PROJECT ALERT NOTICE (MN) Flooding in Minnesota, Day 28

Date: Wed, 10 Apr 2019 16:50:35 EDT

Summary: Flooding from snowmelt continues in northern Minnesota as a powerful spring snowstorm spreads across state.

Up to 24 inches of forecast snow has started falling in southwestern Minnesota, with more than a foot forecast for much of the state into early Friday. We don’t plan to deploy any staff Thursday, when the storm will be most intense. Meanwhile, flooding continues in northern Minnesota with the Red River of the North, Rainy and Mississippi Headwaters basins. One or more gages have been at or above NWS flood stage since March 14. Currently, 20 gages are above minor flood stage, including 2 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 basin gages in southern Minnesota peaked, while flood forecasts have been issued for most gages in the northern half of Minnesota, including the Red River of the North.

Today 3 teams (5 staff) are making flood measurements in northern Minnesota. Since flooding began approximately 140 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. The ADCP has been located on a debris pile in the river and one attempt was made to recover it, but flows will need to recede further.

USGS has been communicating on NWSChat, and directly with the USACE-St Paul District, MNDOT, 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. Scour soundings for a scour-critical bridge over the Mississippi at St Paul was performed at the request of MNDOT. Two interviews have been performed with Minnesota Public Radio concerning USGS flood preparations, monitoring and ice jams.