Dear bloggers and forum users,
I am searching the whole day for a list of all the hydraulic formulas, that are incorporated into SWMM5. The user manual and quality assurance manual don't seem to be the documents containing those... I only know and find the formula for runoff, but I would like to check also the weir and orifice formulas and others.
I am not sure if this whole thread sounds ridiculous, but I really ran out of ideas.
Thanks in advance!
Best regards,
Ivan
Replies
Subject: What are the Equations for Weirs in SWMM 5, Part 2?
There are four types of Weirs in SWMM 5: Transverse, Sideflow, V Notch and Trapezoidal. The trapezoidal weir is a combination of the Sideflow and V Notch Weir and the Sideflow acts like a Transverse Weir when the flow is reversed (Figure 1). The Weirs can have zero, one or two end contractions (Figure 2) and the Weir Length is a function of the Weir Setting and Horizontal Weir Length. A V Notch weir works as Trapezoidal Weir when the Weir RTC Setting is less than 1.0
Figure 1. Weir Equations in SWMM 5
Figure 2. Valid Number of End Contractions
Figure 3. Weir Length Calculations
Dear Mr. Robert Dickinson,
I was about to test the formulas from the American textbooks on hydraulics with some test files in EPA SWMM, but you made it very much easier for me, writing those equations. Thank you again for your time, I appreciate it very much!
Best regards,
Ivan
Subject: What are the Equations for Weirs in SWMM 5?
There are four types of Weirs in SWMM 5: Transverse, Sideflow, V Notch and Trapezoidal. The trapezoidal weir is a combination of the Sideflow and V Notch Weir and the Sideflow acts like a Transverse Weir when the flow is reversed (Figure 1).
Figure 1. Weir Equations in SWMM 5
Thank you for the clarification, Mr. Robert Dickinson!
Note: The main difference between an Bottom and Side Outlet orifice at the same offset elevation and the same diameter is the depth at which the flow in the orifice will switch between weir flow and orifice flow. The Side Outlet orifice has Weir flow until the Orifice is full but the Bottom Orifice has Weir flow until the Critical Height which is usually shorter than the maximum depth of the orifice.
For a circular orifice the Critical Height is:
Critical Height = Orifice Discharge Coefficient / 0.414 * Orifice Opening / 4
For a rectangular orifice the Critical Height is:
Critical Height = Orifice Discharge Coefficient / 0.414 * (Orifice Opening*Width) / (2.0*(Orifice Opening+Width))
SWMM 5 uses the normal Weir equations internally using these conventions
Note: SWMM5 Weir Rules and Head Calculations
This note attempts to explain both how the head upstream and the head downstream of a weir in SWMM 5 is calculated compared to the weir crest elevation and also to explain how the weir is presented in the HGL plot of SWMm 5. There has been confusion in the past concering how the weir is shown compared to the actual weir calculations. The node head is calculated obviously at both ends of the weir but the head over the weir is always based on H1-Crest or H2-Crest (Figure 1) and hence the weir should look flat – to the weir the downstream head is important but NOT the downstream node invert so the weir really is flat and should look flat in the HGL Profile across the weir (Figure 2). The crest elevation is always relative to the upstream node invert elevation NOT the downstream node invert elevaation
Figure 1. How the Head across a Weir is calculated in SWMM 5
Figure 2. HGL Profile across a Weir in SWMM 5.0.022. The black line should be shown flat.