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Engineering Comments 2-7-05 ,. CITY OF CHANHASSEN 7700 Market Boulevard PO Box 147 Chanhassen, MN 55317 Administration Phone 952.227.1100 Fax 952.227.1110 Building Inspections Phone 952.227.1180 Fax: 952.227.1190 Engineering Phone 952.227.1160 Fax 952.227.1170 Finance Phone 952.227.1140 Fax: 952.227.1110 Park & Recreation Phone: 952.227.1120 Fax: 952.227.1110 Recreation Center 2310 Coulter Boulevard Phone 952.227.1400 Fax: 952.227.1404 Planning & Natural Resources Phone: 952.227.1130 Fax 952.227.1110 Public Works 1591 Park Road Phone: 952.227.1300 Fax: 952.227.1310 Senior Center Phone: 952.227.1125 Fax: 952.227.1110 Web Site www.ci.chanhassen.mn.us / MEMORANDUM TO: Bob Generous, Senior Planner FROM: Matt Saam, Assistant City Engineer M~ February 7, 2005 DATE: SUBJ: Preliminary Plat Review for Hidden Creek Meadows LUR No. 04-16 Upon review of the plans dated January 14, 2005, prepared by Ryan Engineering, I offer the following comments: GRADING, DRAINAGE & EROSION CONTROL The plans propose to grade about 75% of the site for the 21 new house pads, proposed street ending with a cul-de-sac, and a storm water pond. The proposed grading will prepare the site for full basement, lookout, and walkout-type dwellings. The plan proposes a retaining wall in the back yard of Lot 7, Block 1. It appears the wall will be approximately six feet in height. The applicant should be aware that any retaining wall over 4 feet will require a building permit and must be designed by a Minnesota Registered Professional Structural Engineer. Also, the retaining wall must be 20 feet off the back of the building pad to provide adequate area for a backyard and deck. The existing site drains to a wetland along the south side of the parcel. Under developed conditions, the street, front yards and most of the house roofs and rear yard drainage will be conveyed via storm sewer to a proposed storm water pond for treatment prior to discharging to the existing wetland. The proposed development will be required to meet the existing stormwater runoff rates for the 10- and 100-year, 24-hour storm events. The applicant has done a good job of attempting to preserve the existing drainage pattern of the wetland on the site through the use of a storm sewer culvert. Where the proposed street has bisected the wetland, the culvert will act as a conduit and allow the water to flow as it would under existing conditions. According to the City's Surface Water Management Plan (SWMP), the proposed culvert must be a minimum size of 36-inches in diameter to facilitate proper drainage through the site. The developer is proposing a 42-inch diameter culvert under the road, which exceeds the minimum size requirement. By oversizing the culvert, the developer is being conservative and helping to ensure that the drainage will flow freely through the site. There is an area in the rear yard of Lot 9, Block 1 that will need to be revised to prevent trapping water behind the curb. Either a catch basin will need to be added or the area will have to be regraded with a minimum slope of 2% to drain from the The City 01 Chanhassen . A growing community with clean lakes, quality schools, a charming downtown, thriving businesses, winding trails, and beautiful parks A great place to live, work, and play Bob Generous February 7, 2005 Page 2 rearyard to the street. Also, staff recommends that the existing culvert across the street from Lot 9, Block 2 be incorporated into the storm sewer for Pipewood Lane. Staff has received drainage calculations for the development and only minor revisions are necessary. The proposed pond is required to be designed to National Urban Runoff Program (NURP) standards. The storm sewer will have to be designed for a lO-year, 24-hour storm event. Drainage and utility easements will need to be dedicated on the final plat over the public storm drainage system including the pond, drainage swales, and wetlands up to the 100-year flood level. The minimum utility easement width must be 20 feet wide. To help address concerns raised at the initial Public Hearing for this development, the applicant has submitted a drainage exhibit for the area in and around the development. This exhibit shows topography and critical HWL (high water level) elevations for the ponds and wetlands on the development site and upstream/downstream of the site. The HWL of the wetlands on the exhibit are from the City's SWMP plan. Most importantly, the exhibit shows the HWL of the wetland on the south side of TH 7. This wetland is upstream of the development site and was one of the areas of concern raised by the public at the last Public Hearing. The exhibit lists the HWL of the wetland as 943.4 which, based on the topography on the exhibit, is 4+ feet lower than any of the adjacent homes. In order for any of these homes to flood, the stormwater in the wetland would have to rise up past elevation 948 which would also begin to flood Minnewashta Parkway. The applicant has included silt fence around the grading perimeter. Adjacent to the pond and wetlands, silt fence type II must be used. A 75-foot minimum rock construction entrance has been shown at the street entrance to the site. In addition, an erosion control blanket will be required for the steep slopes along the north property line of the site. The applicant should be aware that any off-site grading will require an easement from the appropriate property owner. All disturbed areas must be sodded or seeded and mulched within two weeks of grading completion. UTILITIES Municipal sewer and water service is available to the site from existing mains, which run along the eastern property line and from Pipewood Lane on the southwest comer of the parcel. The applicant is proposing to extend sewer and water lines along the proposed streets to service the proposed lots. The watermain will be looped through the site to avoid stagnant water issues. According to the City's Finance Department records, there is a remaining assessment balance of $25,477.05 on the parcel. This balance is required to be paid at the time of final plat recording. Since the developer will be responsible for extending lateral sewer and water mains, the sanitary sewer and water connection charges will be waived. However, the sanitary sewer and water hookup charges will still be applicable for each lot. The 2005 trunk hookup charge is $1,458 for sanitary sewer Bob Generous February 7, 2005 Page 3 and $2,955 for watermain. Each of these charges is based on the number of SAC units calculated by the Metropolitan Council. Sanitary sewer and watermain hookup fees may be specially assessed against the parcel at the time of building permit Issuance. Public utility improvements are required to be constructed in accordance with the City's latest edition of Standard Specifications and Detail Plates. Detailed construction plans and specifications must be submitted at the time of final plat for staff review. The applicant is required to finance all of the proposed improvements. The applicant will also be required to enter into a development contract with the City and to supply the necessary financial security in the form of a letter of credit or cash escrow to guarantee installation of the improvements and the conditions of final plat approval. Permits from the appropriate regulatory agencies must be obtained, including but not limited to the MPCA, Department of Health, MnDOT, etc. STREETS The plans propose to extend Pipewood Lane, as the main road, approximately 1300 feet within the site. The public streets are shown within a 60-foot right-of-way, as per City ordinance along with street pavement widths of 31-feet back-to-back and a 45.5- foot radius on the cul-de-sac. Due to the long length of the proposed cul-de-sac and the practice of having two connections int%ut of developments, staff is recommending that the existing gravel road known as Cartway Lane be connected to the proposed cul-de-sac at the eastern border of the site. This will be an interim connection until the properties to the north develop and Cartway Lane is vacated. RECOMMENDED CONDITIONS OF APPROVAL 1. Prior to final plat approval, a professional civil engineer registered in the State of Minnesota must sign all plans. 2. Prior to final platting, storm sewer design data with a drainage map will need to be submitted for staff review. The storm sewer will have to be designed for a 10- year, 24-hour storm event. The pond is required to be designed to National Urban Runoff Program (NURP) standards. Drainage and utility easements will need to be dedicated on the final plat over the public storm drainage system including ponds, drainage swales, and wetlands up to the 100-year flood level. The minimum utility easement width shall be 20 feet wide. 3. Type II silt fence must be used adjacent to all ponds and wetlands. In addition, an erosion control blanket is requi~ed for the steep slopes along the north property line of the site. The applicant should be aware that any off-site grading would require an easement from the appropriate property owner. All disturbed areas must be sodded or seeded and mulched within two weeks of grading completion. Bob Generous February 7, 2005 Page 4 4. The remaining utility assessment due payable to the City at the time of final plat recording is $25,477.05. In addition, the sanitary sewer and water hookup charges will be applicable for each of the new lots. The 2005 trunk utility hookup charges are $1,458 per unit for sanitary sewer and $2,955 per unit for water. Each of these charges is based on the number of SAC units calculated by the Metropolitan Council. 5. Public utility improvements will be required to be constructed in accordance with the City's latest editions of Standard Specifications and Detail Plates. Detailed construction plans and specifications will be required at the time of final platting. The applicant will also be required to enter into a development contract with the City and supply the necessary financial security in the form of a letter of credit or cash escrow to guarantee installation of the improvements and the conditions of final plat approval. Permits from the appropriate regulatory agencies must be obtained, including but not limited to the MPCA, MNDOT, Department of Health, etc. 6. A registered structural engineer must design any retaining walls in excess of four feet in height. This will also require a building permit. 7. Show all of the existing and proposed easements on the plans. 8. Show all of the existing utilities on the plans. 9. The proposed development is required to meet the existing storm water runoff rates for the 10- and 100-year, 24-hour storm events. 10. The walk-out elevation of the proposed homes must be a minimum of 3 feet higher than the adjacent pond or wetland high-water-level. 11. Show the proposed storm manhole rim and invert elevations on the utility plan. 12. Show all emergency overflow elevations on the grading plan. 13. The existing temporary pavement turnaround for Pipewood Lane just south of this site must be removed when Pipewood Lane is extended. Any disturbed area must be sodded and restored. 14. The retaining wall in the rearyard of Lot 7, Block 1 must be 20 feet off the back of the building pad. 15. Revise the rearyard grading of Lot 9, Block 1 to prevent trapping water behind the curb. Either a catch basin will need to be added or the area will have to be regraded with a minimum slope of 2% to drain from the rearyard to the street. Bob Generous February 7,2005 Page 5 16. The existing gravel road known as Cartway Lane must be connected to the proposed cul-de-sac at the eastern border of the site. 17. The existing culvert across the street from Lot 9, Block 2 be connected to the storm sewer for Pipewood Lane. c: Paul Oehme, Public Works Director/City Engineer Mak Sweidan, Engineer g:\eng\projects\hidden creek meadows\revised ppr.doc