There's no reason why it would be a variation of Ridley, just because they sound similar.
As quoted on www.ancestry.com, the Oxford Dictionary of American Family Names suggests:
Rydell 1) Swedish: ornamental name composed of the place name element ryd ‘woodland clearing’ + the common suffix -ell, from the Latin adjectival ending -elius. 2) English: perhaps a variant spelling of Riddell. Dictionary of American Family Names, Oxford University Press, ISBN 0-19-508137-4
And for Riddell:
Riddell 1) Scottish and northern English: from a Norman personal name, Ridel. Reaney explains this as a nickname from Old French ridel ‘small hill’ (a diminutive of ride ‘fold’, of Germanic origin), but a more probable source is a Germanic personal name derived from the element rid ‘ride’. Dictionary of American Family Names, Oxford University Press, ISBN 0-19-508137-4
I hope this provides some insight into the possible meaning of your name. Like many names, Rydell does not have an 'easy answer' as to its meaning.
♦ Chrisell ♦
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