The thesis deals with the problem of friction during rapid transient 1-D flows in a pipe caused by water hammers. The evolution of the wall shear stress is interpreted in terms of two steps. The first step is the dramatic change of the wall shear stress during the passage of the pressure wave; the corresponding new value of the shear stress is much greater than the value predicted in steady-state. The second step, which begins after the passage of the pressure wave, is a relaxation process; here the shear stress decreases, tending to the new steady-state value corresponding to the new average velocity. The Extended Irreversible Thermodynamics theory is proposed as a tool to model the wall shear stress during the relaxation process. The Friction Relaxation Model presented in this thesis describes both steps of the evolution of the wall shear stress during water hammers, and therefore it enables to take into account the information about the velocity gradient at the wall, which is otherwise not available in 1D modelling.