| With the significantly decreasing of runoff and sediment in the Yellow River and the ecological conservation and high-quality development of the Yellow River Basin,it is of great important to clarify the variation trend of sediment load and its causes in the Yellow River.Check dams are the main engineering measures of soil and water conservation in the reach from Hekou-Tongguan of the Yellow River.And they play a vital role in trapping sediment,reducing soil erosion,regulating flood peaks and dissipating erosion energy.The present research selected small watershed,mesoscale basin and Hekou-Tongguan section of the Yellow River as research subjects.And statistical analysis,soil erosion prediction model and geographical spatial analysis were included in this research.The effects of sediment retention and erosion reduction of check dams were studied in small watershed scale.The calculation model of annual sediment reduction in the check dams were constructed based on mesoscale basin,and analyzed the influence of spatiotemporal distribution of check dams on sediment transport.The contribution rate of check dams to the reduction of sediment transport in the reach of Hekou-Tongguan section of the Yellow River were evaluated in the large scale watershed.Finally,the present research proposed the construction and management measures of check dams in the reach of Hekou-Tongguan section of the Yellow River.The main conclusions obtained were as follows:(1)The regulation effects of check dams on the process of runoff and sediment were analyzed in small watershed.Through comparing and analyzing the characteristics of flood events in the governance river basin and natural river basin,we found that the construction of check dams changed the relationship between runoff and sediment in the basin.Under the same runoff depth,the sediment delivery modulus of the governance river basin was smaller than that of the natural river basin.Check dams had a strong reducing effect on runoff erosion of the rainfall events with low total rainfalls.The "7.26" heavy rainstorm in the Wuding River Basin indicated that check dams played an important role in trapping sediment under rainstorm conditions.Check dams in the Jiuyuangou basin intercepted 71.43×104 t sediment,and the sediment transport ratio in this basin was reduced to 0.16.Check dams changed the model of sediment yield and transport,and significantly reduce the sediment transport ratio in the watershed.(2)The calculation model of the annual sediment reduction in check dams was established.Based on the soil erosion prediction model,the weight coefficients was calculated to distribute the total sediment amount to each year in mesoscale of Dali River basin.And the amounts of trapped sediment in medium and small check dams were determined based on the coefficient of sedimentation effect.According to the calculation model of annual amount of sediment reduction in check dams,the sediment reduction characteristics of check dams in Dali River Basin were analyzed.From 1954 to 2011,the annual sediment reduction amount of the check dams in Dali River Basin increased in a fluctuating tendency.The average annual sediment reduction amount of the check dams was 0.12×108t.The average annual sediment reduction amount in key dams and small and medium-sized check dams were 0.04×108 t and 0.08×108 t,respectively.The contribution rate of sediment reduction of check dams in Dali River basin decreased from 47.35%in 1971-2001 to 31.19%in 2002-2011.(3)The contribution rate of sediment reduction in check dams in the reach of Hekou-Tongguan section of the Yellow River was evaluated.From 1952 to 2011,the annual sediment reduction amount of the check dams in the Hekou-Tongguan section of the Yellow River increased in a fluctuating tendency.The average annual sediment reduction of check dams was 1.50×108 t,and the cumulative amount of sediment was 68.63×108 t.The contribution rate of sediment reduction of check dams in the Hekou-Tongguan section of the Yellow River decreased from 30.03%in 1979-1998 to 19.92%in 1999-2011.In the Hekou-Tongguan section of the Yellow River,the contribution rates of sediment reduction of the check dams showed a decreasing trend in most of the basins,and the key dams in these basins were mainly constructed from 1970 to 1979.The contribution rate of sediment reduction of check dams showed an increasing trend in a small part of basins,and those key dams were mainly constructed from 2000 to 2009.(4)The sediment reduction capacity of key dams in the Hekou-Tongguan section of the Yellow River were predicted,and evaluated the construction potential of check dams.And proposed some measures for the construction and management of check dams.When the capacity of the deposition of the key dams reached to 70%?80%,the efficiency of trapping sediment would be decreased.Based on the 80%of the total deposition capacity to calculation,by the year of 2011,the number of 2,466 out of the total number 4,847 key dams in the Hekou-Tongguan section of the Yellow River had reduced their sediment reduction efficiency.It is predicted that 53.08%of the key dams in Hekou to Tongguan section would be fully deposited in 2030 and this percentage would be increased to 77.49%in 2040.The construction potential of the number of key dams in the Hekou-Tongguan section of the Yellow River was 13813,and the average construction intensity of check dams was 35%.The basins with high intensity of check dams were Xinshui River,Xianchuan River,Jialu River,Zhujiachuan River,Hun River,Kuye River and Wuding River.Finally,some measures for the construction of key dams in 11 main basins of the Hekou-Tongguan section were proposed.Except for choosing the appropriate time to promote the construction of check dam in the Hekou to Tongguan section of the Yellow River,the danger removal and reinforcement projects of check dams should also be considered. |
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