Sub-Grade Structures in Restoration Design
Grant Ginn, PE
Stantec Consulting
Asheville, NC
Although the industry-standard, boulder and log weir structures have proven effective in maintaining channel grade and directing flow vectors away from the near-bank region there remain compelling reasons to move designs toward more natural and wood-dominated structures that can provide additional benefits to restored streams. The design and use of sub-grade structures hold promise for providing the needed interim stability for newly constructed channels while also adding the potential for long-term channel adjustment, improved hyporheic exchange, increased volume of LWD, and improved aquatic habitat, while presenting a more natural channel bed form.
Sub-grade structures can be either configured to mimic a natural bed facet or a subsurface feature. The main distinction from traditional weir or vane structures is that instead of holding grade with a fixed vertical top component, the sub-grade structure is intended to influence flow dynamics through the nature of the material buried into and below the channel bed. This can be accomplished through structured run and glide features in larger rivers, with cascading bed forms in smaller streams, and with buried high or low K zones in headwater channels.
This presentation will provide descriptions of how the design of in-stream structures and bed features can be modified to improve structure functions. Replacing standard boulder and log weirs with structured run and glides can provide more nature flow interfaces, increase initial channel roughness, provide additional critical habitat zones, and increase LWD composition. Additionally, substrate augmentation and configuration can improve necessary subsurface conveyance, promote hyporheic exchange, and provide proper upwelling and downwelling zones.
About Grant Ginn, PE
Grant Ginn is a Senior Principal and Senior River Engineer with Stantec who specializes in stream and wetland restoration. Grant has over 30 years of experience in the morphologic, and hydrologic and hydraulic design of streams and wetlands associated with restoration, mitigation, remediation, and infrastructure projects.