The root ᴹ√SPAL with extended form ᴹ√SPÁLAS appeared in The Etymologies of the 1930s as a variant of ᴹ√PHAL(AS) (Ety/SPAL). Given that both ᴹ√SPAL(AS) and ᴹ√PHAL(AS) produce the same results in both Quenya and Noldorin, it is had to say which derivatives belong to which root, but Ilk. espalass and ᴹT. spalasta- are definitely from ᴹ√SPAL(AS) (Ety/PHAL; EtyAC/PHAL). This root is likely a later iteration of unglossed ᴱ√Palas or ᴱ√Palat from the Qenya Lexicon of the 1910s, where Tolkien indicated it was related to ᴱ√PḶTYḶ “✱strike; flat of the hand” (QL/72); it had derivatives like ᴱQ. palasse “foam, splashing” and ᴱQ. palasya- “splash, foam” (QL/72), and it might be related to G. osp(a) “foam” from the contemporaneous Gnomish Lexicon (GL/63).
In general, however, Tolkien used √PHAL (earlier ᴱ√FALA) much more regularly as the basis for wave/beach/foam words.
A root appearing in both The Etymologies of the 1930s as well as the contemporaneous Tengwesta Qenderinwa (first version, TQ1) appearing as both short ᴹ√PAL “beat” and extended form ᴹ√PALAP of similar meaning (Ety/PALAP; PE18/33). In The Etymologies, the entry for short ᴹ√PAL was erased (EtyAC/PAL), and the extended root ᴹ√PALAP had a couple of derivatives: ᴹQ. palpa- “to beat, batter” and N. blab- “flap, beat (wings etc.)” (Ety/PALAP). A variant root ᴹ√PALAK may have been the basis for ᴹQ. palka- “to beat flat” from the Quenya Verbal System of 1948 (PE22/114).
The short form ᴹ√PAL may also have been the basis for some words in the earlier Gnomish Lexicon of the 1910s: G. palta- “beat” [sic., rather than expected paltha-], G. paltha “blade (of swords, knives, oars, etc.); wide flat leaf; page of book” and G. plados “oar” (GL/63-64), though these words might be blended with root ᴱ√PALA “flatness” from the contemporaneous Qenya Lexicon (QL/71); see that entry for discussion.