Applying flow convergence routing to control sediment erosion and deposition locations in a dam's backwater zone

Despite studies showing that dams have significant effects on the sediment dynamics and evolution of a river upstream of a dam, the knowledge of relationships between river topography and sediment transport in a dam's backwater zone has hardly been applied in reservoir sedimentation management. This study investigated the potential of an alternating sequence of engineered topographic nozzles and oversized landforms, utilizing flow convergence routing theory, to redistribute sediment erosion foci in a dam's backwater zone for remote mountain reservoirs with a sediment storage capacity of ~105 m3. To test scientific ideas and engineering alternatives, the current topography of the backwater zone upstream from Our House Dam on the confined, mountainous Middle Yuba River, California, was virtually re-contoured into different scenarios for numerical experimentation. As most of the dam's backwater zone is filled with sediment (a common global problem) in a narrow, confined canyon, two-dimensional hydrodynamic modeling was useful for evaluating erosion patterns resulting from different manipulations. The results found that high velocity concentrates through nozzles and dissipates through oversized landforms, resulting in the latter exhibiting hydraulics indicative of functioning as sediment settling basins. These basins can be located away from the dam where key infrastructure needs clearance from sedimentation. As flow increases through the sequence of nozzles and oversized landforms, each nozzle's hydraulic jet will persist farther into the oversized area. Moderate in-channel flow (daily recurrence of ~5–30 %) was best for creating conditions to force deposition of sediment in oversized landforms. At high enough discharge (recurrence of <1–5 %) significant sediment erosion can occur throughout the constructed terrain in the backwater zone, so the whole topographic scheme can become overwhelmed and ineffective. Thus, the performance of re-contouring as an aid in reservoir sedimentation management is location and flow-dependent, necessitating careful design refinement for local conditions and assessment of financial benefits and costs. Overall, this study opens a new realm of sediment management for dam operators and regulators hard-pressed to know what to do when a moderate-sized mountain reservoir with poor accessibility has a sediment storage capacity of ~105 m3