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HomeMy WebLinkAbout1 - Response to Engineering Comments 10-16-2020 1091 Stoneridge Drive • Bozeman, Montana • Phone (406) 587-1115 • Fax (406) 587-9768 www.chengineers.com • E-Mail: info@chengineers.com Civil/Structural Engineering and Surveying October 16, 2020 Griffin Nielsen City of Bozeman Engineering Department 20 E. Olive Street Bozeman, MT 59718 RE: Response to Engineering Comments – Norton East Ranch Subdivision, Phase 5 Comments (#18278) Dear Mr. Nielsen: The purpose of this letter is to provide a narrative response to the preliminary plat comments received in your letter dated October 8, 2020. Your comments are listed below, with your most recent comments in red, and our responses are following each comment in bold. BMC 38.220.060.A.11. 1. Currently, Norton Ranch Phase 3 conveys stormwater directly north to a temporary detention pond located on the southwest corner of the proposed Phase 5A development. The following must be provided in order for continued review. i. The Norton Ranch Phase 3 stormwater system is inundated with groundwater, reducing the capacity of the system by add additional and creating significant maintenance changes. The propose stormwater system does not appear to account for any additional offsite from the groundwater present in the upstream system. The applicant must demonstrate that the proposed system can convey the 25-year storm plus additional groundwater flow. The applicant must also demonstrate that the proposed system can meet the City’s storage requirements with the additional groundwater flow and volume. The report includes a narrative noting that the influence from the groundwater with a flow rate 1.765ft3/min. This flow rate is not included in the sizing calculations for Detention Pond #1. The sizing calculation sheet for Detention Pond #1 was revised to more clearly depict the inclusion of this groundwater flow rate into the sizing calculations. Revising the pond sizing to include this calculation resulted in a change in the required storage volume for the detention pond from 37,018 cf to 37,081 cf. The Detention Pond #1 section of the stormwater design report was revised to include this change to the pond volume as well. Both the revised calculation sheet for Detention Pond #1, and the updated stormwater design report are enclosed with this resubmittal. Since no other Civil/Structural Engineering and Surveying figures or calculation sheets were affected by this change, these were the only sheets included in this resubmittal. Engineering Variance Request Comments: 1. Variance Request to the City of Bozeman Design Standards and Specification Policy – Length of tangent at Intersection requirements - The City Engineer has authority to approve deviations for the City’s Design Standard and Specification Policy manual as such a variance may not be required. In order to approve a deviation an applicant’s engineer must provide a certified deviation request explain the why the deviation is request and demonstrating that the deviations ensures the public’s health and safety. If the deviations don’t ensure that public health and safety can be maintained then the City Engineer will not approve the deviation nor would support a variance. The applicant may have their engineering submit a deviation request. The engineer will need to demonstrate that the reduced length of tangency is appropriate for the design speed. The deviation request cannot be supported based on the information presented. The burden of proof falls on the party requesting a deviation. Please demonstrating that reduced length of tangency can support traffic volumes, turning movements, is supported by proposed traffic controls, site design, sight distances location and alignment of other access points and if any additional measures are required to the ensure the health and safety for the public. The applicant will need to present standards from AASHTO, MUTCD, and/or other industry publication demonstrating that the proposed tangent length is safe and have a professional engineering registered in the State of Montana certify that the deviation is safe. The deviation request does not identify the reduced length of tangency. The City design speed for Local Street is 30mph, the posted speed of 25mph will not be considered in review of the deviation. Enclosed with this submittal is a revised deviation request referencing AASHTO standards for vehicle stopping sight distance, and intersection sight distance triangles. This revised deviation request verifies that the intersections in question still meet these sight distance requirements despite not meeting the city’s tangent length requirements from an intersection. Applying these sight distance standards to the intersections of Westgate Ave / Gooseberry Drive, and Boomtown Road / Laurel Parkway verified that these intersections are not impacted by the shorter tangent distances. However, these same standards did confirm that the intersection of South Reliance Ave / Gooseberry Drive did not meet the requirements for intersection sight distance standards as it was previously designed. The short tangent length in question at this intersection is on Gooseberry Drive, which was the through street at this intersection. Since South Reliance Avenue was stop controlled at this intersection, drivers stopped Civil/Structural Engineering and Surveying on South Reliance did not have adequate vision around the curve of Gooseberry Drive to allow them to safely cross or turn onto Gooseberry Drive. In discussing this problem with Abelin Traffic Services it was suggested that the simplest way to address the issue would be to change the stop control on the intersection from South Reliance Avenue to Gooseberry Drive. We therefore changed the intersection so that South Reliance Avenue is the through street and maintains crown through the intersection, while Gooseberry Drive will have stop signs at the intersection. With the vehicles on Gooseberry Drive now required to stop, the required site vision triangle for the intersection is now directed down South Reliance Avenue as drivers on Gooseberry look down South Reliance Ave prior to crossing the intersection. This removes the curve on Gooseberry Drive from conflicting with the vision triangles at the intersection. We did still verify that vehicles on Gooseberry Drive still have adequate stopping sight distance around the curve as they approach the intersection, although this calculation is not typically required at intersections. Since changing the through crown of this intersection from Gooseberry to South Reliance is a very minor change, we will plan to incorporate this into our infrastructure design for that phase of the project. 2. Variance Request to BMC 38.400.060.B.4. – Level of service standard – The Engineering department does not support a variance to BMC 38.400.060.B.4 at the intersection of Laurel Parkway and Durston. The variance was not request with the subdivision pre-application. The variance does support the public interest as the developments traffic impact study shows the development will add loading to the intersection of Durston and Laurel Parkway. Applicant is encouraged to update the traffic impact study to identify the specific year the intersection’s level of service will fall below a C and identify the buildout time frame of the subdivision’s Phase. Phases which can be fully buildout prior to the intersection level of service dropping below a C will be supported. The update provided dated 9/10/2020 identifies the intersection will fall below a LOS C by 2027, but does not identify the building time frame for the development for Phase 5. Buildout of the Norton East Ranch Phase 5 development will be dependent on demand but will likely be completed within the next five years. At this time, we envision an annual absorption of 30-35 units per year. Under this scenario, build- out for Phase 5 is estimated for 2024 or 2025. It should be noted that the traffic projections contained within the most recent TIS took into account both the traffic from Phase 5 and the traffic from the future Phase 6 of this development. The LOS issues at the intersection of Durston Road and Laurel Parkway will likely occur at approximately the same time regardless of the exact level of construction in Phases 5 and Phases 6 of the Norton East Ranch development. These LOS issues will be primarily driven by anticipated traffic volume growth along Durston Road Civil/Structural Engineering and Surveying west of Laurel Parkway. The exact number of trips generated by the Norton East Ranch Development which will use Laurel Parkway is not the controlling factor which will create the anticipated congestion at the Durston Road intersection. This intersection will likely fall below LOS C by approximately 2027 regardless of any traffic from the Norton East Ranch Development based on the anticipated growth to the west on Duston Road projected in the Bozeman Transportation Management Plan. If you require any further information, please give me a call at (406) 587-1115. Sincerely, Ethan Cote, P.E.