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# Difference between revisions of "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 in the fort.13 file, meaning that it allows spatially (and temporally) varying bottom friction in the model.

## Attribute Summary Drag coefficient $C_{d}$ contours for Manning's n and total depth

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:

$C_{d}(t)={\frac {gn^{2}}{\sqrt[{3}]{h+\eta (t)}}}$ where,

• 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 = 0 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, under the assumption the Cd calculated from this formula tends to become too small in deep water.

## Negative n Values

If a combination of Manning's and time invariant bottom friction is desired, users can elect to set the Manning's nodal attribute to a negative value at certain mesh nodes. At mesh nodes where n is negative, the time invariant Cd coefficient, specified via a constant CF in the fort.15 file or the spatially varying quadratic_friction_coefficient_at_sea_floor fort.13 nodal attribute, will be used instead.