HomeMy WebLinkAboutTPK_DJM^1500_Bracketts_Drainage_Review_070207Memorandum
To:Tom Kellogg Project:1500 Bracketts Point Road
Plat # 139-3293 Date:7/3/2007
From:Lance Hoff, Dan
Murphy Client:City of Orono
Re: Drainage Review File No: 139GEN
Remarks:
Existing and proposed grading plans (dated 11-20-06) and a HydroCAD model (dated 6/22/07)
were reviewed for the site at 1500 Brackett’s Point Road in Orono. We offer the following
comments:
1.The proposed rain gardens provide sufficient rate control such that proposed runoff
rates meet existing runoff rates for the 2-, 10-, and 100-yr, 24 hr storm events.
2.The two rain gardens provide sufficient water quality volume to treat the volume of
runoff generated by the 2.5”, 24 hr storm over the entire contributing drainage area
to each (NURP requirement).
3.No water quality treatment is provided for the northwest section of the proposed
private drive. An option would be to construct another rain garden on the south side
of the drive to receive this drainage. This could outlet to the west to prevent runoff
from the proposed north rain garden from flowing over southern driveway stub.
4.The EOF for the south rain garden is noted on the plans at 940.20. The riprapped
overflow is noted at an elevation of 940.00. This discrepancy should be resolved. In
general, the EOFs for the proposed rain gardens should be set at or above the HWLs.
5.The HWLs for the two proposed raingardens were modeled assuming the rain gardens
are dry. When calculating outflow rates and high water levels the rain gardens
should be assumed to be filled to the NWL. We calculate HWLs of 934.4 and 940.3
for the north and south rain gardens, respectively.
6.Riprap or another type of permanent erosion control measure should be provided at
the rain garden outlet pipes.
7.How is runoff routed to existing Bracketts Point Road proposed to be collected and
conveyed?
8.The annualized percentage of runoff infiltrated is approximately 86% and 84% for
the north and south rain gardens, respectively, assuming the rain gardens are
maintained in good working condition.
2335 Highway 36 W
St. Paul, MN 55113
Tel 651-636-4600
Fax 651-636-1311
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