New vents have been installed on both sides of the center divider after the 1999 and prior incidents.
Near 35th Street Underpass on I-35 W
The Minnesota DOT knows about the air surge problem, and here is a PDF that specifically mentions these geysers of stormwater:
Page 68, Environmental Assessment, Interstate 35W and Highway 62, State Project: 2782-281, Minnesota Project: IM 0353(287)
Mississippi River Subwatershed
This drainage area is defined as the portion of the project area north of Minnehaha Creek and draining into the I-35W storm tunnel, which flows northerly to the Mississippi River. In addition to the highway runoff, several municipal storm sewer systems connect to the existing trunk storm sewer within the highway right of way. The existing trunk line connects to the storm tunnel north of the project limits. The storm tunnel currently operates under pressure for storm events larger than the 5-year frequency (Barr, 1999). With this limited capacity, pressure surges have caused geysers at two manhole locations during small but intense rainfall events. Flooding within off-site areas connecting to this system may also be attributed to the lack of capacity.
The impervious surface area of the proposed project within this subwatershed will increase by nine acres over the existing condition, with a corresponding increase in storm water volumes from those of the existing condition. Due to limited right of way adjacent to the roadway corridor in this segment, detention ponds are not feasible for peak flow reduction.
A release of water and air has been observed at the first and second drop shafts in the system at 39th Street and 35th Street respectively. Mn/DOT has been monitoring this condition for a number of years and plans to build diffusers on the top of each drop shaft to redirect the energy. In addition, the flooding of the area served by the I-35W tunnel is a concern for Mn/DOT and the City of Minneapolis. Mn/DOT and the City of Minneapolis are currently performing a hydrologic and hydraulic modeling study of the I-35W tunnel system to evaluate the hydraulic conditions for both the current and future conditions. The study intends to identify potential solutions to the capacity issues, and may include storage, diversion of drainage area, or additional capacity. This study will be completed in early 2005. Mn/DOT and the City of Minneapolis will work together to implement these strategies to protect the neighborhoods served by the tunnel and the I-35W corridor.
A number of strategies are being employed to minimize the risk of flooding in this segment of I-35W, including:
1. Low point elevations at 42nd and 46th Street have been maintained in the proposed condition.
2. Trunk storm sewer within this segment will be designed to current highway standards and will be evaluated for larger storm events to prevent flooding.
3. Other flood protection measures (underground overflow pipe storage, backflow prevention, etc.) will be evaluated during final design to minimize the risk of flooding in the area.