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# 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

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:

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

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

## Specifying n Values

Manning's n is often assigned using land cover datasets, when available. Examples of commonly used land cover data in the US are the National Land Cover Dataset and the Coastal Change Analysis Program. There is a broad literature on specification of n values based on laboratory and fields studies, the most classical example of which is Chow (1959). Ideally, field surveys or review of on-site photography should be done to correlate land cover values to n. Alternatively, review of available literature may provide some basis for selecting values.