Abstract:
In recent times, the rainfall induced landslides have been prominent in the Chattogram
Hill Districts of Bangladesh. Excessive rainfall, deforestation and indiscriminate hill
cutting have made Chattogram extremely vulnerable to topsoil erosion, slope
instability, and thus to landslides.
The existing topography of the hills of Chattogram City was studied and soil samples
were collected from two hills of the city. Their physical and chemical properties were
determined through laboratory and field tests. One of the soils has been characterized
as sandy silt and the other as silty sand. Both the soil samples have very low to low
total nitrogen, potassium and phosphorous, thus were nutrient deficient. These were
used to make six small scale glass models. One of these was bare and vetiver grass
(Vetiveria zizanoides) was planted in other five to study its growth. The prepared
physical models were tested under artificially simulated rainfall to determine the
effectiveness of vetiver grass in rain-cut erosion reduction. Finite Element Modeling
via Plaxis 2D was also conducted with the obtained soil properties to determine the
factor of safety, thus the stability of bare, vegetated and nailed hill slopes in natural
and terraced condition.
The average root length for vegetated models were 55 cm and average number of tillers
increased from 3 to 23 in 12 months. Thus, the growth of vetiver has been satisfactory
even in the nutrient deficit hilly soils. The bare soil generated a sediment yield of 11.7
kg whereas it varied between 0.10-0.63 kg (94.6%-99% reduction in erosion) for the
other five vegetated models. Hence, vetiver is effective in reduction of rainfall induced
erosion and will eventually ensure slope stability. The numerical analysis show that an
increase in slope angle decreases the factor of safety. At maximum stable angles,
where factor of safety is greater than 1.0 in natural condition, incorporation of vetiver
increased the factor of safety up to 16% with the increase of root zone depth and the
factor of safety varied between 1.064 to 1.171 for vegetated condition. However, the
reinforcing effect of vetiver was inhibited when the slope angles exceed the threshold
angle which is 46.33±3.50° and 33.33±1.97° for sandy silt and silty sand.
For stabilizing steeper slopes, terracing it with smaller slope heights has been observed
as a suitable option. The positive reinforcing effect of vetiver was more pronounced
here, than the natural slope and it increased the factor of safety up to 60%. Terracing
also increased the range of threshold angles for both the soil types. This higher range
facilitates restoration of slope with lower amount of earthwork than compared to that
of natural slope. The stability of natural slopes, where only vegetation is not sufficient,
can also be ensured by using nailing. The increase in factor of safety due to nailing
was more prominent for sandy silt than silty sand. Overall, the study suggests ecofriendly
and feasible solutions for the reduction of soil erosion and safeguarding slope
stability. This shall propose a direction to the decision making authorities of
Chattogram City about sustainable measures for restoration of dangerous hills and
mitigating the risks of landslide.
