Post-Construction Stormwater Management

Post-construction stormwater management involves the installation and/or construction of Best Management Practices (BMP's) to collect, control, and treat stormwater at a particular site.  Unlike temporary construction site runoff control measures, post-construction BMP's are more permanent structures designed to collect and treat stormwater runoff after construction has been completed.  Examples are detention or retention basins, swales, underground storage chambers, and associated stormwater pipes, catch basins, and outlet structures. 

There are three (3) aspects of post-construction stormwater management that are typically controlled: water quality, volume, and flow rate.  In the City of Waseca, the water quality and volume control requirements are both satisfied if a volume of at least one inch (1") times the sum of the created and reconstructed impervious surface area for a particular site is captured, retained, and permitted to infiltrate.  (Impervious surfaces are those that allow little to no stormwater infiltration into the ground, such as buildings, pavement, and compacted gravel.)  For rate control, post-construction stormwater runoff rates may not exceed pre-development conditions for the 2, 10, and 100-year, 24-hour rainfall events. 

These water quality/volume and rate control conditions apply to projects that create or reconstruct a combined total of one (1) acre or more of impervious surface area.  Projects that exceed the one (1) acre threshold must submit a complete Stormwater Management Plan (SWMP) for review and approval.  Projects that create or reconstruct less than one (1) acre of impervious surface area are not required to submit a SWMP or meet the water quality/volume or rate control requirements.  However, the site plan review and approval process (City Code Chapter 154.178) may still apply.

These standards and additional post-construction stormwater management information can be found in City Code Chapter 154.156 and the City's Engineering Design Standards for Stormwater Management.