Phase 4 Project Details (Wetwell & Outfall)

Update 29 January 2020


  • Substantial Completion is scheduled for early summer 2022.  Substantial Completion is defined as the project is in a condition such that it fully functions as designed.  
  • Final Completion is scheduled for late summer 2022.  Final Completion is defined as all project elements are fully complete and the contractor is able to demobe.  

Project Phase Overview

Phase 4 of the U.S.17 Spring/Fishburne Drainage Improvement Project will include the construction of an outfall for the pump station (to be constructed in Phase 5), the wetwell that will house the large main stormwater pumps, and installation of dewatering/sump pumps and an automated mechanical trash rack.  

  • Construction and installation of outfall into the Ashley River
    • 550 feet long
    • Triple 8-ft by 10-ft concrete box culverts
    • Supported by pre-stressed concrete piles
  • Construction of pump station wetwell
    • 55-ft wide by 136-ft long and 33-ft below grade (246,840 c.f., 9,142 c.y. of excavated material)
    • Concrete pile foundation
    • Installation of self-cleaning mechanical barscreens
    • Installation of 2 dewatering pumps, 2 mud pumps, 1 sump pump, and 1 silt agitation pump
    • Installation of control equipment

Project Benefits

At the completion of Phase 4, the new system will see moderate improvements under gravity flow scenarios even without installation of the main stormwater pumps.  

  • Currently, the existing conditions allow an estimated maximum flow rate from the basin of 80 cubic feet per second (cfs) or 36,000 gallons per minute (gpm) at low tide.
  • The table below shows the expected flow rates for various tide levels at the completion of Phase 4.

Max Flow Rate (CFS)

Max Flow Rate (GPM)

Percent Improvement Over Ex. Conditions

Low Tide




High Tide




King Tide




  • At the high tide and king tide events, there will still be ponding on the Crosstown, depending on the rain event, however it will drain faster than existing conditions.
  • Full benefits for the 10% design storm (10-yr event) will be realized at the completion of Phase 5 when the pump station is brought on-line with a maximum flow rate of 800 cfs or 360,000 gpm, ten times the existing maximum flow rate.