Hydraulic habitat composition and diversity in rural and urban stream reaches of the North Carolina Piedmont (USA)

Abstract

The differences between urban and rural streams regarding hydrological process, channel morphodynamics and ecosystem functioning have been highlighted by a number of studies in recent decades. The need to understand lotic ecosystem functioning in these environments at scales relevant to individual organisms has led to research focusing on hydraulic composition and structure over small areas of channel bed. In this study we map and analyze the hydraulic biotope composition of two urban and two rural stream reaches in the North Carolina (USA) Piedmont to determine if urban flow regimes and attendant channel modification processes might translate into important differences in hydraulic environment, and if so, what those differences are. Hydraulic biotope assemblages were found to vary only moderately in diversity per unit stream length between sites, but were distinctly different in composition. One important control on the differences between rural and urban streams was found to be the localized incision of urban channels into bedrock and saprolite. Resistant rock outcrop in the beds of urban streams creates rapid and riffle biotopes and long stretches of upstream pool habitat by impoundment. Urban reaches were found to be more homogeneous than rural reaches in hydraulic composition and were dominated by pools. Rural reaches, characterized by copious sandy alluvium in the bed, were dominated by runs or glides. Quantitative differentiation of biotopes based on four hydraulic indices generally yields coherent associations, although these may vary in content. Comparisons between hydraulic and biotic diversities suggest relationships favoring biotic functional group aggregation over species richness-based indices of diversity. Because the majority of published hydraulic diversity analyses are based on coarse-bed streams, further study of hydraulic diversity in streams with finer substrate is likely to be beneficial.

Mapping hydraulic biotopes using terrestrial laser scan data of water surface properties

“…This perhaps suggests an overly complex set of existing biotope classifi cations in relation to overlapping hydraulic conditions (Froude number), a point also previously suggested by Clifford et al (2006), Large and Heritage (2007) and Shoffner and Royall (2008). (2) There is a clear issue regarding scaling within the current biotope classifi cations (Table II) which could explain some of the apparent surface roughness overlap of units composed of several smaller components, that are measurable using TLS and also reported using hyperspectral imagery (Marcus, 2002).…”

“…Clifford et al (2006) highlight overlap between scarcely perceptible fl ow and smooth boundary turbulent units, rippled units and unbroken standing waves, and chute and broken standing waves. Shoffner and Royall (2008) propose three groupings; glideboil-riffl e, rapid-run and pool-slackwater. Wadeson and Rowntree (1998) and Newson et al (1998) further highlight the fuzzy nature of the hydraulic boundary between biotopes, as defi ned by Froude number.…”

“…More recently the proposed links between hydraulics and the biotope unit have been questioned (Clifford et al, 2006;Shoffner and Royall, 2008). Issues remain, including misclassifi cation in the fi eld (Marcus, 2002), the qualitative and subjective nature of biotope mapping (Clifford et al, 2006;, and the fuzzy nature of biotope boundaries (Legleiter and Goodchild, 2005).…”

The development of hydraulic and geomorphic complexity in recently formed streams in Glacier Bay National Park, Alaska

“…The development of hydraulic complexity within older streams is likely to have important consequences for instream biota. Hydraulic complexity had been shown to be an important aspect in creating habitat diversity, yet few habitat assessments take hydraulic habitat into account (Crowder and Diplas, 2006;Shoffner and Royall, 2008). The geomorphic complexity of entire reaches has been studied to allow comparisons along the 200-year chronosequence to be assessed.…”

Hydraulic and Physical Structure of Runs and Glides Following Stream Restoration

“…All sites had rough uneven bed surfaces characteristic of an armoured bed. We confirmed our in‐field designation of the hydraulic biotopes by examining the Froude number (Fr), Reynolds number (Re), velocity/depth ratio (⟨ M ⟩/ Z ), shear velocity ( V *) and roughness Reynolds number (Re*) (Jowett, ; Wadeson and Rowntree, ; Shoffner and Royall, ): where ⟨ M ⟩ is the patch‐mean velocity, g is the acceleration due to gravity, Z is the mean water depth, D 50 is the median bed particle size and v is the kinematic viscosity of water (10 ‐6 m 2 /s).…”