2021 Kitsap County Stormwater Design Manual
2021 Kitsap County
Stormwater Design Manual

5.4.20 Media Filter Drains

5.4.20.1 BMP Description

The media filter drain (MFD, BMP T8.40 in Volume V, Chapter 6 of the Ecology Manual), previously referred to as the ecology embankment, is a linear flow-through stormwater runoff treatment device that can be sited along highway side slopes (conventional design) and medians (dual media filter drains), borrow ditches, or other linear depressions. Cut-slope applications may also be considered. MFDs can be used where available right of way is limited, sheet flow from the highway surface is feasible, and lateral gradients are generally less than 25% (4H:1V). MFDs have a General Use Level Designation (GULD) for basic, enhanced, and phosphorus treatment (see Volume V, Chapter 10 of the Ecology Manual for more information about the TAPE approval process). Updates/changes to the use-level designation and any design changes will be posted in the Post-publication Updates section of the Washington State Department of Transportation’s Highway Runoff Manual Resource web page.

MFDs have four basic components: a gravel no-vegetation zone, a grass strip, the MFD mix bed, and a conveyance system for flows leaving the MFD mix. This conveyance system usually consists of a gravel-filled underdrain trench or a layer of crushed surfacing base course (CSBC). This layer of CSBC must be porous enough to allow treated flows to freely drain away from the MFD mix.

5.4.20.2 Performance Mechanism

MFDs remove suspended solids, phosphorus, and metals from highway runoff through physical straining, ion exchange, carbonate precipitation, and biofiltration.

Runoff flowing through a MFD goes through the following treatment steps:

  1. Stormwater runoff enters the MFD and is conveyed via sheet flow over a vegetation-free gravel zone to ensure sheet dispersion and provide some pollutant trapping.

  2. Next, a grass strip, which may be amended with composted material, is incorporated into the top of the fill slope to provide pretreatment, further enhancing filtration and extending the life of the system.

  3. The runoff is then filtered through a bed of porous, alkalinity-generating granular medium—the MFD mix. MFD mix is a fill material composed of crushed rock (sized by screening), dolomite, gypsum, and perlite. The dolomite and gypsum additives serve to buffer acidic pH conditions and exchange light metals for heavy metals. Perlite is incorporated to improve moisture retention, which is critical for the formation of biomass epilithic biofilm to assist in the removal of solids, metals, and nutrients.

  4. Treated water drains from the MFD mix bed into the conveyance system below the MFD mix. Geotextile lines the underside of the MFD mix bed and the conveyance system.

The underdrain trench is an option for hydraulic conveyance of treated stormwater to a desired location, such as a downstream Flow Control BMP or stormwater outfall. The trench’s perforated underdrain pipe is a protective measure to ensure free flow through the media filter drain mix and to prevent prolonged ponding. 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.

5.4.20.3 Application and Limitations

In many instances, conventional runoff treatment is not feasible due to right of way constraints (such as adjoining wetlands and geotechnical considerations). The MFD and the dual MFD designs are runoff treatment options that can be sited in most right of way confined situations. In many cases, a MFD or a dual MFD can be sited without the acquisition of additional right of way needed for conventional stormwater BMPs or capital-intensive expenditures for underground wetvaults.

MFD BMPs can be designed to meet basic and enhanced runoff treatment requirements (see Table II-5.23). See additional detailed guidelines on MFD applications and limitations in BMP T8.40 in Volume V, Chapter 6 of the Ecology Manual.

Table II-5.23. MFD Applicability.
BMP

MR #5: On-site Stormwater Management

MR #6: Runoff Treatment

MR #7: Flow Control

List

LID Performance Standard

Basic

Enhanced

Oil Control

Phosphorus

Media Filter Drain

  

 

X

X

  

 

  

5.4.20.4 Site Considerations

Since maintaining sheet flow across the MFD is required for its proper function, the ideal locations for MFDs in highway settings are highway side slopes or other long, linear grades with lateral side slopes less than 4H:1V and longitudinal slopes no steeper than 5%. 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. The longest flow path from the contributing area delivering sheet flow to the MFD shall not exceed 150 feet.

If there is sufficient roadway embankment width, the designer shall consider placing the grass strip and media mix downslope when feasible. The project office shall ensure the MFD does not intercept seeps, springs, or ground water.

The dual MFD is fundamentally the same as the side-slope version. It differs in siting and is more constrained with regard to drainage options. Prime locations for dual MFDs in a highway setting are medians, roadside drainage or borrow ditches, or other linear depressions. It is especially critical for water to sheet flow across the dual MFD. Channelized flows or ditch flows running down the middle of the dual MFD (continuous offsite inflow) shall be minimized.

Additional site considerations include:

  • Steep slopes – Avoid construction on longitudinal slopes steeper than 5%. Avoid construction on 3H:1V lateral slopes, and preferably use less than 4H:1V slopes. In areas where lateral slopes exceed 4H:1V, it may be possible to construct terraces to create 4H:1V slopes or to otherwise stabilize up to 3H:1V slopes. (For details, see Geometry, Components and Sizing Criteria, Cross Section in the Structural Design Considerations section below).

  • Wetlands – Do not construct in wetlands and wetland buffers. In many cases, a MFD (due to its small lateral footprint) can fit within the highway fill slopes adjacent to a wetland buffer. In those situations where the highway fill prism is located adjacent to wetlands, an interception trench/underdrain will need to be incorporated as a design element in the MFD.

  • Shallow ground water – Mean high water table levels at the project site need to be determined to ensure the MFD mix bed and the underdrain (if needed) will not become saturated by shallow ground water.

  • Unstable slopes – In areas where slope stability may be problematic, consult a geotechnical engineer.

  • Areas of seasonal ground water inundations or basement flooding: Site-specific piezometer data may be needed in areas of suspected seasonal high ground water inundations. The performance of the dual media filter drain may be compromised due to backwater effects and lack of sufficient hydraulic gradient.

  • Narrow roadway shoulders – In areas where there is a narrow roadway shoulder that does not allow enough room for a vehicle to fully stop or park, consider placing the MFD farther down the embankment slope. This will reduce the amount of rutting in the MFD and decrease overall maintenance repairs.

5.4.20.5 Design Information

Refer to BMP T8.40 in Volume V, Chapter 6 of the Ecology Manual for detailed design guidance and criteria on MFDs.

5.4.20.6 Minimum Construction Requirements

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.

5.4.20.7 Operations and Maintenance

Refer to BMP T8.40 in Volume V, Chapter 6 and Volume V, Appendix V‑A of the Ecology Manual for required maintenance practices for MFDs.