Section 3: Pump Station Hydrology

Methods for Design

In order to design a pump station effectively, the inflow hydrology must be known. The hydrology developed for the associated storm drain system usually will not serve as a firm basis for discharge determination into the pump station. A hydrograph is required because the time component is critical in understanding the inflow which governs the sizing of the wet well. The designer needs to know not only the peak inflow, but the timing and volume. The difference between the input and the output hydrographs is the storage requirement of the pump station wet well. The hydrograph should consider the storage abilities of the storm drain system, which may reduce the required size of the wet well. Governmental regulations or the physical limitations of the receiving waters determine the output discharge from the pump station.
The storm drain system associated with the pump station may have a design basis of less than 2% AEP. However, TxDOT recommends at least a 2% AEP flood design because the pump station is generally used when drainage by gravity from a low point is inadequate or impractical.

Procedure to Determine Mass Inflow

A mass inflow curve represents the cumulative inflow volume with respect to time. In order to determine a mass inflow curve, the hydraulic designer must first develop an inflow hydrograph based on a design storm. The most typical design method is the
NRCS Dimensionless Unit Hydrograph
, discussed in detail in Chapter 4. For the following procedure taken from FHWA Hydraulic Engineering Circular 24 (
HEC-24
) example, the hydrograph data in
Table 4-31
of this manual will be used.
  1. Evaluate the time base of the hydrograph and select a time increment, usually the same time increment as that used for developing the inflow hydrograph.
  2. Develop a table with columns for time, time increment, inflow rate, average inflow rate, incremental inflow rate, cumulative inflow volume, cumulative outflow volume, and storage difference as shown in Table 11-1.
  3. At each time step, extract the inflow rate from the computed inflow hydrograph. (For this example, use
    Table 4-31
    , column Q
    u
    ).
  4. Compute and tabulate the average inflow rate as half of the current and of the previous inflow rates for each time step. (i.e. time step 30: 188/2 cfs + 350/2 cfs = 269 cfs).
  5. Compute the incremental volume for each time step as the average inflow rate multiplied by the time step in seconds.
  6. Compute the cumulative inflow as the sum of each time step and the previous time step.
  7. Plot a curve of cumulative volume versus time. The result is a mass inflow curve, shown as
    Figure 11-1
    .
  8. Determine the allowable discharge to the receiving waters. The pump flow rate must be at or below the allowable discharge rate. For this example, assume the allowable discharge rate is 100 cfs. Notice that the pumping did not start until a sufficient volume was in the wet well.
  9. Multiply the allowable discharge by the time step for the pump flow. Notice that the pumping cannot start until the inflow has developed. The greatest difference between inflow and pump flow is the required storage of the facility. The greatest difference in this example is at time step 80, which is about 691,200 cubic feet. The negative numbers at time steps 230 and 240 indicate that regular pumping should have stopped at about time step 220. The Pump Flow line is also plotted with the inflow curve in
    Figure 11-1
    .
Table 11-1: Mass Inflow Computation Table
1
2
3
4
5
6
7
8
Time (minutes)
Time Increment (seconds)
Inflow Rate Q
u
 (cfs)
Average Inflow (cfs)
Incremental Inflow (cubic feet)
Cumulative Inflow (cubic feet)
Cumulative Outflow (pump flow in cubic feet)
Storage Difference (cubic feet)
0
0
0
0
0
0
0
10
600
58
29.0
17,400
17,400
0
17,400
20
600
188
123.0
73,800
91,200
60,000
31,200
30
600
350
269.0
161,400
252,600
120,000
132,600
40
600
400
375.0
225,000
477,600
180,000
297,600
50
600
358
379.0
227,400
705,000
240,000
465,000
60
600
272
315.0
189,000
894,000
300,000
594,000
70
600
170
221.0
132,600
1,026,600
360,000
666,600
80
600
112
141.0
84,600
1,111,200
420,000
691,200
90
600
77
94.5
56,700
1,167,900
480,000
687,900
100
600
51
64.0
38,400
1,206,300
540,000
666,300
110
600
34
42.5
25,500
1,231,800
600,000
631,800
120
600
22
28.0
16,800
1,248,600
660,000
588,600
130
600
15
18.5
11,100
1,259,700
720,000
539,700
140
600
10
12.5
7,500
1,267,200
780,000
487,200
150
600
7
8.5
5,100
1,272,300
840,000
432,300
160
600
4
5.5
3,300
1,275,600
900,000
375,600
170
600
3
3.5
2,100
1,277,700
960,000
317,700
180
600
2
2.5
1,500
1,279,200
1,020,000
259,200
190
600
1
1.5
900
1,280,100
1,080,000
200,100
200
600
0
0.5
300
1,280,400
1,140,000
140,400
210
600
0
0.0
0
1,280,400
1,200,000
80,400
220
600
0
0.0
0
1,280,400
1,260,000
20,400
230
600
0
0.0
0
1,280,400
1,320,000
-39,600
240
600
0
0.0
0
1,280,400
1,380,000
-99,600
Inflow versus Pump Flow (click in image to see full-size image)
Figure 11-1. Inflow versus Pump Flow