The morphology of coastal and fluvial flood-driven sediment deposits in natural and built environments

Abstract

Extreme flood events are essential for landscape creation, maintenance, and development through the unconsolidated sediment they mobilise and deposit – driving large-scale morphological change. However, hazard-prone coastal and fluvial floodplains offer socio-economically favourable conditions leading to the ubiquity of human presence. When floods extend into these built environments, they can cause widespread damage to infrastructure and pose an immediate threat to human life. Disaster damage costs include expenses associated with removing debris, including vast amounts of sediment, to allow access to stricken communities and permit an appropriate emergency response. To allow suitable disaster management plans to be implemented, there needs to be an increased understanding of sediment routing and deposition in built environments.
Therefore, this research aims to quantitatively investigate flood-driven sediment deposition on natural and built coastal and fluvial floodplains to gain physical insight into their comparative morphometry and explore how such understanding of landscape dynamics may inform risk assessment and management. It achieves this by 1) constructing morphometric scaling relationships for real-world coastal deposits, in natural and built settings, after hurricane strikes in the US, 2) using CAESAR-Lisflood to assess natural controls on fluvial deposits, 3) investigating the impact of built environments on fluvial deposits in CAESAR-Lisflood, 4) synthesising this data, combined with examples from external literature.
Storm size, grain size, and floodplain roughness impart a first-order control on deposit morphology. However, the built environment significantly impacts deposit shape and size, pertinent as many morphological landscapes depend on sediment delivery. Importantly, volume and area scale consistently regardless of the density of the built environment. The ability to derive the 3D properties from 2D metrics could help predict disaster impacts and facilitate the efficient clean-up of debris post-flood. Nevertheless, the scaling relationships depict a snapshot of deposit morphometrics, providing information concerning their morphology at one point in time, rather than the underlying dynamics of how they are formed. Consequently, future work could investigate the impact of built obstacles on flow dynamics by tracking the disturbance imparted by built environments on flow velocities and directions – and the impacts on sediment deposition. Finally, trusting models to accurately predict real-world conditions is critical. Here, CAESAR-Lisflood is quantitatively validated in its creation of deposits. Technological advances will only enable model representations of morphological systems to become more accurate and detailed – with the capability to aid policymakers and engineers in disaster impact mitigation.

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ORCID for Eli Lazarus: orcid.org/0000-0003-2404-9661

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