Study On The Ice Regime And The Prediction Model Of Ice-conditions Information In River Reach Upstream Of Wanjiazhai Reservoir Since Its Completion In 1998

Before Wanjiazhai Reservoir was built,96km from upstream of dam was sTable.freeze-up reach,the lower reach of it was not freeze.Only border ice and frazil slush in river and the whole winter was ice run based.There was no ice jam or ice dam happened during freeze-up period and break-up period,so there was no ice flood disaster.After Wanjiazhai Reservoir was built,ice regime characteristics had changed a lot with the river natural form.Since 1998 when the reservoir started to operate,water surface slope and backwater velocity and ice transport ability was decreased.A large number of frazil slush stranded at the reservoir area and led freeze-up happened from reservoir area to upper reach.Ice flood disaster was serious during freeze-up period and break-up period.This paper first have examined and compared the ice regime in natural river condition and the new ice regime under operation of the reservoir,using the field measurements during 1987-2016,and analyzed ice regime factors:river morphology,meteorological condition,reservoir operation and hydraulic factors.Then built support vector machine model(SVR),BP neural network and partial least squares regression models(PLS)to predict ice-run date,freeze-up date and break-up date in river reach upstream of Wanjiazhai.Finally used negative accumulated temperature method and unified degree day method to simulate ice thickness in the river reach upstream of Wanjiazhai.The results show that in the reach upstream of the reservoir,the ice regime was featured with a freeze-up process advancing downstream to upstream and a freeze-up type either juxtaposed,packed,or mixed in different section along the river.Ice break-up occurred in the packed freeze-up section and its upstream later than in its upstream.In the upstream section it started at the downstream and was advancing upstream.The whole reach took forms of thermal,mechanical,and semi-violent break-up.Since 1998 when the reservoir started to operate,the ice regime in its upstream river has changed greatly due to changes in riverbed form,air temperature,reservoir control,and hydraulic factors.Major changes were observed in three aspects:ice-run date and freeze-up date were delayed while break-up date was shifted earlier,seldom-freeze-up reaches were turned to sTable.freeze-up with longer freeze-up river sections,in freezing period,water level was rising and the occurrences of ice jam and ice dam were increased.Forecasting results of ice-run date,freeze-up date and break-up date show that three models are high in precision and the results are reasonable.The forecast result by PLS is best,and the forecasting projects attain first level.SVR comes second and BP is last.Ice simulated by unified degree day method was better than by negative accumulated temperature method.Unified degree day method can be better used in river reach upstream of Wanjiazhai ice thickness simulation and can provide a basis for ice flood prevention.