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combining hydrographs and reflecting cumulative TC

combining hydrographs and reflecting cumulative TC

combining hydrographs and reflecting cumulative TC

multiple bioinfiltration swales are linked in a series.  How do you model for the changing time of concentration as each subcatchment area contributes runoff, each swale fills up and then spills over into next downstream bioinfiltration swale with its own subcatchment area.  I have tried pairs of subcatchments and ponds and areas are added but change in TC (for the time to fill each pond) does not seem to be reflected.   

RE: combining hydrographs and reflecting cumulative TC

In general, the Tc is calculated up to the next node in the model.  If the swale is being modeled as a pond or reach, the travel time for that element is simulated by the hydrograph routing calculation, rather than explicitly adding it to the Tc.  Adding it to the Tc would basically double-count the effect, and should be avoided.  For details see www.hydrocad.net/tc.htm

Peter Smart
HydroCAD Software

RE: combining hydrographs and reflecting cumulative TC

PS: The time lag and detention time are shown in the pond summary report.  These effects are introduced by the pond routing calculation, so that are not, nor should they be, added to the Tc used for the upstream runoff.

More about detention time at www.hydrocad.net/detentiontime.htm

Peter Smart
HydroCAD Software

RE: combining hydrographs and reflecting cumulative TC

I am a new user of HydroCAD.  My model consists of a series of subcatchments each draining into a pond with discharges from each pond flowing into the next downstream pond.  I provided the TC of each subcatchment by direct input of a time computed by a segmental approach.  Each small pond takes some time to fill up before it overflows into the next pond in sequence. for example the TC for subcatchment A is 19 min.  it talkes about 9 min. to fill pond A at only receiving runoff from subcatchment A.  Subcatchment B has it own TC, say 23 min. and fills pond B.  But pond B receives runoff from subcatchment B and also the overflow from pond A after 28 minutes.  I assume that the hydrograph for Pond B would reflect a TC of 28 minutes since it does not receive runoff from subcatchment A until that time.  Am I missing somthing? How would you see this on a report?

If my TC's are generally about 23 minutes, why does the peak rate of inflow show up at a time of 12 hours?

RE: combining hydrographs and reflecting cumulative TC

The outflow from each pond will be delayed by the routing calculation - roughly the "time is takes to fill up" - which is why you should not add this detention time to the Tc.  The effect is handled automatically by the routing calculation.

With a type II storm, the peak rainfall occurs at 12-hours, so the peak runoff will always occur after 12-hours, depending on the Tc.  For details please see www.hydrocad.net/runoff.htm

Peter Smart
HydroCAD Software

RE: combining hydrographs and reflecting cumulative TC

The hydroCAD 9.0 report shows error message #93 - storage range exceeded by 1.93 feet (100 year storm). The stage storage data cannot be extended to a higher elevation and more storage volume, because of lack of area for a larger pond. With this error message reporting, are the outflow numbers from two primary devices still valid?  The raingarden has a 15" pipe with an invert 1' lower than a 6' wide broad crested wier.  Combined outflow from both primary devices is reported at 23.5 feet per second, and the total outflow volume is 1.706 acre feet atten = 0% and lag = 0.0 min.

What does the atten and lag mean?

RE: combining hydrographs and reflecting cumulative TC

Warning 93 indicates that the pond WSE has exceeded the storage data you have provided, so the results will not accurately represent the physical situation.  To correct the model you need to define storage at higher elevations (typically additional contour elevations.)  Even if you don't want the water to go that high, you cannot accurately model your 100 year event without this information.  Also be sure you're defined storage to some point above the crest of your weir.  There IS storage above the crest - it's the volume being detained when water is flowing over the weir.  For details please see www.hydrocad.net/overfilled.htm

The attenuation indicates the reduction in the peak flow, and the time lag is the delay between the inflow and outflow peak.  For further details please click that portion of the pond summary report.

BTW: Please start a new thread for a new topic, rather than tagging it onto an old one.  This makes it possible for everyone to follow their specific topics of interest.

Peter Smart
HydroCAD Software

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