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Oroville Dam Will Not Fail

Feb 22, 2017 by

Oroville Dam Will Not Fail

By Henry W. Burke

2.22.17

 

The 770-ft.-high Oroville Dam was never in danger of failing. 

The Main Spillway is damaged, but the earthfill dam is safe.

The Department of Water Resources (DWR) can prevent future problems through proper dam management and significant water releases from the Main Spillway.

DWR must immediately design a replacement for the damaged Main Spillway, then award a contract to execute the work this summer (during the dry season).

 

On Sunday, February 12, 2017, California officials ordered the immediate, mandatory evacuation of 188,000 residents from towns below Oroville Dam.  When federal and state officials deemed that the dam was safe on Tuesday, February 14, the evacuation order was rescinded and people were allowed to return to their homes.

What are the facts about Oroville Dam?

 

  1. OROVILLE DAM DESIGN

Oroville Dam is located on the Feather River about 65 miles north of Sacramento, California.  At 770-feet high, Oroville Dam is the nation’s tallest dam.  Unlike the iconic concrete dams, Hoover Dam and Glen Canyon Dam, Oroville Dam is a zoned-earthfill structure.  Oroville Dam is huge, containing 80 million cubic yards of embankment material.  The total crest length of Oroville Dam is over one mile, at 6,920 feet. 

The major design characteristics for Oroville Dam are shown in Table 1.

Table 1 – Oroville Dam Design

Description Oroville Dam
Type of Dam Zoned Earthfill
Height     770 ft.
Height Rank     No. 1
Earth Volume  (cubic yards) 80 million CY
Crest Width        51 ft.
Crest Length   6,920 ft.
Crest Elevation      922 ft.
Number of Spillways   2 EA
Main Spillway Crest Elevation   814 ft.
Emergency Spillway Crest Elev.   901 ft.
Main Spillway Discharge Capacity 150,000 CFS
Emergency Discharge on 2.11.17    12,600 CFS
Reservoir Capacity 3.5 million acre-ft.
Reservoir Capacity Rank   No. 21
Reservoir Surface Area   15,805 acres
Year Built 1962-1967
Construction Cost $123 million
Contractor Oman Construction

 

Oroville Dam was built from 1962 to 1967, at a construction cost of $123 million.  When Oroville Dam was dedicated on May 4, 1968, a California Governor by the name of Ronald Reagan heralded the completion of this major project. 

When engineers are designing a dam, they must assess numerous variables and balance various priorities.  For example, what is the purpose of the dam?  Will it provide flood control and recreation?  Will it generate hydroelectric power? 

A major design consideration is handling all of the river flows during construction (river diversion) and after the structure is completed.  All expected river flows must be accommodated so that the dam is never overtopped.  Even though this is important for concrete dams, it is absolutely crucial for earthfill dams!

Oroville Dam is the centerpiece of the California Water Project.  Oroville Dam is managed by the California Department of Water Resources (DWR).  The Dam provides flood control, power generation, freshwater releases, and recreation.  The reservoir formed behind Oroville Dam (Lake Oroville) furnishes 750,000 acre-feet of flood control for the Feather River. 

The Maximum Operating Storage (MOS) of Lake Oroville is 3,537,577 acre-feet at a Water Surface Elevation of 900 feet (900 feet above mean sea level).  The 3.5 million acre-feet lake places Oroville at the No. 21 Rank in the country.  The water surface area (at MOS) for Lake Oroville is 15,805 acres.

The important Oroville Dam Design Crest Elevations are given in Table 2.

Table 2 – Oroville Dam Crest Elevations

Description Crest Material Elevation

(Feet above

Mean Sea Level)

Oroville Dam Crest  Earthfill     922 ft.
Emergency (Auxiliary) Spillway Crest   Concrete     901 ft.
Main Spillway Ogee Crest  Concrete     814 ft.

 

Water can be released from Lake Oroville in four ways: power plant, river outlets, main spillway, and auxiliary/emergency spillway.  The power plant located at the base of the dam is known as the Hyatt Pumping-Generating Plant.  This plant has six turbine-generators; three of these can be used as pumps in the pumping cycle.  The licensed capacity is 645 mega-watts (645 MW) with 16,950 CFS of water.  Typical water use is about 14,000 CFS.  (“CFS” is cubic feet of water per second.) 

The river outlets (Palermo Outlet Works) were originally rated at 5,400 CFS.  After an accident in 2009 where five workers were injured, the river outlets have been used very sparingly.  DWR Officials estimate that the outlets might be able to release about 2,000 CFS.

http://www.orovillemr.com/general-news/20120912/dwr-planning-study-on-worrisome-river-valves-blamed-in-2009-oroville-dam-accident

The Main Spillway is a concrete-lined chute, 3,050 feet long, which extends from the flood control outlet structure down to the Feather River.  An unlined approach channel connects the lake to the flood control outlet.  The gated headworks at the outlet has eight radial gates to meter water flow into the spillway chute.  Each radial gate is 17 ft.-7 in. wide x 33 ft.-6 in. high.  The Main Spillway is rated at 150,000 CFS.

The final component for releasing flood waters from Lake Oroville is the Emergency Spillway (or Auxiliary Spillway).  This is an un-gated overflow weir with a crest length of 1,730 feet.  This 30-ft. high concrete dam extends from the Main Spillway gated headworks to the parking lot for the boat ramp.  After floodwater passes over this weir, the water tumbles down an unlined slope to the Feather River.

http://www.water.ca.gov/swp/facilities/Oroville/LakeDam.cfm

http://www.water.ca.gov/swp/facilities/Oroville/hyatt.cfm

 

  1. RECENT RAINFALL AND OROVILLE LAKE LEVELS

After large storms hit the Feather River tributaries in early February, Lake Oroville water levels began rising dramatically.  Normally, this does not happen until the snow melts and the spring rains bring additional water into the reservoir. 

These early thunderstorms had a dramatic impact on the reservoir behind Oroville Dam.  On February 7, 2012, about 107,000 CFS of water was entering Lake Oroville.  High inflows continued for the next five days, with three days over 100,000 CFS (102,000 CFS on Feb. 8; 155,000 CFS on Feb. 9; and 128,000 CFS on Feb. 10). 

Please refer to Table 3 – Oroville Dam and Reservoir.  This Table provides information on the Oroville Reservoir for the period from February 1 to February 20.  (The data was taken from DWR tables for Oroville Dam.)

Table 3 – Oroville Dam and Reservoir: Elevation, Storage and Releases

Date

Feb.

Day

of

Wk

Reservoir

Elevation

(ft.)

Distance

Below

Spillway

Crest

(ft.)

Reservoir

Storage

(Acre-ft.)

Available

Reservoir

Storage

(Acre-ft.)

Reserv.

Inflow

(CFS)

Reserv.

Outflow

CFS)

  1 W 850.8     50 2,818,827   718,750     8,644   20,187
  2 Th 849.1     52 2,796,832   740,745     9,381   20,463
  3 F   —      —       —       —   20,210   25,772
  4 Sat 849.2     52 2,797,635   739,942   36,027   30,014
  5 Sun 849.4     52 2,799,641   737,931   30,856   29,911
  6 M 850.9     50 2,819,768 717,809   48,795   38,741
  7 T 862.3     39 2,976,908 560,669 106,845   27,425
  8 W 874.8     26 3,155,684 381,893 101,841   11,687
  9 Th 890.9     10 3,396,134 141,443 155,498   34,253
10 F 899.4       2 3,528,727      8,850 127,679   60,697
11 Sat   902.6      — 3,578,367       —   84,437   59,472
12 Sun 899.8       1 3,533,936      3,641   69,167   69,131
13 M 891.7       9 3,408,430 129,147   14,441 100,000
14 T 882.4     19 3,266,396 271,181   28,388 100,000
15 W 872.2     29 3,117,546 420,031   25,032 100,000
16 Th 863.4     38 2,991,747 545,830   26,535   90,000
17 F 856.7     44 2,899,096 638,481   29,364   76,040
18 Sat 853.3     48 2,852,724 684,853   39,131   62,499
19 Sun 850.2     51 2,810,364 727,213   35,733   57,083
20 M 850.3     51 2,812,914 724,663   61,271   60,000

 

On an average February day, Lake Oroville would normally receive about 7,000 acre-feet of water.  With the Main Spillway shut off on February 8, the reservoir took in a whopping 240,000 acre-feet in a 24-hour period (3,396,134 acre-ft. – 3,155,684 acre-ft. = 240,450 acre-ft.). 

This caused the lake level to jump 16 feet in one day!  The reservoir level was 874.8 on Feb. 8 and 890.9 on Feb. 9 (890.9 ft. – 874.8 ft. = 16.1 ft.).  On Feb. 10, the lake moved up 9 feet.  Over the course of six days from February 6 – 11, Oroville Lake levels rose a gigantic 52 feet! 

On Feb. 11, the lake rose 3 feet and water started flowing over the Emergency Spillway shortly after 8:00 a.m.  This was the first time that water had moved over the Emergency Spillway (Auxiliary Spillway) since Oroville Dam was built in 1967.  Water continued flowing over the Emergency Spillway until 8:00 p.m. on February 12.

 

  1. OROVILLE DAM PROBLEMS

The Department of Water Resources (DWR) estimates that about 6,000 to 12,600 CFS of water passed over the Emergency Spillway on Saturday and Sunday (Feb. 11 and 12).  As the water made its way down the steep slope to the Feather River, significant erosion took place.  Even though DWR crews had removed some trees and brush before the overflow event, much debris was washed into the river.  Huge canyons were carved out of the steep hillside and sections of the boat ramp access road were destroyed. 

Of particular concern was erosion that took place at the toe of the Emergency Spillway.  After the Emergency Spillway stopped flowing, engineers could assess the damage that took place on Saturday and Sunday.  Holes near the 30-ft.-high x 1,730-ft.-long Emergency Spillway were readily apparent. 

Some people feared that this concrete weir would be undermined and fail, thereby releasing a 30-ft-high wall of water onto the towns below the dam.  This is the reason that the authorities called for the immediate and mandatory evacuation of 188,000 people.

Since February 12, DWR crews and contractor forces have been working around the clock to place rock and concrete over the affected areas.  This work is currently underway.  Crews are placing rock with trucks, loaders and excavators.  After the rock is placed, transit-mix concrete trucks deliver concrete for placement by multiple truck-mounted boom concrete pumps.

Because the ground was wet and soft, helicopters were used to place stone in some areas.  One practice is highly questionable; crews are loading plastic bags will small rocks and flying these bags into place with helicopters.  Even though concrete is placed around the rock bags, these bags may not survive the rushing water.  This practice must be stopped!  Instead, large rocks should be used.

The problems with the Main Spillway were brought to light on February 7, when a large hole appeared in the spillway slab.  By February 10, the spillway hole had grown to 500-ft. long x 300-ft. wide x 45-ft. deep.  In the process, the concrete side wall collapsed and was washed away.  As the crater got larger, more and more water shot out of the spillway and carved a new canyon that carries the water down to the Feather River.  Depending on the flow volume, most of the water leaves the spillway chute at the location of this large crater; very little water stays in the chute until the bottom end.

The only feasible way to slow down the growth of this big chasm would be to drop huge boulders into place while the Spillway is temporarily shut down.  Also Tetrapods or similar seashore precast concrete products could be used.

Further checking revealed that the concrete slab had been patched in 2013 and Department of Water Resources (DWR) personnel declared: “It looked like it would be able to hold, be able to pass water.”

What caused the failure of the Main Spillway concrete slab?

On Tuesday, February 14, I sent a letter to the Acting Director for the California State Water Project.  In the letter, I suggested that the concrete in the Main Spillway chute had been destroyed by powerful cavitation forces.  Cavitation can occur in spillways and tunnels when high-velocity water encounters irregularities in the concrete surface.  At these locations, cavitation creates high-pressure shock waves capable of destroying solid concrete.  Proper aeration of the water flow can minimize the harmful effects of cavitation.

The DWR Acting Director placed the rebuilding cost of the Main Spillway at about $100 – $200 million.  The Main Spillway construction project must take place this summer (during the low flow season).  The contractor might have to incorporate a temporary flume into the project to carry the spillway water during the construction period.

With the failure of the Main Spillway, large amounts of earth, rock and concrete have washed into the Feather River.  One estimate places the amount of debris in the river at 150,000 cubic yards.  Because this debris pile acts like a dam, water levels are rising in the tailrace pool leading from the power plant.  To prevent damage to the equipment, the Hyatt Power Plant cannot be used until the debris is cleared out of the tailrace pool. 

This means water cannot be released through the Power Plant.  Because the River Outlets are in poor condition, they cannot be used to route water past the dam.  This leaves only the Main Spillway and the Emergency Spillway (Auxiliary Spillway).

 

CONCLUSION

Oroville Dam in northern California is the nation’s tallest dam at 770 feet.

After thunderstorms dumped heavy rainfall in the area, Oroville Dam sustained a major event from February 7-12, 2017.  Over the course of six days from February 6-11, Oroville Lake levels rose 52 feet. 

The record high water flowed over the Emergency Spillway on Saturday and Sunday, February 11 and 12.  This was the first time that water had moved over the Emergency Spillway since Oroville Dam was built 50 years ago.

As the water made its way down the unlined steep slope to the Feather River, significant erosion took place.  Huge canyons were carved out of the steep hillside and parts of the access road were destroyed. 

 Some people feared that the Emergency Spillway concrete weir would be undermined by the water.  If this occurred, a 30-ft-high wall of water would be released onto the towns below the dam.  Out of caution, the authorities called for the immediate and mandatory evacuation of 188,000 people.

As water was released into the Main Spillway to lower the lake levels, a huge crater opened up in the spillway chute concrete slab.  By February 10, the spillway hole had grown to 500-ft. long x 300-ft. wide x 45-ft. deep.  With water pounding this area, the crater and gorge are continuing to grow in size. 

Cavitation is the likely cause for the spillway concrete chute failure.  Proper aeration of the water flow can minimize the harmful effects of cavitation.

The 770-ft.-high Oroville Dam was never in danger of failing.  Because the earthfill dam crest is at Elevation 922 feet and the lake rose to 903 feet, the dam was 19 feet from being overtopped. 

With the Emergency Spillway crest at Elevation 901 feet, the lake will never overtop the dam!

The Department of Water Resources (DWR) is placing rock and concrete on the surfaces below the Emergency Spillway. 

DWR should continue to clear rock and debris from the Feather River and Hyatt Power Plant tailrace pool.  This will permit the hydroelectric powerhouse to resume operations and allow the passage of 14,000 CFS of water past the dam.

Even though the Main Spillway is damaged, DWR must continue to release significant volumes of water (60,000 – 100,000 CFS) through the Main Spillway radial gates.  Never again, should the agency allow Oroville Reservoir levels to rise anywhere close to Elevation 901 feet (Emergency Spillway crest elevation).

If the lake levels can rise 52 feet in 6 days (like Feb. 6 – 11), an adequate cushion must be provided.  With the strong likelihood of significant spring snowmelt and rains, the Department of Water Resources should maintain the Oroville Lake levels at about 825 feet.  

DWR must immediately design the rebuild of the Main Spillway and award a contract for the work (estimated cost of $100 – $200 million).  During the dry summer season, the contractor must pursue the construction project on an expedited “24/7” basis.

=======================================

Bio for Henry W. Burke

 Henry Burke is a Civil Engineer with a B.S.C.E. and M.S.C.E.  He has been a Registered Professional Engineer (P.E.) for 37 years and has worked as a Civil Engineer in construction for over 45 years. 

Mr. Burke had a successful 27-year career with a large construction company. 

Henry Burke has served as a full-time volunteer to oversee various construction projects. He has written numerous articles on education, engineering, construction, politics, taxes, and the economy. 

Henry W. Burke

E-mail:  hwburke@cox.net

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1 Comment

  1. Jim Piver

    Yeah. And it’s not gonna rain anymore and the snowpack isn’t going to melt. Uh huh. Everything’s just fine. No problem. It’s completely under control. Nothing to see here… Move along….

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