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February 8, 2021
Mr. Alex Jordan - Plan Review Coordinator
City of Carmel Engineering Department
One Civic Square
Carmel, IN 46032
RE: Magnolia Condominiums – Phase 1
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 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
2. 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 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.
3. 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.
4. 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.
5. 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
February 8, 2021 // Magnolia-Stormwater Waiver Requests // PAGE 2 of 2
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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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 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