Dr. Riddick Kakati Research Scholar
Department of Civil Engineering
IIT Guwahati

River bank erosion due to unstable banks and high flow variability is usually controlled using permeable and impermeable structures, which are not studied much yet and also cannot solely provide desired velocity reduction. These structures, along with a combination of porcupine screens followed by geobag (i.e., Hybrid layout), are investigated using CCHE3D model for emerged, transition, and submerged flow conditions with respect to porcupine height. An optimum hybrid layout showed velocity reductions of 35% in submerged and 70% in emerged conditions, which further increased with multiple porcupine screens.

Protection of river banks is an inseparable part of river training works. Permeable and impermeable structures are most commonly used for riverbank protection. Porcupines impose a mild impact on the river by implementing its effect gradually. However, during high flow conditions, these structures are ineffective and often get washed away. On the other hand, impermeable spurs impose a sudden impact on the river system and drastically reduce the velocity in its zone of influence. Due to this, turbulence is generated near the nose of the structure leading to the formation of scour hole, which results in structural instability. Therefore, an attempt has been made to study the effectiveness of the interventions mentioned above in stabilizing and protecting the rivers. Due to several limitations of the physical models, such as scale effect, steady-state flow, and high cost, which make it difficult to carry out in the case of a braided river system, a three-dimensional hydrodynamic model was used. In this study, the performance of the 3D hydrodynamic model CCHE 3D is evaluated in terms of velocity reduction potential by comparing it with experimental results. It was observed that initially, the velocity was in the range of 0.1 m/s under emergent condition, which reduced by more than 50% in the downstream of single porcupine screen, more than 75% in the downstream of two porcupine screens, and more than 94.36% in case of geobag layout. Flow deflection was also observed, but it was not significant.

Riverbank erosion is widespread in alluvial rivers in India and elsewhere. River training works are frequently used to aid in the prevention of these losses by regulating the river and therefore protecting critical human habitats. These structures often become unstable and incapable of performing adequately during periods of heavy flooding. For the first time, the three-dimensional hydrodynamic open-source Open Field Operation and Manipulation (OpenFOAM) model is used to assess the potential of a novel hybrid river training arrangement to reduce downstream flow velocity and divert downstream flow to the opposite bank. The results indicate that for single-phase approximation, algorithms such as the Semi Implicit Method for Pressure-Linked Equations (SIMPLE) with lower computational requirements can satisfactorily reproduce flow patterns discovered in the laboratory (𝑅² > 0.74). The hybrid configuration outperforms the porcupine and geobag layouts. When compared to geobag, dual-screen porcupine, and single-screen porcupine, its downstream velocity decreases by 1.33%, 11.62%, and 13.34%, respectively. Similarly, flow diversion to the opposing bank increases by 0.49%, 0.65%, and 0.92%. Thus, the porcupine structure reduces the intensity of the incoming flow prior to it reaching the impermeable geobag in a hybrid layout. It dissipates the flow energy to the point where it can no longer scour the bed, thereby eliminating the disadvantage associated with the formation of scour holes.

In this study, chlorophyll indices already developed for selected rivers in Brazil, USA and India are applied to a reach in Brahmaputra River in India, to test their applicability in this large braided river system and also to compare their results. Linear, logarithmic, exponential and quadratic relations of Blue, Green, Red and Near Infrared surface reflectance of Sentinel 2A and Landsat 8 imageries are used in this study to determine the chlorophyll indices. It is found that there is a significant variation of the values obtained by these algorithms. For algorithms using Landsat Imageries, concentrations obtained were in the range 40.727-261.836 mg/L and for algorithms using Sentinel Imageries, concentrations obtained were in the range 1.092-25.612 mg/L in the month of February 2021. This clearly indicates that the algorithms already developed for other regions can only be applied after validation, or new site-specific algorithms need to be developed. Thus, google earth engine can be effectively used for real time ecological monitoring of large water bodies (lakes) and rivers provided the algorithms used are first validated for the region of interest.

Existing studies on flow through vegetation have focused mainly on understanding the turbulent structure in vegetated channels of single plant type. However, in natural riverine environments, vegetation also occurs as patches with heterogeneous plant forms. The present paper investigates the flow and turbulent characteristics in heterogeneous vegetation patches at a laboratory scale. Experiments were conducted using different forms (grass, leafy and cylindrical) of natural vegetation planted, alternatively and also as a mixed variety of patches in a staggered pattern. The results show that the presence of other vegetation forms in mixed heterogeneous patch increases the velocity reduction up to 10% compared to flexible grass. Moreover, additional drag due to mixed vegetation reduces shear generated turbulence at the canopy top and shifts its peak above the canopy. In the case of heterogeneous patches, spatial heterogeneity in velocity fields and, varying zones of increased and diminished turbulence were observed. Specifically, patch form and its alignment significantly control the velocity reduction and, momentum transfer between the canopy and overflow regions. These findings and furthermore studies on heterogeneous patches may be helpful for riparian management practices in creating ecological and sediment deposition zones.