Please Landscape Help...

Hi all. I made a large 100 by 100 flat grass landscape using the terrain editor to house my neighbourhood render and I have created lots of layouts such as roads, homes, driveways, fences, etc.

But now for some reason I notice during this whole process I might have accidentally sculpted the very landscape that rests beneath a lot of my homes, roads, sidewalks, etc. And I can’t uniformly use the shift+click buttons to flatten this raised terrain. Usually it will create a hole or crevice.

Can someone please let me know if there is a way to auto flatten a large area uniformly? I thought about just deleting the landscape and easily creating a new one but a lot of road and sidewalks splines rest over the grass landscape and get deleted too if I erase the landscape.

Thanks for reading.