Hello, I'm wondering if anyone has drainage or structural advice for turning what is currently a 2 tiered retaining wall area into a waterfall area. Please focus only on the section outlined in blue. We are planning a large garden pond and to use the dirt dug from the pond to regrade the hill into a waterfall/ stream. The water feature will not go to the edge of the fence! The outline is just for landscaping reasons.
OK, so the idea here is to use the existing grade (away from the patio) as the slope for a stream and waterfall? Yes, I know everyone is going to say that the waterfall should face the patio. Usually that's the way one would orient a pond. On the other hand, you can always build a new patio later on at the other end to face the falls.
@GBBUDD would be the one to advise you on the structural issues, as he's got years of experience. But from your second photo later in this thread, it looks like you could simply step down through your existing retaining wall, making the topmost step into a bog filter with overflow into the pond via a couple of short drops. The pressure of the rocks and water inside the liner would give some pushback to the tendency of the hill to slump.
However, you could add further stability by adding a wall of cinderblocks just off the patio. @GBBud and
@combatwombat can give you some good tips on doing that without needing construction experience. My own quick-and-dirty technique is to stack the cinderblocks, pound a couple of pieces of rebar through the centers into the ground, and then pour in a mix of dirt and cement and sprinkle it with water. Such advanced building methods are a staple here at Grandma Yael's Construction Inc.
For drainage, if you live in an area with a high water table, you may need a French drain under the pond, especially if you plan on going very deep. But if you're well above the water table, it may be enough simply to build up the edges so they're some 15-20 cm above the surrounding soil. Drainage on the slope near the patio is easier to manage, as you can build a rockery around the waterfall to carry the water away from the pond (or toward it, if you use rainwater to top it up).
On the third hand... Your situation is very similar to mine setting out. I have a patio over a yard that slopes away and then drops off a cliff (quite literally). My overall slope is similar to yours, but instead of a retaining wall, the whole yard just sort of slouched off toward one corner of the property, where erosion was gradually carrying it away down the cliff. At first glance it seemed that using the existing slope as the waterfall would simplify things. But then, I realized that I could save myself a lot of digging if I put the deep end just off the patio and used the spoils to build a low berm around the far end. As a 60+ year old granny, the less digging I had to do in our rocky excuse for soil, the better!
In your case, if you did decide to turn the design around, it might actually simplify the project in the same way - you've already got your deep end right there below the patio with very little digging needed (other than to carve out part of your current retaining walls). The spoils of the dig can go to the other end and the sides to build up the edges around the shallower zones.
That would also resolve the issue of drainage and support for the patio, as you could leave the back part of the retaining wall (just below the patio) in place, carving out only part of the steps. The rocks and water pushing against the liner would provide some support to the patio, but you could add a wall of cinder blocks outside the liner as extra support. Drainage for the pond would also be less of an issue, as you would be building up the edges above the surrounding yard, while the floor of the pond wouldn't be much below the current level of the ground. If you decided to go this route, a stock tank bog or something similar could provide your filtration, with a return to the pond via a stream.
But whichever way you decide to build it, you've got the perfect palette for pond building there! And as others have said, you can always build something now, add onto it as time and budget allow.
www.gardenpondforum.com
