According to variations of 137Cs and clay contents, 44 flood couplets were identified in a profile of res- ervoir deposit with a vertical length of 28.12 m in the Yuntaishan Gully. Couplet 27 at the middle of the profile had the highest average 137Cs content of 12.65 Bq·kg?1, which indicated the 1963s' deposits, then 137Cs content decreased both downward and upward in the profile. The second top and bottom couplets had average 137Cs contents of 2.15 Bq·kg?1 and 0.92 Bq·kg?1, respectively. By integrated analysis of reservoir construction and management history, variations of 137Cs contents over the profile, sediment yields of flood couplets and rainfall data during the period of 1958-1970, individual storms related to the flood couplets were identified. 44 floods with a total sediment yield of 2.36×104 m3 occurred and flood events in a year varied between 1 and 10 times during the period of 1960-1970. 7-10 flood events occurred during the wet period of 1961-1964 with very wet autumn, while only 1-2 events during the dry period of 1965-1969. Average annual specific sediment yield was 1.29×104 t·km?2·a?1 for the Yuntaishan Gully during the period of 1960-1970, which was slightly higher than 1.11 ×104 t·km?2·a?1 for the Upper Yanhe River Basin above the Ganguyi Hydrological Station and slightly lower than 1.40 ×104 t·km?2·a?1 for the nearby Zhifang Gully during the same period. Annual specific sediment yields for the Yuntaishan Gully were correlated to the wet season's rainfalls well.
There are many sediment trapping reservoirs in the Hill Loess Plateau.The Huangtuwa small catchment is selected as a research field where samples were collected in a sediment deposit profile with a vertical length of 12.73 m.By the variation of fine particle content and pollen concentration,54 flood couples were identified.Taking the freeze-thawing disturbed texture as the sign of the latest flood couple in a year,it is ascertained that those 54 floods should happen in 31 years.Using pyramid volume calculation formula,the sediment yields of flood ranged between 716―30376 t·km-2.The average sediment yield was 7106 t·km-2.The annual sediment yields varied between 968―55579 t·(km2·a) -1,and the average value is 12629 t·(km2·a) -1.Compared with the modern annual sediment discharge of the Huaining River,the erosion rate in the Huangtuwa region 450 years ago was similar to the modern erosion rate.In the period from the 27th year to the 31st year,the average annual sediment yield increased sharply to 31309 t·(km2·a) -1.It implied that re-reclamation of the abandoned land in the catchment since the landslide disaster,where vegetation had rehabilitated already,caused very severe soil erosion in the catchment.
