You are currently reviewing an older revision of this page.
How is the "Velocity" (average velocity) computed in SewerCAD and StormCAD? (GVF-Convex and GVF-Rational solvers)
How do each of the Average Velocity Methods work?
The velocity (and thus the travel time) for a conduit or channel 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) 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. V = Q/A
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. 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.
Differences between solvers: GVF-Rational vs. GVF-Convex vs. Implicit vs. Explicit (SWMM)