Not sure what you are trying to calculate and plot. If QGIS cannot help you, you may try SMS software from Aquaveo. You could use a community version (free, if I recall correctly). It is a very popular user interface to 2D modeling. However, SMS does not support telemac modeling and therefore the .slf format and you will have to convert it to a .dat format, which is binary but its format is well documented on their website. Some python skills will be needed for the conversion. Good luck!
Hailiang