Pond relocation design memo
Short Elliott Hendrickson Inc., 3535 Vadnais Center Drive, St. Paul, MN 55110-5196 SEH is an equal opportunity employer | www.sehinc.com | 651.490.2000 | 800.325.2055 | 651.490.2150
fax MEMORANDUM TO: Paul Oehme/City of Chanhassen FROM: Rebecca Nestingen/SEH DATE: June 2, 2011 RE: Tower Site Pond Relocation SEH No. CHANH 114852 In the southwest corner of the Minnetonka
Middle School property a proposed water tower would require the relocation of an existing stormwater detention pond. The proposed relocation site of the pond is slightly to the north
into the adjacent hillside. Currently the pond receives stormwater runoff from a drainage area of approximately 5.5 acres, just slightly over half of which is impervious surface. For
the proposed pond relocation, the majority of the drainage from the east has been maintained, however, the drainage area increases slightly to 5.7 acres due to the pond relocation north
and additional drainage area from the grading on the hillside. The impervious surface area to the proposed pond remains consistent with the existing conditions. The existing pond functions
as a dry detention basin which reduces peak flow rates and provides marginal water quality benefits. The objective of the proposed pond design was to maintain the flood storage of the
existing pond so as to not exceed existing peak flow rates and to gain water quality treatment benefits through a two cell filtration basin design. Table 1 below summarizes the peak
flow rates for the existing and proposed conditions which have been modeled using HydroCAD®. Attached is a HydroCAD® report for further details. To quantify the water quality benefits
of the proposed basin in relation to the existing pond, the existing and proposed conditions were modeled in P8. Table 2 below summarizes the P8 results of total suspended solids (TSS)
and total phosphorus (TP) annual load reductions. Table 1 Peak flow rates (in cfs) Storm Event Existing Conditions Proposed Conditions 2-yr, 24-hour 6.25 4.27 10-yr, 24-hour 7.55 7.51
100-yr, 24-hour 8.99 8.98 Table 2 TSS and TP load reductions TSS Load (lbs/year) TP Load (lbs/yr) Existing Proposed Existing Proposed Pond Inflow 166.7 162.7 0.95 0.94 Pond Outflow 50.5
31.6 0.74 0.30 % Removal 70% 81% 22% 68% Please do not hesitate to contact me at 651-490-2175 or rnestingen@sehinc.com if any questions or concerns arise. rsn Attachment s:\ae\c\chanh\114852\5-dsgn\p
ond relocation design memo.docx