Calculation of the separation grid design length in a new water–sediment separation structure for debris flow defense

A new water–sediment separation structure with a herringbone separation grid has been developed for debris flow defense. Previous model experiments showed that, compared to existing structures, this structure can continuously maintain its water–sediment separation function. However, in the structure design, the length of the separation grid is key to its success in separating water and sediment. This paper presents a theoretical formula for calculating the design length of the grid. The theoretical formula shows that the grid length relates to the debris flow velocity v x , the grid width B , and the grid incline angle θ . A series of model experiments were conducted in the laboratory to test the accuracy of the formula. The results show that the experimental value and the theoretical value for grid length form a linear relationship and the design length of the grid may be corrected by a coefficient. Further analysis indicates that the correction coefficient changes with the bulk density of debris flow. Finally, a formula for determining the grid design length is derived from the theoretical formula, corrected using a coefficient related to the bulk density of a debris flow.