Storm water will rush into the pipes and whoosh into the rising river.
Whoosh is the best fit because it is a lot of fast movement; since a lot of water was going into the pipes, one would assume that a lot would flow quickly into the river.
Squelch is like a squishy, oozing sound; one might squelch in wet mud.
Thwack is a sound made by something springy hitting something else, like a branch might thwack against a house.
Pitter-patter is similar to drip; rain may pitter-patter onto a sidewalk.
Slurp is an uptake of a fluid; someone can slurp soda or soup.
Belch is a quick expulsion, like a "burp".
Hope this helps!!
