Tc pipe segments as well as Modeling as No-Storage ponds? 3

[ctbailey]

Hi all,

I'm having a senior moment. I can't recall the BestPractice with regards to Tc segments through a subcatchment.

I prefer to model catchbasins, drain manholes and the piping using insignificant-storage ponds.

But... should these pipe segments be included as Tc segments as well?
Or does the fact that the ponds are included in the routing take care of the time?

Thanks

Craig

___
Craig T. Bailey, PE, LLS

http://www.bailey-associates.com

 

[TerryScan]

It all depends on how one is configuring your hydrology & hydraulics model. If a pipe segment is part of the Tc (hydrology) it should not be modeled in the hydraulics (that would be accounting for it twice). If one models the pipe segment in the hydraulics, the Tc should end before entering the pipe segment.

 

[ctbailey]

TerryScan I believe I now understand your answer. Thank you

If the pipe segments WERE NOT included as Tc segments, the subcatchment would need to be broken up into a smaller subcatchment that is draining towards the inlet of an insignificant storage pond. This way the hydrology model would not "see" the flow in the pipe, but the hydraulics model would be able to report on flow values.

Since the hyraulics in the closed drainage system are more of interest in my current case, I will break the subcatchments into per-catchbasin sub-subcatchements, and route the primary outflow from insignificant ponds to the next lower pond. Each sub-subcatchment will flow the hydrology (overland) into the next lower insignificant pond.

in that way, Tc segments are the simple overland flow segments and need not be concerned with any Tc pipe segments.

My problem is when we are to model an entire closed drainage system with ponds... there are instances where these "sub-subcatchment" could be as small as one half-lane, the length being the distance between catch basins. Surely that approach (tiny tiny subcatchments) is not valid when applying classic SCS methods such as TR-55 and TR-20?

___
Craig T. Bailey, PE, LLS

http://www.bailey-associates.com

 

[psmart]

Tiny subcatchments are not a problem. As the Tc goes to zero your runoff will approximate the precipitation excess, with essentially no time delay between the precipitation and the runoff.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TerryScan]

Craig,

As with a multitude of things we face in engineering, there are "many ways to skin the cat." The typical workflow I have used in over 25 years of experience is this:
Pond level Hydrology & Hydraulics (H&H) is modeled using SCS methods. Most closed drainage systems are not specifically modeled and included in Tc paths. The exception would be drainage systems which connect ponds or are part of pond outfalls.
All other Closed Drainage systems are modeled using FWHA /Rational Methods analysis of flows, gutter spreads, inlet capture & bypass, and pipe capacity.

 

[beej67]

Terry's biprocedural method of cat skinning is the method to which I adhere, and most other professionals I've encountered do it similarly.

One thing I sometimes do, is use the calculated Tc from the proposed pipe network, as reflected in the rational analysis, as my Tc for my single node SCS pond analysis.

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

http://www.campbellcivil.com