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Transient flow models are expected to represent rapid flow changes, whereby acceleration and deceleration significantly influence energy dissipation. Such effects on energy dissipation can be expressed in terms of unsteady friction (UF) losses, which is a well-established process for closed-pipe flow models, but not in the context of mixed-flow models. Mixed flow refers to flow conditions where pressurized and free-surface flow regimes coexist or transition between each other within the same system. Many water systems experience significant flow acceleration and deceleration while in mixed-flow conditions, but current models have only the ability to account for these effects through steady roughness terms. This work builds from existing modeling approaches to adapt mixed-flow models based on the Saint-Venant equations that incorporate unsteady friction losses. The approach used to incorporate unsteady friction losses is a modification of a well-established formula based on local acceleration and spatial velocity gradient. The proposed numerical model, referred to as SVUF, is compared with three experimental data sets, and the results were improved, particularly for longer-duration flow simulations.