Evaluation of E.coli losses in a tidal river network using a refined 1-D numerical model

Guoxian Huang,Roger A Falconer,Binliang Lin

doi:10.1016/j.envsoft.2018.07.009

Guoxian Huang, Roger A Falconer + Show 1 more

Open Access

https://doi.org/10.1016/j.envsoft.2018.07.009

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Abstract

Predicting the rate of Escherischia coli (E.coli) loss in a river network is one of the key conditions required in the management of bathing waters, with well verified numerical models being effective tools used to predict bathing water quality in regions with limited field data. In this study, a unique finite volume method (FVM) one-dimensional model is firstly developed to solve the mass transport process in river networks, with multiple moving stagnation points. The model is then applied to predict the concentration distribution of E.coli in the river Ribble network, UK, where the phenomena of multiple stagnation points and different flow directions appear extensively in a tidal sub-channel network. Validation of the model demonstrates that the proposed method gives reasonably accurate solution. The verification results show that the model predictions generally agree well with measured discharges, water levels and E.coli concentration values, with mass conservation of the solution reaching 99.0% within 12 days for the Ribble case. An analysis of 16 one-year scenario runs for the Ribble network shows that the main reduction in E.coli concentrations occurs in the riverine and estuarine regions due to the relatively large decay rate in the brackish riverine waters and the long retention time, due to the complex river discharge patterns and the tidal flows in the regions.

Highlights

Escherischia coli (E.coli) loss at the river-estuary transition zone is a complex process where decay and production through various sources coexist
The results show the importance of the need for model mass conservation, especially in the lower reaches of the river basin, where the reversing current and the multiple stagnation zones appear extensively, driven by tidal and river flow interactions
The results indicate that the multiple stagnation zones may appear in the river networks in the flood to ebb stage

Summary

Introduction

Escherischia coli (E.coli) loss at the river-estuary transition zone is a complex process where decay and production through various sources coexist. Numerical models are often used, together with limited field measurements and laboratory analysis to evaluate quantitatively the E.coli losses in riverine and coastal waters (Servais et al, 2007). A mass conserved, stable, accurate and computationally manageable model is a prerequisite for E.coli concentration evaluation, since rainfall-runoff intensities enter river channels in pulses, often at minute scales, creating large gradients in pollutant concentrations (Sanders et al, 2001). This is especially important in complex river networks with relatively steep gradients and where highly unsteady tidal currents exist in the estuarine and coastal zones. For long-term simulations, e.g. for up to 100 years, and for a series of scenario runs of the hydrodynamic, sediment and mass transport processes, 1D models are extensively used

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