First Flush is the initial surface runoff from a rainfall event. Pollution wash off for impervious surfaces is likely to be at a higher concentration for this initial period and is referred to as the ‘Pollution Shock’. The purpose of the First Flush Calculator is to allow users to establish these pollution values for an initial depth of water.

The user is given two main options for calculating the pollutant load:

1. If User Input is selected then the user enters values for:

  • Rainfall depth, P
  • Area, A
  • Volumetric runoff coefficient, Cv
  • Pollutant concentration cp for each pollutant p.

When the Calculate button is clicked, the total volume is calculated as:

V = A.P.Cv

and the resultant pollutant load lp, for each pollutant p is then calculated as:

lp = V.cp

The calculated total volume and pollutant load values are then presented as a table under the “Reports” tab, see Fig. 1 below:


 Fig. 1: An example set of inputs for when User Input, and the calculated results under the Reports tab


2. If Use Plan Data is selected then the user only enters a value for rainfall depth, P.

When the Calculate button is clicked, the calculator calculates:

  • For each Inflow Node from the current design, its volume and set of pollutant loads.
  • The total volume and the total pollutant load for all Inflow nodes from the current design.

see Fig. 2 below.

The user can then select, from the list on the left hand side under the “Reports” tab, whether they wish to select the Total volume and pollutant load, or a particular Inflow node’s volume and pollutant load.

For each Inflow node, its volume is calculated depending upon its Runoff Method (in italics):

  • Time of Concentration: Inflow node area * runoff coefficient
  • Time Area Diagram: total area from TAD * runoff coefficient
  • Other Runoff Methods: Inflow node area

For a hydrograph, the rainfall depth from the “Inputs” tab is ignored, the volume of a hydrograph is the area under the hydrograph’s flow vs. time graph.


Fig. 2 An example set of inputs for when Use Plan Data is selected, and the calculated results under the Reports tab