SewerGEMS Maximum Velocity Calculations

Question

In SewerGEMS, the terms "maximum velocity" and "outlet velocity" pertain to the hydraulic analysis of sewer systems. These parameters are critical for assessing the performance and integrity of sewer networks. I am hoping that the technical questions below can be given some guidance. My end goal is to try to understand plunging velocities and trying to make sense of them and I know this outside the scope of the product, as this was mentioned in another post: 
 Maximum Velocity Calculation: a. What is the mathematical formulation or algorithm used to calculate the maximum velocity within a sewer conduit in SewerGEMS? b. How does SewerGEMS account for variations in pipe size, shape, and flow rate when determining the maximum velocity? c. Can you explain the role of boundary conditions and roughness coefficients in the calculation of maximum velocity? 
 Outlet Velocity Calculation: a. How is the outlet velocity from a sewer pipe or junction computed in SewerGEMS? b. What specific hydraulic principles or equations are utilized to determine the outlet velocity? c. Does SewerGEMS consider factors like downstream pipe conditions or outfall characteristics in outlet velocity calculations? 
 Manhole Velocity Calculation: a. What methods or equations are employed in SewerGEMS to calculate velocities at manholes within the sewer network? b. How does SewerGEMS handle the transition of flow velocity between pipes and manholes? c. Can you elaborate on any unique features or considerations in the calculation of velocities entering and exiting manholes, especially in the context of complex sewer geometries? 
 Boundary Conditions and Assumptions: a. What assumptions and boundary conditions underlie the velocity calculations in SewerGEMS? b. Are there scenarios or conditions where the velocity calculations may deviate from real-world conditions, and how does SewerGEMS address such situations? c. How does the software account for unsteady flow conditions or transient events in velocity computations? 
 User Customization: a. To what extent can users customize the velocity calculation methods in SewerGEMS to match specific project requirements or regional standards? b. Are there parameters or settings within SewerGEMS that allow users to fine-tune velocity calculations for particular sewer system designs?

Luca Santos Billis · Answer

Hello, Daniel, I would like to recommend that you check out our explanatory material on all the theory related to SewerGEMS in detail and if this still leaves you with any doubts, please get back to us and we will help you. 
 Help topic > Theory > Fundamental Solution of the Gravity Flow System .

Jesse Dringoli · Answer

To add to Luca's response: 
 
 The calculation of velocity depends on the solver. For the more simplified GVF-Convex solver available in SewerGEMS, see this article . 
 For the Implicit or Explicit dynamic solver, the standard dynamic equations are used (St. Venant - see the theory section of the help for the complex equations) 
 For more information on the differences between solvers, see: Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM) 
 The solvers account for pipe size/shape, flow, slope and other factors when the velocity is calculated. 
 Regarding "outlet" and "maximum" velocity - with the dynamic solvers, velocity is calculated along the length of each pipe and you can see "Start", "Stop" and middle velocity when looking at the results in the properties or in a graph. The maximum velocity over the length of pipe is also shown, based on the highest value over the course of the simulation. 
 For a manhole, the "Velocity (in)" and "Velocity (out)" are calculated and reported and you can see a description of these fields by looking at the bottom of the Properties window when clicking on the field. See more here: Where can I find the definition of a property? Also see the theory documentation for more information on calculations. Generally speaking the velocity in a node like a manhole is based on the controlling velocity of the incoming and outgoing pipes and a detailed analysis of the internal velocity of flow through the structure itself is not done as this is not usually something an engineer needs to look at.
 
 There are several structure loss methods available for manholes and other node types, and the concept of benching can be an impact. See more here .

Regarding "boundary conditions" - with the dynamic solvers, the conditions of the adjacent pipes and nodes (such as the depth of flow) are automatically accounted for (pipes are not just calculated independently). 
 Regarding "unsteady flow conditions" - the dynamic solvers are considered to be performing unsteady simulations (conditions changing over time), so you will be able to view (graphs, profile animations, etc) how the results change over time. 
 Regarding transient events - hydraulic transients (from a sudden change in momentum) are not modeled in SewerGEMS. They occur in the context of pressurized pipes and you would need to use our separate HAMMER software to analyze transient events for example in sewer force mains. See: Can the storm-sewer products perform a transient analysis in the sewer force mains? 
 Regarding customization of velocity calculations - as seen in this article previously mentioned, you can select from a few different average velocity methods with the GVF-Convex solver, but that is for a more simplified analysis. With the dynamic solvers (implicit or explicit) the velocity calculation is part of the standard set of equations, so the adjustment options for you would be based on the more general advanced calculation options (such as the NR Weighting Coefficient for the Implicit solver) described in these articles: Implicit | Explicit These advanced options give you some control over the solvers in general but not directly related to velocity. This type of "customization" is not something we've heard of before. If you are looking to control parameters to achieve some specific result, there may be another way to do this by way of changes to the physical setup of the network, and some details on this would help us guide you. 
 
 If you can explain a bit more about your project and the reason for all of your velocity questions, it would be helpful. 
 Other community members may be able to comment on your engineering practice-related questions.