Non-Proprietary BMPs

Dry swales are essentially bioretention cells that are shallower, configured as linear channels, and covered with turf or other surface material (other than mulch and plants).

The dry swale is a soil filter system that temporarily stores and then filters the desired Treatment Volume (TV). Dry swales rely on a pre-mixed soil media filter below the channel that is similar to that used for bioretention. If soils are extremely permeable, runoff infiltrates into underlying soils. In most cases, however, the runoff treated by the soil media flows into an underdrain, which conveys treated runoff back to the conveyance system further downstream. The underdrain system consists of a perforated pipe within a gravel layer on the bottom of the swale. Dry swales may appear as simple grass channels with the same shape and turf cover, while others may have more elaborate landscaping. Swales can be planted with turf grass, tall meadow grasses, decorative herbaceous cover, or trees. The primary pollutant removal mechanisms operating in swales are settling, filtering infiltration and plant uptake. The overall stormwater functions of the dry swale are summarized in Table 1.

Table 1: Summary of Stormwater Functions Provided by Dry Swales
Stormwater Function	Level 1 Design	Level 2 Design
Annual Runoff Reduction	40%	60%
Total Phosphorus Removal 1	20%	40%
Total Nitrogen Removal 1	25%	35%
Channel Protection	Use RRM Design Spreadsheet to calculate CN Adjustment OR Design extra storage (optional; as needed) on the surface, in the engineered soil matrix, and in stone/underdrain layer to accommodate larger storm, and use NRCS TR-55 Runoff Equations2 to compute CN Adjustment.
Flood Mitigation	Partial. Reduced Curve Numbers and Time of Concentration
1 Change in event mean concentration (EMC) through the practice. Actual nutrient mass load removed is the product of the removal rate and the runoff reduction rate. 2 NRCS TR-55 Runoff Equations 2-1 thru 2-5 and Figure 2-1 can be used to compute a curve number adjustment for larger storm events based on the retention storage provided by the practice(s). Sources: CWP and CSN (2008), CWP, 2007

Both the Dry Conveyance Swale and the Dry Treatment Swale can be configured as Level 1 and Level 2 (see Table 2). The difference is the typical drainage area of a Conveyance Swale is impervious with an adjacent vegetated filter strip serving as pre-treatment.

Table 2 - Dry Swale Design Criteria
Level 1 Design (RR:40; TP:20; TN:25)	Level 2 Design (RR:60; TP:40; TN: 35)
Sizing (Sec. 5.1): Surface Area (ft2) = (Tv– volume reduced by upstream BMP) /Storage depth1	Sizing (Sec. 5.1): Surface Area (ft2) = {(1.1)(Tv) – volume reduced by upstream BMP }/Storage Depth1
Effective swale slope ≤ 2.0%	Effective swale slope ≤ 1.0 %
Media depth minimum = 18 inches; Recommended maximum = 36”	Media depth minimum = 24 inches Recommended maximum = 36”
Sub-soil testing (Sec. 5.2): not needed if underdrain used; Min infiltration rate > 0.5 inch/hour to remove underdrain requirement;	Sub-soil testing (Sec. 5.2): one per 200 lf of filter surface; Min infiltration rate > 0.5 inch/hour to remove underdrain requirement
Underdrain (Sec. 5.7) = Schedule 40 PVC with clean-outs	Underdrain & Underground Storage Layer (Sec. 5.7) = Schedule 40 PVC with clean outs, and a minimum 12” stone sump below invert OR none if soil infiltration requirements are met (Sec. 5.2)
Media (Sec. 5.6) supplied by vendor; tested for acceptable phosphorus index: P-Index between 10 and 30, OR Between 7 and 23 mg/kg of P in the soil media2
Inflow = sheet flow with required vegetated filter strip minimum 10 ft.
On-line design	Off-line or multiple treatment cells
Turf cover	Turf cover, with trees and shrubs
All Designs: acceptable media mix tested for phosphorus index – Section
1 Storage depth is the sum of the Void Ratio (Vr) of the soil media and gravel layers times their respective depths, plus the surface ponding depth. Refer to Section 5.1 2 Refer to Design Specification No. 9: Bioretention for soil specifications

Dry swales can be implemented on a variety of development sites where density and topography permit their application. Some key feasibility issues for dry swales include the following:

Available Space: Dry swale footprints can fit into relatively narrow corridors between utilities, roads, parking, or other site constraints, and are approximately 3 to 10% of the size of the contributing drainage area, depending on the amount of impervious cover.

Site Topography: Dry swales should be used on sites with longitudinal slopes of less than 4%., and preferably less than 2%. Check dams can be used to reduce the effective slope of the swale and lengthen the contact time to encourage filtering and/or infiltration. Steeper slopes adjacent to the swale may generate rapid runoff velocities into the swale that may carry a high sediment loading (Refer to pre-treatment requirements in Section X).

Available Head: A minimum amount of head is needed to implement dry swales, as measured as the elevation difference from the inflow point and the downstream storm drain invert. Dry swales typically require 3 to 5 feet of hydraulic head since they need both a filter bed and underdrain.

Hydraulic Capacity. Dry swales are an on-line practice and must be designedwith enough capacity to convey runoff from the 2-year and 10-year storm at non-erosive velocity, and maintain the 10-year flow within the swale. This means that the size of the swale’s surface dimensions are often driven more by the need to pass the 10-year storm events, which can be a constraint in the siting of Dry Conveyance Swales within an existing right of way (i.e., constrained by sidewalks).

Depth to Water Table: Designers should ensure that the bottom of the dry swale is at least 2 feet above the seasonally high water table to ensure groundwater does not intersect the filter bed, as this could lead to groundwater contamination or practice failure.

Soils: Soil conditions do not constrain the use of dry swales although they normally determine whether an underdrain is needed. Low-permeability soils with an infiltration rate of less than 0.5 inches per hour, such as those classified in Hydrologic Soil Groups (HSG) C and D will require an underdrain. Designers must verify site-specific soil permeability at the proposed location using the methods for on-site soil investigation presented in Appendix A of VA DCR Stormwater Design Specification No. 8 in order to eliminate the requirements for an underdrain.

Community Acceptance: The main concerns of adjacent residents are perceptions that swales will create nuisance conditions or will be hard to maintain. Common concerns include the continued ability to mow grass, landscape preferences, weeds, standing water, and mosquitoes. Dry swales are a much better alternative in that these concerns can be fully addressed through the design process and proper on-going operation and routine maintenance. If dry swales are installed on private lots, homeowners will need to be educated on their routine maintenance needs, understand the long-term maintenance plan, and be subject to a legally binding maintenance agreement (see Section 7). The short ponding time of 6 hours is much less than the time required for one mosquito breeding cycle, so well maintained dry swales should not create mosquito problems or be difficult to mow. The local government my require that dry swales be placed in a drainage or maintenance easement in order to ensure long term maintenance.

The linear nature of dry swales makes them well-suited to treat highway or low and medium density residential road runoff, if there is an adequate right-of-way width and distance between driveways. Typical applications of Dry Conveyance Swales include:

Sizing of the surface area (SA) for Dry Swales is based on the computed treatment volume (Tv) of the contributing drainage area and the storage provided within the swale media and gravel layers, and behind check dams. The required surface area, in square feet, is computed as the treatment volume in cubic feet divided by the equivalent storage depth in feet. The equivalent storage depth is computed as the depth of media, gravel, and surface ponding (in feet) multiplied by the accepted void ratio.

Dry Swale Soil Media Vr = 0.20
Gravel Vr = 0.40
Surface Storage behind check dams Vr = 1.0

The equivalent storage depth for Level 1 (without considering surface ponding) is therefore computed as

The effective storage depths will vary according to the actual design depths of the soil media and gravel layer.

Where:
SA= Minimum surface area of Dry Swale (ft2)
Tv = Treatment Volume (ft3) = [(1.0”)(Rv)(A)/12]

1 The volume reduced by upstream Runoff Reduction BMPs is supplemented with the anticipated volume of storage created by check dams along the swale length

The final Dry Swale design geometry will be determined by dividing the SA by the swale length to compute the required width; or by dividing the SA by the desired width and computing the required length.

In order to accommodate a greater quantity credit for channel protection or flooding control, designers may be able to create additional surface storage by expanding the surface ponding behind the check dams by either increasing the number of check dams, or by expanding the swale width at select areas. However, the expanded surface storage footprint is limited to the ponding area directly behind the check dam, and is limited to twice the channel bottom width. Care must be taken to ensure that the check dams are properly trenched into the side slopes of the swale, and adequate overflow capacity is provided over the weir.

The second key sizing decision is to measure the infiltration rate of subsoils below the dry swale area to determine if an underdrain will be needed. The infiltration rate of subsoils must exceed 0.5” per hour to dispense with an underdrain. The acceptable methods for on-site soil infiltration rate testing procedures are outlined in Appendix A of Bay-wide Stormwater Design Specification No. 8. A soil test should be conducted for every 200 linear feet of dry swale.

Design guidance regarding the geometry and layout of dry swales is provided below:

Side Slopes: The side slopes of dry swales should be no steeper than 3:1 for maintenance considerations (mowing). Flatter slopes are encouraged where adequate space is available to aid in providing pretreatment for sheet flows entering the swale. Swales should have a bottom width ranging from 4 to 8 feet to ensure an adequate surface area exists along the bottom of the swale for filtering. If a swale will be wider than 8 feet, designers should incorporate berms, check dams, level spreaders or multi-level cross sections to prevent braiding and erosion within the swale bottom.

Swale Longitudinal Slope: The slope of the swale should be moderately flat to permit the temporary ponding of the water quality or runoff reduction volume within the channel. An effective swale slope less than or equal to 2% for Level 1, and less than or equal to 1% for Level 2 is recommended, though slopes up to 4% are acceptable if check dams are used. Dry swales with a longitudinal slope less than 1% may be restricted by the locality. The minimum recommended slope for an on-line dry swale is 0.5%. An off-line dry swale may be designed at less than 0.5% and function similar to bioretention, although this option may be limited by the locality. Refer to Table 3 for check dam spacing based on the swale longitudinal slope.

Table 3: Typical Check Dam (CD) Spacing for Achieving Effective Swale Slope
	Level 1	Level 2
Swale Longitudinal Slope	Spacing1 of 12” (max) Checkdams to Create an Effective Slope of 2%	Spacing1 of 12” (max) Checkdams to Create an Effective Slope of 0 to 1%
0.5%	–	200 ft. to –
1.0%	–	100 ft. to –
1.5%	–	67 ft. to 200 ft.
2.0%	–	50 ft. to 100 ft.
2.5%	200 ft.	40 ft. to 67 ft.
3.0%	100 ft.	33 ft. to 50 ft.
3.5%	67 ft.	30 ft. to 40 ft.
4.0%	50 ft.	25 ft. to 33 ft.
4.5%2	40 ft.	20 ft. to 30 ft.
5.0%2	40 ft.	20 ft. to 30 ft.

Drawdown: Dry swales should be designed so that the desired treatment volume is completely filtered within 6 hours or less. This drawdown time can be achieved by using a the soil media mix specified in Section 5.6 and an underdrain along the bottom of the swale, or adequately permeable native soils verified through testing (Section 5.2).

Underdrain: Underdrains are provided in dry swales to ensure they drain properly after storms. The underdrain should have a minimum diameter of 6 inches and be encased in a foot deep gravel bed. Two layers of stone should be used. A choker stone layer should be installed immediately below the filter media that is at least 3 inches deep, consisting of #8 or #78 stone. Below the choker stone layer, the main underdrain layer should be at least 12 inches and composed on one-inch double washed stone. The underdrain pipe should be set at least 4 inches above the bottom of the stone layer.

Pretreatment for a Dry Conveyance Swale is in the form of a required grass filter strip (10 ft. minimum) along the length of the contributing impervious cover. Pretreatment for a Dry Treatment Swale is required at the inflow points along the length of the dry swale to trap coarse sediment particles before they reach the filter bed to prevent premature clogging. Several pretreatment measures are feasible, depending on whether the specific location in the dry swale system will be receiving sheet flow, shallow concentrated flow or concentrated flow:

Initial Sediment Forebay(channel flow):. This grass cell is located at the upper end of the dry swale segment with a storage volume equivalent to at least 15% of the total treatment volume and is designed with a 2:1 length to width ratio.
Check dams(channel flow): These energy dissipation techniques are acceptable as pre-treatment on small swales with a drainage area of less than 1 acre.
Tree Check dams(channel flow): These are street tree mounds that are placed within the bottom of dry swale up to an elevation of 9 to 12 inches above the channel invert. One side has a gravel or river stone bypass to allow storm runoff to percolate through.
Grass Filter Strip (sheet flow): Grass filter strips extend a minimum of 10 feet from edge of pavement to the swale.
Gravel Diaphragm (sheet flow): A gravel diaphragm at the end of pavement should run perpendicular to the flow path to promote settling.
Pea Gravel Flow Spreader (sheetflow): This extends along the top of each bank to pretreat lateral runoff from the road shoulder to the swale and involves a 2 to 4 inch drop from a hard-edged surface into a gravel or stone diaphragm

Dry swales should be designed with dimensions (bottom width) and slope such that a velocity of 3 feet per second will not be exceeded for the one-inch rainfall event. Check dams may be used to obtain the needed runoff reduction volume, as well as serve to reduce the flow velocities (Refer to Grass Swale for additional guidance on channel design). The check dams should be spaced based on the channel slope and ponding requirements per Table 3.

The swale should also convey the locally required design storm (usually the 2 and 10-year storm) at non-erosive velocities with at least 3 inches of freeboard. The analysis should evaluate the flow profile through the channel at normal depth, as well as the flow depth over top of the check dams. Refer to Design Specifcation No. 3 Grass Channels for design criteria for maximum velocities and depth of flow.

Dry swales may be designed as off-line systems with a flow splitter or diversion to divert runoff in excess of the design capacity to an adjacent conveyance system. Or, strategically placed overflow inlets may be placed along the length of the swale to periodically pick up water and reduce the hydraulic loading at the downstream limits.

Dry swales require replacement of native soils with a prepared soil media. The soil media provides adequate drainage, supports plant growth and facilitates pollutant removal within the dry swale. At least 18 inches of soil media should be added above the choker layer to create an acceptable filter. The recipe for the soil media is identical to that used for bioretention and is provided in Table 4 (Refer to Design Specification No. 9 Bioretention for additional soil specification information). The soil media should be obtained from an approved vendor to create a consistent, homogeneous fill media. One design adaptation is to utilize 100% sand for the first 18 inches of the filter and add a combination of topsoil and leaf compost for the top 4 inches where turf cover will be maintained.

Some Level 2 designs will not use an underdrain (where soil infiltration rates meet minimum standards; see Section 5.2 and Section 2 design tables). For Level 2 designs with an underdrain, an underground storage layer of a minimum of 12 inches of stone should be incorporated below the invert of the underdrain. The depth of the storage layer will depend on the target treatment and storage volumes needed to meet water quality, channel protection, and/or flood protection criteria. However, the bottom of the storage layer must be at least 2 feet above the seasonally high water table. The storage layer should consist of clean, washed #57 stone or an approved infiltration module.

Dry Swales should include observation wells along the length of the swale with cleanout pipes if their contributing drainage area exceeds 1 acre. The wells should be tied into any T’s or Y’s in the underdrain system, and should extend upwards to be flush with surface, with a vented cap.

Designers should choose grasses, herbaceous plants, or trees that can withstand both wet and dry periods as well as relatively high velocity flows within the channel. Salt tolerant grass species should be chosen for dry swale applications along roads. Taller and denser grasses are preferable, although the species is less important than good stabilization and vegetative cover. To find a list of plant species suitable for use in dry swales, consult the Virginia Erosion and Sediment Control Handbook.

Table 4 outlines the standard material specifications to construct bioretention areas

Table 4: Dry Swale Material Specifications
Material	Specification	Notes
Filter Media Composition	Filter Media to contain: 85-88% sand 8-12% soil fines 3-5% organic matter in form of leaf compost	Volume of filter media based on 110% of the product of surface area and media depth to account for settling.
Filter Media Testing	P-Index range 10-30, CECs greater than 10 Mixed onsite, or procure from approved media vendors (Refer to Design Specification No. 9 Bioretention for additional soil media information.
Surface Cover	Turf or river stone
Top Soil	4 inch surface depth of loamy sand or sandy loam texture, with less than 5% clay content, corrected pH 6 to 7, and organic matter of at least 2%
Filter Fabric	Non-woven polyprene geotextile w/ flow rate of > 110 gallons/minutes/square foot (e.g., Geotex 351 or equivalent) Apply to swale sides and above underdrain only. For hotspots and certain karst sites only, use appropriate liner on bottom.
Choking Layer	2 to 4 inch layer of sand over a 2 inch layer of choker stone (typically #8 or # 89 washed gravel) over the underdrain stone
Stone and/or Storage Layer	# 57 stone should be double-washed and clean and free of all soil and fines. 9 to 18” layer, depending on desired depth of storage layer
Underdrains, Cleanouts, and Observation Wells	6” rigid schedule 40 PVC pipe, with 3/8” perforations Corrugated HDPE for Raingardens	Perforated pipe for length of dry swale cell. Non-perforated pipe as needed to connect with storm drain system.
Grass Species	Plant species as per landscaping plan
Check Dams	Non-erosive material such as wood, gabions, riprap, or concrete. All check dams should be underlain with filter fabric, and include weep holes. Wood used for check dams should consist of pressure treated logs or timbers, or water-resistant tree species such as cedar, hemlock, swamp oak or locust.
Erosion Control Fabric	Where flow velocities dictate, use woven biodegradable erosion control fabric or mats (EC2) that is durable enough to last at least 2 growing seasons.

Shallow dry swales are an acceptable practice in the active karst regions of the Ridge and Valley province of the Bay watershed. To prevent sinkhole formation and possible groundwater contamination, dry swales should utilize impermeable liners and underdrains (i.e., Level 2 dry swale designs that rely on infiltration are not recommended in any area with a moderate or high risk of sinkhole formation (Hyland, 2005). Shallow dry swales are also recommended to stay above active karst areas.

If a dry swale facility is located in an area of active sinkhole formation, standard setbacks to buildings should be increased.

The flat terrain, low head and high water table of many coastal plain sites can constrain the application of dry swale areas (particularly Level 2 designs). Swales perform poorly in extremely flat terrain because they lack enough grade to create storage cells, and lack head to drive the system. In these situations, the following design adaptations apply.

While these design criteria permit dry swale to be used on a wider range of coastal plain sites, it is important not to force it into marginal sites. Other stormwater practices, such as wet swales, ditch wetland restoration and smaller linear wetlands, are preferred alternatives for coastal plain sites.

Contributing slopes to a dry swale area can be increased to 20% in areas of steep terrain, as long as a multiple cell design is used to dissipate erosive energy prior to filtering, by terracing a series of dry swale cells to manage runoff across or down a slope. The drop in slope between cells should be limited to a foot, be armored with river stone or suitable equivalent. A greater emphasis on properly engineered energy dissipaters and/or drop structures is warranted.

Dry swales can store snow and treat snowmelt runoff when they serve road or parking lot drainage. If roadway salt is applied in their CDA, dry swales should be planted with salt-tolerant and non-woody plant species. Consult the Minnesota Stormwater Manual for a list of salt-tolerant grass species (MSSC, 2005). The underdrain pipe should also extend below the frost line and be oversized by one pipe size to reduce the chances of freeze-up.

Construction Stage ESC Controls. Dry Swales should be fully protected by silt fence or construction fencing, particularly if they will rely on infiltration (i.e., have no underdrains). Ideally, dry swales should remain outside the limit of disturbance during construction to prevent soil compaction by heavy equipment. Dry swale locations may be used as small sediment traps or basins during construction. However, these must be accompanied by notes and graphic details on the ESC plan identifying the maximum excavation depth at the construction stage being at least 1 foot above the post-construction installation, must contain an underdrain, and show the proper procedures for conversion from a temporary practice to a permanent one, including de-watering, cleanouts and stabilization.

The following is a typical construction sequence to properly install a dry swale, although steps may be modified to reflect different site conditions.

Step 1. Protection during Site Construction. Ideally, dry swales should remain outside the limit of disturbance during construction to prevent soil compaction by heavy equipment. However, this is seldom practical given that swales are a key part of the drainage system at most sites. In these cases, temporary ESC controls such as dikes, silt fences and erosion control other measures should be integrated into the swale design throughout the construction sequence. Specifically, barriers should be installed at key check dam locations, erosion control fabric used to protect the channel, and excavation limited to no more than 2 feet above the proposed invert of the bottom of the planned underdrain. Dry swales that lack underdrains (and rely on filtration) must be fully protected by silt fence or construction fencing to prevent compaction by heavy equipment during construction.

Step 2. Installation may only begin after entire contributing drainage area has been stabilized by vegetation. The designer should check the boundaries of the contributing drainage area to ensure it conforms to original design. Additional ESC controls may be needed during swale construction, particularly to divert stormwater from the dry swale until the filter beds and side slopes are fully stabilized. Pretreatment cells should be excavated first to trap sediments before they reach planned filter beds.

Step 3. Excavators or backhoes should excavate the dry swale only from the outside of the swale footprint to the appropriate design depth and dimensions.

Step 4. The bottom of the dry swale should be ripped, roto-tilled or otherwise scarified to promote greater infiltration.

Step 5. Acceptable filter fabric shall be placed on the underground sides of the dry swale with a minimum 6 inch overlap. Place the stone on bed needed for storage layer, perforate the underdrain and check slope, add remaining stone jacket, and then pack #57 stone to 3 inches above the top of underdrain, and then add 3 inches of pea gravel as filter.

Step 6. Add soil media 12 inch lifts until desired top elevation of dry swale is achieved. Wait a few days to check for settlement, and add additional media as needed.

Step 7: Install check dams, driveway culverts and internal pretreatment features as per plan

Step 8. Prepare any planting holes for any trees and shrubs, install erosion control fabric where needed, lay down seed or sod and install any temporary irrigation.

Step 9. Plant landscaping materials as shown in the landscaping plan, and water them weekly in the first 2 months. The construction contract should include a care and replacement warranty to ensure vegetation is properly established and survives during the first growing season following construction.

Step 10. Conduct final construction inspection and develop punchlist for facility acceptance.

Inspections during construction are needed to ensure that the dry swale practice is built in accordance with these specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist for dry swales can be accessed at the CWP website at: www.cwp.org/postconstruction. Go to Tool #6.

Some common pitfalls can be avoided by careful construction supervision that focuses on the following key aspects of dry swale installation.

Check filter media to confirm that it meets specifications and is added to the correct depth.
Check elevations such as the invert of the underdrain, inverts for inflow and outflow points, ponding depth provided between the surface of the filter bed and the overflow structure.
Ensure caps are placed on the upstream (but not the downstream) end of the underdrain.
Make sure desired coverage of turf or erosion control fabric has been achieved following construction, both on the beds and their contributing side-slopes.
Inspect check dams and pretreatment structures to make sure they are properly installed and working effectively
Check that outfall protection/energy dissipation measures at concentrated inflow and outflow points are stable

The real test of a dry swale occurs after its first big storm. The post-storm inspection should focus on whether the desired sheetflow, shallow concentrated flows or concentrated flows assumed in the plan are realized in the field. Also, inspectors should check that the dry swale drains within minimum 6 hour draw down. Minor adjustments are normally needed as part of this post-storm inspection, such as spot reseeding, gully repair, added armoring at inlets or outfalls and check dam realignment.

Section 4VAC 50-60-124 of the regulations specifies a maintenance agreement to be executed between the owner and the local program. The section requires a schedule of inspections, compliance procedures if maintenance is neglected, notification of the local program upon transfer of ownership, and right-of-entry for local program personnel.

All dry swales must be covered by a drainage easement to allow inspection and maintenance. If dry swale is located in a residential private lot, the existence and purpose of the dry swale shall be noted on the deed of record. Homeowners will need to be provided a simple document that explains their purpose and routine maintenance needs. A deed restriction or other mechanism enforceable by the qualifying local program must be in place to help ensure that dry swales are maintained The mechanism should, if possible, grant authority for local agencies to access the property for inspection or corrective action.

Annual inspections are used to trigger maintenance operations such as sediment removal, spot revegetation and inlet stabilization. Several key maintenance inspection points are detailed in Table 5. Ideally, inspections should be conducted in the spring of each year. An example maintenance inspection checklist for dry swales can be accessed at CWP website at: www.cwp.org/postconstruction. Go to Tool #6.

Table 5: Suggested Spring Maintenance Inspections/Cleanups for Dry Swales

Activity

Add reinforcement planting to maintain 95% turf cover on vegetation density. Reseed any salt killed vegetation

Remove any accumulated sand or sediment deposits on the filter bed surface or in pretreatment cells.

Inspect upstream and downstream of check dams for evidence of undercutting or erosion, and remove and trash or blockages at weepholes

Examine filter beds for evidence of braiding, excessive ponding or dead grass

Check inflow points for clogging and remove any sediment.

Inspect side slopes and grass filter strips for evidence of any rill or gully erosion and repair

Look for any bare soil or sediment sources in the contributing drainage area and stabilize immediately.

Once established, dry swales have minimal maintenance needs outside of the spring clean up, regular mowing and pruning and management of trees and shrubs. The surface of the filter bed can become clogged with fine sediment over time, but this can be alleviated through core aeration or deep tilling of the filter bed. Additional effort may be needed to repair check dams, stabilize inlet point and remove deposited sediment from pretreatment cells.

The following references and resources were used to develop this master specification.

Claytor, R. and T. Schueler. 1996. Design of Stormwater Filtering Systems. Center for Watershed Protection. Ellicott City, MD.

CWP. 2007. National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD.

Hirschman, D. and J. Kosco. 2008. Managing stormwater in your community: a guide for building an effective post-construction problem. EPA Publication 833-R-08-001, Tetra-tech, Inc. and Center for Watershed Protection. Ellicott City, MD

Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban stormwater retrofit practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD

Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD www.chesapeakestormwater.net

Virginia Department of Conservation and Recreation (VA DCR). 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA.

VIRGINIA DCR STORMWATER
DESIGN SPECIFICATION No. 10

DRY SWALES

VIRGINIA DCR STORMWATER DESIGN SPECIFICATION No. 10

DRY SWALES

VIRGINIA DCR STORMWATER
DESIGN SPECIFICATION No. 10