Consensus Recommendations To Improve Protocols 2 And 3 For Defining Stream Restoration Pollutant Removal Credits

Two groups of more than 25 stream experts have worked over the last year on how to better apply protocols 2 and 3 to integrated stream and floodplain restoration projects (FR) (Section 1). Floodplain restoration can be achieved using two basic techniques to reconnect incised streams to their floodplains. The first approach, termed legacy sediment removal (FRLSR), removes sediments to lower the floodplain surfaces, increasing out-of-bank flow and re-establishing the hyporheic exchange zone by reconnecting the floodplain with the hyporheic aquifer. The second approach, known as raising the stream bed (RSB), involves several techniques to raise the elevation of an incised stream channel and shallow groundwater, thereby increasing the volume of runoff diverted into the floodplain for treatment. These two approaches are often used in combination. The group came to consensus on the key terms, definitions and qualifying conditions for both floodplain restoration design approaches (Section 3). The groups reviewed the considerable research conducted over the last decade on the sediment and nutrient dynamics associated with FR projects (Section 4) and concluded that: Denitrification can be enhanced when the hyporheic exchange zone is expanded, floodplains are connected to hyporheic aquifers and runoff, and roots and other organic matter provide a carbon source. Denitrification rates are variable in space and time, but tend to increase with greater geomorphic and floodplain complexity, greater supply of nitrogen, and where mature and natural floodplain plant communities exist. Both sediment and nutrients are effectively trapped by floodplains during larger storms, where they may be stored for many decades. Most of the trapping research has occurred in un-restored floodplains of the Chesapeake Bay watershed, but there is strong evidence that FR projects that increase the annual volume of storm flow diverted to the floodplain can mimic this function of natural floodplain trapping zones. The groups recommended changes to the existing crediting protocols to improve their accuracy and reliability in estimating pollutant reduction for floodplain restoration projects.