Stormwater sizing for redevelopment projects

Small redevelopment sites can pose special challenges for stormwater design, given their small size, intensive use, and compacted soils. Redevelopment projects are also not covered under the CGP unless they created more than one acre of new impervious surface or are part of a larger related planned development. Communities may wish to develop special sizing criteria for smaller redevelopment so that the cost to comply with stormwater requirements does not become a barrier to smart growth.

The following guidance is offered for handling redevelopment projects. It has been adapted from several recent manuals that represent a balanced approach to stormwater management for these sites.

The first issue is how to define what is meant by infill and redevelopment, which may be different in each locality. One accepted definition is that redevelopment is “any construction, alteration, or improvement that disturbs greater than or equal to 5,000 square feet of existing impervious cover performed on sites where the existing land use is commercial, industrial, institutional, or residential.” Note that this definition does not fall under the purview of the CGP.

The second issue is to provide some greater flexibility in how redevelopment projects can comply with basic stormwater sizing criteria. This is done by proposing stormwater management guidance that a redevelopment will

• provide a reduction in impervious area; or
• implement stormwater management practices; or
• a combination of both to result in an improvement to water quality.

More specifically, redevelopment projects introduce a chance to reduce existing site impervious area. Where site conditions prevent the reduction of impervious area, stormwater management practices could be implemented to provide water quality control for at least 20 percent of the site’s impervious area as a general guideline.

When a combination of impervious area reduction and stormwater management practice implementation is used for redevelopment projects, the combination of impervious area reduction and the area controlled by a stormwater management practice should equal or exceed 20 percent coverage of the project size.

The MPCA may allow practical alternatives where conditions prevent impervious area reduction or on-site stormwater management. Practical alternatives include, but are not limited to

• fees paid in an amount specified by the approving agency and then dedicated to stormwater management;
• off-site stormwater treatment practice implementation for a drainage area comparable in size and impervious cover to that of the project;
• watershed or stream restoration; or
• stormwater retrofitting.

The recharge, channel protection storage volume, overbank, and extreme flood protection volume requirements specified in the Manual do not apply to redevelopment projects unless specified in an approved and adopted basin plan.