Using gl_FragDepth in WebGL

[TheAnonymousPoster]

I am writing a 3D application in webgl, and need to provide my own depth data that is contained within a texture, my current code does this:

VS:

varying vec2 vUv;

void main() {
vUv = uv;
}

FS:

uniform sampler2D depthTex;

varying vec2 vUv;

void main() {
gl_FragDepth = texture2D(depthTex, vUv).r;
}

however gl_FragDepth is disabled in opengl-es (and therefor webgl) is there anyway to somehow enable it, or any way to provide my own depth data that doesnt involve heavy manipulation of render targets?