The measurement of reaeration in streams

Abstract

A detailed study of the several methods for the determination of reaeration rates in streams has been made. Particular attention has been given to the floating soluble solids technique which appears to be a simple and reliable method for the measurement of reaeration rates in small streams. A comprehensive review of both conceptual and empirical mathematical models for the prediction of the reaeration rate coefficient is presented in an attempt to identify the practical outcome and application of these models to- natural stream conditions. A critical review and discussion of the three primary methods usually employed for the reaeration measurement in streams, namely the oxygen balance, disturbed equilibrium and tracer techniques, is presented to allow the understanding of the main principles, related assumptions, degree of applicability and the errors involved in each of these methods. The theoretical basis of a new method, the floating soluble solids technique is introduced, and previous work results presented. The physico-chemical para'meters effect upon the reaeration rate coefficient are discussed in the last part of the literaýure review. An extensive laboratory programme has been conducted, during which the individual and interaction effects of three important hydraulic variables (velocity, depth and roughness) upon atmospheric reaeration, propane gas desorption and solids dissolution processes have been investigated using statistical experimental design and analysis techniques. The assumption that the ratio between the reaeration and propane desorption rate coefficients js constant and independent of mixing and turbulent conditions has been herein confirmed using a different approach (surface response methodology) from the previously reported tests. A weaker depth dependence of both reaeration and propane desorption rate coefficients has been verified at low velocity levels, and which becomes stronger as average water velocity increases. Natural stram data and energy dissipation response to the same hydraulic factors variation appear to further substantiate the above evidence. The common parameters and main 'limitations controlling both reaeration and dissolution processes have been identified and dimensionless correlation equations were developed for the determination of the reaeration coefficient from' velocity of solids dissolution measurements. Field trials, involving the disturbed equilibrium, steady state propane gas tracer, and floating soluble solids methods, appears to confirm the evidence that reaeration coefficients might be successfully estimated from, velocity of solids dissolution measurements in small streams.