My guess is it's a truly odd cliff hanger avoider.
No wondering if it is or isn't a Great Tidal Bore while awaiting the next volume.
We've been informed that the Guild Master called it correctly, that their prepared defensive plan was successful, that there were casualties, and that this is something the kingdom budgets for.
That it's not some unexpected thing, but rather something the local infrastructure is built to handle at semi-regular intervals.
This kingdom is competent.
Ryo himself may be problematical, but the kingdom he currently finds himself within isn't.