Assessing the Role of Hydrodynamics in Enhancing Height‐Above‐the‐Nearest‐Drainage Derived Synthetic Rating Curves: A Comparative Study in the Wu River Basin, Taiwan

ABSTRACT The conventional approach to generating synthetic rating curves (SRC) using the Height‐Above‐the‐Nearest‐Drainage (HAND) method typically relies on the assumption of uniform flow, such as Manning's equation, to establish stage‐discharge ratings. The zero‐physics application of the uniform flow equation is insufficient for capturing detailed hydraulic features (e.g., backwater effect) and neglects the hydraulic effects from adjacent channels. This lack of hydrodynamic computation can impact the accuracy and effectiveness of riverine flood risk estimation and management. To reduce this foreseeable error, we introduce the HAND‐hd workflow, which integrates sophisticated hydrodynamic computations in the production of HAND‐based SRC with hydrodynamic features (SRChd). The results indicate that SRChd demonstrates consistent agreement with both gauge observations and benchmark solutions. Additionally, the comparative analysis suggests that SRChd provides notable improvements in stage‐discharge ratings over conventional HAND‐based SRCs, particularly in channels with mild bed gradients, where it reduces water stage prediction errors and percent biases. In steeper channel segments, SRChd maintains comparable accuracy to conventional methods. The comprehensive evaluation in this study emphasizes the potential discrepancies and inaccuracies associated with the adoption of the uniform flow assumption in the conventional HAND‐SRCs and addresses the necessity of including hydrodynamic physics in the application of HAND‐based SRC (e.g., inundation map) in channels with mild gradients.