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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.

RE: Time of Concetration

Detention/retention is designed for a specific return interval storm.  Use the time for the design storm; an estimate of overland flow time can be calculated and the total will probably be similar,(120% to 200%), to the 25 year time.

RE: Time of Concetration

Remember, these are models and many approximations are made in an attempt to come up with a "scenario" that will encompass natural events which will NEVER look exactly like synthetic models.

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

I always compute the Tc in the following manner:

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

(OP)
So your Tc for your pond would be about what the final Tc for the storm sewer system is?

RE: Time of Concetration

The Tc is for the basin inflow hydrograph (in this case, the storm sewer directing flow to the basin).  There is no 'pond' Tc.  Rather, the inflow hydrograph gets routed through the pond, which, depending upon the basin control configuration, determines the basin discharge rates and draw-down time.

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