How to make a flood

KELOLAND.com Original

SIOUX FALLS, S.D. (KELO) — If there is a recipe for a flood it seems like a good part of South Dakota has all the ingredients.

The area around Sioux Falls had reports of at least 40 inches of total rain in 2019 with 34.22 inches reported Sioux Falls as of Sept. 23, 2019, KELOLAND meteorlogist Brian Karstens reported.

Add in 2018’s moisture to 2019’s moisture. KELOLAND News reported on Dec. 26, 2018, that an winter storm would make 2018 the wettest year ever in the Sioux Falls area.

Eastern, southeastern and west central South Dakota have had wet weather for two years and now, the soil has significant moisture. River, stream, and lake levels are up.

Mix it all up and there’s high potential for spring flooding.

The National Weather Service offices in Aberdeen and Sioux Falls predicted on Thursday above normal chances for minor, moderate or major levels of flooding for 2020.

The flooding chances only increase if the regions get more moisture in the coming weeks.

“The lesser the better,” when it comes to moisture, said Lance VandenBoogart of the NWS in Sioux Falls. “If it rains, it’s better if it does it in little bits rather than heavy rains of one, two and three inches.”

The NWS tracks water levels of lakes, streams and lakes as well as snow cover and soil moisture.

When soil moisture levels are high, there is less storage for any other water.

“It’s not quite like a parking lot where there is 100% run off,” VandenBoogart said.

The U.S. Department of Agriculture’s most recent crop report was on Dec. 31. The website said said reports for Jan. 2 and Jan. 27 were canceled because of the lapse in federal funding. The Dec. 31 report listed the subsoil moisture for South Dakota as 68% adequate, and 31% surplus with similar numbers for the topsoil. The next report is expect on Feb. 25.

South Dakota is within the Upper Missouri River Basin for the U.S. Army Corps of Engineers. The Corps of Engineers tracks run off and its impact, including determining release of water at Gavins Point Dam near Yankton.

The Upper Missouri River Basin includes parts Wyoming, Colorado, Montana North Dakota, South Dakota, Iowa, Kansas and Missiouri.

The U.S. Army Corps of Engineers released this map in relation to run-off and 2020 flooding on Feb. 5. The map highlights the area involved in the Missouri River Basin.

According to a Feb. 5 news release from the Corps of Engineers, the 2019 calendar year run off was 60.9 million acre-feet (MAF), the second highest run off in 121 years of record-keeping (1898-2018). The highest MAF was 61.0 MAF of run off in 2011. The Corps of Engineers said that based on current soil moisture conditions, current plains and mountain snowpack, and long-term temperature and precipitation outlooks, the 2020 calendar year runoff forecast is 36.3 MAF above Sioux City, Iowa, or 141% of average.

The average annual runoff is 25.8 MAF. 

The Sioux Falls NWS office covers the region of southeast South Dakota, southwest Minnesota, northwest Iowa and northeast Nebraska. The flooding chances vary from one section to another.

Northwest Iowa had less rainfall in the fall and not as much snow as other areas so far this winter, VandenBoogart said.

“There is less risk of major to moderate flooding in that area,” VandeBoogart said.

Snow cover is another factor in determining the flood risk

The NWS office in Sioux Falls said in its Feb. 13 news release on flooding forecast that “the current snow pack is widespread for all areas along and north of Interstate 90. Depths are generally 10 to 20 inches in that area, with some locations between U.S. Highway 14 and U.S. Highway 212 having 20 to 30 inches of snow on the ground. The water content of the snow pack in this area is generally 2 to 4 inches. Areas between I-90 and Highway 18 generally have 5 to 10 inches of snow on the ground, with a water content of 1 to 2 inches. Areas south of Highway 18 generally have less than 5 inches of snow on the ground, with a water content of 1 inch or less.”
The Big Sioux River is a barometer of sorts for flooding. The river starts in northeastern South Dakota and travels 419 miles to North Sioux City where it joins the Missouri River.

A map from the NWS Aberdeen office shows the snow pack in the region as of Feb. 13.

The Big Sioux River can be a barometer with flooding.

This is a map of the Big Sioux Watershed. The map is from the Friends of the Big Sioux River website.

The NWS tracks lake, stream and river levels. The NWS Aberdeen office posts the historic crest levels for the Big Sioux River at various points on its route. The levels included in the list were as of Feb. 13.

Some of gauge readings on Feb. 14 could be affected by ice, according to the NWS.

Flood stages indicate that the Big Sioux widens and carries more water from its start near Florence. Water levels as of Feb. 14 also show that snow melt and rain fall amounts have varied within the river’s path.

At Florence, flood stage is 8 feet and the water level was 4.31 feet as of Feb. 14.

At Castlewood, the flood stage is 9 feet and the water level as of Feb. 14 was 7.62 feet, according to the NWS.

On Feb. 14, the water level of the Big Sioux River at Maple Street in Sioux Falls was 3.3 feet, according to the NWS. The flood stage is 15 feet with action level at 14. Moderate flood stage is 18 feet and major flood stage is 21 feet.

Crest levels and flood levels show the Big Sioux River collects water as it travels through Sioux Falls and toward Sioux City.

The flood stage at Cliff Avenue is 16 feet. As of Feb. 14, the water level was 8.37, according to the NWS.

The NWS lists a crest of 37.42 on the river near Sioux City on March 17, 2019, as the second highest crest in history. The crest figure is preliminary, according to the NWS. The major flood stage of the river is 41 feet, moderate is 38 feet and flood stage is 32.

As of Feb. 14, the NWS listed the water level at 12.69 feet.

If snow, rain, soil saturation aren’t enough for a flood, toss in ice.

Ice jams can cause water to back up on lakes and in rivers and streams. The rate at which ice melts also impacts the flow of water and the possibility of ice jams.

VandenBoogart said warm weather and rain and snow melt can break up the ice. Ice that melts too quickly or in spurts where chunks break off and drift downstream can cause problems. Melting ice can cause flood stage waters. Ice jams can clog up the flow of water.

Imagine melted snow and rain pouring into one part of a river flowing with chunks of ice and collecting more ice chunks as it travels downstream. Eventually, some of the ice chunks will hit debris or slower water and an ice jam is created. Water may still be flowing and an ice jam can cause it find another path, flooding nearby land.

Dave Schaefer, the emergency management director for Hamlin County, said the county has had ice jams at bridges and at several roads in the county.

“I’ve seen it where the ice come off in slabs 2 1/2 feet thick on the road,” Schaefer said.

The county has used backhoes to break up ice jams and debris jams in various locations to help with the flow and ease the back up of water, Schaefer said.

Copyright 2020 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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