6.2.2 bioslope

6.2.2 Bioslope

Description: A bioslope is a specialized media filtration IMP that is typically used in linear applications to treat stormwater along an impervious area (road, parking lot, etc.).

LID/GI Consideration: A bioslope is adaptable to many linear situations, and often a small IMP used to treat runoff close to the source. This is considered to be a green infrastructure IMP.

Augusta Stormwater Management Manual

Version: July 20206.2.2- 1

6.2.2 Bioslope

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KEY CONSIDERATIONS STORMWATER MANAGEMENT SUITABILITY

DESIGNCRITERIA: ` Longitudinal slopes must be less than 5% ` Minimum 2 foot width ` Side slopes 3:1 or flatter; 4:1 recommended ` Length is usually the length of adjacent paved area being treated

` Sized to capture the water quality peak flow rate of discharge ` Pretreatment most commonly provided through a vegetative filter strip

ADVANTAGES/BENEFITS: ` Requires minimal land

` Reduces runoff volume and velocity

DISADVANTAGES/LIMITATIONS: ` Limited to sheet flow uses only ` Not suitable for embankment slopes steeper than 3:1

` Does not meet quantity control stormwater requirements

ROUTINEMAINTENANCEREQUIREMENTS: ` Provide signage for the IMP ` Avoid damaging or rutting the permeable soil layer when mowing grass

` Perform weeding and trash removal as necessary ` Remove sediment and debris from pretreatment area (vegetative filter strip) and adjacent areas

Residential Subdivision Use: Yes High Density/Ultra-Urban: No Regional Stormwater Control: No Drainage Area: Contributing upstream flow must be less than 150 feet Soils: *See Runoff Reduction Credit Other Considerations: Bioslope Media Mixture shall be used in design

RUNOFF REDUCTION CREDIT

` Hydrologic Soils Grou (HSG) A & B: 50% of the runoff reduction volume provided

` HSG C & D: 25% of the runoff reduction volume provided

POLLUTANTREMOVAL85% Total Suspended Solids

60% Phosphorus

25% Nitrogen

75%Metals - Cadmium, Copper, Lead, and Zinc

60% Pathogens – Fecal Coliform

Runoff Reduction

TSS Removal

u Channel Protection

Overbank Flood Protection

Extreme Flood Protection Retrofit

suitable for this practice

u may provide partial benefits

Low Land Requirement

Medium Capital Cost

Medium Maintenance Burden

IMPLEMENTATION CONSIDERATIONS



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6.2.2.1 GeneralDescriptionandStormwaterManagementSuitabilityBioslopes (also referred to as ecology embankments) are IMPs that use a permeable engineered soil media to capture and treat stormwater runoff from adjacent paved areas (Figure 6.2.2-1). Bioslopes are typically installed along embankments or other slopes and designed to treat sheet flow stormwater runoff.

Bioslopes are designed with limited longitudinal slopes to force the flow to be slow and uniform, thus allowing for particulates to settle and limiting the effects of erosion. Once infiltrated into the highly permeable engineered soil layer, an underdrain is typically used to remove the treated stormwater from the embankment or slope. Larger flow rates (from less frequent storm events) in the form of sheet flow bypass the engineered soil media by overtopping and continuing down the embankment or slope.

Figure 6.2.2-1. Example of a Bioslope

6.2.2.2 PlanningBioslopes can be used in a variety of development types; however, they are primarily applicable to linear roadway applications where a rural (no curb and gutter) cross-section is utilized. The impervious cover in the contributing drainage area is relatively small, and consists of paved areas adjacent to the embankment or slope where the practice has been installed.

Since bioslopes require a relatively small amount of land, they are more commonly selected when right-of-way availability is limited. Other practices that would be required at the bottom of the embankment or slope, such as enhanced swales or grass channels, most always require a larger amount of land. Bioslopes may not be desirable in some more aesthetically landscaped or sodded areas, due to the need for keeping the media layer free and clear of plantings and other obstructions.



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The topography and location of a site will determine the applicability of the use of bioslopes. Overall, the topography should allow for the design of a bioslope with sufficient slope and cross-sectional area to maintain non-erosive velocities. The following criteria should be evaluated to ensure the suitability of a bioslope for meeting stormwater management objectives on a site or development.

Physical Feasibility - Physical Constraints at Project Site ` Drainage Area – Contributing upstream flow shall be less than 150 feet.

` Site Slope– Site slope is typically no more than 5%.

` Minimum Depth to Water Table – A minimum 2 feet is required between the bottom of the media layer and the elevation of the seasonally high water table.

` Soils – Bioslope media shall be used. Determination of the HSG soil type shall be made to determine runoff reduction credit.

` Hotspots - Exfiltration should not be allowed in hotspot areas.

The data listed below are necessary for the design of a bioslope area and shall be included with the stormwatermanagementplan(SWMP):

` Existing and proposed site, topographic and location maps, and field reviews

` Impervious and pervious areas

` Roadway and drainage profiles, cross-sections, utility plans, and soil report for the site

` Design data from nearby storm sewer structures

` Water surface elevation of nearby water systems as well as the depth to seasonally high groundwater

` HSG determination of native soils at proposed elevation of bottom of bioslope area.

See Appendix B for more information on required elements for the SWMP.

6.2.2.3 DesignThe following criteria shall be considered minimum standards for the design of a bioslope:

` A bioslope shall be sited such that the topography allows for the design with sufficiently mild slope and cross-sectional area to maintain non-erosive velocities.

` Bioslopes shall have a maximum contributing upstream flow length of 150 feet, or less.

` Bioslopes are designed to treat the volume of water for treatment through a flow rate-based design, and to safely pass larger storm flows by checking velocities. Flow enters the bioslope via sheet flow through a pretreatment filter strip area, or a minimum 2-foot wide grass strip.

` A bioslope shall consist of a bioslope media that overlays an underdrain system. Flow enters the media layer where it is filtered through the soil bed. Runoff is collected and conveyed by a perforated pipe and gravel underdrain system to the outlet. Figure 6.2.2-2 provides a schematic for typical components of a bioslope.



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Figure 6.2.2-2. Bioslope Schematic

` The following equation and variables shall be used for the sizing of a bioslope:

whereW = bioslope width (parallel with flow path) (feet)C = conversion factor = 43,200 [(in/hr)/(ft/s)]Qwq = water quality volume peak flow (ft3/s, refer to Section 5.2.6)SF = safety factor equal to 1 (unit less, typical throughout Georgia)k = infiltration rate, use long-term infiltration rate of 10 (inches/hour)L = bioslope length (perpendicular with flow path) (feet)

Specifications ` Where space allows, vegetated filter strips should be used as the pretreatment device for bioslopes. See Section 6.2.15 for more information on vegetated filter strips.

` Where space does not allow for a full-width vegetated filter strip, a grassed area can still be used as pretreatment to the best extent possible.

` A pea gravel diaphragm can also be used for pretreatment where space constraints do not allow for enough of a grassed area between the bioslope and the paved surface.

` Embankment slopes shall be 3:1 or flatter. When slopes steeper than 4:1 are used, additional measures shall be taken to ensure stabilization of vegetation along the slope.

` Longitudinal slopes (parallel with the embankment) shall be no more than 5%.



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` The area between the impervious surface, or paved area, shall be no more than 30 feet to avoid reconcentration of stormwater runoff, creating erosion and scour through the engineered media.

` The bioslope shall consist of a permeable soil layer of at least 12 inches in depth, above an underdrain.

` The soil media shall contain a mixture of crushed rock, dolomite, gypsum, and perlite as shown in Table 6.2.2-1.

` This mixture has an initial infiltration rate of 50 inches per hour, and an infiltration rate of 28 inches per hour long term. For sizing, an infiltration rate of 10 inches per hour shall be used in calculations as a factor of safety.

` A permeable filter fabric shall be placed between the media and underlying soil.

` The underdrain collection system shall be equipped with at least a 6-inch diameter perforated PVC pipe longitudinal underdrain in a gravel layer.

` The underdrain system shall discharge to a storm drainage structure or a stable outfall.

` Bioslopes shall be adequately designed to safely pass flows that exceed the design storm flows.

` Adequate access shall be provided for all bioslopes for inspection and maintenance.

Table 6.2.2-1 Bioslope Media Mixture

EngineeredMediaAmendment Quantity

Aggregate: #89 stone No recycled material Non-limestone material mineral aggregate

3cubicyards(CY)Note: 3 CY is used as a baseline for other mixture components; adjust total quantity based on bioslope dimensions

Perlite: Horticultural grade, free of any toxic materials 0-30% passing US No. 18 Sieve 0-10% passing US No. 30 Sieve

1 CY per 3 CY of mineral aggregate

Dolomite:CaMg(CO3)2(calciummagnesiumcarbonate) Agricultural grade, free of any toxic materials 100% passing US No. 8 Sieve 0% passing US No. 16 Sieve

10 pounds per CY of perlite

Gypsum: Non-calcined, agricultural gypsum CaSO4•2H2O (hydrated calcium sulfate) Agricultural grade, free of any toxic materials 100% passing US No. 8 Sieve 0% passing US No. 16 Sieve

1.5 pounds per CY of perlite

(Source: Adapted from WSDOT, 2011)



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Landscaping ` Surrounding vegetation is typically a grassed or vegetated filter strip. The surrounding vegetation shall be well established and thriving to prevent erosion and sedimentation.

` Only when the same infiltration rate of the media could be preserved, can grass be planted directly on top of the engineered media. This may include frequently maintained “half-cut” sod or similar vegetation.

` In this instance 100% of the bioslope must be covered by vegetation on a continuous basis.

` A protective landscape design allows visual opportunities of the area, but discourages encroachments into the area by directing foot traffic or vehicles around or away from the area. Examples of protective landscaping are provided in Subsection 6.2.2.4.

` Additional information about landscaping can be found in Appendix D.

Design Procedures

Step 1. Determine if the development site and conditions are appropriate for the use of a bioslope.

Consider the application and site feasibility criteria in this Chapter. In addition, determine if site conditions are suitable for a bioslope. Create a rough layout of the bioslope dimensions taking into consideration existing trees, utility lines, and other obstructions.

Step 2. Determine the goals and primary function of the bioslope.

Consider whether the bioslope is intended to:

` Meet the runoff reduction criterion (RRv) or 80% TSS removal criterion (WQv). For information on the sizing of an IMP utilizing the runoff reduction approach, see Step 3A. For information on the sizing of the IMP utilizing the TSS removal approach, see Step 4A. Note: Minimum infiltration rates of the surrounding native soils must be acceptable and suitable when the bioslope area is designed for purposes of compliance with the runoff reduction criterion.

` Be “oversized” to include partial credit for storage capacity for other stormwater requirements (channel protection volume [CPv ])

` Provide a possible solution to a drainage problem

` Enhance landscape and provide aesthetic qualities

Identify any restrictions or other requirements that may apply or affect the design.

CompleteSteps3A,3B,and3Cforarunoffreductionapproach(RRv)approach,orskipStep3andcompleteStep4 foraTSSremoval (WQv). Refer to Chapter 5 for detailed information on compliance calculations.



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Step 3A. Calculate the RRv.

Calculate the RRv using the following formula:RRv=(P)(Rv)(A)/12

where RRv = runoff reduction volume (ft3) P = runoff reduction rainfall (1.0 inches) Rv = volumetric runoff coefficient which can be found by

Rv = 0.05+0.009(I)and where I = new impervious area of the contributing drainage area (%)

A = area draining to this practice (ft2)12 = unit conversion factor (in/ft)

Using Table 6-4 – IMP Runoff Reduction Credits, lookup the appropriate runoff reduction percentage provided by the practice:

Using the RRvcalculatedabove,determinetheminimumVolumeofthepractice(VP)(VPMIN)≥RRv/(RR%)

whereRR% = runoff reduction percentage, or credit, assigned to the specific practiceVPMIN = minimum storage volume required to provide runoff reduction volume (ft3)RRv = runoff reduction volume (ft3)

Step 3B. Determine the storage volume of the practice and the pretreatment volume.

To determine the actual volume provided in the bioslope, use the following equation:VP=(PV+VES[N])

where VP = volume provided (temporary storage) PV = ponding volume VES = volume of engineered soils N = porosity.

To determine the porosity, a qualified licensed professional should be consulted to determine the proper porosity based on the engineered soils used. The required porosity for engineered soils and gravel is 0.25 and 0.40, respectively.

Provide pretreatment by using a grass filter strip or pea gravel (sheet flow), or a grass channel or forebay (concentrated flow), as needed. Where filter strips are used, 100% of the runoff should flow across the filter strip. Pretreatment may also be desired to reduce flow velocities or assist in sediment removal and maintenance. Pretreatment can include a forebay, weir, or check dam. Splash blocks or level spreaders should be considered to dissipate concentrated stormwater runoff at the inlet and prevent scour. Forebays should be sized to contain 0.1 inch per impervious acre of contributing drainage. Refer to Section 6.2.4 for design criteria for a grass channel and Section 6.2.15 for vegetated filter strips.



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Step 3C. Determine whether the minimum storage volume was met.

When the VP ≥ VPMIN, then the runoff reduction requirements are met for this practice. Proceed to Step 5.

When the VP < VPMIN, then the IMP must be sized according to the WQv treatment method (see Step 4A).

Step 4A. Calculate WQv.

Calculate the WQv using the following formula:WQv=(1.2)(RV)(A)/12

whereWQv = water quality volume (ft3)1.2 = target rainfall amount to be treated (inches)Rv= volumetric runoff coefficient which can be found by

Rv = 0.05+0.009(I)and where

I = new impervious area of the contributing drainage area (%)A = area draining to this practice (ft2)12 = unit conversion factor (in/ft)

Step4B.Ifusingthepracticefor80%TSSremoval,determinethefootprintofthebioslopeareapractice and the pretreatment volume required.

Determine bioslope dimensions:

Refer to Sections 5.2.6 and 5.2.7 to calculate the water quality volume peak flow rate using a reduced CN for any RRv provided, then find either length or width using the equation in Section 6.2.2.3. In most applications, site restrictions limit the available length.

` Longitudinal slope cannot exceed 5% (2 to 4% recommended)

` Width should be a minimum of 2 feet

` Ensure that side slopes are no steeper than 3:1 (4:1 recommended)

See Subsection 6.2.2.3 (Specifications) for more details

Step 5. Calculate the adjusted curve numbers for CPv (2-year, 24-hour storm), and the peakdischargesforthefloodprotectionperformancestandards(Qp2, Qp10, Qp25, Qp50, and Qp100).

See Section 5.2.7 for more information.

Step 6. Size the underdrain system.

An underdrain system is required if the in situ soil infiltration rate is less than 0.5 in/hr. Underdrain should be a 4-6-inch perforated PVC pipe (AASHTO M 252) in an 8-inch gravel layer.



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6.2.2.4 Inspection,ProtectionandMaintenanceRequirementsAll IMPs require proper construction, protection, and long-term maintenance or they will not function as designed, and may cease to function altogether. The design of all IMPs includes considerations for maintenance and maintenance access. A legally binding IMP Maintenance Agreement shall be completed. For Augusta policies, additional guidance and forms pertaining to IMP protection, inspection and maintenance requirements, see Chapter 8 of this Manual.

Requirements DURING Construction

` Construction equipment shall be restricted from the bioslope area to prevent compaction of the native soils.

` A dense and vigorous vegetative cover or other effective soil stabilization practice shall be established over the contributing pervious drainage areas before stormwater can be accepted into the bioslope area. This will prevent sediment from clogging the pores in the engineered media.

` Areas where IMPs will be located shall be readily identifiable and protected from unwanted encroachments during construction, both on SWMPs and at the construction site. Physical protection measures can include, but are not limited to, orange fencing, wood or chain link fencing, and signage. For infiltration-based IMPs, protection of IMP locations during construction of a land development will ensure that native soils that surround (or are within) the stormwater treatment area will remain uncompacted and therefore continue to meet the design parameters that were specified in the approved SWMP. For other IMPs, such as extended detention ponds, protection of IMP locations can reduce the soil compaction that typically occurs during construction, which often leads to poor soil conditions for plant growth once the IMP must be permanently stabilized. Regardless, a lack of IMP protection will most certainly reduce or destroy the stormwater functionality of the IMP once it is installed, often leading to costly corrective actions required by AED.

Protection Requirements ` Providing signage for the IMP will:

` Allow for easy identification and location of the IMP, which is especially important for bioslopes to make roadway maintenance staff aware of the location and need for protection of the IMP

` Serve as a general education tool, making those responsible for property, landscape or IMP maintenance and the general public aware of the water quality features of the IMP and to avoid encroachment

` Consider using natural fencing such as graduated vegetation sizes and densities, dense shrub fencing, rocks or other landscape features placed in a manner that discourages foot, equipment and vehicle traffic in the stormwater treatment area.

` Roadway maintenance staff should be trained on the proper maintenance of bioslopes and the importance of avoiding the use of equipment on the IMP.

` Design the layout of the bioslope area such that maintenance access can be achieved without the need for vehicles or equipment in the stormwater treatment area.



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` Provide clearly marked, easily accessible and well-maintained driveways, sidewalks and pedestrian pathways that lead vehicles, equipment and foot traffic around the stormwater treatment areas.

Inspection RequirementsWhile AED requires an annual inspection, more frequent inspections help identify problems earlier and keep maintenance costs lower. The following are recommended frequent inspection items:

Bioslope Area Typical Routine Maintenance Activities and ScheduleActivity Schedule

` Clear debris in inlets and outlets.

` Mow and stabilize the area surrounding the bioslope. Remove grass clippings.

` Ensure that activities in the drainage area minimize oil/grease and sediment entry to the system.

` Remove trash and debris.

As needed or 4 times during growing season

` Stabilize eroded areas on the bioslope.

` Ensure that flow is not bypassing the facility.

` Ensure that no noticeable odors are detected outside the facility.

` Mow the bioslope grass using a retractable arm mower to avoid compaction. Grass height should be mowed to a height of 6 to 15 inches. Remove grass clippings.

Monthly

` Ensure that gravel spreader or other structural elements of the bioslope are in good condition and free of debris.

` Test the permeability of the bioslope media using a hydraulic conductivity test. Replace the media as needed.

` Flow test the clean outs to look for signs indicating the underdrain system is clogged.

` Evaluate sediment accumulation and remove once it reaches or exceeds a depth of 3 inches.

Annually

For bioslope areas, inspections of the following elements are important for proper function of the IMP, and inspection frequency and diligence are critical.

` Inspect the areas where stormwater flows into or out of the bioslope area for clogging or sediment build-up.

` Ensure that the gravel spreader or other structural elements of the bioslope are in good condition and free of debris.

` Inspect bioslope media for clogging areas.

` Inspect the property that drains to the bioslope area for erosion, exposed soil or stockpiles of other potential pollutants.



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Maintenance RequirementsBioslopes are linear IMPs with a permeable media that allows stormwater runoff to infiltrate and filter through the practice before exiting through an underdrain. High flows bypass the bioslope in the form of sheet flow running over the bioslope. Generally, a filter strip is placed before the bioslope for pretreatment where it captures sediment and debris and prevents premature clogging of the bioslope. If the space available for the bioslope is limited, a grass shoulder or pea gravel diaphragm may be used as an alternate method of pretreatment.

There are some common problems to be aware of when maintaining a bioslope. They include, but are not limited to, the following:

` Sediment build-up.

` Clogging in the inlet and outlet structure as well as the underdrain

` Undesirable vegetation

` Erosion

` Mowing the grass filter strip

` Compaction

Typically bioslopes are indistinguishable from the surrounding areas, so it is recommended that GPS coordinates of the bioslope location be obtained and the IMP be staked with markers. If markers are used, they should be placed at both ends at the toe of the slope and every 50 feet.

Specifically, the following maintenance actions are of critical importance to the long-term performance of the IMP:

` Perform weeding, pruning, and trash removal as needed to maintain appearance

` Mow the bioslope grass (6 to 15 inches) using a retractable arm mower to avoid compaction, and remove grass clippings

` Keep inlets clear of debris to prevent clogging

` Inspect bioslope area for sediment build-up, erosion, vegetative health/conditions, etc. and perform appropriate maintenance as necessary

` Inspect underdrain clean out to ensure stormwater infiltrates properly, and clean out underdrain if necessary



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