Corps’ Flood Damage Reduction Reservoirs Recover from Flooding, Prepare for Spring Rainy Season

Published Feb. 19, 2016
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LITTLE ROCK, Ark. - The six White River basin reservoirs operated by the Army Corps of Engineers Little Rock District were key players in reducing the amount of flooding experienced in December across northern Arkansas and southern Missouri and are being readied for the spring rainy season.

Even though releases from Beaver and Table Rock lakes were more than the “typical” flood control releases, the dams reduced flooding by releasing far less than the inflow coming into the lakes during this significant rain event.

The reservoirs, Beaver, Table Rock, Bull Shoals, Norfork, Clearwater and Greers Ferry lakes, capture and hold upstream runoff during heavy rains.  Without that storage, all that water would pour downriver at one time and add to flood crests already developing from rain falling downstream.  Those flood crests would rise higher, spread over more land, and cause significantly more damage and loss of life. 

Without lakes like Bull Shoals and Norfork near Mountain Home, Ark., working with the other four lakes, many more homes and businesses would have been flooded, the trout fishery would have been destroyed, towns like Cotter, Norfork, Calico Rock, and Batesville would have flooded, and most of the major levees downstream would have overtopped, inundating towns like Jacksonport, Newport, and Des Arc as well as flooding thousands of acres of prime farmland. 

Mike Biggs, chief of the Hydraulics and Technical Services Branch was responsible for regulating all six reservoirs during the flood fight, said flows moving into the Branson, Mo., area from Table Rock Dam would also have been about four to five times greater. 

“Preliminary estimates indicate the flood crest would have been 20 feet higher at Branson, Mo., inundating hundreds more homes and businesses in the Branson area,” Biggs said.  “While the largest release from Table Rock was about 72,500 cubic feet per second, flows coming into the lake exceeded 345,000 c.f.s.” 

Engineers in Biggs’ office reported that flows into Beaver Lake were in excess of 200,000 c.f.s. while releases were limited to about 91,000 c.f.s. 

If the dams were not operated as they were, homes and businesses downstream from Beaver Dam could have experienced flows of more than 200,000 c.f.s. for many hours and the Branson area could have seen a flow rate of nearly 345,000 c.f.s.  Those amounts are vastly different than the flows the Corps released from the dams.

Since the rivers downstream have begun receding, water continues to be released in a controlled fashion following pre-determined plans.  The water control engineers report that several lakes are still high, and it will take several more weeks to empty the flood pools and get the lakes back down to their conservation levels. 

“We hope that the basin doesn’t receive any additional heavy rains fall until we can evacuate quite a bit more water from the flood pools,” added Biggs.

Dams have limitations

Biggs explained that the difficulty with excessive or repeated rain is water control engineers are unable to capture and hold all of the rainfall when it exceeds the design limits of the dams or release all captured water between rains.

            “Most people don’t understand the engineering that went into the design of the dams,” said Biggs. 

It all begins with rainfall.

“The size of the lake was determined by historical rainfall, so, the lakes have capacity limits that Mother Nature can exceed, and from time to time, does, which was the case this past December,” said Biggs. “Our lakes received two and one-half to three and one-half times the average monthly rainfall in less than two days.”

The lakes may hold three to four inches of rainfall spread over 30 days, but when the lakes gets eight to 10 inches or even 12 inches of rain in 30 hours, they simply cannot hold that much.

Biggs explained, “When the lake fills to capacity, water still flowing into the lake must be released from the dam because there is no storage space left.” 

When this happens the dam’s ability to help reduce downstream flood heights is reduced. 

“At this point, the Corps opens the spillway gates that create the large releases downstream, but even in doing this, the lake is temporarily storing more water than it is releasing,” said Biggs.  “That is how we were able to release only 72,500 c.f.s. into Lake Taneycomo when the flow into the lake was 345,000 c.f.s.

“I don’t think people appreciate the benefits that Table Rock Dam was providing at that time,” said Biggs.

Even when making a larger than typical release, a dam does not make conditions downstream worse than the natural condition the river would be in without the dam in place.  It is just that the dam has less capability to reduce downstream flooding under those conditions.

Also, some people think they are fully protected downstream of a big dam. 

“Not so,” said Biggs.  “The lakes are not intended to and cannot prevent all flooding.” 

Rather the dams were designed to reduce the peak of the flood.  Those living around the lakes or downstream next to the rivers and creeks live with the risk of flooding every time it rains heavily.

What the Corps sees happening is that more and more people continue to encroach closer into the flood plain around the lake and in the downstream reaches.  Then when the Corps needs to make a release from its dams, people get angry about how it operates the dams.

Why the Corps doesn’t release water before the rain begins

Corps guidance requires its staffs to operate their lakes based on runoff from rain that has occurred and can be measured.  Operating the dams based upon “water on the ground” allows engineers to make decisions based upon facts and engineering calculations rather than estimates.  This best provides the benefits intended for all the stakeholders. 

Pre-releasing before the rain has occurred puts the areas protected by the dam at greater risk if more rain develops or is more intense than forecasted in the protected areas.  If engineers released water before rain fell and future rain then caused an area to be flooded or flooded to a higher level, then the Corps would have added to that flood level. 

Also, rainfall forecasts are not sufficiently accurate to base operational decisions based on forecasts of future rain.  Engineers have to know the amount of rain, its intensity, and its duration to calculate runoff over very large areas.  Just because the broadcast weather forecast shows rain, it does not provide this detail. 

“How many times have you made plans for a sunny weekend on Monday to find out Friday night that it will be rainy,” asked Biggs.  “This happens all the time because long-range forecasts are not accurate.  By the time a weather forecast becomes accurate, there is not enough time to evacuate enough storage from the lakes to effect lake levels.” 

What has changed?

Many people ask why the Corps has changed the way it operates the lakes.

“We haven’t,” said Biggs. “The water control plan used today is essentially the same as the one used to design the lakes—size of the flood pools, height of the dam, size of the spillway gates, the real estate purchased, etc.—in the 1940’s and 1950’s. The water control plan was part of that design.” 

Changing the operational plan could invalidate the engineering that built the dam, and thus it requires extensive study to enact change. 

“We follow that plan dogmatically because, over the long term, the congressionally authorized plan provides the greatest benefits to the greatest number of people,” said Biggs. “It’s not the best plan for any single person or community, but it is the best plan for all.”

So what has changed?  Since the dams were placed in operation, people have encroached into flood-prone areas.  This includes businesses, homeowners, fishing facilities, agriculture, and about any form of man-made development one can think of.

“It makes our job more difficult with each flood,” said Biggs. “Property exchanges owners and the new owner exclaims that I have never seen the lake (or river) this high before; you must be doing something wrong!” 

Weather has changed.  The floods of 1927, 1935, and 1945 are historic floods occurring before the dams were built.  Shortly after the dams were built, the area experienced an extended dry spell.  Over the decades since the dams were built, the weather has fluctuated between drought and wet patterns.  The floods of 1982, 1990, 2008, 2011 and 2015 have reminded everyone why the dams were built in the first place.

Weather is not static, it is always changing.  The one constant is that the Corps’ flood risk reduction dams are doing the job of reducing flooding downstream of its reservoirs and the methodology used to operate the reservoirs is based on science proven statics.  The long-standing water control plan followed by the Corps’ engineers was designed to provide public safety, protect the Corps infrastructure, and provide the greatest benefit to all. 

Unfortunately, when Mother Nature provides us with more water than we need or want, some living and working in flood prone areas will still experience flood waters.  The bottom line is “always respect the river.”  

Laurie Driver

Release no. 16-016