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Lookup NU author(s): Professor William Sloan,
Professor John Ewen
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A method has been developed to simulate the long-term migration of radionuclides in the near-surface of a river catchment, following their release from a deep underground repository for radioactive waste. Previous (30-year) simulations, conducted using the SHETRAN physically based modelling system, showed that long-term (many decades) simulations are required to allow the system to reach steady state. Physically based, distributed models, such as SHETRAN, tend to be too computationally expensive for this task. Traditional lumped catchment-scale models, on the other hand, do not give sufficiently detailed spatially distributed results. An intermediate approach to modelling has therefore been developed which allows flow and transport processes to be simulated with the spatial resolution normally associated with distributed models, whilst being computationally efficient. The approach involves constructing a lumped model in which the catchment is represented by a number of conceptual water storage compartments. The flow rates to and from these compartments are prescribed by functions that summarize the results from physically based distributed models run for a range of characteristic flow regimes. The physically based models used were, SHETRAN for the subsurface compartments, a particle tracking model for overland flow and an analytical model for channel routing. One important advantage of the method used in constructing the lumped model is that it makes down scaling possible, in the sense that finescale information on the distributed hydrological regime, as simulated by the physically based distributed models, can be inferred from the variables in the lumped model that describe the hydrology at the catchment scale. A 250-year flow simulation has been run and the down scaling process used to infer a 250-year time-series of three-dimensional velocity fields for the subsurface of the catchment. This series was then used to drive a particle tracking simulation of contaminant migration. The concentration and spatial distribution of contaminants simulated by this model for the first 30 years were in close agreement with SHETRAN results. The remaining 220 years highlighted the fact that some of the most important transport pathways to the surface carry contaminants only very slowly so both the magnitude and spatial distribution of concentration in surface soils are not apparent over the shorter SHETRAN simulations.
Author(s): Sloan WT, Ewen J
Publication type: Article
Publication status: Published
Journal: Hydrological Processes
Print publication date: 30/04/1999
ISSN (print): 0885-6087
ISSN (electronic): 1099-1085
Publisher: John Wiley & Sons Ltd.
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