Runoff tab (Catchment Properties Dialogue 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 Time Area, Soil Conservation Service (SCS), EPA SWMM or Santa Barbara Urban Hydrograph (SBUH).

Note: Methods, parameters and settings defined on this tab are only used for Drainage Analysis.

Time Area

A method for estimating runoff by dividing the catchment into sub-areas based on the time of concentration. The runoff from each sub-area is calculated and combined to generate the overall runoff hydrograph.

Catchment Properties

• Area. Displays the calculated area of the catchment.
• Time of Concentration. Displays the sum of the calculated travel time values.
• Calculation Method. Displays the method of runoff calculation.
• 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.

Soil Conservation Service (SCS)

A method from the Natural Resources Conservation Service (NRCS), previously the Soil Conservation Service (SCS). SCS computes a runoff hydrograph by convoluting a rainfall excess hyetograph with a dimensionless unit hydrograph. Rainfall excess is first determined using the SCS Curve Number Method runoff equations across the design storm. The unit hydrograph represents a single unit of rainfall excess over time based on the catchment characteristics.

Catchment Properties

• Area. Displays the calculated area of the catchment.
• Time of Concentration. Displays the sum of the calculated travel time values.
• Calculation Method. Displays the method of runoff calculation.

SCS General Properties

• 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. Represents the lesser depth of precipitation.
  • Initial Abstraction Fraction. 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 = Per cent Impervious
  • PACN= Pervious Area Curve Number

EPA SWMM

An Environmental Protection Agency Storm Water Management Model (EPA SWMM) method for estimating runoff that uses an equivalent width to model the catchment as an idealised rectangular area. It models the pervious and impervious areas separately accounting for infiltration and depression storage.

Catchment Properties

• Area. Displays the calculated area of the catchment.

General Runoff Properties

• Percent Impervious. Specifies the percentage of a catchment area of hard surfaces that prevents water from soaking into the ground.
• Average Slope (x1). Specifies the rise:run slope across the catchment.
• Equivalent Width. Specifies the theoretical width of an equivalent rectangular catchment.
• Internal Routing. Specifies how the runoff flows between pervious and impervious areas within the catchment. Select one of the following.
  • To Pervious. A specified fraction of the runoff from the impervious area is routed onto the pervious area
  • To Impervious. A specified fraction of the runoff from the pervious area can be routed onto the impervious area with depression storage
  • Direct. No internal routing through the other cover type.
• Routing Percentage. Specifies the percentage of the To Pervious and To Impervious internal routing methods.

Impervious Area

• Area. Displays the total impervious area, calculated as: Catchment Area * (Percentage Impervious / 100).
• Depression Storage Depth. Specifies the depth of depression storage on the impervious portion of the catchment. This value accounts for ponding after rainfall.
• Manning's n. Specifies the Manning's roughness coefficient for the overland flow over the impervious portion of the catchment.
• No Depression. Specifies the percentage of the impervious area that has no depression storage.

Pervious Area

• Area. Displays the total pervious area, calculated as: Catchment Area * ((100 - Percentage Impervious)/100).
• Depression Storage Depth. Specifies the depth of depression storage on the pervious portion of the catchment. This value accounts for ponding after rainfall.
• Manning's n. Specifies the Manning's roughness coefficient for the overland flow over the pervious portion of the catchment.

Infiltration

Defines the infiltration methods for the runoff model. Select Horton, Green Ampt or SCS Curve Number.

• Horton
  • Calculation Method
    • Standard. Based on empirical observations showing that infiltration decreases exponentially from an initial maximum rate to some minimum rate over the course of a long rainfall event. Input parameters required by this method include the maximum and minimum infiltration rates, a decay coefficient that describes how fast the rate decreases over time, and a time it takes a fully saturated soil to completely dry.
    • Modified. A modified version of the classical Horton Method that uses the cumulative infiltration in excess of the minimum rate as its state variable (instead of time along the Horton curve), providing a more accurate infiltration estimate when low rainfall intensities occur. It uses the same input parameters as does the traditional Horton Method.
  • Maximum Infiltration Rate. Specifies the maximum rate for infiltration.
  • Minimum Infiltration Rate. Specifies the minimum rate for infiltration.
  • Max Infiltration Volume. Specifies the depth used to determine the maximum infiltration volume from the area.
  • Decay Constant. Specifies the decay constant applied to the infiltration rate.
  • Drying Time. Specifies the time it takes for the catchment to dry. This is typically 2–14 days.
• Green-Ampt
  • Calculation Method
    • Standard. Assumes that a sharp wetting front exists in the soil column, separating saturated soil above from soil with initial moisture content below. The input parameters required are the initial moisture deficit of the soil, the hydraulic conductivity of the soil and the suction head at the wetting front. The recovery rate of moisture deficit during dry periods is empirically related to the hydraulic conductivity.
    • Modified. Modifies the original Green-Ampt procedure by not depleting moisture deficit in the top surface layer of soil during initial periods of low rainfall as was done in the original method. This change can produce more realistic infiltration behaviour for storms with long initial periods where the rainfall intensity is below the saturated hydraulic conductivity of the soil.
  • Suction Head. Specifies the average soil capillary suction.
  • Conductivity. Specifies the soil saturated hydraulic conductivity.
  • Initial Deficit. Specifies the difference between soil porosity and initial moisture content.
• SCS Curve Number
  • Composite Curve Number. Specifies the runoff curve number for the pervious fraction of the catchment, as described in NEH-4 Chapter 9. Typical values vary from 20 for regions with high infiltration and interception capacities to 98 for impervious areas. It is a dimensionless number depending on the hydrologic soil group, cover type, treatment, hydrological condition and antecedent moisture conditions. This number has a valid range of 0 to 100.
  • Drying Time. Specifies the time it takes for the catchment to dry. This is typically 2–14 days.

Santa Barbara Urban Hydrograph (SBUH)

A method from the Santa Barbara Flood Control and Water Conservation District in California that is a simple means of developing runoff hydrographs. Incremental excess rainfall is computed using the SCS Curve Number Method runoff equations. Instead of using the unit hydrograph approach in the SCS Method, an equation directly computes an instant hydrograph which is routed through an imaginary reservoir with a time delay equal to the catchment's time of concentration.

General Properties

• Area. Displays the calculated area of the catchment.
• Time of Concentration. Displays the sum of the calculated travel time values.
• Calculation Method. Displays the method of runoff calculation.

General Runoff Properties

• Composite Curve Number. Specifies a dimensionless coefficient (between 0–100) generally relating the amount of runoff to the amount of rainfall. It is a function of the land cover or land use, and sometimes also based on soil type and drainage basin slope. This represents the average curve number for the entire catchment where the composite is a weighted average based on respective areas.

Rational and Modified Rational Method

Rational-based runoff methods are widely used for estimating stormwater runoff. They calculate peak runoff based on the area of the catchment, the runoff coefficient and rainfall intensity derived from Intensity-Duration-Frequency (IDF) rainfall data.

Important: To use IDF rainfall data for drainage analysis, all catchments in the design must be specified with rational or modified rational runoff methods. If any catchment contains dynamic hyetograph runoff methods, IDF rainfall data sets will not be available for analysis.

General Properties

• Area. Displays the calculated area of the catchment.
• Time of Concentration. Displays the sum of the calculated travel time values.
• Calculation Method. Displays the method of runoff calculation.
• 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.

General Runoff Properties

• Calculation Method. Specifies either Rational or Modified Rational calculations.
  • Rational. A widely used calculation for estimating peak stormwater runoff rates. It calculates peak runoff based on the area of the catchment, the runoff coefficient, and the rainfall intensity at the time of concentration. This generates a triangular hydrograph with peak flow occurring at the time of concentration and extending beyond based on a recession limb multiplier.
  • Modified Rational. A variation of the Rational Method used for sizing stormwater storage facilities. Rainfall intensity that determines the peak flow is based on the duration of peak. This generates a trapezoidal hydrograph that peaks at the time of concentration and extends to the duration of peak, providing a more accurate representation of the runoff volume.
• Recession Limb Multiplier. Specifies a numerical factor used to determine the duration of the receding portion of a storm hydrograph.
• Duration of Peak (Modified Rational only). Specifies the maximum rate of discharge during the event. This must be greater or equal to the Time of concentration property.