Resistance¶
The resistance defines how “easily” the water flows through a cross section. The higher the resistance, the more the water is slowed down and a higher water level slope is required to obtain a certain flow velocity.
The resistance term is introduced in the momentum equation (4.1b) in the form of a friction slope If. The friction slope can be derived as a function of an appropriate combination of the flow parameters (Q, A and R) and the water and resistance properties.
Generally, the friction slope is expressed as:
Equation: Friction slope from generalised friction factor and discharge in momentum equation.
where f is a generalised friction factor. The exact form of the friction factor depend on the resistance formulation, and can be found in Chapter 3.
By these means, the friction slope is explicitly determined as a function of instantaneous values of local flow parameters.