Hi, I need some advice.

I want to build a retaining curved garden wall. 27' long, hight to be
4'1/2' down to 1/2' high and have a square channel on top of it that
carries the water from one end of the wall to the other.

If the wall was just one level - ie all 4'1/2 high that would be
simple, but as it slops downwards and curves, I'm not sure what to
build the wall from.

the options I can think of a-
Block work and render which I've never done before.
I could make the shape of the wall from thin plywood and concrete in,
I have done a simpler/small project like this.
I could combine both options as I don't fancy mixing that much
concrete as we have to move all products by hand to the back of the
house.

for the top of the wall I was going to use 3 blocks of thermalite type
bricks to form the channel shape and render/seal them with something
like flexcrete.

Plumbing side, tank, drainage, water flow is all tested (used
guttering) and working as expected.

Some constructive comments/advice would be great on how to move
forward with the wall

Ta
Dorine