Spring Harbor Outfall Repair

Last Updated: 09/20/2019

Updated Plan Available!

Following the July public meeting, several discussions were had with Parks Commission members, residents, and Parks staff, which resulted in plan modifications to the Spring Harbor Outfall Repair project.  The plan alterations are highlighted below.  Sally Swenson, the Project Engineer, will be accepting comments on these modifications, so please contact her with any concerns or questions.

  • Boat Launch Footprint: The plan shown at the public meeting had the eastern lobe of the boat launch removed. Comments and observations indicate that the full asphalt area is used frequently and will, therefore, remain. The layout will be altered slightly by making the east edge a mountable sidewalk instead of curb and gutter. This will provide a clear walking path, but will also allow vehicles maneuvering room, if needed. The new sidewalk will tie into the existing sidewalk, providing a continuous path to the pier.
  • Limestone Flume/Steps: The suggestion was made to consider overland flow behind the wall, in the likely event that Lake Mendota Drive overtops again in the future. Therefore, limestone steps have been added behind the retaining wall. The steps will primarily serve as a stable flume for water, but will also act as a more protected water access point. The steps will not provide an ideal fishing spot, as they will have a significant riprap toe. They will, however, be safer than the steps that extended directly into the harbor.
  • Tree Removal: A tree near the existing outfall and Lake Mendota Drive will be removed in order to replace a failed storm sewer pipe beneath Lake Mendota Drive. At the time of the July public meeting, it was anticipated this tree would remain. The tree removal on the south side of Lake Mendota Drive, near the springs, was originally called for and will still be removed.
  • Tree Replacement: Due to the tree loss, we will plant at least two new trees.

Proposed Spring Harbor Outfall Repair - Plan View
Proposed Spring Harbor Outfall Repair – Plan View

Proposed Spring Harbor Retaining Wall - Profile View
Proposed Spring Harbor Retaining Wall – Profile View

Project Description

The August 20, 2018 rain event caused the concrete storm sewer outfall structure at Spring Harbor to fail. The heavy rain and high volume of stormwater runoff caused scour beneath the existing retaining wall. The scour was significant enough to undercut and destabilize a portion of the wall, which lead to its rotation and ultimate failure. As an emergency repair, the City placed temporary sheet piling and riprap in order to secure the embankment.  This project will construct a permanent repair for the failed wall.  Additionally, the project will resurface the boat launch area and replace a failed storm sewer pipe beneath Lake Mendota Drive.  Each item is discussed below. 

The proposed project will repair the failed section of retaining wall by replacing it with a concrete-faced sheet piling wall.  The concrete facing will allow the new wall to blend with the existing section of box culvert, while creating a wall that is not susceptible to being undermined by scour.  However, instead of extending the concrete wall 3-4 feet above the adjacent ground surface, the new section of wall will be topped with a tubular steel railing, which will provide fall protection, but will create less of a visual barrier.

The asphalt boat launch is in poor shape due to age.  It is unlikely that the existing surface would hold up well under construction activities.  Therefore, this project will remove and replace the existing asphalt surface.  Curb and gutter will be constructed on the north and west sides of the launch area.  On the eastern edge, a new section of sidewalk will tie into the existing sidewalk.

Finally, a pipe that connects the spring area, south of Lake Mendota Drive, to Lake Mendota and Spring Harbor failed.  This pipe will be replaced as part of this project.

Photogrpah showing the failure of the retaining wall.
Temporary sheet piling being placed behind the failed retaining wall at Spring Harbor.
Photograph showing the temporary wall repair at Spring Harbor.
Temporary wall repair at Spring Harbor.  The proposed wall will align with the existing wall in the foreground.