The Méthode des Pluies approach is commonly used in France to estimate the storage volume needed in a Stormwater Control. Although this is a simple method, it is useful for preliminary studies.
It provides a way of determining the storage requirement based on an inflow curve and a set discharge rate. The inflow is determined based on the rainfall intensity that lands on a specified area (user specified or taken from the Plan). The Rainfall Intensity is derived from the Montana Coefficients as per the Desbordes Rainfall specified by the user. The discharge rate is a constant rate specified by the user.
The Methode des Pluies calculator is divided into an Input and a Results section.
The Input section has the following parameters:
- Input Type: Select either User Input to type in a specific area or Use Plan Data which will use the sum of the contributing area from all the Inflow nodes on the Plan.
- Area: This is specified by the user if Input Type: User Input is chosen, otherwise if Input Type: Use Plan Data is chosen the value is set to the sum of the contributing area from all the Inflow nodes on the Plan (and made read-only)
- Volumetric Runoff Coefficient (Input Type: User Input only). Specify the volumetric runoff coefficient for the specified area.
- Use Montana coefficients from Rainfall: Select the Desbordes Rainfall from which the Montana coefficients are taken. Note that the Méthode Des Pluies uses the selected Desbordes rainfall's Intense a and b coefficients to calculate the total rain depth over a given duration, and does not use the double-triangular rainfall profile. Please note that the convention in InfoDrainage is to specify the Montana B coefficient as positive (some sources specify it as negative).
- Discharge Rate: constant discharge rate leaving the Stormwater Control.
Note that the results section is updated as input values are changed, so long as the inputs values are legal; if any input values are illegal then results are not displayed with the illegal values highlighted by a red triangle and associated warning.
The Results section consists of:
- Two fields which give maximum storage volume and the corresponding storm duration.
- A graph of rain volume, discharge volume and stored volume vs. duration