Control domain of circle to extract sub-surface from pipe-variable

I am trying to extract the lower half of a circle using the constructdomain2 command.

The context of this is that I am modelling a stream in a topographic model. The stream bed has been modelled as a negative of a pipe-variable (topographic model with pipe subtracted from it). I am now looking to ‘fill’ this with water which is to be modelled by the same pipe, but with the waterline just below the edges of the stream (using Isotrim command to remove the top half of the pipe).

Thus, I need say, the bottom two thirds of the pipe. This is well explained in this video: https://www.youtube.com/watch?v=t8ixuJ2t9q8&ab_channel=RhinoGrasshopper

It says that a domain of 0 - 0.5 will extract the bottom half of the circle. I would like a subsurface of approx 0 - 0.4 (so that the water line sits below the edges of the stream). Inputting these numbers as a domain works except that I can’t control where on the circular section of the pipe this domain starts.

How do I control where the domain starts on the circle?

Thank you.

At least share internalized shape, check seam, or you can use sweep surface

Thank you for replying. The trimmed surface you have produced is ideal, except that I would like the inverse of it, so the bottom half, I would then like to patch the rails to create a solid shape that mimics water in the stream.

Can you show how I could retrieve the same shape, but the bottom section, as per image 3 of my original post?

Thank you.

Thanks again. See, the axis that runs across the circle from 0.5-1.0 is on somewhat of an angle, compared to yours. Do you think this is to do with the long, varied pipe-variable geometry mine is based on?

I told you to check seam and it is impossible to know from images !

How do I check seam?

Seam is the start point of the circle if you used a circle to create pipe

No, I used a base curve which is on the XY Plane, and then the pipe-variable command.

Share your file , it’s better to check it and find a solution

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Sweep is probably the easier option for this case since you have control on the orientation of the section planes.

I added a waterstream and a Galapagos optimization for the height of the stream.

The definition could be tweaked for non circular, varying cross sections.

20_10_25_halfpipe_waterstream.gh (30.1 KB)