| Since the flood disasters frequently happened and brought in serious damages in China, a large significant of hydrologic engineering have been constructed greatly to improve the flood control and the water management. Historical flood records and gauge flood data play the important roles in improving the quality of the design flood. However, the data are still limited to estimate the long-scale, extreme, rare flood and always make errors in hydraulic reconstruction. Now, the hydrological palaeoflood discipline in combination with geological and chronological methods provides the new approach solving the data shortage and extrapolation errors. A reverse extrapolation from the Holocene palaeoflood records is made to estimate the return period of 1000-year or 5000-year large flood. Collectively, the palaeoflood reveals the relationship between the rivers hydrological system and global climate change.A field investigation was carried out along the upper Hanjiang River. Typical palaeoflood slackwater deposits were found at the YJP site. They were identified by sedimentary criteria during fieldwork, sampled and analyzed in detail include particle-size, magnetic susceptibility, loss-in-ignition and CaCO3 content compared with eolian loess and soil. Using OSL dating method combined with Holocene pedo-stratigraphic correlations with the previously researches of upper Hanjiang River. They all have recorded the three episodes of extraordinary palaeoflood events in the upper Hanjiang River, China. The hydrological dates based on the slackwater deposits were reconstructed. The flood peak discharges were measured and modelled by slope-area method and HEC-RAS hydraulic model based on palaeostages indicators. Collectively, the results combined with historical data and gauge tested data extend flood frequency curve to the 10000 year scale, improving the precision of flood frequency analysis. This paper provides the 10000-year flood using the palaeoflood hydrology methodology, which is quite significant in flood mitigation, water resources utilization and city hydrological engineering. The main conclusions of this paper are as follow:1) The field characteristics show these palaeoflood slackwater deposits were light brownish gray (2.5Y6/2) silt with abrupt vertical changes. The end-points of the SWDs were obvious, and boundaries were obvious in the stratigraphy. The magnetic susceptibility and loss-in-ignition of the SWD were very low, while the CaCO3 content was high. The physical and chemical indicators results indicate the palaeoflood SWDs were differentiated from as eolian loess and soil, alluvial of the riverbed and floodplain deposit. The former are sediments from the suspended load in floodwater at sites of suddenly reduced flow velocity and well preserved by the eolian loess, soil and other sediments.2) The characteristics of particle-size distribution are shown as follow:(1) Analysis of the particle-size distribution suggests that these SWD consist of silt. (2) Particle-size distribution curve of the SWD is high and thin which indicates they were well sorted. (3) The cumulative probability distribution diagram of palaeoflood SWD indicates that they were suspended and saltation load. (4)They corresponded to the type Ⅵ and Ⅶ in CM patterns which represents a low turbulence deposit. (5) Grain-size of the top part of the palaeoflood SWD1 is coarser than the lower part. The particle-size criteria and analytical characteristics indicate SWD have a significant change in space and hydrodynamic condition.3) Using optically stimulated luminescence method in combination with pedo-stratigraphic correlations of Holocene pedo-stratigraphy in the upper Hanjiang River, these palaeoflood slackwater deposits was recorded the three Holocene extraordinary palaeoflood events which happened during 1000～900 a BP、1800～1600 a BP and 3 200～2800 a BP. These palaeoflood events always occurred with the climate changing abruptly, going worse and fluctuating extremely, leading in abnormal precipitation, and therefore causing big floods and severe droughts. This study deepens the relationship between global climatic change and river hydrology.4) The paleostages and peak discharge of upper reach of Hanjiang River are reconstructed by multiple methods. Paleostages are calculated by the end-point of slackwater deposits and the results are from 176.20 m to 176.73 m. Peak discharge were estimated using slope-area method and HEC-RAS mothed, respectively. The slope-area method shows the highest peak discharges of the three flood events range from 50220 m3/s to 52 620 m3/s. HEC-RAS hydraulic model with ARC/INFO software estimates the peak discharges of these palaeoflood are 53770～55950 m3/s. Then, modern flood date from Baihe gauge in 2010 was used to test the slope-area method and HEC-RAS model and the errors are less than 10%.5) Comparison between HEC-RAS hydraulic model and the slope-area approach illustrates there are three main differences:1) the principles of these two methods; 2) the requirements of geometry of channels and the hydraulics parameters;3) the sensitivity of discharge rate on roughness values. Both the methods are reliable. HEC-RAS model has less roughness values sensitive. The slope-area method is used to calculate the discharge at first when only less hydraulic parameters were investigate, while the HEC-RAS model is suited to estimate the long-distance river channel when hydrological parameters were measured systematically.6) Palaeoflood discharges data added to the Baihe gauge station hydrology data and historical data extend the flood frequency curve. The value for the 10000 y a flood,1 000-year a flood and 100-year flood are 57820 m3/s,45670 m3/s and 33400 m3/s, respectively. This study presents how the palaeoflood hydrology helps extrapolating downward to rare, extreme floods by using natural evidence, rather than.,.extrapolating upscale by those common, small-scale floods. All these results play a very significant part in mitigating flood hazards, water resource management and hydraulic constructions. |
PDF Full Text Request