This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
篊 combines 竹 (bamboo) with 洪 (flood, vast), but the exact historical development is uncertain. It may have originally referred to a bamboo pipe or conduit for water, given the components, but this is speculative.
The top 竹 suggests bamboo, and the bottom 洪 suggests a flood. Imagine a bamboo pipe channeling floodwater, but the exact meaning is unclear.
For コウ, picture a bamboo pipe making a hollow 'koh' sound as water rushes through: koh -> コウ.
brushwood or bamboo piles used for seaweed or oyster farming; cultch; culch