Krishnappan – Environmental Consultant

Providing expert advice on Modelling of River Flows and Sediment and Contaminant Transport.

Example of Previous Studies

Stability of Contaminated sediments in the St. Mary’s River.

A flow and sediment transport modelling approach was proposed for the St. Marys River to determine the physical stability of the surface sediment in the river (top 0-5 cm). This study was able to determine whether the top 0-10 cm of sediment will erode away under various flows, ice scour and changes in water levels exposing deeper more contaminated sediment to aquatic organisms. The results of the study assisted in the development of management strategies for contaminated sediment deposits in St. Marys River.

Based on measured flow and sediment bed characteristics, a two dimensional flow modelling system called TABS-MD with SMS user interface was selected for the modelling approach. TABS-MD consists of a two dimensional hydrodynamic flow model (RMA2) and a fine sediment transport model (RMA4). Details of these models and their application to the St. Marys River are described in Krishnappan, 2008. The predicted bed shear stresses for different flow-rates with their associated water levels were compared with the critical shear stress for erosion of sediment to assess its stability under different flow regimes. Effects of ice cover on flow and sediment stability were also assessed. Potential impacts of ice scour process were discussed. The results of this study indicated that lower layers of contaminated fine sediment deposits near the banks of the river are stable under different flows tested in this study.

Modelling of flows and sediment transport in wildfire impacted watershed in Alberta, Canada.

Large scale land disturbance by wildfire can have a significant and prolonged effect on the source, transport and fate of sediment and associated contaminants in watersheds. The downstream propagation of fine sediment from these impacted watersheds to reservoirs can degrade water quality and increase water treatment and supply costs. In this study, a framework which integrates four existing models (MOBED, RIVFLOC, RMA2, RMA4) was formulated and used to model flow and cohesive sediment transport in wildfire impacted headwater streams of the Oldman River and Oldman Reservoir in southern Alberta, Canada. Based on output from the MOBED model, it was observed that bed shear stresses that prevail in the modelled rivers are considerably higher than the critical shear stress for deposition of fine sediment generated in the watershed even at low flow conditions. Accordingly, fine sediment entering these rivers from the local burned landscapes will be transported through the channel system to the reservoir with little to no likelihood for deposition and storage. The RIVFLOC model predicts transverse dispersion and flocculation of the sediment transported through the river channels and thereby provides sediment concentration and size distribution data needed for the upstream boundary of the reservoir dispersion model, RMA4.

Application of the RMA2 and RMA4 models to the Oldman Reservoir suggests that bed shear stress prevailing in the reservoir even under high flow conditions is considerably lower than the critical shear stress for deposition of sediment entering the reservoir. Accordingly, fine sediment reaching the reservoir will be deposited without any likelihood of transport through the structure. Deposition patterns of the burned sediment are likely to be different from that of the unburned sediment because of the differences in the still water settling of the two materials as observed by Stone et al (2010) in their laboratory study. The modelling system reported in this contribution can be used to examine sediment propagation characteristics through rivers and reservoirs under different flow and land use scenarios and as such, can provide a valuable tool for supporting decision-making for catchment planning.

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