comparing pre-dev calcs using Rational when post-dev used NRCS

[willmark]

I am preparing a drainage report for a client that is having flooding problems. His site existed prior to the development of an adjacent property. A broad swale previously flowed onto my clients property from undeveloped land. The land was developed and the stormwater from that development dicharges directly to my clients parking lot (no easement or any type of agreements)through two 6" pipes (side by side with matching inverts). Durring a large storm, water shoots out of the pipes like a fire hose and floods a large portion of his parking area (we have pictures showing this in action). He has recently renovated his building and had to treat for mold problems, as the flood water reaches his building. I have designed improvements on his site to solve the problem (not yet constructed) and am now generating a report to detail the cause of damage. Several Major "errors" were found on the plans for the adjacent property- apparrent modification of the existing grades to show that a much greater area flowed onto my clients site in the pre-developed condition is just one of the many crazy things.

The locality requires 50/10 detention- (i.e. detain 50yr post back to 10yr pre). The calcs on the plans for the adjacent development shows a computer output (program not listed or known) that used NRCS (TR-20) to generate the 50yr post developed peak flow from the pond. Setting aside the errors I noted in the data on these plans..... The only data shown for the 10-yr pre-developed flow is a small handwritten area on the plan stating a C-value, Tc (coorisponding I value), and site area. The Q-10 is listed below these values, and is simply Q=cia (i.e.-Rational Method)

My question is this- is it appropriate to compare the peak discharge from a pond that was calculated using TR-20 with a pre-development peak discharge from the site that was calculated using the Rational Method? The pre-developed peak is simply that- a peak flow from a site without regard to any "volume" or durration... its just that when preparing pre vs post calcs, I normally use the same method- as one value is being compared to the other (apples to apples sort of thing)- since the post is routed through a pond, the Rational Method isn't used at all- I typically use the NRCS method- which is accepted for all the surrounding localities.

 

[psmart]

As your question implies, stormwater comparisons need to use the same method. Since the NRCS and Rational methods will give different results even under identical conditions, comparing the results for different conditions is next to meaningless.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

hydrologists often use different methods and compare results. Our flood control district even requires that hydrology be checked to see if it is reasonable by using one or more different methods to check it. These may include envelope curves, regression equations or even rational method. Especially when making a lot of assumptions in models with many parameters, a lumped parameter model may provide a "gut check" for your detailed model. However, I agree that to use one method for pre- and another for post-development is not standard practice and will generally give poor results.

 

[willmark]

I'm glad to see these responses. It was one of those things you're fairly sure about, however, you just need to hear it from someone else to be confident. I'll often use different methods to check and make sure my results are reasonable(as cvg noted), however, I had just never seen someone compare pre and post using different methods before.... unfortunately there are lots of things within this set of plans I'm reviewing that I've never seen before... Thanks guys

 

[beej67]

Basin swapping like that is flat illegal in some areas of the country, and basically ignored in others as long as the peak flows match at a study point. And sometimes both - illegal by the law and ignored by the regulatory body. Some research will help. I presume you guys have an attorney signed on for this, since mold abatement ain't cheap.

Make sure you hold their feet to the fire on their allowables. One thing in particular that stands out, is rational Cs for undeveloped areas are often realistically down around 0.1 or 0.2, but municipalities often require a minimum of no lower than 0.3 or 0.35 for pipe design, "to be conservative." The catch is, what's conservative for pipes may have turned into an allowable discharge rate that's too "liberal" (?) for your case.

Other things to check, when bad engineers do hydrology, are whether they monkeyed with their Tcs to make their analysis work, whether they monkeyed with the initial abstraction value behind the scenes without telling anyone, and whether they monkeyed with the shape factor.

If the regulatory body let them get away with mixing methods like this, then they may hold some liability as well. Ask the attorney, it may be worth it to name them in the case.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com