The Study On The Evolution Of Floods And Water Environment In The West Of Guanzhong Plain During The Last Century

The flood usually submerges the field, roads and villages, and also leads to the loss of the properties and lives. Thus, the research about flood is paid high attention. It has important scientific significance to reveal the rule of the flood change, the flood occurrence and the deterioration of the water quality in historical period investigating on the distributing and grain size and the content of the heavy metal in the floodplain sediments in Guanzhong plain about in one hundred years. It also has important practical significance to study those problems on hydrodynamic force and the aspect of water environment. Based on the detailed investigation and systemic design, 189 samples were collected in high floodplain of Zhouzhi (ZZ) profile, Meixian (MX) profile. Granulometric composition, content of trace elements, grayscale, and magnetic susceptibility of samples were determined. Based on the analysis of these indexes combined with the re-analysis of historical literature and power spectrum analysis for flood sequence using the REDFIT software, some conclusions have been summarized as follows:1. The flood deposition of Weihe River can record the sequence of flood occurrence. It is vital role that the grain size can be regarded as the proxy which represent in hydrodynamic force. Therefore, we can investigate the flood cycles by the analysis of flood sediments. The thickness of ZZ and MX profile is respectively1.9m, which was formed by the ancient flood deposit, 16 sediment phases indicating 16 flood depositions differently, and also indicated 16 flood depositions stages, including the cataclysm of 2005 which the topside indicated and the other 15 flood depositions formed in the early times. According to the historical material, the 16 flood depositions which showed in the ZZ and MX profiles are mostly formed in the recent 120 years.2. There is obvious variety rhythm of granularity along with the depth in the ZZ, MX profiles. The granularity in the 1st,2rid,3rd,5th,6th,7th,and 8th sections was thinner than the 4th,9th,10th,11th,12th,13th,14th,15th and 16th sections of the ZZand MX profiles. After eliminating the influence of the landform heighten caused by the deposition thickness on granulometric composition, the order of flood depth and the flood scale of these 16 flood phases was: 15th＞12th＞13th＞16th＞14th＞9th＞10th＞11th＞4th＞1st＞2nd＞6th＞5th＞7th＞8th＞3th. The 15th,12th,13th,16th,14th,9th,10th,11th and 4th were the catastrophic flood phases, and the 1st,2nd, 6th,5th,7th,8th and 3th were the usual flood phase.3. We have done the reanalysis on the data of flood hazards in the middle and low-reaches in Weihe River from 1664 to 1949, using the historical document. As a result, it is found that the flood hazards in this area are mostly local ones in a small scale. It is worthy to point out that power spectrum analysis on the flood sequence indicates that there exists of periodicities of 76 years, 11 years and 2-7 years in this area. Periodicities of centennial and quasi-decennial can be well correlated respectively to those of solar activity; the periodicity of 3.4-2.2 years is probably associated with the activity of tropical air mass, which indicates that both the solar activity and the atmospheric activity in the area of low latitude ocean can influence in the frequency of flood hazards in the study region.4. There is a strong relativity between the content of the heavy metal elements and the grain size of the depositions in the research profiles. The higher contents of the heavy metal elements corresponded to the thinner granulometric composition, while the lower content of the heavy metal corresponded to the thicker granulometric composition. The contents of the heavy metal elements changed obviously and decreased along with the profile depth reflected the influence of humanity activity on the Weihe River water environment. All these indicate the huraan strengthen the soil pollution when the farmers use the fertilizer and the workers dump the dirty water into the river.5. The main cause of the flood is usually due to the increasing of annual mean rainfall. The instant heavy rainstorm can also lead to the flood. Besides those, the human activities prick up the flood occurrence. For example, the human destroy the forest to cultivate the field, which result in soil erosion and raising the flood leve, eventually, the flood disaster is aggravated.6. The change of flood frequency in Weihe area in recent four hundred years has approximate anti-dependence relationship with the number of the sunspots and the change of the air temperature in the east of China in decadal scale. When the sunspots increase, the air temperature raise in the east of China, while the flood frequency decrease in Weihe area, and vise versa.7. Be compare with the flood in the lower reach, the flood in the middle reach have the following characteristics: the sedimentary thickness of the flood are thinner, the grain size are coarser, and the sendimentary level are less.