Land Use Properties (Reclassification) Tool

The surface roughness of a subcatchment affects the overland flow during a runoff event.  GeoSWMM uses Manning’s roughness coefficient (Manning's N or N-value) when computing the overland flow rate. This roughness coefficient varies for different types of land uses. Moreover, they are not the same for the impervious and pervious areas. Generally, the impervious areas have a lower N-value than pervious areas (e.g. 0.011 for smooth asphalt versus 0.24 for dense grass).

Depression Storage is another special characteristic of a subcatchment that represents surface storage provided by ponding, surface wetting, and interception; it is expressed as the depth of the water. Surface runoff accumulates over a subcatchment surface only after the water depth exceeds the value of maximum depression storage. It represents the water volume filled before the occurrence of surface runoff. The value of the depth of the maximum depression storage (D-value) varies from impervious to pervious areas (e.g. 0.05 for impervious surfaces whereas 0.30 for forest areas). 

The " Land Use Properties " pre-processing tool in GeoSWMM enables the user to efficiently handle the land use data for their SWMM project. It provides comprehensive options to reclassify reference land cover data (e.g. the USGS NLCD 2011 land cover data) to the simplified land use types that are representative of the model subcatchments. In addition, it allows the user to assign suitable values for Manning's N and the depth of the maximum depression storage for each land use type. Finally, the tool calculates the area-weighted N- and D- values for each subcatchment and stores the results in a user specified attribute column in the subcatchment layer.

The tool works primarily in three steps:

1. Reclassifies original or source land use data

2. Assigns N- and D- values to the reclassified land uses, and

3. Calculates N- and D- values for each subcatchment

The interface has three pages for three steps. At beginning, the tool opens in a dockable window which looks like the following Fig. 107. Features of each working step have been described below.

 

Fig.107: Land Use Properties Pre-processing Tool window

 

Land Use Reclassification

The first component of the tool allows users to reclassify or simplify the source land cover data. It primarily requires the original land cover layer name as input. This could either be a raster or polygon feature (vector) data layer. If the data is a vector i.e. polygon feature layer, then the attribute field containing land use codes needs to be assigned as an additional input. When the source land use data is assigned, a table displays the list of available land use values in the *Code column * (Fig. 107). In addition, three more columns are shown:

Land Cover Description , which allows users to provide a title or short description for each source land use code. The land cover description should be unique for each code.

User Defined Value , which represents the user-specified land use codes. The values are set to the original land use codes by default.

Reclassified Name , which corresponds to the title or short description for the User Defined Value column. It must be unique for each User Defined Value. Initially, this column is populated with the available User Defined Value by default.

Users may also assign the desired land use codes and descriptions from a previously developed land use reclassification scheme by clicking the Load button. It allows the user to import land use reclassification data from a text file. In this case, users must be aware of the consistency in source land use data.  The Save button allows the user to export the reclassified land use information into a text file. Detailed land use reclassification file formats are provided in Appendix E.1 .

 

Land Use Properties (N- and D- values) Assignment

After the land use reclassification task is complete, the next step is to assign the values of  Manning's roughness (N-value) and the depth of maximum depression storage (D-value) to the reclassified land use types. Users need to click Next after populating a land use reclassification table (Fig. 107) to open the second page of the tool that looks like the following Fig. 108.

 

Fig.108: Land Use Property assigning window

 

The second page of the Land Use Properties tool (Fig. 108) has two data panels. The left panel shows the SWMM reference tables for N- and D- values ( Appendix A.5 and A.6 ). The reference table opens based on the type of land use property specified by the user from a drop-down list in the right panel. The land use properties shown in the drop-down list are N-Imperv, N-Perv, D-Imperv and D-Perv. The suffixes Imperv and Perv stand for impervious and pervious area, respectively. The property table displayed on the right panel has three columns with the headings Code, LU Des, and the property name (e.g. N-Imperv). The Code and LU Des are the user specified and reclassified land use value and a short description as provided on the first page of the tool (Fig. 107). The property name (e.g. N-Imperv) is shown based on the user's choice in the drop-down list.

To assign the value of N-Imperv to a land use type:

1. Select N-Imperv from the drop-down list in the right panel. Instantly the reference table and the property-assigning table will be generated in the left and right panel respectively (Fig. 108).

2. Select the desired one or more land use cells under column N-Imperv (in the right panel). It can be done by dragging the mouse or with the Ctrl-key.

3. Select a suitable N-Imperv value in the reference table (in the left panel).

4. Click on the (

   
   ) button to assign the value to the selected land use cells in the right panel. However, the value can be manually inserted also.

The user can also assign the values of N-Perv, D-Imperv, and D-Perv following successive steps listed above. In the case of a range of reference values (e.g. 0.05-0.10 for D-Imperv), the user has to manually insert the digits in the selected cells.

There are two additional options (Load and Save) which allow users to apply a previously developed land use property table and to save the current table (in text files) respectively. Data formats of these files are provided in Appendix E.2 .

 

Land Use Property (N- and D- values) Calculation for Subcatchments

The final step in the Land Use Properties tool is to calculate the area-weighted N- and D- values for each subcatchment. After assigning the N- and D- values for all (or selected) the land uses, clicking on the Next button will open the third page of the tool which looks like the following Fig. 109.

 

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Fig.109: Subcatchment Land Use Property Calculation window

 

The user needs to provide the following inputs on this page to calculate the land use properties for the model subcatchments.

Subcatchment Layer

This is a polygon feature class layer that represents the model subcatchments.

Subcatchment ID Field

This is the unique ID field in the attribute table of the subcatchment layer.

N-Imperv Field

This is an attribute field available in the subcatchment layer in which the tool will store the calculated N-Imperv values.  If a new field name is assigned, then it will be added to the attribute table of the subcatchment layer.

N-Perv Field

This is an attribute field available in the subcatchment layer in which the tool will store the calculated N-Perv values.  If a new field name is assigned, then it will be added to the attribute table of the subcatchment layer.

D-Imperv Field

This is an attribute field available in the subcatchment layer in which the tool will store the calculated D-Imperv values.  If a new field name is assigned, then it will be added to the attribute table of the subcatchment layer.

D-Perv Field

This is an attribute field available in the subcatchment layer in which the tool will store the calculated D-Perv values.  If a new field name is assigned, then it will be added to the attribute table of the subcatchment layer.

After providing all the required inputs, clicking on the Update button will execute the land use property calculation and finally populate the data columns. Users can see the calculated property values in the attribute table of the subcatchment layer