What exactly do you mean by 'backflow'? The system backs up by design? Or is your subdivision tapping a combined system? If you will have a sanitary system only then put in a structure with a "reed valve' on the pipe coming from your system. Then the system will close for a short time and then reopen and drain. you may want to put in a'holding tank'with a couple hour capacity upstream of the valve.

I am designing a subdivision.  I have an outflow structure at a creek.  I need to consider the hydraulic grade line to be sure that ponding is not created at the upstream and low rim elev. structures.  Is there a quick and efficient way of doing that?  Spreadsheet?

There is lots of hydraulic modeling software out there i.e. Haestads makes StormCad or many people have their own devised spreadsheets or what not. However, as with any program, you need to know what the program doing. Otherwise its the garbage in/garbage out syndrome, which you may be susceptible to since you don't seem to have a real good grasp on hydraulic design and are looking for a quicky fix. My advice is go ask an expert in your office or someone familiar with hydraulic design. If you dont have anyone like this Roberson/Cassidy/Chauldry have an excellent book on hydraulic design you might want to refresh up on.

Remember Land Development may be fast paced but there is still an ethical level of quality and competence that needs to be maintained.

Best of luck to you or the registered Professional Engineer in charge of your work,

BigDog23

Design your system from the top down, calculating the HGL as you go considering all losses in pipes, pits, bends etc.

Then set your start HGL as the obvert of the pipe at the outlet. Now calculate the HGL levels working from the bottom end up. Depending on local rules, you would generally design the piped system for a 1 in 10 year event and allow the flows greater than this to flow aboveground.

For most subdivisions you will need to pay attention to how the above ground flow works as this is also important as people do not like the look of water "flooding" their street.


Good luck

sc

a quick check would be to calculate the headloss in the system from the catch basin all the way to the river.  add up the friction loss in the pipe (assuming flowing full), add losses for flow through manholes, bends, junction structures etc. and add entrance and exist losses at the beginning and end of the run.  add this number to the water surface elevation and compare to the rim elevation.  This could be done in a spreadsheet very easily.  

This is done assuming full flow for simplicity.  Also, since you have water backing up, full flow is a good first assumption.

to determine the change in the hydraulic grade line with a downstrem water level above the pipe flowline you need to use the energy equation. If you have access to "StormCad" it can be easily programed to make these computatioins. Cimmeron