Hey, thanks for all the responses.
You always think when writing these things that you've articulated the problem but it doesn't take long to realise you haven't! My apologies - but you are correct, I am limited by space above the dam to excavate as it's at the top of a valley. I c ould puch a touch further at the top but there are bunch of native trees that I would hate to cut down.
The primary problem is that, because of the slope of the valley is about 1:4, and the fact that the dam height is measured from the crest of the dam to the the point where the dam meets the natural ground level (further down the slope), that 4-metre height limit mean that the dam is actually only about 1m above the natural ground level at the crest of the dam. This means I've got a lot of excavating to do above the dam to get the necessary water depth. I would prefer to avoid this as it's already 2-3m from the valley walls to the valley floor, ideally, I would use that depth for water rather than dig much deeper.
Oddly enough, dam height is regulated but dam slope isn't. If I built a vertical dam (impossible with compacted dirt I know) the crest would be 4m above the natural ground level, the shallower the slope, the lower the final height of the dam. I've attached the same picture but with a 1:2 slope front (on the right) and a 1:3 slope back (on the left) dam overlaid on the slope to help illustrate what I mean. I've also attached a photo of the valley from the top and bottom.
I think I might be misunderstanding this part a touch:
For most embankment dams, the front slope and rear slope should NOT be greater than 3:1. Typically if you have a significant amount of non-cohesive material in the fill (such as sand, gravel, or rocky debris), then the downstream slope should be further reduced to about 2:1.
Wouldn't reducing the slope go the other direction (e.g 4:1). I am presuming when people day 3:1 they mean 3m horizontal, 1m vertical?
In regards to lining - the plan is to use a pool liner or similar as opposed to clay.
