Available and Critical Shear Stress for Sediment Entrainment

Abstract

Causes and mechanisms of sedimentation are explained in connection to the seasonal fluctuation of shear stress. Available and Critical shear stress have been calculated following DuBoys and Shield formula. Critical shear stress of sediment entrainment varies from 0.031 to 0.147 N/m2 in pre-monsoon, 0.041 to 0.169 N/m2 in monsoon and 0.034 to 0.148 N/m2 in post-monsoon season. Available shear stress varies from 0.271 to 0.923 N/m2 in high tide and 0.014 to 0.683 N/m2 in low tide during pre-monsoon season. In monsoon, it varies from 0.275 to 0.965 N/m2 and 0.237 to 0.907 N/m2 during high tide and low tide respectively. It varies from 0.259 to 0.889 N/m2 and 0.022 to 0.521 N/m2 during high tide and low tide in post-monsoon. Lack of available energy to transport the sediment during low tide (in dry season) is the main reason behind the rapid sedimentation in this area. Most of the places (>75%) having deficiency of energy (available shear stress is lower than critical shear stress), during low tide are characterized by deposition of sediments. Presence of mud above the critical limit (30%) in some of the sediment samples generates the cohesive property, restricts sediment entrainment and invites sedimentation. Sheltering of fine grains by coarse grains, biological activity and organic content increase the critical shear stress of sediment entrainment and causes sedimentation.