Evaluation of Sediment Diversion Design Attributes and Their Impact on the Capture Efficiency

Abstract

Many riverine systems have been disconnected from their receiving basins by flood-protection levees and other engineered systems. Reconnecting these alluvial rivers with their receiving basins is a viable option to nourish and sustain existing coastal wetland systems as well as to build new land. This sediment nourishment can be accomplished through direct dredging and placement or through sediment diversions. Efficient design of sediment diversions is important to maximize the land building potential. This study’s objective is to quantitatively identify key design attributes of sediment diversions, influencing their ability to capture sediment. The outfall channel alignment angle (ϕ), intake invert elevation, and diversion size are hypothesized as key parameters. The analysis is limited to sediment grain sizes larger than 63 μm and has been performed using a validated three-dimensional numerical model. A time integrated sediment-water ratio was used as an indicator to reflect efficiency of sediment capture. Analyses indicate ϕ has minor impact on total diverted sediment load and limited to the medium sand (M) fractions (250–500 μm) compared to the fine (125–250 μm) and very fine (63–125 μm) size classes. The sediment water ratio increases as intake invert elevation is deepened to a certain limit, then it plateaus around yInv/yBar∼0.75, implying that deeper invert beyond that limit may not be beneficial. The analysis also shows that the sediment water ratio sharply increases up to a water discharge extraction ratio of 0.1 as the size of the diversion is increased, and plateaus around 0.2.