I now have a specific question to ask which I suspect may have a more general application than just my own case, with respect to the matter of building swales . It is a rather simple question but simple questions do not necessarily have simple answers nor necessarily just one answer. And the question is:
The swales I intend to build will terminate on the north property line at a fence or at the "tractor path" parallel to the fence on my side. What concerns me is that during very heavy rains accompanied by winds the water accumulation in the swale may not be able to overtop the berm but that would not mean that it might not do an end run around the end of the berm if it rises above the native soil level and starts backing up the hill even just a little bit. Stating the concern just a little bit differently; once the downpour has filled the swale and started climbing up the bermit will start covering the ground just up grade from the swale trench. If the swale trench is very deep this might be avoided but this might develop if the the swale has filled in a bit, or if a strong wind runs parallel to the swale. by hooking the berm up the hill at the end or since it is the ditch that is providing the material for the berm is may be much easier to just hook the entire swale and berm up the grade.
While I am on the issue of swales that end at the property line one ought to consider the legalities if ones swale should overflow at the end and discharge its entire contents as aa swift flowing rush of water across a neighbor's land and it should do significant damage in washing out a fence or flooding some valuable feature of the neighbor's land. That could happen at any place on one's own land, but if the neighbor is litigious or not friendly the risk is higher.
In my case I have a primary pathway for the tractor right up the middle of the field I wish to build swales across, so on either side of that pathway / Tractor route, I may wish to terminate the swales on each side. of that pathway. The same technique could prove valuable if the swales are long enough to hold a great deal of water. That way if a spillway should wash out during a deluge the amount of water released against the next swale down grade from the breech would be limited to less than the entire contents of the swale if only the entire length of the swale is not one very long resevoir of water.simply by treating various points along the swale in a similar manner as done with the ends. In all the swale building videos and instructions I have not seen this question addressed.
So as usual any advice will be appreciated.
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The swale ends can be made to curl uphill a little to contain the water, and the overflow be constructed wherever it makes the most sense so each swale can overflow to the next lower down, with the final swale overflowing into a pond or creek, or some other place where the water can safely go on its way.
You've stumbled upon a dread secret of permaculture, Dan. We don't like to talk about it, but the truth is there is no proper way to end a swale; a true swale must continue indefinitely until it reconnects to itself, following the contour line of course. If the contour line doesn't behave this way,, or leaves your property, too bad, you can't use swales.
...thankfully no. There are a few options that come to mind, and you're right to be concerned about it... but I think you're already on the right track. The key is really the low spot which will be either the planned or de-facto spillway, and this certainly doesn't need to be at the end of the swale. To close the end just angle the berm or swale/berm upslope as you described, and then put a deliberate spillway wherever the water will be most useful/least inconvenient.
Some of the so-called 'swales' in my area are slightly altered by addition of a very, very gentle slope, to help move water across the landscape to a more desired area gradually, rather than it all soaking in until reaching overflow level.
Breaking your swales up into sections has some merit in that as you describe, in case of a washout less water will escape. On the other hand, you'll either need more spillways for each section, or more likely make your swale-dividers lower than swale level, so that overflow occurs first between the segments rather than over the swale itself. You would also loose some of the spreading effect of the swale in light rains, if that's an issue. I would probably choose to put the effort into more, bigger swales, and robust spillways, rather than dividing up my swales...
As far as your tractor access, roadways of any sort can really flow a lot of water; you might give some thought, while planning your swales, to modifying this access route so that it will dump any flowing water into swales rather than having it all continue down the access way.
If there is perceived high risk of swale overflow *and* this is an undesirable outcome as it is not practical to channel the water somewhere productive or at least harmless, ideally one would add more swales, or deepen the swales, or use other methods like ditches or drains to get the flow under control, until such an overflow became extremely improbable.
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