When a City of Edmonton Lot Grading Inspector completes a Rough or Final Grade Inspection and issues an approval, it verifies that at the time of inspection, the property was in compliance with the Drainage Bylaw. From that moment on, as the owner of the property, it is your obligation to maintain the lot grading in perpetuity regardless of natural settlement or activity caused by yourself or damage by a third party.
Property owners are required to ensure that others do not trespass and damage grading on their private property. The City of Edmonton is unable to enforce or police such actions or determine cause or fault. That would require a civil action suit against the offending party or you as the owner should consult with the offending party and advise of the damage.
You may also wish to consult with the Builder's Warranty program if damage by a third party is covered.