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Home My WebLink About19 - Design Report - Costco - Max & Catron Roundabout - Stormwater 406-586-8834 ■ 800-865-9847 (fax) ■ 2090 Stadium Drive ■ Bozeman, Montana 59715 ■ www.dowl.com MEMORANDUM Purpose As part of the City’s review of the Planned Unit Development (PUD) and Site Plan (SP) applications the City is requiring the intersection of Max Avenue and Catron Street be improved to an acceptable level of service. A mini-roundabout was selected to mitigate the impacts of the increased trip generation. The proposed roundabout will increase the impervious surface at the intersection by 2,604 FT2, or 0.06 acres. A figure showing the additional area of impervious material is included in Attachment A. This report defines the proposed stormwater improvements to address the additional impervious area. Hydrology The existing configuration of the intersection of Max Avenue and Catron Street does not have curb and gutter. In general, the paved surface drains to adjacent road side swales, with the exception of the southeast corner of intersection, which drains to an adjacent ditch. The other intersection quadrants all eventually drain to the north via a system of existing culverts, ponds, and swales. A culvert (12-inch) runs from the pond at the southwest corner of the intersection, under Catron Street, to the swale in the northwest quadrant of the intersection. The swale then drains to a culvert (12-inch) under Max Avenue which flows into a structure in the northeast corner of the intersection where it changes directions and flows to an outlet to the north. The culvert under Catron Street is approximately 0.3 feet lower than the culvert under Max Avenue, causing some ponding in the swale before stormwater begins to discharge. The collected stormwater eventually drains to an open channel located behind the Holiday Inn Express adjacent to Valley Center Drive. Figure 1 - Current System Configuration TO: Griffin Nielsen, EIT FROM: Erik Garberg, PE – Senior Civil Engineer DATE: 06/14/19 SUBJECT: Roundabout Costco Project - Mini-roundabout Stormwater Design Analysis – Revision 2 MEMORANDUM Page 2 of 4 Figure 2 - Southwest Quadrant Pond Figure 3 - Southeast Quadrant Drainage Figure 4 - Northwest Quadrant Swale MEMORANDUM Page 3 of 4 Figure 5 - Northeast Quadrant Drainage Figure 6 - North Outlet The proposed project will maintain the existing drainage pattern described above with additional storage consisting of a pond and a combination overflow manhole structure in the northwest corner of the intersection. This stormwater management method was selected based on the City direction and review of multiple design iterations. The tables below summarize the inputs and outputs of the rational method, for a 10-yr storm event using City of Bozeman’s IDF curves for the pre and post development conditions. Stormwater calculations are included in Attachment B. Figure 7 – Roundabout Pre-development Stormwater Inputs and Results (10-yr event) Drain Area (ac)0.06 Run-off Coefficient 0.2 Time of Concentration (min)5 10-yr Peak Run off (cfs)0.04 10-yr Volume (cuft)11 Roundabout - Pre-Development MEMORANDUM Page 4 of 4 Figure 8 - Roundabout Post Development Stormwater Inputs and Results (10-yr event) The calculations indicate that the additional impervious area of the roundabout will produce a total of 51 cuft of run-off. A pond will be installed with a volume of 51 cuft in the northwest quadrant of the intersection of Max Avenue and Catron Street to retain the entire 10-year design storm volume with no deduction for the pre-development condition. Additionally, a stormwater manhole will be installed to extend the existing drainage beneath new pedestrian crossing in that quadrant. The proposed structure will act as a stormwater overflow as the rim will be set at an elevation to discharge only after the required 51 cuft volume is retained. See attachment E for proposed improvements. Reviewing the Stormwater Master Plan for The Gallatin Center PUD Final Plan Revised November 1998 indicated the pre-development flow from the pond at the northeast corner of the Target lot is 3.7 CFS. This project will add 0.17 CFS for the 10-y event or .20 CFS for the 25-yr event for a total flow of 3.9 CFS allowing for rounding under both peak flow conditions. The controlling run of 12-inch HDPE discharge pipe is the section flowing north with a slope of 0.41%. Using Hydraflow Express by Autodesk the maximum flow from a 12-inch HDPE pipe with a Manning’s N of 0.015 and slope of 0.41% is 14.49 CFS at 8-inches of depth. Pertinent excerpts from the above referenced Gallatin Center Stormwater report are included in Attachment D. The proposed pond will have a storage volume of 51 cuft and only discharge after the required volume is detained. The discharging piping has sufficient capacity to accommodate the new and existing flows with significant surplus capacity. Conclusion Attachment E contains a figure detailing the existing condition and the pond installed as part the design of the proposed roundabout. The pond will provide stormwater retention necessary to offset the drainage produced by the additional impervious surface. Attachments Attachment A – Impervious Area Figure Attachment B – Hydrology Calculations Attachment C – Miscellaneous Calculations Attachment D – Stormwater Report Excerpts Attachment E – Proposed Conditions Drain Area (ac)0.06 Run-off Coefficient 0.9 Time of Concentration (min)5 10-yr Peak Run off (cfs)0.17 25-yr Peak Run off (cfs)0.2 10-yr Volume (cuft)51 Roundabout - Post Development ATTACHMENT A – IMPERVIOUS AREA FIGURE ATTACHMENT B – HYDROLOGY CALCULATIONS 1. 10-yr Pre-development 2. 10-yr Post Development 3. 25-yr Post Development Hydrograph Report Hydraflow Hydrographs Extension for AutoCAD® Civil 3D® 2016 by Autodesk, Inc. v10.514 Thursday, 01 / 24 / 2019 Hyd. No. 1 Roundabout Pre-development Drainage Hydrograph type = Rational Peak discharge = 0.038 cfs Storm frequency = 10 yrs Time to peak = 5 min Time interval = 1 min Hyd. volume = 11 cuft Drainage area = 0.060 ac Runoff coeff.= 0.2 Intensity = 3.145 in/hr Tc by User = 5.00 min IDF Curve = Bozeman IDF.IDF Asc/Rec limb fact = 1/1 0 1 2 3 4 5 6 7 8 9 10 Q (cfs) 0.00 0.00 0.01 0.01 0.02 0.02 0.03 0.03 0.04 0.04 0.05 0.05 0.06 0.06 0.07 0.07 0.08 0.08 0.09 0.09 0.10 0.10 Q (cfs) Time (min) Roundabout Pre-development Drainage Hyd. No. 1 -- 10 Year Hyd No. 1 Hydrograph Report Hydraflow Hydrographs Extension for AutoCAD® Civil 3D® 2016 by Autodesk, Inc. v10.5 Tuesday, 07 / 17 / 2018 Hyd. No. 1 Roundabout Hydrograph type = Rational Peak discharge = 0.170 cfs Storm frequency = 10 yrs Time to peak = 5 min Time interval = 1 min Hyd. volume = 51 cuft Drainage area = 0.060 ac Runoff coeff.= 0.9 Intensity = 3.145 in/hr Tc by User = 5.00 min IDF Curve = Bozeman IDF.IDF Asc/Rec limb fact = 1/1 0 1 2 3 4 5 6 7 8 9 10 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) Roundabout Hyd. No. 1 -- 10 Year Hyd No. 1 Hydrograph Report Hydraflow Hydrographs Extension for AutoCAD® Civil 3D® 2016 by Autodesk, Inc. v10.514 Tuesday, 06 / 11 / 2019 Hyd. No. 1 25-yr Roundabout Hydrograph type = Rational Peak discharge = 0.196 cfs Storm frequency = 25 yrs Time to peak = 5 min Time interval = 1 min Hyd. volume = 59 cuft Drainage area = 0.060 ac Runoff coeff.= 0.9 Intensity = 3.627 in/hr Tc by User = 5.00 min IDF Curve = Bozeman 10 and 25 IDF.IDF Asc/Rec limb fact = 1/1 0 1 2 3 4 5 6 7 8 9 10 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) 25-yr Roundabout Hyd. No. 1 -- 25 Year Hyd No. 1 ATTACHMENT C – MISCELLANEOUS CALCULATIONS 1. 12-inch HDPE pipe capacity Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc.Tuesday, Jun 11 2019 <Name> Circular Diameter (ft)= 1.00 Invert Elev (ft)= 1.00 Slope (%)= 0.41 N-Value = 0.002 Calculations Compute by:Known Depth Known Depth (ft)= 0.80 Highlighted Depth (ft)= 0.80 Q (cfs)= 14.49 Area (sqft)= 0.67 Velocity (ft/s)= 21.51 Wetted Perim (ft)= 2.22 Crit Depth, Yc (ft)= 1.00 Top Width (ft)= 0.80 EGL (ft)= 7.99 0 1 2 Elev (ft)Section 0.50 1.00 1.50 2.00 2.50 3.00 Reach (ft) ATTACHMENT D – GALLATIN CENTER STORMWATER REPORT EXCERPTS ATTACHMENT E – PROPOSED CONDITION CATRON STREET AND MAX AVENUE ROUNDABOUT GEOMETRY AND TEMPORARY TRAFFIC CONTROL NOTESCONSTRUCTION NOTES: C8.6 TEMPORARY TRAFFIC CONTROL NOTES:COSTCO WAREHOUSE ADDITIONCIVIL SITE IMPROVEMENTS406-586-883414108 PE 999 LAKE DRIVEISSAQUAH, WA 98027TEL: (425) 313-8100PREPARED FORCOSTCO WHOLESALE CORPORATION CONSTRUCTION NOTES: 1 CATRON ST PLAN SCALE: 1" = 20' 2 CATRON ST PROFILE SCALE: H: 1" = 20' V: 1" = 5' WATER SYSTEM NOTES: SEWER SYSTEM NOTES: # 1 CATRON STREET AND MAX AVENUE ROUNDABOUTPLAN AND PROFILEC8.14COSTCO WAREHOUSE ADDITIONCIVIL SITE IMPROVEMENTS406-586-883414108 PE 999 LAKE DRIVEISSAQUAH, WA 98027TEL: (425) 313-8100PREPARED FORCOSTCO WHOLESALE CORPORATION