Notes:
Possibly <res/rys> ‘ford’ + <-el> diminutive suffix. Note that [d] does not normally assibilate before [vowel] + [l]. Thus the spelling would be <Rydel> in any case.
It has also been suggested that there may be a connection with similar names in England e.g. Ruddle (Wiltshire) where the names originate from red ochre.