Runoff Tab (Catchment Properties Dialog Box)

You can use the Runoff Properties tab to change the Runoff Method and various drainage related properties of the catchment.

Runoff Method

Specifies the method of determining rainfall runoff. The selection displays different properties for the tab. Select either SCS (Soil Conservation Service) or Time-Area Diagram (TOC).

Area

Displays the calculated area of the catchment.

SCS

SCS runoff method only.

Shape Types

Specifies the shape type for SCS calculation. Select either Curvilinear or Triangular.

Shape Factor

Specifies the shape factor or peak rate factor as defined in NEH-4 Chapter 16. The typical value is 484 for a hydrograph where the volume under the falling side of the triangular unit hydrograph is equal to 1.67 times the volume under the rising limb of the curvilinear unit hydrograph.

Initial Abstraction Type

The initial abstraction from the precipitation may be represented as the lesser total depth of precipitation, or as a fraction of the amount of precipitation. Specify either Depth or Fraction.

Initial Abstraction Depth

When the Initial Abstraction Type is specified as Depth, this field represents the lesser depth of precipitation.

Initial Abstraction Fraction

When the Initial Abstraction Type is specified as Fraction, this field represents the fraction of the amount of precipitation (between 0 to 1).

Composite Curve Number

The weighted CN used by the calculation. It is calculated as follows:

CN = (PIMP / 100 * 98) + ((1 - (PIMP / 100)) * PACN)

where:

• PIMP = Percent Impervious
• PACN= Pervious Area Curve Number

Volumetric Runoff Coefficient

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. This value is not used; however, it is exported as part of catchment data so it can be used in Storm and Sanitary Analysis.

Time Of Concentration

Time Of Concentration Method

Specifies which method to use for calculating the Time Of Concentration, TR-55 or User-Defined.

• If User-Defined is selected, then the Sheet Flow, Shallow Concentrated Flow, and Channel Flow sections are not shown on this tab.

• By default, flow paths are created with the number of segments as specified in the catchment feature settings. You can use the Edit Flow Segments command to add additional segments of any of the three types to a flow path after creation.

Time Of Concentration

Displays the sum of the calculated travel time values if the Time Of Concentration Method is set to TR-55. This field is editable if the Time Of Concentration Method is User-Defined.

Hydraulic Properties

Hydraulically Most Distant Point

Displays the coordinates of the selected or calculated point that is the upstream end of the flow path.

Hydraulically Most Distant Length

Displays the distance between the specified discharge point and hydraulically most distant point.

This length may not be the longest measurable distance between any two points within a catchment; it can be shorter.

Water moves through a watershed as sheet flow, shallow concentrated flow, swales, open channels, street gutters, storm sewers, or some combination of these. The hydraulically most distant length represents these flows along a path where "distant" relates to time rather than length.

Sheet Flow

TR-55 method only.

Sheet Flow Segments

Displays the number of sheet flow segments defined within the flow path.

Total Sheet Flow Travel Time

Displays the travel time for all sheet flow segments.

Shallow Concentrated Flow

TR-55 method only.

Shallow Concentrated Flow Segments

Displays the number of shallow concentrated flow segments defined within the flow path.

Total Shallow Concentrated Flow Travel Time

Displays the travel time for all shallow concentrated flow segments.

Channel Flow

TR-55 method only.

Channel Flow Segments

Displays the number of channel flow segments defined within the flow path.

Total Channel Flow Travel Time

Displays the travel time for all channel flow segments.