Is Colebrook White formula suitable for shallow gradient of pipes?

Manning’s Equation is commonly used for rough turbulent flow while Colebrook-White Equation is adopted for transition between rough and smooth turbulent flow.

For Manning’s Equation, it is simple to use and has proven to have acceptable degree of accuracy and is the most commonly used flow formula in the world. When using Colebrook-White Equation, it is observed that for very flat gradient (i.e. <1.5%) it tends to underestimate the flow because as gradient approaches zero, velocity also approaches zero. Hence, care should be taken when using Colebrook-White Equation for flat gradients.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.