Getting Started with Bioretention/Infiltration: Design …vaswcd.org/wp-content/uploads/2015/01/VCAP-Module-2C...Level Spreader Vortex Filters Street Sweeping Proprietary Systems Propriety
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Getting Started with Bioretention/Infiltration: Design and Review Virginia Conservation Assistance Program Training Module #2C
Design and Review Virginia Conservation Assistance
Program Training Module #2C
Design Plan
Design Steps Looking for Opportunities 1. Potential Location 2. Drainage Map 3. Available Space 4. Soil Assessment 5. Computations
Presenter
Presentation Notes
Bioretention and Infiltration Design Plans need the following: Soil Assessment for water table/bedrock and Results of infiltration tests, with a minimum rate of ½ inch per hour Erosion Control Plan or Plan to prevent sediments from entering during construction Bioretention needs a landscape plan Miss Utility notification Permitting requirements Drawings and specifications Computations�
Potential Location
Land Cover Existing Drainage Pathways
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Presentation Notes
Level of impervious cover. Type of inflows and outflows expected. Sheet Flow or Concentrated Flow? Is there a need for Overland relief?
Potential Location
Soils Map Topography
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Presentation Notes
Soils Maps can indicate water table/bedrock issues, hydrologic soil group/drainage class, erodibility/pH. Topography whether using USGS or conducting a site visit using a sight level can indicate critical slopes (>15%) or lack of positive drainage.
Drainage Map
Presenter
Presentation Notes
Underground pipes do not mean impervious connection. Find that point of discharge!! Disconnection occurs when the impervious surface is directed to a well vegetated pervious area that is at least 40 feet long and less than 5 percent slope prior to flowing into a conveyance channel or pipe. Some sites might have a close proximity to a waterway or storm drain (i.e. <100 feet), but could be disconnected. Drainage map should show impervious and pervious surfaces. Flow Path lengths and Time of Concentration Runoff flows perpendicular to the contours. Look for barriers that impede positive drainage.
Available Space
• Utility locations – Underground – Overhead
• Setbacks – building – septic field/Well – Property Line
• Geometry – Width/Length – Depth
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Presentation Notes
Call Miss Utility Setbacks: 10-foot building, 35-foot Septic Field, 50-foot well Geometry: Width/Length
Soil Assessment
Texture Infiltration Rate
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Presentation Notes
To determine BMP suitability onsite testing is performed. - USDA Soil texture test Infiltration Test using a perc pit or double-ring infiltrometer Evidence of high water table, look at mottling and color. Must have a separation of 2 feet from water table or bedrock. Resource: Appendix B Soil Assessment. Watershed Stewards Academy Rainscaping Manual. Anne Arundel County, Md.
Bioretention Designing and Reviewing Plans for Bioretention
Presenter
Presentation Notes
Bioretention will have 2 to 3 feet of Engineered Soil Media and 6 to 12 inches of gravel sump with underdrain depending on the design criteria applied (level 1 or level 2).
Bioretention Sizing
Treatment Volume (Tv) Tv = 1.0*Rv*CDA*3630 Rv = runoff value CDA = Drainage Area Surface Area SA = Tv / d d = is the effective depth of the facility; d = ponding d + soil d*0.2 + gravel d*0.4
Presenter
Presentation Notes
Treatment Volume is computed from the Contributing Drainage Area (maximum 2 acres) and runoff value based on land cover and hydrologic soil group. Example: CDA = 0.39 acres with an Rv = 0.77 Tv = 1,090 cubic feet Surface Area of the engineered soil is determined using the effective depth of the facility. Effective depth includes the water depth stored in the ponding area, soil and gravel layers. Soil Depth determined by vegetation and space. Soil depth is typically 3 feet for trees and 2 feet for shrubs and perennials. Gravel sump may be replaced with an open chamber structure for more voids, otherwise gravel is typically 6 to 12 inches to encapsulate underdrain. Ponding depth may be up to 12 inches. Recommend keeping it 6 to 9 inches, since most vegetation may not survive 1-foot inundation. Example: d = 0.5 + 2*0.2 + 1*0.4 = 1.4 feet SA = 1,090/1.4 = 780 square feet Level 2 designs must have a Treatment volume of 1.25 times the baseline volume.
Bioretention Geometry
Shortest Flow Path SFP / L > 0.3 (Level 1) SFP / L > 0.8 (Level 2) SFP = Shortest flow path L = Length
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Presentation Notes
Shortest Flow Path is measured from the closest inlet to the outlet. Length is measured from the furthest inlet to the outlet and is perpendicular to flow. No more than 20 percent of the contributing drainage area may travel the shortest flow path. Length to Width ratio is the bottom of the facility. The Length is perpendicular to the flow path and the Width is parallel to the flow. Side Slopes shall be no steeper than 3:1. Recommend minimum width of 10 feet and length of 15 feet, this produces a SFP/L of 0.667
Filter Strip Slope < 2 % >2% <2% >2% <2% >2% <2% >2% Max = 6% Minimum Length 10 ft 15 ft 20 ft 25 ft 10 ft 12 ft 15 ft 18 ft *GD as
necessary
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Presentation Notes
When runoff is sheet flow from such areas as parking lots, residential yards, etc., is involved, a grass filter strip, often enhanced with a gravel diaphragm, is usually employed. Table B.2 provides sizing guidelines as a function of inflow approach length, land use, and slope. Maximum Overland Flow from Pervious surfaces shall be 150 feet and for impervious surface shall be 75 feet. The first 10 feet of the filter strip must be 2% or less. The minimum filter strip length should be 10 feet. The contributing drainage area should not have more than 5,000 square feet of impervious surface. Use a gravel diaphragm when impervious surface exceeds this limit or when the minimum filter length cannot be met.
Gravel Diaphragm
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Presentation Notes
A gravel diaphragm at the top of the slope is created by excavating a 2-foot wide and 1-foot deep trench that runs on the same contour at the top of the filter strip or grass channel. The diaphragm serves two purposes. First, it acts as a pretreatment device, settling out sediment particles before they reach the practice. Second, it acts as a level spreader, maintaining sheet flow. Maximum Overland Flow from Pervious surfaces shall be 150 feet and for impervious surface shall be 75 feet. The flow should travel over the impervious area and to the practice as sheet flow and then drop at least 3 inches onto the gravel diaphragm. The drop helps to prevent runoff from running laterally along the pavement edge, where grit and debris tend to build up. A layer of nonwoven filter fabric should be placed between the gravel and the underlying soil trench. If the contributing drainage area is steep (4% slope or greater), then larger stone (clean gravel that meets VDOT #57 grade) should be used in the diaphragm. If the contributing drainage area is solely turf (e.g., lawn), then the gravel diaphragm may be eliminated.
Grass Channel Table B.3: Pretreatment Grass Channel Sizing Guidance for a 1.0 Acre Drainage Area (Source: Claytor and Schueler, 1996)
For applications where concentrated runoff enters the practice by surface flow, such as through a slotted curb opening, a grassed channel, often equipped with a gravel diaphragm to slow the velocity and spread out the flow entering the basin, is the usual pretreatment method. The length of the grassed channel depends on the drainage area, land use, and channel slope. Table B.3 provides recommendations on sizing for grass channels leading into a practice for a one acre drainage area. The minimum grassed channel length should be 20 feet. Dimensions of the grass channel shall ensure that the velocity during the 1 inch per hour storm is 1 foot per second or less. Use a gravel diaphragm when the minimum length cannot be met or channel bottom width exceeds 6 feet. Use Check Dams when channel slope exceed 2 percent or erosive flows occur for the 10-year storm.
Engineered Level Spreader with Forebay
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Presentation Notes
An engineered level spreader is an energy dissipater device that is used to convert concentrated stormwater runoff to sheet flow. A forebay is constructed to allow sediment to settle from the incoming stormwater runoff before it is delivered to the treatment or control practice. The engineered level spreader should be located at each point of concentrated incoming flow of the stormwater BMP. Storm drain piping or other conveyances may be aligned to discharge into one forebay or several, as appropriate for the particular site. Engineered level spreaders should be installed in a location which is accessible by maintenance equipment. The length of the level spreader lip shall be 13 feet per every 1 cfs of the contributing 10- year peak flow. The width of the level spreader channel on the up-stream side of the level lip should be three times the diameter of the inflow pipe, and the depth should be 9 inches or one-half the culvert diameter, whichever is greater. The level spreader lip may be set at the treatment volume elevation if no other pretreatment is provided and velocities are non-erosive. The forebay section of the level spreader shall be excavated as shown below. The forebay should be sized to hold 0.25 inches of runoff per impervious acre of contributing drainage area. Each individual forebay should hold a minimum of 0.1 inches per impervious acre.
Q = 2.3x10-5 K (SA) (H/L) K = Measured Infiltration Rate (at least 0.5 iph) H = Total depth above underdrain L = Depth of soil and gravel above underdrain
2. Pipe Size and Number N x D = 16 (Q’ n / S0.5) 3/8 Q’ = 10*Q n = roughness, typically 0.011 S = slope of underdrain
3. Spacing of Pipes S = √(4 K (M2 + 2 A M)/Q’) M = Vertical Distance to top of gravel A = Vertical Distance to bottom of gravel
Presenter
Presentation Notes
If Spacing distance is longer than width of Bioretention, then only one underdrain is needed and the pipe size should be increased.
Bioretention Outlets
Overflow Capacity • Orifice or Barrel
– Depth of 10-year water surface elevation
– Area of orifice
• Weir – Depth of 10-year water
surface elevation – Length
Presenter
Presentation Notes
Drop Inlets and vegetated spillways are considered weir flows. Drop Inlet Weirs will use a coefficient of 3.3 (Brater and King 1976). Vegetated weirs will use a coefficient of 2.7 Elevated Barrels pipes or internal control structures are considered orifice flows with a coefficient of 0.6. The design storm for sizing the outlets shall be the 10-year. Calculate the 10-year peak flow using Appendix A in the VCAP Manual. The Rational Method is adequate using the Runoff Value, Rv as the runoff coefficient C. There should be enough freeboard to safely pass the 25-year storm event. Generally, there should be at least 6 inches of freeboard above the 10-year water surface elevation. When the embankment is greater than 3 feet, 12 inches of freeboard is encouraged. The downstream receiving channel or storm sewer should adequately convey the 10-year storm without eroding or overtopping. Use Manning’s equation in Appendix A to verify adequacy.
Infiltration Designing and Reviewing Plans for Infiltration
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Presentation Notes
Infiltration Practices must have underlying soils that meet the minimum infiltration rates. Level 1 must have infiltration rate of ½ to 1 inch per hour. Level 2 must have infiltration rate of 1 to 4 inch per hour.
Infiltration Sizing Treatment Volume (Tv) Tv = 1.0*Rv*CDA*3630 Rv = runoff value CDA = Drainage Area Surface Area (SA) SA = Tv / d d = is the effective depth of the facility; d = ponding d + gravel d*0.4 *Underground systems do not have a ponding depth. Bottomless chambers offer more void spaces.
Presenter
Presentation Notes
Treatment Volume is computed from the Contributing Drainage Area (maximum 2 acres) and runoff value based on land cover and hydrologic soil group. Example: CDA = 0.39 acres with an Rv = 0.77 Tv = 1,090 cubic feet Surface Infiltration: surface area uses the effective depth of the facility. Effective depth includes the water depth stored in the ponding area, gravel layers. Ponding depth may be up to 24 inches. Typical depths range from 6 to 12 inches. Example: d = 1 + 2*0.4 = 1.8 feet SA = 1,090/1.8 = 610 square feet Underground Infiltration: surface area is determined by the actual volume of the chamber system or gravel. Effective depth does not include a ponding depth. Depths range from 3 to 5 feet. Example: 5 foot of gravel reservoir, d= 5 *0.4 = 2 SA = 1,090/2 = 545 square feet Example: 5 foot bottomless chambers with void ratio of 0.9, d = 5*0.9 = 4.5 ft SA = 1,090/4.5 = 245 square feet Level 2 designs must have a treatment volume 1.1 times the baseline volume.
Infiltration Geometry
• Length, Width and Depth should be sized to provide a drawdown time of 36 to 48 hours 1. Darcy Equation, Q = 2.3x10-5 K (SA) (H/L) K = Measured Infiltration Rate (at least 0.5 iph) H = Total depth above native soil L = Depth of Gravel 2. Time = Tv/ (3600*Q)
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Presentation Notes
Sides slopes 4:1 or flatter. 2-foot maximum depth of surface ponding Example: d = 2 + 2.5*0.4 = 3 feet SA = 1,090/3= 364 square feet H = 2+2.5 = 4.5 ft L = 2.5 ft K = ½ inch per hour Q = (2.3x10^-5) * 0.5 * 364 * (4.5/2.5) = 0.00753 cfs Drawdown Time = 1,090 / (0.00753 * 3600) = 40 hours.
Engineered Level Spreader Vortex Filters Sediment Forebay Street Sweeping Proprietary Systems Propriety Systems
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Presentation Notes
Baseline (Level 1) requires at least two pretreatment per inlet. Enhanced (Level 2) requires three pretreatment per inlet.
Infiltration Pretreatment
Gravel Flow Spreader Inlet Sump
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Presentation Notes
The same pretreatment measures from Bioretention can be used. Gravel Flow Spreader is an option for curb cuts on steep slopes. Inlet Sumps is an option for underground infiltration systems. Propriety filters, screens or separators may also be used for underground systems or in catch basins.
Infiltration Outlets
Overflow Capacity • Orifice
– Depth of Dry Storage – Area of orifice
• Weir – Depth of Dry Storage – Length
Presenter
Presentation Notes
Infiltration does not require an underdrain. Drop Inlets and vegetated or rock spillways are considered weir flows. Drop Inlet Weirs will use a coefficient of 3.0 to 3.3. Vegetated weirs will use a coefficient of 2.7 Elevated Barrels pipes or internal control structures are considered orifice flows The design storm for sizing the outlets shall be the 10-year. Calculate the 10-year peak flow using Appendix A in the VCAP Manual. The Rational Method is adequate using the Runoff Value, Rv as the runoff coefficient C. There should be enough freeboard to safely pass the 25-year storm event. Generally, there should be at least 6 inches of freeboard above the 10-year water surface elevation. When the embankment is greater than 3 feet, 12 inches of freeboard is encouraged. The downstream receiving channel or storm sewer should adequately convey the 10-year storm without eroding or overtopping. Use Manning’s equation in Appendix A to verify adequacy.
DESIGN CONSIDERATIONS
Conveyance and Overflow
Online • 10-year design storm • Maximum water surface
shall be 12 inches • Freeboard
Offline • Preferred • Flow Splitters • Sized for the 10-year design
storm • No Freeboard
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Presentation Notes
Online: The inlet and outlet shall be sized to minimize the impacts from the 10-year design storm Maximum water surface elevation for the 10-year design storm shall be 12 inches above the soil media. Freeboard of 6 inches is required for berms less than 3 feet, otherwise 12 inches of freeboard is required Hydraulic routings may be necessary to verify the design flows. Offline: A funnel design for the curb cut or diversion. Check Dams set at the ponding depth would divert water to the facility, yet allow overflow to bypass. Flow splitter must be designed to be stable and non-erosive during the 10-year design storm.
Karst
• Infiltration – Should be avoided – 4 feet of separation to
bedrock/ water table – Impermeable liner with
underdrain
• Bioretention – Level 2 design is not
recommended. – 4 feet of separation to
bedrock/ water table – Impermeable Liner with
underdrain – Drainage area < 20,000
sq. ft. preferred
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Presentation Notes
Karst topography is prone to sinkhole formation and groundwater contamination. Bioretention and Infiltration should be avoided where sites are close to bedrock and water table. Minimize contributing drainage area and scale of the practice will minimize risks.
distance to water table allowed with large underdrains
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Presentation Notes
Coastal Plain has high water table and tidal influences. Bioretention and Infiltration should be avoided where sites are close to water table. Minimize contributing drainage area and scale of the practice will minimize risks.
Steep Slopes
• Bioretention: Setback 30 feet - slopes >15%
• Infiltration: Setback 200 feet - slopes >20%
• Slope Stability Analysis
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Presentation Notes
Steep Slopes are prone to seepage and sloughing. Slope Stability analysis should be performed as necessary. Terraced Infiltration and Bioretention in a series of cells with retaining walls are an option. Generally setback the practices to avoid the phreatic line from seeping to the slope surface. The phreatic line should pass well under the toe of the slope.
