Manning's n at sea floor
The mannings_n_at_sea_floor nodal attribute is one of the available options for specifying bottom friction in ADCIRC based on the Manning formula. It is a nodal attribute, meaning that it allows spatially (and temporally) varying bottom friction in the model.
If the user elects to use this nodal attribute, which ADCIRC reads in as the ManningsN variable, NOLIBF must be set to 1 or the run will terminate. During execution, the Manning’s n values specified are converted to equivalent quadratic friction coefficients before the bottom stress is calculated. The equivalent quadratic friction coefficient is calculated according to the following formula at each node at each time step:
- Cd drag coefficient
- t time
- g acceleration due to gravity
- n Manning's n
- h depth
- η water surface elevation
The addition of the water surface elevation is conditional upon the setting of NOLIFA: η is treated as zero if NOLIFA is set to zero in the fort.15 file. Finally, the value of CF in the fort.15 is used to set a lower limit on the resulting equivalent quadratic friction coefficient, since the Cd calculated from this formula tends to become small in deep water.