Probing temporal variation of suspended load to bedload ratio using seismic saltation model

Monitoring temporal and spatial changes in sediment volume in the upstream reservoir is one of the important indicators for evaluating the reservoir project life, especially the information carried by bedload and suspended load. According to field condition, direct bedload monitoring is often difficult. Thus, bedload usually can be estimated by a specific proportion of suspended load depended on the flooding magnitude, which can cause a large uncertainty in estimates of total sediment load. In recent years, riverine micro-seismic signals have been applied to study bedload transport. Our study chose the Da-Pu Dam (location: 23.296500°N, 120.644611°E), located at the upstream of the Zeng-Wen Reservoir and the junction of the Zeng-Wen river and Cao-Lan river, which is the last check dam before entering the reservoir area. Its upstream catchment area is 30,312 hectares that comprise approximately 63% of the Zeng-Wen Reservoir catchment area (48,100 hectares). The length of the monitoring section of the Da Pu Dam is 1,100 meters, with an average width of 121 meters and an average slope of 0.36 degrees. With the available data composed of riverbed cross-section survey, sediment particle size distribution, fluvial measurements (water depth, surface flow velocity), orthoimagery, and suspended load measurement, our study applies seismic saltation model to estimate the bedload flux and compares the results with the measured suspended load. Results showed that there are different ratios between bedload and suspended load under similar hydrological condition during the plum rain season(May-June) and typhoon period(July-September). In a case of flooding event considering the flow stage from medium to high discharge, significant temporal changes in the ratio between bedload and suspended load can also be observed, which imply a complex transition process between the bedload and suspension particles. The temporal changes in sediment ratio obtained in this study can be applied to estimate the total volume of sediment load entering the reservoir. Our estimated results are consistent with the survey of sediment accumulation at the end of each year in the reservoir area.