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I am designing a subdivision.  I usually use the rational method to size my pipes.  HOwever, this piping system outlets into a detention pond so my pipes are going to be under pressure.  How should I size my pipes?


It has been my experience that using the Rational Method on culverts yields a pipe diameter that is safe for the particular storm frequency.  However, in drainage systems, it fails to take into account the time of transport, the inlet and outlet conditions of each individual pipe run, and since it yields only a peak flow, is useless for designing detention ponds.  That is unless the method is modified in some way.

I've been using the TR-20 methodology in the form of HydroCAD for the last few years and have found it accounts for almost everything.  It also lets you use a version of the Rational Method.  I'm not trying to sell you on it, but it works for me.  I think they have a downloadable trial version.  Check the AMS:HydroCAD Stormwater Modeling System forum.


I use the rational method to get my flows.  Then I use mannings to size my  pipes.  However this doesnt work when using detention ponds because the pond creates a tailwater on the pipe outlet.  However i do not know what elevation of tailwater to use.  Will hydrocad help me size my pipes too?  


Hydrocad will perform pipe flow hydraulics, however there are many programs out there that will do that.  As far as tailwater is concerned, if the pipe is discharging into the pond, assume a reasonable, conservative pond elevation and use that as your starting point for the hydraulic analysis.  Then try lower or higher elevations to see if your system is sensative to changes in tailwater elevation.  design for the worst case.


Most regulatory agencies will dictate within their criteria the tailwater frequency for storm sewer design (usually 25 or 100-year). Or a variable tailwater curve can be used.

The simpliest is the constant tailwater; this can be found in the design report for the basin. If you can't find the design analysis for the basin, most basins tend to be design with the 100-year at a foot below top of bank. If gravity-drained, at the same elevation as the 100-year within the receiving stream.

A variable tailwater would require the detailed design of the basin. And a program like Pondpack should be used.



First of all, the Rational Method is a hydrologic method for calculating runoff, not a hydraulic method for calculating pipe sizes.  Manning's equation is the most common method for determining pipe sizes in a storm drainage system.  Granted, Manning's equation is an open channel flow equation (free surface flow),  and the surcharged condition you describe is a pressure flow condition, however this can be easily accounted for by using the invert of the upstream end of the pipe and the tailwater elevation at the discharge end of the pipe to calculate the "hydraulic slope" of the last section of pipe in the system, and use that slope in Manning's equation to determine the size.  This method isn't exact, but it certainly will be within the range of accuracy of your rainfall intensity, runoff coefficient, time of concentration, etc.  Drainage design isn't an exact science.  In the words of a former water resources professor at NC State University relating to drainge design:  "we're not making watches, we're totin' water".

Good luck.


All of the above are good answers, but to properly check the design of your system, you will need to know something about headloss in pipes and inlets.

Generally every bend causes the corresponding upstream flow to be a little higher and create a sort of pressure head to get around the bend.  This is due to the friction along the walls in the pipe, and water surface tension.  Every obstacle causes this effect, and every run of straight pipe has friction head.  Keep in mind gravity, and that every thing is at a perpendicular to the forces of gravity.  This is why a level tank has the same level of liquid along the perimeter.  Now tilt the tank, the forces of gravity come into effect converting to motion, and the water will run out of the lower end.  Measure the cross-section and the depth of water will be 1/2 +/- however the cross-section of the tank at the bottom is still full.  Now put a little hole in the bottom of the tank, water will run out, and the tank will empty slowly.  How slow or how fast the tank empties is dependent on the hole size.  Now cut the bottom of the tank off, there is still a time factor in the tank emptying.  The time factor is based on gravity @ 64.4 ft/sec., motion coverted to heat energy.  Yes there is heat build up in a pipe system.  Remember your laws of physics.  Velocity also plays a key role in the amount of water leaving the pipe, and don't forget atmospheric pressure, clog all upstream inlets and you remove the source of air displacement.

What I always suggest to young engineers is, forget the programs and study the phnomenium, than at least when you use the programs for timely design, you will immediately know if the answers do not look right!  I have seen so many mistakes when using computer programs, that I wonder what happened to the pencil in class.  Programs are very usefull when the person using them knows what they are doing, and understands what the program is accomplishing for them.  Remember nothing is perfect and programs do have flaws and unknown results in certain situations,  Check all your work by hand in the beginning, and if possible review the code of the program, especially the subroutines, these are the basic mathematical computation used.  If you do not understand the basic mathematical computations than you will never know if something is right or wrong!!!


I agrre with all that N3REI had to say. If you don't understand what the program is doing, how do you know the answer is right? I remember back in the last century when I went to school, we were taught about 'inlet control and outlet control'. Go back to your textbook and look up outlet control in gravity systems. Get out your slide rule and go to work.


In my design method I follow pretty well what others are suggesting, rational for inflows and mannings for pipe sizes.

However, I tend to design the detention basin with a hydraulic grade line level at the outlet equivalent to or slightly less than the obvert of the outlet and the HGL of the entire system below street level. This ensures that I can get the minor flow (usually 1 in 10) into the basin without flooding streets. I then calculate the ultimate basin level based on major flow (1 in 100). This means that my streets must flow towards the basin with minor ponding (<150mm) in various locations.

Remember that your detention basin is designed for the maximum inflow over a period of time and as result will probably remain near dry for period of time during larger events until the water can flow through the system.

Also I tend to design my systems by hand as I find that the data entry required in programs is often tedious with all of the hard work (areas, slopes of surfaces, etc)having to be done by hand anyway. I usually design systems with less than 30 pits and minimal lateral flows, so the additional lateral inlows are not a major issue for the pipe line hydraulics. The hand design also gives me a good feel for the end numbers.




Is it true that for a film of liquid running down all over the wall of a vertical pipe or vessel, much wider than the thickness of the film, one should take the thickness of the film as the hydraulic radius ?


The rational Method is fine for calculating the Qs and is not relavent to a detention pond, unless the pond is upstream.  So just use the water suface in the pond as starting point and calculate the hydraulic grade through the storm drain.


The problem is that the detention pond is creating a backwater in the pipe this will:

1) Reduce the pipe velocity leading possibly to deposition of silt.
2) increase the storage in the system which is a minor benefit.
3) possibly raise the upstream water levels above the inlet levels causing flooding.

Providing 3) does not cause you a problem you have nothing to worry about. Pipe sizes are calculated the same as for free surface flow. You then calculate the length of pipe that is running submerged and calculate the head loss for full pipe flow. This is iterative. You will then derive the point where the pipeline starts to run submerged. From this point up you can calculate a backwater curve but for an approximation you can take a hydraulic gradient equal to the average between the free flow gradient and the pipe invert slope. You can then project this gradient from the crown of the pipe to the point of free surface flow.

Providing this gradient does not cause upstream flooding your design is sound. You don't need to increase pipe diameters above those that you have calculated using Manning’s equation for free surface flow.


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