12.6 Consider an incompressible steady flow in a stream with constant depth, z = d, bounded on one side by a straight line y = 0, and on the other side by a curve y = h(x), which is slowly varying |dh/dx| < 1. (a) Calculate the average flow velocity in the x-direction (for fixed x). (b) Approximately calculate the comoving acceleration in the flow. (c) What should the shape of the curve be in order for the comoving acceleration to be independent of x?