Teledyne RD Instruments

Estimating longitudinal dispersion in rivers using Acoustic Doppler Current Profilers

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The longitudinal dispersion coefficient (D) is an important parameter needed to describe the transport of solutes in rivers and streams. The dispersion coefficient is generally estimated from tracer studies but the method can be expensive and time consuming, especially for large rivers. A number of empirical relations are available to estimate the dispersion coefficient; however, these relations are known to produce estimates within an order of magnitude of the tracer value. The focus of this paper is on using the shear-flow dispersion theory to directly estimate the dispersion coefficient from velocity measurements obtained using an Acoustic Doppler Current Profiler (ADCP). Using tracer and hydrodynamic data collected within the same river reaches, we examined conditions under which the ADCP and tracer methods produced similar results. Since dead zones / transient storage (TS) are known to influence the dispersion coefficient, we assessed the relative importance of dead zones in different stream reaches using two tracer-based approaches: (1) TS modeling which explicitly accounts for dead zones and (2) the advection–dispersion equation (ADE) which does not have separate terms for dead zones. Dispersion coefficients based on the ADE tend to be relatively high as they describe some of the effects of dead zones as well. Results based on the ADCP method were found to be in good agreement with the ADE estimates indicating that storage zones play an important role in the estimated dispersion coefficients, especially at high flows. For the river sites examined in this paper, the tracer estimates of dispersion were close to the median values of the ADCP estimates obtained from multiple datasets within a reach. The ADCP method appears to be an excellent alternative to the traditional tracer-based method if care is taken to avoid spurious data and multiple datasets ar z used to compute a distance-weighted average or other appropriate measure that represents reach-averaged conditions.

The longitudinal dispersion coefficient (D) is an important parameter that describes the transport of solutes in streams and
rivers. Accurate estimation of the dispersion coefficient is important from human health and public safety perspectives as the parameter is needed to predict contaminant concentrations near drinking water intakes and receiving water bodies such as lakes or oceans. Once a solute is released into the stream and becomes vertically and laterallywell-mixed, longitudinal dispersion is the primary mechanism responsible for spreading the tracer plume and for reducing peak concentrations. The one-dimensional transport of solutes following the initial period of mixing can be described using the advection– dispersion equation (ADE) [1,2]:

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