SWMM "Time of Concentration"

[MapEngr]

We have been committed to using SWMM to perform a flood study for a 2.26 square mile urban watershed. The watershed has been divided in to 26 subcatchments.

It appears SWMM assumes overland flow to determine a "time of concentration" for a subcatchment (from what I can gather, the equation used was developed from a kinematic wave analysis).

With that, I am not entirely comfortable with assuming overland flow in each subcatchment. Although some subcatchments are moderately developed with little drainage infrastructure, others are significantly developed with drainage infrastructure, curbing, et cetera. It is apparent (at least to me) that overland flow in certain subcatchments would be minimal.

I should mention that flow metering data is available for this watershed. Therefore, I do have the ability to adjust the length parameter in the time of concentration equation until a good fit exists between the observed and simulated hydrographs. However, flow metering is not available at the outlet of each subcatchment meaning I could not adjust the length parameter for each subcatchment.

As there are no other options available in SWMM to model a time of concentration, I am looking for some reasoning/verification/etc. behind SWMM's methodology for assuming overland flow.

Thanks in advance!!

 

[tobeace]

It's standard in the engineering world to use overland flow no more than 25% of total. This isn't published very often, because the government doesn't like to state what's not in their favor.



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[ptmoss]

I've never used SWMM so I don't know if it's the primary software typically used for this application (I also haven't modeled such a large urban watershed, only rural). But since time of concentration is such a critical component of the various modeling programs I've used over the years, have you considered using a different program?

 

[mricks]

We currently are using XPSWMM and from what I remember SWMM is based on TR20. A condensed version of TR20 is TR55 (several key assumptions are made and applied). This is typically what we use to calculate Tc. Sheet flow (overland) is restricted to a maximum of 100 ft in TR55. The flow is then classified as shallow concentrated flow. If you work through section 3 in TR55, you can vary the Tc for each individual watershed regardless of whether there is drainage infrastructure or not.

 

[MapEngr]

Thanks everyone for the insight.

I have completed further research on the topic, and it appears that SWMM does not calculate a time of concentration. The hydrograph is developed assuming each subcatchment is a non-linear reservior. Flow discharging from the "outlet" of the reservoir (if you will) represents the hydrograph of the subcatchment.