A Dam Incident at Table Rock Lake Dam

A DAM INCIDENT AT TABLE ROCK LAKE DAM
D. Wade Anderson, P.E.1
Elmo J. Webb, P.E.2
Bobby Van Cleave, P.E.3
ABSTRACT
Table Rock Dam, Power Plant and Auxiliary Spillway are located on the White River
approximately 8 miles southwest of Branson in southwestern Missouri. Significant
releases or failure of this dam would lead to catastrophic loss of life and significant
economic loss in the Branson, MO and White River, MO area.
During the spring of 2011, extreme rainfall occurred over the White River Basin in
Arkansas and Missouri. Table Rock Lake experienced record inflow, record releases,
and a record pool. As the pool approached a new record pool and releases exceeded
previous records, a slide developed on the upper downstream slope and extended to the
crest of the dam. Operations personnel quickly notified the Dam Safety Officer who
mobilized geotechnical and dam safety engineers. Once on site the engineers assured
Operations, Water Management, and the Dam Safety Officer the dam was not at risk of
failure. With the support and leadership of the Dam Safety Officer, the team of engineers
further evaluated the condition of the dam and expeditiously developed a contract to
repair the slide. The emergency contract was awarded within 4 days of the incident and
repairs were completed within 14 days.
The response to this incident highlights the ability of Operations, Engineering and
Construction, and Contracting personnel to respond to dam safety emergencies and how a
focused team of multi-disciplined engineers from two districts were able to quickly
evaluate, inform leadership, and respond to such an incident. Additionally, it emphasizes
the need to have well trained and experienced engineers available to properly ascertain
and design remediation efforts for dam safety deficiencies.
PROJECT DESCRIPTION
Little Rock District operates six flood damage reduction lakes in the White River Basin.
The basin drains 28,000 square mile across southern Missouri and northern Arkansas and
empties into the Mississippi River near Tichnor, AR.
1
Dam Safety Program Manager, U.S. Army Engineer District, Tulsa, OK 74128,
[email protected]
2
Civil Engineer, U.S. Army Engineer District, Little Rock, AR, 72201, [email protected]
3
Geotechnical Engineer, U.S. Army Engineer District, Little Rock, AR, 72201,
[email protected]
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Location
Table Rock Dam is the second most upstream dam and is located approximately 8 miles
southwest of Branson in southwestern Missouri as shown in Figure 1.
Figure 1. Table Rock Dam Location
Project Features
Table Rock Dam, Power Plant and Auxiliary Spillway are located on the White River.
Project structures include a concrete gravity dam with 10 tainter gates flanked by
concrete non-overflow sections, a powerhouse integral with the concrete dam, a concrete
auxiliary spillway structure with 8 tainter gates, and earth-fill embankment sections
abutting the concrete dam structures, as shown in Figure 2.
The main concrete dam is 1602 feet-long with a maximum height of about 260 feet above
stream bed. The concrete gravity dam is controlled by 10 tainter gates, measuring 45
feet-wide by 37 feet-high, flanked by gravity non-overflow sections. Water is fed to the
main power turbines via four gate controlled penstocks measuring 18 feet in diameter. In
addition to the tainter gates, there are four gate controlled flood control passages
measuring 4 feet by 9 feet.
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The earth-fill embankment section is about 3800 feet long on the left bank and about
1020 feet long on the right bank. The embankment consists of compacted, zoned,
engineered fill. Internal zoning includes a massive impervious fill zone flanked by a
downstream random fill zone overlying a horizontal drainage blanket, which extends to
the downstream toe. The embankment has upstream slopes of 1V on 3H and downstream
slopes of 1V on 2H in the upper embankment and 1V to 2.5H on the lower portions of the
embankment. Upstream slope protection consists of 18 inches of riprap on 6 inches of
bedding. The downstream slopes are grassed. The embankment crest is capped with an
asphalt roadway with guardrails
The auxiliary gated spillway consists of a new gated ogee spillway, earthen embankment,
spillway bridge, roadway, training dike, approach channel, and equipment building. The
auxiliary spillway has eight 14.75 meter wide by 13.26 meter height (43.5 feet by 48.4
feet) tainter gates with seven intermediate piers each having a width of 3 meters (10 feet).
Each gate is operated by two hydraulic cylinders. The concrete ogee weir has a crest
elevation of 273.10 (896.0), which matches the crest elevation of the existing dam. The
ogee is founded at approximately El. 267.66 (878.19) and is 26.68 meters (88 feet) in
length (perpendicular to dam axis). Overall, the spillway is approximately 142 meters
(466 feet) wide. A transitional non-overflow section is located at each spillway
abutment. A non-overflow retaining wall section is on the right side of the spillway to
accommodate storage of the spillway maintenance bulkhead.
The embankment portion of the new dam consists of a right embankment which is 230
meters (755 feet) long and a left embankment, which is 115 meters (377 feet) in length.
The right embankment is integrally tied to the existing dam embankment. The left
embankment ties into natural ground. Both embankments have a top elevation of 288.6
(947 feet). Both embankments consist of a clay core with a rock fill shell and serve as
the new roadway. The embankments are constructed with a 1V:2.5H slope and each has
a crown width of 15.24 meters (50 feet). Concrete retaining walls are located at the ends
of each embankment (both upstream and downstream) where the embankments abut the
new concrete structure.
Pertinent Data
Pertinent data for the Table Rock Dam is:
Feature Elevation
Top of Dam
Maximum Pool
Top of Original Gates and
Flood Control Pool
Conservation Pool
Spillway Crest
Previous Pool of Record (April 2008)
New Pool of Record (April 2011)
Dam Incident at Table Rock Lake Dam
947.0
942.0
(ft)
933.0
915.0
896.0
933.25
935.47
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Auxiliary Spillway
Embankment
Main Spillway
Figure 2. Project Features
APRIL 2011 EVENT
Rainfall
During the month of April 2011, southern Missouri and Northern Arkansas experienced
significant rainfall. The 14-day rainfall amounts exceeded 20 inches in many area and 15
inches over most of the White River basin. Most of the rainfall occurred in a 72-hour
period from April 22nd to April 28th. Rainfall amounts measured at Table Rock Dam
exceeded 10-inches between April 22nd and 28nd and 13 inches for the month of April.
Rainfall amounts measured at Beaver Dam, just upstream of Table Rock Dam, exceeded
11.5-inches between April 22nd and 28nd and 15 inches for the month of April.
Table Rock Records
During this event, Table Rock experienced several record events. The rainfall was much
greater than normal. The peak inflow reached 290,000 cubic feet per second (cfs) with a
6-hour average of 240,000 to 250,000 cfs. The maximum release reached 68,000 cfs, as
shown in Figure 3, was well above the previous release of 48,000 cfs.
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Figure 3. Record Release
Table Rock also experienced a record pool elevation of 935.47 feet, as shown in Figure 4
and 5. The pool was just below the cat-walk and wave action was minimal so access for
tainter gate operations was not affected.
Figure 4. Record Pool on Tainter Gates
Figure 5. Record Pool
RESPONSE
Project Personnel
As the pool continued to rise, a slide was detected by the Project Office personnel on the
downstream slope near the top of the dam, as shown in Figure 6. Project personnel
immediately notified Engineering and Construction Division and the Dam Safety Officer
(DSO). At the time of reporting, the magnitude of the slide was unknown. The pool was
continuing to rise and expected to reach a record pool.
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Figure 6. Slide on Downstream Slope detected by Project Personnel
Request Assistance
Due to wide spread flooding throughout Little Rock District, the Dam Safety Officer
(DSO) requested geotechnical engineering assistance from neighboring Tulsa District. A
senior and a journeyman geotechnical engineer were immediately dispatched to Table
Rock Dam from Tulsa District. The Tulsa District engineers worked closely with
engineers and operations staff from Little Rock District and reported directly to the Little
Rock District DSO.
Evaluation
Upon arrival, geotechnical
engineers determined that the
slide was limited to the steep (1V
to 2H) upper slope of the dam
and had not progressed into the
crest of the dam or the
embankment core. The soft wet
material on the lower slope, as
shown in Figure 7, was material
from the upper slope that had slid
over the grassed surface of the
lower slope. Although the slide
continued to grow in length,
Figure 7. Material of Upper Slope slid down
engineers did not expect the
Lower Slope
slide to progress further upstream.
The DSO was advised that increased releases to maintain or lower the pool were not
recommended. This evaluation and recommendation was accepted and additional flood
damage that would accompany required evacuations were avoided.
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Road Closure
The state highway on top of Table Rock Dam was closed upon detection of the slide.
Following evaluation by geotechnical engineers, the traffic control was revised to closure
of the downstream lane only. The state highway department facilitated this road closure
and set up the traffic control, as shown in Figure 8.
Figure 8. Road Closure by Missouri Department of Transportation
REPAIR DESIGN AND CONSTRUCTION
Design
Various alternatives were considered to repair the slide. Initial designs included removal
of the slide material and replacement with a locally available rock quarry run material.
Since this material would not meet filter criteria, a more permanent design was
developed.
The emergency repair consisted of excavation and removal of the slide material,
placement of an 18-inch sand filter against the existing core and bottom of the
excavation, a gravel zone at the base of the sand to allow drainage of any embankment
seepage or infiltrated water, backfill with compacted clay soil and top soil, as shown in
Figure 9. The purpose of the sand filter was to provide drainage of the upper backfill and
serve as a filter and drain to any seepage through the embankment core.
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Figure 9. Upper Slope Slide Repair Design
Emergency Contract
An emergency contract was developed within three days of the event. Two local
contractors were invited to the site to discuss the repairs. Some modifications were made
based on their comments. Contractors were given 24 hours to submit a bid. Both
submitted a bid on time. The contract was awarded the same day.
Construction
Construction began immediately upon award of the contract and began with removal of
the slide material as shown in Figure 10.
Figure 10. Removal of Slide Material
As the repair progressed, additional rainfall occurred. The contractor protected the
excavated slope with a thin geomembrane, as shown in Figure 11. As the excavation
progressed, it was discovered the removal depth of 15-feet was not sufficient. Engineers
completed estimates of excavated slope stability and recommended the excavation could
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safely proceed to a depth of 19-feet. Once the rain ceased and excavation proceeded, a
maximum depth of 17-feet was required to remove the soft and wet slide material.
Figure 11. Protection of Excavated Slope
Once the slide material was removed, the sand filter was placed against the embankment
core and base of the excavation as shown in Figure 12. Following placement of the
granular filter material, the embankment was reconstructed using clay borrow materials.
Figure 12. Placement of Sand Filter against the Embankment Core
The repair was completed in approximately 14 days allowing the highway to be reopened.
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163
Figure 13. Completed Repair with the exception of Surface Erosion Protection
LESSONS LEARNED
Several lessons were learned throughout this incident. They included:
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The Corps Projects on the White River performed as intended, providing flood
risk management to the region and nation.
We should anticipate the unexpected may occur during emergencies. For
example, Table Rock Dam had never experienced a slide in over fifty years of
operation.
Resources will be thin during regional emergencies. Expect that leaders and
decision makers may be out of reach or otherwise occupied and plan accordingly.
The Corps can successfully respond to various emergency events.
Project Personnel dam safety training is critical for successful response to
emergency events.
Communication between various entities required for timely response is critical.
Communication paths between key Project Personnel to the DSO, Engineers on
Site to the DSO, Engineers to the Lake Manager, the Lake Manager and Public
Affairs Officer to the Public and Media need to be maintained.
Constantly educate the public on the purpose and value of Projects and
consequences without Projects or poor performance of the projects
Consulting engineers from other districts for critical dam safety decisions in
regional emergencies relieves pressure on resources and can help ensure good
outcomes.
Experienced dam engineers are invaluable in an emergency.
REFERENCES
USACE, Little Rock District. Table Rock Dam, As Built Drawings.
USACE, Little Rock District. Table Rock Dam, Periodic Inspection.
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