Time of Concentration, Modified Rational Method, and Detention Basin Routing

[MrEase]

Hello, first time poster here so forgive me if these questions have been addressed somewhere else in the forums as I have not been able to locate them through the advanced search feature. Thank you for taking the time to read and respond to my post.

My background is 20 months as an EIT. In my current role, I work mostly with site design and more specifically with stormwater management design. Up until recently I had a fairly knowledgeable PE who specialized in hydrology to work with but they have recently taken a position at another company and thus I am at it learning on my own to the best of my abilities. The project managers I work with are familiar with stormwater design but they each have different preferences with respect to calculation methods.

My first question is which Time of Concentration is correct to use for post-developed drainage areas. Should there be one single time of concentration (and hydrograph) applied across the entire area tributary to a detention basin? Or should there be hydrographs for each individual inlet's tributary area with their own individual time of concentration? Then combine all of these hydrographs and then route them through the outfall structure/pond? I have submitted projects this way to municipalities and the plans have been approved in the past. If I remember correctly, the City of Tulsa requires this. On one hand it seems incorrect to assume you have an intensity of say 6 in/hr on one part of a parking lot and then 20 feet west you have a sub basin with an intensity of 4 in/hr. On the other hand, I would think by combining all of these hydrographs before routing into the outfall structure, you would have a combined hydrograph that represents the varying peaks for each inlet tributary area (since in reality if a cloud starts dumping rain over a site, it will take different duration for the each inlets contribution to make it to the outfall structure). This issue I have is with the intensities being different based on the time of concentration. Using several smaller Times of Concentration pushes up the peak flows but overall would reduce the volumes. So I'm conflicted on which is the "correct" approach.

On another note, when I'm designing inlets or pipes I would naturally have several individual times of concentrations (each stopping at the inlet).

My next question is what is the "correct" way to design detention using the Modified Rational Method? Specifically when the municipality asks to design for say the 20-year, 20 minute storm (with emphasis 20-minutes is the minimum storm duration allowed)... The way I've been told to do this in the past has been to enter the time of concentration as usual but then set the Storm Duration Factor to a multiple of the time of concentration that makes the storm duration 20 minutes long. For example, lets say I've got a time of concentration of 4 minutes and the city is asking for a 20 minute design storm. I would input a Storm Duration Factor of 5.0 since 4 minutes (ToC) x 5.0 (SDF) = 20-minute design storm. The hydrograph looks correct but this pushes the peak flows down and the volumes up. The other issue I have with this is when I use Autodesk Hydrographs Express tool, there is a feature where you can allow the program to determine the Storm Duration Factor that produces the largest detention volumes. There have been instances where I've used this tool to check which SDF produces the critical volume and it has turned out to be less than a 20-minute storm duration. Maybe the city engineer could be convinced that the 20-minute minimum storm is not the most conservative design storm?

As far as detention basin routing, what do you do personally and why? Do you use a single time of concentration and use the area tributary to the basin to generate one post-development hydrograph for routing? OR do you start with hydrographs for each individual inlet or bypass and use several different times of concentration based on the longest duration route from the inlet tributary area to the outfall structure?

Any advice or discussion is greatly appreciated.

 

[psmart]

I would go with your first alternative: Generate a separate hydrograph for each subarea based on it's Tc but using a common rainfall distribution for all subareas. Then combine and route the hydrographs through the drainage system.

This avoids the many patched-up procedures that have been developed in trying to apply the Rational method to all but the simplest pipe-sizing applications.

PS: There are actually lots of threads about the use (and mis-use) of the Rational method. It's an ongoing issue.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[MrEase]

Thanks for your response Peter.

Just to clarify, by "using a common rainfall distribution" you mean each of the subareas will have different intensities (based on the ToC), as long as all intensities are based off the same IDF curves (assuming this is what you mean by rainfall distribution)?

I do agree with you on the overuse of the rational method. I'm part of a professional networking organization in my area and I've had this discussion with several of my colleagues. Most agree that the rational method is good for issues that are peak flow dependent such as pipe capacity and inlet capacity whereas SCS is really the go to for volume sensitive calculations such as detention design. But that's the thing about being an EIT... When the PE you're working under wants you to use the Rational or Modified Rational Method for detention design and the code allows it, you put your head down and do it. This is really why I'm trying to figure out what the generally accepted practices are when the Modified Rational Method is what is requested.

I will take another look around the forums to see what I missed. Thanks for the heads up.

 

[psmart]

Each subarea will have it's own Tc. That's a physical characteristic of the subarea which remains fixed. Since we're assuming a spacialy uniform rainfall event, all the subareas will be evaluated with the same rainfall distribution, such as a 25-year Type II storm.

This is based on the use of the SCS/NRCS runoff procedure, in which a single design storm includes events of all durations, so you avoid the duration selection issue that plagues the Rational Method.

The Tc's are fixed, as is the rainfall applied to all subareas.


Peter Smart
HydroCAD Software

http://www.hydrocad.net