Effective long-term performance of infiltration practices requires an infiltration management plan (HIGHLY RECOMMENDED) and performance monitoring (HIGHLY RECOMMENDED).
The infiltration management plan (operation plan) should address the following items:
The monitoring plan should address the following items:
Elements to be considered for the development of a maintenance plan are broken into the following categories:
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:
The table below provides a summary of common problems for infiltration trenches and basins.
Summary of infiltration practices cost components.
Link to this table
|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|
|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)|
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.
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.
|Construction Sequence||Satisfactory / Unsatisfactory||Comments|
|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|
|3. Filter Fabric Placement|
|Fabric per specifications|
|Fabric per specifications|
|Placed per plan location|
|4. Aggregate Material|
|Size as specified|
|Clean / washed material|
|5. Observation Well|
|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|
|7. Final Inspection|
|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|
|Actions to be taken:|
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.
Excessive sediment loadings can occur without the use of proper erosion and sediment control practices during the construction process.
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.
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.
It is HIGHLY RECOMMENDED that construction of sediment practices be suspended during snowmelt or rainfall, in order to prevent soil smearing, clumping, or compaction.
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.
Typical maintenance problems for infiltration trenches and basins.
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|Problem||Practices Applied To||Comments|
|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.
Link to this table
|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
|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|