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<p>High-resolution flood maps are needed for more effective flood risk assessment and management. Producing these directly with hydrodynamic models is slow and computationally prohibitive at large scales. Here we demonstrate a new algorithm for post-processing low-resolution inundation layers by using high-resolution terrain models to disaggregate or downscale. The new algorithm is roughly 8 times faster than state-of-the-art algorithms and shows a slight improvement in accuracy when evaluated against observations of a recent flood using standard performance metrics. Qualitatively, the algorithm generates more physically coherent flood maps in some hydraulically challenging regions compared to the state of the art. The algorithm developed here is open source and can be applied in conjunction with a low-resolution hydrodynamic model and a high-resolution DEM to rapidly produce high-resolution inundation maps. For example, in our case study with a river reach of <span class="inline-formula">20</span> km, the proposed algorithm generated a <span class="inline-formula">4</span> m resolution inundation map from <span class="inline-formula">32</span> m hydrodynamic model outputs in <span class="inline-formula">33</span> s compared to a <span class="inline-formula">4</span> m hydrodynamic model runtime of <span class="inline-formula">34</span> min. This 60-fold improvement in runtime is associated with a 25 % increase in RMSE when compared against the <span class="inline-formula">4</span> m hydrodynamic model results and observations of a recent flood. Substituting downscaling into flood risk model chains for high-resolution modelling has the potential to drastically improve the efficiency of inundation map production and increase the lead time of impact-based forecasts, helping more at-risk communities prepare for and mitigate flood damages.</p>