Stone Bridge Park Improvement Project

Stone Bridge Park is a quaint neighborhood park along Lake Monona with beautiful views of the Madison skyline. The park is home to Lake Monona Sailing Club and features a dock where approximately twenty boats are kept during the summer, green spaces with mature ash and willow trees, and a historic stone pagoda. In recent years, a large stormwater outfall located at the park became an issue for the city and the local community.

The existing outfall discharged stormwater into the park green spaces and overland flowed to the lake, making much of the park unusable for large portions of the year. The flow and velocity of the runoff could be quite intense at times, causing not just water pollution but also erosion. To solve the issue, the city needed an effective solution to manage and treat runoff, complying with the state and local requirements – WPDES Wisconsin Pollutant Discharge Elimination (WPDES) program and MAMSWAP (Madison Area Municipal Storm Water Partnership) permit.

The main initial project concerns were related to park limitations of area for the construction of a stormwater management system, the runoff velocity (high flow rates), and the compliance with the water treatment requirements. Considering those, the City of Monona and Strand Associates worked together with StormTrap to design a unique underground vault that would fit within the site constraints and effectively remove trash & debris, total suspended solids (TSS), and particularly phosphorus from water before discharging into the lake.

A 5’-8″ SingleTrap wet vault with a 3’-0″ permanent pool was designed to be installed in the middle of the park and work as a “treatment train system.” The structure incorporates an inlet diversion chamber, a high flow bypass channel, a forebay sediment chamber, a floatable baffle, and a main sedimentation area. When water enters the system, it flows into the inlet diversion chamber and is directed into a forebay sedimentation chamber that allows larger particles to settle. Water then travels under two oil and floatable trap devices, mounted to an internal wall, and over a higher weir before flowing to the larger central sedimentation area. When high flow (<25-year event) events occur, water overtops the weir in the diversion chamber and travels directly to the outlet in the integrated bypass channel.

The vault was installed in two days by Daniels General Contractors using excavators to set the pieces in place. Then, it was wrapped with a geomembrane liner system, backfilled with stone, and covered with grass. “The field staff were knowledgeable and had experience with the installation process and the logistics of getting the StormTrap pieces on-site and in the correct order to put together the puzzle,” affirmed Brad Bruun, Project Manager & GIS Specialist for the City of Monona.

Today, the basin stores and treats 11.5 acres of urban runoff from the neighborhood, and erosion is no longer an issue at the park. “The underground wet detention system was a perfect solution for this project,” stated Bruun. “The system allowed more space in the park to be useable while also trapping debris and treating the effluent.” The project was completed in the Fall of 2021.