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Tc channel segment = depth of 2yr flow???

Tc channel segment = depth of 2yr flow???

Tc channel segment = depth of 2yr flow???

(OP)
Hello.  Hydrocad 8.5 user here.

I recently have submitted a storm water runoff analysis/design to a local reviewing agency.

during their review of my work, a review comment was included, directing me to adjust my analysis.

The comment was:  "you must change the flow depth of the channel Tc segments to get the computed discharge of the segment to approximately equal the 2yr discharge from the subcatchment."

The particular segment in my Tc calculation he is referring to is a roadside ditch, that I am including as a portion of my Tc calculation for a subcatchment.

I intend to comply with the reviewer, but I do not understand what this practice of adjusting channel Tc segment depths of flow accomplishes.

As soon as I change the calculation from the 2-yr event to a larger storm, the capacity of that Tc channel segment is "held" as the 2-yr flow.  When I say "capacity" I am refering to the node report.  

What is the theory behind this request?  If I have some idea in my head before I ask the reviewer, I won't feel so behind the 8-ball.

I will attach a simple file that shows the channel Tc segment that I am talking about.

___
Craig T. Bailey, PE
www.bailey-associates.com

RE: Tc channel segment = depth of 2yr flow???

The travel time through a segment of channel flow is calculated based on the expected flow depth, rather than the overall channel depth.  (I believe the entry field says FLOW depth, not channel depth.)

Since the 2-year event is obviously the lowest flow, it will produce the longest travel time.  This also tends to approximate the low-flow conditions during the initial hours of the larger storms.  But in many cases, the velocity won't change much for the larger events.  (Try it and find out!)

Of course, the segment of channel flow is usually a small fraction of the total Tc, so small variations in the travel time will have even less effect on the total Tc.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Tc channel segment = depth of 2yr flow???

CT,

The accepted practice in SCS runoff computations is to calculate Tc based on the 2year flow.  This is somewhat of a compromise for simplicity's sake.  

In the real world of course, the flow in each segment of the Tc path will vary constantly and thus the Tc will vary constantly even during a particular storm event.

But of course, this isn't the real world it is a model.  One could have picked from among various events as the basis for the Tc.  I believe the 2 year event may have been chosen because it may represent the majority of the flow times of larger events too (most of the hydrograph is not peak flow).

Remember - this entry is only being used to calculate a TIME (velocity in the channel).  So, if channel geometry changes a bit, it is a moot point when the velocity and length does not change much.

I have had reviewers get hung up on the capacity of this channel segment of Tc in Hydrocad.  I must remind them it is just a time calculation and the model does not actually ROUTE through this segment.

In fact, when "matching" the depth to achieve the 2 year flow for the Tc, I consider whether the Tc flow patht DOES convey the entire subcatchment flow.  That is to say: if the Tc path only conveys HALF of the subcatchment flow, I set the depth so that the flow shown is only HALF of the total subcatchment.

RE: Tc channel segment = depth of 2yr flow???

(OP)
peter and Terry:

Thank you for the insight.  It now makes sense to me.  Thanks very much,

Craig

___
Craig T. Bailey, PE
www.bailey-associates.com

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