Model A Pump Station in HEC-RAS, HydroCAD, XPSWMM, and HEC-HMS

Model A Pump Station in HEC-RAS, HydroCAD, XPSWMM, and HEC-HMS

A storm water pump station (Figure 1) is needed in low areas of a drainage system where drainage by gravity is not possible, such as a depressed roadway segment. Pump station design is a complicated task which requires engineers from different disciplines to work together including structural, hydrologic and hydraulic, electrical, mechanical, civil, and architecture. FHWA has several publications dedicated to pump station design such as HEC-22 and HEC-24.

Figure 1

From conservation of energy perspective, a pump station is to lift water from a lower elevation to a higher elevation (the difference between the two elevations is the static head) by electrical and mechanical energies. During pumping, some electrical and mechanical energies are converted to heat energy when overcoming various frictions throughout the drainage system. If the velocity head difference is insignificant, the total required dynamic head is expressed as H-total = H-static + H-loss, which is a function of pump flow rate. A pump will operate at the point where the system head loss curve intersects the pump curve (Figure 2).

Figure 2

A lot of today’s H&H modeling tools, such as HEC-RAS and XPSWMM, require a modified pump curve as input by subtracting the system friction losses from the total dynamic head to simplify the computation (Figure 3). For HEC-RAS and XPSWMM, this means the pump curve revision process needs to be done outside the programs. HydroCAD and HEC-HMS give a modeler two options:

  1. similar to HEC-RAS and XPSWMM, enter a revised pump curve as input after subtracting system friction loss externally (friction loss or equipment loss to be entered as zero under this scenario);
  2. Apply the original pump curve. For HydroCAD, supply the discharge pipe length, size and Hazen-Williams coefficient to let the program calculate the system friction loss and subtract it from the original pump curve automatically. For HEC-HMS, enter a fixed friction loss/equipment loss which is independent of pump flow rates.
Figure 3

A simple pump station is to be set up in HydroCAD, HEC-RAS and XPSWMM to examine their modeling capability (Figure 4). The pump curve values in Table 1 are arbitrary for demonstration purpose only.

Figure 4
Table 1
  • Pump Station in HydroCAD

To model the pump station in Figure 4, create a new HydrocAD project file and make sure the pond routing method is “Dyn-Stor-Ind” so the tailwater in the downstream pond can be “felt” by the pump.

Figure 5

Add two ponds with the bottom sizes of 900′ x 1800′ and 200′ x 200′ respectively. The upstream pond has the pump as an outlet (Figure 6). In this example, the original pump curve is entered and the friction loss is calculated by HydroCAD based on the discharge pipe data provided. It is important to enter the pond initial WSE under the “Advanced” tab (Figure 7).

Figure 6
Figure 7

The 48-hr model run results are shown in Figure 8 (upstream pond) and Figure 9 (downstream pond): the pump flow rate is decreased from 3.83cfs@0.0hr to 3.12cfs@48hr while the downstream pond stage rises from Elev 105.00′ to Elev 119.94′ during the same time period.

Figure 8
Figure 9
  • Pump Station in HEC-RAS

A HEC-RAS unsteady state model is established to model the pump station in Figure 4, which consists of two ponds (storage areas, Figure 10 & Figure 11) and a pump station (Figure 12). In Figure 12, the pump curve is the revised one after the friction loss being taken away.

Figure 10
Figure 11
Figure 12

After specifying the initial WSE of the two ponds (Figure 13), run the model for 48hrs. The results of this HEC-RAS model match those of the HydroCAD model pretty well as indicated in Figure 14 in terms of pump flow rates and pond stages.

Figure 13
Figure 14
  • Pump Station in XPSWMM

Similarly, a XPSWMM model in Hydraulics mode is created to model the pump station for comparison (Figure 15 & Figure 16).

Figure 15
Figure 16

In XPSWMM, a pump station is modeled as a multiple link (Figure 17), and a pump curve must be defined first under Global Data… –> (H) Pump Rating Curves (Figure 18), which is the revised one after the friction loss being taken away.

Figure 17
Figure 18

The XPSWMM model established above is run for 48hrs and the results are shown in Figure 19 which are the same as those of the HydroCAD and HEC-RAS models.

Figure 19

In summary, a pump station rated by dynamic head can be modeled by HydroCAD, HEC-RAS, or XPSWMM; for this very simple example pump station, the three programs are able to generate exactly the same results.

If a discharge pipe is raised to pass over a levee system, the static head should be calculated from the highest elevation in the pump line as indicated in Figure 20 (HEC-RAS) and Figure 21 (HydroCAD). As a result, the pump flow rate is smaller (Figure 22) due to the higher required static head.

Figure 20
Figure 21
Figure 22
  • Pump Station in HEC-HMS

It is not common, but if needed, a pump station can be modeled in HEC-HMS as an outflow structure of a reservoir (Figure 23 and Figure 24).

Figure 23
Figure 24

The pump curve is provided as an Elevation-Discharge paired data in HEC-HMS (Figure 25). Supply the original manufacturer’s pump curve if a fixed friction loss is entered separately as Equipment Loss (FT); otherwise, use the revised pump curve by subtracting friction losses externally as explained above.

Figure 25
Figure 26

A tailwater condition in the format of an Elevation-Discharge rating curve can be assigned to the reservoir (Figure 27). Under this scenario, the required dynamic head H-total is to be calculated using the higher value of line elevation and tailwater as illustrated in Figure 26.

Figure 27

It should be noted that it is not convenient (you need to define a rating curve!) or even possible sometimes to adequately define a tailwater condition in HEC-HMS. A modeler shall consider using other modeling tools introduced above if the tailwater condition is complex and critical to properly model a pump station.

The above example models of HydroCAD/HEC-RAS/XPSWMM/HEC-HMS can be downloaded here.

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