Research On Flood Change Indicated By Deposit In High Valley Flat Profile Of Shiyang River In Wuwei

In northwest aridity areas,especially in Minqin oasis at lower reaches of Shiyang River,water resource is deficient badly and climate is dry wondrously,so entironment is flimsy.Such as water table toboggans,and the mineralization of ground water gets higher and higher,and vegetation degenerates,and so on.It has threatened the people's survival and economic development,and counteracted the society's and resource's sustaining development.Therefore,it has important practical significance,on forecasting water environment's evolution trend of Shiyang River and comprehensive treatment to Shiyang River valley,to study the evolvement of water environment of Shiyang River and probe into the climate variance in Shiyang River valley in historical period.Based on the detailed investigation and systemic design,253 samples were collected in high floodplain of WW and WWC profiles in Caiqi Village.Granulometric composition,content of CaCO₃ and magnetic susceptibility of samples were determined. Based on the analysis of these indexes combined with the re-analysis of historical literature,the rule of flood occurrence and its relationship with precipitation were discussed,and some conclusions have been summarized as follows:1.The flood deposition of Shiyang River in recent 200 years records the sequence of floodwater occurrence,so it can be regarded as preferable index which represents the change of flood and precipitation.The grain size of flood deposition in Shiyang River can reflect floodwater's transit force during sediment period.Thick granularity component denotes strong floodwater's transit force,vice versa.2.The thickness of WW and WWC profiles is 2.40m and 2.66m respectively,which was formed by the latter-day flood deposit,22 sediment phases indicating 22 flood depositions differently and also indicating 22 flood depositions stages,including the cataclysm of 1994 which the topside indicated.3.There is obvious variety rhythm of granularity along with the depth in the WW, WWC profiles.After eliminating the influence of the landform height caused by the deposition thickness on granulometric composition,the order of flood depth and the flood scale of these 22 flood phases of WWC profile was:20^th＞4^th＞3^rd＞5^th＞7^th＞2^nd＞8^th＞15^th＞6^th＞10^th＞16^th＞22^nd＞9^th＞11^th＞17^th＞1^st＞13^th＞19^th＞21^st＞12^th＞14^th＞18^th.The 20^th,4^th,3^rd,5^th,7^th,2^nd,8^th,15^th,6^th,10^th,16^th,22^nd,9^th,11^th and 17^th were rather big flood phases comparatively,and the 13^th,19^th,21^st,12^th,14^th and 18^th were the usual flood phases.So it does in the WW profile. 4.CaCO₃ in the sediment profile mostly comes from scrappy calcite which is conveyed by river.To some extent,the low content reflects coarse granularity and denotes strong flood transit force when the sediment layer forms and more rainfall that year,vice versa.5.Flood waterlogging disaster doesn't occur in Wuwei area during 1701～1737, 1781～1800,1857～1883 and 1907～1926.There are more flood record during 1738～1780,1818～1850 and 1927～1949.Flood occurrence frequency increases during 1950～1990,which is related with the more precipitation.Flood waterlogging disaster mostly occurs from May to September,especially in June,July and August,which is the rainy season in that area.And it is consistent with the month with the highest daily precipitation.6.The main cause of the floodwater is usually due to the rainstorm or concentrated downfall.The thickness of sediment layers in WW and WWC profiles mainly reflects the precipitation change that year and it can't reflect the climate directly.7.Shiyang River valley is the area with the outstanding inconsistency between supply and need in Hexi Corridor.Along with the development of social economy and increaser population,water demand will increase unceasingly and water resource's standing using will be faced with austere challenge.Among all the water using styles, agriculture account for 85.57%,so it is the important approach of water resource's standing use and development in Shiyang River valley to build network conveying water piping in all drainage area and develop high-tech agriculture and advanced saving water irrigation technology.