As ocean waves approach shore, their velocity decreases. how does a decrease in velocity affect the frequency and wavelength of the waves entering the shallow water?

A decrease in velocity also causes a decrease in frequency and wavelength because if we manipulate the equation: v = f × wavelength, we can see that:

f = v / wavelength

wavelength = v / f

So in both cases the velocity is on the top. If the velocity decreases by a half for example then the wavelength and frequency will both decrease by a half because the velocity is proportional to the two values.