PROJECT ALERT NOTICE (MN) Flooding in Minnesota, day 21

Date: Wed, 03 Apr 2019 15:42:37 EDT

Summary: Flooding from snowmelt continues in Minnesota as the mainstem Mississippi River crests in southern Minnesota, while streams farther north continue to rise.

Flooding continues in Minnesota with western Minnesota and the Red River of the North Basin receiving most of the attention. One or more gages have been at or above NWS flood stage each day since March 14. Currently, 20 gages are above minor flood stage, including 4 at major flood stage. Ice jams have been more frequent, widespread, and more severe than recent snowmelt events, causing unpredictable backwater flooding. Ice jams caused flow into an interbasin diversion and provisional peak stages of record at two streamgages. Provisional peak discharges have commonly had Annual Exceedance Probabilities (AEPs) of 0.2 to 0.1, but a few gages have had AEPs as low as 0.01. The first wave of Mississippi River gages in southern Minnesota have peaked as of today, while flood forecasts are beginning to be issued for gages in the northern half of Minnesota, including the Mississippi headwaters and Red River of the North.

Today 4 teams (8 staff) are making flood measurements and repairing one gage. Since flooding began approximately 111 discharge measurements have been made in flooding parts of the state, and 4 secondary stage sensors have been installed at gages to improve data during ice-out or in-case ice and debris destroy orifice lines. Damages and losses due to ice-heave, ice flows, and ice jams, include crest-stage gages for peak confirmation, possibly an index-velocity meter, two bank-operated cableways, two DCPs and a gage battery. An ADCP and a field laptop were lost during discharge measurements.

USGS conducted scour monitoring soundings at the request of the MN-Department of Transportation at a scour-critical bridge over the Mississippi River at St Paul, where a 70-80 feet deep scour hole can develop at high flows. In addition, USGS has been communicating on NWSChat, and directly with the USACE-St Paul District, local emergency managers and public works directors in communities of Marshall, Granite Falls, New Ulm, Mankato, Houston, and Jackson. The USACE asked USGS to make discharge measurements at the outlet of a Minnesota River flood structure that is under renovation. Two interviews have been performed with Minnesota Public Radio concerning USGS flood preparations, monitoring and ice jams.