CONTROLS OF HYDROGRAPH SHAPE ON FLOODPLAIN SEDIMENTATION DURING MAJOR FLOODS

Paleoflood analyses associate intervals of floodplain deposition with a specific discharge, operating on the assumption that deposition depends alone on the flood peak. However, this neglects the importance of other flood characteristics—such as time-to-peak and drawdown time—on the amount of sedimentation. Recent research indicates that hydrograph shape controls entrainment of sediment fractionally, so it follows that aspects of the hydrograph would also affect floodplain sedimentation, especially considering the time integral of overbank flow. This paper investigates these relationships using a 2D hydrodynamic and suspended sediment transport model (Telemac) applied to a 70 km reach of the Feather-Yuba Rivers, CA where recent large floods have remobilized sediment produced in 19th c. mining. Total sediment influx per unit channel width (m³/m), and deposition (m³) resulting from nine days of a rapid-rising, and receding flood (1986) were compared to 18 days of an event exhibiting a steady rise and decline in discharge (1997). Differences between hydrograph characteristics reflect antecedent conditions, reservoir operations, and storm centers. Upstream boundary conditions were defined by hourly discharge measurements and suspended sediment concentrations derived from rating curves. Sediment (d₅₀ = 0.03 mm) was transported to, and deposited onto, the floodplain over both the rising and falling limbs of each flood. With only 27 hours to rise, 84% of the total modeled 1986 deposition occurred after the discharge peak. In contrast, 52% of the total 1997 modeled deposition preceded the discharge peak by 10 hours. Although the sediment influx was greater in 1997 compared to 1986, three orders of magnitude less sedimentation resulted (0.0017 x 10⁸ m³ and 5 x 10⁸ m³ respectively). Rapid floodplain filling early in the 1986 flood increased resistance, limiting further incursion, lowering flow velocities, and increasing inundation depths 0.5 m to 11.5 m over the 1997 flood. Deposition was encouraged by the prolonged inundation period early in the flood when suspended sediment concentrations were high. Results suggest that sedimentation hysteresis patterns, as well as the sediment supply and flood peak are relevant to understanding overbank accretion processes during large floods.