Analyzing roughness co-efficient of vetiver planted char land using physical model

Abstract

Natural sedimentation often accelerated artificially for land reclamation in the deltaic environments to support many development activities. To trap more sediment at some strategic locations (to elevate land elevation), the governing parameters flow-sediment regime and resistance to flow are interacting together, more the sediment concentration and higher the resistance to flow are likely to increase sedimentation and land reclamation potentials. We will examine here potential of sedimentation or land reclamation due to the impose of additional roughness, through theoretical analysis complemented by simple hydraulic experiments. Different species can be used as sedimentation enhancing material and controlling surface erosion. Adding vegetation to the bed surface acts to increase hydraulic roughness by decreasing the mean velocity of flow. To achieve the objectives of the study, vetiver grass is chosen as a bio-engineering material and applied to common soils in char land and haor area (char: sand bars inside rivers that need continuous sedimentation to be elevated and haor: bowl shaped depressions in the north eastern part that need sedimentation regularly to cope with the ongoing natural and anthropogenic changes). To implement the hydraulic experiments, char and haor soil collected from Narayanganj and Brahmanbaria are classified as silty sand. The fineness modulus of the char soil and haor soil are 0.60 and 1.49, respectively. The coefficient of permeability of char soil varies between 1.03×10-2 and 5.70×10-3 cm/sec which specifies low to medium degree of permeability and 1.80×10-4 and 9.29×10-5 cm/sec which indicates very low to a low degree for haor soil. It is observed in the direct shear test results that friction angle values lie between 28-40° for char soil and 26-28° for haor soil. From the nutrient test, it is observed that pH, organic matter, Potassium, and Sulphur contents in soil are adequate but the percentage of Nitrogen, Phosphorus, Boron and Zinc indicate that these contents are not well satisfactory according to the required amount for proper vegetation growth. Four rectangular wooden models have been prepared and filled with char soil and haor soil having a dimension of 3.0m× 0.7m× 0.6m namely Bare Char Model (BCM), Char Model with Vetiver (VCM), Bare Haor Model (BHM) and Haor Model with Vetiver (VHM). After 140 days, it is seen that shoot length has grown up to 127 cm and 119 cm, and root length has grown up to 50 cm and 41 cm for char and haor soil, respectively. Root propagation is slightly better in haor soil than in char soil. Theoretical relationships using empirical equations have been established based on char soil and haor soil. Different flow discharges ranging from 0.002 to 0.004 m3/s have been considered and corresponding Manning’s roughness coefficient, flow depth and mean velocity are determined against a specific channel section with a fixed slope. Manning’s roughness coefficient has been found from 0.0116 to 0.0118 for bare conditions and from 0.03 to 0.05 for different roughness heights in both types of soil. Under both conditions, it is seen that vegetation reduces flow velocity but increases surface roughness and flow depth. In bare conditions, bed material acted as a roughness element whereas vegetation acted as a roughness element when vegetation was added. Besides, a condition for initiation of sedimentation under specific roughness height was established. Small scale physical model-based experimental program has been conducted on char soil. Results shows that within a certain flow range, the depth of flow increases in BCM whereas, in case of VCM, the average flow velocity decreases from 0.16 to 0.11 m/s. A physical representation of surface erosion after the experiment has also been shown for both cases. From the study, it can be concluded that vetiver grass plantation is effective for increasing hydraulic roughness and for the initiation of sedimentation, that commences the process of land reclamation. It is important to note flow-sediment regimes are often changes both due to climatic factors and the human interventions, and it is important to consider these while implementing any and reclamation techniques.