Q. 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's simple to use and has proven to have acceptable degree of accuracy and is the most generally used flow formula in the world. When using Colebrook-White Equation, it's observed that for very flat gradient (which is<1.5%) it leads to underestimate the flow since as gradient approaches zero, velocity also approaches zero. Therefore care must be taken when using Colebrook-White Equation for flat gradients.