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# Flatland Detention

## Flatland Detention

(OP)
I've got a subdivision
• The drop from one side of the subdivision to the other is only about two feet.
• My calcs show that the required detention would be about 5.4 acre-ft.
• Basic math then says that I need detention to be 2.7 acres.
This means I'm giving up two entire lots worth of area in a 27 lot subdivision, that's way too much. It makes the subdivision unprofitable.

I can dig down deeper and have a pond. But the problem is that we have no electricity. Nothing will be put in until housing starts. But we need the subdivision approved before we can sell the lots.

Is there some other method that can be used in this situation?

### RE: Flatland Detention

How did you arrive at 5.4 acre-ft and 2.7 acres of detention?

Is infiltration not an option? Much depends on the soils and surrounding area. How deep is the groundwater table?

I don't follow the relation between having electricity and excavating a pond.

Lots of questions need to be answered on this before any helpful solutions can be offered.

### RE: Flatland Detention

(OP)
If I have 2ft of depth for 5.4 acre-ft of detention required, the area of the pond will be 2.7 acres. I'm not sure what you're asking?

I've already included the runoff coefficients in the calculations for total volume.

If we excavate further than natural grade, we'll have to install a pump to get the water out and onto the neighboring lot's entrance elevation. Pumps require electricity.

### RE: Flatland Detention

Agreed, pumping storm water is never a promising option. Is infiltration possible on this site?

If you have 2 feet of drop across the site, why are you planning on a 2 foot deep pond? If the highest point on the project site is at elevation 2500 ft, and the lowest point is at elevation 2498 ft, the bottom of the pond would be at 2496 ft? Or are you planning the pond to have a built up containment berm with a bottom elevation of 2498 ft?

There still needs to be some slope to the outlet pipe/channel from the pond. How will the water leave the site? Will it discharge into an established system of drainage ways? Or, does it need to be discharged in a metered fashion to take an extended time to discharge onto neighboring property?

What size is this property? What is the average slope across the surface? What will the slope be after development?

Maybe sharing a diagram or plan view with elevations would help.

### RE: Flatland Detention

Have you considered using a Siphon system for your pond?

### RE: Flatland Detention

The starting elevation for water storage is typically the area's water table elevation. Are you saying that is just 2' below your site's ground surface? If so, you will likely have to elevate the rest of the site to effectively increase the storage height of that pond. This sounds like a water quality calculation since it is in the range of 1.5 inches of runoff over your subdivision area. Typically there are several storm events which are modeled to ensure the subdivision's roads and houses are elevated high enough, water quality is just one such computation. It's tough to say more without more information about your area, but at the very least I expect you will need to meet a local agency's requirements for storm water attenuation (aka storage and delay of discharge) so you should look into what agency is responsible for that and read into their regulations or have a meeting with them.

### RE: Flatland Detention

First off, lets address the required volume for detention and how that is regulated. Are your requirements to meet pre-project conditions (Balanced Basin Approach) or and overall decrease in outflow (Critical Basin Approach)from your site. Have you computed the pre vs post project hydrographs to establish the differential flow rates? We use the hydrograph method to determine the volume required for detention; which is the column under the proposed condition hydrograph and above the straight line from 0,0 to max allowable discharge (usually existing discharge). Once you know that actual volumes required then one can assess how to store the volume either in a pond or underground vault/storage facility.

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