Loading...
HomeMy WebLinkAbout2.6 Response to Griffin Email Comments 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 September 3, 2020 Griffin Nielsen City of Bozeman Engineering Department 20 E. Olive Street Bozeman, MT 59718 RE: Response to Email Engineering Comments – Norton East Ranch Subdivision, Phase 5 DRC Comments (#18278) Dear Mr. Nielsen: This letter is to provide a narrative response to the email comments from August 30, 2020: Engineering Division, Griffin Nielsen, gnielsen@bozeman.net, (406) 582-2280 1. Please provide the conservation easement used as justification why Boomtown and El Dorado shouldn’t be fully extended to the property line. I recommended provided additional justification for this one as we have required road to be extended to properties in similar circumstance. • A copy of the filed conservation easement (Doc. No. 24009500) is included with this submittal. The applicant does not wish to disrupt the linear park along the western boundary of the project with two dead end streets due to the small chance that these streets ever get extended further west. In the small chance they need to be extended in the future, public right-of-way is being granted to the far western boundary to allow for this extension if necessary. 2. Monitoring well no. 5 (MW-5) was never identified in the groundwater subsection. • A groundwater monitoring well map is included with this submittal showing approximate locations of the seven groundwater monitoring wells installed on site. Additionally, groundwater monitoring well maps are included for both Norton Phase 4 and the Valley West Subdivision as well (see answer to comment #3). 3. Since you haven’t receive confirmation from the County’s Health Department or DEQ please provided additional information demonstrate that the limited groundwater data does in fact capture the season high and low. Your engineer needs to certify that the seasonal high and low has been provided. The highest groundwater elevations were seen with the first sampling event, without a oblivions inflection point in groundwater data it’s not reasonable to assume the “maximum” seasonal high has been observed just using the provided data. You might consider pulling data from the Lakes and the early phases of Norton. Civil/Structural Engineering and Surveying • The data collected by Michael Welch with C&H Engineering during the spring of 2018 shows that the groundwater is approx. 3” below ground surface in some areas. The applicant designed the subdivision assuming the groundwater is at existing grade (most extreme case). Because of this, all roads within the Norton Phase 5 subdivision have been elevated approximately 2’ to eliminate groundwater interference within the road subgrade. Additionally, information from the groundwater monitoring from Norton Phase 4 and the Lakes and Valley West are included in this submittal. Data from the Valley West Subdivision was collected over a three year span and consistently shows that the peak groundwater levels in this area occur between the months of March to May. There is very little fluctuation in the groundwater levels for these three months. Because the data collected for Phase 5 began in April, it is our professional opinion that these peak levels shown with the Phase 5 data represent the seasonal high groundwater level. 4. Updates to the stormwater report does not appear to account for any additional offsite from the groundwater present in the upstream system. In the response it just states that proposed system will be water tight but we know that the phase 3 systems already has groundwater in which will be convey downstream. I at least need to see that is account for in the design with the preliminary plat. Though unless there is real data to back the presented flow my going to condition the infrastructure design to provide flow actual data to ground true the report. • The proposed stormwater detention pond is oversized by 10,000 cubic feet. Additionally, the pond footprint has the ability to expand 3’ in all directions which would provide an additional 3,500 cubic feet of storage if the groundwater influence is determined to be greater than the additional 10,000 cubic feet already provided. The applicant wishes to install the pond as currently designed with the first phase of construction this fall/winter and monitor the groundwater influence during the peak months (March-May). With the phase 1 infrastructure design, preliminary calculations based on infiltration rates and soil conditions will be performed to determine the groundwater seepage into the existing system. A head of 4’ will be used as a conservative measure for these calculations as the storm pipes all have 2’ of cover over them. Based on field observations on the pond during the peak months, a decision will be made at that time whether or not to expand the pond, but this determination will be made after Phase 1 has been constructed. 5. Traffic Comments – Taylor has reach out to Bob asking for additional clarification on the new supplements to the TIS that was provided. • It is our understanding the Taylor Lonsdale is working directly with Bob Abelin to provide the required traffic impact study materials. A copy of the TIS supplement prepared by Bob based on his conversations with Taylor is included with this submittal. This supplement analyzed the Fallon Street level of service, the timing of the northern connection of Laurel Parkway, and the timing of the traffic control improvements to Laurel Parkway. Civil/Structural Engineering and Surveying If you require any further information, please give me a call at (406) 587-1115. Sincerely, Luke Stein, E.I.