Hello Nicholas

I have looked at your question,

Both 洪水 and 洪灾 mean flood.
By looking at the characters the first litterally means flood-water and
the second flood-disaster so the secon might be stronger in meaning.
(the first character stands for flood, the second looks lik a fire under a roof
a house on fire, a disaster! But a flood might put that fire out :-) )

这一带田畴都毁于洪水了，真让人惋惜呀。
All the fields here have been destroyed by floods, which is a real pity

军民团结一致，奋击洪灾。
The army and the local people worked together to fight against the flood.

As for 确定 and 肯定 they have different meaninings.
确定 means to dertermine and 肯定 to confirm

这种病的病源尚未确定。
The cause of this disease is not clear yet
他是否同意，我不能肯定。
I can’t confirm whether he agrees or not.

I hope Benny cna give you a better answer.

陆滥克

I’m a Chinese but I am not sure I can express the difference of the words in English well.

Both stand for flood, but the latter emphasis the disaster caused by flood.
Because 灾-灾难 means diseaster.