Water Services

Low Impact Development

picture: downtown nashville buildings water spraying trees

The LID (Low Impact Development) Manual is a voluntary site design approach that utilizes Green Infrastructure to meet a development site’s post development stormwater runoff water quality requirements. This approach will satisfy the new MS4 Permit requirement to infiltrate, evapotranspire, or harvest and use the first inch of rain, which will be become mandatory in 2016. Until then, sites designed in accordance with the LID Manual can benefit from the incentives listed below. It is an alternate water quality compliance path to the total suspended solids removal efficiency requirement described in Volumes 1 and 4 of our Stormwater Management Manual.

LID Manual 2013 (full version)

Green Infrastructure Practices (GIP's)

Incentives

These are the incentives that are currently offered. Please check back for others.

Table 1. Green Infrastructure Incentives
Incentive	Requirement/Benefit
Waiver of Plan Review Fees	Certain stormwater and water/sewer plans review and application fees will be waived if GIPs are implemented according to this Volume.
Stormwater Fee Reduction*	The stormwater user fee can be reduced 75% through implementation of the methods provided in the LID Manual.
Infill Water Quality*	Sites within the Infill Boundary will have a runoff reduction credit of 60 % (versus 80%) if the site’s pre-development Rv > 0.4.
Green Roof Credit	Bonus Runoff Reduction percentage above the actual reduction rate has been incorporated into the Green Roof GIP to further encourage the use of green roofs
Cisterns	While previously not allowed, in the LID Manual cisterns can be used to meet water quality requirements
Reduced Detention Requirement (see Chapter 3.2.5)	GIPs can reduce the required stormwater detention quantity

* Sites cannot receive both the 75% Stormwater Fee reduction and the Infill Water Quality incentives. Sites receiving the Infill incentive may still receive up to a 50% Stormwater User Fee reduction as outlined in the Stormwater User Fee Credit Manual.

LID Site Design Tool

A Site Design Tool has been created to aid engineers in designing the water quality treatment for a project in accordance with the methodology in the LID Manual. This program can be downloaded onto your computer and used to calculate if a site meets Metro’s water quality requirements. To ensure we are able to send out any updates to this tool, we are now requesting you to enter your e-mail address before this tool can be downloaded.

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Rainwater Harvester

This cistern design tool was designed by Matthew Jones of NC State University and can help size a cistern, determine if it is meeting a site’s runoff reduction goal, and calculate the cost savings.

Rainwater Harvester - Cistern Design Model
Nashville's Rainfall Data - download to your computer
Nashville's User Manual - step by step instructions

picture: building with 3 large cisterns courtesy of the Chesapeake Bay Foundation/cbf.org

LID Manual Training Video

A training video was created to introduce the LID Manual methodology and describe how to design a site in accordance with the Manual. The video includes examples using the LID Site Design Tool. The video lasts approximately one hour and twenty minutes and the breakdown of the topics and their start times is listed below. The presentation is available for download in case you have trouble seeing the slides in the video.

Video time stamps & topics:

0:00:00 Overview of Green Infrastructure and the Runoff Reduction Method
0:28:52 Green Infrastructure Design Steps and Green Infrastructure Practices (GIPs)
0:45:54 LID Site Design Tool and Simple Example
1:02:07 Adjusted Curve Number
1:06:26 Nashville Site Example