As some of you know I have 2 small ponds in close proximity, ~50ft.

I want to connect the two for drainage purposes, but not allow fish to travel between them, as I want a different species set in each pond. I wanted something better looking than a culvert pipe with screen on the end. I came up with this design:

French Drain Culvert

I guess my main question is, do you think fish eggs will still get through this? More so than ducks transferring them between the ponds?

I have a bunch of heavy driveway fabfric that I will use to install the surge rock, I could even put this over the ends of the culvert too, but not sure if over time it would get clogged up.

Nice drawing. Most fish eggs ( LMB , BG , RES , FH and tilapia)will not be a problem as the eggs will not be viable if off the nest. However the scatter spawners ( TShad , GShiners etc ) could be. More likely that yoy/fry swimming through the pipe will be a problem. However be careful if you have GSF as they are known to jump long distances to get to new water.

I would suggest that some type of screening process to keep very small fry from using the pipe is a must. You will have to devise a method to do that without blocking up the water flow.

How fine of a screen?

I'm thinking maybe I will make a concrete manhole box in the middle of the two 20 ft length pipes, and make replaceable screens, sort of like changing a forced hot air furnace filter.

Could you connect them with a vien of sand? Thinking of how well this works when you have one emptying your pond.

Fine gravel should allow water to run between the two BOW's but not allow eggs or small fry to get through...

Agree that pea gravel should work better since it should allow more water movement and less clogging. I hope to use this solution at some later time to allow some of my seasonal pond runoff to leak from the downstream ditch into a wetland that I want to construct. I want to keep out as many fish as possible, however floods will happen. I will probably seal the top with filter fabric and cover with soil.

The trick is to get the screen (whether sand , gravel , wire or whatever) small enough to do the job but big enough to let the water through in a big runoff situation ( keep the fish out but not clog the system).

What is the difference in the elevation for the emergency spillways between the two ponds (how much drops on the line between the two ponds can be expected)? I ask because it may be possible, with sufficient drop, to bring a drainage pipe from the upper pond on top of a gravel-filled pit that has a second pipe in the bottom that feeds the lower pond. This would increase the water handling ability due to maximum gravel surface area and also hydraulic pressure.

My lot is very flat, there is ho hieght delta...

My lot has maybe 2 inches of slope over a 500 ft distance.

I talked to my younger brother today, (owner of the small pond), he just wants a culvert and the ability of the fish to swim through...

So, I'll most likley do that. The option to change things up in the future will always be there I guess.

The idea of having a french drain between two ponds is one that I have never tried before. It sounds great.

I can`t open your link.
But i stil coment on your idea.
If you bild a french trench the water vill inter conect beetwen ponds, if you put drainpipe(s)in the interchange of water speeds up.
The problem is that you can not empty one pond without draining the other.