Permitting of D.B. Benson Water Supply Project

Client: City of Raleigh

Location: Raleigh, NC

Summary: ENTRIX completed an Instream Flow Incremental Methodology (IFIM Study) of Swift Creek for the City of Raleigh to support the permitting requirements for construction of a new 18 MGD water treatment plant and to negotiate a minimum flow downstream of Lake Benson, the water source for the new water treatment plant. The study serves to examine the potential impacts to threatened and endangered mussel species that occur downstream of Lake Benson in Swift Creek.

The proposed project is subject to the North Carolina 1995 Dam Safety Rules – Minimum Stream Flows to Maintain Aquatic Habitat, which required an instream flow study to determine minimum flow requirements downstream of the lake to support native fishes and a population of the Federally endangered dwarf wedgemussel, (Alasmidonta heterodon). In addition, a number of other mussels that are either federal species of concern, state-threatened, or state species of concern are also known downstream of the project in Swift Creek. ENTRIX utilized a two-dimensional hydraulic model to address potential impacts downstream of Lake Benson in Swift Creek. Traditional instream flow studies utilize a one-dimensional modeling approach and have limitations in their ability to address the spatial distribution of habitats for mussel species. This novel modeling approach allowed ENTRIX to address agency concerns regarding the limitations of traditional modeling approaches for examining potential habitat impacts to mussels species downstream of the project. The results allowed the City of Raleigh to develop plans to expand water supply for their utility by providing a scientific study to judge potential project impacts of the proposed project. The study is now being used to write portions of the EA to receive final permitting and approval for the project.

In addition, ENTRIX is leading the Endangered Species Act Section 7 Consultation process for the Federally endangered dwarf wedgemussel (Alasmidonta heterodon) . ENTRIX has completed the scoping process and is preparing a Biological Assessment that will be the basis for the U.S. Fish and Wildlife Service’s Biological Opinion.