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Event Mean Concentration (EMC) method

In the EMC method, a \(C_{\text{EMC}}\) concentration is applied to the runoff of catchment during a rain event. The pollution builds up in the same way as for Build-up/Wash-off method. The difference is that the wash-off is not considered as a dynamic process related to rainfall intensity but instead to the actual rainfall volume. The following is assumed:

  • All pollution accumulated on the catchment surface is washed off by a rainfall event. This assumption is valid if the pollutant concentration does not exceed a specified maximal concentration \(C_{\text{max}}\). In case of exceedance, the pollutant concentration is set to \(C_{\text{max}}\), and the residual surface pollution remains on the catchment surface.
  • Concentration of pollutant in runoff is constant throughout the rainfall event and is calculated as ratio between the available pollutant mass and rainfall event volume. The water volume in a rain event is determined by summing up all the rainfall over the event duration:

\[ V = A\int_{t_{\text{start}}}^{t_{\text{end}}}{\text{d}t\, i_{r}(t)} \]

Equation: Water volume of a rain event, integrated over the event duration.

where

  • \(V\) — water volume in the rain event
  • \(A\) — area of the catchment
  • \(i_{r}\) — rainfall intensity
  • \(t_{\text{start}}\) — start time of the rain event
  • \(t_{\text{end}}\) — end time of the rain event

The EMC is determined as:

\[ C_{\text{EMC}} = \max\left( C_{\text{max}},\frac{M_{\text{start}}}{V} \right) \]

Equation: Event mean concentration, capped by the maximum allowed pollutant concentration.

where

  • \(C_{\text{EMC}}\) — pollutant event mean concentration
  • \(C_{\text{max}}\) — maximum pollutant concentration allowed in the rain event
  • \(M_{\text{start}}\) — mass of pollutant at the start of the rain event

Note

During the rain event, the pollutant mass continues to build up.

Calculation of \(C_{\text{EMC}}\) based on the rain volume and its application to the runoff is inherently an approximation and introduces a small error in the calculated \(C_{\text{EMC}}\). As runoff volume is always smaller than or equal to rain volume, the pollutant mass included in the runoff will in most cases be smaller than the available mass on the catchment surface, the difference being retained on the catchment surface. In a continuous simulation with consecutive events, the method remains mass conserving.

Definition of rain events

A rain event in the EMC method is characterized by two user-specified parameters:

  • \(i_{t}\) — event threshold, (mm/h)
  • \(\text{ADWP}_{\text{min}}\) — minimal Antecedent Dry Weather Period (ADWP), (h)

Two rain intervals separated by a dry interval are considered as one continuous rain event, if the dry interval between two wet weather periods is less than the specified \(\text{ADWP}_{\text{min}}\).

The rain event is included in the simulation when the peak rain intensity exceeds the specified event threshold \(i_{t}\).

Pollutant wash-off routing

The wash-off of pollutants happens during the rain event, but the pollutants follow the runoff routing over the catchment surface and are delayed like the rain water. I.e. pollutant runoff from a catchment continues also in the period after the rain has stopped and until the end of the runoff.