VCAP BMP Standards: 3.11 Infiltration (IF)

Virginia Conservation Assistance Program Training

Module #2B

Infiltration (IF) Components

PresenterPresentation NotesInfiltration practices provide temporary surface or subsurface storage of stormwater, that promotes the infiltration of runoff, without using an underdrain.Surface Infiltration must pond runoff and use stone aggregate, sand or open chambers to create void space for storage of the treatment volume.

Underground Infiltration includes bottomless or perforated chambers surrounded by clean stone aggregate. The storage volume is provided in the chambers.

Surface Infiltration components: sand bed, clean aggregate; surface layer (pea gravel/river stone/sand/sod); Observation well or maintenance port; overflow or bypass structure. Underdrains may be necessary when the infiltration rate is less than ½ inch per hour or drawdown time is greater than 48 hours. A 12-inch sump is provided below the underdrain.

Infiltration does not convey runoff like a Swale, it temporarily ponds runoff.Must be sized to provide a 36 to 48 hour drawdown.

Infiltration Engineering

PresenterPresentation NotesScales of implementations: Small, Medium and Large

Any designs that incorporates the Level 2 criteria from the Clearinghouse standard shall be considered Large or Complex

Clearinghouse’s Level 2 criteria requires a treatment volume based on 1.1 inches of rainfall; THREE Pretreatment; Infiltration rates of greater than 1 inch per hour

Design Criteria

Drainage Area

PresenterPresentation NotesDrainage Area shall be no more than 2 acres and should be treating impervious areas greater than 2,500 sq. ft.

This shows a 27,700 sq. ft. (or 0.63 acre) CDA with 93% impervious cover (i.e. 25,761 sq. ft. Impervious), which makes it eligible for Infiltration versus Dry Well.

The more impervious the better. Pervious surfaces introduce fines that can clog infiltration practices.

Constraints

PresenterPresentation NotesConstraints: Avoid Wetlands and Floodplain locations;

Depth to Bedrock and Water Table > 2 ft; Karst areas need impermeable liner and underdrain in most cases.

Soils should infiltrate at a rate of ½ to 1 inch per hour and drawdown ponded water within 36 to 48 hours.

Not appropriate where there is significant risk for basement/slope seepage or surficial flooding due to mounding. 30 foot setback from 6:1 or steeper slopes.

Pretreatment

Sheet Flow Concentrated Flow

PresenterPresentation NotesInfiltration need at least two types of pretreatment to reduce the quantity of fines entering the voids. Level 2 designs require three pretreatments.

Sheet Flow Pretreatment: Filter Strips or Gravel DiaphragmSurface trenches will be wrapped in nonwoven geotextile with a built-in failure plan of pea gravel or sod.

Concentrated Flow Pretreatment: Grass Channel; Gravel Diaphragm; Engineered Level Spreader; Sediment Forebay; Sumps; or Proprietary filters Underground Infiltration will have sumps and other proprietary filters.

Observation Well

PresenterPresentation NotesObservation wells should be vertical and elevated above the surface with a screw top. The well should be solid with either an open bottom or perforations on the bottom 6 inches.

Maintenance Ports are placed in pavement to observe the capacity of the chamber. A larger access manhole should be provided to perform the maintenance of the chamber.

Overflow Relief

Surface Trench with Weir Underground with Manhole

PresenterPresentation NotesFlow Through designs require an overflow to pass the 25-year design storm. Flow Through or online designs include a drop inlet riser to adjacent storm sewer or weir to channel or dispersed as sheet flow.The drop inlet riser needs to be elevated to ponding elevation and designed to have capacity for 25-year storm event.The weir needs to be elevated to the ponding elevation and designed to have capacity for the 25-year storm and be non-erosive for the 10-year storm.

Bypass or Offline designs need to be sized to bypass the storms greater than the 10-year storm event. The inlets need to be non-erosive for the 10-year design storm. There should be capacity to allow the safe overland relief of the 25-year storm.

Adequate Receiving System

PresenterPresentation NotesThe receiving stormwater conveyance system (channel, storm sewer, or culvert) should have adequate capacity to pass the 10-year design storm (capacity and Velocity).

If it sheet flows, the 10-year flow depth should be ½ the height of the receiving vegetation and should have a velocity of 1 foot per second.If the facility discharges to a channel or storm sewer, those structures should have capacity for the 10-year design storm and resist erosion during the 10-year flows.

∗ Sand Layer – 6 to 8 inches of clean Sand ∗ Stone Aggregate –VDOT#1s or VDOT#57s ∗ Surface Cover –pea gravel, river stone; or Sod ∗ Geotextile Fabric – Non-woven ∗ Observation Well – solid schedule 40 PVC 6 inch

diameter. ∗ Underdrain, if needed – schedule 40 PVC with 3/8-inch

perforations at 6 inches on center

Material Specifications

PresenterPresentation NotesMaterials shall meet the specifications from the Virginia Stormwater Clearinghouse, standard 8 Infiltration (2013 draft specifications preferred).

Stone Aggregate must be double washed without any fines. VDOT#1s used for surface facilities and underground chambers use VDOT#57s.

Geotextile fabric may be used on the side as a separator between the soil and sub-base of adjacent structures. Geotextile may be wrapped over the top to form a choking layer or root barrier.

Observation well should have a screw cap elevated above the surface. Bottom may be capped with perforations up to 6 inches of the bottom.

Underdrains to be used with soil infiltration rate is marginal or drawdown rate exceeds 48 hours.

Plans and Specifications

A professional design is required for Infiltration. Depending on the Scale of the design, the participant may forgo an engineered design certification by a licensed professional.

Engineering Design

PresenterPresentation NotesInfiltration is considered an Engineered Practice. A professional design created and stamped by a licensed professional is required when a participant wants to undertake this practice.

When the participant wishes to forgo an engineer, they are accepting responsibility for the design plan and must sign the Release Agreement for Eligible Practices (VCAP Form – 5). The design plan must still meet all applicable requirements of the Bioretention standard in the VCAP Manual and the standards on the Virginia Stormwater Clearinghouse.

The steering committee will accept the Release Agreement after determining the level of risk the project poses to public safety and welfare and environmental impacts. The following BMP scales will aid the Steering Committee and Districts in determining the appropriateness of the waiver. In general any design that exceeds one of the parameters in the three columns, the design is considered the next scale up.

Generally, a Small scale Infiltration should consider the Dry Well practice first.

∗ Professional Stamp ∗ Infiltration Test (MUST) ∗ Drainage Map; Sketch with Dimensions; Cross Sections; ∗ Computations: Inlet and outlet capacity; Treatment Volume ∗ Construction Sequence; Material Lists; Cost Estimate ∗ Erosion and Sediment Control Plan, if applicable ∗ Statement regarding compliance with any permitting

requirements or local codes

Reviewing Design Plans

PresenterPresentation NotesEach District will be responsible for reviewing Design Plans to ensure they comply with the appropriate standards in the VCAP Manual and any relevant standards of the Virginia Stormwater Clearinghouse.The District staff should have training equivalent to the DEQ certifications for Erosion Control and Stormwater Management.

Designs shall be stamped by a professional, unless a Release Agreement is approved by the Steering Committee.Infiltration test is required 1 per practice; Level 2 design should provide 1 test per 1,000 sq. feet.Plan View Drawings showing Drainage Map; Location with Dimensions; Cross Sections and other construction details for inlets and outlets; Computations: Inlet and Outlet Capacity; Treatment Volume; Pretreatment Sizing as necessaryNarrative including Summary of applicable permits and local codes; Construction Sequence; Material Lists; and Cost EstimateErosion Control Plan as necessary. At least provide inlet protection for receiving systems and note on how the site will be stabilized (i.e. sod side slopes or use matting in channels).

Construction Sequence: Excavation and Layers

PresenterPresentation NotesClearly mark off the excavation area to prevent compaction by heavy equipment.Store materials in a protected area to keep them free from foreign materials.The contributing drainage area should be stabilized with vegetation. Use temporary Erosion Controls as necessary, including blocking necessary inlets or diverting sheet flow away from the excavation area. Call miss utility for existing utilities. Excavation should occur from the sides of the facility. The floor of the facility should be level but equipment should be kept off the floor. Scarify or rip the bottom to a depth of 6 to 12 inches.Geotextile should be applied to the sides of the facility to separate the facility from adjoining soils and prevent root intrusions.Spread 6 inches of sand on the bottom.Install observation wellsCheck aggregate to ensure it is double washed and clean for installation

Construction Sequence: Top Layer

PresenterPresentation Notes11. Check elevations at each stage of installation.12. Spread aggregate in 12-inch lifts.13. Goetextile may be wrapped over the aggregate to create a choking layer of pea gravel or river stone and to create a root barrier with sod

Construction Sequence: Site Stabilization

PresenterPresentation Notes14. Install any necessary Erosion Control matting or sodding on side slopes.15. After facility has been stabilized then pretreatment measures may be built 16. Once stable, activate the inlets with pretreatment

Construction Sequence: Underground Chambers

PresenterPresentation NotesUnderground facilities follow the same procedures.Storage chambers are completely wrapped in non woven geotextile.VDOT 57s form the bedding of the chambers

Construction Sequence: Underground Chambers Completion

PresenterPresentation NotesThe chamber is then buried with aggregate and covered with topsoil and sod.The observation wells and maintenance ports should be visible on the surface.

Operation and Maintenance

Inspections

PresenterPresentation NotesAnnual inspections are very important.Inspect the surface cover; inlets; pretreatments; and outlets

Inspect the observation well and maintenance ports for signs of standing water within the facility.

Routine

PresenterPresentation NotesAnnually: Remove sediment or debris buildup at inlets, pretreatments or bottom of facility.Check for erosion at inlets, pretreatments or outletsCheck for excessive ponding or poor drainage (algae, wetland plant growth, etc.)

Non-Routine

PresenterPresentation NotesNon-Routine maintenance includes all the periodic repairs that may occur.Removing sediment and debris from pretreatment and the surface (every 5 years)Restoring eroded inlets and outlets.Restore clogged aggregateRemove plants

∗ Virginia BMP Stormwater Clearinghouse, Design Standard Number 8 Infiltration. http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.html

∗ Infiltration Test Procedures: Appendix 8-A.

Technical Resources

http://www.vwrrc.vt.edu/swc/NonProprietaryBMPs.html

Questions?

PresenterPresentation NotesInfiltration requires testing of the soil.Infiltration requires more pretreatment measures

VCAP BMP Standards: �3.11 Infiltration (IF)�Infiltration (IF) ComponentsInfiltration EngineeringDesign CriteriaDrainage AreaConstraintsPretreatmentObservation WellOverflow ReliefAdequate Receiving SystemMaterial SpecificationsPlans and SpecificationsEngineering DesignReviewing Design PlansConstruction Sequence:�Excavation and LayersConstruction Sequence:�Top LayerConstruction Sequence:�Site StabilizationConstruction Sequence:�Underground ChambersConstruction Sequence:�Underground Chambers CompletionOperation and MaintenanceInspectionsRoutine Non-Routine Technical ResourcesQuestions?