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<br />PART IV
<br />
<br />PALEOFLOODS
<br />
<br />INTRODUCTION
<br />
<br />Following decades of war and revolution, the People's
<br />Republic of China was faced with the opportunity to mod-
<br />ernize. In 1950 there were 300 hydrologic stations in China,
<br />and most of the record lengths were too short to provide
<br />useful design information (Luo Chengzheng, 1985). In
<br />order to generate information to design numerous major
<br />dam projects, China embarked on a national survey of
<br />historical flood marks (Chen Chia-Chi et aI., 1975; Hua
<br />Shi-Qian, 1985), In part this realization derived from haz-
<br />ardous underdesign of several reservoir spillways based
<br />on flood-frequency analysis of inadequately short systematic
<br />records (Teng Wei-fen and Gu Chuan-zhi, 1985), The
<br />philosophy of the historic flood survey was to collect as
<br />much documentary information as possible to test the rep-
<br />resentativeness of the systematic gauge records and to reduce
<br />the potential error of extrapolation in frequency curves to
<br />extraordinary flood magnitudes,
<br />Examples of Chinese historical flood documentation
<br />include studies of the Chang Jiang (Yangtze River) where
<br />the major floods of 1153, 1227, 1520, 1560, 1788, 1796,
<br />1860, and 1870 are well documented by inscriptions and
<br />flood marks (Luo Chengzheng, 1985), The importance of
<br />such data in China is emphasized by its use in defining
<br />the spillway design discharge for the Gezhouba Dam, a
<br />$2.4 billion project now nearing completion, and the Three
<br />Gorges Dam, a proposed $7.4 billion project (Shih Win-
<br />shing, 1985). Failure of these dams would put a greater
<br />total population at risk than that of the entire United States.
<br />The Huang He (Yellow River) has an even longer his-
<br />torical record, extending back to a recorded flood stage in
<br />223 A,D, The Chinese have incorporated geological studies
<br />of flood sedimentation into design studies on this river in
<br />order to independently verify historical records (Shih Fu-
<br />cheng et aI., 1985). The Chinese have rigorous procedures
<br />to evaluate the uncertainty of indirect discharge estimation
<br />for ancient flood evidence and to incorporate dated ancient
<br />floods into the frequency analysis. They find that the largest
<br />events in the last several hundred to 2000 years exert a
<br />tremendous influence on frequency analysis (Shih Winshing,
<br />1985).
<br />
<br />The floods previously described are historic floods. They
<br />took place before the time of continuous modem hydrologic
<br />measurement, but their occurrence was observed, recorded,
<br />or otherwise communicated by human action for subsequent
<br />hydrologic analysis. Paleofloods are past flow events that
<br />need not have been humanly observed. Rather, their ex-
<br />istence is indicated by various persistent phenomena that
<br />they induce on the landscape or its vegetative coveL Pa-
<br />leoflood hydrology attempts to reconstruct ancient floods
<br />utilizing various procedures of sedimentology, stratigraphy,
<br />geomorphology, and geobotany, combined with principles
<br />of hydraulics and hydrology. As reviewed by Costa (1986)
<br />the development of paleoflood hydrology in the United
<br />States through 1970 has come predominantly from gee-
<br />morphologists and geologists rather than hydrologists,
<br />
<br />PALEOFLOOD HYDROLOGIC RESEARCH
<br />
<br />In geomorphic studies of paleofloods it is very important
<br />to distinguish effects of (I) low magnitude, high-frequency
<br />floods, and (2) high magnitude, low-frequency floods, Type
<br />I effects have been extensively studied in alluvial rivers,
<br />that is rivers with banks and beds of sediment that can be
<br />moved over a relatively broad range of flow conditions.
<br />Chapter 19 by Garnett P. Williams entitled "Paleofluvial
<br />Estimates from Dimensions of Former Channels and Mean-
<br />ders" reviews a major body of empirical data concerning
<br />type I floods and their relationships to alluvial river mor-
<br />phology. Williams details numerous empirical relation-
<br />ships that may be used in estimating paleostreamflow from
<br />the interrelated morphological parameters of paleochan-
<br />nels,
<br />Type 2 effects are of interest for the reconstruction of
<br />rare, large floods. One line of research on this topic employs
<br />sediment transport relationships, such as described by Komar
<br />in Chapter 6, to estimate paleoflood parameters from studies
<br />of coarse-grained flood deposits. Particle sizes are generally
<br />related to shear stress, velocity, or stream power. Examples
<br />of this approach include work by Baker (1974), Baker and
<br />Ritter (1975), Church (1978), Bradley and Mears (1980),
<br />Costa (1983), and Williams (1983). Williams (1984) sum-
<br />marizes most of the relevant equations. As discussed by
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