This thesis presents the problem of dimensioning transitional segments of open channels. In order to better understand a problem, it is necessary to say something about the basic concepts, sizes and phenomena that occur as a result of flow in open channels, and that part is elaborated in the opening two chapters of the thesis. Furthermore, we enter the narrow problems of the performance of transitional structures, where we have observed the general characteristics and problems related to the contraction and expansion of the flow, and finally we mentioned the real examples and various types of transitional structures, whether these structures are outlet or inlet type. After the theoretical presentation of contraction structures, examples were used to prove that the methods are valid. First, the calculation is presented on a few basic examples for some of the basic parameters of contractions and expansions. Thereafter, a few more demanding examples were shown to explain the problems that occur in the case of flow energy losses, as well as problems of dissipation of excess energy. The results showed that the implemented calculation is valid and led us to the solution of the basic problems they may occur with the contraction and expansion of channels.