Highbanks Ravine Bat Hibernaculum

The Highbanks Bat Hibernaculum has historically been studied by SCSU staff starting in 1952 through the 2000's. Surveys indicated the number of bats within the hibernaculum to range from 50 to over 500. Surveys were also completed by Gerda Norquist with the MNDNR in 1992, 2006, 2012 and 2016, who noted that surveys completed prior to 2006 found hundreds of bats roosting in the hibernaculum. The most recent surveys found only two (2) big brown bats (Eptesicus fuscus) on both occasions. Ms. Norquist noted during recent visits the occurrence of more frequent flash flood events and evidence of stormwater flooding to the top of the structure, is detrimental to bats roosting there. The hibernaculum is also an active stormwater conveyance system. Stormwater from approximately 125 acres of residential and commercial land use drain into the hibernaculum, causing the detrimental flooding events. The hibernaculum/stormwater convenance discharges into the 680 linear foot, 40-foot-deep Highbanks Ravine which outlets into the Mississippi River. Stormwater flows through the ravine cause significant erosion and sedimentation into the Mississippi River, impacting fish and aquatic habitat. The plume of sediment is visible on aerials.

According to an article (The Chronicle [January 23, 1979] (stcloudstate.edu)) the hibernaculum should be a cool place (32-44 degrees F) with a water supply as the bats will crawl down and get a drink occasionally. The brick-and-mortar tunnel provides the perfect environment with cool temperatures, cracks for the bats to crawl into for the winter, and a water source (Mississippi River) nearby. The Highbanks Ravine habitats are listed as a Floodplain Forest and Mesic Hardwood Forest. Having this habitat adjacent to the river and hibernaculum is ideal for the bats as they emerge from hibernation. The bats can exit, find food immediately and rest in the trees.

The City has been working closely with the MNDNR over the past eight years to identify a solution to address the negative and detrimental stormwater impacts on the bat hibernaculum and Mississippi River. After a detailed assessment of varying options ranging from eliminating the bat hibernaculum, creating an alternative bat roosting area, re-routing stormwater flow, and many other options, the MNDNR and City have identified the best option to enhance habitat in the area is to remove/route the stormwater flow away from the bat hibernaculum and around the ravine.

The proposed project is unique and a non-traditional habitat enhancement project in that the project includes the installation of a stormwater pipe.

The new stormwater pipe alignment eliminates flows to the brick-and-mortar tunnel by rerouting the lateral stormwater pipes to the south along 4th Street South and would outlet directly into the Mississippi River, and removes downstream inflow to the ravine along 2nd Ave S. The new route will improve water quality by eliminating flow and erosion within the ravine. This re-route is critical in the enhancement of the hibernaculum as it eliminates the flooding of the tunnel and improves fish and aquatic habitat by reducing sediment into the Mississippi River.