Please tell me more about the catchbasin. Is the primary purpose to reduce the churning in the main pond? Haven't seen or heard that discussed much in my research, but it sounds like a good idea.
My original design had 3 goals.
1. Reduce surface ripples. I noticed that whenever I went to someone's house with a Koi pond or to buy Koi the owner would shut off the waterfall so we could see the fish better. The more still the water the better the viewing. I saw one pond with a floating rope around the falls which calmed the surface and that's where I got the basic idea.
2. I wanted to get water current to the bottom of the pond to push low O2 water to the surface. Waterfalls aren't very good at reaching the bottom (depends on flow and shape). By making a container (the basin) I could place a couple of outlet pipes right at the bottom so 100% of the water I was paying to pump did double duty.
3. I could fill the basin with fairly large rocks and get a decent, 100% maintenance free, bio filter for nothing. The incoming water had good O2 and the flow kept the rock surfaces free of muck, both of which nitrifying bacteria need. The outlets being at the bottom meant the bio filter was flushed 24/7.
After the pond was running I found I'd lucked into a couple of more bennies.
4. The outlets at the bottom swept the bottom clean for about 4' out all around the bottom of the catch basin and pushed stuff to the bottom drain. This was before TPRs were well known.
5. One morning I come out to the pond and there's a big pile of foam in the catch basin. The basin was acting as a foam fractionator and I just netted out the foam for a couple of days and no more foam. So it helped reduce DOC levels. Without the catch basin I would have had foam floating all over the pond surface and the foam (which contains dirt) would have stayed in the pond. It was nice to know I would never ever have any foam at all floating on the surface. Not something you think about until you have a problem. I really don't like foam, even in amounts other people find acceptable. Here's a video of a pond with a foam fractionator and to me, for my tastes, still don't like the remaining foam on the surface.
For about $20 in materials and paying once to pump water I got a waterfall, a bio filter, a foam fractionator, still surface and TPRs. Me likey.
In a future pond I'll expand the concept. I'll play around with bio media maybe creating a moving bed filter in the basin. I'll expand the TPR concept so I can increase the size of the falls by adding the water that would normally be used for just the skimmer and TPRs. So maybe 30-50% more water volume for almost free (more head).