CONSTRUCTION CONSTRAINTS FOR GEOMORPHIC-UNIT REHABILITATION ON REGULATED GRAVEL-BED RIVERS

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 +/- 0.15 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. Copyright (C) 2008 John Wiley & Sons, Ltd.