41-3 Geomorphic Effects of Major Floods – Integrating Ancient, Modern and Experimental Data

Session: Philosophy of Extreme Events and Landscape Evolution on Earth and Other Planets: Thinking Geologically in the Spirit of Victor Baker

Presenting Author:

Piret Plink-Bjorklund

Authors:

Plink-Bjorklund, Piret¹, Hansford, Mark², Li, Haipeng³, Ono, Kenya⁴, Castillo-lopez, Amanda⁵, Lenssen, Nathan⁶, Hutson, Arielle⁷

Abstract:

River floods are among Earth’s most common and most destructive natural hazards and have a high impact on society. However, the effect of major floods in shaping the landscapes, and formation of the sedimentary record is much less clear. The moderate (ordinary) events rather than the major floods are commonly thought to shape the rivers and build the sedimentary record, because of the following assumptions: (1) The high frequency moderate events are more common and thus do more geomorphic work; and (2) River feedback loops are negative, as the rare high-magnitude event effects are reworked and channel restored to frequent moderate-event conditions. This results then  in river sedimentary record that is “normal” , which means formed by Froude subcritical flow with bedload transport of sand, and equilibrium conditions for downstream bedform migration that result in ripple, dune and bar scale cross strata. Here we, however, show that reviewing the sedimentary record indicates many river deposits dominated by preserved high-magnitude flood event beds rather than equilibrium bedform strata. Experiments show that these strata were formed by Froude supercritical flow that is efficiently suspends and transports sand and gravel in suspension (in experiments) and causes local rapid erosion and high deposition rates.

Based on a synthesis of ancient and modern river deposits, modern river discharge records and experimental data, we propose that there are fundamental differences in magnitude-frequency relationships and relaxation times in rivers with distinct hydrology, such that in some rivers channel recovery may be virtually non-existent and larger floods may leave permanent and formative imprints on landscape. Only if the ratio of the mean relaxation time (normal conditions) to the mean recurrence interval of extreme channel disturbing events is <1, and the critical shear stress for sediment motion is exceeded during moderate (normal) conditions can a river recover from extreme flood-induced change before the next major disturbance occurs. This concept helps to explain the observed variability in the sedimentary record of rivers, as well as critical differences in river flood hazards.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9723

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