HomeMy WebLinkAboutApplication Files_85048_pg0182 each of the subcatchments are provided in Appendix 3. A summary of the calculated
peak flows is provided in Table 2.
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C. Post Development Hydrology
' The two designpoints selected for calculations of pre-development p the pre development condition were
also used for the calculations of the post-development condition. The post development
site is divided into 22 subcatchments as shown on the Post Development Drainage Area
Map in Appendix 2. Subcatchment 1 includes the on-site areas that discharge directly
into Sullivan Avenue. Subcatchments 1S, 2S, 3S and 6S reflect the same off-site areas
that contribute runoff to the existing drainage system in Sullivan Avenue. Subcatchment
20 includes the entire on-site area that discharges to Design Point #2. The remaining
subcatchments (1-17) include the areas that discharge into the series of basins on-site.
The characteristics of each of these subcatchments are summarized in the attached
HydroCAD data sheets in Appendix 4 and 5.
The project will result in the addition of approximately 24 acres of impervious area. In
order to mitigate the increase in runoff resulting from the increase in impervious area, a
series of six interconnected stormwater management basins are being proposed. Based on
the shallow depth to groundwater at the site, all of the basins will contain a permanent
pool area. Basins 1 through 5 will provide initial treatment (as a sediment forebay) as
' well as storage above the permanent pool elevation for detention. Basin#6 provides the
entire water quality volume for final treatment of the runoff prior to discharge from the
site, as well as excess storage above the permanent pool elevation for detention.
With he '
t exception of Basins 3 & 5, all basins will be equipped with an aquatic bench (5
to 10' wide) that will submerged by 1.5' and be planted with a wetland seed mix to
provide vegetation to aid in treatment of the runoff. Basin outlets will consist of culverts
with trash racks. The side slopes of all basins are 4:1 with the exception of the slopes
below the aquatic bench in Basin#6, which are graded at 3:1. Basin 46 is also equipped
with a perimeter access drive to allow for additional access for maintenance.
' The individual water quality volumes and sediment loads were calculated for all basins.
These calculations are provided in Appendix 6. The calculations were performed to
verify that Basins 1-5 have adequate capacity to serve as sediment forebays. A review of
' the capacity of Basins 1-5 found in the HydroCAD data sheets in Appendix 4 shows that
each basin has more than sufficient storage to meet the forebay requirement. Similarly,
the same calculations were performed for Basin#6. The overall water quality volume for
the site was determined to be 2 acre-feet. As shown in the calculations in Appendix 4,
the ponded areas within Basin#6 exceed this required water quality volume.
Using the characteristics described above, the Post Development peak flow rates for the
entire site were calculated for the design storms. Refer to Appendix 4 and 5 for
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