Back Water analysis for Storm Drain System
Back Water analysis for Storm Drain System
(OP)
Dear all hi
I am sticking with one sub division project within Napa Valley. The project has existing 12" storm drain pipe which has outlet in the creek. The flow line of the creek is below the 100 year flood level.
The Proposed condition is that the pipe should be divided into three segments. At upstream portion it should be 12" diameter as existing. After few feet of pipe the diameter should be changed to 24" pipe to minimise the post construction flow. Again after this segment, the owner wants to keep 12" pipe which drains into the creek. For this I did my best calculations.
But I am thinking about the backwater effect in upstream of the pipe. I mean backwater effect on the catch basin. For this purpose, I am having trouble to find the exact methodology which analyse the backwater effect in storm drain system.
Is there anyone who can suggest or give appropraiate reference of analysis? This will be great if someone knows about it.
I am sticking with one sub division project within Napa Valley. The project has existing 12" storm drain pipe which has outlet in the creek. The flow line of the creek is below the 100 year flood level.
The Proposed condition is that the pipe should be divided into three segments. At upstream portion it should be 12" diameter as existing. After few feet of pipe the diameter should be changed to 24" pipe to minimise the post construction flow. Again after this segment, the owner wants to keep 12" pipe which drains into the creek. For this I did my best calculations.
But I am thinking about the backwater effect in upstream of the pipe. I mean backwater effect on the catch basin. For this purpose, I am having trouble to find the exact methodology which analyse the backwater effect in storm drain system.
Is there anyone who can suggest or give appropraiate reference of analysis? This will be great if someone knows about it.





RE: Back Water analysis for Storm Drain System
"The flow line of the creek is below the 100 year flood level."
What does this mean? Does it mean the water surface on the day you observed it was below the estimated "100 year" water surface elevation? Does it mean the thalweg of the creek is below the 100 year water surface elevation ? Does it mean tht you are using some calculated water surface elevation as a starting point for you calculations ? Or does it mean something entirely different ?
"The Proposed condition is that the pipe should be divided into three segments. At upstream portion it should be 12" diameter as existing. After few feet of pipe the diameter should be changed to 24" pipe to minimise the post construction flow. Again after this segment, the owner wants to keep 12" pipe which drains into the creek...."
Who is proposing this; the Owner?
Why would you want to change pipe sizes three times ?
Why would you want to reduce pope size in the downstream direction ?
How does, or can, changing pipe size reduce the flow coming into the system ?
"For this purpose, I am having trouble to find the exact methodology which analyse the backwater effect in storm drain system."
There are several methods for analyzing this and a number of computer programs available. All are based on the laws of conservation of energy, momentum and mass. Among these are HEC-RAS, HydraFlow Storm Sewers 2008, HydroCad, and others. All will give you answers IF you can correctly model the system you are apparently trying to design. None can tell you how to do that.
good luck
RE: Back Water analysis for Storm Drain System
As a very wise man once told me, there are two kinds of people in the world, (in the least offensive wording possible) dummies that know they are dummies, and dummies that don't. I prefer to be the first kind.
RE: Back Water analysis for Storm Drain System
RE: Back Water analysis for Storm Drain System
How did you come up with a 24" pipe size in order to minimize the post development flow? Is it a requirement that you either detain/retain your post develepment runoff?
For backwatering effects, I typically use EPA SWMM 5.0 for conduits.
RE: Back Water analysis for Storm Drain System
RE: Back Water analysis for Storm Drain System
RE: Back Water analysis for Storm Drain System
My apologies
RE: Back Water analysis for Storm Drain System
good luck
RE: Back Water analysis for Storm Drain System
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SWMM5 will handle this well. It is free from the USEPA. Try searching this site for "SWMM" OR "SWMM5".
Other software applications are also available for this.
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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation
http://hhwq.blogspot.com
RE: Back Water analysis for Storm Drain System
But be as it may, the way I would figure it out is to determine the flood elevation in the pipe to get any idea if the pipe is plugged by flood or is partially filled as an open channel; determine how much surface runoff is to be drained from the subdivision; figure out the minimum pipe size free of any obstruction to show whether or not the 12" pipe is adequate; and figure out another pipe size with the above flood water condition. All the necessary equations can be found in hydraulic and environmental engineering handbooks and on the internet. Do your calculations by hand first to get a fill on the numbers before relying on computer programs.
RE: Back Water analysis for Storm Drain System
Isidor I. Rabi, the Nobel laureate in physics was once asked, "Why did you become a scientist, rather than a doctor or lawyer or businessman, like the other immigrant kid in your neighborhood?"
"My mother made me a scientist without ever intending it. Every other Jewish mother in Brooklyn would ask her child after school: 'Nu? Did you learn anything today?' But not my mother. She always asked me a different question. 'Izzy,' she would say, 'Did you ask a good question today?' That difference - asking good questions - made me become a scientist."
RE: Back Water analysis for Storm Drain System
Anyway, there are lots of programs out there that can model this, whether the 100 year storm is important or not. SWMM5 is a little difficult to use in my opinion, and probably more "muscle" then you need (I think it's free however). I'd use Hydraflow if you can still find it. Otherwise HydroCAD, or, as much as I hate to say it, "StormCAD" would work fine. Just set your outlet water elevation to whatever the design storm is in the channel.
RE: Back Water analysis for Storm Drain System
Again, My apologies.