Time of Concetration
Time of Concetration
(OP)
I have always been a bit confused as to how to develop a proposed Time of Concetration in which to model detention. Most of the work my firm does are small (~5 acres commercial sites). Most time of concetration models assume all water is channeled directly to the pond. For instance, TR-55 assumes sheet flow, shallow concentrated, and channelized. This is easy to figue in an undeveloped state.
However, in a proposed commercial site, water will sheet flow to the storm sewer. The inlets and pipe are generally designed for the 10 year event. In the 100 year event, water will pond above the inlets and flow to the detention basin through overland flow routing. So to get a time of concetraion for the 100 year event is almost impossible. Does it make sence to use the final time of the 10 year storm sewer to model the detention?
I guess, I usually see engineers state that they used the TR-55, LAG, or some other equation to get a Time of Concetration to model their detention. But in a developed state, I do not see how that is possible.
However, in a proposed commercial site, water will sheet flow to the storm sewer. The inlets and pipe are generally designed for the 10 year event. In the 100 year event, water will pond above the inlets and flow to the detention basin through overland flow routing. So to get a time of concetraion for the 100 year event is almost impossible. Does it make sence to use the final time of the 10 year storm sewer to model the detention?
I guess, I usually see engineers state that they used the TR-55, LAG, or some other equation to get a Time of Concetration to model their detention. But in a developed state, I do not see how that is possible.
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RE: Time of Concetration
RE: Time of Concetration
You state that water will pond above all inlets in the 100 year event. This will not happen the instant rainfall starts. Tc will be valid until the flooding does occur.
If you are truly concerned about the effects of ponding in the paved areas, you should model them accordingly. In the work I have done on similar sites, I am not concerned about water ponding on the paving during a 100 yr event and it probably has little effect on a small site. On a "big box" store site, with vast parking areas, the ponding may provide storage and the stormater facility is oversized if the storage is ignored (chalk it up as a safety factor?)
RE: Time of Concetration
Leg 1: Sheet flow across lawn/landscaping for 50 ft or so (or the actual distance, if less) with a Manning's n value of 0.15
Leg 2: Shallow concentrated flow over lawn/landscaping (unpaved) surface, if applicable
Leg 3: Shallow concentrated flow across the parking lot paving to the storm sewer inlet
Leg 4: Channelized flow within the storm sewer until it reaches the basin
Once you reach the basin - Tc ends
RE: Time of Concetration
RE: Time of Concetration