Categorized | IN OUR COMMUNITY

Above average rainfall, saturated soils and aging infrastructure cause sinkholes

Posted on 20 November 2017 by calvin

The City of Minneapolis and the Minneapolis Park and Recreation Board (MPRB) received inquiries in late October from visitors and neighbors of Nokomis-Hiawatha Regional Park about waterlogged parkland and the appearance of sinkholes and settled grounds along streets and in parks.

High precipitation levels over the past two years have resulted in lush grass, beautiful summer gardens, and saturated ground. 2016 was officially the wettest year on record for the Twin Cities, and this year, we are already 4 inches above the average yearly precipitation. By Oct. 3, 2017, the Twin Cities had already exceeded the average rainfall for all of October.
These past wetter-than-normal years have resulted in high lake, river and creek levels, and also mean that groundwater levels and water tables are high.

Ground saturation and mowing
For many years the City of Minneapolis has collaborated with the MPRB to locate stormwater ponds on MPRB property. Since the surrounding soil can no longer absorb additional moisture, many of these ponds are flowing over into nearby parkland. In some areas around Lake Nokomis, parkland near the ponds was too saturated to mow, and the grass grew longer.

Stormwater pipes/sinkholes
During rain events, ground runoff enters underground stormwater pipes. During periods of heavy or extended rainfall, the extra water flow can cause weakened areas of the aging stormwater pipes to fail.

This failure causes water to seep into the ground, where it moves through the spaces and cracks between the soil toward lower elevations. The water may eventually pool, cause soil erosion and result in a sinkhole. The movement of groundwater means that the sinkhole may not appear near the failed pipe. It may be located several feet, yards-or even blocks–away.

The City of Minneapolis is working on a plan to systemically look at their stormwater management system. City officials are aware that there are parts of the system that are undersized. However, putting in larger pipes is very expensive and letting water infiltrate into the ground at various locations is much cheaper. The amount of water that can be infiltrated reasonably is something that needs to be determined.

Settling, not sinkholes
In some areas around Nokomis-Hiawatha Regional Park, parkland is “settling.” The soft, very porous, peat-like material ground that comprises much of the area slowly compresses over time, and patches of compressed soil may “settle” slightly lower than the surrounding ground. These areas are not caused by broken or weakened stormwater pipes and are not sinkholes.

The City of Minneapolis and the MPRB have identified a lack of groundwater monitoring devices in this area. While groundwater models are helpful, monitoring devices are key to understanding the variability of groundwater. MPRB and City staff have identified potential locations of these additional devices, including locations within parkland around Lake Nokomis. Installation was scheduled for the area. Finally, the groundwater pumping and flooding issues at Hiawatha Golf Course are not connected to the ground saturation and sinkholes at Nokomis-Hiawatha Regional Park. Although both share the same soft soil and are affected by high water, Lake Hiawatha does not empty into Lake Nokomis, so Hiawatha does not impact Nokomis.

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