sizing roof drain dry well 1

[dbags]

I have to size dry wells for a residential house roof leader for a 1/2" rainfall. I remember that the calculation is quite simple, but I cannot remember the specific equations. Can anyone help refresh my memory with how to determine the number and size of dry wells needed for this application?

 

[LHA]

1/2" x area of roof = volume. Make sure it is simply 1/2" total accumulation. If the spec is from an ordinance written within the past decade, they are probably requiring either:

Rational analysis (most likely and appropriate): 1/2 in/hr, in which case use a Tc of 5 minutes (for a single roof) and the equation cfs .95 * 0.5 * area of roof; or

SCS analysis: 1/2" of precipitation over a specified strom duration (almost always 24 hours, but could be different). Then inches runoff = ((P-0.04)^2 / (P+.16) for a roof. Inches runoff = 0.32, and use SCS methods.

 

[dbags]

It is the 1/2" precipitation over 24 hours. 1/2" x area of roof will give me the runoff volume. What is the storage capacity of the dry wells?

 

[cvg]

you can construct dry wells with different capacities. It is a function of the infiltration rate of your dry well, size of your roof and the number of wells you will construct.

 

[dbags]

cvg,
Thanks for the info. If I spec a round pit filled with 3/4" to 1-1/2" rock, what is the storage capacity? I've seen calculations in which 25% to 50% of the volume of the pit is used as the storage capacity? Does that assume you have 25% to 50% voids for storage? I have no problem being conservative and ignoring the infiltration rate and just using the storage from the voids?

 

[cvg]

Yes it does, however you should verify this percentage with the gravel you propose to use. Get a sample of the gravel and measure the unit weight. Compare that to the unit weight of the actual rock material to estimate the percent voids.

 

[boo1]

Or fill a five gallon bucket with gravel. Then measure how much additional water it will hold. Ratio the water to the 5 gallon.

 

[LHA]

Here in PA, USA, 1/2" - 2 1/2" stone (AASHTO #3) is always presumed to be 40% void space. I've never had that assumption questioned in review, and I've never asked a designer to verify it to me under review.

Forget the infiltration rate. The "flow" out of a dry well of 313 cf = .5/12 ft runoff * 3000 sf house / 0.4 voids (say 10' * 10' * 3.5' deep) will be imperceptable.

Once again, make sure they only require 1/2" total runoff to be stored, not 1/2" rainfall. They are not the same, and that seems like a very generously low volume, at least it would be in this area.

 

[hexhead123]

I am located in Northeast PA, USA and also am in need of a refresher course on sizing dry wells for a commercial project.

The local code enforcer told me the expected runoff from a ten year storm event at our facility is approximately 4 inches of rain in a 24 hour period. And was told to use this rate for design.

We are interested in paving & concreting a 11,000 sq ft area. I have specified the use of 8' diameter x 9' deep dry wells, which will be surrounded with 2' of clean river stone. I need help determining the amount of dry wells required.

Using 40% void ratio for the stone area surrounding the dry well, I've calculated the total capacity of each dry well to be 678 ft^3. But, I don't know how to compute the volume of runoff the dry wells will need to handle based on the code enforcer's information.

Any help would be appreciated.

 

[bltseattle]

It sounds like you are not familiar with hydrologic methods as LHA described above. So the simple answer for you is take 4" * 11000 sf / 678 cf/well = 5.4 wells. So use 6 or increase the storage to get by with 5.

An experienced engineer could develop a site hydrograph and route it with a level pool analysis, incorporating infiltration rates, and downsize the system possibly since water is released continuously during the storm. But your query implies you are minimally experienced/supervised to conduct this analysis, so the simple but conservative approach outlined above is appropriate.

 

[TerraSouth]

A 9 foot deep dry well sounds mighty deep to me.
That would not fly down South, as we have shallow water tables. Sounds like you might have a "wet" well situation, rather than a dry well. I'd look very closely at the soils and the geology of the site before digging a 9' hole.

Where I'm located I have to design for the 25 yr 24 hour storm of 10.4" YES, 10.4" of rain. With shallow water tables and large roof areas, dry wells seem like a distant dream.

Good luck

 

[hexhead123]

Thank you for your feedback. I am familiar with hydrologic methods, but it has been 12 years since I was taught so my familiarity is a distant memory. I've been reviewing my notes and textbooks but my results left me doubtful. I am hoping that I don't need to provide capacity for 4" of water over the entire area because 6 dry wells seem a tad excessive, not to mention expensive.

Fortunately, the water table in our area allows for the use of 9' deep wells; possibly as much as 15' in some areas but the local contractors prefer the smaller wells.

I have been attempting to get someone to perform a percolation test at our site so that I could incorporate infiltration rates into the equations, but his workload has not given him the opportunity to visit. Perhaps someone could help me get an idea by providing me with average numbers one could expect from different types of soil.

Any additional info or references would be appreciated. In the mean time, I'll attempt to read up on developing a site hydrograph and routing it with a level pool analysis.

 

[TerraSouth]

Instead of percolation "rates", I'd look at saturated hydraulic conductivities. Ksat gives you 3 dimensional data verses the 2D of the perc tests. Contact a local soil scientist in your area and inquire about such. Also, look into getting your counties soil map from you local NRCS office. While not accurate at the single acre level, it can help you get a handle on what your dealing with.

I do alot of on-site engineering consulting and have a constant head permeameter that I use for testing. It gives both perc. rates and a Ksat. Ksat just tells you so much more about the phycics of water movement through soils.

I'd also intertain the idea of using a small detension basin versus those infiltration pits, just to compare cost mind you.

http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=22

An excellent site on hydrology with manuals.

Good luck! I'm having to brush up on detension basins myself so I can relate to your situation.

 

[gtrainor]

A good source to find infiltration rates here in AZ is the local drywell installer. Maybe you have one in PA. The guy here has been installing d-wells for three decades. You can tell him where you need a well and he will provide you with a infiltration reports for drywells nearby.

 

[hexhead123]

Thanks everyone for your help. Since spare time is a rare commodity, I decided to take the easy route and hired the local civil engineer. I paid him to give me a quick refresher course, which worked out nicely.

Just incase your wondering...

I was pleased to learn that the "I" value in the formula for calculating peak runoff is 0.4 iph, which yielded a requirement of one (1) drywell with plenty of room to spare. Actually, there was enough capacity to negate the infiltration rate.