BMP T5.30: Full Dispersion

Purpose and Definition

This BMP allows for "fully dispersing" runoff from impervious surfaces and cleared areas of development sites that protect at least 65% of the site (or a threshold discharge area on the site) in a forest or native condition.

Applications and Limitations

• Rural single family residential developments should use these dispersion BMPs wherever possible to minimize effective impervious surface to less than 10% of the development site.

• Other types of development that retain 65% of the site (or a threshold discharge area on the site) in a forested or native condition may also use these BMPs to avoid triggering the flow control facility requirement.

• The preserved area may be a previously cleared area that has been replanted in accordance with native vegetation landscape specifications described within this BMP.

• The preserved area should be situated to minimize the clearing of existing forest cover, to maximize the preservation of wetlands (though the wetland area and any streams and lakes do not count toward the 65% forest or native condition area), and to buffer stream corridors.

• The preserved area should be placed in a separate tract or protected through recorded easements for individual lots.

• The preserved area should be shown on all property maps and should be clearly marked during clearing and construction on the site.

• All trees within the preserved area at the time of permit application shall be retained, aside from approved timber harvest activities regulated under WAC Title 222, except for Class IV General Forest Practices that are conversions from timberland to other uses, and the removal of dangerous or diseased trees.

• The preserved area may be used for passive recreation and related facilities, including pedestrian and bicycle trails, nature viewing areas, fishing and camping areas, and other similar activities that do not require permanent structures, provided that cleared areas and areas of compacted soil associated with these areas and facilities do not exceed eight percent of the preserved area.

• The preserved area may contain utilities and utility easements, but not septic systems. Utilities are defined as potable and wastewater underground piping, underground wiring, and power and telephone poles.

Minimum Design Requirements for Residential Projects

Developments that preserve 65% of a site (or a threshold discharge area of a site) in a forested or native condition, can disperse runoff from the developed portion of the site into the native vegetation area as long as the developed areas draining to the native vegetation do not have impervious areas that exceed 10% of the entire site.

Where a development has less than 65% of a site available to maintain or create into a forested or native condition, that area may still be used for full dispersion of a portion of the developed area. The ratio of the native vegetation area to the impervious area, which is dispersed into the native vegetation, must not be less than 65 to 10. The lawn and landscaping areas associated with the impervious areas may also be dispersed into the native vegetation area. The lawn and landscaped area must comply with BMP T5.13: Post-Construction Soil Quality and Depth. All design requirements listed also must be met.

The portion of the developed area which is not managed through full dispersion can be considered a separate project site. It must be evaluated against the thresholds in Figure I-2.4.1 Flow Chart for Determining Requirements for New Development and Figure I-2.4.2 Flow Chart for Determining Requirements for Redevelopment, whichever is appropriate, to determine the applicable minimum requirements.

Additional impervious areas above the 10% are allowed, but should not drain to the native vegetation area, and are subject to the thresholds, treatment and flow control requirements of this stormwater manual.

Within the context of this dispersion option, the impervious surfaces that are over and above the 10% maximum can be routed into an appropriately sized dry well or into an infiltration basin that meets the flow control standard and does not overflow into the forested or native vegetation area.

Runoff must be dispersed into the native area in accordance with one or more of the dispersion devices, and in accordance with the design criteria and limits for those devices, cited in this BMP. A native vegetation flow path of at least 100 feet in length (25 feet for sheet flow from a non-native pervious surface) must be available along the flowpath that runoff would follow upon discharge from a dispersion device cited in this BMP. The native vegetated flowpath must meet all of the following criteria:

• The flow path must be over native vegetated surface

• The flow path must be on-site or in an off-site tract or easement area reserved for such dispersion

• The slope of the flowpath must be no steeper than 15% for any 20-foot reach of the flowpath. Slopes up to 33% are allowed where level spreaders are located upstream of the dispersion area and at sites where vegetation can be established.

• The flowpath must be located between the dispersion device and any downstream drainage feature such as a pipe, ditch, stream, river, pond, lake, or wetland.

• The flowpaths for adjacent dispersion devices must be sufficiently spaced to prevent overlap of flows in the flowpath areas.

For sites with on-site sewage disposal systems, the discharge of runoff from dispersion devices must be located downslope of the primary and reserve drainfield areas. This requirement may be waived by the permitting jurisdiction if site topography clearly prevents discharged flows from intersecting the drainfield.

Dispersion devices are not allowed in critical area buffers or on slopes steeper than 20%. Dispersion devices proposed on slopes steeper than 15% or within 50 feet of a geologically hazardous area (RCW 36.70A.030(5)) must be approved by a geotechnical engineer or engineering geologist.

The dispersion of runoff must not create flooding or erosion impacts.

• Roof Downspouts

  Roof surfaces that comply with BMP T5.10A: Downspout Full Infiltration, are considered to be "fully infiltrated" (i.e., zero percent effective imperviousness). All other roof surfaces are considered to be "fully dispersed" (i.e., at or approaching zero percent effective imperviousness) only if they are within a threshold discharge area that is or will be more than 65% forested (or native vegetative cover) and less than 10% impervious (total), AND if they either: 1) comply with BMP T5.10B: Downspout Dispersion Systems, but with vegetated flow paths of 100 feet or more through the native vegetation preserved area; or 2) disperse the roof runoff along with the road runoff in accordance with the roadway dispersion BMP section below.

• Driveway Dispersion

  Driveway surfaces are considered to be "fully dispersed" if they are within a threshold discharge area that is or will be more than 65% forested (or native vegetative cover) and less than 10% impervious (total), AND if they either: 1) comply with BMP T5.11: Concentrated Flow Dispersion and BMP T5.12: Sheet Flow Dispersion - and have flow paths of 100 feet or more through native vegetation; or, 2) disperse driveway runoff along with the road runoff in accordance with the roadway dispersion BMP section below.

• Roadway Dispersion BMPs

  Roadway surfaces are considered to be "fully dispersed" if they are within a threshold discharge area that is or will be more than 65% forested (or native vegetative cover) and less than 10% impervious (total), AND if they comply with the following dispersion requirements:

  1. The road section shall be designed to minimize collection and concentration of roadway runoff. Sheet flow over roadway fill slopes (i.e., where roadway subgrade is above adjacent right-of-way) should be used wherever possible to avoid concentration.

  2. When it is necessary to collect and concentrate runoff from the roadway and adjacent upstream areas (e.g., in a ditch on a cut slope), concentrated flows shall be incrementally discharged from the ditch via cross culverts or at the ends of cut sections. These incremental discharges of newly concentrated flows shall not exceed 0.5 cfs at any one discharge point from a ditch for the 100-year runoff event. Where flows at a particular ditch discharge point were already concentrated under existing site conditions (e.g., in a natural channel that crosses the roadway alignment), the 0.5-cfs limit would be in addition to the existing concentrated peak flows.

  3. Ditch discharge points with up to 0.2 cfs discharge for the peak 100-year flow shall use rock pads or dispersion trenches to disperse flows. Ditch discharge points with between 0.2 and 0.5 cfs discharge for the 100-year peak flow shall use only dispersion trenches to disperse flows.

  4. Dispersion trenches shall be designed to accept surface flows (free discharge) from a pipe, culvert, or ditch end, shall be aligned perpendicular to the flowpath, and shall be minimum 2 feet by 2 feet in section, 50 feet in length, filled with 3/4-inch to 1 1/2-inch washed rock, and provided with a level notched grade board (see Figure V-5.3.2 Sheet Flow Dispersion for Driveways). Manifolds may be used to split flows up to 2 cfs discharge for the 100-year peak flow between up to 4 trenches. Dispersion trenches shall have a minimum spacing of 50 feet between centerlines.

  5. Flowpaths from adjacent discharge points must not intersect within the 100-foot flowpath lengths, and dispersed flow from a discharge point must not be intercepted by another discharge point. To enhance the flow control and water quality effects of dispersion, the flowpath shall not exceed 15% slope, and shall be located within designated open space.

     Note: Runoff may be conveyed to an area meeting these flowpath criteria.

  6. Ditch discharge points shall be located a minimum of 100 feet upgradient of steep slopes (i.e., slopes steeper than 40%), wetlands, and streams.

  7. Where the Local Plan Approval Authority determines there is a potential for significant adverse impacts downstream (e.g., erosive steep slopes or existing downstream drainage problems), dispersion of roadway runoff may not be allowed, or other measures may be required.

• Cleared Area Dispersion BMPs

  The runoff from cleared areas that are comprised of bare soil, non-native landscaping, lawn, and/or pasture of up to 25 feet in flow path length can be considered to be "fully dispersed" if it is dispersed through at least 25 feet of native vegetation in accordance with the following criteria:

  1. The topography of the non-native pervious surface must be such that runoff will not concentrate prior to discharge to the dispersal area.

  2. Slopes within the dispersal area should be no steeper than 15%.

  If the width of the non-native pervious surface is greater than 25 feet, the vegetated flowpath segment must be extended 1 foot for every 3 feet of width beyond 25 feet up to a maximum width of 250 feet.

Minimum Design Requirements for Public Road Projects

Applicability:

These criteria apply to the construction of public roads not within the context of residential, commercial, or industrial site development. They will likely only be implementable on roads outside of the urban growth areas where roadside areas are not planned for urban density development.

1. Uncollected or natural dispersion into adjacent vegetated areas (i.e., sheet flow into the dispersion area)

   Full dispersion credit (i.e., no other treatment or flow control required) for sites that meet the following criteria:

   1. Outwash soils: (Type A – sands and sandy gravels, possibly some Type B – loamy sands) that have an initial saturated hydraulic conductivity rate of 4 inches per hour or greater. The saturated hydraulic conductivity must be based on a Pilot Infiltration Test or the Soil Grain Size Analysis method as identified in Chapter III-3 - Flow Control Design, or another method as allowed by the local government.

      • Up to 20 feet of impervious flow path needs 10 feet of dispersion area width.

      • Each additional foot of impervious flow path needs 0.25 feet of dispersion area width.

   2. Other soils: (Types C and D and some Type B not meeting the criterion in 1a above)

      • Dispersion area must have 6.5 feet of width for every 1 foot width of impervious area draining to it. A minimum distance of 100 feet is necessary.

   3. Criteria applicable to all soil types:

      • Depth to the average annual maximum ground water elevation should be at least 3 feet.

      • Impervious surface flow path must be ≤ 75 ft. Pervious flow path must be ≤ 150 ft. Pervious flow paths are up-gradient road side slopes that run onto the road and down-gradient road side slopes that precede the dispersion area.

      • Lateral slope of impervious drainage area should be < 8%. Road side slopes must be < 25%. Road side slopes do not count as part of the dispersion area unless native vegetation is re-established and slopes are less than 15%. Road shoulders that are paved or graveled to withstand occasional vehicle loading count as impervious surface.

      • Longitudinal slope of road should be ≤ 5%.

      • Length of dispersion area should be equivalent to length of road.

      • Average longitudinal (parallel to road) slope of dispersion area should be ≤ 15%.

      • Average lateral slope of dispersion area should be ≤ 15%.

2. Channelized (collected and re-dispersed) stormwater into areas with (a) native vegetation or (b) cleared land in areas outside of Urban Growth Areas that do not have a natural or man-made drainage system.

   Full dispersion credit (i.e., no other treatment or flow control required) is given to projects that meet the following criteria:

   1. Outwash soils: (Type A – sands and sandy gravels, possibly some Type B – loamy sands) that have an initial saturated hydraulic conductivity rate of 4 inches per hour or greater. The saturated hydraulic conductivity must be based on field results using procedures (Pilot Infiltration Test or Soil Grain Size Analysis Method) identified in Chapter III-3 - Flow Control Design, or another method approved by the local government.

      • Dispersion area should be at least ½ of the impervious drainage area.

   2. Other soils: (Types C and D and some Type B not meeting the criterion in 2a above)

      • Dispersion area must have 6.5 feet of width for every 1 foot width of impervious area draining to it. A minimum distance of 100 feet is necessary.

   3. Other criteria applicable to all soil types:

      • Depth to the average annual maximum ground water elevation should be at least three feet.

      • Channelized flow must be re-dispersed to produce longest possible flow path.

      • Flows must be evenly dispersed across the dispersion area.

      • Flows must be dispersed using rock pads and dispersion techniques as specified under Roadway Dispersion BMPs.

      • Approved energy dissipation techniques may be used.

      • Limited to on-site (associated with the road) flows.

      • Length of dispersion area should be equivalent to length of the road.

      • Average longitudinal and lateral slopes of the dispersion area should be ≤ 8%.

      • The slope of any flowpath segment must be no steeper than 15% for any 20-foot reach of the flowpath segment.

3. Engineered dispersion of stormwater runoff into an area with engineered soils

   Full dispersion credit (i.e., no other treatment or flow control required) is given to projects that meet the following criteria:

   • Stormwater can be dispersed via sheet flow or via collection and re-dispersion in accordance with the techniques specified under Roadway Dispersion BMPs.

   • Depth to the average annual maximum ground water elevation should be at least three feet.

   • Type C and D soils must be compost-amended following guidelines in BMP T5.13: Post-Construction Soil Quality and Depth. The guidance document Guidelines and Resources for Implementing Soil Quality and Depth BMP T5.13 in WDOE Stormwater Management Manual for Western Washington can be used, or an approved equivalent soil quality and depth specification approved by Ecology. The guidance document is available at http://www.soilsforsalmon.org.

     • Dispersion area must meet the 65 to 10 ratio for full dispersion credit.

   • Type A and B soils that meet or exceed the 4 inches per hour initial saturated hydraulic conductivity rate minimum must be compost amended in accordance with guidelines in BMP T5.13: Post-Construction Soil Quality and Depth. Compost must be tilled into the soil in accordance with the guidance document cited above.

     • Up to 20 feet of impervious flow path needs 10 feet of dispersion area width.

     • Each additional foot of impervious flow path needs 0.25 feet of dispersion area width.

   • Average longitudinal (parallel to road) slope of dispersion area should be ≤ 15%.

   • Average lateral slope of dispersion area should be ≤ 15%.

   • The dispersion area should be planted with native trees and shrubs.

4. Other Characteristics for Dispersal areas

   • Dispersal areas must be outside of the urban growth area; or if inside the urban growth area, in legally protected areas (easements, conservation tracts, public parks).

   • If outside urban growth areas, legal agreements should be reached with property owners of dispersal areas subject to stormwater that has been collected and is being re-dispersed.

   • An agreement with the property owner is advised for uncollected, natural dispersion via sheet flow that represents a continuation of past practice. If not a continuation of past practice, an agreement should be reached with the property owner.

Native Vegetation Landscape Specifications

These specifications may be used in situations where an applicant wishes to convert a previously developed surface to a native vegetation landscape for purposes of meeting full dispersion requirements or code requirements for forest retention. Native vegetation landscape is intended to have the soil, vegetation, and runoff characteristics approaching that of natural forestland.

Conversion of a developed surface to native vegetation landscape requires the removal of impervious surface, de-compaction of soils, and the planting of native trees, shrubs, and ground cover in compost-amended soil according to all of the following specifications:

1. Existing impervious surface and any underlying base course (e.g., crushed rock, gravel) must be completely removed from the conversion area(s).

2. Underlying soils must be broken up to a depth of 18 inches. This can be accomplished by excavation or ripping with either a backhoe equipped with a bucket with teeth, or a ripper towed behind a tractor.

3. At least 4 inches of well-decomposed compost must be tilled into the broken up soil as deeply as possible. The finished surface should be gently undulating and must be only lightly compacted.

4. The area of native vegetated landscape must be planted with native species trees, shrubs, and ground cover. Species must be selected as appropriate for site shade and moisture conditions, and in accordance with the following requirements:

   1. Trees: a minimum of two species of trees must be planted, one of which is a conifer. Conifer and other tree species must cover the entire landscape area at a spacing recommended by a professional landscaper or in accordance with local requirements.

   2. Shrubs: a minimum of two species of shrubs should be planted. Space plants to cover the entire landscape area, excluding points where trees are planted.

   3. Groundcover: a minimum of two species of ground cover should be planted. Space plants so as to cover the entire landscape area, excluding points where trees or shrubs are planted.

   Note: for landscape areas larger than 10,000 square feet, planting a greater variety of species than the minimum suggested above is strongly encouraged. For example, an acre could easily accommodate three tree species, three species of shrubs, and two or three species of groundcover.

5. At least 4 inches of hog fuel or other suitable mulch must be placed between plants as mulch for weed control. It is also possible to mulch the entire area before planting; however, an 18-inch diameter circle must be cleared for each plant when it is planted in the underlying amended soil. Note: plants and their root systems that come in contact with hog fuel or raw bark have a poor chance of survival.

6. Plantings must be watered consistently once per week during the dry season for the first two years.

7. The plantings must be well established on at least 90% of the converted area. A minimum of 90% plant survival is required after 3 years.

Conversion of an area that was under cultivation to native vegetation landscape requires a different treatment. Elimination of cultivated plants, grasses and weeds is required before planting and will be required on an on-going basis until native plants are well-established. The soil should be tilled to a depth of 18 inches. A minimum of 8 inches of soil having an organic content of 6 to 12 percent is required, or a four inch layer of compost may be placed on the surface before planting, or 4 inches of clean wood chips may be tilled into the soil, as recommended by a landscape architect or forester. After soil preparation is complete, continue with steps 4 through 7 above. Placing 4 inches of compost on the surface may be substituted for the hog fuel or mulch. For large areas where frequent watering is not practical, bare-root stock may be substituted at a variable spacing from 10 to 12 feet o.c. (with an average of 360 trees per acre) to allow for natural groupings and 4 to 6 feet o.c. for shrubs. Allowable bare-root stock types are 1-1, 2-1, P-1 and P-2. Live stakes at 4 feet o.c. may be substituted for willow and red-osier dogwood in wet areas.

Runoff Model Representation

Areas that are fully dispersed do not use the WWHM or other approved continuous runoff models.