This spring’s flood highest since 2001

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River stage sixth highest ever

By Ted Pennekamp

 

The Mississippi River crested at 21.34 feet on Friday, April 5, according to the National Weather Service.

The river is not expected to bounce back up and have another crest. Daniel Fasching of the U.S. Army Corps of Engineers said when snowpack in northern Wisconsin and Minnesota melts, the river may level off for awhile but then should continue to recede.

At 21.34 feet, the river was the sixth highest in recorded history at the McGregor gauge. The Mississippi has only gone above major flood stage of 22 feet twice, 25.38 feet in 1965 and 23.75 in 2001. This year marks the highest since 2001.

The 1965 flood is the only time the river at McGregor has exceeded the 100-year flood stage of 24.16 feet since records have been kept.

A common misunderstanding is that a 100-year flood is likely to occur only once in a 100-year period. In fact, there is approximately a 63.4% chance of one or more 100-year floods occurring in any 100-year period.

A 100-year flood is a flood event that has a 1% probability of occurring in any given year.

The 100-year flood is also referred to as the 1% flood, since its annual exceedance probability is 1%. For river systems, the 100-year flood is generally expressed as a flow rate. Based on the expected 100-year flood flow rate, the flood water level can be mapped as an area of inundation. The resulting floodplain map is referred to as the 100-year floodplain. Maps of riverine 100-year floodplains may figure importantly in building permits, environmental regulations and flood insurance.

On April 5, the Mississippi River at Lock and Dam 9 near Lynxville had an elevation of 628.33 feet in relation to mean sea level in accordance with the North American Vertical Datum 1988 (NAVD88), the data system now used by the United States Geological Survey. The top of the dam is 633 feet. The streambed is 600 feet.

The flow rate was a whopping 198,340 cubic feet per second (cfs). The threshold at which all gates at the dam need to be wide open is 63,000 cfs. Fasching said that flow rates are estimates, especially at the very high river stages because really high river stages don’t occur very often and therefore there are a lot fewer samples.

The flow rate for a 100-year flood is between 250,000 cfs and 275,000 cfs at Lock and Dam 9, said Fasching in giving a margin for error.

The 100-year flood elevation at any given site is based upon the whole record of all elevations ever recorded at that site and calculating what elevation would have a 1% chance of being reached in any given year.

“Elevation is the only direct measurement,” said Fasching. “Flow rate is an approximation.” He said calculating flow rate involves several factors, some of which include conducting a survey of the river bottom at the site, using a cross section of the river and conducting mathematical calculations.

Fasching noted that the river has been here “forever,” yet man has only approximately 200 years of recorded data.

Some interesting facts about Lock and Dam 9 include:

It was completed in 1937 and has a length of 9,087 feet, a structural height of 46 feet, and a hydraulic height of 32 feet. It has a drainage area of 66,610 square miles.

The locks and dams on the Mississippi River were not designed for flood control. They were built in order to help maintain the navigational channel for commercial barge traffic.

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