Managing lake levels, dam safety prove challenging in the Natural State

by Kim Souza ([email protected]) 2,136 views 

Beaver Lake Dam

Dam safety is a national priority for the U.S. Army Corp of Engineers (USACE) which regulates 716 dams across the country and 25 facilities in Arkansas.

Bradley Clark, a geotechnical engineer with the U.S. Army Corp of Engineers, said USACE owns and oversees 716 dams across the U.S., with 66% being embankment dams and 7% are concrete, and 7% being a combination of both. He said the median height of the dams is 93 feet and the average age is 58. He said some of the oldest dams are about 160 years old and there are 14 dams over 100 years old which require maintenance and vigilance given there are populations living downstream.

Dams are expensive to build and they seldom die of old age if USACE and state regulators can possibly help it, he said.

Clark and several other Arkansas water officials recently convened in Fayetteville to discuss water safety and water quality issues with growing populations during a two-day conference conducted by the Arkansas Water Resources Center and the University of Arkansas Division of Agriculture.

Clark said in the USACE Little Rock District, which includes Northwest Arkansas, there are 25 dam facilities and 13 are lock and dam operations. He said USACE is focused on managing lake levels to avoid downstream flooding which is the corps’ biggest challenge. He said better warning systems that use technology combined with lake instrumentation is providing earlier warning capabilities. In the Little Rock district he said the projects are broken down in groups of 5 that one engineer monitors, which includes routine reporting during the year and a full department audit every five years.

In the past six years USACE has moved from a standardized rating system to more of a risk-assessment rating given the age of the dams and new dam-monitoring technologies. He said the dams are evaluated based on hazards that could occur with extreme conditions and who is at risk downstream. He said this kind of analysis gives the corps a better way to prioritize toward mitigating those risks. He said dams are rated between 1 to 5 with 1 indicating an immediate need or concern and 5 means the dam meets all the USACE guidelines with a very low risk.

Clark said Beaver Lake is rated a 3 and is in the process of possibly being upgraded to a 4. He said as of last year none of the dams in the Little Rock district were rated a 1 or 2, but there were 16 dams elsewhere in the country with 1 ratings and 67 rated at 2. In the Little Rock district there were 5 dams rated a 3 and 5 rated a 4.

Nathaniel Keen, chief reservoir control engineer for USACE, spoke to the group about the lakes in the district. He said Beaver Lake has the smallest flood pools and is the least likely handle a few inches of intense rainfall. He said Beaver Lake does have a large surplus pool of several feet once the gates go up. Beaver Lake is a drinking water source for most of Northwest Arkansas and the lake has a very large conservation pool for that purpose.

Keen said Beaver Lake differs greatly from other lakes in the district, namely Clearwater Lake near Hot Springs. He said Clearwater Lake was originally built to be a hydropower lake but its conservation pool is much too small, though it has a very large flood pool with a very small surplus pool at the top. He said Murray Lock and Dam in North Little Rock is just a small conservation pool, having no surplus, nor flood pools. He said excess water at Murray flows into the Arkansas River and a couple of miles upstream during a flood it’s as if the dam does not exist.

He said the spring rains created a number of challenges for the USACE because of the demands voiced from those who use the lakes and streams for various reasons. He said those who live on Beaver Lake want the lake to be left at the same level all the time, but Keen said if that happened drinking water sources would be sacrificed because inflows must equal outflows. Stable lake levels also sacrifice hydropower.

“There are big challenges in thinking about who is downstream and how they may be impacted by the way water is used upstream. If we fill the lakes really high in the spring then we are increasing our flood risks. Thinking downstream we have farmers who need the water levels to stay low because flooding could cause ecological damage to their farm fields. There are lots of folks who rely on our lakes and streams and they have very different needs based on their locations,” Keen said.

He also reiterated the USACE’s main focus is on safety among those in the path of flooding. He also said earthquakes can trigger the need for dams to be re-examined and the benchmark for that is a 4.5 magnitude quake. But he did say if enough people felt it and and report it then usually a team will do an inspection.

Trevor Timberlake of the Arkansas Natural Resource Commission said the state program is separate from the USACE federal agenda, and it’s necessary because there are 1,257 dams in Arkansas. Timberlake said 409 of them are regulated by the state based on their size and risk downstream, and 114 of those are deemed high hazard, meaning if they were to fail there would be probable loss of life downstream as well as economic losses. There are 92 other dams rated significant hazard and 203 deemed less hazardous.

He said 77 of the state-regulated dams are between 58 and 77 years old. But there is a dam near Hot Springs that is 138 years old. The majority of them (226) were built between 1960 and 1979. Since 1980 there have been 87 dams built in Arkansas which are large enough to require state monitoring.

He said dams fail for numerous reasons most of which begin as little problems that gradually weaken the structure so when flooding occurs and pressure builds behind the dam, the structure integrity of the wall often falls. He said the most notorious flood in U.S. history occurred in Johnstown, Penn., on May 31, 1889 when 16 million tons of water was released into the Little Conemaugh River, 14 miles upstream from Johnstown. When the dam failed on a Sunday afternoon it rushed into the town killing 2,209 people in its path. The damages to the town were equal to $473 million today.

Timberlake said the 1970s also were a time of multiple dam failures in five different states with loss of lives and economic destruction. Because of those failures he said new federal rules were implemented and states like Arkansas began safety programs to monitor and regulate dams outside the federal purview.

All the engineers who spoke on dam and water safety agreed that risks are higher today because more people live the flood paths of very old dams which has USACE and state regulators on vigilant watch.