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Hydrology Report 6-30-04BKBM- ENGINEERS 5930 Brooklyn Boulevard Minneapolis, MN 55429-2518 (763) 843-0420 Fax: (763) 843-0421 E-mail: bkbm@bkbm.com HYDROLOGY FOR CENTURY PLAZA LOT "C" HIGHWAY 5 AT CENTURY BLVD. CHANHASSEN, MN JUNE 30, 2004 CITY OF CHANHASSEN RECEIVED JUL 0 B 2OO4. CHANHA$SEN PLA'NNING DEP? EXIST1NG SITE DESCRIPTION: The site is a 1.95-acre commercial/retail lot located near the northeast corner of the intersection of State Highway 5 and Century Boulevard in Chanhassen, Minnesota. Other than an access driveway for ingress/egress from the developed site to the west, the existing site is essentially undeveloped. Due to the site grades being worked by the previous owner a number of times, the existing site consists of bare earth that has been seeded and partially established. The existing site drains via overland flow and storm sewer toward an existing Mn/DOT pond to the east of the site. Approximately 0.15 acres of the site is paved surface. The remaining 1.80 acres is considered pervious, undeveloped land. PROPOSED SITE IMPROVEMENTS: Site improvements include the addition of approximately 33,500 sq. ft (0.77 ac.) of new bituminous parking lot and sidewalks, and a 15,000 sq. ft. (0.35 ac) commercial/retail building. Changes to the site will increase the impervious site area from 0.15 acres to 1.12 acres. The proposed hard surface drainage on the site, including the building roof, will be directed into a storm sewer system and discharged into the existing Mn/DOT pond to the east. The remainder of the surface drainage on the site not captured by the storm sewer will sheet flow into the existing Mn/DOT pond. METHOD: HydroCAD using the SCS method of abstractions was used for this hydrologic study, modeling with a Type II, 24 hour rainfall. Assumptions used in the calculations include a time of concentration (Tc) of 20 minutes for the pre-developed site and 10 minutes for the post- developed site. SCS runoff curve numbers of 60 for existing pervious areas, 72 for proposed pervious areas and 98 for proposed impervious areas were used to model the land usage types. Peak runoff calculations were done for both pre-development and post-development using the 1- year, 1 O-year and 100-year storm return periods. BKBM ENGINEERS An Equal Opportunity Employer EASTERN MN/DOT POND: Runoff from the proposed property will be treated and rate-controlled by an existing Mn/DOT pond to the east of the site. Over the course of residential and commercial development in the area, it is understood that the pond has been resized to meet NURP treatment requirements and rate control for storm water discharge from a development on this property. Referring to the "Storm Water Management Plan and Drainage Calculations" for the Vasserman Ridge Development prepared by Schoell and Madsen, Inc. dated July 26, 2002, it is understood that the pond was previously sized for drainage from a portion ofW. 78th Street, a portion of State Highway 5, and a 5-acre undeveloped tract of land. The total drainage area for the pond is approximately 11.25 acres. The 5-acre undeveloped tract includes the 1.95-acre site noted in this report and was modeled with HydroCAD using a weighted SCS curve number of 60. The report proposed increasing the size of the pond to handle storm water runoff from the partial development of this 5-acre tract. The modeling of the pond included increasing the SCS weighted curve number for the 5-acre tract from 60 to 72 to account for development of the 1.95-acre portion of the 5-acre tract. Therefore, the amount of impervious surface on the 1.95-acre site needs to be limited such that the weighted SCS curve number for the 5-acre tract does not exceed 72. Assuming that the SCS curve number for impervious surface is 98, we estimated with HydroCAD that the developed impervious area for the 1.95-acre tract cannot exceed 1.52 acres (approximately 78% impervious fraction). Referring to our development plan for the site, the impervious surface area on the site is 1.12 acres, which yields a 57% impervious fraction and a weighted SCS curve number of 69 for the 5-acre tract. The table below compares HydroCAD runoff-rate modeling the 5-acre tract for pre- development and post-development with maximum impervious surface (from Shoell and Madsen report, CN = 72), and post-development using the submitted site plan (CN = 69)~ 24-HOUR PRE DEVELOPMENT POST DEVELOPMENT POST DEVELOPMENT STORM (weighted CN=60) MAXIMUM PROPOSED SITE PLAN EVENT (weighted CN=72) (weighted CN:69) 5-acre tract 5-acre tract 5-acre tract 1-year 0.17 cfs 2.77 cfs 1.92 cfs 10-year 5.38 cfs 12.02 cfs 10.35 cfs 100-year 13.36 cfs 22.68 cfs 20.53 cfs SUMMARY: The computations and narrative indicated that the modified Mn/DOT pond to the east of the site is properly sized for the proposed development. PROP iSubcat:' Drainage Diagram for 04363 - Proposed Prepared by BKBM Engineers 6/30/2004 HydroCAD~ 6.10 sin 002421 © 1986-2002 Applied Microcomputer Systems 04363 - Proposed Prepared by BKBM Engineers HydroCAD® 6.10 s/n 002421 © 1986-2002 Applied Microcomputer Systems Type II 24-hr Rainfall=2.30'' Page 1 6/30/2004 12:29:04 PM Subcatchment PROP: 5-acre tract - proposed 1.95-acre site development Runoff = 1.92 cfs @ 12.05 hrs, Volume= 0.119 af, Depth= 0.29" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr Rainfall=2.30" Area (ac) CN Description 3.880 60 Pervious Acreage of 5 acres 1.120 98 Impervious Acreage of 5 acres 5.000 69 Weighted Average Tc Length Slope Velocity Capacity (min) (feet) (ft/ft) (fi/sec) (cfs) 10.0 Description Direct Entry, 04363 - Proposed Prepared by BKBM Engineers HydroCAD® 6.10 s/n 002421 © 1986-2002 Applied Microcomputer Systems Type II 24-hr Rainfall=4.20" Page 1 6/30/2004 12:28:39 PM Subcatchment PROP: 5-acre tract - proposed 1.95-acre site development Runoff = 10.35 cfs @ 12.02 hrs, Volume= 0.525 af, Depth= 1.26" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr Rainfall=4.20'' Area (ac) CN Description 3.880 60 Pervious Acreage of 5 acres 1A20 98 Impervious Acreage of 5 acres 5.000 69 Weighted Average Tc Length Slope Velocity Capacity (min) (feet) (ft/ft) (fi/sec) (cfs) 10.0 Description Direct Entry, 04363 - Proposed Prepared by BKBM Engineers HydroCAD® 6.10 s/n 002421 © 1986-2002 Applied Microcomputer Systems Type H 24-hr Rainfall=6~O0" Page 1 6/30/2004 12:28:19 PM Subcatchment PROP: 5-acre tract - proposed 1.95-acre site development Runoff = 20.53 cfs @ 12.02 hrs, Volume= 1.034 af, Depth= 2.48" Runoff by SCS TR-20 method, UH=SCg, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr Rainfall=6.00'' Area (ac) CN Description 3.880 60 Pervious Acreage of 5 acres 1.120 98 Impervious Acreage of 5 acres 5.000 69 Weighted Average Tc Length Slope Velocity Capacity (min) (feet) (ft/ft) (fi/sec) (cfs) 10~0 Description Direct Entry, EX ,Subcat, Drainage Diagram for 04363 - Century Plaza - Runoff from Site Prepared by BKBM Engineers 6/30/2004 HydroCAD~ 6.10 s/n 002421 © 1986-2002 Applied Microcomputer Systems 04363 - Century Plaza - Runoff from Site Prepared by BKBM Engineers HydroCAD® 6.10 s/n 002421 © 1986-2002 Applied Microcomputer Systems Subcatchment EX: Existing Site - 1.95 Ac Runoff = 0.16 cfs @ 12.20 hrs, Volume= 0.024 aL Depth= 0.15" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dr= 0.05 hrs Type II 24-hr Rainfall=2.30" Area (ac) CN Description Type II 24-hr Rainfall=2.30" Page 1 6/30/2004 2:18:59 PM 0.150 98 1.800 60 1.950 63 Tc Length (min) (~et) 18.0 Weighted Average Slope Velocity Capacity (ft/ft) (fi/sec) (cfs) Runoff = 3.39 cfs @ Description Direct Entry, Subcatchment PROP: Proposed Site 12.02 hfs, Volume= 0.171 al, Depth= 1.05" Runoff by SCS TR~20 method, Ut4=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs 1Type II 24-hr Rainfall=2.30" Area (ac) CN Description IA20 98 0.830 72 1~950 87 Weighted Average Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (fi/sec) (cfs) 10,0 Direct Entry, 04363 - Century Plaza - Runoff from Site Prepared by BKBM Engineers HydroCAD® 6.10 s/n 002421 © 1986-2002 Applied Microcomputer Systems Subcatchment EX: Existing Site - 1.95 Ac Runoff = 2.08 cfs @ 12.13 hrs, Volume= 0.148 af, Depth= 0.91" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr Rainfall=4.20'' Area (ac) CN Description Type II 24-hr Rainfall=4.20 Page 6/30/2004 2:19:15 PM 0.150 98 1.800 60 1.950 63 Weighted Average Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (fi/sec) (cfs) 8.0 Direct Entry, Subcatchment PROP: Proposed Site Runoff = 8.16 cfs @ 12.01 hrs, Volume= 0.427 af, Depth= 2.63" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type Il 24-hr Rainfall=4.20" Area (ac) CN Description 1~120 98 0.830 72 1.950 87 Weighted Average Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (fi/sec) (cfs) 10.0 Direct Entry, 04363 - Century Plaza - Runoff from Site Prepared by BKBM Engineers HydroCAD® 6.10 s/n 002421 © 1986-2002 Applied Microcomputer Systems Subcatchment EX: Existing Site - 1.95 Ac Runoff = 4.83 cfs @ 12.11 hrs, Volume= 0.319 al, Depth= 1.96" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr Rainfall=6.00'' Area (ac) CN Description Type II 24-hr Rainfall=6.00'' Page 1 6/30/2004 2:19:28 PM 0.150 98 1.800 60 1.950 63 Weighted Average Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (fi/sec) (cfs) 18.0 Direct Entry, Subcatchment PROP: Proposed Site Runoff = 12.77 cfs @ 12.01 hrs, Volume= 0.687 af, Depth= 4.23" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr Rainfall=6.00" Area (ac) CN Description 1.120 98 0~830 72 1.950 87 Weighted Average Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (fi/sec) (cfs) 10.0 Direct Entry,