Subcritical flow in a short expansion tends to become detached from its sidewalls and create turbulent eddies. This phenomenon can cause undesirable losses of flow energy and channel erosion. This paper numerically investigates the effects of a triangular hump fitted at the bottom of the expansion on flow separation reduction. The results are validated using available experimental data. This paper reports quantitative details of the velocity field, eddy structures, and flow streamline separation in expansions of different divergence angles under various conditions of the Froude number. The results show that the flow of water can separate along either of the two sidewalls, possibly due to bistability. The use of appropriate humps can effectively reduce separation and turbulent eddies because they force the flow to accelerate and hence create a favorable pressure gradient. The results from this paper are useful for the optimal design of expansions as an important part of irrigation and water systems.