If a channel connects a basin to the sea, the tide can induce a periodical flow in the channel. When the flow leaves the channel, a dipole can be formed. The dipole can either escape away from the channel, or get drawn back in to the channel as the flow reverses. It was thought that this behavior depended on the Strouhal number, where a critical number of 0.13 indicated the separation between the two characterizing behaviors. This value was also analytically predicted, based on the so-called WH-model (Wells and van Heijst, 2003). However, since errors are found in the WH-model, this report suggests improvements. These flow systems are also further studied in this report, by both numerical simulations and experiments. This is performed for various shapes of the channel, with special focus on corner refinements at the ends of the channel. It has been found that these different shapes are of great influence to the critical Strouhal number and have the capability to more than triple the critical value for certain cases. However, additional influences are found to have an impact on this value as well. For flows that are characterized by sucked back dipoles, observations are made concerning the longevity of the dipoles. If the vorticity from these dipoles survives long enough, several of these vorticity patches can interact with each other. Through the merging of these patches, stronger dipoles can be created which are capable to escape. In certain cases, these interactions also lead to periodical behavior with a period that is twice the tidal period.