Sheet Flow and Curb/Gutter Flow

[RFreund]

I'm slightly confused at how the time of concentration and flow should be calculated when a curb/gutter is combined with sheet flow.
For example say you have 150 ft x 400 ft parking lot that is sloped to a curb on one side this slope Sx=0.03ft/ft and the slope of the curb/gutter, SL=0.003ft/ft. I would assume the time of concentration is equal to the sheet flow time over 135ft to get to the gutter plus the time from the gutter to the inlet. However I'm not sure how to use the appropriate equations for gutter flow in conjunction with sheet flow to arrive at an answer.
Or the if I use the time of concentration for sheet flow to solve for my flow into the channel (Q=CIA where I is based on Tc for sheet flow). However then I'm confused as to how I would find the different Flow rates at the begining and end of the channel.

See attached.
How do you calc spread T1 and T2 to find average velocity?

Thanks again

EIT

 

[francesca]

Tc is the time for the longest path, which would be from the opposite corner to the inlet, depending on the slope. Look in any DOT's drainage manual to see maximum sheet flow lengths on asphalt and for gutter flow calculation details.

 

[bencraddock]

Generally when you're doing Tc calcs you have to make some initial assumptions about the flow characterics for the gutter or pipe flow segments. Since you don't have a Q yet (that's why you need the Tc!) you can't nail down the cross sectional area of flow in the gutter which you need to get the velocity and time of flow. So I'll often make an assumption (say the flow is up to half the curb or the pipe is flowing full). You need to keep your assumptions consistent if you're comparing pre and post and often I'll check other assumptions (flow at the top of the curb or pipe flowing half full) to see how sensitive the calculated Tc is to changes in the assumption. Once you get the Tc and calculate a Q, you could go back and check your Tc assumption, but most engineers that I know don't go to that length.