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Home My WebLink About19 - Design Report - Icon Apartments - West Babcock Street Widening - Storm Drainagememo Page 1 TO: Bob Murray, Jr., PE FROM: Kevin Jacobsen, PE DATE: May 24, 2019 JOB NO.: 5305.006 RE: Icon Apartments – West Babcock Street Widening Storm Drainage Report CC: Will Ralph, PE – Blackridge Companies, LLC Urgent For Review Please Comment Please Reply For Your Use Widening of West Babcock Street was required by the City of Bozeman as part of future phases of the Icon Apartment Project. The widening is occurring on the south side of the roadway and impacts several (three) curb inlets. The purpose of the memorandum is to analyze the storm drainage following the improvements to determine if the project meets city standards. This storm drainage analysis is based on the storm drainage design report for Valley West Subdivision – Phase 1. (Narrative included with this memo). We have included the applicable record drawing sheets from this project as well (see attached). The three inlets that currently exist on the south side of West Babcock Street flow via storm drainage pipe to one of two detention ponds located in the Valley West Park or the west and east sides of Maynard Border Ditch. The two easternmost inlets flow to the north and west to a detention pond on the east side of the ditch. The one westernmost inlet flows to the north and east to a detention pond on the west side of the ditch. For purposes of this analysis the following nomenclature exists. The East Storm Basin flows to Detention Pond #1 located on the east side of the ditch. This is described as Drainage Area 1 and Minor Basin 1 (DA1, MB1) in the original Valley West Phase 1 Storm Drainage Report. The West Storm Basin flows to Detention Pond #2 located on the west side of the ditch. This is described as Drainage Area 2 and Minor Basin 1 (DA2, MB1) in the original Valley West Phase 1 Storm Drainage Report. East Basin Analysis The widening project, as designed, will add twelve feet to the south side of the roadway. The east basin is approximately 1,100 feet in length and has a taper near Ferguson Avenue. This area totals 11,900 sq.ft. of new roadway. This area was analyzed as impervious area being added to the existing drainage basin. See following table. Icon Apartments – West Babcock Street Widening Storm Drainage Report Page 2 Exst. Basin Area (Ac.) Added Area (Ac.) Total Area (Ac.) 2.34 0.27 2.61 The following table illustrates the pre-development and post-development values that were calculated on the original report and now modified to include the additional area. Please note that a composite C value was calculated by adding in the additional area as impervious with a C value of 0.90. The time of concentration was kept the same as there would only be very minute differences in this value with the widening project. All calculations are included. Pre or Post Development “C” Tc (min) Before Widening Q 10yr (cfs) After Widening Q 10yr (cfs) Before Widening Q 25yr (cfs) After Widening Q 25yr (cfs) Pre-Development 0.20 18 0.66 0.73 Post-Development 0.41 11.1 1.57 2.06 1.88 2.47 The original size of the pond was designed to be 692 cu.ft. but was actually built to a larger capacity based on the record drawings (see record drawings attached). The actual volume based on the record drawings is 1,186 cu.ft. The required detention volume is 878 cu.ft. and therefore sufficient volume is available within the pond for the increased runoff. As this project is not a development or redevelopment project and is quite small (less than 1.0ac), we suggest removing the LID requirement so as to limit the potential disturbance to the existing Valley West Park. The existing pipe network has a 15-inch RCP at 0.35% and we analyzed this pipe for the new 25 year flow. The 2.47cfs flows through is pipe at a depth of 0.74 feet. The East Basin analysis illustrates that the existing storm drainage infrastructure can accommodate the widening of West Babcock Street. West Basin Analysis The widening project, as designed, will add twelve feet to the south side of the roadway. The east basin is approximately 670 feet in length and ends west of Resort Drive. This area totals 8,150 sq.ft. of new roadway. This area was analyzed as impervious area being added to the existing drainage basin. See following table. Icon Apartments – West Babcock Street Widening Storm Drainage Report Page 3 Exst. Basin Area (Ac.) Added Area (Ac.) Total Area (Ac.) 1.53 0.19 1.72 The following table illustrates the pre-development and post-development values that were calculated on the original report and now modified to include the additional area. Please note that a composite C value was calculated by adding in the additional area as impervious with a C value of 0.90. The time of concentration was kept the same as there would only be very minute differences in this value with the widening project. All calculations are included. Pre or Post Development “C” Tc (min) Before Widening Q 10yr (cfs) After Widening Q 10yr (cfs) Before Widening Q 25yr (cfs) After Widening Q 25yr (cfs) Pre-Development 0.20 18 0.43 0.48 Post-Development 0.41 8 1.27 1.67 1.52 2.00 The original size of the pond was designed to be 437 cu.ft. but was actually built to a larger capacity based on the record drawings (see record drawings attached). The actual volume based on the record drawings is 781 cu.ft. The required detention volume is 578 cu.ft. and therefore sufficient volume is available within the pond for the increased runoff. As this project is not a development or redevelopment project and is quite small (less than 1.0ac), we suggest removing the LID requirement so as to limit the potential disturbance to the existing Valley West Park. The existing pipe network has a 15-inch RCP at 0.24% and we analyzed this pipe for the new 25 year flow. The 2.00cfs flows through is pipe at a depth of 0.73 feet. The West Basin analysis illustrates that the existing storm drainage infrastructure can accommodate the widening of West Babcock Street. Please note that the portion of this widening project west of the “West Basin” was accounted for with the adjacent Cottonwood project. Please contact RPA for details. END Hydrology Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Friday, May 24 2019 East Pond #2-10 yr pre Hydrograph type = Rational Peak discharge (cfs) = 0.731 Storm frequency (yrs) = 10 Time interval (min) = 1 Drainage area (ac) = 2.610 Runoff coeff. (C) = 0.2 Rainfall Inten (in/hr) = 1.400 Tc by User (min) = 18 IDF Curve = COB IDF.IDF Rec limb factor = 1.00 Hydrograph Volume = 789 (cuft); 0.018 (acft) 0 5 10 15 20 25 30 35 40 Q (cfs) 0.00 0.00 0.10 0.10 0.20 0.20 0.30 0.30 0.40 0.40 0.50 0.50 0.60 0.60 0.70 0.70 0.80 0.80 0.90 0.90 1.00 1.00 Q (cfs) Time (min) Runoff Hydrograph 10-yr frequency Runoff Hyd - Qp = 0.73 (cfs) Hydrology Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Friday, May 24 2019 East Pond #1-10 yr post Hydrograph type = Rational Peak discharge (cfs) = 2.063 Storm frequency (yrs) = 10 Time interval (min) = 1 Drainage area (ac) = 2.610 Runoff coeff. (C) = 0.41 Rainfall Inten (in/hr) = 1.928 Tc by User (min) = 11 IDF Curve = COB IDF.IDF Rec limb factor = 1.00 Hydrograph Volume = 1,362 (cuft); 0.031 (acft) 0 5 10 15 20 25 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 Q (cfs) Time (min) Runoff Hydrograph 10-yr frequency Runoff Hyd - Qp = 2.06 (cfs)Outflow Hyd *Req. Stor = 878 (cuft) * * Estimated Hydrology Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Friday, May 24 2019 East Pond #1-25 yr post Hydrograph type = Rational Peak discharge (cfs) = 2.472 Storm frequency (yrs) = 25 Time interval (min) = 1 Drainage area (ac) = 2.610 Runoff coeff. (C) = 0.41 Rainfall Inten (in/hr) = 2.310 Tc by User (min) = 11 IDF Curve = COB IDF.IDF Rec limb factor = 1.00 Hydrograph Volume = 1,631 (cuft); 0.037 (acft) 0 5 10 15 20 25 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 3.00 3.00 Q (cfs) Time (min) Runoff Hydrograph 25-yr frequency Runoff Hyd - Qp = 2.47 (cfs) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Friday, May 24 2019 East Pond #1-25 yr post - Pipe Flow Circular Diameter (ft) = 1.25 Invert Elev (ft) = 100.00 Slope (%) = 0.35 N-Value = 0.013 Calculations Compute by: Known Q Known Q (cfs) = 2.47 Highlighted Depth (ft) = 0.74 Q (cfs) = 2.470 Area (sqft) = 0.76 Velocity (ft/s) = 3.25 Wetted Perim (ft) = 2.20 Crit Depth, Yc (ft) = 0.63 Top Width (ft) = 1.23 EGL (ft) = 0.90 0 1 2 3 Elev (ft)Section 99.50 100.00 100.50 101.00 101.50 102.00 Reach (ft) Hydrology Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Friday, May 24 2019 West Pond #2-10 yr pre Hydrograph type = Rational Peak discharge (cfs) = 0.482 Storm frequency (yrs) = 10 Time interval (min) = 1 Drainage area (ac) = 1.720 Runoff coeff. (C) = 0.2 Rainfall Inten (in/hr) = 1.400 Tc by User (min) = 18 IDF Curve = COB IDF.IDF Rec limb factor = 1.00 Hydrograph Volume = 520 (cuft); 0.012 (acft) 0 5 10 15 20 25 30 35 40 Q (cfs) 0.00 0.00 0.05 0.05 0.10 0.10 0.15 0.15 0.20 0.20 0.25 0.25 0.30 0.30 0.35 0.35 0.40 0.40 0.45 0.45 0.50 0.50 Q (cfs) Time (min) Runoff Hydrograph 10-yr frequency Runoff Hyd - Qp = 0.48 (cfs) Hydrology Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Friday, May 24 2019 West Pond #2-10 yr post Hydrograph type = Rational Peak discharge (cfs) = 1.672 Storm frequency (yrs) = 10 Time interval (min) = 1 Drainage area (ac) = 1.720 Runoff coeff. (C) = 0.41 Rainfall Inten (in/hr) = 2.371 Tc by User (min) = 8 IDF Curve = COB IDF.IDF Rec limb factor = 1.00 Hydrograph Volume = 803 (cuft); 0.018 (acft) 0 5 10 15 20 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 Q (cfs) Time (min) Runoff Hydrograph 10-yr frequency Runoff Hyd - Qp = 1.67 (cfs)Outflow Hyd *Req. Stor = 578 (cuft) * * Estimated Hydrology Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Friday, May 24 2019 West Pond #2-25 yr post Hydrograph type = Rational Peak discharge (cfs) = 1.997 Storm frequency (yrs) = 25 Time interval (min) = 1 Drainage area (ac) = 1.720 Runoff coeff. (C) = 0.41 Rainfall Inten (in/hr) = 2.832 Tc by User (min) = 8 IDF Curve = COB IDF.IDF Rec limb factor = 1.00 Hydrograph Volume = 959 (cuft); 0.022 (acft) 0 5 10 15 20 Q (cfs) 0.00 0.00 1.00 1.00 2.00 2.00 Q (cfs) Time (min) Runoff Hydrograph 25-yr frequency Runoff Hyd - Qp = 2.00 (cfs) Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Friday, May 24 2019 West Pond #2-25 yr post - Pipe Flow Circular Diameter (ft) = 1.25 Invert Elev (ft) = 100.00 Slope (%) = 0.24 N-Value = 0.013 Calculations Compute by: Known Q Known Q (cfs) = 2.00 Highlighted Depth (ft) = 0.73 Q (cfs) = 2.000 Area (sqft) = 0.74 Velocity (ft/s) = 2.69 Wetted Perim (ft) = 2.17 Crit Depth, Yc (ft) = 0.57 Top Width (ft) = 1.23 EGL (ft) = 0.84 0 1 2 3 Elev (ft)Section 99.50 100.00 100.50 101.00 101.50 102.00 Reach (ft)