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. 

Note: When 2D Landuse polygon areas overlap the attributes from the Landuse highest in the Layer Control Panel order will be used during the simulation – as shown in the image below. The top most Landuse in the layer list takes precedence over all other Landuse Layers.

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

Minimum

Maximum

Vegetated Areas



Dense turf

0.17

0.80

 Bermuda and dense grass

0.17

0.48

Average grass cover

0.20

0.40

Poor grass cover on rough surface

0.20

0.30

Short prairie grass

0.10

0.20

Shrubs and forest litter

0.30

0.40

Sparse vegetation

0.05

0.13

Sparse rangeland with debris



0% cover

0.09

0.34

20% cover

0.05

0.25

Plowed or tilled fields



Fallow – no reside

0.008

0.012

Conventional tillage

0.06

0.22

Chisel plow

0.06

0.16

Fall disking

0.30

0.50

No till – no residue

0.04

0.10

No till – (20 -40% residue cover)

0.07

0.17

No till – (100% residue cover)

0.17

0.47

Other surfaces



Open ground with debris

0.10

0.20

Shallow flow on asphalt or concrete

0.10

0.15

Fallow fields

0.08

0.12

Open ground, no debris

0.04

0.10

Asphalt or concrete

0.02

0.05

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:

Land Use

Mannings n

Industrial and commercial (roads, paths, car parks, some grass)

0.020

Residential (roads, alleys, gardens)

0.030

Playing fields and parks (soil grass, isolated vegetation)

0.035

Brownfield land underdevelopment (soil, grass, isolated vegetation)

0.035