Considering the motion of rigid bodies in an ordinary (Newtonian) viscous fluid, we saw in the preceding chapter, how naturally fractional derivatives (semiderivatives) arise in framework of classical hydrodynamics. Here, we bring four more such examples. One of them is linked with the design of a weir notch [92, 98, 113]. A weir is a small overflow dam used to alter the flow characteristics of a river or stream. In most cases weirs take the form of a barrier across the river that causes water to pool behind the structure, but allows water to split through a weir notch over the dam. Weirs are commonly used to alter the flow regime of the river, prevent flooding, measure discharge and to help render a river navigable. Schematically, the dam element is depicted in Fig. 2.1.