Heretic's default terrain and custom flats

I'm getting into making Heretic maps and was wondering if there was a way to use the default terrain that comes with Heretic on custom flats. I have a custom water flat that I'd like to act like Heretic's normal water.

If your wad is intended for ZDoom, use ANIMDEFS and TERRAIN. There is also a vanilla compatible trick for making custom flats out of others, but I'm not sure if it would transfer Heretic's terrain qualities too. Or you would just replace the stock water graphics with your custom ones, at the cost of losing the stock ones.

Figured it out. Heretic's default splash block is Water for water (go figure), so you only need to put

terrain (name)
{
	splash Water
	footclip (pixels)
	liquid
}

floor (texture) (name)

This sounds cool. If you can make it vanilla compat that would be awesome.

Unfortunately the terrain types are hardcoded into the executable itself, so there's no way to assign terrain to added custom flats in vanilla without completely modifying the game (at that point you're better off just using ZDoom). You'll have to replace Heretic's default water flats if you want a water effect which comes with its own slew of issues (e.g. color palette, graphic format, etc.).

If you're talking about vanilla Doom, it doesn't have terrain so it's impossible. Only Heretic and Hexen have it.