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Partnering on Lower Missouri River flood risk management solutions

Published June 22, 2020
Between March 13 and March 23, 2019, per preliminary analysis by the National Weather Service, over 50 gages on the Missouri River and its tributaries in the region set new stage records. Nearly all of these new records were on unregulated areas of the Missouri Basin -- tributaries and the Missouri River downstream of Gavins Point Dam.

Between March 13 and March 23, 2019, per preliminary analysis by the National Weather Service, over 50 gages on the Missouri River and its tributaries in the region set new stage records. Nearly all of these new records were on unregulated areas of the Missouri Basin -- tributaries and the Missouri River downstream of Gavins Point Dam.

normous inflows from this flood event came from every major tributary that enters the Missouri River from Niobrara, Nebraska to the confluence of the Platte and Missouri Rivers. The Niobrara River quickly filled the very limited flood storage capacity at Gavins Point Dam forcing increased releases out of the dam.

A bombogenesis (or bomb cyclone) storm dumped up to 2.25 inches of warm rainfall on frozen soils, and a heavy, wet snowpack over much of northern Kansas, eastern Nebraska, western Iowa, and southeastern South Dakota from Tuesday, March 12 through Thursday, March 14. The rapidly melted snowpack had a liquid water content ranging from 2-4 inches in Nebraska and Iowa and higher amounts into South Dakota. The combination of rain and melting snow on frozen ground resulted in minimal infiltration and record runoff. Enormous inflows from this flood event came from every major tributary that enters the Missouri River from Niobrara, Nebraska to the confluence of the Platte and Missouri Rivers. The Niobrara River quickly filled the very limited flood storage capacity at Gavins Point Dam forcing increased releases out of the dam. Missouri River flood levels dramatically increased south of Omaha due to the record flows in the Platte River. The combined flows of the Platte and Missouri Rivers greatly exceeded the capacity of several levee systems.

The breach on the Clear Creek Levee System along the Platte River March 17.

The breach on the Clear Creek Levee System along the Platte River March 17, 2019.

Aerial photo of the L-611-614 Breach Location April 4.

Aerial photo of the L-611-614 Breach Location April 4, 2019.

Record Runoff

March 2019, and the months that followed, proved to be a challenging time for communities along the lower Missouri River due to record flooding. Many factors contributed to the flooding:  (1) a wet fall in 2018 that saturated soils and filled tributaries throughout the Missouri River Basin, (2) an extremely cold and wet winter, that resulted in deeply frozen ground, above-average snowfall and thick river ice, and the (3) the “Bomb Cyclone”, which dumped rain and snow on frozen ground in Nebraska, Iowa, and South Dakota, and included a rapid warm-up that caused snow to quickly melt over frozen, fully saturated soils.

The runoff entered the Missouri River upstream and downstream of the Missouri Mainstem Reservoir System (System).  The lowermost System reservoir is Gavins Point Dam.  During March, April and May, 20.8 million acre-feet (MAF) of runoff entered the Missouri River upstream of Gavins Point Dam, more than two times average.  The unregulated area downstream of Gavins Point to Sioux City, Iowa received about 6.6 MAF of runoff during that same 3-month period, which was more than six times average.  The largely unregulated area between Sioux City, Iowa and Nebraska City, Nebraska received nearly 7 MAF of runoff in this 3-month period, just under the long-term annual average of 7.7 MAF.

The Missouri Basin runoff during 2019 was at near-historic levels. The runoff in the upper basin (above Sioux City, Iowa) was 60.9 MAF, just 0.1 MAF less than the 2011 record runoff of 61.0 MAF (1898-2019).  The runoff in the lower basin (Sioux City, Iowa to the mouth) was 91.6 MAF, more than two times average and exceeded only in 1973 and 1993.  Total runoff in the Missouri Basin was 152.5 MAF, more than two times average and 0.1 MAF less than the highest runoff of record, which was observed in 1993. 

Between March 13 and March 23, 2019, per preliminary analysis by the National Weather Service, over 50 gages on the Missouri River and its tributaries in the region set new stage records. Nearly all of these new records were on unregulated areas of the Missouri Basin -- tributaries and the Missouri River downstream of Gavins Point Dam.  This severely limited the Corps’ ability to regulate the flows on the Missouri River downstream of Gavins Point. For example, the USGS estimated that the Platte River peak flow at Louisville, Nebraska, its most downstream gaging location, was a record 250,000 cubic feet per second (cfs).  The average annual peak flow at this location is approximately 50,000 cfs. This unprecedented amount of runoff resulted in the lower Missouri River being above flood stage at multiple locations for nearly nine months and caused billions of dollars of damage to homes, businesses, agricultural production, levees and natural resources across five states, including Nebraska and Iowa.   

Solutions for Change

The historic nature of the 2019 flood, in addition to severe flooding that has occurred over the past decade, served as a catalyst for the governors of Nebraska, Iowa, Kansas and Missouri to come together to discuss solutions for improving the resiliency of the lower Missouri River Basin. 

One of the solutions was to partner with the U.S. Army Corps of Engineers on a Planning Assistance to States (PAS) Study. A PAS study is a technical assistance study cost-shared 50-50 between the Corps and a non-federal sponsor that includes developing a plan that can help states and local entities make informed floodplain management decisions.

In November 2019, the Nebraska Department of Natural Resources (DNR) and Iowa DNR signed a PAS agreement with the Corps’ Omaha District to cost share a study that includes (1) working with stakeholders to identify problem areas and flood impacts along the lower Missouri River, (2) using existing data and hydraulic models, along with stakeholder input, to define existing conditions and develop conceptual-level solutions for identified problem areas, and (3) developing a flood risk management plan.

The Missouri DNR and Kansas Water Office signed a similar agreement with the Corps’ Kansas City District.  Since the study kickoff in February, the four states and two Corps Districts have been working as one integrated team in developing a plan that will provide a system-wide view of the problems and potential solutions.

Study Steps

The first step in the Lower Missouri River PAS study is to identify problem areas along the lower Missouri River (below Gavins Point Dam). These may be areas where: (1) there are known pinch points (bottlenecks in the river that could cause water to back up and worsen flooding), (2) recurring flooding causes repeat damages to property, crops, businesses, etc., and (3) flooding severely impacts critical infrastructure such as roads, bridges, water and wastewater treatment plants.

Once problem areas have been identified by the state partners and stakeholders, a set of criteria will be developed to rank and prioritize them. That prioritized list, along with any other relevant background information such as the indirect impacts of flooding (e.g., impeded access, loss of revenue, high groundwater or underseepage issues), and ideas for potential solutions will be provided to the Corps for further analysis.

Analysis will include using existing hydraulic modeling to investigate conceptual-level solutions proposed by the states and stakeholders. Solutions for a particular area might be structural (e.g., levee raise, levee setback, channel widening) or non-structural (e.g., raising or relocating buildings) or could include off-channel detention or road raises. Advantages and challenges associated with each potential solution will also be considered.

All of the information gathered and analysis completed will be documented in a flood risk management plan which can be used at the state and local level to help inform flood risk management decisions moving forward.