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HomeMy WebLinkAbout018 Appendix Q - Storm Drainage Engineers ReportEngineering Report Stormwater Design Aaker Phase 1 Subdivision Preliminary Report for Preliminary Plat Review October 2022 Rev. March 2023 Submitted to: City of Bozeman Engineering Department 20 East Olive Bozeman, MT 59715 Prepared by: Stahly Engineering and Associates 851 Bridger Drive, Suite 1 Bozeman, MT 59715 (406) 522-9526 Stormwater Plan 1 3/28/2023 Engineering Report Table of Contents General Information and Design Criteria Existing Site Conditions Storm Drainage Plan Description Estimation of Runoff Retention Facilities Conveyance Capacity Flood Conveyance Aaker Phase 1 Subdivision Stormwater Maintenance Plan Figures and Tables Figure 1. Vicinity Map Table 1 – Storm Runoff Calculations for Retention Table 2 – Retention and Bio-Retention Basin Capacities Table 3 – Storm Runoff Calculations for Conveyance Table 4 – Curb and Gutter Conveyance Calculations Table 5 – Storm Runoff Rate and Pipe Conveyance Calculations Table 6 – Retention Pond Overflow Conveyance Calculations Table 7 – Culvert Conveyance Calculations Attachments/References C5.0 Phase 1 Grading and Drainage Plan C5.1 Phase 1 Stormwater Details C5.2 Phase 1 Stormwater Details Geotechnical Report Aaker Phase 1 Subdivision Flood Hazard Evaluation Stormwater Plan 2 3/28/2023 11.1 Engineering Report – General Information The Aaker property consists of approximately 95 acres of vacant land located west of South 19th Avenue, north of Stucky Road, and extending west and north to the MSU BART Farm. Concept master planning and initial REMU and B-2M zoning of the Aaker Property envision a mixed-use development with commercial uses along South 19th Avenue grading to predominantly residential uses toward the north and west. The Phase 1 Subdivision includes the extension of Kagy Boulevard and the land to the north, comprising 31.73 acres, with a 63.68-acre restricted future development lot south of Kagy Boulevard. Phase 1 will create 7 lots for development with REMU zoning. Planned land use includes moderate density residential with some commercial uses integrated into lots adjacent to Kagy Boulevard. Water and sewer service for the subdivision will be provided by extensions from existing City of Bozeman water and sewer mains adjacent to the site. The developer’s contact information is: West University, LLC 113 East Oak, Suite 4A Bozeman, MT 59715 Water supply and wastewater treatment will be provided by the City of Bozeman’s municipal systems. The system owner’s contact information is: City of Bozeman Public Service Department PO Box 1230 Bozeman, MT 59771-1230 Figure 1 shows a vicinity map of the Aaker Property and Phase 1 Subdivision. Figure 1. Vicinity Map, Bozeman, Montana Stormwater Plan 3 3/28/2023 Existing Site Conditions The Aaker Phase 1 property was previously used for agriculture. The site slopes generally to the north and contains a stream and wetland in the western portion of the site. A geotechnical investigation was conducted, and the report is provided separately with the subdivision submittal. In summary, the geotechnical investigation found a variable depth of fine-grained soils overlying gravel sediments. The depth to gravel varies between 1 and 4 feet. Groundwater monitoring was conducted throughout the 2022 season. Depth to seasonal high groundwater varied from near the surface in the wetlands to about 3-4’ below ground at the proposed lots. Groundwater is generally flowing to the north. Proposed street and lot grading will raise the developed portions of the site approximately 2-3’ resulting in a typical depth to seasonal high groundwater in developed areas of about 5-7’. Storm Drainage Plan Description The Storm Drainage Plan for the Aaker Phase 1 Subdivision consists of separate mitigation systems for subdivision streets (public) and lots (private). The Aaker Phase 1 Subdivision stormwater collection area is divided into 5 separate drainage areas for evaluation of retention requirements and sizing. Stormwater from drainage areas 1, 2 and 4 is conveyed in curbs and storm drainage pipes to retention basins. Stormwater from drainage areas 3 and 5 is conveyed in curbs to bio-retention swales in the roadway boulevards. The City is responsible for maintaining storm piping within City rights-of- way. The Aaker Subdivision Property Owners’ Association is responsible for monitoring and maintaining the subdivision stormwater retention and bio-retention facilities. Storm runoff from individual lots will be mitigated on-site as determined during each lot’s site design phase and is the responsibility of the individual lot owners to maintain. The Overall Storm Drainage Plan for Aaker Phase 1 Subdivision is depicted on Plan Sheet C5.0 Phase 1 Grading and Drainage Plan, attached to this report. The plan shows proposed grading, drainage areas, stormwater piping, retention basins, and culverts. The subdivision stormwater retention systems are sized to contain the storm runoff from the 10-year, 2-hour storm, in accordance with City of Bozeman Design Standards. All lots are required to have private on-site stormwater systems to meet the City’s design standard for stormwater mitigation. The retention basins will contain 96% of all rainfall events and significantly reduce discharges from larger events. The retention basins are in areas that facilitate overflow to natural water ways to prevent property damage during large rainfall events. Stormwater Plan 4 3/28/2023 The five (5) subdivision storm drainage areas are further divided into 8 reaches or sub- areas with discrete catchment areas to determine peak flow rates, required pipe sizing, and curb capacity. Storm drainage curbs, pipes and overflows are sized to convey the peak flow from a 25-year storm in accordance with City of Bozeman Design Standards. Stream crossings are sized to convey the 100-year flood flows without causing flooding of adjacent property. Estimation of Runoff Runoff estimates were obtained for each drainage area using City of Bozeman Standards. The drainage area for the subdivision streets shown on Plan Sheet C5.0 includes a 10’ front yard which typically is sloped toward the street. The runoff coefficient was selected at 0.65. Runoff coefficients for a local street are 0.58 and for the full buildout Kagy section are 0.64. Since most drainage basins have a mix of street sections a value of 0.65 was used for all basins for simplicity. The design storm event is a 10-year, 2-hour storm. The Bozeman IDF (Intensity, Duration, and Frequency) curves show a rainfall intensity of 0.41 in/hour for the 10-year, 2-hour storm, totaling a 0.82- inch storm event. The rainfall event is the 96th percentile event, meaning 96% of rainstorms are equal to or less than this amount. Table 1 – Storm Runoff Calculations for Retention Table 1 shows the 10-year storm runoff calculations for the 5 drainage areas of the subdivision stormwater collection area. Retention Facilities Stormwater from subdivision streets is mitigated by individual stormwater retention systems for each drainage area as shown on Plan Sheet C5.0 Phase 1 Grading and Drainage Plan. Stormwater mitigation for drainage areas 1, 2, and 4 is primarily by ponding retention, with bio-retention media in the pond bottom. The bio-retention component provides improved infiltration to shorten the duration of standing water and allows subsurface retention of smaller storms. Drainage areas 3 and 5 utilizes bio- retention swales in the street boulevard to retain stormwater. All bio-retention areas consist of an engineered soil media that has consistent permeability but allows for plant growth. Table 2 shows the storage capacities of each basin exceed the 10-year, 2-hr storm runoff. Taking into account the improved infiltration provided by the bio-retention Basin Characteristics 1 2 3 4 5 Mitigation Type Retention Retention Bio-Retention Retention Bio-Retention Area (sf)159,917 17,320 44,317 222,107 99,849 Area (acre)3.67 0.40 1.02 5.10 2.29 Impervious Area (sf)103946 11258 28806 144370 64902 Pervious Area (sf)55971 6062 15511 77737 34947 Weighted (C)0.65 0.65 0.65 0.65 0.65 Runoff Volume (cf) 10-yr 2-hr storm 7103 769 1968 9865 4435 Stormwater Plan 5 3/28/2023 media, the proposed retention basins will contain storm events considerably larger than the 10-year, 2-hour storm. Table 2 – Retention and Bio-Retention Basin Capacities Construction details of the retention systems are shown on Plan Sheet C5.2. Retention basins located in City Park have 6:1 side slopes and a maximum ponding depth of 1.5 feet to be compatible with adjacent park uses. Bio-retention basins have concrete inlet chases designed to capture sediment for easy removal before water enters the bio- retention basin. To protect groundwater and provide reliable infiltration, retention systems are situated to provide 3’ of separation between basin bottom and seasonal high groundwater. To accommodate discharge from storm events larger than the design event all retention systems have overflow provisions sized to meet the 25-year peak flow event. The retention basins will contain a significant amount of the runoff even from large events, dramatically reducing the actual overflow flow rate and occurrence. To reduce impacts to the adjacent wetland and stream the retention basins in the City Park have a broad rip-rap overflow that allows overflows to disperse into the wetlands adjacent to the stream. The bio-retention basins all include an overflow structure at the downgradient side that conveys the overflow out of the street. The west bioretention overflow (Basin 3) discharges just outside of the wetlands. The east bio-retention overflow (Basin 5) discharges directly to the stream crossing culvert beneath the roadway. Overflows are anticipated to rarely occur, but even so all the basin overflows are situated to minimize impacts to the adjacent wetlands and stream. Conveyance Capacity The peak flow from the 25-year storm event is determined for the purpose of sizing conveyance facilities. With a time of concentration of 10.9 minutes, the 25-year storm has an intensity of 2.32 in/hour, but due to its short duration, only represents a 0.32-inch event. Drainage Area 1 2 3 4 5 Retention Storage Volume Ponding Depth (ft)1.50 1.50 N/A 1.50 N/A Pond Bottom Area (sf)2870 304 N/A 4146 N/A Pond Top Area (sf)5513 872 N/A 7442 N/A Retention Storage (cf)6287 882 0 8691 0 Bio-Retention Storage Volume Media Area 2870 304 5760 4146 7600 Media Depth 1.5 1.5 2 1.5 2 Media Volume 4305 456 11520 6219 15200 Media Porosity 0.30 0.30 0.30 0.30 0.30 Media Storage (cf)1292 137 3456 1866 4560 Total Retention Storage 7579 1019 3456 10557 4560 Stormwater Plan 6 3/28/2023 The peak flow from the 25-year storm shown in Table 3. Note that the two largest drainage areas, 1 and 4, are broken into sub-areas to identify intermediate flow rates from pipe branches. Conveyance facilities (curbs, pipes, and overflows) are designed to have capacities exceeding the 25-year peak flow shown in Table 3. Table 3 – Storm Runoff Calculations for Conveyance Curb and gutter collects storm runoff and conveys it to the storm drainage pipe network. The capacity of a typical City of Bozeman Standard Curb and Gutter at the minimum slope of 0.5% is shown in Table 4 below. A typical curb and gutter has a conveyance capacity of 3.47 cfs, and there are two curbs conveying stormwater in each drainage area (one on each side of the street). The curb capacity of a single curb exceeds >50% of the 25-year peak flow from any single sub-area as shown in Table 3. Table 4 – Curb and Gutter Conveyance Calculations Curb and Gutter Capacity Calcs Right-side Slope X:1 0.06 Left-side Slope X:1 33.00 Channel Bottom Width (ft) 0 Flow Depth (ft) 0.300 Flow Area (ft^2) 1.488 Wetted Perimeter (ft) 10.205 Width 9.919 Hydraulic Radius (ft) 0.146 Manning's Roughness 0.013 Slope (ft/ft) 0.005 Average Velocity (ft/sec) 2.33 Flow (ft^3/sec) 3.47 Table 5 below shows the stormwater flow in each pipe reach as flow from sub-areas combine. For drainage area 2, the entire basin is collected in a single pipe. For drainage areas 1 and 4, the storm drainage pipe collects runoff along its length from Inlet Subarea Basin #1 1A 1B 2 3 4 4A 4B 4C 5 Drainage Area (acres)3.67 1.20 2.47 0.40 0.93 5.10 3.08 1.11 0.90 2.3 Drainage Area (sf)159917 52195 107722 17321 40428 222107 134256 48441 39410 99850 Slope (%)1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Runoff Coefficients 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 Frequency Adjustment Factor, Cf 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 Basin Length (ft)300 300 300 300 300 300 300 300 300 300 Time of Concentration (min)10.9 10.9 10.9 10.9 10.9 10.9 10.9 10.9 10.9 10.9 Peak Flow Calculations Design Storm 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr 25yr Time of Concentration (min)10.9 10.9 10.9 10.9 10.9 10.9 10.9 10.9 10.9 10.9 Intensity at Tc (Figure I-2 pg. 29) (in/hr)2.32 2.32 2.32 2.32 2.32 2.32 2.32 2.32 2.32 2.3 Peak Runoff Rate at Tc (Q = CIA) (cfs)5.55 1.81 3.74 0.60 1.40 7.70 4.66 1.68 1.37 3.46 Stormwater Plan 7 3/28/2023 contributing sub-areas shown in Table 3, requiring pipe size increases. Table 7 below also shows the minimum pipe sizes and slopes of each reach required to carry the storm runoff at that point in the system. For drainage areas 3 and 5, the OF designates the flow rate and capacity of the overflow pipes for these basins. The capacity of all pipes and overflows exceeds the flowrate generated by the 25-year storm event in each basin or subbasin. Table 5 – Storm Runoff Rate and Pipe Conveyance Calculations Table 6 below shows the capacity of the overflows for retention basins 1 and 4 in the City Park. The overflow is 10’ wide to provide a broad shallow overflow condition, allowing water to dissipate into the wetlands. The capacity of the overflow is 12.63 cfs which exceeds the peak flow from the 25-year storm runoff of these two drainage areas. Table 6 – Retention Pond Overflow Conveyance Calculations Flood Conveyance A Flood Hazard Evaluation (FHE) was conducted for the Aaker Phase 1 Subdivision and is included separately with the subdivision submittal. The FHE identified three Drainage Subarea #1 1A 1B 2 3 OF 4 4A 4B 4C 5 OF Pipe Material PVC PVC PVC PVC PVC PVC PVC PVC PVC PVC Pipe Size (in)18.00 15.00 15.00 15.00 15.00 21.00 18.00 15.00 15.00 15.00 Manning's "n" (PVC)0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 0.013 Area (ft2)1.77 1.23 1.23 1.23 1.23 2.41 1.77 1.23 1.23 1.23 Wetted Perimeter (ft)4.71 3.93 3.93 3.93 3.93 5.50 4.71 3.93 3.93 3.93 Hydraulic Radius (ft)0.38 0.31 0.31 0.31 0.31 0.44 0.38 0.31 0.31 0.31 Slope (ft/ft)0.0033 0.0033 0.0033 0.0033 0.0033 0.0033 0.0033 0.0033 0.0033 0.0033 Full Flow Capacity (cfs)6.05 3.72 3.72 3.72 3.72 9.13 6.05 3.72 3.72 3.72 Velocity (ft/sec)3.42 3.03 3.03 3.03 3.03 3.79 3.42 3.03 3.03 3.03 Contributing Inlet Basin 1A,1B N/A N/A N/A N/A 4A,4B,4C 4A,4C N/A N/A N/A Stormwater Flow (cfs)5.55 1.81 3.74 0.60 1.40 7.70 6.02 1.68 1.37 3.46 % Capacity 92%49%100%16%38%84%100%45%37%93% Right-side Slope X:1 4.00 Left-side Slope X:1 4.00 Channel Bottom Width (ft)10 Flow Depth (ft)0.500 Flow Area (ft^2)6.000 Wetted Perimeter (ft)14.123 Width 14.000 Hydraulic Radius (ft)0.425 Manning's Roughness 0.040 Slope (ft/ft)0.010 Average Velocity (ft/sec)2.10 Flow (ft^3/sec)12.63 Pond Overflow Capacity Calcs Stormwater Plan 8 3/28/2023 channels that convey floodwater through the Aaker property. The three channels are the East Fork of Catron Creek (near South 22nd Avenue), an irrigation ditch from Middle Creek Ditch (near South 23rd Avenue), and the West Fork of Catron Creek (west of South 25th Avenue). The FHE identified potential future 100-year flood flows for both the east and west forks of Catron Creek. These flows are 22.1 cfs and 50.7 cfs, respectively. The irrigation ditch is within the West Fork of Catron Creek drainage basin and represents a potential flow split of the west drainage basin. The FHE shows the areas of inundation for both the existing and proposed conditions. The areas inundated by the 100-year flood event are within the watercourse setback and do not extend into areas proposed for development. The Middle Creek Ditch irrigation ditch intercepts the West Fork of Catron Creek south of the Aaker property and could potentially divert the west drainage flood flows into the center of the Aaker property. To reduce the flood hazard created by this ditch, the perennial stream flows, unused irrigation water, and flood flows are proposed to be returned to the west channel at the south side of the Aaker property. A headgate on this return pipe is proposed to allow the existing irrigation ditch to continue to supply downstream users while limiting overflows in a high flow event. Because the proportion of flood flow split is unknown, the return pipe is sized to carry 100% of the west drainage basin 100-yr flood flows. Kagy Boulevard crosses both the east and west forks of Catron Creek. The east fork crossing will be by an extension of the existing 30” RCAP to the south right-of-way of Kagy. The west fork crossing is proposed to be a 6’ wide open bottom box culvert, with a 4’ high opening. Retaining walls at the road right-of-way are proposed to minimize impacts to existing wetlands at the Kagy crossings. The hydraulic capacities of the flood conveyance pipes is shown in Table 7. All pipes have capacity to transmit the 100-year flood flows. Table 7 – Culvert Conveyance Calculations Drainage Basin West Drainage West Drainage East Drainage Location Kagy Crossing W. of 25th Ave.Irrigation Ditch Return Pipe Kagy Crossing E. of 22nd Ave. Pipe Description 6'x5' Conc. Box - Cobble Bottom 27" x 44" RCAP 23" x 36" RCAP Pipe Width (ft) (rectangular pipe)6 n/a n/a Pipe Equalivalent Diameter (in) (arch pipe)n/a 36 30 slope (ft/ft)0.01 0.006 0.005 coefficient of roughness (n)0.05 0.013 0.013 Depth in pipe (in)29 36 30 depth in pipe (ft)2.42 3.00 2.50 pipe radiaus (ft)n/a 1.50 1.25 wetted perimeter (ft)10.83 9.42 7.85 area of flow (sf)14.5 7.07 4.91 hydraulic radius (ft)1.34 0.75 0.63 Avg. Velocity 3.61 7.31 5.91 Flowrate Pipe (cfs)52.3 51.7 29.0 Stormwater Plan 9 3/28/2023 Aaker Phase 1 Subdivision Storm Water Maintenance Plan General Information The Aaker Phase 1 Subdivision utilizes storm water retention basins and bio-retention swales to mitigate storm water impacts from subdivision streets. The stormwater retention basins are located in City parks, and the bio-retention swales are located in City rights-of-way. Stormwater is collected and conveyed to the retention systems through City streets and storm drainage pipes. The City is responsible for maintaining storm piping within City rights-of-way. The Aaker Subdivision Property Owners’ Association is responsible for monitoring and maintaining the subdivision stormwater retention and bio-retention facilities. Stormwater mitigation for private lots is not part of the subdivision infrastructure and is the responsibility of the individual lot owners to maintain. The subdivision storm water facilities are designed to operate without excessive maintenance. However, like all infrastructure, periodic monitoring and maintenance will prevent costly repair and replacement. This Maintenance Plan has been prepared in accordance with City of Bozeman guidelines. Over time, recommended maintenance guidelines may evolve. Please contact the City of Bozeman Storm Water Division if any questions arise. Storm Water Facilities Maintenance Schedule 1. Site Housekeeping. (Continuously as needed) The main cause of storm water facility damage is poor site housekeeping. Sediment tracked onto pavement can be washed into storm water bio-retention basins and conveyance piping and damage these facilities. Trash can clog pipes and inlet structures causing property damage. • Keep sidewalk and pavement areas clean • Pick up trash • Restore damaged landscaping in order to prevent sediment runoff 2. System Monitoring. (Quarterly, except in winter) The storm water facilities shall be inspected quarterly to quickly identify small issues before expensive damage can occur. In addition to regular monitoring, the best time to inspect the performance of storm water facilities is during runoff events. • Observe system during runoff. Look for ponding outside of retention areas. This can indicate a clogged inlet or pipe. • Inspect Bio-retention and Retention Basins • Inspect Inlets, Manholes and Pipes Stormwater Plan 10 3/28/2023 3. Bio-retention and Retention Basin Maintenance. (Quarterly) The bio-retention and retention basins are designed to provide long-term sustainable treatment of storm water. However, with poor housekeeping sediment can clog these facilities and reduce infiltration capacity. Proper and timely cleaning of incidental sediment in these basins can prevent these facilities from being damaged. The bio-retention and retention basins are intended to be vegetated, since the vegetation provides microbiological communities that can treat common pollutants in storm water. Unhealthy vegetation can diminish the performance of these basins. The landscaping of the bio-retention basins shall follow typical landscape maintenance guidelines. The vegetation on the bottom of the retention basin should not be regularly mowed, since mowing can cause plant debris to build up in the basin. • Remove sediment, trash, and debris • Inspect for healthy vegetation • Inspect for uniform ponding and water disappears in 3 days 4. Bio-retention and Retention Basin Maintenance. (Annually) The vegetation should be maintained annually to reduce plant debris build-up. The retention basin vegetation should be cut to a height of 6” in the fall with the clippings removed from the basin. • Remove dead plant materials from bio-retention basins. • Cut retention basin vegetation and remove clippings. • Clean inlet piping or chases, remove sediment if present. 5. Bio-retention and Retention Basin Maintenance. (Long-term) If regular housekeeping and maintenance is not performed adequately, sediment and debris can accumulate in the basin and reduce the required storage capacity. If this occurs the basins should be excavated back down to original plan grade. • Hire a contractor to return basin(s) condition to initial design found on City engineering plans. • Dredge basin if sediment build-up is greater than 6”. 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