I am writing to provide three comments on Guidance Memo No. GM23-2004 – Accelerated Release of Nonpoint Source Nutrient Credits for Stream Restoration Projects.  

1. The justification for the release of 50% of the credits prior to the occurrence of a bankfull flood event or end of Year 1 monitoring is justified based on crediting guidance that allows only 50% of the estimated nutrient load reduction to be applied as credited nutrient reduction ("50% efficiency").  The decision to only allow 50% of the estimated load reduction be "counted" as credit was made to provide a "factor of safety" or margin of error, in recognition that the load calculations are estimates.  This factor of safety provides a conservative estimate of the likely achieved load reduction to provide additional assurance that water resources are improved.  I served on the committee that made this decision for the Chesapeake Bay watershed TMDL.  Increasing the initial credit release from 25% to 50% is less protective of water resources, not more protective.  Therefore, the "50% efficiency" is not a justification or basis for an increase in the amount of credits initially released and is instead simply the maximum amount allowed by SB 187.  
2. Having conducted an assessment of over 60 stream restoration projects and more than 500 instream structures, it is evident that a major failure mode for instream structures is when stream flows exceed bankfull discharge and cause erosion behind structures, at the interface of the bank and the structure.  Since most structures are built to the bankfull stage, the greatest risk of structure failure is during floods that exceed the bankfull stage.  Therefore, it is recommended that accelerated credit release only be allowed if there is at least one flood that exceeds bankfull stage, rather than just meets bankfull stage.  
3. While having the accelerated criteria be applicable only to low-risk projects is more protective of water resources, it is possible that this requirement will encourage conservative designs that utilize large rock structures that prevent natural channel adjustments.  It is important that the criteria for what constitutes a "low risk design" not discourage innovative designs that could produce more ecological benefits than traditional, form-based, static designs.  My research has shown that projects constructed in urban watersheds or watersheds that have recently undergone urbanization have greater risk than projects in rural watersheds or watersheds with stable landuse/land cover.  Additionally, projects with more frequent floodplain access (lower relative bank height and higher width-to-depth ratio) are more likely to be successful.  

I am happy to discuss these points further and/or share my research findings.

Best regards,

Tess Thompson

Associate Professor

Department of Biological Systems Engineering

Virginia Tech