Area Types in SWMM 5 for Links with Offsets

Subject:  Area Types in SWMM 5 for Links with Offsets

 

 There are six types of link flow in SWMM 5:

 

1.   Dry Conduit at both Ends

2.   Upstream End is Dry

3.   Downstream End is Dry

4.   Subcritical Flow

5.   SuperCritical Flow

6.   Free Fall at Upstream End

7.   Free Fall at Downstream End

 

Figure 1.  Six types of Link Area Allocation

 

 

 

 

 

Figure 2.  Example Link Area Type in SWMM 5

Force Main Friction Loss in SWMM 5


Subject:  Force Main Friction Loss in SWMM 5

You can model Force Mains in SWMM 5 using either Darcy Weisbach or Hazen Williams as the full pipe friction loss method (see Figure 1 for the internal defintion of full flow).   No matter which method you use for full flow the  program will use Manning’s equation to calculate the loss in the link when the link is not full (see Figure 2 for the equations used for calculating the friction loss – variable dq1 in SWMM 5).   The regions for the different friction loss equations are shown in Figure 3.
Figure 1.  How the full pipe condition is defined in SWMM 5










Figure 2:  Friction equations used in SWMM 5 for a Force Main.

 

 
Figure 3:  Regions of Friction loss equations in SWMM 5.








Capacity Limited Links in SWMM 5

Subject: Capacity Limited Links in SWMM 5


The table in the text report file of SWMM 5 called “Conduit Surcharge Summary” has a column entitled “Hours Capacity Limited” Here is how a link is defined asCapacity Limited in SWMM 5. Two factors have to be present:

1. The upstream end of the link has to be full and

2. The HGL Slope has to be greater than the Slope of the Link

What is the Area of a Node in SWMM 5?

Subject: What is the Area of a Node in SWMM 5?

 The minimum area of a node is the default area of the node, which is the user defined minimum surface area or 12.566 square feet if the user does not define the surface area of the node. The surface area of a node is usually the half of the surface area of all of the connecting conduits but if the link is dry then the sum of all of the surface areas may be less than the default surface area. If you look at a scatter plot of the area versus the depth then you will see that it always has the minimum surface area.

 

 

What is Node Convergence in SWMM 5?

Subject: What is Node Convergence in SWMM 5?

 If a link has converged within two iterations then the hydraulic computations will be bypassed for the link in new iterations. A link is deemed converged if BOTH the upstream and downstream node depth has converged between successive iterations as long as two iterations have occurred during the time step. The nodes are considered converged if the depths between successive iterations is less than the stop tolerance of the program (the default stop tolerance is 0.005 inches). The new node depths are ALWAYS computed for each iteration but the links connected to the converged nodes may bypass computations and save on simulation time. Note: If the nodes ever become non converged after 2 iterations then the link flows will be computed.