An evaluation of surface flow types as a rapid measure of channel morphology for the geomorphic component of river condition assessments
Highlights
► We examine the relationship between surface flow types and channel morphology. ► At low flows, flow type diversity reflects cross-sectional geometry variability. ► Flow type diversity correlates well with the diversity of the channel bed (depth). ► Flow types can provide rapid and meaningful information on channel morphology. ► Flow types can assist with tracking changes in morphologic diversity over time.
Introduction
River condition assessments are widely employed for determining the physical state of river ecosystems. Notable examples include the Sustainable Rivers Audit (Davies et al., 2008), the Index of River Condition (Ladson et al., 1999), the Australian Rivers Assessment System (AusRivAS) (Parsons et al., 2002), and the River Habitat Survey (Fox et al., 1998). These assessments incorporate a suite of parameters including aspects of hydrology, aquatic life, vegetation, water quality, and geomorphology. However, many of these assessments rely on limited and largely subjective geomorphic input because of the time-consuming and expensive nature of collecting channel morphology data.
Geomorphic assessments of channel morphology traditionally involve measuring cross sections and longitudinal profiles along a river reach and observing changes in geometric parameters over time. These parameters include cross-sectional area, depth, wetted perimeter and Froude number (Table 1).
Monitoring changes in geometric parameters can assist in determining if channel incision (deepening and widening) is active, or if any erosion is localised (e.g., slumping or meander migration), or if the channel is aggrading (developing bars or benches), and associated rates of change and sediment transport. Alternatively, many of these geomorphic processes may also be assessed qualitatively, by observing visual changes to the channel over time. For example, the extent of channel incision can be determined based on a visual assessment of channel form, by noting the width-depth ratio, presence/absence of an inset floodplain, benches, and scour or aggradation of in-channel features. However, these visual assessments require specialised geomorphic knowledge and experience.
Both of these approaches to geomorphic assessments, quantitative and qualitative, are not well suited for broad-scale river condition assessments. River condition assessments typically involve relatively rapid surveys of all aspects of river condition across numerous reaches dispersed over a broad geographical area. For cross section surveys, an adequate characterisation of geomorphic variability along a river reach typically requires somewhere in the order of 15 cross sections per 1 km for a conservative survey (Stewardson and Howes, 2002). Subsequently, a cross section survey (and calculation of geometric parameters) typically is too time consuming for most river condition assessments. Visual geomorphic assessments can be less time consuming; however, they require specialised knowledge and skills to assess current channel condition and active processes. Two examples of these visually based assessments are Geomorphic River Styles® (Brierley et al., 2002) and Phase of Incision (Schumm, 2005). These approaches are useful for the purposes of monitoring geomorphic condition over time; however, they both require specialised knowledge and skills. Most broad-scale river condition assessments and monitoring programmes are conducted by field assessors who have a more general knowledge base to cover all areas of river health (e.g., hydrology, ecology, riparian vegetation), and, therefore, a specialist approach to geomorphic assessments is not appropriate.
Recently, airborne LiDAR (Light Detection and Ranging) surveys have become popular for a range of landscape evaluation purposes, including the geomorphic survey of rivers and their condition (e.g., Bull et al., 2010, De Rose and Basher, 2011, Mitasova et al., 2011). At low altitudes, LiDAR can provide a high resolution survey of the three-dimensional topography of river channels without the need for detailed ground surveys and monitoring. While in many cases this has reduced the need for extensive channel surveys (cross sections and feature surveys), LiDAR still has several limitations. Current commercially available forms of LiDAR cannot survey bathymetry (below the water surface). Extracting information on in-channel morphology (variability in the bed and cross section form) in a consistent way from LiDAR data can also still be challenging.
And so a need exists for an alternative, more rapid approach to assessing and monitoring channel change over time. There is growing popularity in the ecohydraulic literature for relatively rapid methods of characterising the in-channel environment into uniform patches of surface flow type and channel bedform. The definition of these visually discrete units has progressed from earlier classifications of pool, run, and riffle (Jowett, 1993) into a more formal set termed hydraulic or physical biotopes (Wadeson, 1996, Padmore, 1998, respectively). The surface flow type, as shown for Padmore's (1998) classification scheme in Table 2, is the primary assessment tool for identification of various biotope units. This method of ecohydraulic characterisation has been adopted for broad-scale, mesohabitat mapping of instream hydraulics (e.g., Maddock and Bird, 1996, Wadeson and Rowntree, 2001, Maddock and Lander, 2002, Newson et al., 2002), incorporated into the River Habitat Survey in the UK (Fox et al., 1998) and used in the setting of environmental flow requirements for the Cotter River, Australia (Dyer and Thoms, 2006). Research is ongoing to assess specific ecological relevance of biotope units (e.g., Newson et al., 1998, Harper et al., 2000) and looks promising given our increased understanding of species preferences for particular depth-velocity environments (e.g., Fjellheim, 1996, Hart and Finelli, 1999, Maddock et al., 2004).
In addition to their ecological and hydraulic relevance, flow types and associated biotopes may also prove useful as a rapid measure of channel morphology. Previous work in New Zealand, the UK, and South Africa has shown good relationships between flow types and geometric parameters (such as those listed in Table 1) — in particular Froude number (ratio of kinematic to potential energy) (Jowett, 1993, Padmore, 1998, Wadeson and Rowntree, 1999).
In our investigation we examine the strength of the relationship between surface flow types and channel morphology (i.e., the depth-velocity environment). A strong relationship would favour the use of surface flow types as a rapid measure of channel morphology for river condition assessments (in addition to their flow ecology relevance). In this way, rapid assessments of surface flow types could potentially be used to monitor changes in channel form such as depth diversity and scour or deposition of in-channel features (e.g., bars, benches, and in-filling of pools). We examine two components of geomorphic form: (i) depth variability and (ii) channel geometry.
Depth variability is a useful geomorphic indicator as it reflects in-channel heterogeneity. An assessment of depth variability at multiple locations can detect changes to the channel bed, which may be caused by the presence of sediment migration through the system (e.g. a sand slug) or by other disturbances over time. Traditionally, an assessment of depth variation would require data from the time-consuming surveys of detailed cross sections, multiple depth measurements, or topographic feature surveys (bathymetry). In the first part of this study, we assess the suitability of using surface flow types as a surrogate for depth measurements by examining the relationship between depth variability and point assessments of surface flow types along a reach.
Channel geometry, as discussed previously (Table 1), is another common geomorphic variable for characterising and assessing temporal changes to river morphology. In the second part of this study, we examine the relationship between cross-sectional channel geometry and dominant cross-sectional biotope (determined from point assessments of surface flow types). From this we assess the potential for using cross-sectional biotope assessments as a surrogate for a more detailed channel survey.
The combined results from this two-part study are used to inform our evaluation of surface flow types as a rapid measure of channel morphology (geometry, depth diversity) for the geomorphic component of river condition assessments.
Section snippets
Field survey
Depth variability and flow type data were collected from April to July 2004, at six sites in Victoria, Australia. Three upland sites (the Delatite River, King Parrot Creek, and the Ovens River) and three lowland sites (the Loddon River, Seven Creeks, and Boosey Creek) were used to encompass a range of geomorphic and hydraulic conditions (Fig. 1). Upland regions were considered to be located in confined valleys with substrate dominated by bedrock, cobble, and gravel; whereas lowland regions were
Part (i) Depth variability
Flow type presence and proportions varied markedly with both site type (upland and lowland) and between surveys at a site (Fig. 4). A greater variety of flow types were present in the upland sites, as higher velocities and coarser bed material produced rarer types, such as chute flow and broken standing waves (Fig. 4). The diversity of flow types at uplands sites corresponded with greater depth variability than was calculated for lowland sites (Fig. 5). The Seven Creeks site borders on midland
Part (i) Depth variability
Flow type composition is notably sensitive to increases in discharge. While the nature of this variation is largely site specific, some general trends were observed both in the results and in the field. For example, as discharge increased, smooth boundary turbulent flow frequently transformed into upwelling flow and rippled flow often changed into broken standing waves (as the crests begin to break under higher velocities). These observations correspond with those expected for a generalised
Acknowledgements
The authors wish to thank the participants of the former Cooperative Research Centre for Catchment Hydrology (CRCCH) vacation studentship programme of December 2003–February 2004 for collection of the data set used in part (ii) of this study (flow types and channel geometry). This broader research project was funded through the former CRCCH post-graduate scholarship programme and through resources provided by the University of Melbourne.
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