, whereas the completely unstable slopes of plane failure mode are Rb2, Ml1 Slope 1 and Ml2. The unstable slope of toppling failure mode is Ml2. When wedge failure mode is considered, the slopes at Ti1 and Ka1 are unstable while the slopes at Kh1, Ka1, Ml1 Slope 1 are completely unstable. The rock slopes where unstable to completely unstable states exist are considered bad (SMR Class IV: 21–40) to very bad (SMR Class V: 0–20) rock mass conditions with fair to poor rock mass rating, respectively. These bad to very bad rock mass slopes are vulnerable to slope movements and prone to river, therefore requiring mitigative measures.