4.9 Grass Channel. Volume 2 (Technical Handbook) Limited Application Structural Stormwater Control. Formatted: Font: 10 pt

Transcription

1 4.9 Grass Channel Limited Application Structural Stormwater Control Description: Vegetated opengrass channels designed to filter stormwater runoff and meet velocity targets for theenhance water quality through the settling of suspended solids. design storm and the 2-year storm event. LID/GI Considerations: If properly incorporated into overall site design, grass channels can possibly reduce the impacts of impervious cover, partially infiltrate runoff with pervious soils while complementing the natural landscape, and render aesthetic benefits. Formatted: Font: 10 pt Formatted: Font: 10 pt Volume 2 (Technical Handbook) Georgia Stormwater Management Manual4.9-11

2 Design Criteria DESIGN CRITERIA Can be used as part of the runoff conveyance system to provide pretreatment Grass channels can act to partially infiltrate runoff from small storm events if underlying soils are pervious Less expensive than curb and gutter systems Should not be used on slopes greater than 4%; slopes between 1% and 2% recommended Potential for bottom erosion and resuspension Standing water may not be acceptable in some areas Contributing Ddrainage area - <less than 5 acres Channel Length - Minimum residence time of 5 min Runoff velocity - Water quality rainfall event flow velocity less than 1.0 ft/s and flow depth less than 4 inches Side slopes are 3:1 or flatter Minimum soil infiltration rate of in/hr Minimum 2-foot clearance from groundwater ADVANTAGES/BENEFITS Lower Cost Reduces impervious area Ideal for linear environments (along roadways) Stormwater collection and conveyance Aesthetic benefits Well suited for linear environments, interchanges, and facilities May be contained within the roadway right-of-way DISADVANTAGEES/LIMITATIONS Cannot achieve the 80% TSS removal target alone, and must be used in series with other BMPs for removal credit Limitations for drainage area, flow, velocity, and flow depth Design dependent on existing site conditions and topography ROUTINE MAINTENANCE REQUIREMENTS Provides access to BMP and appropriate components Sediment cleanout, trash and debris removal, revegetation, and repair of erosion must be completed as necessary to maintain functionality POLLUTANT REMOVAL Total Suspended Solids 50% Nutrients Total Phosphorus/Total Nitrogen Removed 25%/20% Metals Cadmium, Copper, Lead, and Zinc Removal 30% Pathogens Fecal Coliform Insufficient Data STORMWATER MANAGEMENT SUITABILITY Runoff Reduction Water Quality Channel Protection Overbank Flood Protection Extreme Flood Protection -suitable for this practice in certain situationsmay provide partial benefits IMPLEMENTATION CONSIDERATIONS Land Requirements Capital Cost Maintenance Burden Residential Subdivision Use: Yes High Density/Ultra-Urban: Yes Roadway Projects: Yes Other Considerations: Curb and gutter replacement Pollutant Removal Total Suspended Solids 50% Total Phosphorus 25% Total Nitrogen 20% Fecal Coliform insufficient data Heavy Metals 30% Formatted: Font: Century Gothic, 11 pt, Bold, Underline Formatted: Indent: Left: 0", Hanging: 0.31", No bullets or numbering Formatted: Left Formatted: Centered Formatted: Left Formatted: Left Formatted: Indent: Left: 0.13", Hanging: 0.13" Formatted: Left Georgia Stormwater Management Manual Volume 2 (Technical Handbook)

3 / % POLLUTANT REMOVAL Total Suspended Solids Nutrients - Total Phosphorus / Total Nitrogen removal Metals - Cadmium, Copper, Lead, and Zinc removal Pathogens Fecal Coliform Runoff Reduction Credit: 25% of the runoff reduction volume provided for A & B hydrologic soils 10% of the runoff reduction volume provided for C & D hydrologic soils Formatted: Centered Formatted Table Formatted: Left, Bulleted + Level: 1 + Aligned at: 0.24" + Indent at: 0.49" Formatted: Font: Not Bold, No underline Formatted: Font: Not Bold Formatted: Left, Indent: Left: 0.69" General Description Grass channels, also termed biofilters, works like a filter designed to enhance water quality by settling suspended solids through filtration, infiltration, and biofiltration. Grass channels are also able to help meet runoff velocity targets for the water-quality design storm of small drainage areas. Vegetative practices are able to offer a method to manage pollution while conveying stormwater runoff. Grass channels are well suited to a number of applications and land uses, including treating runoff from roads and highways and pervious surfaces. They can also be used in a variety of ways including, but not limited to, a single BMP, a pretreatment to another BMP, or link other BMPs together. Grass channels differ from the enhanced dry swale design in that they do not have an engineered filter media to enhance pollutant removal capabilities and therefore have a lower pollutant removal rate than for a dry or wet (enhanced) swale. Grass channels are different from the traditional roadside ditch in that they are designed for water quality purposes and help increase residence time while decreasing velocities. When properly incorporated into an overall site design, grass channels can reduce impervious cover, accent the natural landscape, and provide aesthetic benefits while still removing pollutants. When designing a grass channel, the two primary considerations are channel capacity and minimization of erosion. Runoff velocity should not exceed 1.0 foot per second during the peak discharge associated with the water quality design rainfall event, and the total length of a grass channel should provide at least 5 minutes of residence time. To enhance water quality treatment, grass channels must have broader bottoms, lower slopes and denser vegetation than most drainage channels. Additional treatment can be provided by placing check-dams across the channel below pipe inflows, and at various other points along the channel Pollutant Removal CapabilitiesStormwater Management Suitability Grass channels, a form of biofilter, are trapezoidal or parabolic shaped vegetated channels that work as a vegetative filter designed to enhance water quality through the settling of suspended solids through filtration, infiltration, and biofiltration. Only vegetative filters provide an acceptable pollution management measure while conveying stormwater runoff. Runoff Reduction Grass channels are an effective low impact development (LID) practices that can be used in Georgia to reduce post-construction stormwater runoff and improve stormwater runoff quality. Like other LID practices, they become even more effective the higher the infiltration rate of the native soils. A grass channel can be designed to provide 25% of the runoff reduction volume for type A and B hydrologic soils or 10% of the runoff reduction volume for type C and D hydrologic soils. Performance is dependent on vegetation density and contact time for settling, filtration, and infiltration. Water Quality Grass channel is a stormwater treatment practice that can be used to remove a variety of pollutant sremoval mechanisms. Grass channels are typically used as the pre-treatment component to reduce incoming runoff velocity and filter particulates from the runoff. Volume 2 (Technical Handbook) Georgia Stormwater Management Manual4.9-13

4 Channel Protection For smaller sites, a grass channel may be designed to capture the entire channel protection volume (Cp v). Given that a grass channel facility is typically designed to completely drain over hours, the requirement of extended detention of the 1-year, 24-hour storm runoff volume will be met. For larger sites, or where only the WQ v is diverted to the grass channel, another controlpractice must be used to provide Cp v extended detention. Overbank Flood Protection Another controlpractice in conjunction with a grass channel will likely be required to reduce the post-development peak flow of the 25-year storm (Q p) to pre-development levels (detention). Extreme Flood Protection Grass channels must provide flow diversion and/or be designed to safely pass extreme storm flows and protect the ponding area, mulch layer and vegetation. Credit for the volume of runoff reduced in the grass channel may be taken in the overbank flood protection and extreme flood protection calculations. If the practice is designed to provide Runoff Reduction for Water Quality compliance, then the practice is given credit for Channel Protection and Flood Control requirements by allowing the designer to compute an Adjusted CN (see section 3.2 for more information). For additional information and data on pollutant removal capabilities for bioretention areas, see the National Pollutant Removal Performance Database (2 nd Edition) available at and the National Stormwater Best Management Practices (BMP) Database at Design Criteria and SpecificationsPollutant Removal Capabilities Pollutant removal from filter stripsgrass channels is highly variable and depends primarily on density of vegetation and contact time for filtration and infiltration. These, in turn, depend on soil and vegetation type, slope, and presence of sheet flowcontact time. Research on fecal coliform removal has been inconclusive, but suggests that filter stripsgrass chnnels are generally not considered to be effective BMPs for treating bacterial loads. For additional information and data on pollutant removal capabilities for grass channels, see the National Pollutant Removal Performance Database (2 nd Edition) available at and the National Stormwater Best Management Practices (BMP) Database at Application and Site Feasibility Criteria Grass channels are best suited to treat smaller drainage areas. Flow must enter the practice as sheet flow spread out over the width (long dimension normal to flow) of the channel. For longer flow paths, special provision must be made to ensure design flows spread evenly across the filter strip. Because of design constraints, grass channels should have a drainage area of less than 5 acres. Grass channels should have a side slope of 3:1 or flatter. The maximum flow depth through the channel should be no more than 4 inches. The base width of the channel should be between 2 and 6 feet wide. The minimum width ensures a minimum filtering surface for water quality treatment, and the maximum width prevents braiding, which is the formation of small channels within the swalechannel bottom. The bottom width is a dependent variable in the calculation of velocity based on Manning's equation. If a larger channel is needed, the use of a compound cross section is recommended. The depth from the bottom of the channel to the groundwater should be at least 2 feet to prevent a moist swalechannel bottom, or contamination of the groundwater. Formatted: Outline numbered + Level: 3 + Numbering Style: 1, 2, 3, + Start at: 4 + Alignment: Left + Aligned at: 0" + Indent at: 0.5" Formatted: Normal, No bullets or numbering, Tab stops: 0", Left ", Left ", Left ", Left ", Left ", Left ", Left ", Left ", Left ", Left ", Left ", Left Formatted: Font: Arial, Not Expanded by / Condensed by Formatted: Font: Arial, Not Expanded by / Condensed by Formatted: Font: Arial, Not Expanded by / Condensed by Formatted: Font: Arial, Not Expanded by / Condensed by General Feasibility Georgia Stormwater Management Manual Volume 2 (Technical Handbook)

5 Suitable for Residential Subdivision Usage YES Suitable for High Density/Ultra Urban Areas NO Regional Stormwater Control NO Physical Feasibility - Physical Constraints at Project Site Drainage Area 5 acres or less. If the practices areis used on larger drainage areas, the flows and volumes through the channel become too large to allow for filtering and infiltration of runoff. Side Slope Slopes of the channel should be 3:1 or flatter Longitudinal slope - Between 1% and 4%; slopes between 1% and 2% are recommended. Base width - Between 2 and 6 feet. Minimum Depth to Water Table A separation distance of 1 foot is recommended between the bottom of the vegetated filter strip and the elevation of the seasonally high water table. Runoff Velocities Most not be erosive. Maximum flow velocity of 1.0 ft/s Other Constraints / Considerations Location requirements The following is a list of specific setback requirements for the location of a vegetated filter strip: Filter strips should be constructed outside the natural stream buffer area whenever possible to maintain a more natural buffer along the streambank. Filter stripsgrass channels should not be used on soils that cannot sustain a dense grass cover with high retardance. Designers should choose a grass that can withstand relatively high velocity flows at the entrances, and both wet and dry periods. See Appendix FD for a list of appropriate grasses for use in Georgia. Pedestrian traffic across the filter strip should be limited through channeling onto sidewalks. The filter strip should be at least 15 feet long (25 preferred) to provide filtration and contact time for water quality treatment. 100 feet is recommended for the maximum strip length.. Grass channels can be used on most soils with some restrictions on the most impermeable soils. Grass channels should not be used on soils with infiltration rates less than inches per hour if infiltration of small runoff flows is intended. A grass channel should accommodate the peak flow for the water quality design storm Q wq (see subsection 2.1.7). Incorporation of check dams within the channel will maximize retention time. Designers should choose a grass that can withstand relatively high velocity flows at the entrances, and both wet and dry periods. See Appendix F for a list of appropriate grasses for use in Georgia. A 5-minute residence time is recommended for the water quality peak flow. Residence time may be increased by reducing the slope of the channel, increasing the wetted perimeter, or planting a denser grass (raising the Manning s n). Grass channels should generally be used to treat small drainage areas of less than 5 acres. If the practices are used on larger drainage areas, the flows and volumes through the channel become too large to allow for filtering and infiltration of runoff. Grass channels should be designed on relatively flat slopes of less than 4%; channel slopes between 1% and 2% are recommended. Formatted: Font: Bold Formatted: Font: Bold Formatted: Font: Bold Formatted: Font: Bold Formatted: Indent: Left: 0.25", Bulleted + Level: 1 + Aligned at: 0" + Indent at: 0.25", Tab stops: 0.5", Left + Not at 0.36" Formatted: Indent: First line: 0.06", Tab stops: 0.5", Left + Not at 0.7" " Formatted: Indent: First line: 0.06", Bulleted + Level: 1 + Aligned at: 0" + Indent at: 0.25", Tab stops: 0.25", Left + 0.5", Left + Not at 0.7" " Formatted: Bulleted + Level: 1 + Aligned at: 0" + Indent at: 0.25", Tab stops: 0.25", Left + Not at 0.36" Formatted: Bulleted + Level: 1 + Aligned at: 0" + Indent at: 0.25", Tab stops: 0.25", Left + Not at 0.75" Formatted: Bulleted + Level: 1 + Aligned at: 0" + Indent at: 0.25", Tab stops: 0.25", Left + Not at 0.36" Formatted: Font: Arial, 10 pt, Not Bold, Condensed by 0.15 pt Formatted: No bullets or numbering Grass channels can be used on most soils with some restrictions on the most impermeable soils. Grass channels should not be used on soils with infiltration rates less than 0.27 inches per hour if infiltration of small runoff flows is intended. A grass channel should accommodate the peak flow for the water quality design storm Q wq (see subsection 2.1.7). Grass channels should have a trapezoidal or parabolic cross section with relatively flat side slopes (generally 3:1 or flatter). Volume 2 (Technical Handbook) Georgia Stormwater Management Manual4.9-15

6 The bottom of the channel should be between 2 and 6 feet wide. The minimum width ensures a minimum filtering surface for water quality treatment, and the maximum width prevents braiding, which is the formation of small channels within the swale bottom. The bottom width is a dependent variable in the calculation of velocity based on Manning's equation. If a larger channel is needed, the use of a compound cross section is recommended. Runoff velocities must be nonerosive. The full-channel design velocity will typically govern. A 5-minute residence time is recommended for the water quality peak flow. Residence time may be increased by reducing the slope of the channel, increasing the wetted perimeter, or planting a denser grass (raising the Manning s n). The depth from the bottom of the channel to the groundwater should be at least 2 feet to prevent a moist swale bottom, or contamination of the groundwater. Incorporation of check dams within the channel will maximize retention time. Designers should choose a grass that can withstand relatively high velocity flows at the entrances, and both wet and dry periods. See Appendix F for a list of appropriate grasses for use in Georgia. Formatted: Highlight See Section 4.4 (Open Channel Design) for more information and specifications on the design of grass channels Georgia Stormwater Management Manual Volume 2 (Technical Handbook)

7 Grass Channels for Pretreatment A number of other structural controls, including bioretention areas and infiltration trenches, may utilize a grass channel as a pretreatment measure. The length of the grass channel depends on the drainage area, land use, and channel slope. Table provides sizing guidance for grass channels for a 1-acre drainage area. The minimum grassed channel length should be 20 feet. Table Bioretention Grass Channel Sizing Guidance (Source: Claytor and Schueler, 1996) Parameter Slope (max = 4%) Grass channel minimum length* (feet) *assumes 2-foot wide bottom width <= 33% Impervious Between 34% and 66% Impervious >= 67% Impervious < 2% > 2% < 2% > 2% < 2% > 2% Planning and Design Criteria Before designing the grass channel, the following data is necessary: Existing and proposed site, topographic and location maps, and field reviews. Impervious and pervious areas. Other means may be used to determine the land use data. Roadway and drainage profiles, cross sections, utility plans, and soil report for the site. Design data from nearby storm sewer structure. Water surface elevation of nearby water systems as well as the depth to seasonally high groundwater. Infiltration testing of native soils at proposed elevation of bottom of grass channel. Formatted: Font: Century Gothic, 12 pt Formatted: Font: Century Gothic, 12 pt, Bold Formatted: Font: Century Gothic, 12 pt Formatted: Font: Century Gothic, 12 pt, Bold The following criteria are to be considered minimum standards for the design of a grass channel. Consult with the local review authority to determine if there are any variations to these criteria or additional standards that must be followed LOCATION AND LAYOUT Grass channels vary based on site constraints such as proposed and existing infrastructure, soils, existing vegetation, contributing drainage area, and utilities. Grass channels are designed for intermittent flow and must be allowed to drain between rain fall events. They should not be used on sites with a continuous flow from groundwater, sump pumps, or other sources. Grass channel locations should be integrated into the sitie planning process and aesthetic and maintenance considerations should be taken into account in their sitting and design. Formatted: Font: Bold Formatted: Font: Bold Inspection and Maintenance Requirements Table Typical Maintenance Activities for Grass Channels (Source: Adapted from CWP, 1996) Maintenance Activity Water as necessary during establishment period and also during dry conditions Mow grass to maintain a height of 3 to 4 inches. Schedule As needed Volume 2 (Technical Handbook) Georgia Stormwater Management Manual4.9-17

8 Table Typical Maintenance Activities for Grass Channels (Source: Adapted from CWP, 1996) Maintenance Activity Schedule Inspect vegetation for signs of stress, treat problem or replace vegetation Inspect for erosion or bare spots, repair as necessary Remove sediment build-up within the bottom of the grass channel once Semi-annually during it has accumulated to 25% of the original design volume. establishment and Inspect grass alongside slopes for erosion and formation of rills or annually thereafter) gullies and correct. Remove trash and debris accumulated in the channel. Based on inspection, plant an alternative grass species if the original grass cover has not been successfully established. Inspect for erosion or bare spots, repair as necessary After main rain event Georgia Stormwater Management Manual Volume 2 (Technical Handbook)

9 4.9.5 Example Schematics Figure Typical Grass Channel Figure Schematic of Grass Channel Volume 2 (Technical Handbook) Georgia Stormwater Management Manual4.9-19

10 Design Example Step 1. Determine the goals and primary function of the grass channel Consider whether the grass channel is intended to: Meet a runoff reduction* target or water quality (treatment) target. *Note that minimum infiltration rates of the surrounding native soils must be acceptable and suitable when used in runoff reduction applications. Be oversized to include partial credit for storage capacity for other stormwater requirements (Channel Protection Volume (Cp v), Overbank Flood Protection Volume (Q p), or the Extreme Flood Volume (Q f)) Provide a possible solution to a drainage problem Check with local officials and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply. In addition, consider if the grass channel has any special site-specific design conditions or criteria. List any restrictions or other requirements that may apply or affect the design. The design of the grass channel should be centered on the restrictions/requirements, goals, targets, and primary function(s) of a grass channel. By considering the primary function, as well as, topographic and soil conditions, the design elements of the grass channel can be determined (i.e. planting media, underdrain, inlet/outlet, overflow, etc.) Step 2. Determine if the development site and conditions are appropriate for the use of a grass channel Consider the application and site feasibility criteria in this chapter. In addition, determine if site conditions are suitable for a grass channel. Create a rough layout of the grass channel dimensions taking into consideration existing trees, utility lines, and other obstructions. Step 3. Calculate the minimum length (feet) of the grass channel using a 5-minute (300 seconds) residence time: Where: L = minimum length of channel (ft) V = velocity through the channel (ft/s) ec L=V X (5 minutes) X (300 seconds) Step 4. Assume a base width and side slopes and calculate the depth and velocity. Modify base width value until to make sure that the flow depth is less than 4 inches and that the flow velocity is less than 1 ft/sec. Formatted: No underline Formatted: Centered Formatted: No underline Formatted: No underline, English (United States) Step 5. Prepare Vegetation and Landscaping Plan A landscaping plan for the grass channel should be prepared to indicate how it will be established with vegetation. Basic Data See section (Landscaping) and Appendix D for more details. Small commercial lot 300 feet deep x 145 feet wide located in Athens Drainage area (A) = 1.0 acres Impervious percentage (I) = 70% Water Quality Peak Flow See subsection for details Georgia Stormwater Management Manual Volume 2 (Technical Handbook)

11 Compute the Water Quality Volume in inches: WQ v = 1.2 ( * 70) = 0.82 inches Compute modified CN for 1.2-inch rainfall (P=1.2): CN = 1000/[10+5P+10Q-10(Q *Q*P) ½ ] = 1000/[10+5*1.2+10* ( *0.82*1.2) ½ ] = (Use CN = 96) For CN = 96 and an estimated time of concentration (T c) of 8 minutes (0.13 hours), compute the Q wq for a 1.2-inch storm. From Figure , I a = 0.083, therefore I a/p = 0.083/1.2 = From Figure for a Type II storm (using the limiting values) q u = 950 csm/in, and therefore: Q wq = (950 csm/in) (1.0ac/640ac/mi 2 ) (0.82") = 1.22 cfs Utilize Q wq to Size the Channel The maximum flow depth for water quality treatment should be approximately the same height of the grass. A maximum flow depth of 4 inches is allowed for water quality design. A maximum flow velocity of 1.0 foot per second for water quality treatment is required. For Manning s n use 0.15 for medium grass, 0.25 for dense grass, and 0.35 for very dense Bermuda-type grass. Site slope is 2%. Input variables: n = 0.15 S = 0.02 ft/ft D = 4/12 = 0.33 ft Then: Where: Q wq = Q = VA = 1.49/n D 2/3 S 1/2 DW Q = peak flow (cfs) V = velocity (ft/sec) A = flow area (ft 2 ) = WD W = channel bottom width (ft) D = flow depth (ft) S = slope (ft/ft) (Note: D approximates hydraulic radius for shallow flows) Then for a known n, Q, D and S minimum width can be calculated. (nq)/(1.49 D 5/3 S 1/2 ) = W = (0.15*1.22)/(1.49*0.33 5/3 *0.02 1/2 ) = 5.5 feet minimum V = Q/(WD) = 1.22/(4.0 * 4/12) = 0.92 fps (okay) Volume 2 (Technical Handbook) Georgia Stormwater Management Manual4.9-21

12 (Note: WD approximates flow area for shallow flows.) Minimum length for 5-minute residence time, L = V * (5*60) = 201 feet Depending on the site geometry, the width or slope or density of grass (Manning s n value) might be adjusted to slow the velocity and shorten the channel in the next design iteration. For example, using a 9.3-foot bottom width* of flow and a Manning s n of 0.25, solve for new depth and length. Q = VA = 1.49/n D 5/3 S 1/2 W D = [(Q * n)/(1.49 * S 1/2 * W)] 3/5 = [(1.22 * 0.25)/(1.49 * /2 * 9.3)] 3/5 = 0.33 ft = 4 (okay) V = Q/WD = 1.22/(9.3 * 0.33) = 0.40 feet per second L = V * 5 * 60 = 120 feet * In this case a dividing berm should be used to control potential braiding. Refer to Section 4.4 (Open Channel Design) to complete the grass channel design for a specified design storm event Inspection and Maintenance Requirements All best management practices require proper maintenance. Without proper maintenance, BMPs will not function as originally designed and may cease to function altogether. The design of all BMPs includes considerations for maintenance and maintenance access. For additional information on inspection and maintenance requirements, see Appendix XX. 2jor Georgia Stormwater Management Manual Volume 2 (Technical Handbook)

13 Volume 2 (Technical Handbook) Georgia Stormwater Management Manual4.9-23