Dakota Beacon

Summary of Flooding Cause “Outlier” from Part I

Heavy snow melt in the mountains of Montana is a usual phenomenon. Heavy rains are also to be expected in the springtime on the plains. The difference, what we will call the “outlier”, distinguishing the flooding of 2011 from prior years was the uniform heavy snow cover over the upper great plains. The prairie winds usually re-distribute the snow, leaving vast areas of flat land with little snow cover and the low areas downwind of ridges and other obstacles filled to the brim with wind-blown snow to depths as high as 5 feet to 15 feet. In 2011, as high daily air temperatures approached 32 degrees, the soils underneath the snow melted the snow from the heat of the earth below and the soils of the upper great plains became uniformly and deeply super-saturated without any significant runoff overland. Normally soils absorb the majority of any rainfall, with the remainder running off as overland flow. When the heavy rains fell in Montana and North Dakota, the reverse was true with the majority of the rainfall running off as overland flow. This is why the dams filled quickly, causing the problems we are still dealing with.
The locations that were flooded that lie outside of the Missouri dam system are already in recovery mode, a good thing. The Missouri system’s future, however, is troublesome because of delta formation.

Formation of a Delta at the Tail Waters of Oahe Reservoir

The tail waters of the Oahe reservoir are now considerably further north than in prior years because the reservoir is full of water. As river flows reach the Oahe tail waters, velocity is reduced and a “delta” forms from solids that were in suspension before the water slowed down. We will call this the “Oahe delta”.

Because of the Oahe delta, the Missouri water surface elevations in the Bismarck area are not receding in proportion to the reduction of outflows from Garrison dam. The Oahe delta serves as an underwater dam that will only be breached once flows decrease enough to form one or more underwater channels through the Oahe delta.
So what must be done to insure we have time to repair flooded homes along the Missouri AND to insure against a repeat of the flooding next year?


Plan of Action for the Missouri River System

During the flooding, coordination of individual dam outflows of the six dams was critical to keeping the maximum river water surface levels minimized throughout the system of dams. The first point to be made is that each of the six dams will soon have the opportunity to vary their outflows more independent of the other dams than was possible during the flood event that is coming to a close.

The “trick” will be to allow nature to “cut one or more deep channels through the Oahe delta that has formed. As outflows from Garrison Dam are reduced, the tail waters must be monitored closely.

This will be a critical time to “manage the outflows of Garrison dam. Once one or more underwater channels are naturally cut through the Oahe delta, river levels through Bismarck may drop more quickly. River location/depth monitoring equipment will need to be employed to document channel formation that will occur just below the water surface at the Oahe delta.


The Critical Management Technique

Once flow through the Oahe delta approaches 95% to 100% channel flow, the Garrison outflows must be managed (perhaps even raised somewhat) to maximize the underwater channel formation. As the underwater channel through the Oahe delta grows, upstream water surfaces will be lowered. It may take weeks of monitoring and Garrison dam outflow variations (increases & decreases) to maximize the ability of the underwater delta channel to pass flows.


Preparing for Spring 2012

Subsequently setting a minimum water surface elevation through Bismarck to maximize outflows in preparation for spring 2012 will be necessary once Oahe delta channel flows are maximized.
Maximizing flow within the banks of the Missouri will be critical to insure no repeat of flooding in 2012. The more water we remove from Garrison dam this year, the less chance of a repeat of the fiasco in 2012.


A Groundwater Control Model

Some will suggest reducing Missouri River flows to historic flow rates to allow groundwater near the river to recede as much as possible. But lowering the river to historic flows prior to the winter freeze would be a huge mistake.

 Once again, the groundwater situation will need to be “managed”. “Lowest Floor Elevation” should be established for each of the flooded structures and placed in a database to find the Missouri river’s surface water profile that falls a few feet below the majority of such tabulated elevations. That water surface profile should become the target to seek prior to the winter freeze. It may correspond closely to Bismarck flood stage 14 feet, but that is just a guess. The database of lowest floor elevations should be used to decide the target. With a channel profile “target”, outfalls from Garrison dam can be maximized until river freeze-up occurs. To repeat, allowing flood stage to go down to 8 or 10 would be a huge mistake we could very likely pay for next year when moisture conditions could easily be even worse than they were this year.


Resisting the EASY WAY OUT

While the above may seem quite complicated, it is what hydrologists, engineers and other specialists GET PAID TO DO. Such professionals must be called upon to do their jobs. And politicians should restrict their activities to insuring that the best minds are employed to do what is right for the victims of flooding and to prevent flooding in the future. Politicians’ only legitimate role is to demand the best minds be employed to manage the situation. Perhaps most importantly, beware of politicians who suggest self-determined or knee-jerk solutions.


Mr. Bergman was born and raised in Grand Forks, receiving his Civil Engineering degree from UND. He worked as a municipal engineer in four cities (Grand Forks, Bismarck, Yuma, Arizona, and Colorado Springs, Colorado) over a period of almost 20 years. He then worked for a mining company in North Dakota for another 15 years, retiring in 1998. Development and demonstration of Portland-pozzolan cement and optimally durable concrete has been one of his life’s passions. Political activism is another interest and he is a Director of “Citizens for Responsible Government”, an organization devoted to fiscal responsibility in government.