The only earth pond drain system I have any experience with is just an overflow pipe.
Clay will seal around a pipe. I don't think encasing the pipe in cement helps since you still have the clay to cement bond, same issue, just larger diameter. Cement is used where there could be erosion from moving water. Search for "water scouring".
And cement is poisonous right?
Cement is not toxic.This is why I get a little peeved with myths. For backyard hobby ponds it's not that big a deal that these myths get repeated over and over. But then someone reads the volumes of text repeating the myth and it's taken as fact. And you're trying to do something good for the world and have to deal with misinformation.
The myth says cement will raise pH to levels that would kill fish. Although after being repeated enough I have seen people morph it into cement being a poison. The idea is lime is used to create concrete, lime raises pH, therefore concrete must raise pH. Believers don't want to be bothered with any actual research. Search for cement used in waste water systems. Sewage is very acidic, and flowing through concrete pipes would erode the concrete. So there are a lot of studies that look at pH levels and effects on concrete. The bottom line is as you get below 6 pH the concrete doesn't last as long and these things are really expensive to replace. Ponds aren't kept at low pH. Concrete does react at pH under 14, but is so slow it isn't really an issue. We talking the concrete lasting 100 years instead of 200 years type of deal.
It certainly doesn't raise pH in any measurable way in normal ponds. There are thousands of concrete ponds around the world. Been used for decades. A very easy myth to test and disprove. Not that this myth will ever go way. Luckily the myth is spread way less than it was 10 years ago.
Not to drive you away...but there are other online forums for earth ponds and also for growing catfish, shrimp and tilapia. Many people from around the world too who might understand your local conditions better. I'm strongly thinking of raising tilapia in my next pond so I've been researching in those forums for the past couple of years. They're pretty no nonsense because it's their livelihood. They don't really like myths and have serious in depth discussions on whether something works or not. In hobby pond forums you get a lot of repeated things people heard some where and very unscientific opinions that can be wildly wrong. It does no harm here and seems to be great fun. But you're looking at spending a lot of time and money, so misinformation would be very expensive.
