Flood risk control of dams and dykes in middle reach of Huaihe River

Three stochastic mathematical models for calculation of the reservoir flood regulation process, river course flood release, and flood risk rate under flood control were established based on the theory of stochastic differential equations and features of flood control systems in the middle reach of the Huaihe River from Xixian to the Bengbu floodgate, comprehensively considering uncertain factors of hydrology, hydraulics, and engineering control. They were used to calculate the flood risk rate with flood regulation of five key reservoirs, including the Meishan, Xianghongdian, Nianyushan, Mozitan, and Foziling reservoirs in the middle reach of the Huaihe River under different flood frequencies, the flood risk rate with river course flood release under design and check floods for the trunk of the Huaihe River in conjunction with relevant flood storage areas, and the flood risk rate with operation of the Linhuaigang Project under design and check floods. The calculated results show that （l） the five reservoirs can withstand design floods, but the Xianghongdian and Foziling reservoirs will suffer overtopping accidents under check floods; （2） considering the service of flood storage areas under the design flood conditions of the Huaihe River, the mean flood risk rate with flood regulation of dykes and dams from Xixian to the Bengbu floodgate is about 0.2, and the trunk of the Huaihe River can generally withstand design floods; and （3） under a check flood with the flood return period of 1 000 years, the risk rate of overtopping accidents of the Linhuaigang Project is not larger than 0.15, indicating that it has a high flood regulation capacity. Through regulation and application of the flood control system of the Linhuigang Project, the Huaihe River Basin can withstand large floods, and the safety of the protected area can be ensured.

Real-time flood forecasting of Huai River with flood diversion and retarding areas

A combination of the rainfall-runoff module of the Xin’anjiang model, the Muskingum routing method, the water stage simulating hydrologic method, the diffusion wave nonlinear water stage method, and the real-time error correction method is applied to the real-time flood forecasting and regulation of the Huai River with flood diversion and retarding areas. The Xin’anjiang model is used to forecast the flood discharge hydrograph of the upstream and tributary. The flood routing of the main channel and flood diversion areas is based on the Muskingum method. The water stage of the downstream boundary condition is calculated with the water stage simulating hydrologic method and the water stages of each cross section are calculated from downstream to upstream with the diffusion wave nonlinear water stage method. The input flood discharge hydrograph from the main channel to the flood diversion area is estimated with the fixed split ratio of the main channel discharge. The flood flow inside the flood retarding area is calculated as a reservoir with the water balance method. The faded-memory forgetting factor least square of error series is used as the real-time error correction method for forecasting discharge and water stage. As an example, the combined models were applied to flood forecasting and regulation of the upper reaches of the Huai River above Lutaizi during the 2007 flood season. The forecast achieves a high accuracy and the results show that the combined models provide a scientific way of flood forecasting and regulation for a complex watershed with flood diversion and retarding areas.

Assessment of climate change impact on high flows in a watershed characterized by flood regulating reservoirs

A climate-induced extreme flow event such as flooding is one of the most devastating natural hazards,which can significantly damage human lives and properties.This study examined the effects of climate change on the high flow conditions in the Great Miami River Watershed in Ohio under two emission scenarios(RCP 4.5 and RCP 8.5).Streamflow for the 21st century was simulated by utilizing a watershed model-SWAT(Soil and Water Assessment Tool)and 10 different climate outputs from the Coupled Model Intercomparison Project phase 5(CMIP5).The future streamflow was divided into three equal periods:2016-2043(early century),2044-2071(mid-century),and 2072-2099(late century)and independently analyzed to compare high flows of respective intervals with baseline periods(1988-2015).The analysis predicted that 7-day,10-year high-flow(7Q10)would increase by 38%under RCP 4.5 and 44%under RCP 8.5.Similarly,the annual peak flows for study periods were predicted to increase by 26%under RCP 4.5 and 38%under RCP 8.5 from the base period.However,the analysis demonstrated an erratic response for monthly peaks indicating that the peak flow would increase in summer months-May and July to October.Meanwhile,the result did not show any significant increase during the winter season,especially from November to April.The analysis of the four major dams located in the watershed showed that the dam’s peak discharges increase in January,May,and September.Even though increasing peaks were projected in September for the 21st century,the monthly peaks from the watershed outlet were found to be lowest in September as compared to other months.The frequency of future flooding compared to the historical record was found to be increasing in the mid-century under RCP 4.5 and the late century under RCP 8.5.As the future flood is projected to increase,this study finds the reasonable impact of climate change on flood regulating reservoirs/dams in monthly flows.However,daily high flows(90th percentile flow)would be increasing significantly(44%to 250%)under RCP 8.5.