How does WaterGEMS/WaterCAD treat the discharge to atmosphere element during EPS/steady state?
This element is primarily for use with HAMMER, but also has a hydraulic effect in WaterGEMS and WaterCAD. The reason why this element (and other transient related elements) are present in WaterCAD and WaterGEMS is because these programs share a file format with HAMMER. Meaning, you can lay out your transient analysis protective equipment when developing your steady state/EPS hydraulic model, so that you can later directly open it in HAMMER and have it ready for transient modeling.
The typical flow and pressure drop are used to calculate a discharge (orifice) coefficient. The program uses that value to calculate the other flows through the element during the simulation and their corresponding pressure drop. The flow and pressure drop are based on the HGL in the system during the steady state (initial condition) run where the solver finds the flow needed to drop that HGL to the discharge to atmosphere element's elevation (where it meets the atmosphere) given the calculated discharge coefficient.
The following methods can be used to either assume a flow and solve for head, or assume a head and solve for flow.
Method 1: If there is no restriction/contraction at the pipe outlet, then consider using the minor loss equation
H = K * (V^2) / 2g
K: Minor loss coefficient. 1.0 is a typical value for a pipe exit (to atmosphere, not submerged).
V : Velocity at the pipe exit (ft/s, m/s)
g : Gravitational acceleration (32.2 ft/(s^2), 9.81 m/(s^2))
V = Q/A
V : Velocity (ft/s, m/s)
Q : Flow (cfs, cms)
A : Cross-sectional Area (ft^2, m^2)
Method 2: If the outlet orifice is smaller than the pipe diameter, then consider using the orifice equation.
Q = C A (2 g H)^0.5
C : Orifice coefficient. This value is assumed by the engineer. A typical value for a typical orifice is 0.6. For in-depth information on orifice coefficients for different situations, see Brater and King’s Handbook of Hydraulics (1996).
A : Orifice Area (ft, m)
Q : Discharge (cfs, cms)
H : Head (ft, m)
The program will not produce valid results if the "Flow (Typical)" and a corresponding "Pressure Drop (Typical)" are set to zero. If you are using this element, you will need to enter valid values for these two fields.
Options for modeling an outflow that varies with pressures
How do WaterGEMS/WaterCAD treat the discharge to atmosphere element?
Modeling Reference - Discharge To Atmosphere [TN]
Stormwater Conveyance Modeling and Design - Page 204
Advanced Water Distribution Modeling and Management - Page 40