HomeMy WebLinkAboutCrimson Maple Townhomes - Storm Water Design Report (2)Crimson Maple Townhomes
STORM WATER DESIGN REPORT
Prepared for the City of Meridian
December 16th, 2013
Howard Consulting Engineers
a
5983 W State St., Ste D
Boise, Idaho 83703
Ph: (208) 846-8937 Fax: (208) 846-8822
PROJECT DESCRIPTION
Crimson Maple Townhomes is a 1.59 acre multi -family residential development in
Meridian, Idaho. The site is located approximately 100 feet north of Maple Ave. on NW
0' Street. This storm water report is to accompany the civil construction documents
prepared by JJ Howard Engineers included with the application submittal. The purpose
of this report is to show the hydrologic and hydraulic methodologies used for design and
how the proposed improvements will comply Meridian storm water policy.
EXISTING CONDITIONS
This site has approximately 80 feet of frontage along NW 4th street that will be improved
to the current ACRD standards. Analysis of this portion of the site has been analyzed
and reported to ACHD. The remainder of the site consists of undeveloped ground with
natural vegetation. Soil profiles were determined by studying the well driller's log of an
adjacent property and consist of a layer of top soil followed by brown sandy clay and
then light brown sand. Groundwater was encountered between 14 and 19 feet.
The proposed development will include a driveway approach and private drive with
attached sidewalk to service the individual townhome units. This development will have
8 buildings at full build -out with foundation footprints ranging from 1000+ sf for the
management office to 4,000+ sf for the one-story duplex unit. Two car driveways are
included with each unit and an additional 4 car guest parking area will also be
constructed.
Storm water runoff will sheet flow across the driveways and private drive to designated
retention/infiltration beds. These beds will consist of 2" drain rock and filter sand and
will be depressed approximately 5- inches below the roadway edge for additional
volume. Details can be found on sheets C.5 and C.6 and calculations for basin analysis
and facility sizing are included in the attached calculations and Drainage Area map.
Methodology and Facility Sizing
The Rational Method was used for all peak flow and volume calculations as allowed by
the City of Meridian. The 100 -yr storm was used for volume calculations and
retention/infiltration bed sizing. Composite runoff coefficients were calculated for each
drainage basin and are specific to the site improvements (see attached Drainage Basin
Map for individual drainage basin areas). The proposed homes and driveways will
account for a large percentage of the site area and the Composite `C' values reflect this
high density. The time of concentration for all basins was assumed to be 10 -minutes
which is the typical value used as a minimum for small drainage basins. See attached
calculations spreadsheets for peak volume determinations as well as for facility sizing.
Retention/infiltration facilities were designed with 15% additional storage for sediment
build up. Based on the assumed infiltration rate of 4 inchesihour, these facilities will
easily be able to fully drain within 24 -hours of the design storm. Testing should be
performed during construction to verify percolation rates at the base of the facilities.
Conclusions
This storm water analysis and report shows that the proposed improvements for the
Crimson Maple Townhomes development are in compliance with standard engineering
practices and City of Meridian policy.
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