PROJECT ALERT NOTICE (MN)

Date: Thu, 11 Apr 2019 16:28:43 EDT

Summary: Floods begin to recede in Minnesota as a major spring snowstorm spreads across state.

Flooding continues in Minnesota with most rivers in recession at least until the snow falling today begins to melt. Sixteen gages are above aNWS minor flood stage, including 2 at major flood stage, with one or more gages above flood stage each day since March 14. Ice jams have been frequent, widespread, and more severe than recent snowmelt events, causing unpredictable backwater flooding and damage to gages. Ice jams caused flow into an interbasin diversion and provisional peak stages of record at two streamgages. One tributary to the Red River with 80 years of record had a provisional peak discharge of record. Most peak discharges have commonly had Annual Exceedance Probabilities (AEPs) of 0.2 to 0.1, with a few sites had AEPs as low as 0.01.

No field teams are deployed today due to a spring snowstorm making its way across the state. Since flooding began more than 145 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 floes, and ice jams, include many crest-stage gages for peak confirmation, index-velocity meters at up to 4 sites that have not reported during ice-out but cannot be inspected until after rivers recede, two bank-operated cableways, two data collection platforms, a gage battery. An ADCP and a field laptop were lost during discharge measurements. The ADCP was located on a debris pile in the river and an unsuccessful recovery attempt was made; flows will need to recede further.

USGS has been communicating on NWSChat, and directly with the USACE, MNDOT, local emergency managers and public works directors in communities of Marshall, Granite Falls, New Ulm, Mankato, Houston, and Jackson. Many discussions with communities included anticipated changes to NWS flood stages at selected discharge levels due to changes in stage-discharge ratings, either as a result of gage relocations or channel changes since previous floods. 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.

With a snowstorm today and cooler temperatures slowing snowmelt for the next few days, we hope to keep field crews in until at least Monday except for gage failures, so will pause Project Alerts. If next week’s peak forecasts are similar to what we’ve measured and accordingly adjusted and extended stage-discharge ratings during the past 28 days, then we hope to transition to maintenance and repair mode and cease project alerts unless conditions warrant otherwise.