Newly installed swales put to the test by monsoon rain fails

We recently completed a series of newly installed swales on our property. The swales are each between 150 ft to 300 ft long, 4 feet wide, and 18 inches deep. Each swale has an overflow spillway that directs overflow into the next swale, down slope. The swales are all spaced 40 feet apart. We estimated these new swales can hold a combine total of about 40,000 to 50,000 gallons of water if completley full. Our average rainfall amount is around 40 inches a year, so we never imagined the swales would ever be full. Less than a week after completing the swales we had a monsoon rain event over a two week period that dropped 20 inches of rain. That is half our annual rain fall, in just two weeks. While this is somewhat a rather extreme event, the swales held beautifully and did their job of capturing, slowing, and soaking.... until the overflow went over the spillway lips. At each sill point there was so much water rushing over the spillway and flowing down into the next swale that it cut deep revines in the landscape and dumped a lot of sediment into the next down slope swale.
Even though this is somewhat of an extreme event, can anyone think of something I could do to cut down  on the soil erosion and slowdown overflow from the  spillway before the water flow dumps into the next swale?

Thanks for any advise.
Jason

How wonderful that they filled so soon after construction! Makes it quick to identify issues with the design.

I typically armor spillways with cobbles: rocks too big to wash away during the maximum runoff event.

Also, wider spillways carry more water with less erosion. Construction of single layer rock checkdams in the ravines can  help slow the flow of water and sediment.

Vetiver grass, planted on contour in closely spaced hedges. It forms a dense barrier that slows water flow, prevents erosion, and traps sediment. The roots go very deep and bind soil together.  It makes huge quantities of biomass for mulching, or other use.

It is used throughout the tropics to help moderate rainfall events, stabilise slopes, protect and build terraces for cultivation etc…

In some conditions - where soil moves during rainfall events - it can build natural terraces, by trapping the sediment on the upslope side.

To be honest, no too bad for the first tough test.

You cannot expect that manmade trenches, swales will work perfectly.
Just do some repairs and the next damage will be smaller and so on until it is a natural design.

I dug my lake and river in the dry season and expected eroison of 30% serious damages, we ended up with about 15%.

Vetiver Grass is sure one of the best if not the best barrier you can have.
I planted 60.000 slips (also in the dry season) and expected a loss of 50%, but that stuff is tough, even the slips were looking dead, many of them came right after the first rain, so the loss was also about 15%....

@Joseph Lofthouse
I like the idea of coble stones, in the vein of something like an ancient french drain. The only issue that would bring up is that bunching stones around here tend to act like a mulch and produce huge patches of weeds which are then hard to control in rocks without spraying.
Any thoughts on making that in a way that would prevent weeks from taking root?

Plant something tough that you want amongst the cobbles before weeds can take over.

If that third picture is indicative of your washouts I would call the swales a raging success. 20 inches in two weeks will wash away whole farms and form some real gullies. Seems like it will all be working fantastically once you get better vegetative cover. Any good stand of grass sod will help prevent the washouts.

J Campbell wrote:bunching stones around here tend to act like a mulch and produce huge patches of weeds

That sounds perfect. The weeds would provide an extra layer of erosion protection. If you seed the rocks at the same time you install them, then you'll have more influence over what species grow there.