5.9 Bottom Drag

5.9.1 Command Status

Required.

5.9.2 Description

Bottom roughness is modelled by setting the Bottom Drag Model command. Supported bottom drag model implementations are summarised in Table 5.14, with links to the relevant implementation sections below. Commands are listed in Table 5.15. Roughness values are defined globally using the Global Bottom Roughness command. Global values are typically overridden locally through use of material specific values (Section 5.14.2).

Bottom drag model science is provided in Appendix B.4.8.4.

Table 5.14: Bottom Drag Implementations

Model Implementation	Description
Manning	Manning roughness formulation for bottom drag.
ks	Equivalent sand roughness formulation for bottom drag.

Table 5.15: Bottom Drag Commands

Command	Description
Bottom Drag Model	Required - Sets the bottom drag model for bed friction calculations.
Global Bottom Roughness	Required - Sets the default global Manning’s ‘n’ coefficient or Nikuradse roughness length.

5.9.3 Manning

This model represents bottom roughness through the use of the Manning’s equation and is the default bed roughness model. It requires the spatial definition of Manning’s ‘n’ roughness coefficients.

! Manning Bottom Drag Model
Bottom Drag Model == Manning ! {Manning} | ks
Global Bottom Roughness == 0.03 ! {1.0E-6} Manning’s ‘n’

5.9.4 ks

This model implements a log-law velocity profile model. It requires the spatial definition of Nikuradse roughness length scale (ks) values.

! ks Bottom Drag Model (Metric Units)
Bottom Drag Model == ks ! {Manning} | ks
Global Bottom Roughness == 0.1 ! {1.0E-6} Nikuradse roughness length (m)

Example (ks model, US customary units).

! ks Bottom Drag Model (US Customary Units)
Bottom Drag Model == ks ! {Manning} | ks
Global Bed Roughness == 0.3 ! {1.0E-6} Nikuradse roughness length (ft)