Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations IDS on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Proper Hydrologic Model to Size Pumps 2

Status
Not open for further replies.

Trackfiend

Civil/Environmental
Jan 10, 2008
128
In sizing pumps, I understand that it is an iterative process between choosing an outflow rate and what the available storage is in the drainage system and sump. My question is, what is the proper method to determine the inflow rate and volume of runoff getting to that sump? I realize that the rational method is meant to produce a peak runoff rate and the NCRS (formerly SCS) method is meant to produce a volume of runoff, so I would assume that the NCRS method controls in this case. However, in digging through certain county and state stormwater guidelines (particularly Texas DOT), I see that the rational method seems to be approved to size pumps. Is this an acceptable method? I've read several of the past threads on avoiding the use of the rational method for volume calculations so I may be answering my own question.

Another question is what duration to use (if using the NCRS method), 3hr, 6hr, or 24hr? Obviously the 24hr will produce more volume than the shorter events. Would judgement as to the type of storms and their durations in that particular region (gulf coast) give any hint as to what duration to use?

What I've done to help expedite some of the work is compile hydrograph information into an excel spreadsheet and then copy/paste this information into SWMM 5.0, connect a sump (with a ponded area equal to the drainage basin), a pump with certain controls, and an outflow. Varying the pump on/off levels gives me a good feel for how much volume is being stored at any given time. My concern is in choosing the right hydrologic method to determine the proper runoff volume. Since garbage in = garbage out, I want to make sure that I am not improperly using a method that will give improbable results. Any comments are welcome.
 
Replies continue below

Recommended for you

rational method is of very little use, a hydrograph is necessary. Choice of storm duration and recurrence interval would be dependant on your location, weather patterns and design objectives.

see chapter 5 of HEC 24, attached below:

 
 http://files.engineering.com/getfile.aspx?folder=e0ce2e42-b794-4f07-b577-3dd739969088&file=hec24.pdf
As cvg says: "Choice of storm duration and recurrence interval would be dependant on your location, weather patterns and design objectives. "

Your choice will also depend in how often you can tolerate hydraulic failure of your pump station during its useful life. Note that the question is not IF it will fail, but how often it is likely to fail over the next 50 or 100 years.

For example, suppose you wish to design your station for a useful life of 50 years. Also suppose that you would like to feel 90 % (0.90) confident that the design inflow will NOT be equaled or exceeded on the next 50 years.

You can then calculate the "return period" (n) of your design storm based on the following:

Px=1-(1-1/n)^x

where Px = the probability of being equaled or exceeded in x years
n= return period of the desired design storm

Since we want Px to be less than or equal to 10% (0.10) we can solve for "n".

In this case, "n" turns out to be 465. ( I'd round this to 500 )

You can do this for any desired confidence level, as long as it is less than 100%.

You will also have to find, through trial and error, the critical storm pattern ( duration )for your geographic area.

If you'd like to see an example of how this might be done go to:


and see

"H131 Regional Detention Pond Design, an Eclectic Approach ...


Sorry, there is no "cookbook answer" to your question.
 
I understand that there is no "cookbook" answer, but would like to start off on the right path, i.e., correct method. The Texas DOT is using the rational method and creating a hydrograph by assigning a time of concentration (utilizing the FAA equation or equal). What I don't understand is, if this is an incorrect method, then why are state agencies allowing it? I'm not arguing for using the rational method to generate hydrographs, but I am merely wanting the verification behind it.

FYI, the pumps are for a temporary installation (2yr). I'm comparing the results between designing against a 10yr event and a 100yr event (what would be the required storage volume if the pumps were to run at "x" outflow).

Thanks for the prompt replies, I've read over a lot of threads and it seems as though both of you are quite knowledgeable of this material.
 
The Rational Method and Variations

About the year 1873 in the United States Emil Kuichling, City Engineer of Rochester, New York proposed the so called "Rational Formula"; Q = CIA, for calculating the peak runoff from an urban drainage basin. He presented his ideas in a paper published in the ASCE Journals along with the data from five test basins he had monitored. His simple equation tries to model the complex series of events which take place during a storm. The method is easy to use, requires only a small amount of data and, for small watersheds, appears to give reasonable answers. It is thought by many to be conservative by overestimating the flow.

This simple equation ( Q=CIA ) uses three variables; namely:

C a guess at the runoff coefficient ( dimensionless) [ probably +/-20%]
I a statistically probable intensity ( inches/hour) [ probably +/-30%]
A an estimate of the drainage area in acres [ probably +/- 5%]

Probable Total Error of Estimate...................................................................+/- 55%

Also, because acres times inches per hour does NOT equal cubic feet per second there is an additional error of about 1 percent which is usually ignored. So, can a guess times a statistic times an estimate yield a certainty ? Not likely At best then, we can only hope that our calculated result will be in error no more than plus or minus 55 percent.

To see if this matters let’s calculate the 10, 25, and 100 year flows for our example

Basin 1 using the Rational Method.
You should get the following values:

Q10 = (0.80)(1.4)(10.3)= 11.5 cfs
Q25 =..(0.80)(1.6)(10.3)= 13.2 cfs
Q100 = (0.80)(2.0)(10.3)= 16.5 cfs

Now, assuming all we want to do is size a storm drain pipe for a minimum self cleaning velocity of 2.5 feet per second using the Q100 just calculated, the process is easy:

Required pipe area = Q100/v= 16.5 ft3/sec / 2.5 ft/sec = 6.60 ft2

This turns out to be a circular pipe having a diameter of 2.9 feet or about 35 inches. The closest commercially available pipe would be 36 inches. So far, so good. But let’s see how confident we can feel about this answer.

If our calculated flow is in error by 55% then it could be as high as 25.6 cfs or as low as 9.1 cfs. This might lead us to select a pipe as large as 48 inches in diameter or as small as 24 inches. For a small project this might be acceptable. For larger projects such differences can be significant., and costly.

Despite it’s obvious drawbacks, the method continues to be widely used today. Sometimes it is misapplied to larger( > 25 Acres ) watersheds for which it has never been demonstrated to be applicable. Some would add even more assumptions and stretch the method beyond believability.


One Variation – The Modified Rational Method

The Rational method predicts only the peak runoff according to the formula: Q=CiA. In order to generate a complete runoff hydrograph, it is assumed that the runoff begins at the start of the storm and increases linearly to the peak value. The peak runoff is sustained until the event duration has elapsed, and then decreases linearly to zero.

The rate at which the hydrograph rises and falls is assumed based on the Tc and a rise/fall factor. For the "standard" Rational method, the rise and fall factors are both

assumed to be one. That is, the rise and fall occur over the exact interval Tc. Other Variations of the Rational method (often called the Modified Rational method), may assume different rise and fall factors.

When using the Rational method, the correct intensity and duration must be specified. If an IDF file is defined, the intensity may be calculated for each duration. An IDF file also allows use of the duration analysis report, which simplifies the process of determining the critical duration at each node.

Since a hydrograph produced by the Rational method does not reflect the total runoff or the intensity variations of a real storm, many do NOT recommend it for the design and analysis of detention ponds. It is strongly advised that the SCS-UH or SBUH methods be used when pond routing calculations will be performed.

Despite all of this, many regulatory agencies REQUIRE the Modified Rational Method be used.










































 
Status
Not open for further replies.

Part and Inventory Search

Sponsor