City of Madison

City of Madison Engineering | Photo Credit: Archie Nicolette




On October 1, 2004, Chapter NR 151 of the Wisconsin Administrative Code went in effect. This code requires that a portion of land in new developments shall be used to infiltrate stormwater into the soil, subject to a number of conditions. To meet this code, we expect developers to propose the use of rain gardens as one of the potential management practices.

Apart from the code, some City Boards and Commissions have required the inclusion of rain gardens as part of developments that are seeking City approval. The requirement to include rain gardens has not been uniform and there is not a standard for the size, capacity, or maintenance for rain gardens specified.

The purpose of this specification is to provide a provisional standard using the requirements of NR 151 as a basis. This code is intended to meet the standards of both the Wisconsin Department of Natural Resources (WDNR) and the Department of Commerce, as they are currently understood. This provisional code may be supplemented or replaced in the future as additional guidance is provided by these state agencies.

Sizing the Garden
NR 151 requires that for residential areas (one- and two- family dwellings), 1% of the site area be dedicated to active infiltration. For more dense residential and commercial properties the requirement is 2% of the site area dedicated for active infiltration. NR 151 defines the site as the area being developed before roads and other areas are dedicated to the public. Therefore, when meeting the requirements of NR 151 on a lot-by-lot basis, the following sizing criteria shall be followed:

ACTIVE AREA ON EACH LOT (X) = [(A-B) * (C/(A-(B+D)))] * E


X is the Area for Active Infiltration (acres);

A is the Total Site Area (acres);

B is the area dedicated to the public for park & stormwater managmenet (acres);

C is the percentage specified by NR 151 for active infiltration (0.01 or 0.02);

D is the area dedicated to the public for road right-of-way (acres);
E is the Individual Lot Area (acres)

Example: Given a 100-acre commercial development with 25% dedicated to open space & 8% dedicated to street right of way, how much area must be dedicated to active infiltration for a 10-acre lot?

A=100; B=25; C = 0.02; D = 8; E = 2


Given the above you can calculate the following:

X = [(100 - 25) * (0.02 * (100 - (25 + 8)))] * 2

X = 0.4477 acres (1,950 sq ft)

Note: This area is only for the active infiltration area (this is the flat area in a rain garden); it is expected that the total area for the garden with side slopes and berms will add another 1% to the total area required.

IMAGE: Sizing a rain garden for commercial property
Click on the image at left for more information on sizing a commercial rain garden

Depth of the Garden

Rain gardens shall have depths such that stormwater is expected to infiltrate in 24 hours or less, upon completion of a rain event. This is normally determined by the type of soil at the bottom of the garden. However, in no case shall the depth be greater than twelve (12) inches.

Area Draining to the Rain Garden
The impervious area draining to the garden shall be at least three (3) times but no greater than five (5) times the active infiltration area. This active infiltration area calculated above may be split up into multiple gardens to accomplish this requirement.

Conditioning the Garden

Gardens receiving runoff water from rooftops only:
The garden shall be excavated to the design depth plus two (2) inches. At that depth four (4) inches of compost shall be tilled into the existing soils such that the soils are well mixed.

Gardens receiving runoff water from parking lots:
The garden shall be constructed in accordance with the Standards Oversight Council draft standard for bio-retention systems. Contact Greg Fries at 267-1199 for a copy of this standard.

The gardens shall be planted with dormant or actively growing nursery stock, and the entire garden shall be mulched with hardwood mulch after planting. Plants shall be native perennials in at least a 2.5-inch pot and planted at a minimum rate of one plant per 2 square feet. Plants shall be chosen from Attachment #1 (Approved Rain Garden Plant List). Other plant sizing, spacing and species may be approved on a case by case basis.
Initial Establishment
During the first 2-3 months of establishment the garden will require, at a minimum, watering on a weekly basis (depending on weather). After the first season of establishment, the garden should not require watering.
Annual Maintenance
In the spring of each subsequent year, the remaining dead vegetation shall be removed to allow for new growth, and any accumulated sediment (normally at the entrance to the garden) shall also be removed. At least two (2) times during the growing season the area should be weeded and additional hardwood mulch shall be added as needed to assist in weed suppression.
This document has been prepared by Greg Fries, City of Madison Engineering Division. Please contact Greg (267-1199) with questions.