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Storm Water - Low Impact Development (LID) Recommended Practices and Details

WET SWALES

Brief Description

Wet Swales are a type of filtration BMP and occur when the water table is located close to the surface.  This wet swale acts as a very long and linear shallow wetland treatment system.  Like dry swales, the entire water quality treatment volume is stored within a series of cells created by check dams.  Cells may be planted with emergent wetland plant species to improvement pollutant removal.  It is recommended to be preceded by pre-treatment to capture and remove coarse sediment particles

Cost Range

Construction:   $-$$

O&M:  $-$$

Ideal Conditions for Installation

Precipitation:  (Long duration)

Soils:  (Type C & D)

Groundwater:  (Shallow)

Where/Applicability

Residential:   (Limited)

Commercial:   (Yes)

Ultra Urban:  (No)

Industrial:  (Yes)

Highway/Road:   (Yes)

Retrofit:  (Yes) 

Feasible:  Do not use over impervious soils.

Maintenance

Frequency:  (Twice Yearly)

Type:   (Vegetation maintenance, cleaning of structures)

Monitoring:  (Monitor following storm events) 

Permit Renewal:  (Permit renewal as required for MS4)

Performance

Total Suspended Solids (TSS):   20% removal with no check dams, 40% with check dams

Total Nitrogen:  15% removal

Metals:  35% removal

Oils and Grease:   (No Data)

Pathogens:  (Low/Medium/High) or % removal

Specifications

Pretreatment is recommended to remove sediments and floatables.

 

1.   Install all temporary erosion control measures  prior to the start of any construction operation that may cause any sedimentation or siltation at the site.

2.   Install storm drain inlet protection to prevent clogging of the storm sewer and sediment loads to downstream storm water facilities or waterbodies.

3.   Size bottom width, depth, length, and slope necessary to store WQV (Water Quality Volume) with less than 18 inches of ponding at the downstream end.

4.   Grade to the depth (elevation) specified in the construction documents unless otherwise directed by the engineer.

5.   Slope should not exceed 5 percent (1 to 2 percent recommended).

6.   Bottom width should range from 2 to 8 feet.

7.   Ensure that side slope are no greater than 3:1 (4:1 recommended).

 

8.   If the system is on-line, channels should be sized to convey runoff from the overbank flood event (Vp10) safely with a minimum of 6 inches of freeboard and without damage to adjacent property.  The peak velocity for the 2-year storm must be non-erosive for the soil and vegetative cover provided.

 

9.   Inlets to swales must provide for energy dissipaters such as riprap or geotextile reinforcement.

10. Energy dissipaters such as riprap must be used at drops from check dams. Check Dam spacing shall be approximately 50 feet spacing.

 

11. The channel and under-drain excavation should be limited to the width and depth specified in the design.  The bottom of the excavated trench shall not be loaded in a way that causes soil compaction, scarification prior to placement of gravel and permeable soil.  The sides of the channel shall be trimmed of all large roots.  The sidewalls shall be uniform with no voids and scarified prior to backfilling.

 

12. Grading of the swale shall be accomplished using low-impact earth-moving equipment to prevent compaction of the underlying soils. Small tracked dozers and bobcats with runner tracks are recommended.

 

13. Excavate the swale to the specified depth (elevation). It is recommended that all sub material below the specified elevation shall be left undisturbed, unless otherwise directed by the engineer.

 

14. In the event that sediment is introduced into the bmp during or immediately following excavation, this material will need to be removed from the swale prior to initiating the next step in the construction process. This is especially important if the swale has been designed to infiltrate storm water: sediment that has been washed into the swale during the excavation process can seal the permeable material, significantly reducing the infiltration capacity of the soils.

 

15. Material excavated from the swale(s) shall be disposed of on-site at locations (stockpile areas) designated by engineer.

 

16. Non-standard component:  clean, washed 1.5 to 3.5-inch gravel shall be placed in the bottom of the swale to the depth specified in the construction documents.  Gravel should be placed in lifts and lightly compacted with plate compactors.

 

17. Non-standard component:  the perforated pipe (underdrain) shall be laid directly on the gravel bed.  Grade and alignment shall not vary from the prescribed grade by more than 0.03 feet at any point.  The joints between sections of pipe shall be connected in a fashion acceptable to engineer.  Once the pipe is in place, it shall be covered immediately with granular material as specified in the construction documents.  The granular material shall be of uniform depth on both sides of the pipe.  Special inlets and special devices at the outlet end of the pipe shall be constructed as shown in the plans.

18. Portions of swale to be planted shall receive 3" of woodchip mulch .

19. Portions of swale to be seeded shall be mulched with clean grain straw ) at a rate of 2 tons per acre.

20. Seeding and installation of erosion control blanket shall be completed within 48 hours of final grading.

Construction Sequence

1.   Perform continuous inspections of erosion control practices.

 

2.   Install silt fence along the perimeter of the site to prevent sediment from Leaving the site during the construction process.

 

3.   All downgradient perimeter sediment-control BMPs must be in place before any up  gradient land-disturbing activity begins.

 

4.   Remove topsoil from the site and place in temporary stockpile location. Temporary seed the stockpile.

 

5.   Install underground utilities (water, sanitary sewer, electric and phones) taking the location and function of storm water BMPs into consideration.    

  

6.   Seed and mulch disturbed areas on site.    

  

7.   Construct the roads taking the location and function of storm water BMPs into consideration.  

  

8.   Perform all other site improvements taking the location and function of the storm water BMPs into consideration.  

  

9.   Final grade the site.    

  

10. Stabilize the site by implementing the native seeding and planting portion of the landscaping plan.  

  

11. Install the erosion control blanket and coir roll/check dams.    

  

12. Remove the silt fence after the site is stabilized per project engineer approval.

Design Standard

Cross section

WetSwaleCrossSection.jpg