Working from and conserving a coastline?

I’ve been working on building an eroded height map that has a very specific coastline. The problem I’ve been running into is that when I run it through the filters the coastline is changed and is no longer recognizable or if I mask the filter then I get strange anomalies that don’t look natural at the coast line. Any suggestions on how to start with a specific coastline and get realistic results?

Thanks.

Depends on which filters you’re using, but a very soft mask to keep the filters from influencing the coastline should actually work.
Is creating the coastline afterwards (eg. through the coastal erosion device) an option?
Can you post some images to show your exact problem?