concerns about Swales and erosion

Hey everybody. I've been studying permaculture for a while and recently just came across my first project. This guy owns 20 acres which is situated in a small canyon with hills on either side. there is about an acre of hillside behind the house which is perfect for putting swales on the contour. The hill is at about a 3:1 ratio (For every 3 ft, the hill goes 1 ft up). We are in the high desert of Southern California and get hot hot summers and snowy winters. That being said, sometimes mudslides can be a problem. Are there any tips some of the more experienced people have? How do I stop erosion before the plants roots take hold? I plan on using soil stabilizing plants such as Autumn Olive, but I don't want any mudslides before they have a chance to stabilize. Any tips or advice would be appreciated. Thanks!

Seems like you'd want to know the real risk of mud slide right now to manage risk as you apply techniques... Is this a real risk or just a passing concern, or passing concern that should be a real risk?

#There is a difference between a shallow slump, and a big rotational event where a chunk of the hillside comes loose, which is a results of groundwater and stratigraphy.  If you are going to start messing with putting water into the soil on a steep hillside above a house in landslide country, consider a consult with a good experienced geotech engineer as part of the contract.
#Earthwork in spring rather than fall so you have a whole growing season before rain comes, consider strategic irrigation to establish strong root systems.
#Swales that seem on contour can concentrate water to a low spot resulting in gully erosion.  Be sure to manage water through its entire flow path, assuming that your swales will overtop (imagine a big rain on snow event with slushy dams forming everywhere...)
#Only disturb narrow contours leaving existing veg intact, than replace veg in increments.  Start on the bottom and work up so that the 'receiving system' is solid before you capture water up high.
#Look for evidence of slump risk - cracking, tree 'growing sideways', seeps

All advice for a situation like this is suspect without real understanding of slope hydrology, texture, stratigraphy, percolation rate, etc...

In many of SoCal's mud slide/flooding areas, the County Fire Dept gives away free sandbags.

A single sandbag (or two in an inverted "V") above a planting will divert the majority of the water from washing over the plant.  These could be temporary, and moved as needed.

Those are some good tips! I'm not too worried about large mudslides, but I'd like to prevent any minor ones from damaging future plantings, so this question is more about preventing any possible erosion. I'm going to create an overflow from each swale to the next lower swale, and eventually down to a pond at the bottom of the slope.
As for tests, neither I nor the landowner have the money for any experts. He knows I'm new and we are both willing to do trial and error. He's a builder by trade, so he has the machines and tools necessary to repair any damage.
So basically I either want to swale or terrace this hilly land, catch as much water as possible at the top, and let it slowly flow down to the crops/pond at the bottom. I am probably going to be moving on to his land and gaining self-sufficiency too. He's never heard of permaculture and once I started piling ideas on top of him he wanted to put it into action. We are in the pre-planning stages and I'm just looking for any tips from people who have done it, before I just find the contour and take his bulldozer up there... lol
He has many existing "tracks" on his land which he made for his daughter's BMX course. I don't see the roads giving out too much just by observasion, but all remaining vegetation still exists. A previous owner planted 1000 oak trees on the property and they are about 10 feet tall. Definitely overcrowded, but also helping stabilize the slope.
So leaving remaining vegetation in the hill and starting my plants in Spring so they have a growing season to stabilize seems to be the best option, rather than doing the swales right now to catch this winter's moisture. This guy got a major paycut at work and is gung-ho about becoming self-sufficient, so he's willing to try anything.

If you have strong vegetation and are only worried about surface erosion, than my post was probably overcautious.

How did this project turn out pedro? Any pictures?

#Swales that seem on contour can concentrate water to a low spot resulting in gully erosion. Be sure to manage water through its entire flow path, assuming that your swales will overtop (imagine a big rain on snow event with slushy dams forming everywhere...)
#Only disturb narrow contours leaving existing veg intact, than replace veg in increments. Start on the bottom and work up so that the 'receiving system' is solid before you capture water up high

I will be starting a new project in Portugal, zone 9, mild rainy winters, and hot dry summers. Some years have lots of winter heavy rain, some summers are very dry, other years are mild throughout the year. I must plan for all of this.

Our terrain is generally quite humid, and has many terraces about 3m wide, 2m high from one to another, from past abandoned agriculture now covered with pine forest, obviously showing a need to rearrange the system (the slope has 70º and some places showing erosion). The place has a high water table, therefore I am unsure about adding swales in there too. The terraces make a big C, which encircle further down a lower terrace which is wide and flat and receives the water from its 3 sides uphill, and obviously shows some springs here and there (because of all collected water).

First, I plan replacing (gradually) the pine by fruit trees, adding some deep root species, and many ground covers, berries and species to stop erosion.

I just do not know whether I run swales along the terraces zigzag from one to another one downwards, to avoid water running directly from one terrace to another. However, I fear that with heavy rain, rains might wash soil along the swale. Those terraces will be the typical forest garden.

In the lower terrace we plan to have the vegetable sunny spot, with perhaps raised beds.

Any suggestions for the swale system?