I have to admit to blocking the waste from my bath, with muddy gritty crap. Living in a flat, on my own, I became a little complacent.
My advice is a quick one bucket rinse on the pavement, to get rid of the heavy stuff, then loving care in the shower or bath. When I say a one bucket rinse, I mean exactly that. Just a very quick going over with hot water and a soft brush.
I don't mean standing over your bike, in the street, and then leaving a vulnerable bike alone as you go and get a fresh bucket of water.
This seems to work. I have not blocked the bath again...