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November 25, 2019
Mr. Jeremy Kashman, P.E., Director of Engineering
City of Carmel
City Hall, 1st floor
1 Civic Square
Carmel, IN 46032
317‐571‐2441
Re: Variance Request for New Carmel Elementary School detention release rates
Dear Mr. Kashman,
Based upon our discussions with your Department we are formally requesting a waiver from the City of
Carmel Engineering Department. Summary of discussion is as follows:
Existing site has a central high point which causes the drainage pattern to disperse in a pattern that fans
out to four (4) different sub‐basins (4 different points of storm water collection). All these sub‐basins
drain to Williams Creek Regulated Drain. Two (2) of the sub‐basins go to Ames Run tributary to Williams
Creek (NE and SE in the report). The drainage area of these 2 sub‐basins is 16.46 acres (11. 25 ac NE and
5.21 ac SE). The other 2 sub‐basins are S (17.03 ac) and SW (1.52 ac). The total area of the site is 35.01
acres.
We are able to design one storm detention pond which collects 29.36 acres of developed area. The City
of Carmel Stormwater Technical Standards Manual requires post‐developed release rates of 0.1 cfs per
acre of developed area for a 10‐year storm and 0.3 cfs for a 100‐year storm. We are proposing to
release at 2.62 cfs in 10‐year storm and 7.13 cfs in 100‐year, which is acceptable for a developed area of
23.76 acres or more (7.13 cfs / 0.3 cfs/acre). The downstream conveyance has more than enough
capacity for the 100‐year peak flow. The capacity of the downstream conveyance is 31.19 cfs.
The City of Carmel Engineering Department has said that the drainage area which applies to the 0.1 / 0.3
rule is existing condition. The City also had expressed a stance that we could only count the 2 existing
sub‐basins that go to Ames Run as our developed area calculation. We expressed that the S (south) sub‐
basin discharges so close in proximity to the SE (southeast) sub‐basin that we should be able to include
the SE sub‐basin into our developed area total, which would sum (NE + SE + S) to 33.49 acres.
We would like to formally request a waiver to release 2.62 cfs in the 10‐year storm event and 7.13 cfs in
the 100‐year storm event for the school site.
Sincerely,
Karen K. Collins, PE
Site Division Manager
A&F Engineering