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HomeMy WebLinkAbout013_FloodHazardReport2023 MEMO 32 Discovery Drive  Bozeman, MT 59718  (406) 582-0221  Fax (406) 582-5770 To: City of Bozeman From: Rory Romey, PE, Tom Chingas, PE, CFM, Eric Foss, EI Project Name: North Park Development, Phase 1 - Flood Study Update Project Number: 14-167.10 Date: 6/20/2023 Re: Update to 2016 Flood Hazard Evaluation for Mandeville Creek 45.705525°, -111.055547°, Bozeman, Montana Introduction The original flood hazard evaluation for Mandeville Creek was completed in 2016 using 1-D modeling. The model used surveyed cross sections gathered throughout the length of the study area. Elevation data from the 2007 City of Bozeman LiDAR mapping was utilized between the surveyed cross sections. This model focused on the North Park Development property extending from Frontage Road upstream to Mandeville Drive. The original flood hazard evaluation discussed flood water being directed to the northwest along the southern side of the railroad right-of-way (ROW) due to backwater of the existing culverts under Frontage Road, the main railroad track, and Red Wing Drive. This flow path was not studied or mapped because it was beyond the Master Site Plan extents for the development. The purpose of this Flood Study update is to discuss the phases of the North Park property development along with the required hydraulic improvements associated with each phase and to show the corresponding effects to the floodplain inundation boundary and water surface elevations (WSEs). Project Phasing Existing Conditions For this study the existing condition of the North Park Development property is considered to be the site conditions prior to any development. This was the site condition analyzed for the 2016 Flood Study. Figure 2 shows the existing condition topography and site features. This model run is intended to show the comparison of the 1-D modeling from the original flood hazard evaluation to the new modeling. Phasing of the North Park Development will include three new creek crossing locations that will impact the flood hazard boundary. No structures exist near the flood hazard boundary. Phasing also includes upsizing the two remaining existing culverts under Frontage Road and the main railroad track. 6/20/2023 Allied Engineering Services, Inc. Flood Hazard Evaluation Page 2 As-Built Condition (as of 3/27/2023) In 2022 there was a significant amount of earthwork that took place within the railroad ROW to add siding tracks and stubs for future spur lines. Some of this earthwork filled in the low-laying areas that convey floodwater to the northwest. The Red Wing Drive crossing was removed as part of this work and the existing 36” cast iron railroad culvert was extended approximately 40’ upstream to account for the additional railroad tracks. Figure 3 shows the As-Built conditions topography and site features. Intermediate Condition The next phase of the project is to add three crossings to Mandeville Creek. The first crossing will be for a rail spur line (Track C) to access Tract 2-B. This crossing is anticipated to be constructed in late 2023 or 2024 and is proposed as a precast concrete box culvert with a span of 12’ and a height of 7’ with a 1’ embedment. The remaining two crossings are for Wheat Drive and Cultivar Street. These two are anticipated to be constructed in 2024 or 2025 and are proposed as precast concrete box culverts with a span of 18’ and rise of 6’ with embedment depths of 1’. This phase of the project also involves upsizing the existing 36” cast iron culvert at the main rail line. The upsized culvert will need to be equal to the capacity of the upstream culvert (for Track C) and the future upsized downstream culvert at Frontage Road. We anticipate the existing 36” cast iron culvert will be replaced with a CMP arched culvert with an embedment depth of 1’. The construction of this culvert replacement is anticipated to take place in late 2023 or 2024. We have been coordinating with Montana Rail link about the design of this replacement culvert. Additional rail spurs are proposed to be constructed to the west of Mandeville Creek as well. Figure 4 shows the intermediate condition topography and site features. The intermediate condition assumes the existing culvert at Frontage Road is left as the 36” RCP/CMP. We do not recommend upsizing the existing 36” cast iron culvert at the main rail line before upsizing the Frontage Road culvert because the new overflow path that would be activated (see the Intermediate Condition described in the Hydraulic Analysis section of this report). However, it is ultimately up to Montana Rail Link on what they want to decide for replacing their culvert and the associated risks. Final Condition The final phase of the project involves replacing the existing 36” RCP/CMP culvert at Frontage Road. We anticipated this replacement will be a precast concrete box culvert with a span of 12’, rise of 8’, and embedment depth of 2’. The Frontage Road culvert is anticipated to be replaced in 2024 or 2025. We have been coordinating with Montana Department of Transportation about the design of this culvert replacement. Figure 5 shows the final condition topography and site features. All timelines outlined above are subject to change. As of now, all crossing improvements outlined above are being included under a single general permit with the Army Corps of Engineers being a primary driver due to wetland impacts. Hydrology Analysis A hydrologic analysis was performed for the 2016 Flood Study. The same flow values were adopted for this study which are 390-cfs for Mandeville Creek and 127-cfs the Farmer’s Canal. The 2016 Flood Study is included in this report as Attachment 1. 6/20/2023 Allied Engineering Services, Inc. Flood Hazard Evaluation Page 3 Hydraulic Analysis The updated Flood Study was performed using The Army Corps of Engineer’s Hydrologic Engineering Center’s River Analysis System (HEC-RAS) version 6.3.1 (USACE, 2022). Mandeville Creek and the surrounding floodplain area was modeled using a steady-state 2D model consisting of a cell mesh, inflow and outflow boundary conditions, and 2D connections for the hydraulic structures. Elevation data for the model was obtained from survey data provided by Allied Engineering, Inc. and LiDAR data obtained from the 2018 City of Bozeman mapping project. The data’s vertical datum is NAVD88. Figure 1 shows the model extents and geometry of the existing conditions. Manning’s n values were obtained by referencing aerial imagery and from “Open-Channel Hydraulics” (Chow 1959). A Manning’s n value of 0.045 was appointed to the main channels describing a winding channel with some weeds and stones. The model was extended far enough downstream so the downstream boundary conditions did not affect the hydraulics in the development area. However, a normal depth boundary condition with a friction slope of 0.004 was used for the East Gallatin River outflow location. Flow hydrograph boundary conditions with constant flow values were used for the inflow locations where Mandeville Creek and the Farmer’s Canal enter the model area. A 12 hour initial conditions time was used to ramp up the flows to increase model stability. A model simulation time of 12 hours was used for all plans and the models were checked to ensure a steady-state condition was met. A Courant controlled time step was selected with a maximum and minimum courant values of 1.0 and 0.4 respectively with a base time step of 1 second. Existing Conditions The existing conditions were modeled using the 2D methodology to compare to the 1D model from the 2016 Flood Study. Figure 2 shows a comparison of the 1D and 2D flood hazard boundaries and WSEs for the existing condition. As depicted in Figure 2, the flood hazard boundary and WSEs of the 1D and 2D model are similar. The 2D model flood hazard boundary will be used as the existing condition flood hazard boundary for the remainder of this study. Where the 1D model ended, the 2D model was extended further downstream to the confluence with the East Gallatin River as well as further to the northwest and eventually to the East Gallatin River. The overflow path that flows along the southern side of the railroad ROW to the northwest from the main track railroad crossing of Mandeville Creek is shown in the 2D model. Approximately 438-cfs of the total 517-cfs is being directed along this overflow path because the culverts at Frontage Road, the main railroad track, and Red Wing Drive are undersized. As-Built Condition (as of 3/27/2023) A plan was created with an updated geometry and terrain to reflect the As-Built conditions and the flood hazard boundary was remapped. The resulting flood hazard boundary was the same from the upstream end of the model to within approximately 400-ft of the railroad ROW. In this case, the grading associated with the railroad siding tracks requires backwater to build up more head to overtop to the northwest. This increased WSE causes backwater to extend further upstream and laterally in the adjacent floodplain. The overflow path to the northwest is maintained in this configuration with 6/20/2023 Allied Engineering Services, Inc. Flood Hazard Evaluation Page 4 approximately 423-cfs being directed that way. Figure 3 shows a comparison between the existing condition and the as-built condition flood hazard boundary. Intermediate Condition The modeling for this phase of the project was completed using the anticipated culvert sizing and grading discussed in the Project Phasing section. The three new crossings upstream of the main rail line in this intermediate condition have minimal impact to the flood hazard extents because each culvert is sized to handle the 517-cfs. However, there is a small backwater area at the Cultivar Street crossing where the flood hazard extent increases due to a localized increase in the WSE. This backwater area extends approximately 400’ upstream of the proposed Cultivar Street crossing. The areas are considered to be ineffective flow areas not providing conveyance. The areas will be impacted by the approved (but not yet constructed) shared use trail. The areas are noted Figure 5 and are considered to be filled as part of future development. Because the areas are considered to be ineffective flow areas, impacts to the BFEs are not anticipated. Figure 4 shows a comparison between the existing conditions flood hazard boundary and this intermediate condition. It is important to note that upsizing the main rail line culvert before the Frontage Road culvert creates a new overflow path on north side of the railroad tracks. In previous iterations the northwest overflow path remains on the south side of the railroad tracks until crossing under N 19th Ave and I-90. In this scenario, a large amount of backwater would form just upstream of the existing Frontage Road culvert and would overtop Frontage Road near the intersection with Reeves Road. This new overflow path has the potential to flood existing structures before the flood water reaches the East Gallatin River. Final Condition The final condition will allow the full flow from the 100-year event to reach the East Gallatin River without backwatering upstream of the railroad or the Frontage Road crossing as well as without activating the northwest overflow path. In this configuration the flood hazard boundary along Mandeville Creek is very similar to the existing condition with the exception of the area immediate upstream of Cultivar Lane as discussed in the intermediate condition section above. Figure 5 shows the flood hazard boundary for the final condition. It should be noted that the elevation data used for the current study has been updated since the original study took place. The most recent (year 2018) COB Lidar was used for the current study along with finished design grading for the proposed conditions. Table 1 shows a comparison of base flood elevations at key locations for the various conditions previously outlined. Table 1. BFE Comparisons (Units = Feet, Datum = NAVD 88) Location* Existing Condition (2016 Flood Study) As-Built Condition (as of 3/27/3023) Intermediate Condition Final Condition Cultivar Lane 4701.9 4701.9 4706.2 4706.2 Wheat Drive 4675.0 4675.1 4677.1 4677.0 Track C 4670.5 4673.8 4673.2 4671.3 Railroad Main Track 4670.5 4673.8 4671.8 4668.7 *The WSE for these were taken just upstream of the indicated location FIGURE: 1DRAWN BY: TWCDATE: 5/17/2023PROJECT: 14-167³0 540 1,080 1,620Feet NORTH PARK2D MODEL SET UP - EX. CONDITIONSBOZEMAN, MT P:\2014\14-167 North Park\08 ArcGIS\2023 - Flood Study\Zoomed In\Figure 1 - Model Set Up.mxd1 inch = 900 feet Mandeville Creek Legend 2D Model Area Boundary Condition Lines (Inflow/Outflow Locations) 2D Connections (Hydraulic Structure Locations) 2D Existing Conditions - 1% AC Flood Hazard Boundary East Gallatin River 46684692466647084698467647004695469047154 7 1 04680 47204670473047254700468546654660466547454740473047254675467046654 6 5 54650 47254 7 2 0 470547004 6 8 0 4655 46454730472046654 67 04660 47354735473047304725472547 1 54695 4 6 7 5 46704665466 04655 4655473046604680.44709.84707.84706.24702.947004697.54694.64691.54686.74683.54715.74672.44669.84675.5FIGURE: 2DRAWN BY: TWCDATE: 5/17/2023PROJECT: 14-167³0 240 480 720Feet NORTH PARKEX. CONDITIONS - 1D/2D COMPARISON BOZEMAN, MT P:\2014\14-167 North Park\08 ArcGIS\2023 - Flood Study\Zoomed In\Figure 1 - Existing Conditions.mxd1 inch = 400 feet Mandeville Creek Legend 1D Cross Sections 2016 Flood Study 1D - 1% AC Flood Hazard Boundary 2D WSE Contours (2-ft Contour Interval) 2023 Flood Study 2D - 1% AC Flood Hazard Boundary Northwest Overflow Path Frontage Road Main Railroad Track Red Wing Drive 47154710469046854705470046954660465547454740473547304725467046654665466047204 7 1 54675467046554650 472546704 7 1 5 46954 6 8 0 4 67 046554650 47354730472547304730473047254725468046704665466546 604660 46554730FIGURE: 3DRAWN BY: TWCDATE: 5/17/2023PROJECT: 14-167³0 240 480 720Feet NORTH PARKAS-BUILT/EX. COND. COMPARISON BOZEMAN, MT P:\2014\14-167 North Park\08 ArcGIS\2023 - Flood Study\Zoomed In\Figure 3 - As-Built Conditions.mxd1 inch = 400 feet Mandeville Creek Legend Siding Tracks Existing Conditions - 1% AC Flood Hazard Boundary As-Built (3/27/2023) - 1% AC Flood Hazard Boundary Northwest Overflow Path Frontage Road Main Railroad Track Red Wing Drive crossing removed and railroad culvert extended 471547104705469046854670466547454740473546754 6 7 0 4730472546704665466546604 6 5 54650 4 7 2 5 4720 4700468046754 7 1 5 46954 6 8 046554 6604645472047054 6 8 0 4 67 04655 473547304725473047304730472547254 7 1 546954670467046654665 46 604660 46554730FIGURE: 4DRAWN BY: TWCDATE: 5/23/2023PROJECT: 14-167³0 240 480 720Feet NORTH PARKINTERMEDIATE COND. COMPARISON BOZEMAN, MT P:\2014\14-167 North Park\08 ArcGIS\2023 - Flood Study\Zoomed In\Figure 4 - Intermediate Condition.mxd1 inch = 400 feet Legend Intermediate Condition Features Existing Conditions - 1% AC Flood Hazard Boundary Intermeditate Condition - 1% AC Flood Hazard Boundary Mandeville Creek Exisintg FrontageRoad culvert Main Railroad Track Red Wing Drive crossingremoved and railroad culvertreplaced with box culvert Cultivar LaneWheat Drive Additional railroad tracks Track C with box culvert Northwest overflowpath is eliminated New flow path createdon north side of railroad ROW 471547104705469046854670466547454740473546754 6 7 0 4730472546704665466546604 6 5 54650 4 7 2 5 4720 4700467546804 7 1 5 46954 6 8 046554 660464547204 6 8 0 4 67 04655 473547304725473047304730472547254 7 1 546954690467046654665 46 604660 46554730FIGURE: 5DRAWN BY: TWCDATE: 6/20/2023PROJECT: 14-167³0 240 480 720Feet NORTH PARKFINAL/EX. COND. COMPARISON BOZEMAN, MT P:\2014\14-167 North Park\08 ArcGIS\2023 - Flood Study\Zoomed In\Figure 5 - Final Condition.mxd1 inch = 400 feet Mandeville Creek Legend Final Condition Features Shared Use Trail Existing Conditions - 1% AC Flood Hazard Boundary Final Condition - 1% AC Flood Hazard Boundary Frontage Road culvertreplaced with box culvert Main Railroad Track Red Wing Drive crossingremoved and railroad culvertreplaced with box culvert Cultivar Lane Wheat Drive Additional railroad tracks Northwest overflowpath is eliminated Track C with box culvert Area of ineffective flowand shallow flooding,anticipated impact from approvedshared use trail location Area of ineffective flowand shallow flooding Shared use trail