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Stormwater Mgmt & CalcsSTORMWATER MANAGEMENT AND CALCULATIONS FOR Frontier Second Chanhassen, MN July 2, 2004 Prepared For: Charles R. Stinson 18304Minnetonka Blvd. Deephaven, MN55391 952-473-9503 CtTYOFCHANHASSEN RECEIVED JUL 0 2 2004 CHANHA$SEN PLANNING DEPT Prepared By: Kallio Eneineering LLC 1240 E. 3r~'Ave. Suite 103 Shakopee, MN 55379 952-445-7354 STORMWATER bIANAGEMENT Existing Site The existing site is heavily wooded with steep slopes. An existing 18" storm outlets towards the property on the easterly side, flows through, and exits to the north. Erosion is currently occurring along this drainage way or ditch. The more severe portions are "head cutting" their way towards the pipe outlet. There is very little under story vegetation to retard the drainage and washing. Proposed Improvements Improvements to this site include constructing a new ditch across this property to the north, thus passing the more severe washout areas. The reasons for doing this are to create more cover for the sanitary sewer crossing of the ditch and to also perform the ditch stabilizations on the properb'. To reduce the head cutting, riprap will be placed just upstream of where the more sever washing is occurring. A rock check dam will be placed across the ditch prior to the storm water exiting the property.. A double net coconut blanket will be used to stabilize the new ditch bottom and slopes, and also stabilize the existing ditch that will be filled in. It may be difficult to establish vegetation in the area due to the lack of sunlight as a result of the tree canopy. Calculations and Summary The pre-development and post-development runoff calculations and were performed using HydroCAD software which utilizes SCS TR-2o methodologies. There were no pond calculations or storm sewer design associated with this project. All runoff calculations were performed using a Type II 24-hour rainfall distribution with a lo-year rainfall of 4.1 inches and loo-year rainfall of 6.0 inches. Curve numbers were taken from the table using a hydrologic soil group of C. The stormwater management addresses only the pre-development and post-development runoff of the 2.61 acre site. The results are attached, but summarized below: Storm Pre-Development Post-Development ~o-year 4n cfs 5.5 cfs loo-year 8.~ cfs 9.9 cfs As you can see, the development has very little impact on the peak flows. This is primarily due to the large wooded lots and steep terrain. The site was assumed to have a 25% impervious fully developed area. The increase in the impervious surface of this site is approximately 0.5 acres. The developer has indicated they previously constructed a stormwater basin on the development immediately to the north that takes the stormwater from this site into account. Therefore, because of the small increase in peak flows and the stormwater basin to the north, the new ditch and erosion stabilization should be considered an acceptable stormwater treatment for this site. I hereby certify that this plan, report, or specification was prepared by me or under my direct supervision and that I am a duly licensed engineer under the laws of the State of Minnesota. Curtiss J. Kallio 0423PRE Prepared by Kallio Engineering HydroCAD® 7.00 s/n 002095 © 1986-2003 Applied Microcomputer Systems Subcatchment lS: SITE Runoff = 4.05 cfs @ 12.18 hfs, Volume= 0.314 af, Depth= 1.44" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hfs Type II 24-hr 10YR Rainfall=4.10" Area (ac) CN Description Type II 24-hr 10YR Rainfall=4.10" Page 3 7~2~20O4 2.610 73 Woods, Fair, HSG C Tc Length Slope Velocity Capacity Description (min) (feet) (ftJft) (ffJsec) (cfs) 9.0 130 0.0500 0.2 Sheet Flow, Grass: Short n= 0.150 P2= 2.80" 13.3 160 0.2000 0.2 Sheet Flow, Woods: Light underbrush n= 0.400 1.5 350 0.0600 3.9 Shallow Concentrated Flow, Unpaved Kv= 16.1 fps 23.8 640 Total P2= 2.80" Subcatchment lS: SITE Hydrograph 0 5 Type II 24-hr 10YR Rainfall=4.10" Runoff Area=2.610 ac Runoff Volume=0.314 af Runoff Depth=l.44" Flow Length=640' Tc=23.8 min CN=73 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time (hours) 0423PRE Prepared by Kallio Engineering HydroCAD® 7.00 s/n 002095 © 1986-2003 Applied Microcomputer Systems Subcatchment lS: SITE Runoff = 8.05 cfs @ 12.17 hrs, Volume= 0.615 af, Depth= 2.83" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr 100YR Rainfall=6.00" Area (ac) CN Description Type II 24-hr IOOYR Rainfall=6.00" Page 5 7/2/2004 2.610 73 Woods, Fair, HSG C Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 9.0 130 0.0500 0.2 Sheet Flow, Grass: Short n= 0.150 P2= 2.80" 13.3 160 0.2000 0.2 Sheet Flow, Woods: Light underbrush n= 0.400 1.5 350 0.0600 3.9 Shallow Concentrated Flow, Unpaved Kv= 16.1 fps 23.8 640 Total P2= 2.80" 9 6 5 4 3 2 1 0 6 7 8 9 10 11 Subcatchment lS: SITE Hydrograph Type II 24-hr 100YR Rainfall=6.00" Runoff Area=2.610 ac Runoff Volume=0.615 af oO: Depth=2.83" Length=640' Tc=23.8 min CN=73 12 13 14 15 16 17 18 19 Time (hours) 2O 0423POST Prepared by Kallio Engineering HydroCAD® 7.00 s/n 002095 © 1986-2003 Applied Microcomputer Systems Subcatchment lS: SITE Runoff = 5.54 cfs @ 12.17 hfs, Volume= 0.423 af, Depth= 1.95" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr 10YR Rainfall=4.10" Area (ac) CN Description Type II 24-hr IOYR Rainfall=4.10" Page 1 7~2/2004 2.610 80 1/2 acre lots, 25% imp, HSG C Tc Length Slope Velocity (min) (feet) (ft/ft) (ft/sec) 9.0 130 0.0500 0.2 13.3 160 0.2000 0.2 1.5 350 0.0600 3.9 23.8 640 Total Capacity (cfs) Description Sheet Flow, Grass: Short n= 0.150 P2= 2.80" Sheet Flow, Woods: Light underbrush n= 0.400 Shallow Concentrated Flow, Unpaved Kv= 16.1 fps P2= 2.80" Subcatchment lS: SITE Hydrograph 4 Type II 24-hr 10YR Rainfall=4.10" Runoff Area=2.610 ac Runoff Volume=0.423 af Runoff Depth=1.95" Flow Length=640' Tc=23.8 min CN=80 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time (hours) 0423POST Type II 24-hr IOOYR Prepared by Kallio Engineering HydroCAD® 7.00 s/n 002095 © 1986-2003 Applied Microcomputer Systems Rainfall=6, 00" Page 2 7/2/2004 Subcatchment lS: SITE Runoff = 9.86 cfs @ 12.17 hrs, Volume= 0.761 al, Depth= 3.50" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr 100YR Rainfall=6.00" Area (ac) CN Description 2.610 80 1/2 acre lots, 25% imp, HSG C Tc Length Slope Velocity (min) (feet) (ft/ft) (ft/sec) 9.0 130 0.0500 0.2 13.3 160 0.2000 0.2 1.5 350 0.0600 3.9 23.8 640 Total Capacity (cfs) Description Sheet Flow, Grass: Short n= 0.150 P2= 2.80" Sheet Flow, Woods: Light underbrush n= 0.400 Shallow Concentrated Flow, Unpaved Kv= 16.1 fps P2= 2.80" 11 10 9 8 7 6 5 4 3 2 1 o 6 7 8 9 lO Subcatchment lS: SITE Hydrograph Type !1 24-hr 100YR Rainfall=6.00" Runoff Area=2.610 ac Runoff Volume=0.761 af Runoff Depth=3.50" Flow Length=640' Tc=23.8 min CN=80 11 12 13 14 15 16 17 18 19 20 Time (hours)