Debris flow sediment control using multiple herringbone water-sediment separation structures

Single herringbone water-sediment separation structures (HWSS) have limited sediment control effectiveness in debris flows. A series of such structures in a debris flow channel to form a multiple structure system (M-HWSS system) should be more effective in debris flow mitigation. Hydraulic model tests reveal that a M-HWSS system does perform better in coarse sediment separation and has better stability in differing debris flow situations. The mean particle size of discharged sediment is gradually and significantly decreased down channel by M-HWSS system. The separated sediments are moderately sorted and this can be improved by optimizing the structure design parameters and increasing structure numbers. The fraction separation ratio (λi), coarse separation ratio (λc) and total sediment separation rate (Pt) are suggested parameters to express the sediment control effectiveness. All are closely related to the herringbone opening width and the input sediment grain size distribution. The quantitative relationships among them are proposed. On the basis of the tests, conclusions and guidelines for effective M-HWSS design include: (1) three structures in the M-HWSS located in succession upstream, midstream and downstream, each with substantially different in sediment control functions, (2) a structure's performance is strongly influenced by that of the preceding one so that every structure is designed to fully implement the sediment control function, especially for those in the upstream and midstream, (3)the suggested herringbone opening width in a structure should be set at the percentile of d50 ~ d84 of the input sediment grain size distribution so that 20 ~ 60 % of the effective separation rate can be achieved.