Event Mean Concentration EMC method

In the EMC method, a C_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_max. In case of exceedance, the pollutant concentration is set to C_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:

\[ V = A \int_{\text{t\_start}}^{\text{t\_end}} i_r(t) dt \]

Equation: Recursive innovation model dependent on prior innovations and forecast model state.

where

Table: Variables in the rainfall event volume equation.

V	=	water volume in the rain event
A	=	area of the catchment
ir	=	rainfall intensity
t_start	=	start time of the rain event
t_end	=	end time of the rain event

The EMC is determined as:

\[ C_{E M C}=\operatorname*{max}\Bigl(C_{m a x},\frac{M_{s t a r t}}{V}\Bigr) \]

Equation: Event mean concentration bounded by maximum pollutant concentration.

where

Table: Variables in the event mean concentration equation.

CEMC	=	pollutant event mean concentration
Cmax	=	maximum pollutant concentration allowed in the rain event
Mstart	=	mass of pollutant at the start of the rain even

Note that during the rain event, the pollutant mass continues to build up.

Calculation of C_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_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:

Table: Rain event characterization parameters.

it	=	event threshold, (mm/h)
ADWPmin	=	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 ADWP_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.