Quantification of the sinking conditions of large wood and maximum backwater rise at trash racks

The accumulation of large wood (LW) is a significant concern for river management. The sinking behavior of LW plays a crucial role in altering the shape and submerged volume of LW accumulation, which in turn changes the flow condition and poses a high risk to hydraulic structures. Through laboratory model tests, this work innovatively introduces the critical Froude number as an indicator for the initiation of sinking in a large wood row (LWR) with uniform density floating on the water surface. Factors such as increased LWR length, a higher diameter-to-flow depth ratio, and reduced LW density all lead to a higher critical Froude number. Based on these findings, a formula is proposed to estimate the critical Froude number, which is parameterized by the diameter, density, and length of LWR. Additionally, the sinking of LW simultaneously increases backwater rise and decreases incoming flow velocity, eventually halting further sinking, and giving rise to the maximum backwater rise under the condition of certain discharge. By modeling this special state, a prediction formula for maximum backwater rise is derived and analyzed. Overall, this study enhances the understanding of LW sinking dynamics and provides deep insights into LW management in rivers.