Date: Fri, 14 May 2021 18:38:46 -0700 (PDT) Message-ID: <1758970066.37842.1621042726188@HILLINCONF.xps.xpsolutions.com> Subject: Exported From Confluence MIME-Version: 1.0 Content-Type: multipart/related; boundary="----=_Part_37841_1549086770.1621042726188" ------=_Part_37841_1549086770.1621042726188 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Content-Location: file:///C:/exported.html Desbordes Rainfall

# Desbordes Rainfall

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The Desbordes Rainfall refers to the double-triangle hyetograph that was= proposed by Prof. Michel Desbordes (Universit=C3=A9 de Montpellier, France= ) in the late 1970s. One key principle behind the approach is that the rain= fall hyetograph is not trying to replicate observed rainfall patterns, inst= ead its characteristics are derived from a sensitivity analysis of the resp= onse of a simple linear reservoir model. The shape of the double-trian= gle hyetograph aims to reduce the number of sensitive parameters to a minim= um. This design rainfall method is the most commonly used in France fo= r hydrological modelling of urban catchments.

The rainfall intensities can be specified using calculated Intensity-Dur= ation-Frequency (IDF) curves for the area of interest. In France the most c= ommon way of using IDF data is via the so-called Montana coefficients. Thes= e 2 coefficients (a and b) are used in the Montana formul= a that gives the maximum mean intensity for an event of a given duration an= d return period. In practice a table of coefficients a and b can be obt= ained from Meteo France for a numb= er of return periods (usually up to 100 years at least), from the meteorolo= gical station nearest to a study area. However if this is not available, th= e common approach is to use the Montana coefficients given in a guidance do= cument from 1977 (the "Instruction Technique 77") where the coefficients ar= e given for 4 return periods (up to 10 years) on the basis of 3 different z= ones covering mainland France and Corsica. This is normally not the favoure= d approach because it is widely acknowledged that better data is available = nowadays (much longer rainfall time series are now used to derive the Monta= na coefficients). Also the subdivision of France in 3 zones is very crude a= s it has been shown that there are in practice large variations in rainfall= intensities within any one of the 3 zones. However this approach can still= be useful for a preliminary investigation.

Return Period (years)
The software contains the Mont= ana coefficient values for the 4 return periods specified in the original g= uidance document (Instruction Technique 77) - 1, 2, 5 and 10 years. However= 'Custom' can be selected to enter your own return period if you wish to en= ter your own coefficient values for another return period.

Region
Select which area of France the site is locat= ed in (according to the sub-division shown in the Instruction Technique 77)= . The Map button allows this to be selected graphically by hovering over th= e region required and left clicking.

French Region Picker

Coefficients a and b
There are two sets of a and b coefficients= , one for the shorter intense period triangle and the other to describe the= overall duration triangle. These are automatically assigned for the built-= in 1, 2, 5 and 10 year return periods (as given in the Instruction Techniqu= e 77), for other return periods they need to be entered manually. However a= ny value may be over typed if a known value(s) has been supplied, most like= ly from Meteo France
The= calm period values for a and b default to values ap= proximately equivalent to a 0.5 year return period.

Note the convention for the sign of b: b is = considered a positive value.

Position of Peak
This pa= rameter describes the position of the peak of the intense period triangle w= ithin the whole event duration. By default a value of 0.5 is selected which= means that the peak is in the middle of the storm (symmetrical hyetograph)= . The specified value applies to all the selected durations. Two example rainfall hyetogra= phs. This first for a position of peak value of 0.5 the second at 0.75.

Storm Durations
A set of= Storm Durations and run times must be specified for the analysis to run ba= ck to back. All duration values can be edited, and new rows can be added fo= r additional durations.

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Section Pages

Workflow - What's next...? Connect&= nbsp;Inflows&nb= sp;to your S= tormwater Control, specify = Inlets or Outlets&= nbsp;or connect to another Stormwater Control or Junction.

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