Catchment Properties Tab (Catchment Properties Dialog Box)

General

General Properties

• Area. Displays the calculated area of the catchment.
• Runoff Coefficient. Specifies the runoff coefficient for the catchment. Acceptable values are between 0.01 and 1.00. A runoff coefficient is used when calculating runoff with the Rational methods.
• Exclusionary. Specifies whether a catchment will exclude other catchments.

  Exclusionary catchments exclude any other catchments within the same Catchment Group whether the other catchments are exclusionary or not. If a catchment is exclusionary, its boundary (and flow path) can be modified either by existing catchments as it is being created or by new catchments as they are being created.

  As a catchment is created, it will check all other catchments within the same Catchment Group for two situations:

  • If another catchment is exclusionary, and if the new catchment has some (or all) of its boundary within it, the other exclusionary catchment will exclude the new catchment from itself.
  • If the new catchment is exclusionary, and if the other catchment has some (or all) of its boundary within it, the new catchment will exclude the other catchment from itself.
• Reference Object. Optionally specifies an object to use as a reference for the catchment. Select from the list or click Pick from drawing to manually select a reference object in the drawing area. Catchments can reference ponds, underground storage, channels, and structures for discharge.
  Note: Catchments can be associated with a single pipe network structure. The reason for making this association is to enable catchment data to be exported to Storm and Sanitary Analysis. When catchments are associated with a structure, they can be exported and converted into a Storm and Sanitary Analysis subbasin. With no association, however, catchment data cannot be exported.
• Reference Pipe Network. (Referenced structures only) Displays the name of the pipe network that contains the pipe network structure you select in the Reference Pipe Network Structure list.
• Catchment Group Name. Displays the group in which the catchment was created. To change the catchment group, you can use the Move To Catchment Group command.

Flow Path

• Create flow path. Allows creation of a flow path. If flow paths already exist for the catchment, they will be overwritten on creation.
  • Create flow path from object. For more information, see To Create a Flow Path From an Object.
  • Create flow path from surface. For more information, see To Create a Flow Path From a Surface.
• Flow path length. Displays the total length of the catchment flow path.
• Discharge Point. Displays the specified discharge point of the catchment. If you defined the flow path from a polyline, this is the downhill end point of the selected flow path polyline. If you defined the flow path from a surface, this is the discharge point you selected in the drawing. Click to zoom to the Discharge Point in the drawing.
• Hydraulically Most Distant Point. Specifies the point in the catchment that experiences the greatest total pressure loss from the source. Click to zoom to the Hydraulically Most Distant Point in the drawing.
• Reference Surface. Specifies the surface upon which the catchment area is calculated. If the catchment is creating using the Create Catchment From Object command, this value is <none>. Select from the list or click Pick from drawing to manually select a reference surface in the drawing area.
  Important: The surface associated with a catchment cannot be changed after the catchment is created. The catchment does not update dynamically if the surface changes after the catchment is created. However, you may select another surface to use for calculating flow paths when using the Edit Flow Segments command.

Time of Concentration

• Calculation Method. Specifies which method to use for calculating the Time Of Concentration: TR-55 or User-Defined.
• Time of Concentration(User-Defined only). Specifies the sum of the calculated travel time values.

Flow segments

Flow segments in TR-55 define location along the flow path line where there is a change in flow type, ground cover, and slope. Use the following tools to edit flow segment data and to add flow segments of the type required for the catchment area.

Add Flow Segment. Starts the Create Flow Segments command at the command line and adds a new segment to the flow segments table.

Note: The Create Flow Segments and Delete Flow Segments buttons are not enabled if the flow path Time of Concentration Calculation Method is set to User‐Defined. User-defined flow paths cannot have multiple segments. Multiple segments can be used to represent varying terrain for use in the TR-55 Time of Concentration equation. If you are specifying a User-Defined Time of Concentration, multiple segments are not required.

Delete Flow Segment. Deletes the selected flow path segment. Deleting a flow segment will merge the lengths of the flow segments; slopes will be reread from the surface (if applicable) or recalculated. Velocity and time of travel will be recalculated as well.

Flow segment data

• No. Displays the identification number of the flow segment. Specifies the identification number of the flow segment.
• Flow type. Displays the flow type of the segment. Either Sheet flow, Shallow concentrated flow, or Channel flow.
• Length. Displays the length of the flow path segment. Note that the sum of flow segment lengths displays at the bottom of the table.
• Slope. Displays the average slope of the flow path segment.
• 2-yr 24-hr rainfall (Sheet flow types only). Displays the rainfall depth for the 2-year, 24-hour storm event
• Surface type (Shallow concentrated flow types only). Displays the type of ground surface along the flow segment. Either Paved, Unpaved, Grassed waterway, Nearly bare and untilled, Cultivated straight rows, Short grass pasture, Woodland, or Forest with heavy litter.
• Manning roughness. (Sheet flow and Channel flow types only). Displays the coefficient representing the roughness of the ground along the flow segment.
• Cross-sectional area (Channel flow types only). Displays the area of flow measured in a perpendicular plane to the flow segment.
• Wetted perimeter(Channel flow types only). Displays the wetted length along the cross-section of a channel.
• Velocity (Shallow concentrated flow type and Channel flow types only). Displays the calculated velocity in the channel segment of the flow path using the Manning's equation. In Shallow concentrated flow types, the calculation is further simplified by using a variable based on the surface type selected.
• Time. Displays the number of minutes it takes a drop of water to flow along the flow segment. Note that a sum of time for all flow segments displays at the bottom of the table.