The 2D Landuses dialog sets the Mannings n values, rainfall abstraction, and 2D hydraulic Infiltration values for polygons in the 2D domain.
The Description is the name as it appears in the Layer Control Panel.
Mannings n may be defined as constant or as a function of depth in the 2D cell. Select the appropriate radio button. A variable roughness curve is entered into a table as ‘n’ values and the cutoff depths, ft (m).
Check Rainfall Abstraction if it is to be included in the calculation (i.e. if a 2D Rainfall/Flow Area is used in the simulation). For the Rainfall Abstraction the Initial Loss, in (mm) and the Continuing Loss in/hr (mm/hr) is entered.
Input the Percent Impervious to set the percentage of the landuse that is impervious.
2D hydraulic infiltration can be applied to the model, termed ‘transmission loss’ to differentiate from Infiltration within the Runoff Mode. This 2D infiltration or transmission loss can be applied to the model using the Green Ampt and Initial/continuing loss methodologies. These loss values are applied within the Global Database – (2D) Soil Type record.
To adjust the order of priority of the Landuse layer list, right-click a landuse and select Move this Layer Up, or Move this Layer Down.
The following table presents ranges of roughness coefficients for overland flow (Sheetflow):
Type of cover
Values of roughness coefficient, n
Bermuda and dense grass
Average grass cover
Poor grass cover on rough surface
Short prairie grass
Shrubs and forest litter
Sparse rangeland with debris
Plowed or tilled fields
Fallow – no reside
No till – no residue
No till – (20 -40% residue cover)
No till – (100% residue cover)
Open ground with debris
Shallow flow on asphalt or concrete
Open ground, no debris
Asphalt or concrete
Adopted from Mays, 2001.
A 2D hydrodynamic study in a highly urbanized area of Yorkshire, UK, Crowder, et al (2006) used the following roughness coefficients:
Industrial and commercial (roads, paths, car parks, some grass)
Residential (roads, alleys, gardens)
Playing fields and parks (soil grass, isolated vegetation)
Brownfield land underdevelopment (soil, grass, isolated vegetation)