Construction constraints for geomorphic‐unit rehabilitation on regulated gravel‐bed rivers

Abstract

The emergent practice of applied river restoration uses best available equipment and contouring methods to construct detailed designs with some features at scales as small as 0.5-m relief. As part of adaptive management, it is necessary to determine the practicability of design features and construction methods before widespread adoption. In this study, we compared design versus as-built topography for five salmonid spawning habitat rehabilitation projects at riffle-pool geomorphic units on the lower Mokelumne River, California, USA. These were built instream using rubber tire front loaders. Digital elevation models (DEMs) of each site were produced for pre-project, design and as-built conditions. DEM differencing was used to compare the as-built surface against corresponding design surfaces at each site to identify deviations. Causes of each identified deviation were assessed based on subjective observations by a team during construction. Across the projects, 70% of as-built topography was within AE0.15 m of design specifications. Of the 30% deviating from the design, 41% was overfilled and 59% underfilled. The 30% of rehabilitated channel area that deviated from designs did not affect predicted areas of high-quality spawning habitat. On-site factors that hindered accurate construction of designs included front loader fording depth, poor operator elevation estimation, operator spatial disorientation and wood obstructions. In addition, funding and project management uncertainties caused gravel supply deficits and gravel bulk density estimation errors. It is concluded that constructing broad (>0.5-m relief) features of process-based salmonid spawning habitat rehabilitation projects by gravel augmentation is practicable. However, uncertainties attributed to human error and available methods inhibit detailed (<0.5-m relief) rehabilitation. Despite uncertainties, limitations and errors, following the recommendations reported in this study would improve the as-built adherence to design specifications of future projects.

The Topographic Design of River Channels for Form-Process Linkages

“…However, it's been 562 demonstrated that sinusoids approximate many channel features, including bed profiles, 563 as a form of minimum variance (Hallet, 1990. Further, consideringthat there can be inherent error in implementing river restoration designs with heavy 565 equipment (Sawyer et al, 2009), this is not considered a problem as sub channel width 566 scale variations can occur through operator error and variations in bed and bank 567 materials. There has been no research on suitable analytical functions for riffle-pool bed 568 topography, or channel width for that matter, and future research may identify better 569 functions.…”

Emerging concepts in temporary‐river ecology

“…In terrestrial experiments, connectivity is altered by creating or eliminating dispersal corridors, and patch structure is altered by subdividing, removing, enlarging and fusing patches (McGarigal & Cushman, 2002). In temporary rivers, connectivity can be altered by artificially connecting or disconnecting pools, and aquatic and terrestrial patches can be manipulated through sediment excavation and deposition (Sawyer et al. , 2009).…”

“…Restoration of habitat mosaics in degraded temporary-river channels may require excavation and construction of geomorphic structures (e.g. pools, bars, islands), sediment augmentation, and⁄or the provision of large floods (Kondolf & Wilcock, 1996;Sawyer et al, 2009). These geomorphic structures interact with shifting water levels to produce shifting aquatic-terrestrial habitat mosaics.…”

Comparison of methods for analysing salmon habitat rehabilitation designs for regulated rivers

“…Based on observed deficiencies of past projects, there is a growing recognition of the importance of design and implementation of complex alluvial features that utilize channel non-uniformity to promote habitat heterogeneity Wheaton et al, 2004b,c;Elkins et al, 2007;Sawyer et al, 2008). There is also a growing need for accurate and cost-effective means to represent habitat (Maddock, 1999;Moir and Pasternack, 2008).…”