Modeling wetwell overflow

Product(s): SewerGEMS, CivilStorm
Version(s): 08.11.XX.XX, 10.XX.XX.XX
Area: Modeling

Problem

How do I model wet well overflow or backup? For example as a result of pump failure.

Background

When a pump fails to operate, the upstream wetwell may overflow and cause flooding. SewerGEMS and CivilStorm can provide you the volume of overflow encountered within the wet well with the use of the Implicit or Explicit solver.

Basically, Implicit or Explicit solvers are best for analyzing problematic systems or flooding since they use dynamic numerical solvers that can calculate overflow rate and many other things.

Contrast with the GVF-Convex solver which uses the standard EPANET solver for pressure networks, allowing for solid stability with complex pumping situations like manifolds and pumps that cycle faster than the hydrologic time step. However, it is not intended for use with quantifying overflow, so you wouldn't be able to see the overflow with convex solver.

Solution

The backing up of water as a result of a wetwell overflow is accounted for automatically with the implicit and explicit solvers. If you are modeling a pump failure situation, a pump/outfall is not required downstream of the wetwell, although you can include it if you'd like, and set the pump on/off controls such that they never turn on. The backup effects from the overflow can be seen in profile view and other places.

The wet well overflow volume can be graphed and seen in the hydraulic reviewer with Implicit engine.

You can quantify overflow volume under Analysis > Analysis Tools > More > Hydraulic Reviewer (Or in V8i, Analysis > Hydraulic Reviewer)

Overflow could include flow at the surface area above the wet well, as it can influence the overflow rate and potentially pond up and recede.

To account for the surface above the wet well top:

  • Change the wet well type to depth-area and set up the table so as to include the volume above the actual wetwell top. It would be narrow at the bottom where the wetwell actually is, and wide at the top where “overflow” would pond up above the wetwell. This only works if the surface above the wetwell top slopes uphill. If there are depressions at the surface that are lower than the wetwell top, or if the surface slopes downhill and away from the tank, this approach won’t work.
  • If the land slopes away but if you need to account for tailwater effects, then you could try modeling it as a conduit with a start control structure, attached to the wetwell, with an outfall at the other end. The control structure could consist of a weir to simulate the water spilling over the side, and the conduit would represent the slope of the surface. That way the depth in the conduit could influence the overflow rate.
  • Or simply accept the free-outfall overflow over the wet well top.

A note on stability

Models with a large amount of overflow and/or surcharging can be difficult for the numerical solver to achieve stability. If you are experiencing high mass balance error (continuity error) or otherwise unexpected results, please see the tips in the below article written for a similar situation.

See Also

Modeling a flow restriction at an outfall

Storage volume of wetwell and pipe

http://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/11852.sewercad-gvf-convex-solver-vs-sewergemscivilstorm-implicit-and-explicit-dynamic-solvers-vs-stormcad-gvf-rational-solver 

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