Outfall Constructibility

[pwade1]

Drainage/Marine Engineers Needed:

I’ve inherited a design that proposes a new storm sewer system outfall with an invert elevation below mean low water (MLW), located adjacent to the existing outfall (36" and 30" RCP, respectively). The storm sewer system includes a combination of existing and proposed structures and pipes, which have been constructed up to the last manhole before the outfall. The two outfalls (existing and proposed) branch out from a junction manhole 200 feet from the sea.

The proposed outfall is situated at the timber wall of the bulkhead of a 1950s concrete berth, which is reported to be in good condition based on quality assurance (QA) inspection reports.

To construct the new outfall, significant demolition of the concrete deck berth is required, along with the installation of a sheet pile cofferdam to dry and secure the work area. Can the contractor install the outfall without removing the deck, despite potential modifications?

Solutions Investigated and Ruled Out Through Storm and Sanitary Analysis (SSA):

• Not Constructing the Outfall:
  Using the adjacent existing outfall was explored, but the capacity of the upsized upstream system would result in flooding of lower elevation structures 1200 feet upstream.
• Raising the Outfall Invert:
  • a) Back-pitched Pipe:
    This would cause the structure to always contain water, with flow flushing downstream and upstream depending on the tide elevation and the rain event return period.
  • b) Higher Invert than the Upstream Invert at the Last Manhole:
    This would create a "charging manhole," which would flood until the system's hydraulic grade line (HGL) reaches the higher elevation and allows flow to the outfall. This would raise the invert from -3.50' (MLW is -2.5') to 1.5' (mean high water is 2.5'). The system would only use this "new outfall route" when there is enough head in the system.

Note: Installation of a tide gate is recommended (though it’s not in the contract yet). However, the tide gate would only affect nearby structures. The additional capacity provided by the new outfall is crucial during large storm events.

What is your approach to ensure a durable storm sewer design while minimizing damage to the berth structure?