River mouths along sandy coastlines are influenced by alongshore transport of littoral sands. Here, we study the mechanisms of alongshore sediment bypassing at two river mouths along the French Mediterranean coast. Both river mouths are exposed to the same wave energy, and have similar hydraulic behaviors. We find that bypassing rates are strongly dependent on river discharge and wave energy. When the river mouths are closed, longshore transport occurs as on an open beach for both sites excepted when the shoals are located upstream inducing a discontinuity with adjacent shorelines. The main difference is observed during the most important floods. At the natural Tet river mouth, bypass is mainly associated with spit platform breaching, while at the Agly river, the presence of jetties induced an export of sediment in the nearshore, then transport by waves to the downdrift coast.
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