Effective long-term performance of infiltration practices requires an infiltration management plan (HIGHLY RECOMMENDED) and performance monitoring (HIGHLY RECOMMENDED).

Warning: Dedicated and routine maintenance schedule with clear guidelines and schedules are REQUIRED

The infiltration management plan (operation plan) should address the following items:

• periods of inundation;
• wet/dry cycling of soils; and
• operating instructions for drawdown valves gates and removable weirs.

The monitoring plan should address the following items:

• inspection and efficiency assessment;
• water quality monitoring; and
• monitoring of groundwater elevations, long-term infiltration capacity and plant tolerances.

Elements to be considered for the development of a maintenance plan are broken into the following categories:

• Design Phase Maintenance Considerations;
• Construction Phase Maintenance Considerations; and
• Post-Construction Maintenance Considerations.

In general terms, the most frequently cited maintenance concern for infiltration practices is clogging caused by organic matter and fine silts. Common operational problems include:

• clogging and sediment deposition;
• erosion of contributing land or in channels leading to the practice; and
• maintaining appropriate surface vegetation.

The table below provides a summary of common problems for infiltration trenches and basins.

Summary of infiltration practices cost components.

Implementation Stage Primary Cost Components Basic Cost Estimate Other Considerations
Site Preparation Tree & plant protection Protection Cost ($/acre) * Affected Area (acre) Removal of existing structures, topsoil removal and stockpiling Infiltration area protection Silt fence cost ($/’foot) * Perimeter of infiltration area
Clearing & grubbing Clearing Cost ($/acre) * Affected Area (acre) Topsoil salvage Salvage Cost ($/acre) * Affected Area
Site Formation Excavation / grading X-ft Depth Excavation Cost ($/acre) * Area (acre) Soil & rock fill material, tunneling Hauling material offsite Excavation Cost * (% of Material to be hauled away) Structural Components Vault structure (for underground infiltration) ($/structure) Pipes, catchbasins, manholes, valves, vaults
Media (for infiltration trenches) Media cost ($/cubic yard) * filter volume (cubic yard) Geotextile Geotextile cost ($/cy) * area of trench, including walls
inlet structure ($/structure) Overflow structure ($/structure)
Observation well ($/structure) Site Restoration Soil preparation Topsoil or amendment cost ($/acre) * Area (acre) Tree protection, soil amendments, seed bed preparation, trails
Seeding Seeding Cost ($/acre) * Seeded Area (acre) Filter strip Sod cost ($/square foot) * filter strip area
Planting / transplanting Planting Cost ($/acre) * Planted Area (acre) Annual Operation, Maintenance, and Inspection Sediment removal Removal Cost ($/acre) * Area (acre) * Frequency (1 time per 5 years) Vegetation maintenance, cleaning of structures
Debris removal Removal Cost ($/acre) * Area (acre) * Frequency (2 time per year) Inspection Inspection Cost ($) * Inspection Frequency (6 times per year)
Mowing (for some vegetative filters) Mowing Cost (\$) * Mowing Frequency (6 times per year)

## Design phase maintenance considerations

Implicit in the design guidance in the previous sections is the fact that many design elements for infiltration systems can minimize the maintenance burden and maintain pollutant removal efficiency.

• Open lawn areas are RECOMMENDED locations for infiltration practices because of their accessibility.
• It is HIGHLY RECOMMENDED that every dry well, infiltration trench and subsurface infiltration system design include an observation well consisting of an anchored 6-inch diameter perforated PVC pipe fitted with a cap to facilitate periodic inspection and maintenance. It is also HIGHLY RECOMMENDED that infiltration basins include a draw down device that can be used for winter diversion and to conduct regular maintenance.
• It is HIGHLY RECOMMENDED that a mechanism such as a multi-stage outlet structure be incorporated into the design of the pre-treatment and infiltration practices to facilitate draining for maintenance purposes.
• It is RECOMMENDED that a minimum of 3 soil borings or pits be dug in the same location as the proposed infiltration practice.
Warning: It is REQUIRED that a way to visually verify proper system operation be installed with each infiltration practice.
Warning: Providing easy access (typically 8 feet wide) to infiltration practices for routine maintenance is REQUIRED

## Construction phase maintenance considerations

Infiltration practices are particularly vulnerable during the construction phase for two reasons. First, if the construction sequence is not followed correctly, construction sediment can clog the practice. In addition, heavy construction can result in compaction of the soil, which can then reduce the soil’s infiltration rate. For this reason, a careful construction sequence needs to be followed. Critical construction elements for infiltration practices are shown in the following table.

Infiltration Trench and infiltration basin - Construction inspection checklists.
To access an Excel version of form (for field use), click here.

Construction Sequence Project: Location: Site Status: Date: Time: Inspector: Pre-construction meeting Runoff diverted Soil permeability verified Groundwater / bedrock verified Project benchmark established Facility location staked out Temporary erosion and sediment control established Size and location per plans Side slopes stable Depth adjusted to soil layer with specified soil type and permeability Sub-soil not adjacent to excavation area and stabilized with vegetation and/ or silt fence Stockpile location not adjacent to excavation area and stabilized with vegetation and/ or silt fence Fabric per specifications Fabric per specifications Placed per plan location Size as specified Clean / washed material Placed properly Pipe size per plans Under-drain installed per plans Inlet installed per plans Pre-treatment devices installed per plans Complies with planting specifications Topsoil complies with composition and placement in specifications Permanent erosion control measures in place Dimensions per plans Check dams operational Inlet / outlet operational Effective stand of vegetation and stabilization Contributing watershed stabilized before flow is routed to the facility Comments: Actions to be taken:

### Avoid excessive compaction

Warning: It is REQUIRED that in order to prevent soil compaction, the proposed infiltration area be staked off and marked during construction to prevent heavy equipment and traffic from traveling over it.

In addition, it is HIGHLY RECOMMENDED that the side walls of dry wells and infiltration trenches be roughened if they have been smeared by heavy equipment.

### Stabilize vegetation before and after construction

Excessive sediment loadings can occur without the use of proper erosion and sediment control practices during the construction process.

Warning: It is REQUIRED that upland drainage areas be properly stabilized with a thick layer of vegetation, particularly immediately following construction, to reduce sediment loads.
Warning: If infiltration practices are in-place during construction activities, it is REQUIRED that sediment and runoff be kept away the infiltration area, such as with diversion berms and soil-stabilizing vegetation around the perimeter of the practice.

### Correctly install filter fabrics

Large tree roots should be trimmed flush with the sides of dry wells and infiltration trenches to prevent puncturing or tearing of the filter fabric during subsequent installation procedures. When laying out the geotextile, the width should include sufficient material to compensate for perimeter irregularities in the dry well or trench and for a 6-inch minimum top overlap. The filter fabric itself should be tucked under the sand layer on the bottom of the dry well of infiltration trench, and stones or other anchoring objects should be placed on the fabric at the trench sides to keep the excavation open during windy periods. Voids may occur between the fabric and the excavated sides of the practice. Natural soils should be placed in any voids to ensure fabric conformity to the excavation sides.

Initial infiltration basin excavation should be carried to within 2 feet of the final elevation of the basin floor.

Warning: It is REQUIRED that infiltration systems not be excavated to final grade until the contributing drainage area has been constructed and fully stabilized.

The final phase excavation should remove all accumulated sediment and be done by light tracked equipment to avoid compaction of the basin floor and provide a well-aerated, highly porous surface texture.

### Keep infiltration practices “Off-line” until construction is complete

Warning: It is REQUIRED that sediment and runoff be kept completely away from the infiltration area during construction. Thus, infiltration practices should never serve as sediment control devices during site construction.

It is HIGHLY RECOMMENDED that construction of sediment practices be suspended during snowmelt or rainfall, in order to prevent soil smearing, clumping, or compaction.

### Establish permanent vegetation

• Establishing dense vegetation on the basin side slopes is HIGHLY RECOMMENDED, to reduce erosion and sloughing and
• Provide a natural means of maintaining relatively high infiltration rates. Vegetative cover at inflow points to the basin is also HIGHLY RECOMMENDED to provide erosion protection and reduce sediment accumulation. The use of native grasses is RECOMMENDED for seeding primarily due to their adaptability to local climates and soil conditions.
• Inspections during construction are needed to ensure that the infiltration practice is built in accordance with the approved design and standards and 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 designer.

### Post-construction operation and maintenance

Warning: A maintenance plan clarifying maintenance responsibility is REQUIRED.

Effective long-term operation of infiltration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Some important post-construction maintenance considerations are provided below.

• A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority.
• Adequate access must be provided for all infiltration practices for inspection, maintenance, and landscaping upkeep, including appropriate equipment and vehicles.
• General infiltration trench maintenance activities and schedule are provided in the tables below.

Typical maintenance problems for infiltration trenches and basins.

Clogging, sediment deposition Both Key issue for infiltration practice. Requires vigilant inspection and maintenance.
Surface Vegetation Both Often important to maintain vigorous growth at the base of infiltration practices (basins). Important to restrict woody vegetation from the surface of infiltration trenches.
Erosion of contributing land or in channels leading to practice Both In these practices, it is important to monitor not only the practice itself, but also upland infiltration to minimize the sediment load.
Damage to filter fabric Trench Infrequent but important maintenance concern.
Scouring at Inlet Both Similar issues to Ponds. Need to promote non-erosive flows that are evenly distributed
Access Issues Both Similar issues to Ponds. Need access for inspection and maintenance.
Concrete Failure Basins, if they include a riser structure Similar issues to ponds and wetlands.
Problems with the Embankment Basins Similar issues to dry ponds.

Typical maintenance activities for infiltration trenches and infiltration basins.

Activity Schedule
Replace pea gravel/topsoil and top surface filter fabric (when clogged). As needed

Ensure that contributing area, practice and inlets are clear of debris.

Ensure that the contributing area is stabilized.

Remove sediment and oil/grease from pre-treatment devices, as well as overflow structures.

Mow grass filter strips should be mowed as necessary. Remove grass clippings.

Repair undercut and eroded areas at inflow and outflow structures
Monthly
Inspect pre-treatment devices and diversion structures for sediment build-up and structural damage. Remove trees that start to grow in the vicinity of the trench. Semi-annual Inspection
Disc or otherwise aerate basin bottom. De-thatch basin bottom. Annually
Scrape basin bottom and remove sediment. Restore original crosssection and infiltration rate. Seed or sod to restore ground cover. Every 5 years
Perform total rehabilitation of the trench to maintain design storage capacity. Excavate trench walls to expose clean soil Upon Failure