HomeMy WebLinkAboutRe: drainage review '/�'BOnCSi/00
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Project Name: Hill School Client:City of Orono
To:Tom Kellogg File No: 139-06-000
From: Lance Hoff Date:8-22-06
Re: Drainage Review
Summary of Recommendations
Based on our review of the Hill School Expansion, we offer the following comments:
1. A riprapped outlet should be provided for the proposed filtration basin. We estimate that a 2'-wide
EOF at an elevation of 959 will help to control peak rates while maintaining the City's freeboard
requirements.
2. Peak rates leaving the site have been modeled and are summarized below:
Peak Flows(cfs)
Location Existing Proposed*
. 2-yr 10-yr 100-yr 2-yr 10-yr 100-yr
NW 0.3 0.7 1.3 0.0 0.0 0.6
N E 0.4 0.7 1.2 0.8 1.2 1.8
Overall 0.7 1.4 2.5 0.8 1.2 1.9
"Includes an addition to the filtration basin of a 2'-wide riprapped
emergency overflow at an elevation of 959.
Existing peak rates to the NW have been adequately managed by utilizing the proposed filtration
basin. Peak rates to the NE are shown to exceed existing conditions. While this would not typically
meet City requirements, the additional flow the NE is likely manageable. The increase in peak rates
is relatively small, largely due to the relatively small lot size (�0.6 acre). Routing of runoff from the
east side of the site (parking lot) to the filtration basin would be difficult and unwanted due to
existing drainage facilities located to the NW of the site. Just north of the site, a private draintile
system conveys flows from a low area to the north. Our primary concern for this site is to ensure
that any existing drainage problems in that area are not exacerbated. Modeling shows that peak
rates and runoff volume will be decrease to the NW under proposed conditions. Overall existing
peak flow rates have also largely been met for the site. Therefore, we do not recommend any
modifications of the proposed site aside from Comment 1.
Bonestroo, Rosene,Anderlik and Associates, Inc.
2335 West Highway 36+ St. Paul, MN 55113+ Phone: 651-636-4600+ Fax: 651-636-1311