Velocity Calculations and the Average Velocity Methods in SewerCAD and StormCAD

Applies To 
Product(s): SewerCAD, StormCAD, SewerGEMS, CivilStorm
Version(s): CONNECT Edition, V8i
Area:  Calculations
Original Author: Scott Kampa, Bentley Technical Support Group


How is the "Velocity" result field for a conduit (average velocity) computed in SewerCAD and StormCAD? (or the GVF-Convex and GVF-Rational solvers)

How do each of the Average Velocity Methods work? 


The "Velocity" result field for a conduit or channel represents the average velocity and is used to determine the flow travel time. It is computed based on one of for available methods. The method is selected in the Calculation Options field called "Average Velocity Method":

Here are the assumptions for each method:

1) Actual Uniform flow velocity - This is obtained by calculating the velocity in the pipe at normal depth. Meaning, it solves the Mannings (or other selected method in the calculation options) for normal depth, based on the flow, slope and other characteristics, to determine the corresponding velocity of flow at that depth. If the normal depth corresponds to a surcharged condition, the full flow velocity is used instead.

2) Full flow velocity - This corresponds to the velocity when the pipe is flowing full (depth equal to the top of the pipe). The flow area is equal to the entire cross-sectional area of the pipe. This method takes the Capacity (Full Flow) and divides by the cross-sectional area to find the velocity.

3) Simple Average Velocity - This is simply the average of the velocity at the upstream and downstream end of a conduit. This does not account for any depth changes between the two ends of the pipe as the weighted average velocity method does. The "velocity (In)" and "velocity (out)" used here (and also reported separately for conduits and channels) is based on the flow and flow area corresponding to the depth at the upstream and downstream node, respectively. See note further below. The depth at the node is determined based on the gradually varied flow (GVF) backwater profile calculations (see documentation).

4) Weighted average velocity - With this method, the simple average velocity of each profile segment is considered and given a weight based on its length.

5) Full Depth velocity - This is the relationship with the reported flow and the full cross-sectional area of the conduit. 

Note: There are separate "velocity (In)" and "velocity (out)" results for a conduit as well. These are based on the flow and flow area corresponding to the depth at the upstream and downstream node, respectively. The depth at a node is determined based on the Gradually Varied Flow (GVF) backwater profile calculations (see help documentation for more).

See Also

Differences between solvers: GVF-Rational vs. GVF-Convex vs. Implicit vs. Explicit (SWMM)