HomeMy WebLinkAboutStormwater Waiver Requests (Revised)
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April 20, 2021
Mr. Alex Jordan - Plan Review Coordinator
City of Carmel Engineering Department
One Civic Square
Carmel, IN 46032
RE: Magnolia Condominiums – Phase 1
Revised Stormwater Waiver Requests
Mr. Jordan:
On behalf of Old Town Design Group, please accept this letter as the formal stormwater waiver
request from the following sections of the City of Carmel Stormwater Technical Standards Manual
(STSM):
STSM Reference:
1. Section 104.02: Grading and Building Pad Elevations – as shown on the plans, the proposed
project includes six (6) condominium buildings with garages on the first floor of each
building. Said garages provide direct access to the private interior alleys providing ingress
and egress for each building. The desired density and relatively small project site results in
the garage doors being located approx. 5 feet off the edge of the alley travel lane. As a result,
the site cannot be graded such that the finished floor elevations are 1 foot above the adjacent
roadway/private alleys. Additionally, the location of the required public street connection to
Pawnee Road; as well as, the topography of the existing site and street, result in the finished
floor elevations of buildings #1 and #2 being below the existing streets. As such, the
developer respectfully requests a waiver from the finished floor elevation requirements.
2. Section 302: Stormwater Detention Design – although it’s not explicitly stated in the STSM,
it’s our understanding that the City of Carmel allows redevelopment sites to provide
stormwater detention designed to release runoff resulting from the 10-year and 100-year
storm events under redevelopment conditions at the existing 2-year and 10-year rates as
opposed to the general release rates established in Section 302.03. Given the City’s intent to
construct a large, regional detention pond as part of the Japanese Garden expansion project,
which is located downstream of the Magnolia site and within the same watershed, the
developer respectfully requests a waiver to allow a detention system designed assuming a
“no net increase” policy as follows:
Overall Allowable Release Rates:
Redeveloped 2-yr ≤ Existing 2-yr
Redeveloped 10-yr ≤ Existing 10-yr
Redeveloped 100-yr ≤ Existing 100-yr
3. Section 302.06(2): 100-yr Detention Separation from Buildings – as shown on the plans, the
proposed project includes construction of six (6) condominium buildings in the full built-out
with an underground detention system. Given the layout and size of the site, 25-feet of
horizontal separation cannot be provided between the proposed buildings and the 100-year
April 20, 2021 // Magnolia-Revised Stormwater Waiver Requests // PAGE 2 of 3
detention elevation while providing the desired density. As such, the developer respectfully
requests a waiver to allow 6-feet of horizontal separation between the condominiums and
the underground detention system footprint.
4. Section 302.06(4): Detention Separation from Right-of-Way – as shown on the plans and
secondary plat, the developer intends to dedicate the proposed alleys between the
condominiums as public right-of-way; therefore, no space is available to locate the proposed
underground detention system outside the right-of-way. As such, the developer respectfully
requests a waiver to allow detention within the public right-of-way as shown.
5. Section 302.06(10): Permanent Stormwater Easement – as indicated in #2 above, the
developer respectfully requests a waiver to allow 6-feet of horizontal separation between the
condominiums and the underground detention system footprint due to the size and layout
of the site which is needed to meet the desired density.
6. Section 302.06(12): Detention of Right-of-Way Runoff – since it has been completely
reconstructed with all storm sewer draining to the north and west of the roundabout, it is
highly unlikely that runoff from 126th Street (City Center Drive) will ever be routed into the
proposed onsite Magnolia storm sewer and detention system. Furthermore, the topography
of Pawnee Road makes it infeasible to convey most of the runoff from right-of-way into the
proposed storm sewer. As such, the developer respectfully requests a waiver from this
requirement.
7. Section 303.07: MLAG/MFPG Requirements – the desired density and relatively small project
site results in a layout with condominium buildings that are separated from the proposed
storm sewer inlets located in the proposed alleys by less than 20 feet. Due to the attached
garages that have access off the proposed alleys, the buildings cannot be raised to meet the
MLAG/MFPG. Additionally, the location of the required public street connection to Pawnee
Road; as well as, the topography of the existing site and street, result in the finished floor
elevations of buildings #1 and #2 being below the existing streets. As such, the developer
respectfully requests a waiver from the MLAG/MFPG requirements.
8. Section 306.02(D): Easement Requirements – the size, layout, and desired density of the site
results in an underground detention system that is separated from the condominium
buildings by as little as 6-feet; therefore, the minimum 15-foot easement cannot be provided.
Furthermore, the proposed underground detention system is intended to be located entirely
within public street right-of-way. As such, the developer respectfully requests a waiver from
this section.
9. Section 401.01: Pipe Materials – the proposed site includes multiple ADS Nyloplast drain
basins which are PVC structures; therefore, it’s not feasible to use reinforced concrete pipe
throughout the project. As such, the developer respectfully requests a waiver to allow the use
of HDPE pipe for some structures.
10. Section 501.01: Pipe Cover – due to the elevation of the existing storm sewer where the
proposed system will outlet, the minimum 2.5-foot pipe cover requirement cannot be met
from Str. No. 5 to Str. No. 3. Although minimal pipe cover is provided, these pipes will not be
located in areas subject to vehicle loads. As such, the developer respectfully requests a waiver
from the minimum pipe cover requirements for Str. No. 4 and 5.
11. Section 700: Post-Construction Stormwater Quality – as shown on the plans, post-
construction water quality treatment will be provided by a treatment train consisting of 1)
isolator rows in the underground detention system, 2) manholes with sumps located
immediately upstream of the isolator rows, and 3) a Contech Cascade CS-4 hydrodynamic
separator unit located downstream of the underground detention system. Due to the site
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layout, there are multiple locations where runoff enters the detention system making it
impractical to locate the hydrodynamic separator upstream of the system. The site layout
also makes it impractical to utilize other pre-approved BMPs such as bioretention, infiltration
basins, media filtration, etc. in lieu of the isolator rows. As such, the developer respectfully
requests approval of the proposed post-construction treatment train as shown.
Please contact me at (317) 780-1555 ext. 135 or dsnyder@crossroadengineers.com if you have any
questions.
Sincerely,
CrossRoad Engineers, P.C.
Derek M. Snyder, P.E.
Project Engineer