Modern depositional processes in a confined, flood-prone setting: Benches on the Shoalhaven River, NSW, Australia

- We use chrono-stratigraphic analysis and hydraulic modelling to examine benches.
- Depositional processes are determined to be complex and multi-faceted.
- Drape deposits are identified over multiple elevation surfaces.
- Suggestions of bench formation by discrete recurrence intervals are not upheld.
- Channel dimensions and hydraulics enable construction of multiple-elevation benches.

Past research has suggested that in-channel alluvial landforms, often termed benches, are associated with flow events with specific return periods and that such landforms are inset within terraces, floodplains, or higher bench surfaces. Chronostratigraphic analysis and hydraulic modelling in a bedrock-confined reach of the lower Shoalhaven River show that they have been extensively eroded and at least partially reformed during the historical period. Lower benches are inundated by flood events with average recurrence intervals (ARI) of 2 or less years, middle benches by events ca. 5 years ARI, and upper benches by events ca. 10 years ARI. The lower benches commonly share over- or onlapping stratigraphic units, demonstrating that deposition can occur on multiple bench surfaces simultaneously. This is in contrast to earlier suggestions of discrete bench surfaces being associated with, and formed by, events that have a specific return period. In the Shoalhaven River, Pleistocene and Holocene terraces determine the large alluvial channel geometry for a wide range of discharges up to the 50-year return interval with benches forming temporary sediment storages within this larger channel. The large channel dimensions and associated hydraulics and high annual flood variability (flash flood magnitude index of 0.53) for the Shoalhaven River facilitate the construction of multiple bench surfaces across a range of elevations. Benches are formed of a wide range of facies from decimetre-thick massive sand units through interbedded sands and silts.