A subgrid channel model for simulating river hydraulics and floodplain inundation over large and data sparse areas

TLDR

The study introduces a computationally efficient hydraulic model that simulates water surface elevation, wave speed, and inundation over large, data‑sparse domains. The model extends LISFLOOD‑FP with a subgrid‑scale representation of channelized flows, enabling simulation of channels narrower than the grid, and was evaluated against four model structures (2‑D, 1‑D, 1‑D/2‑D, and the subgrid approach) on an 800 km stretch of the Niger River. The subgrid model proved numerically stable and scalable, and its inclusion of both channel networks and floodplain was essential for accurate simulation of the Niger River delta, improving water‑level, wave‑speed, and inundation predictions and yielding consistent calibration against gauge and ICESat data.

Abstract

This paper presents a new computationally efficient hydraulic model for simulating the spatially distributed dynamics of water surface elevation, wave speed, and inundation extent over large data sparse domains. The numerical scheme is based on an extension of the hydraulic model LISFLOOD‐FP to include a subgrid‐scale representation of channelized flows, which allows river channels with any width below that of the grid resolution to be simulated. The scheme is shown to be numerically stable and scalable, before being applied to an 800 km reach of the river Niger in Mali. The Niger application focused on the performance of four different model structures: a model without channels (two‐dimensional (2‐D) model), a model without a floodplain (one‐dimensional (1‐D) model), a model of the main channels and floodplain (1‐D/2‐D model), and the subgrid approach developed here. Inclusion of both the channel network and the floodplain was shown to be essential, meaning that large scale models of this region, including routing models for land surface schemes, will require a floodplain component. Including subgrid‐scale channels on the floodplain changed inundation dynamics over the delta significantly and increased simulation accuracy in terms of water level, wave propagation speed, and inundation extent. Furthermore, only the subgrid model showed a consistent parameterization when calibrated against either gauge or ICESat water level data, suggesting that connectivity provided by small channels is a strong control on the hydraulics of the floodplain, or, at the very least, that low resolution gridded hydraulic models require additional connectivity to represent the delta flow dynamics.