Back Water analysis for Storm Drain System 7

[philoshophy]

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.

 

[RWF7437]

Although many people will know, you should explain that the Napa Valley is in California, USA.

"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

 

[Rookie2]

Wow RWF7437, you're difficult. I assume everyone is here for the same reason... to learn (I'm guessing even you). These kind of responses make people afraid to ask questions. This is a great resource, don't ruin it.

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.

 

[jartgo]

I disagree Rookie2, RWF's response is not meant to be negative (at least I didn't take it to be). I believe he's only pointing out that what is being described is an odd way to layout a storm drain system and if it is in fact to be laid out in this manner, you won't find many spreadsheets that can handle this sort of hydraulics, accurately. It is better suited for a computer model. We are all here to learn and RWF's responses are usually helpful, I don't believe this response was intended to be anything other than asking for additional information, and maybe pointing out that the original poster was asking for a lot while only giving a little.

 

[Trackfiend]

I concur with jthompson. RWF's response is merely pointing out the lack of info provided, and hopefully shedding some light (via all of the questions provided) into what philosophy should be looking into.

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.

 

[philoshophy]

Thanks all of you for responsing my question. Actually I messed up in my writing. So you couldnot get my point. Actually the flow line of drain outlet is below the 100 year flood level. I mean the outlet is submerged during flood. At that time ther is a chance of backwater through the pipe towards upstream catchbasin. I am just asking what is the methods for analysing such cases.

 

[cvg]

typically, you will enter a starting water surface elevation for the outlet of the pipe which may be the 100-year flood elevation in the river. If you are designing the pipe for the 10-year storm, you might use the 10-year flood elevation in the river for the starting condition. Either way, you will begin your backwater computation using the flood elevation in the river as the starting water surface elevation for the storm drain. You can then do the analysis by hand methods or by computer program. I recommend computer.

 

[Rookie2]

Jthompson, your right. The thing I hate about these forums is once you hit the submit button, you can't take it back. Believe me, I tried.
My apologies

 

[RWF7437]

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

 

[tsgrue]

.

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.

.



tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[chicopee]

It seems to me that a 12" storm drain pipe for a subdivision is small and I would not be surprised if flooding does not occur during thunderstorms.
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.

 

[RWF7437]

"Izzy, Did you ask a good question today?"

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."

 

[655321]

Storm pipes and catch basins are generally not intended to handle the 100-year storm, large events like those usually just need to be carried within the right-of-way, drainage ways or easements (overland). You may want to check your local regulations, the 100-year storm may not even be an issue with these pipes and inlets.

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.

 

[Rookie2]

RWF7497, Point taken. Even if I disagreed there is no way I could argue.. Being that your quote is so much more eloquent than mine.

Again, My apologies.