Detention Basin Using Rational 2

[jeb6294]

Not sure if my brain hasn't started working or what (I just got back from vacation)....

How can I determine what size detention basin I need with a calculated Qin and a specified Qout using the rational method. I can do it using TR55, but all my other calcs are based on rational so I'd rather not switch back and forth.

i.e. I have detention basin with ~25 cfs coming into it and I want to limit the discharge to, let's say 10 cfs irregardless of what the governing body's requirements are (the basin is going to be on a ridge so really there is no predeveloped runoff).

 

[ccor]

The FAA outlined a procedure in "Airport Drainage," 1970.

http://www.airweb.faa.gov/Regulator...f26314836c9a516a86256c750060f2fe?OpenDocument

Easy to code into a spreadsheet, and pretty good results.

 

[RWF7437]

You can't size a detention basin using only the Rational Method. There are some modifications to the method which can be used; e.g Los Angeles County ( sse below ).

But! If the basin is on a ridge how does runoff reach it ? Do you pump 25 cfs ( 12,000 gpm !) ?

Here is the method as described by Los Angeles County:


"MODRAT is a modified rational method computer program developed by the Los Angeles County Department of Public Works (LACDPW) to compute runoff rates under a variety of conditions common to the area of Los Angeles, California…….. MODRAT may be used to find flow rates for any watershed with any combination of existing or proposed channels and drains. Further, the watershed may be undeveloped, partially developed, or completely developed. The model will compute runoff rates for any frequency design storm (storm patterns developed by LACDPW), as well as any other storm which can be represented by a rainfall mass curve. Given any combination of the above variables, MODRAT will compute a hydrographs (sic) for each subarea and mainline or lateral collection point in the watershed……

"Modifications to the Rational Method in MODRAT

"As a method of urban hydrology, the rational method falls short in several ways. First, the method does not produce a hydrograph, only a single flow rate. Second, the rational method does not account for changing (time dependent) conditions such as soil condition or rainfall intensity. Finally, results are not very accurate for large areas. Due to these problems, MODRAT contains the following modifications:

* Rainfall intensity, i, is a variable dependent on rainfall frequency, storm time, and time of concentration. The variation of i is represented by a temporal distribution curve (rainfall mass curve).
* C, the runoff coefficient, varies with soil type, rainfall intensity, and imperviousness.
* The time variation of C and i allow the flow, Q, to vary with time, thus producing a hydrograph. The area under the hydrograph represents the total volume of flow from a watershed, a variable which the rational method does not provide.
* Hydrographs may be computed for a number of subareas, for each lateral to the main channel, and for each collection point on the main channel. These hydrographs are routed and combined as computation progresses downstream.

"The above modifications to the rational method allowed (sic) for the computation of storm hydrographs for any size watershed. With such improvements, the modified rational method (MODRAT) has been adopted by LACDPW as the preferred method of hydrologic analysis."

 

[jeb6294]

"But! If the basin is on a ridge how does runoff reach it ? Do you pump 25 cfs ( 12,000 gpm !) ?"

Part of the development we're designing is above this basin so once the stormwater system is in place it will drain to the basin. Right now they are calling for it to be placed on a ridge between two existing drainage features. This basin will then dump into another basin that is in one of these drainage features.

 

[blueoak]

They need to stop making hydrology programs without some pre-test. I doubt you will kill anyone, but you are really outside your expertise.

I never understood LA for trying to make the rational more than just a guess with a correction factor. Why not use a standard, real hyetograph to hydrograph producing method and not have to deal with the rational at all. To still allow it with modern computers seems just stubborn.

 

[RWF7437]

JEB6294:

Still noy quite sure how this is all supposed to work. "Drainage feature" is a term I've never heard before in 46 years of practice. That aside, if your design flow is 25 cfs you probably have a fairly large drainage basin area. If it is larger than 20 or 30 acres you should probably not be using the Rational Method at all. For larger basins TR-55, NFF, local regression equations or other Unit Hydrograph methods are your best available choices.

blueoak:

Its not clear who your complaints are addressed to. I believe, however that LA County has made some effort to calibrate their ModRat method using hyetographs from actual historical storm events. In my experience using it, it appears to yield peak flows and volumes that are comparable to the Santa Barbara Urban Hydrograph and the SCS-484 hydrograph, but slightly higher. Calibrating hydrology models is difficult and expensive but some jurisdictions do it and it provides a level of confidence in the calculated results. I think the City of Denver is an example of that.

Computers or no, we all start with rainfall data that is no more accurate than plus or minus 30% at best.

What is a "standard real hyeotograph to hydrograph method" ?

 

[cvg]

rwf7437
you are correct that LA has calibrated their model - but it calibrated to give acceptable results in California - possibly not in other regions, unless they are hydrologically similar. So, to take this model and apply it to a watershed in Florida for example might not generate acceptable results. I agree that a rainfall runoff model which generates a hydrograph would probably be a better approach.

 

[blueoak]

RWF7437,
My concern with altering the rational equation is that instead of more accurately modeling the different physical aspects of a runoff event, you are taking an equation that lumps a lot of the processes together and trying to calibrate that.

The rational is a quick method to find a peak discharge, whether it is used appropriately or not. With modern computers, why not look at calibrating your rainfall, loss, watershed transforms and other aspects separately. Your point on the investment cost is well made and I highly respect the work by Denver's flood commission for hydrology, stormwater quality, and stream protection.

I think your suggestions to jeb6294 are extremely useful. However, I doubt any comparisons to NFF or local experience will be performed. The intro remark implies that running TR-55 and the rational is too much work. A frequent event (100 yr) in an urban area is a real danger. I like using this forum to get help as you have given, but jeb6294's intro remark raises concerns that the real impact of design decisions is not being considered.

I understand your 30% remark on rainfall and all dikes fail during the 0.98% event. However, too many engineers use that uncertainty to call hydrology voodoo and then justify using anything and calling it good. As a new engineer(<<46 years experience), I hope to maintain the profession earlier engineers built. And I get concerned when it appears that the public welfare is overlooked by poor engineering decisions.

 

[RWF7437]

Hello Blueoak and CVG,

Perhaps I've misled you both. The LA County method can be used almost anywhere. It is the method which is important, not the specific software developed by the County. That progran is only directly applicable in the County and I believe the County is NOT offering it for sale but only using it internally.

The method requires only a few, easily obtained bits of information and may be "programmed" in Excel with very little effort. The County's curves of C vs I depend pimarily on land use, cover and soils. Soils may be characterized simply by identifying to which of the 4 common hydrologic soil groups it belongs. Once you know that, and the land use ( e.g. Single Family Residential- 1/4 acre lots)you are ready to compute the peak flow and volume under the hydrograph. See your local SCS Soil Survey. These curves can probably be used from Maine to Florida to Alaska.

You also need a rainfall pattern for a specific storm. Usually this is chosen from a local storm event which caused significant flooding in your area. Data for such storms is available from NOAA or your local Weather Service Office. It is easiest to convert this data to a dimensionless mass curve. Doing so will allow you to analyze storms of any duration, frequency or depth. Rainfall depths are available from NOAA's website or from their many published maps.

Using the method is as easy as usng HydroCad or SMADA or SWMM. Easier, actually, once you've setup the spreadsheet.

I have used the method several times and it provides answers comparable to TR-55 and SBUH.

Finally, the missng link I find in nearly all local regulations is CALIBRATION. No model can be better than the data used to verify that is giving reasonable answers. But many agencies would rather spend their time, and waste mine, arguing about whose arbitrary assumptions are better. If that time were spent setting out stream and rain gages and collecting data from actual storms we wouldn't need to spend so much time in such fruitless discussions. Then we would have real would data to base our design decisions on.

Sorry about the rant.

In answer to JEB6294's original question:
"
No, you can't size a detention basin using ONLY the Rational Method.

good luck