Subcatchment Characteristic Width Calculation

Fig.98: Concept of the Characteristic Width of an ideal rectangular shaped subcatchment

Fig.98: Concept of the Characteristic Width of an ideal rectangular shaped subcatchment

Fig.99: Representation of the Characteristic Width for a non-uniform subcatchment

Fig.99: Representation of the Characteristic Width for a non-uniform subcatchment The characteristic width can be estimated in the tool using all of the following methods. However, there are some other empirical techniques also applied in several hydrologic modeling applications.

Fig. 100: Irregular subcatchment shape for width calculations (DiGiano et al., 1977, p.165)

1. Area and Overland Flow Length ratio method

2. DiGiano Method

3. Guo and Urbonas Method

Fig.101: Subcatchment Characteristic Width Calculation Tool window using Area/Over Land Flow method

1. The subcatchment layer and the conduit layer are the primary inputs required for width calculation. These layers must be present in the TOC of ArcMap before launching the width calculation tool. These layers must have the same projected coordinate system to perform the width calculation accurately. The tool window (Fig. 101) can be opened by clicking on the Width calculation (

   
   ) button from the GeoSWMM Tools.

2. Then the user will need to specify the required inputs such as the subcatchment layer, subcatchment width field, and maximum overland flow length.

3. The method estimates the subcatchment width based on the user specified values of the overland flow length.

4. Finally, clicking on the Calculate Width button will execute the computations for the subcatchment characteristic width and assign the results in the user assigned field in the respective subcatchment layer.

Fig.102: Options for calculating the width

Fig.103: Subcatchment Characteristic Width Calculation Tool window using DiGiano method

1. The subcatchment layer and the conduit layer are the primary inputs required for width calculation. These layers must be present in the TOC of ArcMap before launching the width calculation tool. These layers must have the same projected coordinate system to perform the width calculation accurately .The tool window (Fig. 103) can be opened by clicking on the Width calculation (

   
   ) button from the GeoSWMM Tools.

2. Then the user will need to specify the required inputs such as the subcatchment layer, subcatchment width field, and the conduit layer.

3. Checking the Adjust Head-watershed Channel Length option will enable users to edit the conduit lengths of the head-watersheds before performing the width calculation. There are two options provided (Use Channel length and Use Longest Flow Length) for adjusting the head-watershed channel lengths as shown in the following Fig. 104. The Use Channel Length option allows the user to edit the actual conduit length values in a table. On the other hand, the Use Longest Flow Length option allows the user to calculate the longest flow lengths for the head-watersheds using a DEM raster and assign them for the width calculation.

4. The Generate Head-watershed Table button processes the channel lengths for the head-watersheds and displays them in a table (Fig. 104). The three fields shown in this table are the head-watershed feature IDs (FID), calculated channel length (actual channel length or the longest flow length), and the adjusted channel length. The user can edit the channel lengths only in the Adjusted Channel length field, which will subsequently be used in the width calculation.

5. Finally, clicking on the Calculate Width button will execute the computations for the subcatchment characteristic width and assign the results in the user assigned field in the respective subcatchment layer.

Fig.104: Options for adjusting the Head-watershed Channel length

Fig.105: Subcatchment Characteristic Width Calculation Tool window using Guo and Urbonas method

1. The subcatchment layer and the conduit layer are the primary inputs required for width calculation. These layers must be present in the TOC of ArcMap before launching the width calculation tool. These layers must have the same projected coordinate system to perform the width calculation accurately . The tool window (Fig. 105) can be opened by clicking on the Width calculation (

   
   ) button from the GeoSWMM Tools.

2. Then the user will need to specify the required inputs such as the subcatchment layer, subcatchment width field, and the conduit layer.

3. Checking the Adjust Head-watershed Channel Length option will enable users to edit the conduit lengths of the head-watersheds before performing the width calculation. There are two options provided (Use Channel length and Use Longest Flow Length) for adjusting the head-watershed channel lengths as shown in the following Fig. 106. The Use Channel Length option allows the user to edit the actual conduit length values in a table. On the other hand, the Use Longest Flow Length option allows the user to calculate the longest flow lengths for the head-watersheds using a DEM raster and assign them for the width calculation.

4. The Generate Head-watershed Table button processes the channel lengths for the head-watersheds and displays them in a table (Fig. 106). The three fields shown in this table are the head-watershed feature IDs (FID), calculated channel length (actual channel length or the longest flow length), and the adjusted channel length. The user can edit the channel lengths only in the Adjusted Channel length field, which will subsequently be used in the width calculation.

5. Finally, clicking on the Calculate Width button will execute the computations for the subcatchment characteristic width and assign the results in the user assigned field in the respective subcatchment layer.

Fig.106: Options for adjusting the Head-watershed Channel length