Comparison of Direct and Indirect Boundary Shear Stress Measurements along Vegetated Streambanks

Abstract

Estimates of boundary shear stress along vegetated streambanks are needed to predict streambank fluvial erosion. Because fluvial shear stress cannot be directly measured in the field, reliable estimation techniques using field instrumentation are needed. This study evaluated local bank shear stress estimation methods applicable to sloping, vegetated streambanks. Two reaches of a second order stream were modelled in a flume using a fixed-bed Froude-scale modelling technique. One reach was dominated by dense shrubs while the other reach was located in a mature forest. Direct measurements of local bank shear stress using a hot-film anemometer were compared to estimates based on velocity measurements (logarithmic method, Reynolds stresses, and turbulent kinetic energy). For channels with no or widely spaced vegetation, the velocitybased estimates underestimated the bank shear stress due to secondary flow contributions. For banks with dense vegetation, Reynolds stresses and turbulent kinetic energy estimates were statistically similar to direct measurements on average, but substantial error occurred when making point comparisons. Velocity-based estimates generally over predicted bank stress in areas of high shear at the vegetation edge and underpredicted stress within dense vegetation. Ultimately, results suggest that none of tested techniques can be broadly applied to streambanks, and flow structure is critical in selecting the appropriate estimation technique.

The role of increasing riverbank vegetation density on flow dynamics across an asymmetrical channel

“…Shields shear stresses at various locations of the main channel (y = 0.245 to 0.645 m) can be computed (Table 2). Overall, the highest It is seen in Table 2 that bed shear stresses at the main channel are generally increasing with vegetation density, which is overall satisfying considering the large uncertainties (of up to an order of magnitude when comparing bed shear stresses [3]). For example, an increase of about 90% can be seen when the maximum values of bed shear stress for the case of no vegetation and =0.99% are compared.…”

“…Further analysis using novel event-based criteria for sediment transport through or past vegetation, appropriately accounting for the size and energy content of turbulent flow structures [60][61][62]64] or turbulent kinetic energy [59], may be useful to pursue further Table 2 Shields bed shear stresses, for specific transverse locations y and for various riverbank vegetation densities (φ = 0-0.99%) * × 10 −3 = 0 = 0.06% = 0.08% = 0.17% = 0.28% = 0.55% = 0.99% in future studies. The importance of utilizing such criteria is perhaps even more apparent when the variation of the mean bed shear stresses is relatively small (as is the case for the experiments reported herein) and the uncertainties inherent in its calculation (also highly depending on the method of estimation) can be quite considerable [3,63,64].…”

“…However, future research should further aim to assess the stability of the vegetated riverbank, in addition to the main channel. As recent studies have shown [3,59] bed shear stresses may not be the most robust criterion for assessing the stability of vegetated riverbanks. Further analysis using novel event-based criteria for sediment transport through or past vegetation, appropriately accounting for the size and energy content of turbulent flow structures [60][61][62]64] or turbulent kinetic energy [59], may be useful to pursue further Table 2 Shields bed shear stresses, for specific transverse locations y and for various riverbank vegetation densities (φ = 0-0.99%) * × 10 −3 = 0 = 0.06% = 0.08% = 0.17% = 0.28% = 0.55% = 0.99% in future studies.…”

“…Re-vegetation of riverbanks is one of the common river restoration activities, which for the continental United States averaged about 1$ billion annually [1]. A significant body of contemporary research studies in the wider research field of vegetation hydrodynamics has investigated the effects of individual element or patches of vegetation on flow velocity at various locations across the stream, ranging from the floodplain, riverbank and main channel [2][3][4]. Several studies have assessed the effect of vegetation density such as for emergent aquatic vegetation [5,6], and for submerged vegetation [7].…”

Impact of Maintenance Methods of an Overgrown Lowland River on its Hydraulic Conditions

“…When most of the water flows through the trough at high velocity, plants do not have the opportunity to use these substances, which flow unused downstream. The obtained trends in velocity distribution are in line with similar past studies [3,8,14]. However, it is difficult to compare the velocity values themselves, as they usually depend on the individual characteristics of the study area (geometric characteristics of the channel, flow volume, plant density).…”