433 / 2024-03-15 23:30:56

HYDRAULIC 1D MODELLING FOR ALTERED SEDIMENT TRANSPORT REGIME IN A SAND–GRAVEL MINED REACH OF THOUBAL RIVER

sediment deposition and erosion, quasi-unsteady, NSE, channel invert replenishment

Subtheme 7: Sediment Regulation and Management > Topic 3: River Regulation and Sand Mining

Rivers, essential for water supply, undergo continuous changes due to natural and human factors. Understanding erosion and deposition patterns is crucial for river projects. Mismanagement of catchment areas increases sediment loads, impacting river channels. Sediment erosion can destabilize channels, causing bank failures. Erosion in the upper reaches leads to sedimentation. Conversely, sediment deposition reduces channel size and elevates water levels. Therefore, obtaining a realistic estimate of sediment erosion and deposition in alluvial channels is crucial for effective water management.

Thoubal River, a significant tributary of the Imphal or Manipur River, holds prominence within the Manipur River Basin. The study reach extends from Nongpok Keithelmanbi U/S (River Station, RS 28560) to the Thoubal bridge Crossing D/S (RS 5), covering a distance of approximately 29 km, where unregulated sand–gravel mining taking place in large extents has drastically altered the hydraulic regime of the river system and its floodplains. To mitigate this impact, it is essential to estimate sediment deposition and erosion depths at various locations, ensuring sustainable sand–gravel mining without disrupting the river’s fluvial system.

In this particular study, HEC-RAS version 6.1 is utilized. The sediment transport model requires input files related to geometry, steady flow profiles, quasi-unsteady flow, and sediment conditions. Geometric data is prepared using cross-sectional data extracted from CARTOSAT-1 Stereo imagery derived Digital Elevation Model of 10 m spatial resolution and interpolated with ground surveyed cross-sections at selected reach stations. Manning’s roughness coefficient is calibrated to determine river hydraulics, and sediment transport calculations are performed using a “quasi-unsteady” flow approach in HEC-RAS. At the upstream cross-section flow series boundary condition is applied and rating curve is selected as the downstream boundary condition.  Sediment discharge is used as the criteria for calibration of the model. Model run was carried out recursively using varied sediment transport functions for the calculation of sediment transport capacity. The best fit transport function was selected which computed the value of sediment discharge closest to the observed sediment discharge.

The model was simulated for the sedimentation condition of Thoubal River within study reach using HEC-RAS for the period 2021–2023. The model was first calibrated for hydraulic conditions of the river and the Manning’s roughness coefficient of 0.083 selected that gives the best value of Nash-Sutcliffe efficiency (NSE= 0.990) and Coefficient of Determination (R²= 0.998). For the selection of sediment transport function model is calibrated for sediment discharge. Engelund- Hansen sediment transport function with the fall velocity method of Toffaleti was found to give better sedimentation result with NSE of 0.935 and R² of 0.938. Channel invert variations were estimated for erosion and deposition at different stations. The maximum effective depth of erosion in the reach is about 2.19 m at RS 9149 having average sediment discharge of 267.533 tons/day and the maximum effective depth of deposition is about 2.07 m at RS 23476 having average sediment discharge of 351.269 tons/day. It can be inferred that the allowable depth of sand-gravel mining in-stream of Thoubal river at a station should not exceed 2.0 m in order to conserve the replenishment rate of sediment flows and maintain the hydraulic regime of the river.