I believe if you check the California building code, any retaining wall over 3' in height (measured from the base of the footings to the top of the wall) requires engineering and permitting. It's difficult to judge the height of the top wall but the lower would certainly appear to fall under those regulations. And even more so since it is supporting a surcharge.

How do you know the walls have no drainage behind them? Granted, it is possible the code requirements were ignored at the time but it is just as likely they were followed and the wall was constructed in compliance with code. In which case, proper drainage is accounted for and would be adequate.

Check. All the permits and engineered/construction drawings associated with that property should be on file with the county. If you do decide to alter either wall, be aware that any changes you make now will require engineering and permitting according to current code requirements.

Otherwise, I agree with all the advice provided previously. There are lots of CA native groundcovers that are excellent choices for slope stabilization and erosion control.

1. Stone all the way down to base block. If you are doing this, add the drain pipe, so you don't have dirty blocks (at least at bottom courses if you have a stone drain plane) all the time.

2. Bottom, if you are only doing 1 wall.

3. Counting 6 (8" tall) courses on the lower wall, I would guess it was installed 48" high maximum to avoid structural engineering requirements. That then required an upper wall for the slope, which looks to be <36" high.

If you are looking for longevity, removing hydrostatic pressure will reduce chance of leaning that so many improperly installed retaining walls encounter. That would entail the drain plane behind it. Though, if there is not a good base (foundation of stone, buried blocks), the wall may move a lot if you have expansion of soil or earthquakes anyway.

" installed 48" high maximum to avoid structural engineering requirements. "

California code requires engineering - and permitting - for ANY wall over 3' in height. And for any changes made to existing walls.

Thank you for your replies. Code is 3' max measured from the top of the footer. These walls are exactly 3' (6" tall blocks) plus a cap stone. The hillside was regarded and walls built shortly before we bought the house a few years ago, so the soil and blocks are relatively young. I can confirm there is no drainage behind either of these walls -- I've dug in numerous places.

I've met with a soils engineer, a structural engineer, 2 paver/wall building companies and 3 landscape designers. All have given me slightly different information ranging in price and complexity. We had some erosion and mudslides during this big storm that just came through California (above the upper wall and between the two walls) so we have to add drainage somewhere to prevent further erosion and collapse above the walls. For what it's worth, I'm located in the SF Bay Area.

Option 1: Dig a 3' trench alongside the upper wall across our entire hill to capture surface water from the upper side of the hillside and divert before it gets to the walls. This doesn't account for the water/pressure that would come from rain in the middle section between the walls or damp soil under the walls.

Option 2: We tear out the upper wall in the photo shown and regrade the hillside back, removing a few truckloads of dirt in the process and bring it closer to a 30 degree slope to reduce the pressure on the lower wall and reduce the chances of dirt shifting.

Option 3: We just add drainage behind the lower wall.

I've been considering a combination of options 1 and 2.

I'm a homeowner, not a contractor, so please take this with the biggest grain of salt... but!

Here in SoCal, the recent rains ended up doing a lot of damage to our property because there was just SO MUCH water that the problem went well beyond "diverting it as it came down the hill" and instead was more like: "The soil is almost fully saturated and thus there is no place else for the water to go".

Who knows if we'll ever get the heavy rains we recently got again (I think I read that this was the worst rain SoCal has gotten in decades?)... but fwiw, we're currently adding a lot more drainage (your option 3).

That is, (and again I am not even close to being a contractor here, just a regular homeowner!), the trench / diversion thing would have helped us in a moderate rainstorm, but it probably wouldn't have been enough for us during this last deluge where basically all of the soil was wet and was thus exerting tremendous pressure against the retaining wall.

And a sincere apology if I mis-understood your question, and/or gave bad advice!