Microsoft C3 Workshop_Track 2_Sizing Calcs_6 5 12 [Compatibility Mode] 000 Gpd To 288 Workshop Track 2 Sizing Calcs 6
User Manual: 000 gpd to 288
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C.3 Workshop – Track 2: Sizing Calculations and Design Considerations for LID Treatment Measures Jill Bicknell, P.E., EOA, Inc. Santa Clara Valley Urban Runoff Pollution Prevention Program Presentation Overview Determining Water Quality Design Flow and Volume (“QBMP” and “VBMP”) Sizing Bioretention and Flow-Through Planters Sizing Pervious Paving and Infiltration Trenches Sizing Rainwater Harvesting Cisterns Sizing Non-LID Components C.3.d Sizing Criteria Volume-based sizing criteria: • URQM Method - use formula and volume capture coefficients in “Urban Runoff Quality Management”, WEF/ASCE MOP No. 23 (1998), pages 175-178 • CASQA BMP Handbook Method - Determine volume equal to 80% of the annual runoff, using methodology in Appendix D of the CASQA BMP Handbook (2003) using local rainfall data – Sizing curves specific to Santa Clara Valley provided in Appendix B of C.3 Handbook Unit Basin Storage Volume for 80% Capture (inches) San Jose Rain Gage, 1% Slope Percent Imperviousness C.3.d Sizing Criteria Flow-based sizing criteria: • Factored Flood Flow - 10% of the 50-year peak flow rate, determined using Intensity-Duration-Frequency curves published by the local flood control agency • Percentile Rainfall Intensity - Flow of runoff produced by a rain event equal to two times the 85th percentile hourly rainfall intensity – Data for Santa Clara Valley rain gages in Sizing Worksheets (Appendix B of C.3 Handbook) • Uniform Intensity - Flow of runoff resulting from a rain event equal to 0.2 inches per hour intensity Intensity-Duration-Frequency Curve (50-Year Return Period) C.3.d Sizing Criteria 85th Percentile Rainfall Intensity Data: Reference Rain 85th Percentile Design Rainfall Gages Hourly Rainfall Intensity (in/hr)* Intensity (in/hr) San Jose Airport 0.087 0.17 Palo Alto 0.096 0.19 Morgan Hill 0.12 0.24 *Design rainfall intensity = 2 X 85th percentile hourly rainfall intensity By comparison, Uniform Intensity = 0.2 in/hr C.3.d Sizing Criteria Flow-based sizing criteria: • Simplified Sizing Approach – Variation of Uniform Intensity Method (0.2 in/hr) – The surface area of a biotreatment measure is sized to be 4% of the contributing impervious area – Based on a runoff inflow of 0.2 in/hr (assume equal to the rainfall intensity), with an infiltration rate through the biotreatment soil of 5 in/hr (0.2 in/hr ÷ 5 in/hr = 0.04) – Conservative approach because does not account for surface ponding – good for planning purposes C.3.d Sizing Criteria Combination Flow & Volume Design Basis: • Treatment systems can be sized to treat “at least 80% of total runoff over the life of the project” • Option 1: Use a continuous simulation hydrologic model (typically not done for treatment measures) • Option 2: Show how treatment measure sizing meets both flow and volume-based criteria – Used for bioretention and flow-through planters – Appropriate where drainage area is mostly impervious Flow- or Volume-Based Sizing for Treatment Measures? Table 5-1 Flow and Volume Based Treatment Measure Sizing Criteria Type of Treatment Measure LID? Hydraulic Sizing Criteria Bioretention area Yes Flow-through planter box Yes Tree well filter Infiltration trench Subsurface infiltration system Rainwater harvesting and use Yes Yes Yes Yes Flow- or volume-based or combination Flow- or volume-based or combination Flow-based Volume-based Volume-based Volume-based Media filter Extended detention basin No No Flow-based Volume-based Sizing Criteria Worksheets Appendix B of SCVURPPP C.3 Handbook • Worksheets for determining water quality design flow and volume • Figure B-1: Soil Texture and Mean Annual Precipitation (MAP) Depths • Figures B-2 – B-7: Unit Basin Storage Volume for 80% Capture (3 gages, 1% and 15% slopes) • Figure B-8: Intensity-Duration-Frequency Curves for 50-year Return Period (4 gages) Figure B-1: Soil Texture and Mean Annual Precipitation (MAP) Depth Sizing Example #1 Parking lot in Santa Clara • Area = 35,000 sq. ft. (0.80 acres) • 100% impervious • Slope = 1% • Mean annual precipitation (MAP) = 15 inches 35,000 sq. ft. Use the sizing worksheets to determine QBMP and VBMP Answer: VBMP = 1,819 cu. ft.; QBMP = 0.103 cfs Sizing Bioretention Facilities Simplified Sizing Approach • Surface area is 4% of contributing impervious area • Does not consider storage in surface ponding area Volume Based Approach • Store VBMP in just surface ponding area • Store VBMP in ponding area, soil media & drain rock Combination Flow and Volume Approach • Compute both QBMP and VBMP • Route through facility, allowing ponding Sizing Bioretention Facilities: Volume-Based Approach V1 V2 V3 Sizing Bioretention Facilities: Volume-Based Approach Method 1: Store entire volume in surface ponding area V1 Depth (ft) Porosity Volume per sq. ft. (cubic feet) 0.5 1.0 0.5 Surface Area = VBMP (cu.ft.) ÷ 0.5 cu.ft./sq.ft. Sizing Bioretention Facilities: Volume-Based Approach Method 2: Store volume in ponding area and media Depth (ft) Porosity Volume per sq. ft. (cubic feet) V1 0.5 1.0 0.5 V2 1.5 0.30 0.45 V3 0.5* 0.40 0.20 Total 1.15 *Depth below bottom of underdrain Surface Area = VBMP (cu.ft.) ÷ 1.15 cu.ft./sq.ft. Sizing Bioretention Facilities: Flow & Volume Approach “Hydrograph Approach” • Runoff is routed through the treatment measure • Assume rectangular hydrograph that meets both flow and volume criteria Sizing Bioretention Facilities: Flow & Volume Approach 5 in/hr Determine VBMP Assume constant rainfall intensity of 0.2 in/hr continues throughout the storm (rectangular hydrograph) Calculate the duration of the storm by dividing the Unit Basin Storage by the rainfall intensity Calculate the volume of runoff that filters through the biotreatment soil at a rate of 5 in/hr over the duration of the storm and the volume that remains on the surface Sizing Bioretention Facilities: Flow & Volume Approach 5 in/hr To start the calculation, you have to assume a surface area “AS” -- use 3% of the contributing impervious area as a first guess Determine volume of treated water “VT” during storm: VT = AS x 5 in/hr x duration (hrs) x 1 in/12 ft Determine volume remaining on the surface “VS”: VS = VBMP – VT Determine depth “D” of ponding on the surface: D = VS ÷ AS Repeat until depth is approximately 6 inches Sizing Example #1, continued Parking lot in Santa Clara • • • • Area = 35,000 sq. ft. (0.8 acres) 100% impervious VBMP = 1,819 cu. ft. UBS Volume = 0.63 in. 35,000 sq. ft. Use the combination flow and volume sizing worksheet to determine the bioretention surface area Answer: 1,000 sq. ft. (depth of 0.5 ft.) Sizing Bioretention Facilities: Comparison of Methods Example: 35,000 sq. ft. parking lot in Santa Clara MAP= 15 inches, 100% impervious VBMP = 1,819 cu. ft. (80% of annual runoff) Sizing Method Surface Area (sq. ft.) Simplified Method (flow-based) 1,400 Volume ponded on surface 3,638 Volume stored in unit (V1+V2+V3) 1,580 Combination flow & volume 1,000 Sizing Pervious Paving and Infiltration Trenches General Principles • Store the WQD Volume in void space of stone base/subbase and infiltrate into subgrade • Surface allows water to infiltrate at a high rate • Any underdrains must be placed above the void space needed to store and infiltrate the WQD volume Sizing Pervious Paving and Infiltration Trenches Pervious Paving • May be self-treating area or self-retaining area (accept runoff from other areas) • Can only be considered a “pervious area” if stone base/subbase sized to store the WQD volume • Can work where native soils have low infiltration rates (stored water depths are relatively small) • Surface area is usually predetermined • Base and subbase thickness usually determined by expected traffic load and saturated soil strength • Slope should be ≤ 1% (or use cutoff trenches) Pervious Paving Typical Section Paving surface Bedding No. 8 stone 4 in. Thickness varies Base No. 57 stone Subbase No. 2 stone Base and subbase layers available for water storage Both typically have 40% void space Pervious Paving Approach to Sizing Pervious Paving • Self-Treating – Check the depth of the WQD volume in base/subbase: UBS volume (in.) ÷ 0.40 = Depth (in.) Example: UBS volume = 1.0 in., depth = 2.5 in. (Minimum depth for vehicular traffic is 10 in.) – Check the time required for stored water to drain: UBS Vol. (in.) ÷ Infiltration rate (in/hr) = Drain time (hrs) ( recommend < 48 hrs) Pervious Paving Approach to Sizing Pervious Paving • Self-Retaining – Check the depth of the WQD volume in base/subbase: UBS volume (in.) ÷ 0.40 = Depth (in.) Example: UBS volume = 1.0 in., depth = 2.5 in. (Minimum depth for vehicular traffic is 10 in.) – Check the time required for stored water to drain: UBS Vol. (in.) ÷ Infiltration rate (in/hr) = Drain time (hrs) ( recommend < 48 hrs) Sizing Rainwater Harvesting Cisterns Rainwater Harvesting and Use • Types of Demands – Irrigation – Toilet flushing – Other non-potable • Volume based sized criteria in C.3.d is 80% capture of the annual runoff • Key concept is drawdown time • Barriers: lack of plumbing codes, treatment, recycled water preference Storage values are per one acre of impervious surface Estimate Actual Demand Daily Use Rates for Toilets and Urinals1 Land Use Type User Unit User Unit Factor2 Daily Use/Unit (gal/day/unit) Residential Resident 2.9 residents per dwelling unit 8.6 Office or Retail Employee (non-visitor) 200 SF per employee 6.9 Schools Employee (not including students) 50 SF per employee 33.9 1References: CCCWP Stormwater C.3 Guidebook, 6th edition, 2012; BASMAA LID Feasibility Report, 2011; California Plumbing Code, 2010. 2Use project-specific data if available Example: 2-story Office Building 3,000 SF 10,000 SF (Interior floor area = 20,000 SF) 4,000 SF Screening Worksheet Results Potential rainwater capture area = area of one building roof = 10,000 SF Convert to acres: 10,000 SF ÷ 43,560 SF/acre = 0.23 acres Demand for commercial building: Interior floor area = 20,000 SF Minimum floor area to meet toilet flushing demand = 70,000 SF per acre of impervious surface Minimum floor area for this project to meet demand = 70,000 SF/ac X 0.23 acres = 16,100 SF 20,000 SF > 16,100 SF B Building will have minimum toilet flushing demand Determine Building Toilet Flushing Demand Building interior floor area = 20,000 SF Estimate no. of employees: • 200,000 SF ÷ 200 SF/employee = 100 employees • 100 employees × 6.9 gpd/employee = 690 gpd Convert to equivalent demand per impervious acre (to allow use of sizing curves): • 10,000 SF roof area ÷ 43,560 SF/ac = 0.23 ac. • 690 gpd ÷ 0.23 = 3,000 gpd per impervious acre Determine Required Cistern Size From sizing curves, find right combination of drawdown time, tank size and required demand: • 480-hr (20-day) drawdown B 49,000 gallon tank B 2,450 gpd • 360-hr (15-day) drawdown B 40,000 gallon tank B 2,667 gpd • 240-hr (10-day) drawdown B 32,000 gallon tank B 3,200 gpd • 288-hr (12-day) drawdown B 36,000 gallon tank B 3,000 gpd √ Adjust tank size back to actual impervious area: • 36,000-gallon tank per 1 acre impervious area • 36,000 × 0.23 acres = 8,300-gallon tank Sizing Non-LID Components Media Filters (cartridge type) Flow-based Treatment Measure Determine QBMP From manufacturer’s specifications, determine the design flow rate per cartridge Divide QBMP by the cartridge flow rate to calculate the number of cartridges required (round up) Sizing Non-LID Components High Flow Rate Tree Box Filters Flow-based Treatment Measure Determine QBMP From manufacturer’s specifications, determine the appropriate size of unit or combination of units A tree box filter that uses biotreatment soil can be sized like a bioretention area or flow-through planter Sizing Non-LID Components Detention Basin Volume-based Treatment Measure (can only be used in treatment train) Determine VBMP Design outlet for 48-hour detention time If sizing for hydromodification management, use Bay Area Hydrology Model to determine size to meet HM standards ??? Questions ??? Contact Information: Jill Bicknell 408-720-8811, X 1 jcbicknell@eoainc.com www.scvurppp.org
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