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Incomplete mixing in a small, urban stream
Robert J Ryan1, Michel C Boufadel
1Department of Civil and Environmental Engineering, Temple University, 1947 N 12th Street, Philadelphia, PA, 19122, USA.
Journal of Environmental Management
|April 18, 2006
Summary
Even in small urban streams, complete mixing is not guaranteed. Solute tracer experiments revealed vertical stratification, challenging assumptions and increasing uncertainty in flow and dispersion estimates.
Area of Science:
- Environmental Science
- Hydrology
- Water Resource Management
Background:
- Small urban streams are often assumed to be completely mixed vertically and horizontally.
- This assumption is critical for accurate hydrological modeling and water quality assessments.
Purpose of the Study:
- To investigate the mixing dynamics of a small urban stream under realistic flow conditions.
- To determine if vertical and horizontal complete mixing assumptions hold true in such environments.
- To assess the impact of violated mixing assumptions on hydrological parameter estimations.
Main Methods:
- Conducted conservative solute tracer experiments in Indian Creek, Philadelphia.
- Measured stream flow rates, width, and depth.
- Analyzed tracer cloud behavior, specifically vertical stratification within a pool section.
Main Results:
- Observed significant vertical stratification of the tracer cloud in a 1.0m deep pool, despite initial complete mixing.
- Demonstrated that the assumption of complete vertical mixing was invalid.
- Identified a balance between groundwater inflow and transverse dispersion as a potential cause for limited mixing.
Conclusions:
- The assumption of complete mixing is not universally valid, even for small streams.
- Unsupported complete mixing assumptions lead to increased uncertainty in estimating stream flow and longitudinal dispersion coefficients.
- Field verification of mixing assumptions and one-dimensional modeling is crucial, even in small urban streams.