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HomeMy WebLinkAbout210847_Storm Report (Full) INTRODUCTION The Big Sky Benefits Solutions Site Plan consists of two (2) construction phases. Phase 1 will consist of an office building, Building A, being 5,118 sq. ft., and associated infrastructure improvements. Phase 2 is a future phase that in undetermined at the time of this report. Both phases are located on Lot 10, Block 2 of Nelson Meadows Subdivision in Bozeman, Montana. A combination of site grading, curb and gutters, valley gutters, swales, curb inlets, drywells, and underground Stormtech storage chambers consisting of the SC-310 model will be used to manage stormwater runoff for the development within the site. The proposed underground Stormtech SC- 310 stormwater storage systems and dry wells were sized for the 10-year, 2-hour storm and checked for the half inch requirement. Supporting stormwater calculations are attached to this report. A Drainage Area Map is included in Appendix A. Calculations for each individual drainage area are included in Appendix B. Groundwater depth information is included in Appendix C. A Stormwater Facilities Inspection and Maintenance Plan is included in Appendix D. DRAINAGE AREAS Drainage Area 1 Drainage Area 1 (DA#1) will be utilizing a drywell system (DW 1A) that will be located at the southwest corner of property within the landscaped area. Here the drywell will collect and disburse the runoff generated by the southern part of Building A as well as the landscaped area around this part of Building A. The runoff will be routed utilizing a swale on the western and southern sides of Building A. It was determined that DA#4 requires a minimum storage volume of 192 cubic feet. This structure will utilize a 4.5’ radial disbursement of washed, well-drained rock, around the well providing a total storage of 209 cubic feet. Drainage Area 2 Drainage Area 2 (DA#2) consists of the eastern sidewalk tied to Building A and the associated drive aisle. The 627 cubic feet of stormwater generated in this area will be controlled using a Stormtech SC-310 system, where the system for DA#2 (RT 2) will have an approximate storage volume of 657 cubic feet. The proposed SC-310 Stormtech infiltration chambers were designed to retain stormwater runoff using the arch-shaped chambers and void space in the surrounding washed rock while the runoff infiltrates into the ground. The footprint of these chambers will be over-excavated down to native sands and gravels while removing any existing slow-draining layers of soil beneath the chambers. This excavation will be backfilled with a well-draining and washed gravel to ensure proper infiltration. The chambers were sized by applying an infiltration rate for native gravel subgrades to the footprint area of the chambers to determine the discharge (infiltrate rate) from each system. Drainage Area 3 Drainage Area 3 (DA#3) consists of the northwestern portion of the parking lot, landscaping, and the northern portion of Building A. DA#3 will be using the Stormtech SC-310 model, following the same design approach as DA #2. The system for DA #3 (RT 3) was sized to retain the generated storm volume of 863 cubic feet, providing 894 cubic feet of storage. The generated runoff from this area will be directed on site via sheet flow and curb and gutters, while also utilizing valley gutters and grading. Drainage Area 4 Drainage Area 4 (DA #4) is located in the northeast corner of the property comprised of the eastern half of the parking lot, sidewalk, and surrounding landscaped area. Within this area, runoff will be directed to a low point via sheet flow, curb and gutters, valley gutters, and overall site grading. It was determined that DA#4 requires a minimum storage of 946 cubic feet and with the proposed SC-310 Stormtech system, a storage volume of 985 cubic feet is provided. DEPTH TO GROUNDWATER Groundwater monitoring was performed by Morrison Maierle in 2018 for Nelson Meadows Subdivision. The monitoring wells nearest to Lot 10, Block 2 were used to verify that the proposed stormwater infrastructure for Big Sky Benefits Solutions is above the high groundwater table. Monitoring Well (MW) #6 and MW #2 were used in determining the seasonal high ground water levels. It was recorded that MW #6 measured a high-water level of approx. 5.0 feet below ground surface and MW #2 measured a high-water level of approx. 5.5 feet below ground surface, both recordings were takin in April of 2018. For design purposes, a groundwater depth of 5.5 feet was used for DA 1 and a groundwater depth of 5.0 feet was used for DA’s 2, 3, and 4 given the relative locations to each well location. Table 1-A below summarized the bottom of system/structure and groundwater separation. System Structure/Groundwater Separation DW 1A 1.59’ RT 2 1.64’ RT 3 2.09’ RT 4 0.71’ Table 1-A: Separation Summary APPENDIX A DRAINAGE AREA MAP PHASE 1PHASE 2PHASE 1PHASE 2 PHASE 1PHASE 2PHASE 1 PHASE 2 APPENDIX B DRAINAGE AREA AND RETENTION SYSTEM CALCULATIONS DRAINAGE AREA #1 RUNOFF VOLUME FROM DA#1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2) C * Area Landscape 0.2 5842 1168 Hardscape 0.95 1773 1685 Total 7615 2853 A = Area (acres)0.17 C = Weighted C Factor 0.37 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.37 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.17 Q = RUNOFF (cfs)0.03 V = REQUIRED VOL (ft3)192 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #1 Contributing Area Area (ft 2) Hardscape 1773 2. Calculate 1/2" runoff volume over hardscape (aka Runoff Reduction Volume [RRV] as calculated in Montana Post- Construction Storwater BMP Manual - Equation 3-1) RRV = [P*Rv*A]/12 P = Water quality rainfall depth 0.50 inches Rv = Dimensionless runoff coefficient 0.26 0.05 + 0.9*I I = Percent impervious cover (decimal)0.23 decimal A = Entire drainage area 0.17 acres RRV = Runoff Reduction Volume 0.0019 acre-ft RRV = Runoff Reduction Volume 82 cubic feet DRAINAGE AREA #1 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Landscape 0.2 5841.79 1168 Hardscape 0.95 1773.36 1685 Total 7615 2853 A = Area (acres)0.1748 C = Weighted C Factor 0.37 2. Calculate Required Volume Q=CIA V=7200Q C = Weighted C Factor 0.37 I = Intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.17 Q = Runoff (cfs) 0.03 V = REQUIRED VOL (ft3)192 3. Calculate Drywell Volume Existing Surface (ft) 4599.91 (Lowest Point) Proposed Rim Elevation (ft) 4600 (Lowest Point) Groundwater Depth (ft) 5.50 Min Design Elevation (ft) 4594.41 Max Design Depth (ft) 5.59 Existing Soil Condition NA Percolation Rate (min/in) 0 (see Circular DEQ 4, Percolation Rate (ft/hr) 0.00 Table 2.1-1) Porous Media in Drywell Gravel Void Ratio of Media 30.00% Gravel Offset Dist. From Drywell (ft) 4.5 Infiltration Drywell Gravel Area (ft2)150.2 Infilitration Volume (ft3)0.00 Gravel Void Volume Gravel Bed Depth (below MH) 0.00 Gravel Volume (ft3)527.60 Gravel Storage Volume (ft3)158.28 Manhole Volume Manhole Depth (ft) 4.00 Manhole Volume (ft3)50 Provided Volume Inc. Perc. (ft3)209 DRAINAGE AREA #2 RUNOFF VOLUME FROM DA#2 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2) C * Area Landscape 0.2 1432 286 Hardscape 0.95 9490 9015 Total 10921 9301 A = Area (acres)0.25 C = Weighted C Factor 0.85 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.85 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.25 Q = RUNOFF (cfs)0.09 V = REQUIRED VOL (ft3)627 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #1 Contributing Area Area (ft 2) Hardscape 9490 2. Calculate 1/2" runoff volume over hardscape (aka Runoff Reduction Volume [RRV] as calculated in Montana Post- Construction Storwater BMP Manual - Equation 3-1) RRV = [P*Rv*A]/12 P = Water quality rainfall depth 0.50 inches Rv = Dimensionless runoff coefficient 0.83 0.05 + 0.9*I I = Percent impervious cover (decimal)0.87 decimal A = Entire drainage area 0.25 acres RRV = Runoff Reduction Volume 0.0087 acre-ft RRV = Runoff Reduction Volume 379 cubic feet RETENTION TANK # 2 RETENTION TANK #2 REQUIRED VOLUME 1. Calculate Weighted C Factor for Right-of-Way Component Width C ROW Hardscape 41 0.95 ROW Landscape 19 0.2 Weighted C Factor =0.71 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 9490 9015 Landscape 0.2 1432 286 Total 10921 9301 C=Weighted C Factor 0.85 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.85 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.25 Q = runoff (cfs)0.09 V = REQUIRED VOL (ft3)627 3. Storm Tech Design Existing Surface (ft) 4599.92 (Lowest Point) Proposed Elevation (ft) 4600.39 (Lowest Point) Groundwater Depth (ft) 5.00 Lowest Design Elevation (ft) 4594.92 Available Depth (ft) 5.47 Type SC-310 Length (ft) 27.87 Width (ft) 21.50 Required Depth (ft) 3.83 Provided Volume (ft3)657 DRAINAGE AREA # 3 DRAINAGE AREA #3 RUNOFF VOLUME FROM DA#3 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2) C * Area Landscape 0.2 4907 981 Hardscape 0.95 12444 11821 Total 17350 12803 A = Area (acres)0.40 C = Weighted C Factor 0.74 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.74 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.40 Q = RUNOFF (cfs)0.12 V = REQUIRED VOL (ft3)863 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #1 Contributing Area Area (ft 2) Hardscape 12444 2. Calculate 1/2" runoff volume over hardscape (aka Runoff Reduction Volume [RRV] as calculated in Montana Post- Construction Storwater BMP Manual - Equation 3-1) RRV = [P*Rv*A]/12 P = Water quality rainfall depth 0.50 inches Rv = Dimensionless runoff coefficient 0.70 0.05 + 0.9*I I = Percent impervious cover (decimal)0.72 decimal A = Entire drainage area 0.40 acres RRV = Runoff Reduction Volume 0.0115 acre-ft RRV = Runoff Reduction Volume 503 cubic feet RETENTION TANK # 3 RETENTION TANK #3 REQUIRED VOLUME 1. Calculate Weighted C Factor for Right-of-Way Component Width C ROW Hardscape 41 0.95 ROW Landscape 19 0.2 Weighted C Factor =0.71 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2 )C * Area Hardscape 0.95 12444 11821 Landscape 0.2 4907 981 Total 17350 12803 C=Weighted C Factor 0.74 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.74 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.40 Q = runoff (cfs)0.12 V = REQUIRED VOL (ft3)863 3. Storm Tech Design Existing Surface (ft) 4598.08 (Lowest Point) Proposed Elevation (ft) 4599.00 (Lowest Point) Groundwater Depth (ft) 5.00 Lowest Design Elevation (ft) 4593.08 Available Depth (ft) 5.92 Type SC-310 Length (ft) 63.45 Width (ft) 11.50 Required Depth (ft) 3.83 Provided Volume (ft3)894 DRAINAGE AREA # 4 DRAINAGE AREA #4 RUNOFF VOLUME FROM DA#4 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2) C * Area Landscape 0.2 3100 620 Hardscape 0.95 14120 13414 Total 17220 14034 A = Area (acres)0.40 C = Weighted C Factor 0.81 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.81 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 0.40 Q = RUNOFF (cfs)0.13 V = REQUIRED VOL (ft3)946 Check the half inch requirement (per DSSP II.A.4) 1. Determine Area of Hardscape within Drainage Area #1 Contributing Area Area (ft 2) Hardscape 14120 2. Calculate 1/2" runoff volume over hardscape (aka Runoff Reduction Volume [RRV] as calculated in Montana Post- Construction Storwater BMP Manual - Equation 3-1) RRV = [P*Rv*A]/12 P = Water quality rainfall depth 0.50 inches Rv = Dimensionless runoff coefficient 0.79 0.05 + 0.9*I I = Percent impervious cover (decimal)0.82 decimal A = Entire drainage area 0.40 acres RRV = Runoff Reduction Volume 0.0130 acre-ft RRV = Runoff Reduction Volume 565 cubic feet RETENTION TANK # 4 RETENTION TANK #4 REQUIRED VOLUME 1. Calculate Weighted C Factor for Right-of-Way Component Width C ROW Hardscape 41 0.95 ROW Landscape 19 0.2 Weighted C Factor =0.71 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2 )C * Area Hardscape 0.95 14120 13414 Landscape 0.2 3100 620 Total 17220 14034 C=Weighted C Factor 0.81 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.81 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres)0.40 Q = runoff (cfs)0.13 V = REQUIRED VOL (ft3)946 3. Storm Tech Design Existing Surface (ft) 4600.96 (Lowest Point) Proposed Elevation (ft) 4600.50 (Lowest Point) Groundwater Depth (ft) 5.00 Lowest Design Elevation (ft) 4595.96 Available Depth (ft) 4.54 Type SC-310 Length (ft) 99.03 Width (ft) 8.17 Required Depth (ft) 3.83 Provided Volume (ft3)985 APPENDIX C GROUNDWATER DEPTH INFORMATION LOT 10, BLOCK 2 APPENDIX D STORMWATER FACILITIES INSPECTION AND MAINTENANCE PLAN INSPECTION AND MAINTENANCE FOR STORMWATER MANAGEMENT FACILITIES The owner shall be responsible for the maintenance of the stormwater drainage facilities within The Big Sky Benefit Solutions development. Storm Water Facilities: 1. Underground Stormtech SC-310 Infiltration System collect stormwater runoff and store the water until it infiltrates into the ground. 2. Pipe Networks convey stormwater to different discharge locations underground. 3. Inlets are facilities where stormwater runoff enters a pipe network. Inlets include storm water manholes and drains. 4. Catch Basins are sumps typically located directly below storm water inlets and allow sediment to settle before storm water enters the pipe network. 5. Outlets are points where storm water exits a pipe network. 6. Drywells are underground storage systems that collect stormwater runoff and disperses the runoff radially through evenly spaced slots on the side of the structure by means of gravity Post Construction Inspection: 1. Use the attached Stormtech Isolator Row Operation & Maintenance manual to determine if maintenance is required on the system after construction is completed. 2. Observe that catch basins are clear of any material or obstructions in the drainage slots. Inspect these structures to ensure proper drainage following a storm event. Immediately identify and remove objects responsible for clogging if not draining properly. Semi-Annual Inspection: 1. Use the attached Stormtech Isolator Row Operation & Maintenance manual to determine if maintenance is required on the system semi-annually. 2. Check for grass clippings, litter, sediment, and/or debris in inlets, catch basins, and drywells. Flush and/or vacuum storm water pipes if excessive material is observed in the facilities. Standard Maintenance: 1. Inspect and remove debris from catch basins. Use a vacuum truck to clean catch basins and Stormtech system. 2. Inspect for the following issues: differential accumulation of sediment, drain time, signs of petroleum hydrocarbon contamination (odors, oil sheen in pond water), standing water, trash and debris. Sediment accumulation: In most cases, sediment in a catch basin or a retention system does not contain toxins at levels posing a hazardous concern. However, sediments should be tested for toxicants in compliance with current disposal requirements and if land uses in the drainage area include commercial or industrial zones, or if visual or olfactory indications of pollution are noticed. Sediments containing high levels of pollutants should be disposed of in accordance with applicable regulations and the potential sources of contamination should be investigated and contamination practices terminated. Cost Estimate: Depending on the amount of rainfall in the given year, the cost to maintain the stormwater infrastructure will vary. The underground Stormtech system will need to be cleaned according to the attached Stormtech Isolator Row Operation & Maintenance manual. Cost of maintenance will be dependent on the frequency and estimates should be obtained from a local vacuum truck company. The applicant will be responsible for financing the maintenance of the stormwater infrastructure Isolator® Row O&M Manual 2 Looking down the Isolator Row from the manhole opening, woven geotextile Fabric is shown between the chamber and stone base. StormTech Isolator Row with Overflow Spillway (not to scale) The Isolator® Row Introduction An important component of any Stormwater Pollution Prevention Plan is inspection and maintenance. The StormTech Isolator Row is a technique to inexpensively enhance Total Suspended Solids (TSS) and Total Phosphorus (TP) removal with easy access for inspection and maintenance. The Isolator RowThe Isolator Row is a row of StormTech chambers, either SC-160, SC-310, SC-310-3, SC-740, DC-780, MC-3500 or MC-7200 models, that is surrounded with filter fabric and connected to a closely located manhole for easy access. The fabric-wrapped chambers provide for sediment settling and filtration as stormwater rises in the Isolator Row and passes through the filter fabric. The open bottom chambers and perforated sidewalls (SC-310, SC- 310-3 and SC-740 models) allow stormwater to flow both vertically and horizontally out of the chambers. Sediments are captured in the Isolator Row protecting the adjacent stone and chambers storage areas from sediment accumulation. ADS geotextile fabric is placed between the stone and the Isolator Row chambers. The woven geotextile provides a media for stormwater filtration, a durable surface for maintenance, prevents scour of the underlying stone and remains intact during high pressure jetting. A non-woven fabric is placed over the chambers to provide a filter media for flows passing through the chamber’s sidewall. The non-woven fabric is not required over the SC-160, DC-780, MC-3500 or MC-7200 models as these chambers do not have perforated side walls. The Isolator Row is designed to capture the “first flush” runoff and offers the versatility to be sized on a volume basis or a flow-rate basis. An upstream manhole provides access to the Isolator Row and includes a high/low concept such that stormwater flow rates or volumes that exceed the capacity of the Isolator Row bypass through a manifold to the other chambers. This is achieved with an elevated bypass manifold or a high-flow weir. This creates a differential between the Isolator Row row of chambers and the manifold to the rest of the system, thus allowing for settlement time in the Isolator Row. After Stormwater flows through the Isolator Row and into the rest of the chamber system it is either exfiltrated into the soils below or passed at a controlled rate through an outlet manifold and outlet control structure. The Isolator Row may be part of a treatment train system. The treatment train design and pretreatment device selection by the design engineer is often driven by regulatory requirements. Whether pretreatment is used or not, StormTech recommend using the Isolator Row to minimize maintenance requirements and maintenance costs. Note: See the StormTech Design Manual for detailed information on designing inlets for a StormTech system, including the Isolator Row. ECCENTRICHEADER MANHOLEWITHOVERFLOWWEIR STORMTECHISOLATOR ROW OPTIONAL PRE-TREATMENT OPTIONAL ACCESS STORMTECH CHAMBERS 3 Inspection The frequency of inspection and maintenance varies by location. A routine inspection schedule needs to be established for each individual location based upon site specific variables. The type of land use (i.e. industrial, commercial, residential), anticipated pollutant load, percent imperviousness, climate, etc. all play a critical role in determining the actual frequency of inspection and maintenance practices. At a minimum, StormTech recommends annual inspections. Initially, the Isolator Row should be inspected every 6 months for the first year of operation. For subsequent years, the inspection should be adjusted based upon previous observation of sediment deposition. The Isolator Row incorporates a combination of standard manhole(s) and strategically located inspection ports (as needed). The inspection ports allow for easy access to the system from the surface, eliminating the need to perform a confined space entry for inspection purposes. If upon visual inspection it is found that sediment has accumulated, a stadia rod should be inserted to determine the depth of sediment. When the average depth of sediment exceeds 3 inches throughout the length of the Isolator Row, clean-out should be performed. Maintenance The Isolator Row was designed to reduce the cost of periodic maintenance. By “isolating” sediments to just one row, costs are dramatically reduced by eliminating the need to clean out each row of the entire storage bed. If inspection indicates the potential need for maintenance, access is provided via a manhole(s) located on the end(s) of the row for cleanout. If entry into the manhole is required, please follow local and OSHA rules for a confined space entries. Maintenance is accomplished with the JetVac process. The JetVac process utilizes a high pressure water nozzle to propel itself down the Isolator Row while scouring and suspending sediments. As the nozzle is retrieved, the captured pollutants are flushed back into the manhole for vacuuming. Most sewer and pipe maintenance companies have vacuum/JetVac combination vehicles. Selection of an appropriate JetVac nozzle will improve maintenance efficiency. Fixed nozzles designed for culverts or large diameter pipe cleaning are preferable. Rear facing jets with an effective spread of at least 45” are best. JetVac reels can vary in length. For ease of maintenance, ADS recommends Isolator Row lengths up to 200" (61 m). The JetVac process shall only be performed on StormTech Isolator Rows that have AASHTO class 1 woven geotextile (as specified by StormTech) over their angular base stone. Isolator Row Inspection/Maintenance StormTech Isolator Row (not to scale) Note: Non-woven fabric is only required over the inlet pipe connection into the end cap for SC-160LP, DC-780, MC-3500 and MC-7200 chamber models and is not required over the entire Isolator Row. Isolator Row Step By Step Maintenance Procedures Step 1 Inspect Isolator Row for sediment. A) Inspection ports (if present) i. Remove lid from floor box frame ii. Remove cap from inspection riser iii. Using a flashlight and stadia rod,measure depth of sediment and record results on maintenance log. iv. If sediment is at or above 3 inch depth, proceed to Step 2. If not, proceed to Step 3. B) All Isolator Row i. Remove cover from manhole at upstream end of Isolator Row ii. Using a flashlight, inspect down Isolator Row through outlet pipe 1. Mirrors on poles or cameras may be used to avoid a confined space entry 2. Follow OSHA regulations for confined space entry if entering manhole iii. If sediment is at or above the lower row of sidewall holes (approximately 3 inches), proceed to Step 2. If not, proceed to Step 3. Step 2 Clean out Isolator Row using the JetVac process. A) A fixed floor cleaning nozzle with rear facing nozzle spread of 45 inches or more is preferable B) Apply multiple passes of JetVac until backflush water is clean C) Vacuum manhole sump as required Step 3 Replace all caps, lids and covers, record observations and actions. Step 4 Inspect & clean catch basins and manholes upstream of the StormTech system. ADS “Terms and Conditions of Sale” are available on the ADS website, www.ads-pipe.com The ADS logo and the Green Stripe are registered trademarks of Advanced Drainage Systems, Inc. Stormtech® and the Isolator® Row are registered trademarks of StormTech, Inc. © 2022 Advanced Drainage Systems, Inc. #11011 2/22 CS  )( Sample Maintenance Log Date Stadia Rod Readings Sedi- ment Depth (1)–(2) Observations/Actions InspectorFixed point to chamber bottom (1) Fixed point to top of sediment (2) 3/15/11 6.3 ft none New installation. Fixed point is CI frame at grade DJM 9/24/11 6.2 0.1 ft Some grit felt SM 6/20/13 5.8 0.5 ft Mucky feel, debris visible in manhole and in Isolator Row, maintenance due NV 7/7/13 6.3 ft 0 System jetted and vacuumed DJM adspipe.com 800-821-6710