Post-construction

The five-step construction process for sub-surface and ground-level LIDs.

The construction guidance in this section applies to all sub-surface and ground-level LID practices: bioretention gardens, bioswales, rain gardens, exfiltration trenches, enhanced swales, permeable pavements, infiltration trenches, infiltration chambers, and soakaways.

Overview

This section gives guidance for project managers, engineers, and contractors on post-construction verification procedures.

Addressing deficiencies

Prior to passing responsibility to the landowner, contractors must ensure that any deficiencies in the LID feature’s construction are addressed. Keep in-mind what mistakes must be avoided during each construction task.

Final certification

Final inspection requirements are in-place to ensure that a final site-wide inspection is performed and that mitigation can occur before the contractor is relieved of responsibility. Final certifications are performed by knowledgeable personnel, such as the design engineer, who are qualified to ensure that LID features are operating as intended. This stage is critically important, as it is the last opportunity for issues to be resolved before the property owner assumes responsibility of the practice. Four (4) levels of inspection exist for final inspections of LID infiltration and filtration practices. The level of inspection, or combination of tests, that is selected is dependent on the goals of the assessment and feasibility of each inspection. A substantial amount of guidance is available in Low Impact Development Certification Protocols: Bioretention Practices.

Visual Inspection (Level 1)
This qualitative method is low cost and requires minimal effort to conduct. Visual inspection should be used as an initial step to determine if the LID practice is operating properly. Visual inspections can be performed during dry weather and wet weather to examine various aspects of LID performance, such as the following:

• draw-down time
• ponded water
• soil properties
• plant cover and plant health
• sedimentation

Surface Infiltration Capacity Testing (Level 2)
Level 2 includes the employment of quantitive methods of testing and data collection. These tasks are also cost-effective and can establish if the LID feature was built in conformance with the design specifications. Some level 2 tests include the following:

• soil sampling and testing
• elevation surveys
• infiltration testing
• synthetic runoff testing

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  Flood test for a bioretention LID facility. (Photo Source: CVC, n.d.)

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  Flood test for permeable pavers. (Photo Source: CVC, 2013)

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  Double-ring infiltrometer. (CVC, 2012)

Continuous Water Level Monitoring (Level 3)
Site infiltration rates can be measured with greater accuracy through continuous water level monitoring. This monitoring approach evaluates seasonal infiltration performance through the monitoring of temperature and groundwater levels. The following conditions and equipment are necessary for continuous water level monitoring:

1. A perforated observation well
2. A water level logger
3. A rain gauge within 5 km of the site

Using rainfall and water level data can provide an understanding of drawdown times after variously sized storms.

rainfall data and water level measurements can be used to assess infiltration performance. (Photo Source: CVC, 2015)

Long-term Monitoring (Level 4)
High-intensity monitoring is a comprehensive but expensive approach to assessing peak flow reduction and water volume reduction during natural runoff events. These reductions can be quantified by completing a water budget during natural storm events, specifically through the measurement of inflow and outflow. Pollutant load reductions can also be quantified with the appropriate technology. Long-term monitoring is especially recommended if the LID feature that is the subject of the test is the first of its kind in its given jurisdiction, if geologic conditions pose a concern, or if the feature is being implemented to protect sensitive and significant natural features.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The technique of removing pollutants from runoff as it infiltrates through the soil.

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Credit Valley Conservation

The mathematical expression of the water balance.

The total mass of a pollutant entering a waterbody over a defined time period.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.