HomeMy WebLinkAboutStorm Drainage ReportPrepared For: Tradewinds Subdivision
Phase 2
SDN, LLC
& Meridian, Idaho
City of Meridian
Storm Drainage Report
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Prepared By:
Matthew Derr, P.E.
Design Engineer
XM Engineering, LLP
9233 West State Street
Boise, ID 83714
208.639.6939
mderr@kmenglip.com
September 19th, 2013
Project No: 13-097
Engineers . Surveyors . Planners
Table of Contents
Introduction.........................................................................................................................................1
ProjectDescription..............................................................................................................................1
TributaryArea.....................................................................................................................................1
Scopeand Methods.............................................................................................................................1
InfiltrationBasin...................................................................................................................................1
InletCapacity.......................................................................................................................................2
Summary...........................................................................................
....................................................2
INTRODUCTION
The purpose of this report is to show that the storm drainage facilities are designed to
meet Ada County Highway District standards. All drainage calculations were derived
using the standard of one inch of storm water over the impervious area. The Modified
Rational Method is the standard method for small catchments. Based on our
calculations, we verified that for the 100 -year storm the infiltration basin is adequately
sized.
PROJECT DESCRIPTION
This 10.78 -acre project is located on the southeast corner of Victory Road and Locust
Grove Road in Meridian, Idaho. It is a residential subdivision consisting of 45 lots: 39
buildable lots and 6 common lots. The proposed improvements to the site include
roads, sidewalks, and site utilities. This area was previously studied with the previously
submitted and approved Tradewinds Subdivision Storm Drainage Report, by KM
Engineering, revised July 12, 2013 (the Phase 1 report)
TRIBUTARY AREA
We determined the tributary areas for this project based upon the grading of the
roadway and lots. Since the roadways are draining to an infiltration swale, the tributary
areas include the entire roadway plus half of each lot. The exhibit included with this
report illustrates the tributary areas. As noted, this drainage was accounted for in the
previous report.
SCOPE AND METHODS
The Phase 1 report utilized the Modified Rational Method provided in the ACHD
calculation sheets to calculate the storm water volumes and flow rates for this project.
Flow rates and storm volumes were established for each basin for the 25 -year and 100 -
year storms. Calculation of the infiltration basin, inlets and street sections were
completed to verify capacity.
INFILTRATION BASIN
The infiltration basin (POND -1) built with the previous phase will be incorporated with
the drainage system designed for Tradewinds Subdivision Phase 2. Based on previous
calculations, the infiltration basin is adequately sized to ensure that no ponding occurs
on the surface and that storm water will infiltrate within a 24-hour period. The
geotechnical report prepared by STRATA, Inc. dated October 12, 2012, encountered
ground water at depths of approximately 2.5 to 3 feet below existing ground surface.
1
STRATA, Inc. recommends high seasonal ground water to occur at a depth of 2.5 feet
below existing ground surface in non -fill areas.
See Tradewinds Subdivision Storm Drainage Report, revised July 12, 2013, for additional
information.
INLET CAPACITY
There are no additional inlet for this phase of construction.
The gutter capacity of Fathom Drive was verified to ensure that overtopping of the curb
would not occur in the 25 -year storm. Curb capacity was calculated at 5.98 cfs with a
maximum 25 -year flow of 4.56 cfs.
See Tradewinds Subdivision Storm Drainage Report, date revised July 12, 2013, for
additional information.
SUMMARY
This Phase 1 report has accounted for the additional flow anticipated by the Phase 2
construction and the infiltration facility (POND -1) has been constructed to accept these
flow. No changes to the drainage pattern have been proposed for the Phase 2
construction, therefore the calculations provided in the Phase 1 report will adequately
convey and detain the proposed storm drainage.
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BASIN MAP
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