They are called “fiorituras”, and they are a kind of ornament (literally: “Flourishes”). You can find a similar one in the left hand of Chopin’s Etude in C#m op. 25 no. 7 bar 27, or in Chopin’s Nocturne in C#m (Op. Posth.), right hand, bars 58 – 59.
Basically a fioritura is a series of additional notes played against an steady accompaniment (as in the two example aboves), or against a steady beat (in the case of your score). The actual notation (tuplets, semiquavers, etc.) is not that important. Instead count the number of notes in the fioritura and distribute them as uniformly as possible amongst the beats they cover. You may also relax the tempo (that is slow down or accelerate). In fact in some case you may have to slow down the tempo.
Most embellishments cannot be notated precisely, so remember that you must be faithful to the composer’s intentions, not to the composer’s score, which is really just an approximation of his intentions. Usually the best bet is to listen to a superlative pianist playing the passages in question and see how they do it.
For instance, in the last bar of the score you sent, there are 50 notes of the fioritura against 3 beats. You must spread these 50 notes over the three beats. Here is a suggestion:
1. Because the number of fioritura notes (50) does not divide evenly by the number of beats (3), simply partition the notes into similar size groups (50/3 = 16 +17 + 17).
2. Pencil in a line connecting the first note of each partition to the beat it corresponds to.
3. Play the passage to determine which group will have 16 notes and which will have 17 notes. Your ear and your hand will decide this matter.
4. After you can play it rhythmically accurate (use a metronome if you have to), then relax the rhythm and by all means use as much rubato as you think necessary.
I hope this helps.
Best wishes,
Bernhard.
Topic: What's this strange stuff?