HomeMy WebLinkAboutCrimson Maple Townhomes - Storm Water Design ReportCrimson Maple Townhomes
STORM WATER DESIGN REPORT
Prepared for the City of Meridian
Revised
April 14th, 2014
�j J.J. Howard Consulting Engineers
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
4th 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 ACHD standards. Analysis of this portion of the site has been analyzed
and reported to ACRD. 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. For the
purposes of design and for this analysis, a design high groundwater depth of 14 feet from
natural ground was used, or elevation 2579.00, NAVD 88.
PROPOSED CONDITIONS
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
vegetated swales. These swales will consist of an approximately 1.5 -ft depression with
sandy top soil, hydroseeding and native, adaptive vegetation suitable for wet conditions.
Additional storage volume will be provided by 2" drain rock and filter sand beneath the
swales topsoil, separated by a geotextile. The swales will also have a meandering
pathway of river cobbles to create a dry river bed aesthetic and provide a direct
connection to the drain rock beneath.
The grass and other vegetation in the swales will provide adequate pretreatment for the
storm water prior to infiltration. The filter sand layer will provide another level of
treatment before the water enters the native free draining soils and ultimately recharges
the groundwater aquifer. Vegetation as a means of pollutant removal from stormwater
has been found to be more effective than more traditional means such as sediment boxes.
Details of the swales 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 Sizin
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 swale
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.
Based on the soil profiles from well logs in the area, it is reasonable to assume that
infiltration rates of 8-inches/hour or even higher could be achieved at depth in the free
draining native material. However, because we do not have in field percolation testing, a
conservative estimate of 4 -inches per hour was used for the design rate. Using this rate,
the proposed drainage swales should be capable of draining completely within 24-hrs of
the design storm as can be seen in the attached calculations. Testing should be performed
during construction to verify that this design rate can be achieved.
All drainage swales have been designed with 0.5 -ft of free board as measured from the
top of swale.
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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