Near-bed Hydraulics Research Articles

Hydraulic and Physical Structure of Runs and Glides Following Stream Restoration

Hydraulic units are often linked to ecological habitat through geomorphic structure, and a better understanding of the turbulent characteristics of the units is needed. Our work examined the near-bed turbulent structure of runs and glides in a restored river and investigated the physical characteristics that influenced the near-bed hydraulics in these units. The research was completed in three restored reaches and one reference reach at the Virginia Tech Stream Research, Education, and Management Laboratory. The laboratory is unique because three different restoration treatments were applied contiguously along a stream, and the restoration practices ranged from passive to active. The passive reach included cattle exclusion, while the active reaches included cattle exclusion as well as vegetation plantings, bank sloping and the construction of inset floodplains. Three-dimensional velocities were measured near the channel bed in run and glide biotopes within the three restored reaches, as well as an upstream reference reach. The velocities were utilized to analyse and compare near-bed turbulent structure across the reaches. While the restoration activities did not address the channel bed directly, differences in physical structure of the two physical biotopes were observed among restoration treatments, likely because of changes in bank shape and roughness due to vegetation differences. Differences between reference and restored reaches were still evident approximately 3 years after cattle exclusion and construction activities. Few differences were observed in the hydraulic structure between runs and glides, and the near-bed flow structure in both runs and glides was related to local roughness. Copyright © 2016 John Wiley & Sons, Ltd.

‘Sticky business’: The influence of streambed periphyton on particle deposition and infiltration

Strong feedbacks exist between physical and ecological components of aquatic systems. Aquatic plants can alter flow and sedimentation patterns, in turn influencing habitat condition and organism responses. In this study, I investigate the interactions between streambed periphyton, particle deposition and infiltration, and flow hydraulics to determine the influence of these organisms on the local environment. In a series of flume experiments, I measured the effects of two contrasting forms of periphyton at several densities and growth stages on near-bed hydraulics, particle loss from the water column, surface deposition, and subsurface infiltration. Data show that periphyton assemblages altered the rate and quantity of particle deposition via several mechanisms, including shear stress modification, surface adhesion, and bed clogging. Although trends varied for different size classes within a suspension of fine sediment, diatoms and algae had distinctly different effects on hydraulics, deposition, and infiltration. In general, diatoms increased the rate of decline in suspended particle concentrations relative to non-periphyton surfaces by reducing shear stresses and enhancing surface deposition via adhesion. Increases in diatom biomass, however, reduced the quantity and depth of particle infiltration, presumably by clogging interstitial pore spaces, in turn lowering rates of concentration decline. In contrast, all algal growth stages had slower or similar rates of concentration decline compared to non-periphyton conditions, due to partial clogging by high biomass and a lack of adhesion at the bed surface. Clogging effects were counteracted at later growth stages, however, as late-stage algal structures increased shear stresses and downward advection, in turn increasing amounts of infiltration. Compiled data from several field studies and experiments demonstrate a positive relation between periphyton biomass and inorganic mass, but also show a wide range in the relation because of external factors such as discharge and suspended sediment concentration. My analysis indicates that the decrease in the concentration of suspended particles under controlled conditions is strongly influenced by periphyton structure, through direct mechanisms such as adhesion and clogging as well as indirect changes to hydraulics, but that external factors in field settings such as discharge and sediment supply may outweigh the effects of periphyton structure.

Surface flow types, near-bed hydraulics and the distribution of stream macroinvertebrates

Abstract. Spatial variation in hydraulic conditions in streams often results in distinct water surface patterns, or surface flow types. Visual assessments of the distribution of surface flow types have been used to provide rapid assessment of the habitat heterogeneity. The efficacy of this approach is predicated on the notion that surface flow types consistently represent a distinct suite of hydraulic conditions with biological relevance. This study tested this notion, asking three specific questions. First, do surface flow types provide a characterisation of physical habitat that is relevant to macroinvertebrates? Second, how well do near-bed hydraulic conditions explain macroinvertebrate distributions? Third, what components of near-bed hydraulic conditions exert the strongest influence on macroinvertebrate distributions? Results show that hydraulic conditions (incorporating direct measurements of near-bed velocity and turbulence in three dimensions) and substratum character (incorporating estimates of particle size distribution, and biofilm and macrophyte cover) within each surface flow type were largely distinct and that macroinvertebrate assemblages differed across flow types in taxon richness and assemblage composition, thus supporting the notion that rapid assessments of surface flow type distributions provide biologically relevant information. Macroinvertebrate assemblages were most strongly correlated with water depth, size of a flow type patch, near-bed velocity in the downstream direction, turbulence in the transverse direction, % pebble, % sand, % silt and clay and macrophyte cover. This study suggests that surface flow type mapping provides an assessment of physical habitat that is relevant to macroinvertebrates. The strong relationship detected between macroinvertebrate assemblages and transverse turbulence also highlights the value of directly measuring near-bed hydraulics. Further investigations are required to test the mechanisms underlying this relationship.

A casting procedure for reproducing coarse‐grained sedimentary surfaces

AbstractIn the field, the measurement of near‐bed hydraulics remains problematic. Greater precision is possible in the laboratory, but, in the case of gravels, it is difficult to create a water‐worked channel‐bed that is realistic enough to replicate faithfully the conditions found in nature. In this paper, a technique to reproduce coarse‐grained sedimentary fabrics of large areal extent is described. It involves moulding natural river‐bed surfaces from which facsimiles are cast. Remarkably realistic casts with dimensions of 1 m by 2 m have been produced and their quality assessed using spatial data derived using automated digital photogrammetry. The casts reproduce the prototype surfaces with errors at millimetre scale (0·5 per cent of the microrelief). The technique has facilitated the introduction of sedimentary surfaces that incorporate natural, complex structures of grains up to cobble size into experimental channels where detailed studies of near‐bed hydraulics can be carried out. Copyright © 2003 John Wiley & Sons, Ltd.