Examining the root-soil interactions within live Vetiver grass buffers towards their deployment for riverbank stabilisation in the Gangetic delta lowlands of India

The role of vegetation as a bioengineering tool for mitigating riverbank erosion has long been acknowledged but is comparatively less researched. Grasses, in particular, are effective in combating erosion by providing dual support. While their aboveground biomass reduces flow velocities, the underground root network binds detachable soil particles, thereby consolidating bank slopes. The effectiveness of installing Vetiver grass (Vetiveria zizanioides) hedges/buffers for this purpose has been investigated in plot level studies along the River Ketia, a spill-channel of the River Silabati in Paschim Medinipur district of West Bengal, which is part of the Gangetic delta lowlands of India. The impact of this grass' root network on the local soil physicochemical properties was studied along with its contribution towards slope stabilisation. The mechanical reinforcement properties of the Vetiver grass roots were examined from root tensile strength and root morphological parameters. While the uppermost (0-10 cm) soil layer showed an increase in the soil organic carbon content, the saturated hydraulic conductivity, decreased bulk density and increasing size of surface macro-aggregates (>0.25mm), all attested to the increasing stability of soil aggregates brought about by root reinforcement. With an increased root tensile strength in the surface layers, the root area ratio was also discerned as being satisfactorily high and Vetiver grass roots were observed to enhance soil cohesion. A channel cross-section survey across the Ketia in the pre- and post-monsoon seasons of 2019–2020 showed changes in channel dimensions, with the Vetiver-protected right bank getting slightly aggraded while the left flank (bereft of such protection) suffered erosion. Flood modelling in HEC-RAS was done to examine the extent to which flow velocities could be reduced by such grass buffers, using roughness coefficients garnered from similar simulation exercises elsewhere. The overall results attest that Vetiver grass can be a sustainable riparian erosion conservation tool along lowland rivers.