Experiments on mass exchange between groin fields and main stream in rivers

Laboratory experiments are presented that model the exchange processes between a river and dead-water zones created by groin fields. The measurement methods include particle image velocimetry (PIV) at the water surface to capture the dynamics of the flow field as well as planar concentration analysis (PCA) to determine the depth integrated mass fluxes of dissolved tracer. Idealized groins with variable length-to-width ratios and inclination angles relative to the flow direction are investigated. Two-dimensional large coherent structures (2DLCS) are found in the mixing layer between the groin field and the river, and these flow structures control the majority of mixing with the main stream. The PIV data allow quantification of the turbulence intensity distribution in these structures throughout the groin field and mixing region. The PCA data provide measurements of the exchange rate between the groin field and the main stream. Both methods give comparable measures of the entrainment coefficient for the exchange flow. The entrainment coefficient is found to depend on the groin field hydraulic radius (a measure of the average size of 2DLCS in the mixing layer) and the water depth (a parameter controlling the stability of the shallow water flow). These data provide a link between the exchange parameter in groin field dead zones and the river properties that can be used to efficiently develop river alarm models to predict contaminant transport without requiring a large amount of field dye studies for calibration.