13.6 BMP T900 - Media Filter Drain
The media filter drain (MFD), previously referred to as the ecology embankment, is a linear flow-through stormwater treatment device that can be sited along road side slopes (conventional design) and medians (dual media filter drains), borrow ditches, or other linear depressions. Cut-slope applications may also be considered. The MFD can also be used in an end-of-pipe application where surface runoff is collected and conveyed to a location where flows can be redispersed to the MFD.
MFD configurations are separated into seven typical installations. MFD Type 1 though Type 5 have the option of placing a 3-inch medium compost layer with grass over the MFD mix area. If the 3-inch compost layer with grass is used on the MFD mix area, the BMP does not qualify for phosphorous treatment. MFD Types 1 through 7 are shown in Table 4-20 through Table 4-26.
The different MFD types are briefly described below:
MFD Type 1 - V Sheet flow application with underdrain.
MFD Type 2 - V Sheet flow applications; flows are from both sides of the median.
MFD Type 3 - V Sheet flow application without underdrain; drains to slope.
MFD Type 4* - V End-of-pipe application, redispersed to MFD with underdrain.
MFD Type 5* - V End-of-pipe application, redispersed to MFD without underdrain.
MFD Type 6* - V End-of-pipe application that is downstream of a detention BMP, redispersed to MFD with underdrain. MFD Type 6 doesn’t have the no-vegetation zone or grass strip because of the sediment storage in the upstream detention BMP. MFD Type 6 must have a 3-inch medium compost blanket with grass over MFD mix area. MFD Type 6 must have 8-inch-diameter compost socks, spaced at a minimum of 4-foot intervals, along the bottom of the MFD media mix.
MFD Type 7* - V Same as Type 6, except MFD doesn’t have an underdrain; it drains to the adjacent side slope.
Figure 4 - 20: Media Filter Drain Type 1 - Side Slope Application With Underdrain
Figure 4 - 21: Dual Media Filter Drain Type 2 - Median Application
Figure 4 - 22: Media Filter Drain Type 3 - Side Slope Application Without Underdrain
Figure 4 - 23: Media Filter Drain Type 4 - End-of-pipe Application With Underdrain
Figure 4 - 24: Media Filter Drain Type 5 - End-of-pipe Application Without Underdrain
Figure 4 - 25: Media Filter Drain Type 6 - End-of-pipe Application With Underdrain
Figure 4 - 26: Media Filter Drain Type 7 - End-of-pipe Application Without Underdrain
The MFD removes suspended solids, phosphorus (MFD without 3-inch medium compost blanket), and metals from highway runoff through physical straining, ion exchange, carbonate precipitation, and biofiltration
Stormwater is conveyed to the MFD via sheet flow or is redispersed to a vegetation-free gravel zone (MFD Type 1 – Type 5) to ensure dispersion and provide some pollutant trapping. Next, a grass strip provides pretreatment, further enhancing filtration and extending the life of the system. The runoff is then filtered through a bed of porous, alkalinity-generating granular medium—the media filter drain mix. Treated stormwater and surface water flows away from the MFD mix bed into a downstream conveyance system. Geotextile lines the underside of the MFD mix bed and the underdrain pipe and trench (if applicable).
The underdrain trench is an option for hydraulic conveyance of treated stormwater to a desired location, such as a downstream flow control facility, discharge location, or stormwater outfall. The trench’s perforated underdrain pipe is a protective measure to ensure free flow through the MFD mix. It may be possible to omit the underdrain pipe if it can be demonstrated that the pipe is not necessary to maintain free flow through the MFD mix and underdrain trench.
It is critical to note that water should sheet flow across or be redispersed to the MFD. To ensure sediment accumulation does not restrict sheet flow, edge of pavement installations should include a 1-inch drop between the pavement surface and nonvegetation zone where there is no guardrail or include a 1-inch drop where there is guardrail. Note that MFD Types 4 through Type 7 include a 3-inch drop between the flow spreader and the MFD mix bed to ensure sheet flow continues over time.
13.6.3 Applications and Limitations
Media filter drains can be used to ensure compliance with Minimum Requirement 6 - Stormwater Treatment. Depending on the configurations, MFD can provide basic, phosphorus (MFD without 3-inch medium compost blanket on MFD mix area), and enhanced stormwater treatment.
MFD Type 1 and Type 3 - Ideal along highway side slopes, when adjacent to wetlands, and in narrow right of way locations.
Dual MFD for Highway Medians (MFD Type 2) - Prime locations for the MFD Type 2 are in highway medians, roadside conveyance ditches, or other linear depressions. It is especially critical for water to sheet flow across the MFD Type 2. Channelized flows or ditch flows running down the middle of the MFD Type 2 (continuous off-site inflow) should be minimized.
MFD Type 4 and Type 5 - Ideal where stormwater needs to be or already is captured and conveyed to a discharge location that can accommodate this BMP. These options provide maximum flexibility for placement where sheet flow off the edge of pavement is not feasible. Catch basins and pipes are used to convey stormwater to the MFD Type 4 and Type 5.
MFD Type 6 and Type 7 - Ideal where stormwater needs to be collected and conveyed for both stormwater treatment and flow control. The MFD is downstream of the detention BMP.
Ensure lateral MFD side slopes adjacent to the roadway pavement (MFD Type 1 - Type 3) are less than 4H:1V. As side slopes approach 3H:1V, without design modifications, sloughing may become a problem due to friction limitations between the separation geotextile and underlying soils.
Where the MFD is built away from the roadway (MFD Type 4 - Type 7), ensure the lateral MFD side slope is less than 8H:1V.
Ensure longitudinal MFD slopes are no steeper than 5%.
Ensure the longest flow path from the contributing area delivering sheet flow to the MFD (Type 1 - Type 3) does not exceed 150 feet.
Do not construct in wetlands and wetland buffers.
Shallow groundwater - Determine seasonal high groundwater table levels at the project site to ensure the MFD mix bed and the underdrain (if applicable) will not become saturated by shallow groundwater. The hydraulic and stormwater treatment performance of the MFD may be compromised due to backwater effects and lack of sufficient hydraulic gradient due to shallow groundwater or pooling at the discharge location.
Unstable slopes - In areas where slope stability may be problematic, consult a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist.
Narrow roadway shoulders - In areas where there is a narrow roadway shoulder (width less than 10 feet), consider placing the MFD farther down the embankment slope. This will reduce the amount of rutting in the MFD and decrease overall maintenance repairs. Also, consider using a MFD Type 5 or Type 6.
Ensure the upstream conveyance system to a MFD Type 4 - Type 7 has adequate hydraulic head to push flows through the redispersal structure and not create upstream flooding problems.
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
No-Vegetation Zone - The no-vegetation zone (vegetation-free zone) is a shallow gravel zone located directly adjacent to the highway pavement. The no-vegetation zone is a crucial element in a properly functioning MFD or other BMPs that use sheet flow to convey runoff from the highway surface to the BMP. The no-vegetation zone functions as a level spreader to promote sheet flow and a deposition area for coarse sediments. Make sure the no-vegetation zone is between 1 foot and 3 feet wide. Depth will be a function of how the roadway section is built from subgrade to finish grade; the resultant cross section will typically be triangular to trapezoidal.
Grass Strip - The width of the grass strip is dependent on the availability of space within the highway side slope and MFD type. The grass strip is required on MFD Type 1 - Type 5. The minimum grass strip width is 3 feet, but wider grass strips are recommended if the additional space is available. At a minimum, the existing embankment will be scarified 2 inches and covered with a 3-inch blanket of medium compost and seeded. Consider adding aggregate to the soil mix to help minimize rutting problems from errant vehicles. The soil mix should ensure grass growth for the design life of the MFD.
Media Filter Drain Mix Bed - The MFD mix is a mixture of crushed rock (sized by screening), dolomite, gypsum, and perlite. The crushed rock provides the support matrix of the medium; the dolomite and gypsum add alkalinity and ion exchange capacity to promote the precipitation and exchange of heavy metals; and the perlite improves moisture retention to promote the formation of biomass within the MFD mix. The combination of physical filtering, precipitation, ion exchange, and biofiltration enhances the water treatment capacity of the mix. The MFD mix has an estimated initial filtration rate of 50 inches per hour and a long-term filtration rate of 28 inches per hour due to siltation. With an additional safety factor, the rate used to size the length of the MFD should be 10 inches per hour. Internal 8-inch-diameter medium compost socks are required along the bottom of the MFD Type 6 and Type 7 installations at even 4-foot spacings. Make sure there is a minimum of one row of compost socks for each MFD Type 6 or Type 7 installation.
3-Inch Medium Compost Blanket and Grass - Place a 3-inch medium compost blanket with grass over the media filter drain bed area to reduce noxious weeds and unwanted vegetation. Compost shall be medium compost and conform to WSDOT Standard Specification 9-14.5(8). Do not use this compost blanket in phosphorous-sensitive areas or phosphorous total maximum daily load (TMDL) areas. If this option is used, the MFD will not be considered as a phosphorous treatment BMP. Do not use MFD Type 6 and Type 7 in phosphorous-sensitive areas since the 3-inch compost blanket is required.
Conveyance System Below Media Filter Drain Mix - The gravel underdrain trench (MFD Type 1, Type 4, and Type 6) provides hydraulic conveyance when treated runoff needs to be conveyed to a desired location such as a downstream flow control facility, discharge location, or stormwater outfall. In Group C and D soils, an underdrain pipe helps ensure free flow of the treated runoff through the MFD mix bed. In some Group A and B soils, an underdrain pipe may not be necessary if most water percolates into subsoil from the underdrain trench. Evaluate the need for underdrain pipe in all cases. The PEO may eliminate the gravel underdrain trench if flows can be conveyed laterally to an adjacent ditch or onto a fill slope that is properly vegetated to protect against erosion (MFD Type 3 and Type 5). Keep the MFD mix free draining up to the 50-year storm event water surface elevation represented in the downstream ditch.
The length of the MFD (Type 1 - Type 3) is the same as the length of the contributing pavement.
The length of the MFD (Type 4 - Type 7) depends on the sizing procedures. (See the Design Method section below.)
The surface of the MFD (Type 1 - Type 3) should have a lateral slope less than 4H:1V (<25%). On steeper terrain, it may be possible to construct terraces to create a 4H:1V slope, or other engineering may be employed to ensure slope stability up to 3H:1V.
The surface of the MFD (Type 4 - Type 7) should have a lateral slope less than 8H:1V (<12.5%).
For MFD (Type 1 - Type 3), the resultant slope from the contributing area should be less than or equal to 9.4%, calculated using the equation below:
SCFS = resultant slope of the lateral and longitudinal slopes (%)
e = lateral slope (superelevation) (%)
G = longitudinal slope (grade) (%)
The MFD mix consists of the amendments listed in Table 4 - 19: Media Filter Drain Mix. Mixing and transportation must occur in a manner that ensures the materials are thoroughly mixed prior to placement and that separation does not occur during transportation or construction operations.
These materials should be used in accordance with the following WSDOT Standard Specifications:
Gravel Backfill for Drains - 9-03.12(4)
Construction Geotextile for Underground Drainage, Moderate survivability, drainage class A, nonwoven - 9-33.1
Crushed Surfacing Base Course (CSBC) - 9-03.9(3)
If the MFD is configured to allow the treated runoff to flow laterally into a ditch (MFD Type 3 and MFD Type 5), the crushed surfacing base course below the MFD should conform to WSDOT Standard Specification 9-03.9(3).
13.6.4.3.1 Media Filter Drain Mix Bed Sizing Procedure for MFD Type 1 - Type 3
The width of the MFD mix bed is determined by the amount of contributing pavement routed to the embankment. The surface area of the MFD mix bed needs to be sufficiently large to fully infiltrate and filter the stormwater treatment design flowrate using the long-term filtration rate of the MFD mix. For design purposes, incorporate a 50% safety factor into the long-term MFD mix filtration rate to accommodate variations in slope, resulting in a design filtration rate of 10 inches per hour. The MFD mix bed should have a bottom width of at least 2 feet in contact with the conveyance system below the MFD mix.
The MFD mix bed should be a minimum of 12 inches deep, including the section on top of the underdrain trench.
For stormwater treatment, base the sizing of the MFD mix bed on the requirement that the stormwater treatment flowrate from the pavement area, QHighway, cannot exceed the long-term infiltration capacity of the MFD, QInfiltration:
For western Washington, QHighway is the water quality design flowrate calculated by an Ecology approved continuous simulation model, assuming a 15-minute timestep.
Base the long-term infiltration capacity of the MFD on the following equation:
LTIR = Long-term infiltration rate of the media filter drain mix
(use 10 inches per hour for design) (in/hr)
L = Length of media filter drain (parallel to roadway) (ft)
W = Width of the media filter drain mix bed (ft)
C = Conversion factor of 43200 ((in/hr)/(ft/sec))
SF = Safety Factor (equal to 1.0, unless unusually heavy sediment loading is expected)
Assuming that the length of the MFD is the same as the length of the contributing pavement, solve for the width of the media filter drain:
Western Washington project applications of this design procedure have shown that, in almost every case, the calculated widths of the MFD Type 1 and Type 3 do not exceed 1.0 foot. Therefore, Table 4 - 18: Western Washington Design Widths for Media Filter Drains (Type 1 and Type 3) was developed to simplify the design steps; use it to establish an appropriate width.
13.6.4.3.2 Media Filter Drain Mix Bed Sizing Procedure for MFD Type 4 and Type 5
The length (perpendicular to the direction of flow) and width (parallel to the direction of flow) of the MFD mix bed (Type 4 and Type 5) is determined by many factors. The design procedure is outlined below:
Determine the total tributary pervious and impervious area (ft2) and flowrate (cfs) that will be sent to the MFD.
For MFD Type 4 and Type 5, divide the tributary area determined in Step 1 above by the “pavement area to MFD media area” ratio of 19.5. This determines the area of MFD needed, and applies to on-line and off-line Type 4 and Type 5 MFDs.
From A300 - Flow Spreading Devices, choose Option F (slotted flow dispersal pipe) or Option G (perforated pipe in a gravel-backfilled trench with notched grade board) as the redispersal/flow spreader structure type to be used upstream of the MFD. For on-line Type 4 and Type 5 MFDs, the number of flow spreaders and the flow spreader mounding analysis (Option F) is based on the full 100-year rate from the tributary area coming to the MFD. For off-line Type 4 and Type 5 MFDs, the number of flow spreaders and the flow spreader mounding analysis (Option F) is based on the water quality design flowrate.
Determine the length (perpendicular to the direction of flow) and width (parallel to the direction of flow) of the MFD mix bed by the following:
The flow spreader length shall be between 50 feet and 200 feet. The number of flow spreaders and their lengths are calculated based on the criteria in Step 3 above.
The width of the MFD mix bed = (flow spreader length)/5 for flow spreader lengths of 50 feet to 100 feet.
The width of the MFD mix bed = 20 feet for flow spreader lengths of 101 feet to 200 feet.
Check to make sure the total area of MFD mix bed(s) calculated in (4) is greater than or equal to the area determined in (2) above.
13.6.4.3.3 Media Filter Drain Mix Bed Sizing Procedure for MFD Type 6 and Type 7
MFD Type 6 and Type 7 are designed as on-line BMPs only. The design procedure is outlined below:
From A300 - Flow Spreading Devices, choose Option F (slotted flow dispersal pipe) or Option G (perforated pipe in a gravel-backfilled trench with notched grade board) as the redispersal/flow spreader structure type to be used upstream of the MFD. The number of flow spreaders and the flow spreader mounding analysis (if using Option F) shall be based on the 100-year release rate from the detention BMP (MGSFlood, 15-minute time steps). Determine the length of each flow spreader.
Determine the MFD mix bed area (L x W) using the long-term infiltration capacity of the MFD based on (Equation 4 - 19), with the following clarifications:
LTIR = Long-term infiltration rate of the media filter drain mix (use 10 inches per hour for design) (in/hr)
L = Length of media filter drain (parallel to spreader) (ft)
W = Width of the media filter drain mix bed (ft) measured parallel to the flow
C = Conversion factor of 43200 ((in/hr)/(ft/sec))
SF = Safety Factor (equal to 2.0)
Q2year = 2-year release rate (determined from an Ecology approved continuous simulation model, assuming 15-minute time steps) from the detention facility (ft3/sec)
The number of flow spreaders and length of each flow spreader was determined in Step 1. The length of the flow spreader(s) is equal to the length of the MFD. The width of the MFD follows the same ratios stated in Steps 4b and 4c of the MFD Type 4 and Type 5 design. Determine the total MFD mix bed length (L) and width (W). Check to make sure the calculated MFD mix bed area (L x W) is greater than or equal to the MFD mix bed area calculated in Step 2.
Underdrain pipe can provide a protective measure to ensure free flow through the MFD mix and is sized similar to stormwater conveyance systems. For MFD underdrain sizing, an additional step is required to determine the flowrate that can reach the underdrain pipe. This is done by comparing the contributing basin flowrate to the infiltration flowrate through the MFD mix and then using the smaller of the two to size the underdrain. The analysis described below considers the flowrate per foot of MFD, which allows the flexibility of incrementally increasing the underdrain diameter where long lengths of underdrain are required. When underdrain pipe connects to a stormwater conveyance system, place the invert of the underdrain pipe above the 25-year water surface elevation in the stormwater conveyance system to prevent backflow into the underdrain system.
Figure 4 - 27: Media Filter Drain Underdrain Installation
The following describes the procedure for sizing underdrains in a MFD Type 1, 2, 4, and 6.
Calculate the flowrate per foot from the contributing basin to the MFD.
= contributing flowrate per foot (cfs/ft)
LMFD = length of MFD contributing runoff to the underdrain (ft)
Calculate the MFD flowrate of runoff per foot given an infiltration rate of 10 in/hr through the MFD mix
= flowrate of runoff through MFD mix layer (cfs/ft)
W = width of underdrain trench (ft); the minimum width is 2 ft
f = infiltration rate though the MFD mix (in/hr) = 10 in/hr
Size the underdrain pipe to convey the runoff that can reach the underdrain trench. This is taken to be the smaller of the contributing basin flowrate or the flowrate through the MFD mix layer.
= underdrain design flowrate per foot (cfs/ft)
Determine the underdrain design flowrate using the length of the MFD and a factor of safety of 1.2.
QUD = estimated flowrate to the underdrain (cfs)
W = width of the underdrain trench (ft); the minimum width is 2 ft
LMFD = length of MFD contributing runoff to the underdrain (ft)
Given the underdrain design flowrate, determine the underdrain diameter. Round pipe diameters to the nearest standard pipe size and have a minimum diameter of 6 inches.
D = underdrain pipe diameter (inches)
Table 4 - 19: Media Filter Drain Mix Mineral aggregate shall meet all requirements for the WSDOT Standard Specifications 9-03.4 Aggregate for Bituminous Surface Treatment - Crushed screenings 3/8-inch to No.4 with the exception of: The fracture requirement shall be at least two fractured faces and will apply to material retained on the U.S. No. 4 sieve in accordance with FOP for AASHTO T 335.
Landscape the grass strip the same as BMP T1010 - Basic Biofiltration Swale.
13.6.4.6 Construction Criteria
Keep effective erosion and sediment control measures in place until grass strip is established. Do not allow vehicles or traffic on the MFD, to minimize rutting and maintenance repairs.
If MFD is in a critical aquifer recharge area for drinking water supplies, provide signage prohibiting the use of pesticides.
Provide a stormwater facility sign. Sign shall conform to A800 - Signage.