| How
Clearwater Lake Helps Reduce Flood Damage
PIEDMONT,
Mo. April 29 – The effects of repeated rain in the White River Basin in
Arkansas and Missouri are prompting many people to ask how the Army Corps
of Engineers operates Clearwater Dam and Lake to reduce flood damage.
Others want to know what to expect as the rainy season continues.
Clearwater, while located on the Black River, is one of six Corps dams
in the White River basin that help reduce downstream flood damage during
heavy rains. The Black River is a major tributary to the White River.
The confluence of the two rivers is just upstream of Newport, Ark.
How These Lakes Work
Flood damage reduction lakes like Clearwater work by capturing and holding
upstream runoff during heavy rain. After rivers downstream begin receding,
water is released in a controlled fashion following pre-determined plans.
Without the lakes, all that water would rush downriver at one time. Flood
crests would rise higher and spread over more land, thus causing more
damage and possibly loss of life. The lakes are not able to be operated
with a particular house or address near the river in mind, but instead
take into account water levels for many miles downstream.
Reservoir Gate Operations
Flood control projects have designated space to temporarily hold rain
runoff. After a rain the stored water must be evacuated in preparation
for the next storm as quickly as possible. Downstream conditions limit
the amount that can be released. The releases are planned to limit downstream
flooding. Releases are typically made through a conduit at the bottom
of the dam. When the lake is very high (above top of flood pool) water
will flow through the spillway.
The release schedule is directed by engineers, who are often balancing
a system of reservoirs emptying into the same river system. For that reason,
the regulating team is located at a central location to the system of
lakes, but not at the lake. The release plan must consider two basic concepts:
(1) when is there room in the river for more water, and (2) when will
rainfall take away that room.
Figuring out how much room is in the river is fairly simple, but planning
for rainfall is very difficult. Shut the gates too soon, and the lake
fills and unnecessarily loses capacity to deal with the next rain. Wait
too long and flood peaks are not decreased as much. The compromise between
those choices is to actively follow the rainfall with a series of predetermined
gate shut-backs.
At Clearwater, the gate shut-back protocol is governed by a "shut-back
index." The shut-back index is the amount of rainfall at which the
dam tender is instructed to begin automatically shutting the gates. The
index is the amount of rainfall that would reasonably be conceived to
bring the downstream river to bankfull. The measurement of the index is
based on upstream conditions and adjusted for the time of year, amount
of preceding rainfall, time since last rainfall, and the excess downstream
capacity. The most important concern is the downstream capacity. As a
rule of thumb, for a full release about a 1" index is typical. Indices
may even approach 3" during the dry times and 0.1" in the days
following a large spring storm. Reaching the shut-back index doesn't mean
completely shutting the gates at once; but rather they are closed in phases
dictated by additional rainfall. Most often, the phases are completed
in 0.5" rainfall increments and take three shut-backs to reach minimum
release.
Ultimately,
the object is to get stored rainwater out of the lake as quickly are possible.
That fast response makes it possible for the reservoir to be ready for
the next big rainfall.
Dams Have Limitations
The lakes are not intended to and cannot prevent all flooding. Some people
think they are fully protected downstream of a big dam. Not so. The lakes
have capacity limitations that Mother Nature can exceed, and from time
to time she does.
There have been too few dry days between rains this spring. The difficulty
with repeated rains is engineers are unable to release all captured water
between them. This caused lake levels to rise with each new rainfall.
All six Corps lakes in the White River basin have been full or near full.
Once full, a lake has little ability to reduce downstream flood crests.
Water still flowing into the lake must be released through gates, powerhouse
turbines or a spillway at nearly the same rate. This is referred to as
“passing the inflow” or “open river.” At Clearwater this begins to occur
when the pool rises to elevation 567 feet. If the inflows are high enough
a combination is used, such as making releases through gates and a spillway
at the same time.
When passing the inflow, a dam does not make conditions downstream any
worse than the natural condition the river would be in without the dam
in place. It is just that the dam is unable to do very much (if anything)
to reduce downstream flooding under those extraordinary conditions.
Also, there are large areas of each river basin that have no dams to capture
and hold the runoff. For instance, the six dams in the White River basin
only control about 40 percent of the drainage area. About 60 percent of
the basin drains into the rivers downstream of the dams.
The White River Basin lakes, including Clearwater, are still basically
full. While no water has passed over Clearwater’s spillway yet this spring,
it could. This has occurred at several other district lakes.
What
You Should Do
And since it is still early in the rainy season, people who live in the
flood plains of the rivers downstream are not out of the woods. It is
possible locations that have not flooded in many years could flood.
People who live or work downstream in low-lying areas near a river should
maintain awareness by monitoring weather and news media. If more heavy
rain comes, we could see repeat episodes of flooding downstream of any
of the dams similar to that experienced below Beaver, Table Rock and Norfork
Dams in early April. In fact if heavier rains should fall, flooding could
be worse.
If heavy rains are forecast, people should decide beforehand whether to
move belongings to higher ground. Once rain begins, flows can increase
quickly with only short advance notice, perhaps no more than an hour our
two. People in at-risk areas should also stay in contact with local emergency
officials. Warnings will go out through local emergency management officials,
and they may not know everyone who requires notification unless you have
told them so.
The Lakes Are Doing Their Jobs
Corps officials understand it may be hard to see this now, but Clearwater
and the other five White River Basin lakes have done a good job reducing
flood damage. While it will be some time before a dollar amount of damage
prevented can be calculated, it is already obvious Clearwater has reduced
damage downstream substantially.
For instance, in March when the Black River hit a record high stage at
Poplar Bluff, Clearwater kept the main city levee and floodwall from overtopping.
In April, Clearwater substantially reduced the crest experienced in the
vicinity of the Highway 49 Bridge when McKenzie Creek flooded. Had the
dam not be there, the estimated peak flow April 10 at Leeper would have
been 54,000 cubic feet per second, four and a half times more water than
actually experienced.
Many
communities throughout this basin are still affected by the extreme weather.
Many employees of the Corps’ Little Rock District are working long hours
traveling to riverside communities providing technical assistance and
flood fighting materials, as well as coordinating with emergency response
officials. Still other employees are intensively managing the lakes.
Work Continues on Clearwater Dam
Meanwhile, construction on the Clearwater Dam Major Rehabilitation Project
has been going well. Workers continue to drill holes into the bedrock
beneath the dam. Those holes along with any cavities discovered are then
filled with grout. This has helped reduce problems associated with chronic
seepage beneath the dam.
Geotechnical engineers say, by all measures, the dam is in better condition
than in 2003. However, the dam will not be considered fully repaired until
the cutoff wall is completed in about five years. While declining throughout
construction, risks will still remain until the work is finished.
For More Information
Do not rely on rumors. If you have questions about Army Corps of Engineers
dams, contact your local Corps office. For more information about water
levels at Clearwater, call (573) 223-7777, or write the Corps at RR 3
Box 3559-D, Piedmont, MO 63957, or e-mail Clearwater.Project.Info@usace.army.mil.
The Corps has detailed water management information available on the Internet
at http://www.swl-wc.usace.army.mil/.
For more information and photos about the Clearwater Dam rehabilitation
project, visit the Corps website http://www.swl.usace.army.mil
and click “CW Lake Major Rehabilitation” under Key Projects and Issues.
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