what comes first - storm sewer or detention facility

[civilbjp]

I am an engineer in Wisconsin. I have recently undertaken the task of designing subdivisions and got overwhelmed with the stormwater management. Here is my dilemma: If I have an urban section of road, I can size the storm sewer based on watersheds to determine the size of pipe. Knowing that the storm sewer needs to drain to a detention pond, I then size the detention pond, but I have a hard time getting inverts to work. Looking back, I wonder if I should have designed the detention pond first but then I may have issues with the storm sewer. Maybe I am looking at this wrong, but I would like some advice as to the general steps one would take to develop a grading plan, design the storm sewer, and design appropriate detention facilities to maximize efficiency. In addition, is there any good web sites or literature to assist designers to select appropriate BMPs to meet the new Phase II construction requirements by DNR and EPA.

 

[francesca]

Every residential subdivision site is different.

We generally design the storm sewer using the Rational Method and based on a 10-year, 5-minute storm, while the detention/retention basins are sized on a 100-year, 24-hour storm. One is a matter of flow rates, the other primarily a matter of volume (though obviously flow rates are also a factor).

Our storm sewer inverts are usually fixed by minimum cover under our roads. Occasionally, we'll buck a grade and drain through a high point, but this practise is generally frowned upon.

Unless an entire site drains to a single point, it isn't possible to design the detention/retention basin until we know where the storm sewer will discharge.

Frequently, we'll look at our overall drainage and design our roads accordingly; other times we design our roads and make the drainage fit. Individual site conditions dictate which approach we take. On a flat site, discharge of a detention basin might dictate the road elevations; on a steep site, balancing cut-and-fill might dictate the depth of our storm sewers.

Regardless of which aspect of design we start with, however, drainage design is always iterative.

 

[cvg]

It is an iterative process, and generally works out better if the road designer and the drainage designer are working closely together.

If possible, can you take most of your runoff in the streets as far as possible and then into a basin? Or if need be, into a drain?

Common practice around here, most subdivisions have little in the way of storm drains, relying mostly on surface drainage. Unless an arterial road, storm drains aren't required.

 

[francesca]

You didn't say where you were, cvg, but here in Middle TN, curbs and gutters are required even in the county.

Having never been to Wisconsin, I can only guess that it's flat because sufficient fall is an issue. In that case, I would've thought that sanitary sewers would've been a problem, too.

We designed one particular subdivision of 135 lots that drained to five sink holes, so fall wasn't a problem with the storm drainage; however, the site drained to an existing sewer lift station and we had to put a substantial part of the road in fill in order to achieve positive drainage in our sanitary sewer. Fortunately, one of the sink holes had a drainage area of over 50 acres so we were able to get a lot of dirt out of our retention basin in order to get an earthwork balance.

My suggestion to our Wisconsin friend is to take an intial "master planning" view of each site, in order to determine which will be the controling factor. If retention basins or wet ponds are not a possibility, it may very well be necessary to begin with the detention basin design in as far as to set the invert of the basin outlet; then to visit the storm sewer design and finally fix road grades.

I guess I'm lucky in that I do all of the design in the subdivisions I work on.

 

[cvg]

curbs and gutters are nearly always required here (except in alleys, private roads or easements. But piped storm is not required on local streets.

 

[Maury]

When I do a development, I always start with the design of the storm water detention facility, then routing any off-site flow. If the storm water detention doesn't work, there won't be a development. Storm sewer design would be part of an overall grading plan which would also include streets. As cvg said it's an iterative process, and the first design of the detention is rarely the last.

 

[LHA]

I start with the outfall elevation of each storage volume, because, as Maury said: "If the storm water detention doesn't work, there won't be a development."

Get whatever storm swales and pipes you can to go there, the remaining subareas are (by default, for the reason above) bypass. I then size the storage to overdetain for the bypass.

As for the Phase II NPDES requirements, those vary by state, so go to your DEP website. Vendors have a wealth of useful free info, but be advised: their job is to sell things, not to ensure compliance at minimal cost.

ads-pipe.com is a helpful vendor site.

Here in PA, USA, our BMP manual is located here:

http://www.dep.state.pa.us/dep/subject/advcoun/stormwater/stormwatercomm.htm

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