# 5.3. Bottom friction#

Related CPP options:

 LIMIT_BSTRESS Bottom stress limitation forstability BSTRESS_FAST Bottom stress computed in step3d_fast BBL Bottom boundary layer parametrization

Specification in croco.in:

bottom_drag:     RDRG [m/s],  RDRG2,  Zob [m],  Cdb_min, Cdb_max
3.0d-04      0.d-3    0.d-3     1.d-4    1.d-1

• General form for 3D equations (cf get_vbc.F) :

• If $$z_{0,b} \ne 0 \; \rightarrow$$ quadratic friction with log-layer ($$C_{d,\min} \le C_d \le C_{d,\max}$$)

$\boldsymbol{\tau}_b = C_d \| \mathbf{u}_{k=1} \| \mathbf{u}_{k=1}, \qquad C_d = \left( \frac{ \kappa }{ \ln \left( [z_{1} - H] / z_{0,b}] \right)} \right)^2$
• If $$r_{\rm drg2} > 0 \; \rightarrow$$ quadratic friction with $$C_d$$ = constant}

$\boldsymbol{\tau}_b = r_{\rm drg2} \| \mathbf{u}_{k=1} \| \mathbf{u}_{k=1},$
• Otherwise $$\; \rightarrow\;$$ linear friction

$\boldsymbol{\tau}_b = r_{\rm drg} \mathbf{u}_{k=1},$
• In the barotropic mode (cf step2D.F) :

$\qquad \boldsymbol{\tau}_{b}^{\rm 2d} = (r_{\rm drg} + r_{\rm drg2} \| \mathbf{\bar{u}} \|) \mathbf{\bar{u}}$

to be continued here for BSTRESS_FAST and BBL â€¦

BBL parametrization is detailed in the Sediment and Biology models section of the Doc.