Careening water treatment plant

Without knowing it, we all know " the docks ", these slipway arrangements that give direct access to the sea with a gentle slope. They are used in particular for maintaining and cleaning boats. At high tide, the vessels go up these slopes, then are positioned on the invert when the sea drops back down, at the same time as a layer of vase and sediment. How can we prevent sending the cleaning water, chips of paint and various pollutants into the sea? The experience with the Cale de la Salinière at La Baule.