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HomeMy WebLinkAboutDESIGN~1 TO City of Bozeman, Engineering Dept. PROJECT S & S Apartments (#23273) HE PROJ. # 213601 DATE December 2, 2025 FROM Brett Megaard, PE SUBJECT Stormwater Design & Existing Irrigation Facilities The purpose of this memo is to provide additional clarification and justification on the proposed stormwater system design as it relates to the S&S Development, as well as the impacts that it may or may not cause on the existing irrigation ditch. Per the Stormwater Engineering Report that was submitted with the Site Plan application as well as the Engineering Infrastructure submittal, the stormwater system was designed in accordance with the City of Bozeman Engineering Design Standards (dated 3/13/2020). Existing Irrigation Ditch Per recent discussions with the City of Bozeman, it has been requested that further insight and justification for discharging to the existing irrigation ditch be provided, in regards to the proposed S&S development. Currently, with the existing/pre-developed storm runoff, the site drains to the northwest corner of the site where any runoff joins into the existing irrigation ditch prior to it conveying under the existing driveway/railroad berm on the north side of the property. This ditch currently operates at about ¼ full, depending on storm events, leaving plenty of room for expanded flow. From the original topographic survey that was completed in June of 2021, it appears as though the ditch is capable of supporting 7+ cfs (≈3,000 gpm) rather easily, especially when the downstream culvert is a 36” Ø CMP capable of supporting up to 53 cfs or 24,000 gpm at normal slope. So even with our site having a 100-year event at roughly 3cfs (≈1300+ gpm being the theoretical max design storm), being controlled by 2 separate weir designs, and also designed to release at the pre-developed rate with 1’ of head on the structure, the ditch itself would operate at a manageable flow/depth. In regards to the proposed ditch outlet and the existing culvert under the railroad grade, it will function similar to how it does today such that the new outlet won’t change much from what is existing, rather than just converting the ditch to a piped segment, resulting in less sediment transport and irrigation water loss through infiltration of the ditch. The new pipe is ended short of the outlet culvert so that in the event of future development to the north, the ditch pipe could be extended straight north through the ROW/ditch easement, as would be ideal in future situations. PROPOSED DEVELOPMENT SECTIONLINE DITCH 2 © HYALITE ENGINEERS, PLLC Proposed Stormwater System A stormwater system designed as a detention system, in accordance with the City of Bozeman's engineering requirements, is specifically engineered to prevent downstream flooding, even during a 100-year storm event in this instance. This is achieved because the system is designed to temporarily store stormwater on-site and release it at a controlled rate, typically at or below the pre-development flow rates as is shown within the Engineering Report for the site, updated November of 2025. As stated above, the max flow from the site during the 100-year storm event would be controlled through two separate weirs designed to release at a pre-developed rate with 1’ of head on the structure, allowing the existing ditch to operate properly. However, to ensure that there is no downstream flooding, each basin has been examined to ensure the detention basins are properly sized for the 100-year storm event. An additional 41 Cubic Feet of storage will be added to Basin A, and an additional 47 Cubic Feet of storage will be added to Basin B. Basin C is already properly sized for the 100-year storm event. With the additional storage added to each basin, the site will have the capacity to store and properly discharge stormwater produced from the 10-year and the 100-year storm events. Additionally, the City of Bozeman emphasizes minimizing non-stormwater discharges to storm drains and reducing pollutants in stormwater discharges. In a detention system, stormwater is collected in a basin or other storage facility and then the basin is engineered with an outlet structure that carefully regulates the outflow of water, ensuring that the rate of discharge does not exceed the capacity of downstream infrastructure. By slowly releasing the water over time, the detention system mitigates peak flows, preventing a surge of water that could overwhelm downstream drainage systems and cause flooding. With the slight additional storage added to Basin A and Basin B, as well as the over-conservative routing of Basin B through Basin C, the design ensures that stormwater is managed effectively, thereby eliminating the risk of contributing to downstream flooding. Therefore, as our site is developed and broken out into multiple post-development stormwater basins, two are designed as detention systems and one is retention. Meaning, only two are releasing stormwater back into the irrigation ditch, at or less than the original rate of .55cfs, thus not increasing the downstream flow during the normal storm events. As mentioned above, even in the event of a 100-year storm event, with the outlet-controlled structures and 1’ of head pressure, the systems will continue to function adequately. Finally, Basin D will be open space and landscaped with a proposed gravel walking trail. Therefore, post-development conditions will be that of pre-development conditions, or even slightly better, thus not requiring any additional storage. Conclusion/Downstream Assurance Throughout the design of the project, additional concern & analysis was specified to the existing irrigation ditch as well as the railroad berm, to ensure that the standard of practice was given to the stormwater design. By mimicking the existing drainage paths that are present today, from some site infiltration, and taking the natural gradient into consideration to the NW corner of the site where the only outlet exists and will continue to operate that way, with only the pre-existing outflow being matched with this layout (as per City Standards). The engineering design shows that it successfully meets the engineering standards for stormwater runoff control and manages the peak flow, therefore ensuring to the engineering standards that no downstream property should experience flooding attributable to an increase in peak runoff from this development during a 100-year storm event.