In Geopak Drainage, will the software automatically accumulate separate drainage areas and time of concentrations in determining the accumulative flows for a particular system?
For instance at the beginning of the basin you have a sub-basin No. 1 with a pipe as an outlet. This portion of the system has an area A-1 and time of concentration Tc-1 resulting in a calculated flow of Q-1. Now further downstream, you have open channel from the previous pipe discussed to the next pipe that is an outlet to sub-basin No. 2. In this instance with flow being a factor of time and area the following would hold true for its flow calculations in sub-basin 2: Area=(A-1) + (A-2); Time of Concentration=(Tc-1) + (Tc-Pipe) +Tc-Openchannel); yielding a flow of Q-2. And this continues until you have developed all of the flows you want for a given drainage basin. So, moral of the story is flow (Q), is a function of time (Tc) and area (A) accumulated from sub-basins upstream. Of course there are other factors such as land surface coefficients and etc...but that's not important to my question.
Now back to my question....Will Geopak automatically accumulate this information in calculating flows as it moves downstream if the operator inputs separate drainage areas for each inlet node, or will the operator have to "stair-step" through the system including previously developed sub-basins?
To try and be more clear, I have attached a hand drawing of what I'm talking about. I'm no artist, but maybe it will get the point across if my awful attempt of wordsmithing has confused you.
As stated in previous posts this week about Drainage, I'm new to the software and trying to learn as much about it as possible to make sure I fully understand its capabilities, kwirks, and limitations. The software looks to be awesome, but is only as good as the information put into it.
Thanks for any advice given...it will be much appreciated.
Matthew
Hi Matthew,
apologies for the delayed response... Friday was busy!
Ok, I know there are better qualified folks out there to answer this question... If we could get DTM (Dan Calistrat) or Mr Vose involved, we'd get a better quality answer. But, in their absence, here goes.
I believe the answer is yes. GEOPAK drainage does consider the time within the conveyancing system when looking at the Tc and Intensities of downstream catchments. I went to the Help file to see what I can find.. below is the explanation given. The last paragraph (specifically the highlighted sentences) is applicable to your question (I think).
HTHs,
Michael
Rational Method
GEOPAK Drainage computes discharges for individual drainage areas and the collective network peak discharges from multiple areas using the Rational Method. The Rational Method is one of the most common methods of peak discharge calculations in storm drain design. The Rational Equation is given as the following:
where:
Q = peak discharge (ft3/s)
C = runoff coefficient
I = rainfall intensity associated with a specific frequency (in/hr)
A = area of the watershed (acre)
The Rational Method for individual areas is computed using the rainfall intensity (I) for the watershed's time of concentration. The time of concentration is the time required for water to flow from the most hydraulically remote point of the watershed area to the point of the storm drain system under consideration. The discharges for pipe and network hydraulic computations utilize an intensity which is based on the determined longest time of concentration to that point in the network. If there are several branches in the conduit network leading to the subject run, each is analyzed and the longest time of concentration of all the approaches used as a basis for rainfall intensity. This accounts for the additional time for runoff to travel through the conduit system (or travel time). A new calculation of peak discharge occurs at that location in the network.
Great! Thanks for the help. I am slowly realizing how the software works, but now have another questions. When the software analyzes your piping system, it is calcuating the time of concentration throughout the system cummulatively. When I go and double check velocities through pipes to calculate Tc, I am coming up with much different answers then the software. For instance, it looks to be that the software just uses the calculated cummulative flow divided by the flow area through a pipe to calculate the velocity. But, the way I have done this in the past was using Manning's Equation to determine the velocity through the pipe. Two different answers.......Which is right? i would think that using Manning's equation would be conservative enough assuming that whatever the pipe couldnt handle (max flow through pipe), would just bypass the pipe and accumulate at the next, thus giving a faster time of concentration and more concervative number. But assuming all of the water is being forced through the pipe pushes the velocity and Tc way high and feels too conservative. What's your thoughts?
I think that we are now beyond both my drainage knowledge and memory range. We are not using the Drainage app at my current job and I can't recall all the details of how these calcs are performed by the tool from when I last used it.
There is a technical section in the Drainage Help that documents all the calculations being used by the software, you could start there.
I'll see if I can rattle the cage of DTM for some expert advice.
Thanks again. It would be great if you could rattle some cages and find out something. I have looked through the Drainage Help till my eyes have crossed and cant find anything that I can use.
Hello again Matthew,
You're right - I don't see the calculation method documented for that Tc through the system.
I do recall asking the same question many years back when I first came into contact with the drainage tool, got an answer, followed the calculation myself and being satisfied it made sense.
Of course, now that I no longer use Drainage and don't even have a dataset to look at, working though it to provide an answer isn't possible.
You could zip your dataset up and send it and I'd be happy to take a look.
Or you can contact the experts directly at Bentley through their support.