Optimization of Minimum Instream Flow Needs Along the White Clay Creek at Stanton, Delaware

Abstract

The January 17, 2003 Fifth Report to the Governor and General Assembly Regarding the Progress of the Delaware Water Supply Coordinating Council recommended that United Water Delaware (UWD) modify the operating plan for the Tidal Capture Structure (TCS) to provide additional water supply capacity during drought at the White Clay Creek at Stanton Water Treatment Plant. The Delaware River Basin Commission is scheduled to hold a public hearing on January 19, 2005 to review UWD Docket D-96-50 CP-2 to modify the TCS operating plan to “include incremental passby flow targets in conjunction with salinity monitoring to protect the applicant’s water supply and preserve depth of water downstream for aquatic habitat protection.”

The modified TCS operating plan is designed to: 1. Operate the TCS more efficiently. 2. Increase water supply reliability during low flow conditions and provide more water supply during drought. 3. Provide sufficient stream flow for fishery and habitat at the tidal/freshwater interface. 4. Maintain tidal chloride levels in the creek below the USEPA secondary standard of 250 ppm during drought.

This report summarizes a technical basis for maintaining a tidal instream flow standard based on a principle of (a) minimum depth of flow for fishery habitat and (b) attenuation of chlorides. The proposal is to replace the existing 7Q10 flow rate standard as required by the DRBC with a 7Q10 flow depth equivalent to protect fishery habitat and chloride standards to address water quality concerns. The current 7Q10 flow rate standard is difficult to measure due to the two – way tidal nature of the White Clay Creek.