Construction Planning

Natural drainage patterns on a site should be identified and incorporate into the site’s stormwater plan. Strategic LID areas, such as wetlands, streams, and filter strips should be preserved in the site design, and restrictions to minimize disturbance of these areas must be observed during construction to maintain the infiltration capacity of the soil. Planning to disturb smaller, more manageable areas helps reduce erosion potential and facilitates the success of post-construction LID stormwater treatment.

Construction Sequence

For infiltration-type devices, avoid compacting native or installed soils with heavy equipment. Compaction will likely cause the device to fail by preventing infiltration. Likewise, engineered soils for infiltration devices should not be installed until the construction site is fully stabilized. Fine particles in runoff will clog the engineered soil and prevent infiltration, causing the device to fail. Observation wells and/or clean outs should be installed in order to provide access to determine whether soils and/or underdrains become clogged and offer a means of clearing clogged drain lines. Design guidance and construction sequence for treatment types in

LID Maintenance

Maintenance for LID devices, just like any infrastructure, should be anticipated. Ideally, a maintenance plan for each LID device should be developed and followed.

The following tips may be useful:

• For the first year after installation, inspections should be conducted monthly and after major storms. Check for sinkholes, erosion, washouts, or excessive ponding.
• Bioretention areas should be kept free of weeds.
• Stormwater wetlands may need to be treated to remove unwanted invasive species such as cattails.
• An adequate layer of mulch (2-3 inches) should be maintained.
• Clogged soils must be replaced to restore function to the device.
• Pruning of plants should be done as needed to maintain proper infiltration.
• Do not add additional fertilizer. Stormwater runoff and compost in the soils should provide adequate nutrients for plantings.

Bioretention System Maintenance Guide
Porous Pavement Maintenance Guide
LID Operations and Maintenance Manual - Seattle Public Utilities

LID Manuals

Design Manuals are helpful in site planning and implementation of specific devices. Below is a list of design guidance manuals from this region:

• South Carolina Coastal Low Impact Development Manual
• South Carolina Stormwater Best Management Practices Handbook
• North Carolina Low Impact Development Guidebook
• Georgia Stormwater Management Manual and Coastal Stormwater Supplement
• Low Impact Development Strategies - Prince Georges County, Maryland
• LID for Big Box Retailers

LID Cost Analysis vs. Conventional Development

A common question about low impact development is, "What does it cost?" Compared to conventional development techniques, a low impact development design can often cost equal to or even significantly less than conventional development due to savings on infrastructure costs. Also, because LID designs frequently incorporate open and green space, sale prices in LID residential developments are often higher due to demand for the neighborhood features offered by this type of residential setting. Below are some resources analyzing the costs of low impact development design relative to conventional development:

• Cost-Benefit Resources for LID
• Reducing Stormwater Costs through LID - EPA report

LID Design Tools and Calculators

There are many design tools and calculators that can assist in the design of low impact development devices and project planning. Below are a few:

• Urban Design Tools
• Green Values Stormwater Toolbox - National Stormwater Management Calculator
• North Carolina State University - Rainwater Harvesting Model
• North Carolina LID BMP Spreadsheet Tool

LID Design Principles

LID design recognizes the value of the natural landscape and native vegetation. Protecting a site's sensitive natural features is central to LID planning and design. Therefore, a thorough site analysis prior to planning is critical. LID design capitalizes on the functions and aesthetics of the natural landscape - providing stormwater management solutions, along with aesthetic and recreational benefits for the community. Overall, LID principles focus on directing development to the most suitable areas on a site, while reducing the distance of runoff flow paths and managing stormwater as close to its source as possible.

Treatment Train

The concept of a treatment train uses multiple treatment devices prior to discharging to local streams and waterways so that the combined effect on water quality and runoff volume is more than each individual practice by itself. Some helpful documents detailing design and planning considerations are:

• Low Impact Development in South Carolina : A Planning and Design Guide
• LID Practices for Stormwater Management - National Assoc. of Homebuilders Research Center
• The Practice of Low Impact Development - U.S. Dept. of Housing and Urban Development