Permeable mounds made with concrete armor units are placed in front of some of the cooling water outlets of steam or nuclear power generating stations in Japan, which not only diffuse discharge flow, but also prevent the channel systems connected to outlets from severe wave effect. Both high and low water levels at the upstream side of the energy dissipating mounds should be determined for the design of such channel systems. However, the prototype values of the loss coefficient of armor units have not been reported. A new method to determine the coefficient from the values of up-and downstream water depths and discharge per unit width is proposed, and applied to a set of prototype data. An evaluation of prototype loss coefficient for tetrapods and the method to calculate upstream water depth are shown. The drag coefficient of tetrapods to be used in the dynamic equations in the combined porous body and VOF model is mentioned as well.