They're not being dumped down a manhole. More than likely, things like that are being dumped into the larger open storm drain canals. And often storm drains run

They're not being dumped down a manhole. More than likely, things like that are being dumped into the larger open storm drain canals. And often storm drains run along the courses of creeks or rivers that have been covered up/paved over, so it's more like throwing your garbage into the nearby river.