HomeMy WebLinkAboutDRAINAGE ENGINEERING - 16-00554 - 433 Airport Rd - Barney Storage - Bldg 3G tru s s`4
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Southfork Design Group LLC, 1066 N. Yellowstone Hwy., Rexburg, ID 83440, 208-852.6236
D.J. Barney Storage Units Drainage, Building 3.
8/22/16
By: Ray McDougal, P.E.
Given:
Calculate storm water volume collected from the parking area and roof area for building 3 of the
proposed project. The project is located in Rexburg, ID near highway 20. Coefficient of runoff = 1.0
for roofs and 0.95 for paved areas. Although the parking and drive ways will not be paved initially,
this analysis assumes pavement so that the infiltrators will be able to handle runoff if the areas are
paved in the future. We are using a 25 year storm event and 24 -hr duration for the sizing of the
detention facility and rainfall intensity (i=-0.9 in/hr). No off-site discharge allowed.
Required:
Calculate the size of the detention area using the infiltrators. Since the system will consist of (4)
separate infiltrators that each service different areas, we will look at just one infiltrator that services the
largest area and use the resulting infiltrator design for all for infiltrators. The breakout of the areas is
as follows: Total Roof 8,000_sf, Total paved areas 9,325_sf. Limiting infiltrator roof area: 2,000_sf,
Limiting infiltrator paved area 2,331_sf
Design:
Total runoff flow per 24_hours=[(1.0)(2,000)+(0.95)(2,331)](0.9/12)_ft(7.48_gal/cf)=2,364_gal/24-hr
period after the time of concentration is reached. Or 99_gaVhr.
Using Rational Method:
Calculate the area of retention required in limiting infiltrator. Assume free water will percolate
through existing soils at a rate of 3"/hr. Washed rock will be placed two feet around the manhole. Try
a 4' diameter manhole with one 3' sections plus a 2' riser and 1.5' over -dig. The surface area of native
earth is: 27c(3.5)(3)=66 sf. Total amount of water permeated through walls =
(66)(4/12)(7.48_gal/cf)=165_gal/hr. 165 > 99 Therefore, this design is adequate.
Oldcastel or similar 48" diameter perforated precast manhole (one section), manhole base, cone,
grade rings as necessary and perforated cover. Average 18" over -dig backfilled with washed
rock for the perforated manhole section and AASHTO M288 class 2 non -woven geotextile over
the rock Bottom section to have gravel bottom for free flow of water from manhole to over -dig
gravel.
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f.,.: 2crr*, 2. IW Fm: Z3�5n-0701 ywwAM=dbPNNlmm COi3Vlaht0 2011