This is the 16176th most frequent Korean word.
불어난
"불어난" means "swollen" or "increased."
In this sentence, "불어난" describes the increased amount of rainfall indicating its cause for the river's overflow.
불어난 비로 인해 강이 범람했다.
The river overflowed due to the increased rainfall.
Here, "불어난" refers to the growth or increase in the amount of household debts.
불어난 가계부 채무를 해결해야 한다.
The increased household debts need to be resolved.
In this case, "불어난" portrays the expansion of the river's water level.