氵 is the water radical 水 in left-side form. It places 涝 firmly in the water family, here pointing at water that has accumulated to harmful excess — flooding and waterlogging.
劳 supplies the sound (láo drifted to lào) and a faint flavor of trouble or hardship. Same phonetic appears in 捞痨唠. The toil-of-labor association reinforces the negative sense of waterlogged fields — wasted effort lost to flood.
