| The scientific discipline of paleoflood hydrology combines geological and hydrological approaches. Slackwater Deposit-Paleostage Indicator (SWD-PSI) paleoflood hydrology employs recent technological advances in geochronology and hydraulic flow modeling to reconstruct the magnitudes and frequencies of paleofloods from evidence of their paleostages. Conventional flood frequency analysis is conducted by fitting functions (the Pearson Type III is given priority in China) to peak annual discharges from gauged records for a particular catchment. Extrapolations from the instrumented record are then made to estimate the return period or discharge of larger floods. In other words, estimates are based on statistical properties of the relatively common, small-scale flooding, and extrapolated upscale to the predict those of the unknown extreme, rare ones. A reverse kind of scaling, extrapolating downward to rare, high-magnitude floods by using whatever evidence is preserved in nature, is used in palaeoflood hydrology. Thus, the data generated from palaeoflood hydrology achieve a better estimate for large events and provide an interpretation of the regional response of the hydrological system to global climatic change at longer time scales.This paper presents the results of our palaeohydrological investigations in the Jin-Shaan Gorges of the middle Yellow River, the Baojixia Gorges of the Weihe River, the Binxian reach of the middle Jinghe River and the Wufeng reach of the upper Hanjiang River. It presents descriptions of paleoflood slackwater deposits (SWDs) and identification of paleoflood events, surveys of these SWDs to provide palaeostage indicators, modelling the palaeoflood hydraulics associated with these flood events to estimate flood peak discharge, and dating the sediments to provide a chronological framework. Collectively, the results are used to investigate the interactions between the regional hydro-climatic systems and global climatic change. Particularly, this paper is to review different one-dimensional flow techniques, i.e. slope-area method, HEC-RAS mothed, and critical-depth method, with an emphasis on their appropriateness in paleoflood studies. The main conclusions of this paper are as follow:(1) Sedimentary records of the Holocene extraordinary flood were investigated in the middle Yellow River basin and the upper Hanjiang River basin. A series of paleoflood slackwater deposits (SWDs) were identified on the bedrock cliffs at the PDG sites along the Jin-Shaan Gorges in the middle Yellow River, the GCZ sites along the Baojixia Gorges in the Weihe River, the CJC sites in the middle Jinghe River and WFD sites in the upper Hanjiang River. These SWDs, fine-grained sediments deposited at high flood stage from a suspended load in overbank floodwater, are natural evidence of the past extreme flood that really occurred.(2) Ages of these paleoflood events originated from several lines of evidence from each sites. Pedo-stratigraphical correlations with other dated loess-soil profiles in the catchment provided an initial chronological framework for these paleoflood events. Archaeological identification of the retrieved pottery shards and the settlement ruins in the relation with the SWDs enabled approximating the ages of paleoflood events. OSL dating on the sediments and pottery shards refined the ages of the paleofloods. These Holocene extraordinary flood events occurred in the middle Yellow River basin and the upper Hanjiang River basin were synchronous with the climatic event defined by various paleoclimatic proxies from other parts of China and the world. It is therefore inferable that these extraordinary flood events represent obvious responses to the global climatic events during periods of highly variable and instable climate.(3) Paleostages and peak discharge estimation were calculated using different methods. Paleostages are indicated by the highest end-point of the specific flood slackwater deposit, or otherwise the peak stage figured out using the thickness of the SWD and the estimated suspended sediment load in the flood water. Peak discharge of these paleoflood were estimated using slope-area method, HEC-RAS mothed, and critical-depth method.(4) Case studies in the middle Yellow River basin and the upper Hanjiang River basin review different one-dimensional flow techniques, i.e. slope-area method, HEC-RAS mothed, and critical-depth method, with an emphasis on their appropriateness in paleoflood studies. The underlying hydraulic theory behind these methods is similar, but each method uses different amounts of data and different solution schemes that affect their usefulness in the paleoflood studies. Preliminary estimates of the discharge of the paleofloods can be calculated in the few cross sections using the slope-area method during the fieldwork when channel bed topography and depth were unknown. Then a HEC-RAS analysis can be done for the entire cross sectional geometry, incorporating the underwater data. The critical-depth method is particularly advantageous if PSIs are available in the critical flow reach and a particular advantage to this method is its insensitivity to roughness and energy loss coefficients, which cannot be objectively calculated. Special attention must be given to the control section at which critical flow is assumed and additional cross sections at approximately channel width are needed when using the critical-depth method.(5) Flood frequency analysis in the Wubu gauge station on the Yellow River, the Linjiacun gauge station on the Weihe River, the Jingcun gauge station on the Jinghe River and the Baihe gauge station on the upper Hanjiang River, are conducted using the paleoflood, historical flood and systematic data. Combination of systematic and non-systematic data applying in flood frequency analysis is of significance for improvement for the accuracy of the unknown extreme, rare floods.All these results play a very important role in hydraulic engineering, in water resource development, and in mitigating flood hazards in the middle Yellow River basin and the upper Hanjiang River basin.(6) The research progress of hydrological frequency analysis is described. Advantages of the extrapolating downward to rare, high-magnitude floods by using SWD-PSI paleoflood hydrology are discussed. The ages of the Holocene paleoflood events, especially those very ancient ones, and the peak discharges of these floods have been met by new technology for geochronology and for hydraulic modeling. Extraordinary paleofloods are one of the hydrology phenomenon, which consist in the dialectic unity between necessity and contingency. The combination of this logical foundation and the progress in the environmental change research, i.e. physical mechanisms of Holocene paleoflood events, are encouraging and further proof that archives of SWD-PSI and other paleoflood evidence offer a preferable alternative to conventional flood-frequency analyses. Finally, it is pointed out that further improvement and optimization of the statistic methods, and enhancing combination of the analyses of physical mechanisms and mathematical statistics, are the key not only to improving the results of hydrological time series analysis, but also to studying and discovering the magnitude of extraordinary floods, and its space-time distribution, and relationship between these great floods and global change. |
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