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Southfork Design Group LLC, 1066 N. Yellowstone Hwy., Rexburg, ID 83440, 208-852-6236
D.J. Barney Storage Units Drainage 3/30/16
By: Ray McDougal, P.E.
Given:
Calculate storm water volume collected from the parking area and roof area in 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 paring 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 infiltrators and rainfall intensity
(i=0.9 in/hr for one-hr period or i=0.2_in/hr for 24-hr period). No off-site discharge allowed.
Required:
Calculate the size of the required infiltrators: The breakout of the areas is as follows: Total Roofs
areas = 6,000_sf, Total paved areas = 9,570_sf.
Design:
Total runoff flow per hour=[(1.0)(6,000)+(0.95)(9,750)](0.2/12)(7.48_gal/ft^3)=1,903_gal/hr
Using Rational Method:
Calculate the area of infiltration required to handle runoff at time of concentration. Assume free water
will percolate through existing soils at a rate of 2”/hr. Washed rock will be placed two feet around the
manhole. Try a 6’ diameter manhole with two 3’ sections plus a 2’ riser and 2’ over-dig at (6)
locations. The top portion of the section is not included in the height of the permeation area but 1’ of
the riser section is used for permeation area giving a height of 7’. The total surface area of native earth
is: [2π(5)(7)+π(5)2](6)=1,789_sf. Total amount of water permeated through walls and bottom =
(1,789)(2/12)(7.48_gal/cf)=2,231_gal/hr. 2,231>1,903_gal/hr Therefore, this design is adequate.
Design Summary:
Oldcastel or similar 72” diameter perforated precast manhole (two sections), gravel base, cone,
grade rings as necessary and perforated cover. Average 24” over-dig backfilled with washed
rock and AASHTO M288 class 2 non-woven geotextile over the rock.
Southfork Design Group LLC, 1066 N. Yellowstone Hwy., Rexburg, ID 83440, 208-852-6236
Southfork Design Group LLC, 1066 N. Yellowstone Hwy., Rexburg, ID 83440, 208-852-6236