HomeMy WebLinkAboutTreasure Valley Pediatrics - Storm Water System Narrative, Calculations and Soils ReportTreasure Valley Pediatrics
Storm Water System
Narrrative, Calculations
And Soils Report
March 2013
Building Addition
1620 South Celebration Avenue
Meridian, Idaho
B & A Engineers, Inc.
Consulting Engineers & Land Surveyors
5505 West Franklin Road. Boise, ID 83705
Telephone 208+343+3381 Facsimile 208+342+5792
Storm Water Narrative
The existing Treasure Valley Pediatrics building located on the northeast corner of Overland
Road and Celebration Avenue is proposed to have a building addition added to the north side of
the facility. The addition (roof area) will be approximately 2,309 square feet in area.
The existing building and site improvements do not discharge storm run-off to any area in
proximity to the proposed addition. The proposed addition will not interfere with any existing
drainage patterns from existing impervious surfaces.
Minimal change is proposed to the existing site. The new building addition will slightly encroach
into an existing earthen berm and cover some existing pervious (landscaped) surface. Since the
existing berm would essentially engage the north line of the new addition, it is proposed to peel
back part of the berm and slightly increase the slope in the area. This will allow the creation of a
biofiltration Swale just north of the building addition. This Swale will intercept new roof run-off
and any nuisance water running toward the building from the berm area.
The new shallow swale (approximately 15 -inches deep) will have a bottom that is sloped toward
two drainage windows located at each end of the swale. Site soils are resistant to natural
infiltration at depths above approximately four feet.
Site Soil Report
Since a shallow infiltration swale is the suggested method the storm water control and mitigation
for the proposed building addition, and the size of the new addition is relatively small, a new site
specific geotech report has not been conducted. However site soil information is available from
the Natural Resource Conservation Service (MRCS), originally created by the Soil Conservation
Service, as published in the Ada County Area, Idaho Soil Survey. Said survey provides the
following information:
• The site is located in an area designated as soil 448, Elijah Silt Loam
• These soils are moderately deep to an underlying hardpan
• The slope is generally relatively flat ranging from 0 to 2 percent.
• The average annual precipitation is eleven inches.
• The general soil profile is:
0 to 11 inches Silt Loam (CL -ML)
11 to 26 inches Silty Clay Loam, Loam (CL -ML)
26 to 31 inches Silt Loam (CL -ML)
31 to 43 inches Indurated - Hardpan
43 to 96 inches Gravelly Loamy Sand (GP -GM)
• Permeability of the subsoil below the hardpan is fairly rapid — in excessive of a
sustained 20 inches per hour, however for design, the infiltration rate of 8 inches per
hour is used
• Water table unknown, but greater than 6 feet
For the purposes of the Treasure Valley Pediatrics building addition, the following design
information was used:
1. Seasonal High Water Table: Greater than 6 feet
2. Sustained Infiltration Rate at depth with Filter Sand: 8 inches/hour
Storm Water Calculations
The method of storm water conveyance and containment for the proposed building addition is as
follows:
• Regrade the earthen berm north of the proposed building addition to create positive slope
away from the addition.
• Construct a landscaped swale as a primary retention area to capture all design storm run-
off from the roof of the addition and the sloped berm area adjoining the swale.
• Place two drainage windows in the Swale to assure swale drain time in less than 24 hours.
Storm water calculations are as follows:
• New Roof Area:
• Swale Surface Area:
• Berm Area Grading Toward Swale:
• Design Storm (60 Minute Duration)
• Roof and Swale Run-off Coefficient:
• Berm Area Run-off Coefficient:
• Roof and Swale Run-off Volume:
• Berm Area Run-off Volume:
• Total Design Storm Run-off Volume:
• Swale Volume Available:
• Infiltration Occurring During One -Hour Storm Event:
• Total Retention Volume Available:
• Approximate Swale Drain Time:
2,309 Square Feet
618 Square Feet
974 Square Feet
1 Inch per Hour
0.95
0.60
232 Cubic Feet
49 Cubic Feet
281 Cubic Feet
335 Cubic Feet
12 Cubic Feet
347 Cubic Feet
23.4 Hours
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