Effective Collector Efficiency of Sparse and Dense Arrays of Emergent Cylindrical Collectors in Laminar-Turbulent Transitional Flows

Abstract:

Aquatic vegetation plays a major role in the ecology and hydrogeomorphology of rivers, wetlands, and estuaries. Plant surfaces collect suspended sediment via direct interception, affecting the sediment transport budget and patterns of erosion and deposition. For Stokes flows, analytical expressions exist to predict capture efficiency (η). However, for transitional flows (approx. 1 < Re < 1000), η must be estimated empirically. Studies have yielded varying results about the effect of collector Reynolds number (Re_c) on η. While transitional turbulence is theorized to increase η due to increasing particle-stem interactions, we hypothesize that it also increases the rate of particle resuspension from collectors. This makes experimental observations of a negative effect of Re_c on effective capture efficiency (ηʹ), which factors in the probability of retention, compatible with a positive effect of Re_c on η. Furthermore, we hypothesize that in systems with many collectors, upstream collectors increase turbulence for those downstream, which could help explain the negative relationship some previous studies have observed between collector frontal area density (I_c) and ηʹ. We present results from a laboratory flume experiment, in which we estimated ηʹ for emergent cylindrical collectors and measured turbulence kinetic energy (TKE) over a fully crossed parameter space of I_c (0.88, 2.54, and 4.60 m²/m³) and Re_c (67, 133, and 200). We found negative effects on ηʹ for both predictors, which is consistent with our hypotheses. We found that the positive effect of I_c on TKE, which was consistent across flow velocities, leveled off above the intermediate collector density treatment. This could be explained by an accelerating decline in collectors’ drag coefficients due to increased sheltering as intervening distances grow smaller. The difference in ηʹ among treatments at or above this collector density was also slight, lending credence to the existence of a relationship between TKE and ηʹ. Although further research is needed to fully characterize the complex relationships between these aspects of vegetated streams, our results highlight the knock-on effects between collector density, turbulence, and ηʹ, which have important implications for hydrological models and management on the watershed scale.