XPSWMM Confusions and Clarifications (1 of 2)

XPSWMM Confusions and Clarifications (1 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 1 of 2 posts to clarify some topics that quite often confuse new XPSWMM users (for other topics covered by Post 2 of 2 click here). Note that sanitary sewer modeling is not covered by either post.

1. XPSWMM has two basic modes: runoff mode and hydraulics mode, which are inherited from the historical EPA SWMM RUNOFF and EXTRAN blocks. XPSWMM runoff mode and hydraulics mode can be turned on or off by clicking the tool bar icon Rnf and Hdr (Figure 1). Runoff mode and hydraulics mode have their respective Job Control setups (Figure 2).

Figure 1
Figure 2

2. A node or a link can be added or removed from any mode by using the “+” and “” tool on the right side of the main XPSWMM window (Figure 3). Usually a node should be added on to runoff mode if it receives runoff flows from a catchment in XPSWMM; otherwise, it should be removed from runoff mode; A link should ONLY belong to hydraulics mode and the benefit for a link to be in runoff mode is not clear, if there is any.

Figure 3

3. A catchment element is really not a must for a node to receive runoff flows in XPSWMM. If drainage areas have been delineated by GIS or other tools and the hydrology parameters are calculated outside XPSWMM, simply right click a node and then choose “Edit Data” to type in its hydrology/runoff calculation required parameters as shown in Figure 4. There is no catchment element involved in this process.

Figure 4

4. Historically EPA SWMM (up to SWMM4) conducted runoff and hydraulics calculations separately in RUNOFF, TRANSPORT, and EXTRAM blocks and the results of RUNOFF block is transferred to TRANSPOR/TEXTRAN blocks through interface files. Although XPSWMM inherits this feature, it is always recommended to calculate runoff and hydraulics simultaneously in current XPSWMM versions unless a modeler has a special need to use interface files to transfer results. To set up runoff and hydraulics calculation to run concurrently, first to check on the option of “Solve Runoff & Hydraulics Mode Simultaneously” in Hydraulics Job Control (Figure 5); and secondly, enable both RUNOFF and HYDRAULICS in Mode Properties setting (Figure 6).

Figure 5
Figure 6

5. Both runoff mode and hydraulics mode have their own time controls (start date and time, end date and time). When XPSWMM is set up to solve runoff mode and hydraulics mode simultaneously, it simply applies the instantaneous results of runoff mode to hydraulics mode without checking whether their respective date (it appears XPSWMM does check runoff and hydraulics start time’s hour values) match or not, and for this reason, it is a good practice to keep them the same as shown in Figure 7; otherwise the modeling results of runoff and hydraulics are very confusing. It should be noted that the time steps of the two modes can be different and usually hydraulics mode requires a much smaller time step due to model stability requirement.

Figure 7

It is highly recommended that the start time is set to be zero (0.0) hour (midnight) of a day for both runoff and hydraulics time control, even though XPSWMM allows a non-zero hour start time. When a user supplied inflow or stage hydrograph (hours: flow in cfs or hours: stage in ft) is defined in XPSWMM, its zero (0.0) hour is always meant to be at midnight of a day. For this reason, if a hydraulics job control start time is non-zero, for example, set it at 5 (am), XPSWMM will only read the user defined inflow or stage hydrograph from 5.0 hours and ignore values from 0.0 hour to the time before 5am.

Using interface files to transfer data between runoff mode and hydraulics mode is a different story: XPSWMM will check data and time to make sure they match before applying runoff mode results to hydraulics mode.

6. Water can only be lost through nodes in XPSWMM due to flooding or surcharge (Figure 8) and water loss does not happen within a link.

Figure 8

6. In XPSWMM, the HGL of a link is NOT calculated within the link, but assumed as a straight line connecting the water head of the upstream and downstream nodes. For this reason, if a link (pipe) is too long (100ft or more) or the length is not that long but the slope is very steep, you may want to add additional “dummy” nodes to divide the link into multiple shorter ones to get better and more realistic HGL results. Adding additional “dummy” nodes may also help increase a model’s numerical stability.

Figure 9

7. After each successful model run, a detailed output log can be reviewed by clicking Analyze—>Show Output Logs—>1D Log or 2D Log or 2D Extreme Log as shown in Figure 10.

Figure 10

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