The impact of bridge piers on the flow conditions can be included in the hydrodynamic calculations by activating the pier resistance option.
The MIKE 21 solution method involves the use of a finite difference grid with a selected grid mesh size. A typical choice of say 100-1000 metres implies that bridge piers with a typical horizontal dimension of say 5-10 metres are not directly resolvable in the computational grid. Therefore, the presence of piers must be modelled by a subgrid scaling technique.
The resistance to the flow due to the piers is modelled by calculating the current induced drag force on each individual pier and equate this force with a shear stress contribution compatible with the MIKE 21 momentum formulation.
Thus
(6.17)
where
tp: equivalent shear stress
F: drag force on one pier (the sign of F is such that tp acts against the current direction)
n: number of piers allocated to one grid point (density of piers)
Dx, Dy: grid spacing
The resulting shear stress at the bottom is then implemented as the sum of tp and the bottom shear stress, to.
The drag force is determined from
(6.18) 
where
CD: drag coefficient
r: density of water
Be: effective width of pier
He: height of pier exposed to current
v: current speed
