the development of surface water hydrology in the

Transcription

Colloque International OH2 « Origines et Histoire de l’Hydrologie », Dijon, 9-11 mai 2001
International Symposium OH2 ‘Origins and History of Hydrology’, Dijon, May, 9-11, 2001
The development
of surface water hydrology
in the United States
Le développement
de l’hydrologie de surface
aux États-Unis
Martin REUSS
Senior Historian
Office of History, Headquarters, U.S. Army Corps of Engineers
Kingman Building, 7701 Telegraph Road, Alexandria, Virginia 22315-3865 (USA)
[email protected]
Abstract
At the beginning of the nineteenth century, hydrology in the United States was as much
myth as science. Despite scientific progress, some misconceptions — especially the impact of
forests on precipitation — hounded the subsequent development of hydrology. The focus of
hydrology on practical and often immediate applications also raised skepticism among scientists
about hydrology’s claim to scientific legitimacy. Doubts about hydrology continue to the
present day. Some people still prefer water dowsing to hydrologic investigations.
Scientific development — at first mainly empirical — resulted from political and
economic pressures as the United States became industrialized and its population expanded
westward. These advances responded to demands for better measurements of discharge and
rate of flow, especially in early factory towns. In the late nineteenth and early twentieth
centuries, increased use of waterpower and the expansion of western irrigation called for better
mapping of drainage basins. Finally, in the 1930s a massive federal flood control program led to
the development of storm studies, standard project floods, and better instrumentation. Much of
this work involved close coordination between the Army Corps of Engineers and the Weather
Service. Concepts such as standard project storms built on the earlier work of the Miami
Conservancy District. In 1931, the American Geophysical Union established a section on
hydrology after much debate, thus acknowledging that hydrology was a true science.
Yet, hydrologists continue to seek ways to gain acceptance from the public and scientific
communities. Within the Corps of Engineers, this attempt took the form of developing a new
center — the Hydrologic Engineering Center — that explicitly pronounced that there could be
hydrologic as well as hydraulic engineers. Working with university laboratories and other
federal agencies, the Center applies computer modeling and satellite imagery to hydrologic
© Université de Bourgogne
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problems. As hydrology evolves from primarily a descriptive science to one that employs
rational analysis to answer important questions about continental and global water processes, it
is likely that lingering suspicions about its scientific integrity will disappear.
Résumé
Au début du XIXe siècle, l’hydrologie aux États-Unis tenait plus du mythe que de la
science. Malgré le progrès scientifique, certains malentendus, en particulier au sujet de l’effet
des forêts sur le régime des précipitations, ont poursuivi l’hydrologie dans son développement.
L’intérêt de l’hydrologie pour les applications pratiques et souvent directes a aussi contribué au
scepticisme des scientifiques quant à la revendication de l’hydrologie à la légitimité scientifique.
Le doute envers l’hydrologie demeure encore aujourd’hui. Certaines personnes d’ailleurs
préfèrent encore la radiesthésie aux études hydrologiques.
Le développement scientifique, tout d’abord principalement empirique, résulte de
pressions politiques et économiques à mesure que les États-Unis s’industrialisaient et que la
population se propageait vers l’ouest. Ces progrès répondaient aux besoins d'une meilleure
évaluation des débits et vitesse d’écoulement, en particulier dans les premières villes
industrielles. Vers la fin du XIXe siècle et au début du XXe siècle, l’augmentation de l’utilisation de
l’énergie hydraulique et l’extension des surfaces irriguées ont nécessité une meilleure
cartographie des bassins hydrographiques. Finalement, dans les années 1930 un très important
programme fédéral de contrôle des crues a conduit au développement des études sur les
tempêtes, au concept des crues de projet et à une meilleure instrumentation. Une grande partie
de ce travail reposait sur une collaboration étroite entre le Génie militaire et les Services
météorologiques. Les concepts, tels que les crues de projet, ont été développés à partir des
travaux antérieurs de la Commission de conservation du District de Miami. En 1931, l’Union
géophysique américaine a mis en place une section Hydrologie après un long débat,
reconnaissant ainsi que l’hydrologie était une véritable science.
Malgré cela, les hydrologues en sont encore à tenter de se faire accepter par le public et
la communauté scientifique. Pour ce qui est du Génie militaire, cette acceptation s’est faite par
la mise en place du Centre d’ingénierie hydrologique qui reconnaît explicitement l’existence
d’hydrologues et d’hydrauliciens. Le centre, dans le cadre de sa collaboration avec les
laboratoires universitaires et d’autres agences fédérales, applique aux problèmes hydrologiques
la modélisation par ordinateur et l’imagerie satellitale. À mesure que l’hydrologie se
transformera en une science moins descriptive et faisant plus appel à l’analyse rationnelle pour
répondre à des questions importantes sur les processus continentaux et globaux, il est probable
que les suspicions qui subsistent au sujet de l’intégrité scientifique de l’hydrologie disparaîtront.
***
The subtitle of this paper might aptly be “From Myth to Science.” The problem
is that this description suggests a linear development, and both life and hydrology are
more complicated than that. In the United States, empirical and theoretical advances
in hydrology occurred at the same time that politicians and numerous scientists
continued to appeal to myths that rested more on hope than on reality.
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In the early nineteenth century United States, many engineers eagerly embraced
notions that had gained popularity in eighteenth century Europe. Of these, perhaps
the most enticing related to the ability of forests to attract rainfall and to turn deserts
into fertile farmlands. Erie Canal commissioners in the United States feared that
cutting too much timber would leave the region around the canal a dry wasteland.
Later in the arid Western parts of the United States, engineers, hydrologists, and land
developers offered hope that forests planted on dry prairie land would lead to
increased moisture and greater soil fertility. More sceptical scientists were unsure
whether hydrology was science or magic.
Yet another and in the end more fruitful approach to hydrology — empiricism
— achieved impressive results at the same time and gained hydrology increased respect
among practical minded engineers and entrepreneurs. Bluntly put, American economic
and political expansion pushed theory to the background and stimulated an enormous
number of valuable empirical studies. Most of the early studies sought answers to two
related questions — how much water does a stream or canal discharge and what is the
rate of flow? Canal builders needed to know whether the discharge from feeder
streams would supply sufficient water to canals. Water power engineers required
similar information to assess a canal or stream’s capability to supply power to grain
and textile mills — or to assess payments for the use of the water. River engineers
studied discharge and current data to determine the proper height of levees or the
capability of rivers to support navigation. Along the way, they developed new
measuring tools. In all cases, economic and political demands dictated the questions
and established the objectives.
During the so-called Progressive Era at the beginning of the twentieth century,
the ultimate conservationist goal was to put America’s tremendous water resources to
work as efficiently as possible.
Multipurpose river development challenged
hydrologists to identify the quantity, quality, and seasonal availability of water
supplies. Measurement was more important than ever as towns grew and more land
was put under the plow. Yet, measurement explained nothing. Knowing the amount
of rainfall or runoff was rather like knowing the temperature. The thermometer told
how hot the air was, but not that the heat resulted from molecular energies. Runoff
totals told how much surface flow there was, but not why that amount of flow
occurred. The more hydrologists learned about the nature of the hydrologic cycle —
about infiltration into the earth, for example, or transpiration and evaporation — the
more complicated the cycle appeared. In an earlier age, scientists sought immutable
laws to explain nature. Attempts to explain flood frequencies in terms of fixed
formulas continued well into the nineteenth century. However, with so many variables
involved in surface water hydrology, accurate and universally reliable answers did not
appear. Instead, many hydrologists and engineers sought correlations and resorted to
statistics and probability theory to measure them. They worked with “probability
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error”, “standard deviations”, and other statistical tools. This was no universe
described with finite laws, but it was a universe that presumably eliminated human
subjectivity.
Probability analysis seemed strangely out of sort with the rigorous certainty
traditionally sought in science. It allowed hydrologists to describe relationships
without searching for precision, nor even providing anything more than an acceptable
range in showing correlations.
Statistics allowed the extrapolation of physical
relationships (experiences) into the future. Though this was not the science of the
past, it increasingly became acceptable science under various conditions. Physical and
natural scientists accepted, even promoted, statistical correlations as useful scientific
tools. Therefore, those who employed them properly were scientific, if not always
scientists. Their proper use also allowed some control over nature--an obvious appeal
to the engineering community--although the extent of control depended on faith in
numbers.
Yet, hydrologists themselves came to doubt the reliability of statistical analysis
and frequency curves based on inadequate data, and they questioned the development
of huge structures designed to withstand supposed 8,000 or 10,000 year floods. The
debate reached a crescendo in the 1930s, as the United States initiated a huge public
works program to provide work for the unemployed and to provide flood control,
navigation, and hydropower throughout much of the country.
Politically and
economically, the debate balanced cost considerations against concerns for safety and
durability. Theoretically and technically, the debate assessed the value of emerging
tools, such as the unit hydrograph, developed by L. K. Sherman to calculate runoff and
streamflow for similar watersheds, or the overland flow theory developed by Robert E.
Horton. Empirically, the debate reflected a vastly increased amount of available data,
particularly in the Eastern part of the country, partly the result of major data
collection efforts on the part of the U.S. Geological Survey in the mid-1930s.
Extrapolation could give way to interpolation between known values.
New
meteorological techniques, pioneered by the Miami Conservancy District in Ohio, led to
sophisticated storm transposition methods. This so-called “rational” approach took
four or five of the worst storms that hit a particular area or similar areas in terms of
size, climate, topography, and other characteristics and combined them into a short
sequence of storms over the one subject area. From this storm series, rainfall and
runoff could be calculated. The aim was not to reproduce the worst possible storm
series but to identify the standard project storm, or the storm series that reasonably
characterizes an extreme storm in an area. This flood could then be used as the basis
for developing design criteria for flood control structures in a region. Most federal
and state water resource agencies adopted this approach in the coming years.
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The U.S. Weather Bureau went a step further. Since the water development
agencies building storms, levees, and other structural flood control devices required
“worst case scenarios”, engineers needed to plan for the extreme event, no matter how
unlikely. In the 1930s, meteorologists and hydrologists developed a concept called the
“maximum probable storm.” After a 1942 Ohio River flood that the Weather Bureau
badly miscalculated, the term was changed to “probable maximum storm” (sometimes
called “probable maximum precipitation” or PMP), a somewhat more modest claim in
accordance with available data. Although the probable maximum storm used storm
transposition techniques, it differed from a standard project storm in that it requires
the transposition of the most extreme storms and then often makes the storms even
more extreme. Obviously, both procedures require a sizable amount of data. They also
require some simplification of meteorological processes. In the post-World War II
years, hydrologists and meteorologists attempted to overcome lack of data by
frequency analysis (what else!), with mixed results.
In the United States, the
preference is still to rely on the transposition of actual storms.
A look at changes in the U.S. Army Corps of Engineers illustrates the impact of
both politics and evolving theory on hydrologic procedures. With passage of a major
flood control act in 1936, the Corps of Engineers emerged as the major flood control
agency in the United States, charged with the responsibility to develop over 200 flood
control projects, principally levees, reservoirs, and drainage channels, located in thirty
one states. Shortly after the act’s passage, the Corps of Engineers and the Weather
Bureau developed a cooperative arrangement that, modified and enlarged, continues
to the present day. The two agencies agreed to study storm potential in various
sections of the nation, and a Weather Bureau meteorologist was assigned to the Office
of the Chief of Engineers. However, Gail Hathaway, chief of the newly created
Reservoir Regulation and Hydrology Section within the Corps, realized in 1937 that far
more resources were needed to complete all the storm studies necessary to design the
flood control projects. In particular, he was concerned about the correct design of dam
spillways, which carried the excess flows from the reservoirs downstream during floods.
Consequently, he convinced his superiors in late 1937 to transfer War Department
funds to the River and Flood Division of the Weather Bureau to organize a
Hydrometeorological Research Section. At about the same time, the Corps initiated a
nationwide study of major flood producing storms. All the Corps regional offices
supported the effort. Beginning in 1938, the Office of the Chief of Engineers
published regulations detailing the objectives and manner of execution of the storm
studies. The instructions covered the development of rainfall curves that would show
intensity, quantity, average and maximum precipitation, and other factors.
Hathaway had first developed these procedures during 1935-36, while Chief of
the Hydraulic and Hydrologic Design Section of the Missouri River Division of the Army
Corps of Engineers, located in Omaha, Nebraska. A particularly devastating flood on
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the Republican River in Kansas in 1935, which killed over a hundred people, evidently
impressed Hathaway with the importance of developing a conservative approach
towards flood estimates and spillway size. It also showed him the futility of convincing
some people to leave their homes despite imminent disaster. Finally, in 1939-1940,
the Corps helped the Weather Bureau and, along with the Soil Conservation Service,
the Geological Survey to fund a network of precipitation and stream-gauging stations.
Both programs continue to the present.
Along the way, a significant change occurred in the design criteria for spillways.
In the late 1920s, the Corps of Engineers generally designed for a flood that was 2 or
2.5 times greater than the maximum flood of record. Thus, for example, available
records showed that the maximum flood ever recorded for the Missouri River above
Fort Peck Dam, then under construction, was 4,248 cms (150,000 cfs). The Corps
proceeded to multiply that figure by 2.5 and based its Fort Peck dam design on a
maximum probable flow into the reservoir of 10,760 cms (380,000 cfs), or, it was
calculated, a one in 8,000 year flood. The spillway was to accommodate 7,221 cms
(255,000 cfs). In 1936, the Deputy Chief of Engineers, Brigadier General Max Tyler,
personally directed that spillways should be designed so that they can discharge at
least fifty percent in excess of the estimated maximum flow without impairing dam
safety. Tyler’s guidance focused on the safety factor required for high earthen dams.
Then, sometime in 1939, the Corps modified Taylor’s policy to permit a lower
percentage if studies and data justified it. A year or two thereafter, the percentage
factor of safety was entirely eliminated. As a Corps publication later put it, “Because
of the increased reliability of the estimates now obtainable, the need for arbitrary
safety factors has been eliminated.” The Corps’ policy became, in Hathaway’s words, to
“provide complete security against overtopping of the dam during the most severe
flood or sequence of floods considered reasonably possible.” The changes also
mirrored the Corps’ greater reliance on storm transposition methods and less on pure
probability analysis.
The Corps of Engineers, charged with so much of the federal water resources
program, dedicated an increased amount of attention to hydrology, and the results
showed. By 1950, the agency had nearly completed the investigation of 1,500 major
storms of record. It had accumulated rainfall curves for areas ranging from 2,590
hectares (ten square miles) to 25,900,000 hectares (100,000 square miles) or more.
The Weather Bureau hydrometeorological section funded by the Corps had
comprehensively studied maximum possible rainfall for 26 river basins, made special
studies of about 75 reservoirs, and had provided technical assistance to Corps
personnel in the evaluation of wind and hurricane data. Cooperative agreements with
the Weather Bureau and the Geological Survey had resulted in the establishment of
over 7,000 stream-gauging stations and 4,600 rain fall stations funded partially by the
Corps of Engineers, and the agencies maintained a cooperative flood forecasting
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service. The Corps also had transferred funds to various university laboratories to
study sedimentation in reservoirs and streams channels and to examine wave and
wind-tide heights in reservoirs. These investigations enabled the Corps to save
substantial funds by showing ways in which levee and dam heights could be reduced
and riverbanks protected without increasing risk.
The floods and droughts of the 1930s showed the pressing need for hydrologic
data, and stimulated far more cooperation between government and academia than
had previously existed. The 1936 Flood Control Act accelerated the search for better
data and analysis to support a major flood control program. At the same time, the act
did not compel interagency coordination, nor did it insist that water agencies agree
about amount of data to be obtained or analytical approaches to be followed. The
procedures developed jointly by the Corps and the Weather Service were not
universally accepted. The Bureau of Reclamation argued with some justification that
the approach worked fine when agencies had substantial data and when the funding of
large, conservatively built, projects raised no problems. However, in the arid West
historical rainfall data was often scant, forcing the Bureau to extrapolate. More than
likely, this led to less conservative hydrologic parameters. Moreover, the Bureau
needed to design projects that non-federal interests could repay over time, unlike the
Corps’ flood control dams, which were totally federally funded until 1986. Designers
either had to develop smaller projects or accept greater risk. Finally, whereas flood
control constituted a major justification for numerous Corps dams, Bureau dams were
built for irrigation and hydropower. Bureau engineers did not, therefore, share the
Corps’ concern to build for the largest probable flood. Consequently, the Bureau
developed its own in-house meteorological competence and often preferred to develop
“site specific” storm studies rather than the large drainage basin studies characteristic
of Corps and Weather Bureau efforts. Only in the late 1970s did the Bureau change its
position, partly in response to the 1976 Teton Dam disaster and partly the result of
retirements. New people trained in Weather Service procedures came into the Bureau
and adopted procedures more in accord with those they had been taught.
In the United States, no national water policy ever emerged, and no agency
assumed overall responsibility for advances in hydrology. Instead, various agencies
assumed responsibility — often without much debate or thought — for examining and
monitoring different parts of the hydrologic cycle: groundwater to the Geological
Survey, surface water to the Army Corps of Engineers and the Bureau of Reclamation,
meteorology to the Weather Service, etc. The coherence and integrity of hydrology,
then, is not reflected in governmental structure and responsibilities. Similarly trained
professionals become hydrologists in the Geological Survey and hydraulic engineers in
the Corps of Engineers, according to whatever professional category appears to offer
the greatest chance of promotion in the various agencies. Today, agency prerogatives
and career patterns continue to influence the development of hydrology at various
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governmental levels in the United States. Despite this division of responsibilities,
hydrologists in the United States have made impressive gains and obtained increased
respect within the scientific community.
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the development of surface water hydrology in the

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