In this thesis the equilibrium cross-sectional areas of a tidal inlet are investigated. The most common used methods to study cross-sectional stability are the Escoffier method (Escoffier, 1940) and the Modified Escoffier method (Van de Kreeke, 2004). The Escoffier methods are mainly empirical and lack physical justification. Therefore, in this thesis a process-based model, as proposed by Hinwood et al. (2012), is used to show that in the long-term the inlet cross-sectional area approaches a stable equilibrium or closes. Additionally, the influence of higher harmonics on the equilibrium cross-sectional areas is studied. The stable equilibrium cross-sectional area of the Frisian inlet in the Dutch Wadden Sea is computed and the results are compared to earlier findings.