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Letter from T. Schwalbe 8-31-04 '. ~vUlf~¿7 Lower Minnesota River Watershed District RecEIVED SEP 0 '2 2004 CITY OFCHANHASSEN IV'r ~ '" ~~ ~ .~ 5 Edward A. Schlampp, President Helmepin County Ron Kraemer, Vice President Dakota County Lawrence Samstad Treasurer Scott County ..~. Leo Forner, Secretary Carver County Len Kremer Assistant Treasurer Hennepin County Terry L. Schwalbe, Administrator Cell (952) 221-1089 August 31, 2004 Bob Generous, Planner City of Chanhassen 7700 Market Blvd. PO Box 147 Chanhassen, MN 55317 RE: Moon Valley Gravel Mine, Chanhassen Dear Bob: We have previously mailed this letter of comments on the above referenced project. However, at that time we were unable to get board input due to the deadline. We are re-submitting the . previous letter dated August 14, 2004 under the new date of August 31,2004 and including the board's comments on page three. The Lower Minnesota River Watershed District (LMRWD) thanks you for the opportunity to review the Moon Valley Gravel Mine in Chanhassen. We are submitting a summary of our engineering review of the grading, drainage, and erosion control plan for the Moon Valley Gravel Mine located in Chanhassen, Minnesota. The plans that were reviewed included the grading, drainage, and erosion control plan dated July, 16, 2004, and a Stormwater Calculations report dated July 21, 2004. Both were prepared by the Sathre-Bergquist firm. Hydrology and water quantity related comments are as follows: 1. The curve number selection of 69 for existing conditions seems high given the sandy soils ,110v£ CÐ'!~v¡¡H ~ prevalent on the site. The curve number should be lowered to 61 to better reflect current site conditions. 2. The curve number selected for proposed land use conditions appears low. (CN=76) For clarity and to better model small storm runoff from the site, the proposed medium density/high density residential areas should be modeled separately from the overall watersheds, rather than lumping them together. This will assist in future reviews in determining if the proposed development is consistent with the current proposal. 3. Times of concentration used in the model from the sheetflow calculations appear to be too long, resulting in underestimated peak discharges. The Manning's "n" value used in the calculations Page 1 of3 Scott County Government Center 200 4th Avenue West Shakopee, MN 55379 Phone: (952) 496-8842, Fax: (952) 496-8844 E-mail terrys@lowermn.com '.J " Lower Minnesota River Watershed District corresponds to dense grass conditions, which are likely optimistic given the slopes of the site. A better estimate can be made using either short grass or unpaved conditions to be conservative, especially given the steep slopes on the project site. 4. Overall, the rate controls provided by the proposed ponds meets the 10 year and 100 year I' t- .().. c;.: N predevelopment discharge rates. However, it does not meet the 2-year predevelopment discharge rv Ù Øl. r rate. This could be accommodated by installing a small orifice and overflow weir outlet control V· ~ structure without drastically raising the high water levels from the 10 and 100 year events. Changes resulting from comments #1-3 may change these results. If;\ Drainage to Pond 1 from a portion of WS-5P is not accounted for in the rate control calculations II . (IV provided for Pond 1. Although this area is not proposed for development, runoff from this area will ,t-i1v affect pond operation. Modeling of peak flows and rate control for the 2, 10, and 100 year events should be re-evaluated with a revised watershed for Pond 1. , ~Watershed WS-1 P does not have clearly defined drainage boundaries. Although it does not enter i~ the portion of the site that is to be regarded, it will neeçl t¡9",be JI~tter defin. e""d ,iT)¡ Cffder to quantify ¡ ..\:( whether or not flooding will occur over TH212. Ð~'«! b1Y ~{\.,. TV1JU..'l 7. The proposed drainage diagram appears tö show runoff from the Settlers West Development on top of the bluff draining to a concentrated point at the top of the proposed 2.5 :1 constructed slopes. These concentrated flows must be avoided and prevented in order to prevent severe erosion of the steep slopes. Runoff may need to be piped down the face of the slope at this location. Þ., (, ,_/>'" jy" ; 8. The west end of watershed WS-3P will naturally drain to the channel/wetland to the west, while the ultimatE;¡ proposed plan indicates it will be directed east to Pond 2. Water quality treatment should be provided prior to discharging to the channel/wetland complex. A temporary sediment basin may need t9,be ins~all~d ~T this ¡IORí?tion ,until. fwal development occurs and redirects runoff to Pond 2. IV Dt MLR£ul W lii ~ ~gvlCC ~ 9. It is not clear how runoff from watershed WS-6P will be directed to the proposed Pond 1. The i (,('.... grading plans show sheet flow to the south, where it will intersect the proposed site access road. I' . . Based solely on contours, it appears runoff will sheetflow over the road and then will continue on to TH 212. ft,L~gcrl~: h&,\ lætr i1~ ~~ to~~~ bS~OUld be included for review. 19,'No details. fegar~ing. the propo~ed pond outlets were suþ;mit{ed fo._,~~e.~iew... .....~. a minj~Um,..they .j_ r /~st provide sklmmmg up to the 5-year storm event. LJl.:¡ (~;'(/ ,. (/ - ~JI<I!/J1 W ('/l; !11(J)\ ¡t f :' \ ~ \ /. . The plans do not make it clear how runoff from watersh~d WS-5P leave~ the site. High wa~er ( · ( .. levels at any proposed or current outlet may affect the high water levels In Pond 1. A (~0htl~ ~ f12 1?:The steep slopes proposed for the site (2.5H:1V) will require substantial efforts be placed on / erosion control during construction and vegetation establishment. The site should be thoroughly reviewed for erosion control measures as the current single lines of silt fence shown on the plan are likely not enough given the height and length of the proposed slopes. Water quality related comments are as follows: 13. The stormwater ponds should be designed to achieve a 60% reduction in average annual ~_~Scl/" phosphorus loads. No details regarding the proposed nutrient removal efficiency of the ponds were provided. At! 0fJ . k1' I c /A)., < ¿., './1 \ V. . ".- Page 2 of3 Scott County Government Center 200 4'h Avenue West Shako pee, MN 55379 Phone: (952) 496-8842, Fax: (952) 496-8844 E-mail tel.rys@lowel.mn.com -<, , ··t -~) Lower Minnesota River Watershed District < yond 1 appears to have a wet volume of 0.71 acre-feet and a mean d~pth of 2.45 feet. The ~'lT)inimUm recommended mean depth is 3 feet for a water quality pond, in order to prevent scour and re-suspension of captureä sedimènt. fZ V \' (J ·é In addition, at the LMRWD's Board of Manager's meeting held on August 18, 2004, the board made a motion of additional comments as follows: It was moved and seconded to: 1. Send the letter out with recommendations that the Pemton land Company's plan be altered to meet these requirements laid out in the August 14,2004 memorandum and Pemtom Land Company should reply to the District for each item in the memorandum. 2. Encourage the City of Chanhassen to work with us in the development of a mutually acceptable agreement to enforce the provisions of thelMRWD 509 Plan and express the expediency of getting the plan in place. If we can't reach a mutual agreement, then the LMRWD will be forced to adopt rules to enforce stormwater management within the City of Chanhassen. 3. We want to hear back from you that through your permitting process you have implemented these standards and if not, why not, and have them send us a copy of the final permit with a plan or conditions adopted. VOTE: Ayes 5; Nays O. Thank you. Sincerely. erry L. Schwalbe District Administrator cc: Lori Haak, Water Resource Coordinator Page 3 of3 Scott County Government Center 200 lh Avenue West Shakopee, MN 55379 Phone: (952) 496-8842, Fax: (952) 496-8844 E-mail terrys@lowernm.com