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# Backwater curve for a rock-ramp fishpass
This module can be used to simulate the backwater curve of a
[simple macro-roughness fishway](macrorugo.md) in order to determine the
downstream flooding level of the fishway.
## Operating principle
The parameters of this module are :
- The choice of the ["simple" rock-ramp fishpass](macrorugo.md) module among
those present in the work session which will be used to perform the
calculation
- The water level downstream of the pass
- The space step used to discretise the curve calculation.
N.B. : As the backwater curve can only be calculated for one set of
parameters, the [simple rock-ramp fishpass](macrorugo.md) module
cannot contain [varied parameters](../../general/principe_fonctionnement.md).
The theoretical calculation carried out in the macro-roughness channel
corresponds to the calculation of the water line in a uniform regime where the
slope of the water is equal to the slope of the bottom of the channel.
The rock-ramp fishpass module is used here to calculate the slope of the water
in the non-uniform case.
The fluvial backwater curve is then calculated from the water level
downstream using the [trapezoid integration method](../../methodes_numeriques/integration_trapezes.md).
This module is based on [the module for calculating the backwater curve of a
section](../hsl/courbe_remous.md) to calculate and display the results.