How can I divert storm-water and stop erosion behind my retaining wall?

[57414]

Hi all,

I’ve just put in a trampoline pad on a sloping block, and the recent heavy rain has exposed a problem I hadn’t planned for: a mini “river” forms behind the retaining wall and back fence, about 150 mm wide. Each downpour has carved it a little deeper, so I need a practical fix before the next storm season.

What I’d like to achieve

• Divert or slow the runoff so it doesn’t keep eroding the soil.

• Keep the solution low-maintenance and kid-safe around the trampoline pad.

Ideas so far

1. Simple garden edge + rock channel

   • Install a 100 mm plastic garden edge (e.g. Icon Plastics Jarrah Garden Edge) on the right side to steer water straight down the back.

   • Line the trench with plastic or geotextile, fill with small drainage rock, and top with larger rock to resist scouring.

2. Surface drain

   • Lay a polymer channel drain with grate (e.g. Everhard EasyDrain 3 m), then pipe it to the stormwater system.

   • Problem: there’s a 100 mm drop from the fence line to the top of the existing rock, so the channel won’t sit flush without extra excavation or a concrete haunch.

3. Agg-pipe (French drain)

   • 100 mm slotted pipe wrapped in geotextile, buried in coarse gravel, discharging downhill.

   • Might be overkill, but could future-proof the area if rainfall events keep getting heavier.

Site sketch

I’ve attached a rough diagram showing the current flow path, the proposed garden edge on the right, and the main inflow point along the top fence line.

Any experience with similar slopes? I’m torn between the quick garden-edge solution and digging in proper drainage. Keen to hear what’s worked (or failed) for you.

Thanks!

— Dylan

Where water runs in.River trench ive cleaned out a bit. Showing errosion on back fenceSlight errosion on back cornerleft side, minor errosion, not too worriedPadRight angle bend at back of propoerty, water needs to run around this corner.

[EricL]

Hello @57414 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about stopping erosion behind your retaining wall.

Looking at your photos it appears you already have the Everhard 3m EasyDRAIN Polymer Grate And Prejoined Channel in your possession along with Reln Stretch Ag-Pipe 100mm x 8m Slotted and Socked. Please note that linking your agi-pipe to your storm water drain will need to be carried out by a licensed plumber for safety and compliance. If you have another set of the Easydrain channels or would like to do a combination with a French drain both systems I believe will work together nicely.

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

[Dave-1]

Afternoon @57414 

Love playing with water and what it does even if its going to be a pain. Something about finding a solution and implimenting it so you dont need to worry in future really brings a smile to my face.

Love those sandstone blocks Serious effort into getting them in place. Do you know what is behind them if anything? 

Changing the course of water for you might be a little hard so I would work with it. Excavate behind each "join" of the sandstone blocks and then fill them with large gravel of a size that cant come through the join lines. Place some geotex material between the stones and the sandstone itself to stop fines coming through. I have used my gabion walls the same way and have not lost any soil (tho have had a build up on the high side from a few bad instances of rain.

Large gabion retaining wall and if you scroll down you can see how I placed the repurposed upholstery material behind the cage and between it and the soil. With your blocks I would do the same type of deal as far down as I could. (Its a narrow spot by the look of it) and then instead of soil, backfill with a mixture of largish stomes at the join and then further away they can be finer. but the large stones will effectivly block the smaller stones from comething through while still allowing water to come through.

On the right hand side you could create a shallow dish drain if you wanted or continue with the stones. Does the area around the trampoline fill with water? I have a feeling it will drain naturally down hill through that fenceline. I would also do the whole large pebbles to small pebbles along the sandstone next to the timber fence. Just to aid water flow without loosing soil. As a side note I would keep all debris out from between the sandstone and timber fence otherwise it will rot.

Another member @Liv95 had a similar issue with sand coming through her sandstone blocks. Ive included the discussion as it may help you out
How to fill gaps in Sandstone retaining wall? 

If you dont want the water to come through at all then you will be back to drains and drainage channels. (I can enlarge upon that if you want but will wait and see as it will require more digging )

I Would actually scrape back teh white pebbles and then dig a small trench 100mm deep and wide against the sandstone side wall (to the park) and fill with gravel) bring that same trench all the way around to the low side retaining wall (with the timber fence) and do the same plus the larger rocks to block the joins. This way the water will naturally drain and drain downhill. You wont be changing the watercourse so cant be disturbing the lower neighboours with extra water

Dave