Operation and maintenance of Infiltration trench

Operation and Maintenance

Overview

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, operating instructions for drawdown valves, gates and removable weirs.

The monitoring plan should address the following items: inspection and efficiency assessment, water quality monitoring, monitoring of ground-water 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
• Maintaining appropriate surface vegetation

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
	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. Among them are:

Warning: Providing easy access (typically 8 feet wide) to infiltration practices for routine maintenance is REQUIRED

• Open lawn areas are RECOMMENDED locations for infiltration practices because of their accessibility.

Warning: It is REQUIRED that a way to visually verify proper system operation be installed with each infiltration practice.

• It is HIGHLY RECOMMENDED that every dry well, infiltration trench and subsurface infiltration system design include an observation well consisting of an anchored six-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.

Information: ...it is RECOMMENDED that a minimum of three soil borings or pits be dug (in the same location as the proposed infiltration practice)...

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 as follows:

Example construction phase inspection checklist here.

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 iinfiltration trench maintenance activities and schedule are provided in the table below

Typical maintenance problems for infiltration trenches and basins.
Link to this table

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

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