HomeMy WebLinkAbout06070008.27I. INTRODUCTION
The proposed project is tile development of a 3.76acres tract of land located at
15005 North Pennsylvania Avenue. Carmel, Hamilton County, Indiana. The,
property is a c(unmercial site. The project is to construct a two story building of
approximately 33,062 total square feet and associated parking and infrastructure.
1be project will disturb 2.72 acres of the site.
EXIST INC. DRAINAGE CONDITIONS
']'he existing condition of the property is rough graded, re-vegetated (grass) soil,
The site utilizes overland flow as the primary drainage system. The runoff flows
towards the south- southeast comer of the property into a regional detention pond.
The current detention pond has 3.905 acre-feet of storage. The existing normal
pool of the detention facility is approximately 841. 18. The normal pool elevation
is controlled by the outlet pipe and the downstream pipe inverts. approximately
685 If of 12" RCP 0% grade. At the time of the topographic survey, the pool
level of the pond was approximately 841.73.
The original drainage report by'rranimel-Crow Company filed on Mav 26, 1987,
states that the pond is to have 5.84 acre-feet of storage over 5 feet of vertical
elevation. From additional information provided by the original engineering firm.,
the entire parent parcel uses 4.28 acre-feet of the available detention at a depth of
3.77 fad of vertical storage.
The original land prior to the Trammell-Crow drainage approval of 1187 and
development had two discharge points. The site was broken into a northern and
southern basin. the north basin had 12.35 acres and the south basin had 5.64
acres. The original proposed discharge was to die north with a 100 year storm
event discharge rate of 0.25 efs'acre. Thereby. the allowable release rate on the
north outlet basin is 12.35 acres * 0.25 cfs, acre = 3.09 efs, and die south release
rate is 0.25 cfs -acre * 5.64 acres - 1.41 cfs.
From the Trammell -Crow drainage report the developed discharge for the site on
the I00 -year storni, is 2.94 efs. which is less than the allowed 3.09 cfs, The
developed conditions per the Trammell -Crow Drainage report are listed below:
2.67 cf.,; Pond Elevation -- 84-4.19
The as-hui It design of the pond gives the following release rate and pond
elevation:
Q4.25%stcva -- 2,47 efs Pond Elevation = 845.43
,As an additional limitation on the development for this parcel. the available
storage on the existing pond is constrained by an existing pipe that is at the
northeast corner of the pond. `Ibis pipe allows water to flow in both directions.
This occurrence has backflow into the swalc that collects water for the pipe to
discharge into the pond. But. as die pond rises in elevation, tall water is created,
surcharges said pipe and creates a secondary escape path for the detained water.
This secondary discharge point conveys storm water into the swale at time cast
property tine of Fireman's Insurance parcel, The build-up of water at this area
will eventually over top the lowest point in Washington Boulevard. The elevation
at which this happens is an elevation ol'844.28. This gives a consavaii-C storage
volume of 2.22 acre-feet per existing conditions.
These two scenarios are done using an SCS Type 11 storm. The analysis of the 2-
year, 10-year and 100-year storm with the corresponding rainfall amounts will
give 311 overtopping of the pond in time 100-year event.
111. PROPOSED ")NDITIONS
The new parking area and building runoff will drain to the re-constructed
detention pond. The weighted runoff C factor will vary from each inlet.
The original design of the detention pond called for 5.84 acre -fact of storage. The
current detention pond has 2.73 acre-feet of storage over 3.82 feet of vertical
flUCtUati011.
.-Uler discussion with the City of Carmel Engineering Department, the 100 year
storm event elevation in the retention pond resulting from this project must be
below the lowest adjacent grade of Washington Boulevard, which is 844.28. In
order to do this, the existing area not collected in existing stoma sewers and
detained in the existing pond will not be collected in the proposed storm sewers,
and detained in the modified pond. This existing area is approximately 6.49 acres
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