The only way to transition between a gravity conduit and a force main pressure pipe in SewerGEMS and SewerCAD is through an intermediate wet well. This establishes a boundary condition between the upstream gravity subnetwork and the downstream pressure subnetwork. During a Steady State analysis the wet well level can be calculated based on generating the required HGL for the outflow to either match or exceed the inflow, or the wet well level can be fixed to a user-specified level. During an Extended Period Simulation the wet well level is determined for a time step by calculating the change in storage over time.

The inflow into the wet well is determined by summing all loads flowing to that wet well.

Hydraulic (HGL) Transition from Gravity to Pressure Network

Gravity hydraulic calculations upstream of a wet well are based on the wet well hydraulic grade, just as they are for standard calculations within gravity systems.

During a Steady State analysis there is a difference, however, in determining the hydraulic grade within the wet well itself. The wet well level may be set by the user to either be fixed or not fixed.

Fixed Wet Well Level during Steady State Analysis

If the wet well level is fixed, the wet well's starting hydraulic grade is used for pressure calculations. No adjustments are made, and this grade is used as the tailwater grade for the upstream gravity systems.

Non-Fixed Wet Well Level during Steady State Analysis

If the wet well level is not fixed, the pressure calculations will attempt to balance the wet well level such that the total flow out of the wet well is equal to or greater than the total flow into the wet well.

The wet well's starting grade is used for the first iteration. If the calculated flows out of each wet well are greater than or equal to each wet well's incoming flow, the iterations stop there. If not, the wet well levels are increased by the increment specified in the calculation options, and the pressure subnetwork is recalculated. When the wet well level is increased, it changes the static heads and increases the discharge for connected pumps, and may also trigger additional pumps to turn on.

This process continues until the level in each non-fixed wet well either meets the flow criteria, or is prevented by rising to the maximum elevation of the wet well.

Wet Well Level During Extended Period Simulations

During an Extended Period Simulation the fixed wet well level options are not available. The wet well level for a time step is actually determined by the change in storage due to inflows and outflows over a single time step.

Hydrologic (Flow) Transition from Gravity to Pressure Network

In the hydrologic transition from a gravity system into a pressure system, gravity loads upstream from the wet well are accumulated and combined with the wet well's local load to determine the total load entering the pressure system at that location. This can be imagined as the total load "dumping into" the wet well. The GVF solver will determine the impact of the wet well on the HGL upstream but the wet well will not back flow up into the inflow conduit in the GVF solver. If the user wants to study this occurrence, one of the dynamic wave solvers should be used.

Connecting Gravity Conduits to Pressure Pipes

If a user attempts to connect a gravity conduit to a pressure pipe, a user notification "Gravity elements do not have downstream wet well or outfall. Cannot flow directly to pressure pipes. Find with network navigator." will be issued. In complex systems, it may be difficult to locate the node where this occurs. This can be found using the Network Navigator query "Isolated Gravity Elements". This will identify the elements in the gravity subnetwork connected to pressure subnetworks.

Users may want to model a gravity conduit discharging into a pressure pipe in an inverted siphon. In that situation, it is best to model the pipes in the inverted siphon as gravity conduits with bolted manhole covers.