The Chittagong City Corporation (CCC) has undertaken numerous projects for years to address chronic waterlogging problem of the port city but its efforts have yet to yield any significant outcome.
To make matters worse, around one-third of the port city goes under water during every monsoon even after a little rainfall.
To address the issue, three successive mayors of the CCC have already spent around Tk324 crore in the last 14 years.
Against this backdrop, the corporation has now come up with an unconventional and low-cost approach to deal with the longstanding problem. It plans to plant vetiver grass that helps reduce soil erosion on steep slopes of hills.
As part of the pilot project of vetiver grass plantation, the authorities are now planning to cover the four major hills of the port city with vetiver grass, locally known as “Binna” grass, to control slope erosion.
Vetiver (Vetiveria zizanioides), a clumping grass without stolons or rhizomes, is known for its ability to reduce siltation and stabilize soil. Its non-fertile seeds ensure that the plants will not spread beyond where they are planted. Engineers liken the vetiver root to a “living soil nail” with an average tensile strength (used to describe the extent to which something can stretch without breaking) of 1/6 mild steel. The deep root system makes vetiver extremely drought tolerant and difficult to dislodge.
CCC Chief Engineer Lt Col Mohiuddin Ahmed told the Dhaka Tribune that initially four hills - Burma Haji Pahar, Khulshi Pahar, and hills on both sides of Tigerpass intersection - have been earmarked for vetiver grass plantations.
“The port city’s water congestion is inseparably connected with soil erosion from the hills. Indiscriminate deforestation, hill cutting and unplanned human settlements result in massive soil erosion. Following torrential rainfall, the soil eroded from the hills clog up the drains and nearby water bodies and trigger water stagnation in the city,” he said.
If the pilot project yields positive results the corporation will send a project proposal to the Ministry of Local Government, Rural Development, and Cooperatives, he added.
According to experts, when a raindrop falls, it is usually absorbed into the pore spaces of the soil. But when the pore spaces become saturated (full), the raindrops will either remain on the surface as puddle or flow downhill. As the water flows downhill, it carries with it soil particles and other materials, which is called erosion.
Md Rashidul Hasan, assistant professor at the department of urban and regional planning of Chittagong University of Engineering and Technology (CUET), told the Dhaka Tribune that unlike other parts of the country, the hills of Chittagong region constitute of crumbling soil and become vulnerable after a heavy rainfall if the surface is not covered with vegetation.
“The soil becomes heavy after absorbing rainwater and the steep slopes cannot bear the mass weight of the wet soil or mud, resulting in landslides.”
The less vegetative (plant) cover and leaf material on the ground, the more open the soil will be to the runoff water. Slope erosion can therefore be prevented by protecting the soil surface with some form of cover and controlling runoff before it develops into an erosive force.
Prof Dr Md Shariful Islam of Department of Civil Engineering, BUET, who is providing technical assistance to the CCC in this regard, said that vetiver is a miracle grass. It is being used as an efficient bio-technology for slope protection in many countries for its special attributes like longer life, strong and finely structured root system, and high tolerance of extreme climatic conditions.
“It has been used in many tropical countries for being a simple and economical method to conserve soil by slowing the velocity of water and stabilizing steep slopes. The grass can survive extreme weather conditions ranging from -14 degree Celcius to 55 degree Celsius,” said Prof Islam who has been conducting a research on vetiver for over a decade.
“The extremely deep and massively thick root system of vetiver binds the soil and at the same time makes it very difficult for it to be dislodged under high velocity water flows. This deep and fast growing root system also makes vetiver drought tolerant and highly suitable for steep slope stabilization,” he further said.
“Vetiver grass is available in Bangladesh. The grass can grow in both silt and clay types of soil. Apart from saline soil, the grass can even grow in heavy metal contaminated soil. It is not a weed. This grass is non-competitive with adjacent crops and cost-effective. So, vetiver grass plantation could be a sustainable, green, and cheaper bio-engineering solution to slope protection,” said Prof Islam.
Aminul Hoque Chowdhury, deputy director of Department of Agricultural Extension, Chittagong said that vetiver grass is used as a cover crop (crop grown for the protection and enrichment of the soil) all over the world.
“The concept is quite new in our country. If the rate of nutrient and water uptake is higher by vetiver grass, then it might affect other plants. So we have to be careful in this regard,” said the agriculturist.
Prof Dr Md Al Amin said that any type of vegetation slows down the runoff velocity.
“Vetiver grass is one of the tried and tested methods all over the world for slope stabilization. We should also adopt approaches like bolting system to reduce slope failures,” said Prof Amin who teaches at the Institute of Forestry & Environmental Sciences (IFES) of Chittagong University.
CCC Mayor AJM Nasir Uddin said that they were considering planting vetiver grass to stop erosion.
“Hill cutting should be stopped, othrwise vetiver grass plantataion will be of no use,” he said.
