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Old 05-20-2011, 09:44 PM   #1
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french drain question


Hello,
my backyard 37x18 (yeah , i know its tiny) now that my above pool is gone, i am planning on laying down some patio slabs 18x18. while I'll be back there working was thinking of installing a french drain and draining into a dry well. My facing neighbour land is approx 3-4 feet above mine, and during rainy days , have some water accumulation. I have the info on laying the patio slabs and preparation required . My question is on the drywell that I want to insatll in my backyard. How deep and wide should I go ? Any info or suggestions greatly appreciated.
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Old 05-22-2011, 07:08 AM   #2
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french drain question


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Originally Posted by turbofrank View Post
Hello,
my backyard 37x18 (yeah , i know its tiny) now that my above pool is gone, i am planning on laying down some patio slabs 18x18. while I'll be back there working was thinking of installing a french drain and draining into a dry well. My facing neighbour land is approx 3-4 feet above mine, and during rainy days , have some water accumulation. I have the info on laying the patio slabs and preparation required . My question is on the drywell that I want to insatll in my backyard. How deep and wide should I go ? Any info or suggestions greatly appreciated.
You'd plan the drywell based on the potential water accumulation you expect plus a fudge factor allowance. You would also need to know how well the soil drained. Here on Long Island, it's sand 18" down so water goes through it fast. I could dig a dry well smaller then someone in a clay packed soil.
Ron
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Old 05-22-2011, 08:33 AM   #3
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french drain question


Ron is on the right track, do the math!

You have a drain area of only 666 sq ft (37x18), but you indicate there is water coming from your neighbors. For an example, let's say there is an area of 5000 sq ft (50x100) additional drainage from the higher neighboring property (backyards plus roof ?). The dry well must provide temporary storage for the rainfall from 5666 sq ft.

Consider a 1 inch rain with a 50% runoff. 5666 / 12 = 472 cu ft * 50% = 236 cu ft of dry well storage needed. Since the gravel in a dry well takes up two thirds of the space, you need a dry well 3 times as large as the water storage needed. 236 * 3 = 708 cu ft. dry well

Thus the needed dry well becomes ...

Width * Length * Depth = 708 cu ft

One way to get get the volume would be a hole 10ft wide, 23.6ft long, 3ft deep. Were you thinking the dry well would be the size of a 55 gal drum? ROTFLMAO

The sad part about introducing this amount of water to the subsoil is that it can result in saturated soil and a high water table. A water table near the soil surface can lead to dead trees and plants, and new basement leaks.
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Old 05-22-2011, 08:43 AM   #4
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french drain question


The other issue with a dry well is that if the water table is near the surface, you will have a wet well, with no usable storage. You need to determine the high water table mark before you dig the hole. This can be done by digging a test hole, and observing the water level in the hole over the course of a year or so, or you can estimate the high water level using soil mottling. In any case, the only usable storage would be the volume of the dry well above the high groundwater level.
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Old 05-23-2011, 03:42 PM   #5
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I would also talk to the neighbor to encourage him to do something on his side of things, so you are working together. Technically, you are not supposed to move your water onto the neighbor's, but if it was build this way, you just deal with it. Perhaps a small retaining wall --you might have to build it--would help too.
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Old 05-27-2011, 12:54 PM   #6
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TurboFrank,

Instead of dumping the water into the ground, you may want to consider redirecting the water to the street. I always try to control the runoff with catch basins and 4"PVC piping to direct the water to the curb.

This way you don't have to ever worry about how much water ends up in your yard. It'll all make it's way into the local river or stream.

It's pretty common in areas with small lots with very little area for drainage to allow homeowners to use the city's infrastructure to control water. I'd check with your local authorities to make sure.
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