XPSWMM Confusions and Clarifications (2 of 2)

XPSWMM Confusions and Clarifications (2 of 2)

XPSWMM is a popular 1D and 2D hydrologic and hydraulic modeling tool developed by Innovyze. Though originally XPSWMM was designed primarily to model storm sewer network system, it has gained the capability to model open channels, rivers, 2D overland flows and 1D-2D coupled systems in recent years. This post is 2 of 2 posts to clarify some topics that quite often confuse new XPSWMM users (for other topics covered by Post 1 of 2 click here). Note that sanitary sewer modeling is not covered by either post.

1. There are two ways to model dual-pipe, or multiple- pipe system between two nodes (such as double box culvert) in XPSWMM.

  • Set the Number of Barrels to 2 or whatever the total number of parallel pipesbetween two nodes in Conduit Factors setting window. To use this option, the multiple pipes must possess the same sizes, materials, and upstream/downstream flowlines.
Figure 1
  • Use the Multi-link option of XPSWMM to set up multiple parallel pipes between two nodes. (Figure 2A). The pipes do not have to be the same size and flowlines as required by the first option. This option has more flexibility for modeling and it is the recommended method for multiple-pipe modeling in XPSWMM.
Figure 2A

The Multi-link option of XPSWMM can be used to model a dual drainage system (minor system – underground storm sewer; major system – street/overland/ditch). Usually the node spill crest should be raised to match the highest elevation the major system link is representing. For example, if the major system is represented by a link using a natural section from road ROW to ROW, the node spill crest should be raised from its normal position by D as shown in Figure 2B and Figure 2C. Under certain scenarios, the spill crest may need to be raised by more than D, which is explained in more details in #5 below.

Figure 2B
Figure 2C

In a dual drainage system, the node’s inlet capacity feature may need to be enabled (Figure 3A, maximum capacity – the simplest inlet capacity setting) to model an inlet’s restriction to incoming flow: only the flow not exceeding the inlet capacity will drop into the inlet chamber to be carried by the underground storm sewer, while the remaining will flow down the major system – a street or an overland natural channel section. Enabling a node’s inlet capacity feature will split the original node into two by XPSWMM automatically: a “top” node (connecting major system) and a “bottom” subsurface node (connection minor system) which is denoted by “$I”. The two nodes are connected by a rating curve from the inlet capacity feature setting (Figure 3B and Figure 3C).

Figure 3A
Figure 3B
Figure 3C

2. Model culverts in XPSWMM: to turn on culvert calculation routine in XPSWMM, click and check on “Conduit Factors” option in Conduit Data Edit window, and select Inlet Type for inlet control calculation and provide Entrance Loss and Exit Loss Coefficients for outlet control calculation as shown in Figure 4. The Entrance Loss Coefficient can be found at Table C.2, Appendix C of HDS-5. The Exit Loss Coefficient usually is 1.0 and it can vary between 0.3 and 1.0 per HEC-RAS Hydraulic Reference Manual. Refer to this post for more information about culvert modeling by XPSWMM and other tools.

Figure 4

3. To export modeling results as time series data, right click a node or link’s graphic result view window and open “Export Dialog…” to choose Export File type as Text/Data and pick a file name and location (Figure 5). In the next window, various export file options are available for pick up (Figure 6). Usually it is just convenient to save the file as a CSV file (Delimited by Comma) which can be opened and edited by Notepad (Figure 6) or Excel.

Figure 5
Figure 6

4. Node Spill Crest Elevation: node spill crest elevation determines the maximum elevation water can rise in a node (upper boundary of HGL ). Normally it is set up as the ground or top of node cover elevation. Unless a node is sealed/bolted or has enabled the ponding option, any excess water above node spill crest elevation will get lost from the system through nodes (no water loss through links in XPSWMM). For this reason, if a force main is to be modeled, it is recommended that the upstream and downstream node spill crest elevations be set high enough to force the pipes in between the nodes under pressure flow to mimic a force main.

5. Node Spill Crest Elevation for Open Channel: setting spill crest elevations for nodes connecting an open channel can become tricky. First of all, it should be noted that XPSWMM was originally developed to model a closed pipe network and therefore there are limitations on open channel modeling by XPSWMM, for example, there is no actual node for a river, but in XPSWMM river segments have to be connected by nodes just like a closed pipe system. Furthermore, when a river is overtopped, water will be lost from the lowest points of river banks in a real world, but in XPSWMM, water can only get lost through nodes. There are several scenarios for setting up node spill crest elevations of nodes connecting an open channel or a river:

  • 5.1 – If we assume the water above the top of cross section will be lost from the system, set up the node spill crest at the same elevations as the top of channel cross section and the ponding option of the node will be “None“. Or, if you want to store the lost water in the system for it to return to nodes/river at a later time, enable the ponding option.
  • 5.2 – If we want to keep the water above the top of cross section (top of bank) in the system, set up the node spill crest at a higher elevation than the top of channel cross sections. When overflow happens, XPSWMM by default will assume a vertical wall on both sides of the cross section and the water in the open channel can rise all the way up to the upstream/downstream node spill crest elevation but is contained by the vertical walls (Figure 7B). VERT_WALLS=ON does not need to typed in manually in Configuration Parameters since it is XPSWMM’s default setting. In an 1D-2D coupled system where an 1D open channel is connected hydraulically to 2D overland areas, the node spill crest is usually set to an elevation higher than top of cross section so that the 1D open channel water surface can rise above top of cross section (top of bank) to mimic flooding, which would also allow flood water exchange with 2D overbank areas. For a river link’s node when being modeled in an 1D-2D coupled system, the spill crest elevation will automatically be raised by 20% in order to account for flooding (Figure 8).
Figure 7B
Figure 8
  • 5.3 – If we don’t wish the water in Scenario 5.2 to raise above top of bank, the default vertical wall option can be turned off by typing in VERT_WALLS=OFF manually in Configuration Parameters (Figure 9). This configuration is like to put a lid on top of the open channel and it can effectively set the open channel under pressurized flow (water will still rise in the nodes to push water through the lidded open channel, but the water in the channel will not rise above top of the bank due to the imaginary lid). Scenario 5.3 should be used rarely in XPSWMM modeling unless you need to model the open channel as a bridge.
Figure 9

The difference between Scenario 5.2 and 5.3 can be confirmed by checking the E11 Tables of the output log files. In Scenario 5.2, water in the open channel can rise above top of the bank so the X-Sect Areas are pretty big (Figure 10); while in Scenario 5.3, water in the open channel is confined by the imaginary lid and flow in the open channel is pressurized and therefore the X-Sect Areas are only up to the top of the bank (Figure 11).

Figure 10
Figure 11

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