Define the ampere in terms of charge flow rate?

Although an ampere corresponds to a coulomb per second, this is not the way in which it is defined. Instead, it is defined in terms of the force between two, parallel, conductors, as follows:

The ampere is defined as 'the constant current that, if maintained in two straight parallel conductors of infinite length and negligible cross-sectional area and placed one metre apart in a vacuum, would produce between them a force equal to 2 × 10-7 newtons per unit length'.

The importance of this definition is the fact that it provides a connection between mechanical and electrical units of measurement.

The reason that an ampere cannot be defined in terms of a coulomb is that the ampere is a Base Unit, whereas a coulomb is a Derived Unit which, itself, is defined in terms of an ampere and a second.