# How to calculate drywell size needed?



## Alldayrj

I have a customer with a drainage problem along the side of his house, the neighbor is about 3' higher and he has puddling and water damage beginning to happen in the basement. 
he has about 1000 sf of lawn (permeable surface) and 1500 sf of roof draining to the area via two downspouts.
I dont want to undersize the system, he has had two other contractors half ass the repair so i definitely want to provide a final solution but its a tight area so banging in 10x10 wells with a 320 cat isnt really that easy. 

Is there a chart for how many cubic foot of wells i would need based on permeability of soil etc? 
Thanks
RJ


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## SLSTech

I am sure someone has a chart available, but first I got to ask is there anyplace else you can divert the water, get it out of that one area? Grading / Sloped permeable pavers with a French drain system under it or... Can you run the HO's gutters away from the area utilizing PVC pipe that drains to sunlight?

As a standard rule of thumb we use for rainwater harvesting - For every SF of roof (plus land in this case) & inch of rain, one will get .623 gallons of water so a 2" rainstorm will give you over 3000 gallons of water that needs to go somewhere - granted most is not heading directly to the problem area


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## Alldayrj

I could run it all to the front but it would daylight in his front yard, run down the hill cutting a gully over time im pretty sure, across the sidewalks making a nasty ice rink in winter and then dumping in the street. I thought about adding this as a sort of overflow if the wells get overwhelmed


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## hdavis

You'd need to measure actual perc rate where you intend to set the tiles and at the depth you'll set them. This can be very sensitive to soil strata. You can only design these to handle a maximum rainfall rate, everything else has to go into a catchment or drain away. 

SLS gave you what you'd need in gallon terms to handle 1-2" per hour ( very heavy rain). First step is to dig some holes (post holes are fine) to see what you have for strata and test actual perc rates, or have an engineer do it all for you. Then look up rainfall info for your area and calculate what you'll need for capacity. Perc rate and needed capacity lets you calculate how big of a system you'll need. Frequently, handling all the water from a heavy rain results in a bigger system than will fit in a tight space....

From a construction view, you can make these like a leach field. Make whatever size hole you need down to the strata you want to drain into, geotextile and crushed rock for the leach field, place a perforated tile to feed it and act as a small catchment, maybe a solid tile on top of that. 

Each site is unique...


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## Alldayrj

Long island is mostly sand, once you puch through the top 2 or 3 feet of dirt. You can run a hose all day into it and it wont fill up. I just like to size my systems so they dont have to really rely on percing for all the containment. I came up with two 6' precast rings 6' deep and a popup for the overflow. Does this sound way off?

Do you have the formula that takes total volume of rain and perc rate into account?


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## Upchuck

The only way to know perc rate is to do perc test and see how quickly ground takes water. Around here engineers would do that for septic, subdivisions, etc. 

I would always recommend engineering because if you design it and it doesn't work, you will hear about it. As far as small drain jobs w/o engineering, I use experience to figure it out based on soils in our area.


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## Alldayrj

I hear you on the perk, once i have it, then what? I need the formula to plug the rate into


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## Upchuck

Never seen a chart based on perc rate. I've done a bunch of drain jobs this year. Everyone had a plan designed by engineer. I know client may not like it but it could save you future problems.


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## Alldayrj

So where does the engineer get it? Surely he has a formula?


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## hdavis

Alldayrj said:


> So where does the engineer get it? Surely he has a formula?


The last one I did the engineer used simulation software for the rainfall, roof and lawn runoff. Basic design doesn't even need a perc test, as long as you're sure of the soil. For no perc test designs, engineers use the soil maps for the area to verify the soil type. If this is actually sandy / gravel soil, then the design value is 5 gallons per day per square foot of leach field.

Now you have to figure out how many gallons you'll get in 24 hours from a pretty good rain, and it has to include runoff from neighboring properties - that's where the simulation software is really handy. Ignoring the neighbor and just looking at 3000 sq ft for the subject property, SLS's number would get you ~1900 gallons per inch of rainfall. A 6" rainfall in 24 hours (you have to check NOAA for actual rainfall patterns in your area) would get you 11400 gallons to get rid of. That's 2280 sq ft of leach field if I did the math right.

If you convert the 1500 sqft of lawn to leach field, that gets you 7500 gallons capacity, so you'd need 4000 gallons of tanks to hold the excess. Any runoff from the neighbors is on top of that.

From a practical standpoint, you can use swales and taco drains to divert surface water and perc it, and situations like this may need a ponding area if you can't put in enough tanks.

Figure out what you want for a design rainfall, and work through the numbers.


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## peteo

Instead of day lighting it in the front yard why not tap the city storm system? I'm not sure of the area but I would think long island would have a storm system just about everywhere


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## Alldayrj

On long island the rule is if it falls on your property it has to stay there. 
I see pics of guys cutting holes in the curb and just dumping all the water in the street. That would be awesome but they would probably sue you here.


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## hdavis

If the neighbor is 3" higher, put a block wall along that side to dam it out. Most places, that would still require a permit since it's changing the surface water runoff pattern, and I don't think the neighbor will like it. It's a lot cheaper than tasking the neighbor to court to force them to fix their water problem, and a lot faster.

I just got through one of these where the neighbor regraded and sent 23000 gallons of water (6" rain) into someone else's back yard. Lots of money and 3 years in court.


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## Alldayrj

Its 3' higher. Not 3". Theres already a wall there, the neighbors property slopes to the back of the wall. Theres heavy efflorescence coming through the wall with the water, its an all around hack job. These houses we're only built around 10 years ago too


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## hdavis

Alldayrj said:


> Its 3' higher. Not 3". Theres already a wall there, the neighbors property slopes to the back of the wall. Theres heavy efflorescence coming through the wall with the water, its an all around hack job. These houses we're only built around 10 years ago too


I'd say they're hosed, unless there is something about the site that gives a decent out. You've seen the rough numbers, you have to get creative to even come close.


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