Cut a slice of ground out of a slight sloped area, fill it with clay. You have a dam below grade. This will slow down the seepage, raise the water table, and force the water around the edges of the "dam" which will cover a broader area for a longer time.
This should avoid blowouts also.
2 things led to this idea.
1. I believe my swales are more of a depth guage than a water retainer. I can get 1" of rain and it fills up, but is empty the next day. If i get 12" of rain (it has happened twice), the swale stays full for weeks. The first week it is full, like water sheeting out the down hill side of the berm. As weeks pass, the level drops "slowly". The only water the swale retained was the first filling. After that, it's was showing how far below the surface the water was still seeping. Had the swale not been there, the water would still be flowing underground. So the swale is actually doing a lousy job, collecting 100's of gallons instead of thousands of potential gallons. This is my opinion and would like opinions if this is correct.
2. Best video for anyone contemplating a pond. Watch Zach Weiss presentation on Paul Wheatons 2017 PDC course. Its on Paul's youtube page and is free! When he builds a pond, he looks for the clay layer that the seepage is flowing on. His dam is dug down to this layer. and has a clay core. NO OTHER PART OF THE POND IS SEALED ! brilliant. He is catching the seepage, everything up hill is greener. You can see it in the pictures. Without the dam into this clay layer, you have a hole in the ground. I think he called it a basin. I have a basin. The seepage is going under my pond. My pond is filled by runoff only and needs to be sealed to be effective. If it caught the seepage, the thousands of gallons instead of the hundreds.........
Combine both thoughts and it seems viable.