The Historic Hoover Dam in Las Vegas Nevada Historical and Biggest Bridge w/Spectacular design inUSA

Hoover Dam is a concrete arch-gravity dam
in the Black Canyon of the Colorado River, on the border between the U.S. states of Nevada
and Arizona. It was constructed between 1931 and 1936 during the Great Depression and was
dedicated on September 30, 1935, by President Franklin D. Roosevelt. Its construction was
the result of a massive effort involving thousands of workers, and cost over one hundred lives.
Originally known as Boulder Dam from 1933, it was officially renamed Hoover Dam, for
President Herbert Hoover, by a joint resolution of Congress in 1947. Since about 1900, the Black Canyon and nearby
Boulder Canyon had been investigated for their potential to support a dam that would control
floods, provide irrigation water and produce hydroelectric power. In 1928, Congress authorized
the project. The winning bid to build the dam was submitted by a consortium called Six
Companies, Inc., which began construction of the dam in early 1931. Such a large concrete
structure had never been built before, and some of the techniques were unproven. The
torrid summer weather and lack of facilities near the site also presented difficulties.
Nevertheless, Six Companies turned the dam over to the federal government on March 1,
1936, more than two years ahead of schedule. Hoover Dam impounds Lake Mead, the largest
reservoir in the United States by volume (when it is full).[6] The dam is located near Boulder
City, Nevada, a municipality originally constructed for workers on the construction project, about
30 mi (48 km) southeast of Las Vegas, Nevada. The dam’s generators provide power for public
and private utilities in Nevada, Arizona, and California. Hoover Dam is a major tourist
attraction; nearly a million people tour the dam each year. The heavily traveled U.S. Route
93 (US 93) ran along the dam’s crest until October 2010, when the Hoover Dam Bypass opened.
As the United States developed the Southwest, the Colorado River was seen as a potential
source of irrigation water. An initial attempt at diverting the river for irrigation purposes
occurred in the late 1890s, when land speculator William Beatty built the Alamo Canal just
north of the Mexican border; the canal dipped into Mexico before running to a desolate area
Beatty named the Imperial Valley.[7] Though water from the Imperial Canal allowed for
the widespread settlement of the valley, the canal proved expensive to maintain. After
a catastrophic breach that caused the Colorado River to fill the Salton Sea,[8] the Southern
Pacific Railroad spent $3 million in 1906–07 to stabilize the waterway, an amount it hoped
in vain would be reimbursed by the Federal Government. Even after the waterway was stabilized,
it proved unsatisfactory because of constant disputes with landowners on the Mexican side
of the border.[9] River view of the future dam site, c. 1904
As the technology of electric power transmission improved, the Lower Colorado was considered
for its hydroelectric-power potential. In 1902, the Edison Electric Company of Los Angeles
surveyed the river in the hope of building a 40-foot (12 m) rock dam which could generate
10,000 horsepower (7,500 kW). However, at the time, the limit of transmission of electric
power was 80 miles (130 km), and there were few customers (mostly mines) within that limit.
Edison allowed land options it held on the river to lapse—including an option for what
became the site of Hoover Dam.[10] In
the following years, the Bureau of Reclamation (BOR), known as the Reclamation Service at
the time, also considered the Lower Colorado as the site for a dam. Service chief Arthur
Powell Davis proposed using dynamite to collapse the walls of Boulder Canyon,[11] 20 miles
(32 km) north of the eventual dam site, into the river.[12] The river would carry off the
smaller pieces of debris, and a dam would be built incorporating the remaining rubble.
In 1922, after considering it for several years, the Reclamation Service finally rejected
the proposal, citing doubts about the unproven technique and questions as to whether it would
in fact save money.[11] Planning and agreements Sketch of the proposed dam site and reservoir,
c. 1921 In 1922, the Reclamation Service presented
a report calling for the development of a dam on the Colorado River for flood control
and electric power generation. The report was principally authored by Davis, and was
called the Fall-Davis report after Interior Secretary Albert Fall. The Fall-Davis report
cited use of the Colorado River as a federal concern because the river’s basin covered
several states, and the river eventually entered Mexico.[13] Though the Fall-Davis report called
for a dam “at or near Boulder Canyon”, the Reclamation Service (which was renamed the
Bureau of Reclamation the following year) found that canyon unsuitable.[14] One potential
site at Boulder Canyon was bisected by a geologic fault; two others were so narrow there was
no space for a construction camp at the bottom of the canyon[14] or for a spillway. The Service
investigated Black Canyon and found it ideal; a railway could be laid from the railhead
in Las Vegas to the top of the dam site.[15] Despite the site change, the dam project was referred to as the “Boulder
Canyon Project”.[16] With little guidance on water allocation from
the Supreme Court, proponents of the dam feared endless litigation. A Colorado attorney proposed
that the seven states which fell within the river’s basin (California, Nevada, Arizona,
Utah, New Mexico, Colorado and Wyoming) form an interstate compact, with the approval of
Congress. Such compacts were authorized by Article I of the United States Constitution
but had never been concluded among more than two states. In 1922, representatives of seven
states met with then-Secretary of Commerce Herbert Hoover.[17] Initial talks produced
no result, but when the Supreme Court handed down the Wyoming v. Colorado decision undermining
the claims of the upstream states, they became anxious to reach an agreement. The resulting
Colorado River Compact was signed on November 24, 1922.[18] Legislation to authorize the dam was introduced
repeatedly by two California Republicans, Representative Phil Swing and Senator Hiram
Johnson, but representatives from other parts of the country considered the project as hugely
expensive and one that would mostly benefit California. The 1927 Mississippi flood made
Midwestern and Southern congressmen and senators more sympathetic toward the dam project. On
March 12, 1928, the failure of the St. Francis Dam, constructed by the city of Los Angeles,
caused a disastrous flood that killed up to 600 people. As that dam was a curved-gravity
type,[19] similar in design to the arch-gravity as was proposed for the Black Canyon dam,
opponents claimed that the Black Canyon dam’s safety could not be guaranteed. Congress authorized
a board of engineers to review plans for the proposed dam. The Colorado River Board found
the project feasible, but warned that should the dam fail, every downstream Colorado River
community would be destroyed, and that the river might change course and empty into the
Salton Sea. The Board cautioned: “To avoid such possibilities, the proposed dam should
be constructed on conservative if not ultra-conservative lines.”[20] On December 21, 1928, President Coolidge signed
the bill authorizing the dam.[21] The Boulder Canyon Project Act appropriated $165 million
for the Hoover Dam along with the downstream Imperial Dam and All-American Canal, a replacement
for Beatty’s canal entirely on the U.S. side of the border.[22] It also permitted the compact
to go into effect when at least six of the seven states approved it. This occurred on
March 6, 1929, with Utah’s ratification; Arizona did not approve it until 1944.[23] Design, preparation and contracting Hoover Dam architectural plans
Even before Congress approved the Boulder Canyon Project, the Bureau of Reclamation
was considering what kind of dam should be used. Officials eventually decided on a massive
concrete arch-gravity dam, the design of which was overseen by the Bureau’s chief design
engineer John L. Savage. The monolithic dam would be thick at the bottom and thin near
the top, and would present a convex face towards the water above the dam. The curving arch
of the dam would transmit the water’s force into the abutments, in this case the rock
walls of the canyon. The wedge-shaped dam would be 660 ft (200 m) thick at the bottom,
narrowing to 45 ft (14 m) at the top, leaving room for a highway connecting Nevada and Arizona.[24] On January 10, 1931, the Bureau made the bid
documents available to interested parties, at five dollars a copy. The government was
to provide the materials; but the contractor was to prepare the site and build the dam.
The dam was described in minute detail, covering 100 pages of text and 76 drawings. A $2 million
bid bond was to accompany each bid; the winner would have to post a $5 million performance
bond. The contractor had seven years to build the dam, or penalties would ensue.[25] The Wattis Brothers, heads of the Utah Construction
Company, were interested in bidding on the project, but lacked the money for the performance
bond. They lacked sufficient resources even in combination with their longtime partners,
Morrison-Knudsen, which employed the nation’s leading dam builder, Frank Crowe. They formed
a joint venture to bid for the project with Pacific Bridge Company of Portland, Oregon;
Henry J. Kaiser & W. A. Bechtel Company of San Francisco; MacDonald & Kahn Ltd. of Los
Angeles; and the J.F. Shea Company of Portland, Oregon.[26] The joint venture was called Six
Companies, Inc. as Bechtel and Kaiser were considered one company for purposes of Six
in the name. The name was descriptive and was an inside joke among the San Franciscans
in the bid, where “Six Companies” was also a Chinese benevolent association in the city.[27]
There were three valid bids, and Six Companies’ bid of $48,890,955 was the lowest, within
$24,000 of the confidential government estimate of what the dam would cost to build, and five
million dollars less than the next-lowest bid.[28] The city of Las Vegas had lobbied hard to
be the headquarters for the dam construction, closing its many speakeasies when the decision
maker, Secretary of the Interior Ray Wilbur, came to town. Instead, Wilbur announced in
early 1930 that a model city was to be built in the desert near the dam site. This town
became known as Boulder City, Nevada. Construction of a rail line joining Las Vegas and the dam
site began in September 1930.[29] Construction
Labor force Workers on a “Jumbo Rig”; used for drilling
the Hoover Dam’s tunnels “Apache Indians employed as high-scalers on
the construction of Hoover Dam.” – NARA Soon after the dam was authorized, increasing
numbers of unemployed people converged on southern Nevada. Las Vegas, then a small city
of some 5,000, saw between 10,000 and 20,000 unemployed descend on it.[30] A government
camp was established for surveyors and other personnel near the dam site; this soon became
surrounded by a squatters’ camp. Known as McKeeversville, the camp was home to men hoping
for work on the project, together with their families.[31] Another camp, on the flats along
the Colorado River, was officially called Williamsville, but was known to its inhabitants
as “Ragtown”.[32] When construction began, Six Companies hired large numbers of workers,
with more than 3,000 on the payroll by 1932[33] and with employment peaking at 5,251 in July
1934.[34] “Mongolian” (Chinese) labor was prevented by the construction contract,[34]
while the number of blacks employed by Six Companies never exceeded thirty, mostly lowest-pay-scale
laborers in a segregated crew, who were issued separate water buckets.[35] As part of the contract, Six Companies, Inc.
was to build Boulder City to house the workers. The original timetable called for Boulder
City to be built before the dam project began, but President Hoover ordered work on the dam
to begin in March 1931 rather than in October.[36] The company built bunkhouses, attached to
the canyon wall, to house 480 single men at what became known as River Camp. Workers with
families were left to provide their own accommodations until Boulder City could be completed,[37]
and many lived in Ragtown.[38] The site of Hoover Dam endures extremely hot weather,
and the summer of 1931 was especially torrid, with the daytime high averaging 119.9 °F
(48.8 °C).[39] Sixteen workers and other riverbank residents died of heat prostration
between June 25 and July 26, 1931.[40] General Superintendent Frank Crowe (right)
with Bureau of Reclamation engineer Walker Young in 1935
The Industrial Workers of the World (IWW or “Wobblies”), though much-reduced from their
heyday as militant labor organizers in the early years of the century, hoped to unionize
the Six Companies workers by capitalizing on their discontent. They sent eleven organizers,[41]
several of whom were arrested by Las Vegas police.[42] On August 7, 1931, the company
cut wages for all tunnel workers. Although the workers sent the organizers away, not
wanting to be associated with the “Wobblies”, they formed a committee to represent them
with the company. The committee drew up a list of demands that evening and presented
them to Crowe the following morning. He was noncommittal. The workers hoped that Crowe,
the general superintendent of the job, would be sympathetic; instead he gave a scathing
interview to a newspaper, describing the workers as “malcontents”.[43] On the morning of the 9th, Crowe met with
the committee and told them that management refused their demands, was stopping all work,
and was laying off the entire work force, except for a few office workers and carpenters.
The workers were given until 5 p.m. to vacate the premises. Concerned that a violent confrontation
was imminent, most workers took their paychecks and left for Las Vegas to await developments.[44]
Two days later, the remainder were talked into leaving by law enforcement. On August
13, the company began hiring workers again, and two days later, the strike was called
off.[45] While the workers received none of their demands, the company guaranteed there
would be no further reductions in wages. Living conditions began to improve as the first residents
moved into Boulder City in late 1931.[46] A second labor action took place in July 1935,
as construction on the dam wound down. When a Six Companies manager altered working times
to force workers to take lunch on their own time, workers responded with a strike. Emboldened
by Crowe’s reversal of the lunch decree, workers raised their demands to include a $1-per-day
raise. The company agreed to ask the Federal government to supplement the pay, but no money
was forthcoming from Washington. The strike ended.[47] River diversion Overview of dam mechanisms; diversion tunnels
shown Before the dam could be built, the Colorado
River needed to be diverted away from the construction site. To accomplish this, four
diversion tunnels were driven through the canyon walls, two on the Nevada side and two
on the Arizona side. These tunnels were 56 ft (17 m) in diameter.[48] Their combined
length was nearly 16,000 ft, or more than 3 miles (5 km).[49] The contract required
these tunnels to be completed by October 1, 1933, with a $3,000-per-day fine to be assessed
for any delay. To meet the deadline, Six Companies had to complete work by early 1933, since
only in late fall and winter was the water level in the river low enough to safely divert.[50] Tunneling began at the lower portals of the
Nevada tunnels in May 1931. Shortly afterward, work began on two similar tunnels in the Arizona
canyon wall. In March 1932, work began on lining the tunnels with concrete. First the
base, or invert, was poured. Gantry cranes, running on rails through the entire length
of each tunnel were used to place the concrete. The sidewalls were poured next. Movable sections
of steel forms were used for the sidewalls. Finally, using pneumatic guns, the overheads
were filled in. The concrete lining is 3 feet (1 m) thick, reducing the finished tunnel
diameter to 50 ft (15 m).[49] The river was diverted into the two Arizona tunnels on November
13, 1932; the Nevada tunnels were kept in reserve for high water. This was done by exploding
a temporary cofferdam protecting the Arizona tunnels while at the same time dumping rubble
into the river until its natural course was blocked.[51] Following the completion of the dam, the entrances
to the two outer diversion tunnels were sealed at the opening and halfway through the tunnels
with large concrete plugs. The downstream halves of the tunnels following the inner
plugs are now the main bodies of the spillway tunnels.[49] The inner diversion tunnels were
plugged at approximately one-third of their length, beyond which they now carry steel
pipes connecting the intake towers to the power plant and outlet works.[48] The inner
tunnels’ outlets are equipped with gates that can be closed to drain the tunnels for maintenance.[48] Groundworks, rock clearance and grout curtain
To protect the construction site from the Colorado River and to facilitate the river’s
diversion, two cofferdams were constructed. Work on the upper cofferdam began in September
1932, even though the river had not yet been diverted.[52] The cofferdams were designed
to protect against the possibility of the river’s flooding a site at which two thousand
men might be at work, and their specifications were covered in the bid documents in nearly
as much detail as the dam itself. The upper cofferdam was 96 ft (29 m) high, and 750 feet
(230 m) thick at its base, thicker than the dam itself. It contained 650,000 cubic yards
(500,000 m3) of material.[53] Looking down at “high scalers” above the Colorado
River When the cofferdams were in place and the
construction site was drained of water, excavation for the dam foundation began. For the dam
to rest on solid rock, it was necessary to remove accumulated erosion soils and other
loose materials in the riverbed until sound bedrock was reached. Work on the foundation
excavations was completed in June 1933. During this excavation, approximately 1,500,000 cu
yd (1,100,000 m3) of material was removed. Since the dam was an arch-gravity type, the
side-walls of the canyon would bear the force of the impounded lake. Therefore, the side-walls
were excavated too, to reach virgin rock, as weathered rock might provide pathways for
water seepage.[52] Shovels for the excavation came from the Marion Power Shovel Company.[54] The men who removed this rock were called
“high scalers”. While suspended from the top of the canyon with ropes, the high-scalers
climbed down the canyon walls and removed the loose rock with jackhammers and dynamite.
Falling objects were the most common cause of death on the dam site; the high scalers’
work thus helped ensure worker safety.[55] One high scaler was able to save a life in
a more direct manner: when a government inspector lost his grip on a safety line and began tumbling
down a slope towards almost certain death, a high scaler was able to intercept him and
pull him into the air. The construction site had, even then, become a magnet for tourists;
the high scalers were prime attractions and showed off for the watchers. The high scalers
received considerable media attention, with one worker dubbed the “Human Pendulum” for
swinging co-workers (and, at other times, cases of dynamite) across the canyon.[56]
To protect themselves against falling objects, some high scalers took cloth hats and dipped
them in tar, allowing them to harden. When workers wearing such headgear were struck
hard enough to inflict broken jaws, they sustained no skull damage. Six Companies ordered thousands
of what initially were called “hard boiled hats” (later “hard hats”) and strongly encouraged
their use.[57] The cleared, underlying rock foundation of
the dam site was reinforced with grout, forming a grout curtain. Holes were driven into the
walls and base of the canyon, as deep as 150 feet (46 m) into the rock, and any cavities
encountered were to be filled with grout. This was done to stabilize the rock, to prevent
water from seeping past the dam through the canyon rock, and to limit “uplift”—upward
pressure from water seeping under the dam. The workers were under severe time constraints
due to the beginning of the concrete pour, and when they encountered hot springs or cavities
too large to readily fill, they moved on without resolving the problem. A total of 58 of the
393 holes were incompletely filled.[58] After the dam was completed and the lake began to
fill, large numbers of significant leaks into the dam caused the Bureau of Reclamation to
look into the situation. It found that the work had been incompletely done, and was based
on less than a full understanding of the canyon’s geology. New holes were drilled from inspection
galleries inside the dam into the surrounding bedrock.[59] It took nine years (1938–47)
under relative secrecy to complete the supplemental grout curtain.[60] Concrete Columns of Hoover Dam being filled with concrete,
February 1934 (looking upstream from the Nevada rim)
The first concrete was poured into the dam on June 6, 1933, 18 months ahead of schedule.[61]
Since concrete heats and contracts as it cures, the potential for uneven cooling and contraction
of the concrete posed a serious problem. Bureau of Reclamation engineers calculated that if
the dam were to be built in a single continuous pour, the concrete would take 125 years to
cool, and the resulting stresses would cause the dam to crack and crumble. Instead, the
ground where the dam would rise was marked with rectangles, and concrete blocks in columns
were poured, some as large as 50 ft square (15 m) and 5 feet (1.5 m) high.[62] Each five-foot
form contained a set of 1-inch (25 mm) steel pipes; cool riverwater would be poured through
the pipes, followed by ice-cold water from a refrigeration plant. When an individual
block had cured and had stopped contracting, the pipes were filled with grout. Grout was
also used to fill the hairline spaces between columns, which were grooved to increase the
strength of the joints.[63] The concrete was delivered in huge steel buckets
7 feet high (2.1 m) and almost 7 feet in diameter; Crowe was awarded two patents for their design.
These buckets, which weighed 20 short tons (18 t) when full, were filled at two massive
concrete plants on the Nevada side, and were delivered to the site in special railcars.
The buckets were then suspended from aerial cableways which were used to deliver the bucket
to a specific column. As the required grade of aggregate in the concrete differed depending
on placement in the dam (from pea-sized gravel to 9-inch or 23 cm stones), it was vital that
the bucket be maneuvered to the proper column. When the bottom of the bucket opened up, disgorging
8 cu yd (6.1 m3) of concrete, a team of men worked it throughout the form. Although there
are myths that men were caught in the pour and are entombed in the dam to this day, each
bucket deepened the concrete in a form by only 1 inch (25 mm), and Six Companies engineers
would not have permitted a flaw caused by the presence of a human body.[64] A total of 3,250,000 cubic yards (2,480,000
cubic metres) of concrete was used in the dam before concrete pouring ceased on May
29, 1935. In addition, 1,110,000 cu yd (850,000 m3) were used in the power plant and other
works. More than 582 miles (937 km) of cooling pipes were placed within the concrete. Overall,
there is enough concrete in the dam to pave a two-lane highway from San Francisco to New
York.[48] Concrete cores were removed from the dam for testing in 1995; they showed that
“Hoover Dam’s concrete has continued to slowly gain strength” and the dam is composed of
a “durable concrete having a compressive strength exceeding the range typically found in normal
mass concrete”.[65] Hoover Dam concrete is not subject to alkali–silica reaction (ASR),
as the Hoover Dam builders happened to use nonreactive aggregate, unlike that at downstream
Parker Dam, where ASR has caused measurable deterioration.[65] Dedication and completion The upstream face of Hoover Dam slowly disappears
as Lake Mead fills, May 1935 (looking downstream from the Arizona rim)
With most work finished on the dam itself (the powerhouse remained uncompleted), a formal
dedication ceremony was arranged for September 30, 1935, to coincide with a western tour
being made by President Franklin D. Roosevelt. The morning of the dedication, it was moved
forward three hours from 2 p.m. Pacific time to 11 a.m.; this was done because Secretary
of the Interior Harold L. Ickes had reserved a radio slot for the President for 2 p.m.
but officials did not realize until the day of the ceremony that the slot was for 2 p.m.
Eastern Time.[66] Despite the change in the ceremony time, and temperatures of 102 °F
(39 °C), 10,000 people were present for the President’s speech, in which he avoided mentioning
the name of former President Hoover,[67] who was not invited to the ceremony.[68] To mark
the occasion, a three-cent stamp was issued by the United States Post Office Department—bearing
the name “Boulder Dam”, the official name of the dam between 1933 and 1947.[69] After
the ceremony, Roosevelt made the first visit by any American president to Las Vegas.[67] Most work had been completed by the dedication,
and Six Companies negotiated with the government through late 1935 and early 1936 to settle
all claims and arrange for the formal transfer of the dam to the Federal Government. The
parties came to an agreement and on March 1, 1936, Secretary Ickes formally accepted
the dam on behalf of the government. Six Companies was not required to complete work on one item,
a concrete plug for one of the bypass tunnels, as the tunnel had to be used to take in irrigation
water until the powerhouse went into operation.[70] Construction deaths Oskar J. W. Hansen’s memorial at the dam which
reads in part “They died to make the desert bloom.”[71]
There were 112 deaths reported as associated with the construction of the dam.[72] The
first was J. G. Tierney, a surveyor who drowned on December 20, 1922, while looking for an
ideal spot for the dam.[72] The last death on the project’s official fatality list occurred
on December 20, 1935, when an “electrician’s helper,” Patrick Tierney, the son of J. G.
Tierney, fell from an intake tower. Included in the fatality list are three workers, one
in 1932 and two in 1933, who took their own lives onsite.[73][74][75] Ninety-six of the
deaths occurred during construction at the site.[72] Of the 112 fatalities, 91 were Six
Companies employees, three were BOR employees, and one was a visitor to the site, with the
remainder employees of various contractors not part of Six Companies.[76] Not included in the official number of fatalities
were deaths that were recorded as pneumonia. Workers alleged that this diagnosis was a
cover for death from carbon monoxide poisoning (brought on by the use of gasoline-fueled
vehicles in the diversion tunnels), and a classification used by Six Companies to avoid
paying compensation claims.[77] The site’s diversion tunnels frequently reached 140 °F
(60 °C), enveloped in thick plumes of vehicle exhaust gases.[78] A total of 42 workers were
recorded as having died from pneumonia and were not included in the above total; none
were listed as having died from carbon monoxide poisoning. No deaths of non-workers from pneumonia
were recorded in Boulder City during the construction period.[77] Architectural style
The initial plans for the facade of the dam, the power plant, the outlet tunnels and ornaments
clashed with the modern look of an arch dam. The Bureau of Reclamation, more concerned
with the dam’s functionality, adorned it with a Gothic-inspired balustrade and eagle statues.
This initial design was criticized by many as being too plain and unremarkable for a
project of such immense scale, so Los Angeles-based architect Gordon B. Kaufmann, then the supervising
architect to the Bureau of Reclamation, was brought in to redesign the exteriors.[79]
Kaufmann greatly streamlined the design and applied an elegant Art Deco style to the entire
project. He designed sculpted turrets rising seamlessly from the dam face and clock faces
on the intake towers set for the time in Nevada and Arizona — both states are in different
time zones, but since Arizona does not observe Daylight Saving Time, the clocks display the
same time for more than half the year.[80] Tile floor designed by Allen Tupper True Hansen’s bas-relief on the Nevada elevator
At Kaufmann’s request, Denver artist Allen Tupper True[79] was hired to handle the design
and decoration of the walls and floors of the new dam. True’s design scheme incorporated
motifs of the Navajo and Pueblo tribes of the region.[81] Although some were initially
opposed to these designs, True was given the go-ahead and was officially appointed consulting
artist.[82] With the assistance of the National Laboratory of Anthropology, True researched
authentic decorative motifs from Indian sand paintings, textiles, baskets and ceramics.[83]
The images and colors are based on Native American visions of rain, lightning, water,
clouds, and local animals — lizards, serpents, birds — and on the Southwestern landscape
of stepped mesas.[81] In these works, which are integrated into the walkways and interior
halls of the dam, True also reflected on the machinery of the operation, making the symbolic
patterns appear both ancient and modern.[84] With the agreement of Kaufmann and the engineers,
True also devised for the pipes and machinery an innovative color-coding which was implemented
throughout all BOR projects.[85] True’s consulting artist job lasted through 1942; it was extended
so he could complete design work for the Parker, Shasta and Grand Coulee dams and power plants.
True’s work on the Hoover Dam was humorously referred to in a poem published in The New
Yorker, part of which read, “lose the spark, and justify the dream; but also worthy of
remark will be the color scheme”.[86] Complementing Kaufmann and True’s work, sculptor
Oskar J.W. Hansen designed many of the sculptures on and around the dam. His works include the
monument of dedication plaza, a plaque to memorialize the workers killed and the bas-reliefs
on the elevator towers. In his words, Hansen wanted his work to express “the immutable
calm of intellectual resolution, and the enormous power of trained physical strength, equally
enthroned in placid triumph of scientific accomplishment”, because “[t]he building of
Hoover Dam belongs to the sagas of the daring.”[71] Hansen’s dedication plaza, on the Nevada abutment,
contains a sculpture of two winged figures flanking a flagpole. Hoover Dam memorial star map floor, center
area Surrounding the base of the monument is a
terrazzo floor embedded with a “star map”. The map depicts the Northern Hemisphere sky
at the moment of President Roosevelt’s dedication of the dam. This is intended to help future
astronomers, if necessary, calculate the exact date of dedication.[71][87] The 30-foot-high
(9.1 m) bronze figures, dubbed “Winged Figures of the Republic”, were both formed in a continuous
pour. To put such large bronzes into place without marring the highly polished bronze
surface, they were placed on ice and guided into position as the ice melted.[88] Hansen’s
bas-relief on the Nevada elevator tower depicts the benefits of the dam: flood control, navigation,
irrigation, water storage, and power. The bas-relief on the Arizona elevator depicts,
in his words, “the visages of those Indian tribes who have inhabited mountains and plains
from ages distant.”[71] Operation
Power plant and water demands Hoover Dam releasing water from the jet-flow
gates in 1998 Excavation for the powerhouse was carried
out simultaneously with the excavation for the dam foundation and abutments. The excavation
of this U-shaped structure located at the downstream toe of the dam was completed in
late 1933 with the first concrete placed in November 1933. Filling of Lake Mead began
February 1, 1935, even before the last of the concrete was poured that May.[89] The
powerhouse was one of the projects uncompleted at the time of the formal dedication on September
30, 1935; a crew of 500 men remained to finish it and other structures.[90] To make the powerhouse
roof bombproof, it was constructed of layers of concrete, rock, and steel with a total
thickness of about 3.5 feet (1.1 m), topped with layers of sand and tar.[91] In the latter half of 1936, water levels in
Lake Mead were high enough to permit power generation, and the first three Allis Chalmers
built Francis turbine-generators, all on the Nevada side, began operating. In March 1937,
one more Nevada generator went online and the first Arizona generator by August. By
September 1939, four more generators were operating, and the dam’s power plant became
the largest hydroelectricity facility in the world. The final generator was not placed
in service until 1961, bringing the maximum generating capacity to 1,345 megawatts at
the time.[89][92] Original plans called for 16 large generators, eight on each side of
the river, but two smaller generators were installed instead of one large one on the
Arizona side for a total of 17. The smaller generators were used to serve smaller communities
at a time when the output of each generator was dedicated to a single municipality, before
the dam’s total power output was placed on the grid and made arbitrarily distributable.[3] Generators beneath Hoover Dam
Before water from Lake Mead reaches the turbines, it enters the intake towers and then four
gradually narrowing penstocks which funnel the water down towards the powerhouse. The
intakes provide a maximum hydraulic head (water pressure) of 590 ft (180 m) as the water reaches
a speed of about 85 mph (140 km/h). The entire flow of the Colorado River passes through
the turbines. The spillways and outlet works (jet-flow gates) are rarely used.[3] The jet-flow
gates, located in concrete structures 180 feet (55 m) above the river and also at the
outlets of the inner diversion tunnels at river level, may be used to divert water around
the dam in emergency or flood conditions, but have never done so, and in practice are
used only to drain water from the penstocks for maintenance.[93] Following an uprating
project from 1986 to 1993, the total gross power rating for the plant, including two
2.4 megawatt Pelton turbine-generators that power Hoover Dam’s own operations is a maximum
capacity of 2080 megawatts.[3] The annual generation of Hoover Dam varies. The maximum
net generation was 10.348 TWh in 1984, and the minimum since 1940 was 2.648 TWh in 1956.[3]
The average power generated was 4.2 TWh/year for 1947–2008.[3] In 2015, the dam generated
3.6 TWh.[94] The amount of electricity generated by Hoover
Dam has been decreasing along with the falling water level in Lake Mead due to the prolonged
drought in the 2010s and high demand for the Colorado River’s water. Lake Mead fell to
a new record low elevation of 1,071.61 feet (326.63 m) on July 1, 2016 before beginning
to rebound slowly.[95] Under its original design, the dam would no longer be able to
generate power once the water level fell below 1,050 feet (320 m), which might have occurred
in 2017 had water restrictions not been enforced. To lower the minimum power pool elevation
from 1,050 to 950 feet (320 to 290 m), five wide-head turbines, designed to work efficiently
with less flow, were installed.[96] Due to the low water levels, by 2014 it was providing
power only during periods of peak demand.[97] Water levels were maintained at over 1,075
feet (328 m) in 2018 and 2019.[98] Control of water was the primary concern in
the building of the dam. Power generation has allowed the dam project to be self-sustaining:
proceeds from the sale of power repaid the 50-year construction loan, and those revenues
also finance the multimillion-dollar yearly maintenance budget. Power is generated in
step with and only with the release of water in response to downstream water demands.[99] Lake Mead and downstream releases from the
dam also provide water for both municipal and irrigation uses. Water released from the
Hoover Dam eventually reaches several canals. The Colorado River Aqueduct and Central Arizona
Project branch off Lake Havasu while the All-American Canal is supplied by the Imperial Dam. In
total, water from Lake Mead serves 18 million people in Arizona, Nevada and California and
supplies the irrigation of over 1,000,000 acres (400,000 ha) of land.[99][100] In 2018, the Los Angeles Department of Water
and Power (LADWP) proposed a $3 billion pumped hydro storage project—a “battery” of sorts—that
would use wind and solar power to recirculate water back up to Lake Mead from a pumping
station 20 miles (32 km) downriver.[101][102][103] Power distribution
Electricity from the dam’s powerhouse was originally sold pursuant to a fifty-year contract,
authorized by Congress in 1934, which ran from 1937 to 1987. In 1984, Congress passed
a new statute which set power allocations to southern California, Arizona, and Nevada
from the dam from 1987 to 2017.[104][105] The powerhouse was run under the original
authorization by the Los Angeles Department of Water and Power and Southern California
Edison; in 1987, the Bureau of Reclamation assumed control.[106] In 2011, Congress enacted
legislation extending the current contracts until 2067, after setting aside 5% of Hoover
Dam’s power for sale to Native American tribes, electric cooperatives, and other entities.
The new arrangement began on October 1, 2017.[104] The Bureau of Reclamation reports that the
energy generated under the contracts ending in 2017 was allocated as follows:[3] Area Percentage
Metropolitan Water District of Southern California 28.53%
State of Nevada 23.37% State of Arizona 18.95%
Los Angeles, California 15.42% Southern California Edison 5.54%
Boulder City, Nevada 1.77% Glendale, California 1.59%
Pasadena, California 1.36% Anaheim, California 1.15%
Riverside, California 0.86% Vernon, California 0.62%
Burbank, California 0.59% Azusa, California 0.11%
Colton, California 0.09% Banning, California 0.05% Tourists gather around one of the generators
in the Nevada wing of the powerhouse to hear its operation explained, September 1940 A worker stands by the 30 ft (9.1 m) diameter
Nevada penstock before its junction with another penstock that delivers water to a turbine Spillways Water enters the Arizona spillway (left) during
the 1983 floods. Lake Mead water level was 1,225.6 ft (373.6 m)
The dam is protected against over-topping by two spillways. The spillway entrances are
located behind each dam abutment, running roughly parallel to the canyon walls. The
spillway entrance arrangement forms a classic side-flow weir with each spillway containing
four 100-foot-long (30 m) and 16-foot-wide (4.9 m) steel-drum gates. Each gate weighs
5,000,000 pounds (2,300 metric tons) and can be operated manually or automatically. Gates
are raised and lowered depending on water levels in the reservoir and flood conditions.
The gates cannot entirely prevent water from entering the spillways but can maintain an
extra 16 ft (4.9 m) of lake level.[107] Water flowing over the spillways falls dramatically
into 600-foot-long (180 m), 50-foot-wide (15 m) spillway tunnels before connecting to the
outer diversion tunnels, and reentering the main river channel below the dam. This complex
spillway entrance arrangement combined with the approximate 700-foot (210 m) elevation
drop from the top of the reservoir to the river below was a difficult engineering problem
and posed numerous design challenges. Each spillway’s capacity of 200,000 cu ft/s (5,700
m3/s) was empirically verified in post-construction tests in 1941.[107] The large spillway tunnels have only been
used twice, for testing in 1941 and because of flooding in 1983. Both times, when inspecting
the tunnels after the spillways were used, engineers found major damage to the concrete
linings and underlying rock.[108] The 1941 damage was attributed to a slight misalignment
of the tunnel invert (or base), which caused cavitation, a phenomenon in fast-flowing liquids
in which vapor bubbles collapse with explosive force. In response to this finding, the tunnels
were patched with special heavy-duty concrete and the surface of the concrete was polished
mirror-smooth.[109] The spillways were modified in 1947 by adding flip buckets, which both
slow the water and decrease the spillway’s effective capacity, in an attempt to eliminate
conditions thought to have contributed to the 1941 damage. The 1983 damage, also due
to cavitation, led to the installation of aerators in the spillways.[108] Tests at Grand
Coulee Dam showed that the technique worked, in principle.[109] Roadway and tourism
See also: Hoover Dam in popular culture View of Hoover Dam from Mike O’Callaghan–Pat
Tillman Memorial Bridge The bypass in front of the dam Mike O’Callaghan–Pat Tillman Memorial Bridge
as visible from Hoover Dam There are two lanes for automobile traffic
across the top of the dam, which formerly served as the Colorado River crossing for
U.S. Route 93.[110] In the wake of the September 11, 2001 terrorist attacks, authorities expressed
security concerns and the Hoover Dam Bypass project was expedited. Pending the completion
of the bypass, restricted traffic was permitted over Hoover Dam. Some types of vehicles were
inspected prior to crossing the dam while semi-trailer trucks, buses carrying luggage,
and enclosed-box trucks over 40 ft (12 m) long were not allowed on the dam at all, and
were diverted to U.S. Route 95 or Nevada State Routes 163/68.[111] The four-lane Hoover Dam
Bypass opened on October 19, 2010.[112] It includes a composite steel and concrete arch
bridge, the Mike O’Callaghan–Pat Tillman Memorial Bridge, 1,500 ft (460 m) downstream
from the dam. With the opening of the bypass, through traffic is no longer allowed across
Hoover Dam; dam visitors are allowed to use the existing roadway to approach from the
Nevada side and cross to parking lots and other facilities on the Arizona side.[113] Hoover Dam opened for tours in 1937 after
its completion, but following Japan’s attack on Pearl Harbor on December 7, 1941, it was
closed to the public when the United States entered World War II, during which only authorized
traffic, in convoys, was permitted. After the war, it reopened September 2, 1945, and
by 1953, annual attendance had risen to 448,081. The dam closed on November 25, 1963, and March
31, 1969, days of mourning in remembrance of Presidents Kennedy and Eisenhower. In 1995,
a new visitors’ center was built, and the following year, visits exceeded one million
for the first time. The dam closed again to the public on September 11, 2001; modified
tours were resumed in December and a new “Discovery Tour” was added the following year.[106] Today,
nearly a million people per year take the tours of the dam offered by the Bureau of
Reclamation.[114] Increased security concerns by the government have led to most of the
interior structure being inaccessible to tourists. As a result, few of True’s decorations can
now be seen by visitors.[115] Environmental impact View upstream from Hoover Dam, Sept. 2009.
Water elevation on this date was 1093.77 ft (333.38 m).
The changes in water flow and use caused by Hoover Dam’s construction and operation have
had a large impact on the Colorado River Delta.[116] The construction of the dam has been implicated
in causing the decline of this estuarine ecosystem.[116] For six years after the construction of the
dam, while Lake Mead filled, virtually no water reached the mouth of the river.[117]
The delta’s estuary, which once had a freshwater-saltwater mixing zone stretching 40 miles (64 km) south
of the river’s mouth, was turned into an inverse estuary where the level of salinity was higher
close to the river’s mouth.[118] The Colorado River had experienced natural
flooding before the construction of the Hoover Dam. The dam eliminated the natural flooding,
threatening many species adapted to the flooding, including both plants and animals.[119] The
construction of the dam devastated the populations of native fish in the river downstream from
the dam.[120] Four species of fish native to the Colorado River, the Bonytail chub,
Colorado pikeminnow, Humpback chub, and Razorback sucker, are listed as endangered.[121][122] Naming controversy Los Angeles Times political cartoon commenting
on the attempts of Ickes to keep “Hoover” off of the dam.
During the years of lobbying leading up to the passage of legislation authorizing the
dam in 1928, the press generally referred to the dam as “Boulder Dam” or as “Boulder
Canyon Dam”, even though the proposed site had shifted to Black Canyon.[16] The Boulder
Canyon Project Act of 1928 (BCPA) never mentioned a proposed name or title for the dam. The
BCPA merely allows the government to “construct, operate, and maintain a dam and incidental
works in the main stream of the Colorado River at Black Canyon or Boulder Canyon”.[123] When Secretary Wilbur spoke at the ceremony
starting the building of the railway between Las Vegas and the dam site on September 17,
1930, he named the dam “Hoover Dam”, citing a tradition of naming dams after Presidents,
though none had been so honored during their terms of office. Wilbur justified his choice
on the ground that Hoover was “the great engineer whose vision and persistence … has done
so much to make [the dam] possible”.[124] One writer complained in response that “the
Great Engineer had quickly drained, ditched, and dammed the country.”[124] After Hoover’s election defeat in 1932 and
the accession of the Roosevelt administration, Secretary Ickes ordered on May 13, 1933, that
the dam be referred to as “Boulder Dam”. Ickes stated that Wilbur had been imprudent in naming
the dam after a sitting president, that Congress had never ratified his choice, and that it
had long been referred to as Boulder Dam.[124] Unknown to the general public, Attorney General
Homer Cummings informed Ickes that Congress had indeed used the name “Hoover Dam” in five
different bills appropriating money for construction of the dam.[125] The official status this
conferred to the name “Hoover Dam” had been noted on the floor of the House of Representatives
by Congressman Edward T. Taylor of Colorado on December 12, 1930,[126] but was likewise
ignored by Ickes. When Ickes spoke at the dedication ceremony
on September 30, 1935, he was determined, as he recorded in his diary, “to try to nail
down for good and all the name Boulder Dam.”[69] At one point in the speech, he spoke the words
“Boulder Dam” five times within thirty seconds.[127] Further, he suggested that if the dam were
to be named after any one person, it should be for California Senator Hiram Johnson, a
lead sponsor of the authorizing legislation.[69] Roosevelt also referred to the dam as Boulder
Dam,[90] and the Republican-leaning Los Angeles Times, which at the time of Ickes’ name change
had run an editorial cartoon showing Ickes ineffectively chipping away at an enormous
sign “HOOVER DAM,” reran it showing Roosevelt reinforcing Ickes, but having no greater success.[128] In the following years, the name “Boulder
Dam” failed to fully take hold, with many Americans using both names interchangeably
and mapmakers divided as to which name should be printed. Memories of the Great Depression
faded, and Hoover to some extent rehabilitated himself through good works during and after
World War II.[129] In 1947, a bill passed both Houses of Congress unanimously restoring
the name “Hoover Dam.”[130] Ickes, who was by then a private citizen, opposed the change,
stating, “I didn’t know Hoover was that small a man to take credit for something he had
nothing to do Recognition
Hoover Dam was recognized as a National Civil Engineering Landmark in 1984.[131] It was
listed on the National Register of Historic Places in 1981, and was designated a National
Historic Landmark in 1985, cited for its engineering innovations.

12 thoughts on “The Historic Hoover Dam in Las Vegas Nevada Historical and Biggest Bridge w/Spectacular design inUSA

  1. ang ganda naman jan pasyalan brother sana makarating din kami ni misis dyan balang araw

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