Effectiveness of vetiver for landslide prevention in hilly areas of Chattogram metropolitan

Abstract

Hills of tertiary-aged sedimentary rocks prone to landslides and surface erosion characterize the Chattogram metropolitan cityscape. The primary focus of this study is the application of a vegetative method using vetiver grass (Chrysopogon zizanioides) on this terrain to reduce soil loss, improve slope stability, and increase landslide resistance. In addition, vetiver's effect on sediment deposition in the city drainage system due to slope erosion has also been investigated.

Three vetiver-planted plots and a bare hill slope plot were developed on a selected urban hill slope section (Tigerpass Hill). The growth performance was evaluated for three consecutive rainy seasons, from July 2019 to August 2021. The slope top soil up to 1.5 m depth was silty loam (CL-ML), having intermediate dispersive characteristics, low nutrient contents, a pH of 6.4, and an average slope angle of 34.80. Simulating artificial rain, the experimental field method was mirrored in the scaled-down mode.

The field study found that vetiver root-shoot growth outperformed native herbs and shrubs in developing a widespread root network on slopes. The generation of surface runoff was noticed when slope moisture neared the limit of 26-27% with roughly 90% saturation under varying rainfall intensities. Average sediment output on the vetiver slope decreased from 63 t/ha to 18 t/ha in 2021, despite annual rainfall increasing by 2 to 10 times that of 2019 and the estimated cumulative vetiver root coverage increasing from 13% to 44%. Within this period, erosionmeasurements indicated that in a liter of turbid runoff water in 2019, the highest 1.81 g of sediment yield was reduced to 0.5 g (a 73% reduction) in 2021. Moreover, at rainfall intensities greater than 35 mm/h, the trend for silt loss exhibited an exponential rise. Four developed and experimentally fitted equations for four plots demonstrated a linear regression relationship between sediment yield and rainfall intensity. The vetiver growth rate was sufficient to offset the effects of the rising trend in yearly rainfall influencing surface erosion and sediment loss. Therefore, vetiver was found to be a good bioengineering resource for reducing soil loss from mass surface runoff.

In the small-scale model, the average sediment loss differential between a bare and vegetated slope was 2.18 times, while that for the hill slope was 1.22 times, warranting an actual field study for better accuracy in erosion measurement. This study also identifies that the installation of approximately 70,000 m3 of silt traps on the foothills, where major city canals originate, has a beneficial impact on reducing sediment burdens on the city drainage system. The combined effect of vetiver plantation and silt trap installation was expected to result in a 72% decrease in sediment drop in the municipal drainage system for the observed period of 2019 to 2021. This decrement may significantly relieve the municipality's overstretched drainage maintenance load. Thus, vetiver performed outstandingly in resolving the long-standing siltation issue associated with city drainage systems.

A PLAXIS 3D numerical analysis of field slopes for bare and vetiver root-reinforced conditions with variable rainfall intensities between 17.2 and 41.5 mm/h revealed that the mean slope FS improvement between rooted and non-rooted states was 11% by 108 days in 2019, rising to 21% by 470 days in 2020. Thus, vetiver plantation along hill slopes reduces soil erosion and increases slope stability, making this bioengineering technique effective for reducing the landslide susceptibility of hills.