Contour dams & swales in sandy soil

Though my lot is in soggy New England, the Connecticut River Valley is infamous for its extremely sandy soil that quickly becomes parched between rain. It's referred to as Windsor soil, and you can read the breakdown here:

http://www.ct.nrcs.usda.gov/soil-windsor.html

While mentally going over plans for swales and a retention dam on contour of a hillside that presently suffers from decades of topsoil erosion, I thought back to Geoff Lawton's video where he demonstrates his own work cutting a dam into a slope. In that video, he has the guy operating the excavator dig down deep enough to get to the hard clay subsoil to provide the strength necessary to prevent the dam from failing. However, my local soil is virtually devoid of clay, and very loose all the way down. Makes me nervous of a dam break just thinking about it. What's the best course of action for building swales & dams in these conditions to maintain structural integrity? Would I simply be better off cutting Holzer-style terrace steps and building hugel beds on contour?

On the coast, sea oats hold certain beaches in place with their very long roots. Redwoods hold mountains and hills in place with their long roots.
Now, I'm guessing that neither would be particularly happy in your area, but you might use the concept and plant things with long taproots to hold your dam or swale in place.
I suppose you could also use rocks or wood to help hold them in place, like an iron spine <laugh>. On beaches it seems like rocks and large driftwood are the only permanents, but in your case, you shouldn't have to deal with the wave action. Sand can certainly absorb a lot of water quickly, but it can also erode scarily fast. You might very well do better with terraces and hugel beds, though I think you might be able to somewhat combine the concepts. (For instance, build your swale like a hugelbed, thereby giving it stability.)
Well, certainly you'd get better tilth with the hugel beds over time, assuming that they hold moisture long enough to rot. You can also use the scaled down version of hugel beds, what I call rough raised beds, and just use layers of fresh compost, soil, finished compost and dry plant material (in no particular order, but it speeds up the decomposition to layer soil between the other layers) instead of logs and large branches. (Assuming that these are seperate from your swales. I don't know how it'd work in a swale on sand, because literal weight can make a difference in soil retention.)

yup, would use the desert trick, and bury as much wood as you can. planting fungus will really help hold the sand together, and trapping the water in buryed wood should help further down hill too.
just build the swales so they let the heaviest rain overflow the sides easily, but only flow down to the next swale, so you don't let the water get up much speed. get some thyme planted as soon as you can in the edges next to the pathways and openings to the next level. wooly thyme works best out here, can handle direct Arizona sun, and poor soil. YMMV.

Big log downhill side, med wood, brush. will end up terraced as it breaks down.
Grapes will grow well as starters is sand too, if they have a water bank.

Wouldn't try retention dam on the slope until you have trees to hold the hill together.
Once all the swales are rotting down, would be tempted to cut into the hill above, building a buried rock reservoir, with a courtyard over it. sandstone pavers with drains to fill it.

Might stagger them so you can get a tiller or quad or Bobcat up at a diagonal thru them, so you aren't tempted to cut straight paths.

if you could get SOME clay, to put on the backside of the swales, to hold everything in place, would be nice.

did you see this onsite?

https://docs.google.com/document/edit?id=1lDqAvhi90M9B4Klcw8ae6A3u1CjaLG8I7oal21YewyA