Omaha World-Herald question/answer with the U.S. Army Corps of Engineers on the management of the reservoir system during the March 2019 storm and subsequent flooding.

The following Q/A is taking from several interviews with personnel from the U.S. Army Corps of Engineers. These answers and other interviews with Corps personnel were incorporated into the abbreviated version on Omaha.com.

Q. Talk, for a minute, about some of the actions taken by the Corps that succeeded in lessening flooding along the Missouri River.

First, the Corps shut down releases from Fort Randall , which is immediately upstream of Gavins Point, on March 13, during the time that the basin was peaking. As a result, the water in the upper Missouri River basin remained impounded during the worst of the flooding. The flooding that prompted the surge in releases from came from the unregulated Niobrara River and the smaller watersheds immediately around the dam. None of it came from that portion of the Missouri River impounded by the five upstream - the central and western Dakotas, Montana, Wyoming and far southern Canada.

"We stopped the upper 1440 miles of the Missouri River, the longest river in the nation (2321 miles), for four days," said Col. John Hudson, commander of the Omaha District. In the end, Fort Randall releases were at 0 cfs for 10 days, except for a 48-hour period on March 16-17 when a total of 20,000 acre-feet (AF) was released.

Second, the Corps operated the Gavin Point spillway gates to surcharge the pool. The pool level peaked 1.6 feet higher than its previous peak of 1210.7 feet in 1960. In 1960, the water spilled over the closed spillway gates. Surcharging has never been done at Gavins Point and has only been done a few times at other mainstem projects during major floods (for example, we surcharged Fort Peck and Garrison in 2011). To surcharge, all spillway gates need to be in operation. Doing so required breaking loose ice that had locked two of the fourteen spillway gates in place. The top of the exclusive flood control pool is 1210.0 feet, which is the height of the spillway gates when closed. By raising all 14 spillway gates to make releases, the storage in the reservoir increased. When the reservoir elevation peaked at 1212.3 feet, all 14 gates were each open 5 feet. The elevation at the top of the gates was about 1214 feet. The gates are curved, so a gate opening of 5 feet raises the elevation of the top of gates about 4 feet. By surcharging, we were able to maintain regulation of the project even though the peak pool elevation was above the top of the Exclusive Flood Control Zone. The gates were not designed to operate with water spilling over them, so the gates needed to be raised, which results in an increased release, to ensure that the water did not overtop them.

Q. How much of a difference did these efforts make?

The changes allowed the Corps to hold back some of the floodwater flowing into the Gavins Point reservoir. At its peak, the Corps estimated that the Niobrara River and its tributaries were sending more than 180,000 cfs into the Gavins Point reservoir. The maximum Gavins Point releases was 100,000 cfs (for a 6-hour period). The average daily inflow on March 14 was 125,000 cfs, which equates to 250,000 acre-feet of storage.

Typical Gavins Point daily inflow during March is 4,000 cfs, which equates to 8,000 acre-feet of storage.

The flow in the Missouri River between Gavins Point Dam and Omaha comes from many sources. Aside from the releases from Gavins Point Dam, water enters the river from the many unregulated tributaries (e.g., rivers that have no flood control dams), such as the Big Sioux, James, Vermillion, Floyd, Little Sioux, Boyer, Maple and Soldier rivers. ) The peak flows from those rivers were significant:

Big Sioux, 68,900 cfs on March 16 James, 23,400 cfs on March 15 Vermillion, 16,800 cfs on March 17 Floyd, 33,500 cfs on March 15 Maple, 26, 800 cfs on March 15 Little Sioux, 58,400 cfs on March 15 Soldier, 27,500 cfs on March 14 Boyer, 35,400 cfs on March 14

Q. How did inflows into Gavins Point compare to its flood storage capacity?

The entire Gavins Point reservoir has a storage capacity of 428,000 acre-feet or 0.428 million acre-feet (MAF). In comparison, Fort Peck, Garrison and Oahe (the big storage reservoirs) have storage capacities of 18.5, 23.5 and 23.0 MAF, respectively.

For Gavins Point, the Annual Flood Control and Multiple Use Zone extends from elevation 1204.5 to 1208 feet (79,000 acre-feet); the Exclusive Flood Control Zone extends from 1208.0 to 1210.0 feet (54,000 acre-feet).

On March 13, the Gavins Point pool was 1205.8 feet (107,000 AF of flood control storage space available from 1205.8 to 1210.0 feet). If no releases were made from Gavins Point, the March 14 inflow of 250,000 acre-feet would have filled the available flood control storage space (1205.8 to 1210.0 feet) in about 10 hours. If all of the flood control storage space (133,000 AF from 1204.5 to 1210.0 feet) were available, it would have been about 13 hours.

Q. You've described Gavins Point Dam as a re-regulation dam, what does that mean?

A. Gavins Point, the most downstream of the System projects, is primarily used for flow re- regulating to smooth out the hydro-power release fluctuations from Fort Randall Dam.

Q. OK, but why can't Gavins Point base elevation be lowered so that it holds back more floodwaters in addition to re-regulating Fort Randall's discharges?

Gavins Point Dam is one project in the Missouri River Mainstem system of six dams. To make a change like this at Gavins Point Dam would affect the entire system of six dams and require an examination of the potential effects to each of the authorized purposes throughout the system and along the full length of the Missouri River. As a runoff driven system, regardless of the size of the flood control storage, each year all the runoff must clear the system before the start of the next runoff season.

For this event, it is important to note that all releases from Gavins Point have been driven by the flood control purpose.

The Gavins Point reservoir extends from elevation 1160 (original streambed) to 1210 feet (top of gates when closed), which contains 428,000 acre-feet of storage. The lowest the pool could have been drawn down to was elevation 1180.0 feet, the spillway crest elevation. Drawing the pool down below 1204.5 feet would impact all the authorized purposes throughout the system. The lower its drawdown, the bigger the impact.

Regardless of the available storage, significant releases would have been needed from Gavins Point. The amount of runoff from the Niobrara River basin far exceeded available storage. The volume of runoff between Fort Randall and Gavins Point was:

1-day maximum volume (March 14): 235,000 AF 3-day maximum volume (March 14-16): 504,600 AF 7-day maximum volume (March 13-19): 731,700 AF

In other words, even with Fort Randall releases of 0 cfs and the Gavins Point reservoir empty, significant releases from Gavins Point would have been necessary.

Q. How did the failure of the Spencer Dam on the Niobrara affect operations of the Gavins Point Dam?

Minimally. A: With any dam failure, there is a sudden release or surge of water. The issue here is how much additional water entered the Gavins Point reservoir due to the failure of spencer Dam. Any water the was stored behind any ice dam/jam upstream of the Spencer Dam would have runoff into the Gavins Point reservoir with or without the failure of Spencer Dam. Therefore, it is only the water that was stored in the Spencer Dam pool that added to the total volume entering Gavins Point. The design storage of the Spencer Dam pool was about 40,000 acre-feet. The Spencer Dam storage had been significantly decreased by sedimentation making the additional water released from the dam failure less than 40,000 acre-feet.

There are streamgaging stations located along the Niobrara River. The large flows knocked one of the streamgaging stations that the Corps uses to monitor the Niobrara River offline.

Q. How did flows coming out of Gavins Point compare to the flows entering the Missouri from the Platte River?

A. The Platte River, and all other rivers and creeks in the Platte River basin (e.g., Elkhorn, Loup, Salt) contributed significantly more flow to the Missouri River than Gavins Point or any of the other tributaries between Gavins Point and Plattsmouth. At its peak on March 16, the USGS estimated that the Platte River contributed over 190,000 cfs into the Missouri River. The estimated average daily flows from the Platte River at Louisville, NE (the most downstream Platte River station):

1-day average flow (March 16): 167,000 cfs (331,000 AF) 3-day average flow (March 15-17): 162,000 cfs (964,000 AF) 5-day average flow (March 14-18): 145,000 cfs (1,438,000 AF)

Q: During February and early March, as snowpack built up in the Gavins Point Dam watershed, how come the Corps didn't start boosting releases from the dam to make room for additional runoff?

A. The Corps was making above-average winter releases of about 20,000 cfs (normal range is 12,000 to 17,000 cfs) throughout the winter season to finish evacuating all the 2018 stored flood waters. This was accomplished in late January when the System storage reached 56.1 MAF, leaving all 16.3 MAF of flood control storage available for the 2019 runoff season. On March 1, System storage was 56.1 MAF.

We monitor basin conditions on an ongoing basis, including mountain and plains snowpack. Our March 1 upper basin runoff forecast indicated that runoff would likely be slightly above normal and that we might need to start initiating flood storage evacuation measures once the river ice conditions between the reservoirs no longer presented a risk for ice jams from increased releases (in mid- to late March).

Q. In mid-March, when the so-called 'Bomb Cyclone' was forecast - rain on top of the snowpack - how come the Corps didn't, at that point, ratchet up releases from Gavins Point to adjust for the impending glut of run off? A. The National Weather Service forecasts precipitation amounts and river levels. The amount of rain in the forecast was not an unusually large amount. The forecast changed quickly in the final days before the storm hit, with successive forecasts increasing the expected temperatures and precipitation amounts. Additionally, the heaviest rains fell farther north than originally expected. The record runoff came from a combination of three things: 1) moderate-to-heavy rainfall, 2) on top of a heavy plains snowpack, 3) on top of frozen soils. The soil normally absorbs (e.g., infiltration) some of the rainfall and snowmelt. During this event, it’s estimated that the top 1 to 2 feet of soil was frozen, and nearly all of the rainfall and snowmelt became direct runoff. Assessing the volume of runoff that was going to coming into Gavins Point reservoir was extremely challenging, both for the NWS and the Corps. The inflow forecasts increased as more flow information became available.

"At the end of the day, it was an event that, every time we got the new forecast and expectation of what was going to happen on the river, it kept going up," Col. Hudson said. "There was very little time to react."

Q. When did the surge in releases from Gavins Point start reaching Plattsmouth and when did it subside (in other words, how long did it last)?

Release changes from Gavins Point reach Plattsmouth in 3-4 days.

The large increase in Gavins releases began started on early March 14 (from 33,000 cfs) to the early hours of March 15 (peak of 100,000 cfs). From early March 15 through March 20 we lowered the Gavins Point releases in a step-wise fashion to 24,000 cfs. As the inflows to Gavins Point lessened, we lowered Gavins Point releases in such a way that also resulted in the Gavins Point pool elevation to decline out of the Surcharge (below 1210.0) and Exclusive Flood Control zones (below 1208.0), respectively.

Q. As early as March 7, the National Weather Service in Valley was warning of major flooding, and by March 15, it was warning that people shouldn't assume levees on the Platte, Loup and Elkhorn would hold. Why didn't the Corps have similar concerns about the Missouri River levees in the flood-prone stretch of river between Plattsmouth, , and St. Joseph, Missouri?

The NWS’s MBRFC had issued Hydrologic Outlooks on February 22 and March 7. They hosted a webinar to present their outlooks. During both webinars, the NWS indicated a strong possibility of major flooding in this area. The State and County Emergency Operation Centers are the primary audiences of this webinar.

The L-575 levee near Hamburg was overtopped around midnight March 14, less than a day after the storm hit the area. While the NWS was assessing the extent of runoff in the basin and the Corps was increasing releases from Gavins Point to account for the extraordinary flows in the Niobrara River, the L-575 levee had already overtopped – stage of 27.0 and estimated flow of 214,000 cfs. The Gavins Point release on March 10 (travel time from Gavins Point Dam to Nebraska City is about 4 days) was 20,000 cfs.

Regarding the overall management of the river:

Q. When was the last time the Corps adjusted flows in the Missouri River to provide for endangered and threatened species?

In the spring of 2017 we performed two 3-day cycles from Gavins Point dam. A “cycle” involves lowering the Gavins Point releases by 6,000 cfs for 2 days to lessen downstream peaks, then increasing the release back to the “steady release” rate to discourage T&E nesting in lower portions of sand bars.

The Gavins Point bimodal spring pulse, which was only conducted in 2006, 2008 and 2009, was removed from the 2018 Master Manual.