SWM an storm sewer design

[JrEngCa]

Hi everyone this is my first tread. This is a really cool forum! Here is my question:

In my city we restrict the flow from a site to pre-developed conditions for a 1:2 year storm based on SWM requirements. Since the system is designed to surcharge, what kind of capacity should the storm sewers have? On private property I usually design for 1:2 year developed flows. Some engineers design for more than that even when releasing at1:2 year pre-developed flows. Wouldn't sizing the sewer based on 1:2 year pre-developed flows be enough since we are restricing the flow to this level? The flow control at the outlet is going to cause surcharging so does it really matter if the sewer pipes themselves only provide a level of service equal to what the outlet is giving?

I hope soeone can help me. I don't like over designing especially for industrial sites that does not really require a high level of service.

Thanks,
Pete

 

[TerryScan]

I feel I am missing the precise point of your question, but here's a go at it.

When I do stormwater design, flow control and runoff collection are two different things. If I must reduce flow, typically one must provide some kind of storage. Typically the storage is in a pond. It is possible to provide the storage in the pipe system (oversized).

Now, assume I must reduce my post development flow for the 2 year storm event, and I provide a pond for storage. During a 10 year storm event, do I want my collection system that conveys runoff to the pond to flood? Probably not.

 

[JrEngCa]

Thank-you for your reply.

My question is really about sizing storm sewers when SWM is in place. If I am releasing at a rate of a 1:2 year pre-developed flow, would it be wrong to size storm pipes based on 1:2 year pre-development flows?

 

[cvg]

you need to look at the effect this has on your development. are the streets flooded during the 5, 10-year storm? What happens during the 25- or 100-year storm? Will upsizing the drains provide better management of storm water during these larger events? If so, is this a benefit to the community that is required by ordinance, or desired by the developer? Especially since the incremental cost to upsize may be relatively small?

 

[francesca]

If you want to stay on the right side of the law, all storm recurrance intervals should be accommodated in your design. It's no good to release the 1:2 year storm at the same rate as pre-developed if the 1:100 year storm is going to wash the neighbors away.

Many cities require storm sewers to be designed for the 1:10 year storm event, while others require the 1:25 year storm event be used. A storm sewer designed to surcharge in the 1:2 year storm is not really very useful. While that may be the de facto situation in the ground, it's no reason not to design your system to a higher standard. It'll certainly make it cheaper for the city when they eventually upgrade the existing storm drains.

The only reason I can think of to design storm sewers for a 1:2 year storm is if it's a combined sanitary/storm sewer that is very flat and there is a concern that the pipes will block if 2 fps velocity cannot readily be achieved. Of course, you wouldn't want a combined sewer surcharging, though.

 

[JrEngCa]

cvg, thank-you for your reply.

If I can only release storm water from the site at a certain rate why would it be beneficial to to provide more flow capacity in the sewers than the flow control will allow at the outlet of the site?

I forgot to mention that I'm dealing with site servicing on private property. Most of the storage takes place as surface storage above cb's, loading docks, etc.

I am expecting flooding in a 2 year or greater storm event. The depth and location of the flooding is controlled for major and minor storms.

I'm a young engineer w/ limited civil engineering experience. I would really appreciate your assistance.

Thanks,
pete

 

[JrEngCa]

francesca thank-you for your reply. I'm italian too

I sometimes provide on-site storage for the 1:100 year storm based on a 1:2 year release rate. The flooding and escape route is controlled so I know where the water would go if leaves the site overland. When possible, I restrict the 1:100 year storm to pre-developed flows with a separate restrictor.

Again, why would I need to provide more flow capacity in the sewer pipes than the flow control at the outlet can handle?

The majority of my site servicing projects are for industial sites. Not much damage can be done. I also don't over spending on sewers if it is not necessary. I would rather be conservative in the pipe sizes. So based on this how should the pipes be sized?

Thanks.

 

[bltseattle]

"Again, why would I need to provide more flow capacity in the sewer pipes than the flow control at the outlet can handle?"

You would size the downstream pipes for more capacity than the restricted site release because if the flow control device you are designing gets clogged, it could overtop or cause flooding (depending on the facility configuration) from the site to flow uncontrolled to another inlet downstream of your flow control system, at which point the downstream system will experience a higher flow than the controlled release rate.

 

[francesca]

You would also size your downstream pipes larger because the outlet will be surcharged during a larger storm event and therefore would release more water than is possible merely by gravity.

I'm sorry to disappoint you but I don't have a drop of Italian blood -- I'm South African and got my name via my Afrikaner great grandmother who chose it in preference to "Francina!" (I'm eternally grateful, though I doubt my parents would've done that -- or "Petronella," another family name -- to me!)

 

[JrEngCa]

Sorry for making an assumption. Francesca is my sisters name

I am concerned with sizing pipes upstream of my flow control device. Should they be sized according to release rate of the minor system?

 

[JrEngCa]

bltseattle, thanks, but I was refering to the on-site sewers upstream of my flow control device. If you respond again that would be great!

 

[francesca]

If your pipes are going to be maintained privately then you can design them for whatever storm event you choose. Just be sure to check what's going to get flooded in big storm events. If the flooding can be contained within your client's property and won't cause any significant damage (electrical boxes, flooding of buildings, etc.) then you're fine. Don't flood the neighbors, though (any more than they are already being flooded).

Remember, that your client may expand business operations at a later date and almost certainly won't upgrade the storm sewer at the same time. You don't want to get sued because your client built something where you were storing 10-year storm runoff and the new building flooded.

 

[JrEngCa]

Francesca thanks for helping me further. Will providing more flow capacity in my pipes than my flow control device will allow actually provide any benefits in terms of service level?

I may seem crazy but I've been asking ppl this question over and over and I'm not getting an answer.

My friend (an engineer) tells me to design based on gravity flow using 1:2 year developed flows. I think there is no point in that since the flow control device is limiting the flow to pre-development flows anyways. Basically, I think the water needs to get to the flow control device at the the same rate as it is letting out. A little extra capacity will be good but not too much more because there is no benefit. Am I right or wrong. I may be wrong because I see others designing based on developed flow (With SWM in place) but I just don't understand why. I've asked many ppl and I've never got a resonable answer.

Please help!

 

[TerryScan]

JrEngCa

I thinke part of the problem with the responses you have received is that few are accustomed to designing to your criteria.

The only other concern I can see as far as oversizing any of the piping for your design is maintenance. I do not know the size range of pipes you are proposing, but small pipes may be prone to clogging from debris and may be more difficult to clean if obstructed. If one keeps a minimum pipe size and it is larger than required for the design storm, theoretically the debris will pass until it reaches the flow control restriction and can be maintained at a single location.

Otherwise, I agree that any other oversizing is a waste if the client and regulators are satisfied with the flooding on larger events.

 

[JrEngCa]

TerryScan, I think I am finally getting somewhere! I'm glad you agree with me, but I am still troubled by this.


I have reviewed sewer design calculations from our office and calculations done by others. The sewer pipes are always being sized according to flows generated by the development of the site only to get restricted by the flow control device to pre-devleoped flow levels. It is always the case with subdivisions, private site servicing, etc. When I ask why I don't get an answer. I realize that requirements for restricting flows maybe different in the US and maybe is causing confussion.

How do you size sewer pipes? Based on the flows generated from dveloped contitions or are they related to the restricted flow required in SWM.

An end is near, I can feel it. Thanks TerryScan.

 

[waterguy]

I don't think you're going to get a definitive answer on this one. Sizing your storm sewers based on the 2-yr post-development flow (or whatever your minor system storm is) is a reasonable method even though you may not utilize the capacity due to the downstream restriction. It's at least a quantifiable method of sizing the pipes.

However, if the pipe sizes are too large you might consider sticking with at least a minimum pipe size for your system. I generally use 250mm or 300mm diameter pipe because it is readily availably and won't get plugged easily by debris or snow/ice buildup in the catchbasins. If possible, I use the same diameter for the entire system...easy for the contractor and inspector.

A couple of other things to consider.

1. Maintenance - If your pipes are too small there will be greater opportunity for blockage.

2. Make sure your pipes aren't so small that they become the restriction rather than your downstream control.

3. Does the approval agency (city, town, county) have any critieria for minimum pipe sizes and slope for storm sewers? This could at least give you some "backup" if somebody asks.

John

 

[francesca]

If you're designing your upstream pipes for the downstream release flow, then you're obviating the need for that downstream control; you're using the upstream pipes as flow control.

Downstream, once you get off your clients' property, you're bound by the city's drainage ordinance and will likely have to provide pipes to convey the 10- or 25-year storm.

On private property you may size pipes however you see fit, but you must be fully aware of where the water will go in larger storms. There will be two components: flooding upstream of the pipe (storage) and water overtopping the pipe at a much faster rate than it's coming out the pipe. In the latter case, you need to be sure that your downstream control will not be deluged and overtop, releasing water from the property faster than in the pre-developed condition.

In a nutshell, you run the risk of being penny-wise, pound-foolish because the cost of flooding a building or neighbor will certainly outstrip the savings in pipe sizes. Careful analysis of where ponded water will go is required if this is your chosen course of action.

Nobody on this forum is going to give you a definitive answer because they only have the information you've presented and not a detailed view of the site. Further, you're the design engineer and the design decisions are up to you. We can point out the pitfalls of taking a certain course (pipe blocking, flooding, cost, etc.) but we cannot tell you which course to take.

"The internet told me to do it like that" just doesn't hold up in court!

 

[JrEngCa]

Francesca, your first point is exactly my point. Large pipes with a flow control device and using pipes to control flow only differ in cost of installation if storage and maintenance are not factors. The same level of service exists if designed properly although costs can be reduced.

SWM was never my issue I just needed to mention it was the reason flow is restricted. I understand the risk and importance of understanding how the system behaves and where the water will go etc.

I know I have not provided any specifics about projects and realize it may be difficult to answer my question properly.

Now thanks to you I have my answer.

I've been dealing with an extremely large industrial site where storage is plentiful and risk of damage due to flooding is extremely small. I have to be economical so that's why I've been pushing with questions.

Thank-you to everyone your input.

 

[cvg]

in addition to risk of damage, you may want to consider risk to health and safety. Maintaining emergency access to your industrial site is necessary and may require more than a 2-year storm system. Typically, at least 1 access point with 100-year flood protection is required here for all developments.

 

[JrEngCa]

cvg, that's a good point also.