Swale OFF Contour

I am wanting to install swales on our property.  This is an established home site with gardens,  orchards,  house,  shop,  and many mature trees.  Following perfect contour over the property will not be possible due to all the obstacles.  I can get somewhat close,  maybe within a foot or two of elevation.  If the sale stays the same depth throughout it will be a ditch.  

What if I change the depth of the swale as needed so it remains on contour while the ground around it slopes to and fro? Surly someone has done this but a search of this site turned up nothing.  Are there drawbacks to this?  

Environment conditions are 54 inches of rain per year.  Average of 3 to 5 inches per month with winter being heavier. Zone 8 in central mississippi.  Heavy clay soil with a little gravel mixed in the clay in some areas. No sand.  Just clay.   Water is SLOW to sink in.

Because of the heavy clay I was thinking of back filling the swale with wood mulch to cut down on mosquitoes.  I have free access to tons of wood ground up trees.  

I can't see why that would cause any problems.  As far as filling the swale with wood mulch, I plan on doing the small thing, although I am just going to make small fish scale swales a few places.  I have very heavy clay too, but I want to fill mine in just so my clumsy ass doesn't trip and fall on them while I'm wandering around looking at bees and things

One thing to think about with this setup is erosion. As the swale fills, it's going to fill up the "low" side of the swale much faster than the high side and spill over unevenly, creating a sort of wave-effect toward the high side. At the same time, the high side will have a lot more pressure on the soil (water pressure increases with depth), increasing chances of blow-out. I can't say whether or not these things would actually happen, but these are some of the principles behind swales — even pressure and slowed water velocity in heavy events.

Greg B Smith wrote:Average of 3 to 5 inches per month with winter being heavier.......Heavy clay soil with a little gravel mixed in the clay in some areas. No sand.  Just clay.   Water is SLOW to sink in.

From what I gather, swales might not be the best solution with this mix of factors.

My summation of swales:
- good for summer rain events (big ones)
- good with well draining soils

You have neither. Obviously do your own research for your specific situation, but the more I look at them, the less suitable they are for many people.

What happens with cold and clay? the water sits there and causes rot issues. Several threads on permies about this, have a hunt around.

Over here in Texas zone 8 I have similar annoying clay soil and useful frequent rain. Instead of digging proper swales, (My property sloped inwards) I dug a hole into the lowest spot and the high clay content created a slowly draining pond. Then I bought 4 Muscovy ducks and this stuff... https://www.amazon.com/gp/product/B0006IGZCI/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1 The microbes keep the pond from getting "ducky" and the pond keeps the rain from drowning my yard every time it rains. My next step is to create a shrub willow hedge around the edge of the pond to help stave off erosion, soak up additional yard swamp, and feed the rabbits.

I have clay soil and set my swales off contour, with the option to open and drain depending on how much I swivel a 90 degree elbow. People are obsessed with using lazers, and making swales perfect. From experience, it sounds like they have never installed or managed a swale system. Erosion exists, as long as there are no points higher than the highest elevation, within 2 years, the contour of the berm will remain the same, the basin will settle out to where it likes. Measure and find the average depth of your topsoil. Make the basin deep enough that the lens will always be lower, simple as that. To answer your question, yes, if you need to dig deeper to keep on contour, do it. Just use that soil removed from the deeper area, to make the berm at the low spots, wider.

I built my swales off-contour. You could think of  them as ditches which direct the water to where it is more useful to me. At intervals along the swales, I build small dams, which trap sediment, and water and allows it to seep into the soil. You could also think of them as on-contour swales, with an overflow spillway on one end, and a series of these swales installed end-to-end cascading down the hillside...

Sounds like we have a similar design Joseph. The idea is, slow, spread, sink. Not STOP , spread, sink. I feel that all swales should be off contour. I have serpentine, or as you said"cascading" swales. The lowest being the largest, where I can use it as a renatation pond. A 90 degree elbow on a drain pipe, can be lowered, to allow draining. Or if I know that it will be dry, I turn it upwards. To store. I can drain water down to the gardens in times of need. I also believe swales should be in series. Otherwise, you got yourself a large linear seasonal wetland, or a DIVERSION swale.

Chad: The slope of my swales, dug by hand with low-tech ad-hoc leveling devices is about 1%. The slope of the land perpendicular to the swales is around 17%. That really slows the water, and gives plenty of time for seepage. It also really collects a lot of sediment that is useful to patch construction errors.