When they dig the trenches or canals, they leave the silt on the parapets. When it rains, most of it gets washed back into the water ways. My guess is, there is no budget to haul away the garbage.
I understand that they are filling up trenches and canals to create housing areas.
Budget my rear end! The government just didn't think of the need! Like Woody opined, it's progress! We goin tuh.de moon!