Regarding Time of Concentration using TR55 4

[philosophy]

I am recently working in drainage studies for residential and commercial sites Within Napa Valley, California. TR 55 computes the time of travel as the summation of three main components: Sheet Flow, Shallow concentrated flow and channel flow. However, in some of the hand book, overland flow or gutter flow are also introduced. These conceptions really make me confuse. Besides, it makes me confused while choosing the flow length for each term. Is there anybody who can give clear idea about these stuff. Also, how the flow lengths should be separated for sheet flow, shallow concentrated and channel flow.
Further, it is really futile assumption to have channel flow in underdeveloped areas, how we can identify the size and shape of channel if there is none of the artificial channel within the area.

 

[TerryScan]

Responding to your first point of confusion, "overland flow" usually means the same as "sheet flow". To me, gutter flow may be shallow concentrated if it is near the peak of a road and has not collected much runoff, otherwise it is usually channel flow.

One reference you may check out is NRCS Hydrology Technical Note No. N4 "Time of Concentration". If you search this forum, you may find other good references from past threads.

Technote N4 begins with Overland Flow "This includes thin sheet flow over plane surfaces and nonconverging flows over rilled and irregular surfaces."..."The range of mean depth is 0.002' for paved areas to 0.02' for vegetated areas."

So, if you imagine (or better yet, observe) runoff during a storm event, after some distance of sheet flow, water will start to concentrate into small "channels" but not really be in a defined channel. It could be areas between clumps of vegetation or poorly defined "valleys" in the terrain. These segments of time of concentration are computed using "shallow concentrated" flow. We know it is not sheet, and there is not a defined channel to compute channel flow.

Even when there is not an absolutely defined stream or channel in undeveloped conditions, we know that, given enough watershed area and runoff, flow will channelize in whatever "valley" features exist. In these situations, I can usually define a parablolic channel using the contours.

 

[RWF7437]

In addition to the excellent suggestions provided by PSmart, try this:

Calculate Tc by several methods. There are at least 7 or 8 such methods.

Then,using whichever result you think is most realistic, double Tc and take Tc divided by 2. Then calculate the peak flow and volume using each of the three values of Tc you now have.

How much difference do these three values make in your answers ?

How confident are you in the precipitation data you have used ?

How confident do you want to be in your final "design", whatever that is ?

No need to tell me; answers these questions for yourself.

good luck

 

[RWF7437]

Sorry TerryScan,

I meant to credit you for the excellent suggestions.

Russ

 

[psmart]

I didn't think I'd replied to that one...

 

[RWF7437]

Also visit HydraFlow's website and Help screen. They have FAQs and will respond to specific questions if given enough information.

good luck