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time of concentration

time of concentration

time of concentration

when doing a storm water study for a subdivision, how do you know when to stop the length of travel for a water course when calculating the time of concentration. If I am able to collect all runoff at a single pond located in the project. Do I use the travel length from the furthest point in the subdivision i.e. accross the lawn, down the gutter, into a catch basin, through the storm drain, and finally into the detention pond where the final outfall is located?  Or is the length just to the first catch basin where it is possable for water to back up and become detained because of the restriction plate/orifice being used at the pond outfall.  It makes a big difference in the runoff volume when using shorter travel lengths within the sub catch basins of the subdivision.  I would appreciate all coments on how other people calculate the travel length.
Thanks in advance  

RE: time of concentration

Use the longest flowpath for both pre and post, in your case it is the first scenario...I am confused, though; how does the detention volume (in your second scenario) in your pond back the whole way up to the first inlet in the pipe system?  If that is happening, make sure you are not flooding inlets in larger storms, or at least that you have provided an alternate emergency flowpath away from structures and utilities.

RE: time of concentration

As a general rule, the flow length should extend from the most hydologically remote point of the subcachment to the next node on the diagram.  A shorter path would ignore some of the flow path, while a longer path would double-count some of the path.

This rule still leaves some discretion in how you construct the model, which will depend on the objectives of your analysis.  For example, if the subcatchemnt drains into a ditch, the ditch could modeled as part of the subcat flow patch, or as a separate reach.  But the Tc flow path should generally end when the subcatchment meets the next node.

RE: time of concentration

Sometimes I'll model everything as one large subcatchment with a single outlet point (i.e. pond, stream etc.).  Swales, culverts etc. would then just be part of the Tc calculation.  Other projects I need to justify sizes of every ditch and cross-drain culvert to some reviewing authority so I have individual subcatchments above all these structures and they all become modeled as individual reaches instead of all lumped into a single Tc.  There shouldn't be an appreciable difference in the runoff at your ultimate outlet point using either approach.  The key is, when using the single large subcatchment, determining that most hydraulically remote point and flow path.

RE: time of concentration

There are usually several valid ways to model any given site.  As the last post indicates, it all depends on the objectives of your analysis.  Your model might consist of just one subcat and a pond (for a basic pond design) or a complex routing diagram (if you're trying to analyze other aspects of the system.)  In general, use the least complex network that will do the job.

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