Detension Ponds on flat sites? 3

[TerraSouth]

What are other Engineers doing for stormwater detention on relatively flat sites?

I have a 40 acre residential subdivision in the works. 1/4 acre lots, 106 cfs predevelopment flow, approximately 226 cfs post development flow. This site has about a 1% fall from North to South. Good soils, deep sandy clay loams, approximate perc rate of 40 min per inch.

This flat site is difficult proving difficult for building a detention pond. I'm aware of underground storage but not sure about the cost? Developer is TIGHT.. (!)

Any advise?

 

[RCEJD]

Check the web. Several companies, ADS, Contech, NDS and several other companies have good, economical underground detention and retention facilities. Hard to estimate a cost based on what you provided but on the order of 15 to 40k type numbers.

Just do a search on Google. The companies will actually size the pipe and provide you software to check, they will price them and even build them in many cases.

I have literally installed dozens with no glitches.

Good luck.

 

[bltseattle]

You say detention, but it seems like the site would be well suited for infiltration (retention). Hopefully you are well above the groundwater, because that is conducive for the most options. I recently developed a distributive stormwater system for a similar situation that had an infiltration system incorporated into the landscape medians at each parking bay. You could consider:

1. Drain to an infiltration pond, via surface flow or conveyance collection system
2. Porous pavement
3. Use an open-graded gravel layer beneath the parking, distribute collected stormwater through manifolds to allow infiltration beneath parking.
4. Underground infiltration trenches/tanks/etc, with conveyance collection system

I encourage you to consider if porous pavement/pavers could work for you. While it is not as conventional of an approach, your TIGHT developer would save money on CBs and piping and retain use of the site. You could use regular asphalt in drive aisles, sloped to shed water into porous parking bays. Easy to find examples with a web search. Use a gravel bed or Rainstore or equal if you need additional storage beneath the parking. One caution, however, is conduct due diligence regarding any potential water quality issues of infiltration; local agencies typically have guidelines...

 

[TerraSouth]

Thanks for the advise.

The soils are good and a retention pond might just be the way to go. I shall also investigate other methods of infiltration as you have suggested.

I do have experience with porous pavers, but need to look at the bottom line very closely.

Thanks!

 

[francesca]

I agree that you should consider retention, or perhaps a combination of retention and detention. With your site being so flat, chances are that you wouldn't have fall from the outlet of your underground detention offsite. Also underground detention can be prohibitively expensive. Have you considered attenuating flow by routing it through open ditches? If you have to meet water quality criteria you could construct these as dry swales.

 

[TerraSouth]

I don't have any water quality issues to contend with on this project.

On the retention pond issue, how do you design the pond when you could have a varying water level? (infiltration, evapotranspiration etc.) It seems to me that the water level in the pond is going to be contantly fluctuating, so it would be difficult to determine the exact amount of storage that you have during a given storm event. Do you design assuming the pond is dry? If not, how much water do you assume is in the pond. I am required to design for a 100 year storm event (13.4 inches in 24 hours)
I could have a pond that has a mud hole for a bottom most of the year, and that is a maintainance headache.

If the pond should dry up during a dry period, I guess you have a large dried up pond to look at? That doesn't sound very appealing. As stated above I have good soils, a perc. rate of approximatley 40 min/inch and no water table with in 8' of the surface.

Seems logical to fence off the pond and plant a screen of plants to hide the eye sore?

Thanks for all the advice... I'm still kicking this issue around.. (I think I have a headache)

 

[gbam]

OK,
Can you create a park like setting with in the pond? For instance place grass, swings and tennis courts ect... knowing that when the design storm hits it will under water. In the Phoenix/Mesa area many of the local parks are detention facilities as well as golf courses in Tucson. However, this is the arid southwest.

The starting condition of the pond depends on the region you are in and if the ground water table is above or below your invert elevation. I would consult the governing entity for this project to get their requirements for deten/reten design.

 

[LHA]

Here in PA, USA, we often design raingardens. I just did one. Plant lots of water tolerant plants, and mulch or plant meadow grasses, and only mow once a year or so. That way, it will not be a mud-hole, and will look nice with almost no maintenance. Plants can cost a bit, but no more than an outlet structure would for a dry pond, and probably less.

Just design for the increased volume from pre to post, and allow a spillway which will pass the desing storm with minimal headwater, so you don't have scour issues.

With those soils, water table and perc rate, you are OK assuming an empty pond at the start of the storm.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[francesca]

In the Middle TN town I worked in, there are a lot of sink holes and retention basin design is common. The design philosophy there is that the pond will drain in 24 hours; not an unreasonable assumption in most cases as Class V injection wells are installed in the retention basins to aid infiltration. Calculations assume the pond is dry at the beginning of the storm event and that there is no out flow during the storm event. You should check with your local authority on what they require; it ought to be in their drainage ordinance.

You appear to have seepage rates and can probably calculate the time required for infiltration and can add evaporation to your calculations. Whether your pond will hold water or not also depends on the drainage basin area. The larger the basin, the more likely it is to hold water. Where I was working, a minimum drainage basin of 10 acres was require for any wet storage design, implying that less than 10 acres is difficult to maintain a wet pond. (Looking at farm dams in the area, this isn't necessarily true; it depends on the soils.)

 

[TerraSouth]

Excellent advice gents!

I've got to do some calculations, contact the county engineering department for more information.

I'll report back more soon

Thanks for all your valuable input!

 

[unclesyd]

Not being well versed in the art, here in NW Florida we have a lot of retention ponds that use percolation to take the water in all except a hurricane. They just built two right near my place that appear to have no overflow drains. They are quite deep and have a thick layer of gravel in the bottom.
Using the road for a percolation bed has been used around here in very sandy areas where the hardpan is fairly deep.
The most interesting one I've seen lately is a system in a parking lot for new fast food joint being placed on small plot of land. One whole side of the property was excavated to about 6' and series pipes and with large round disks placed about every two feet were laid on a bed of gravel. They were about ten rows of this arrangement. The whole thing was covered with gravel and capped. I first thought this was some type septic system, but they were installing a sewer connection today. I’ll try to stop by and see if I can find out any information about this system. The contractor may not be inclined as my neighbor, the deputy, told me that someone stole all the plumbing fixtures last night, even the installed ones..

 

[francesca]

Sounds like ADS StormTech chambers -- Underground detention (i.e. they have an outlet).



(Original image at

http://www.hjpertzborn.com/PIC00025.JPG.)

 

[unclesyd]

The case is thus or somewhat similar.

This is the first time I’ve seen the system installed around here. I stopped by today and there wasn’t anyone around that knew what they had installed. The expedition wasn’t a loss as I did find out that the place is a Kentucky Fried Chicken joint.

Thanks now I know. Now I've got to get someone to ask me where I can look smart.

 

[Bayou]

I have to ask "why is building the basin difficult"?
Of course I don't know the design considerations that you have, but just because the overland is flat doesn't mean that the basin floor has to be as flat.

Most communities require some sort pilot channel in the basin bottom as well as sloping the floor towards the pilot channel. Mud or wet bottom ponds are not really favorable because they turn into mosquito breeding areas quickly.

Also, 1% isn't really that flat. Based on your 100yr rainfall, you must be near the Gulf Coast (if you are in the US); similar to my region. In my region, the average ground slope is 0.1%. We tend to set pilot channels to a 0.2% slope and slope the basin floor at 1%.

With 13.4 inches per 24 hours, retention is not an option.
40 acres is going to give approximately 35-40 ac-ft of runoff volume. The detention pond will probably require 15-20 ac-ft of storage to mitigate the runoff, depending on local criteria and tailwater conditions.

 

[TerraSouth]

Bayou,

I was hoping for detention, but there is no way to get a positive outlet to drain the pond. This leads to retention, as others have pointed out. Land is expensive and I have to minimize the area of my pond, within reason of course. I don't have my design numbers in front of me at this time, but my preliminary pond design is approximatly 3 acres in size and around 8 foot deep with 4:1 sides slopes. The soils are a sandy clay loam with no water table within 6 feet of the surface.

Yes, I am on the Gulf Coast and have just begun designing detention/retention facilities. I understand the concern with wet bottoms and insect problems, but I see no way to design "detention" on this site.

Am I missing something?

 

[unclesyd]

A little follow up on the ADS Stormtech Chambers mentioned above.
The gravel cap over the Stormtech Chambers was about 18"-24" thick. I don' know if a membrane was used but then a cap of 3"-4" asphalt was applied. After this they came back with a concrete cover, essentially over the entire site. The concrete cover appeared to be more than 4" but less than 6" thick. I said this was a chicken place but it turns out it is a hamburger joint.
It appears that another new construction down the road is going to use this approach as they are putting gravel in large excavation.

 

[francesca]

The StormTech chambers also require a 12"-18" gravel bed (if I remember correctly). You are given credit for detention volume in the voids of the gravel.

Once you add up the cost of excavation, cost of gravel and cost of the StormTech chambers (and fittings), it gets pretty expensive.

At the 2-acre apartment building site we considered StormTech for, we ended up going with underground detention in 48" pipes, at about half the cost.

 

[Beantown89]

Try constructing multiple smaller ponds. Add the rates to determine the peak rates.

I saw a homeowner plant a garden in a portion of a detention basin once. Proximatey to homes does have to be considered, but generally detention basin aren't that much of nuisance.