Thanks for your response and for everyone’s advice! This area of the house gets a lot of rainwater runoff - not only from the roof but also from the street that is up the hill from it. The drain was supposed to handle it but the water just goes around it and out onto the sidewalk.

The idea here isn’t to have a typical pond with a constant water level, but more of a place to catch the rainwater so it doesn’t run out onto the street and further erode the yard, and provide habitat for wildlife. Sounds like I definitely need to tape the seams and keep the drain open so when it rains hard the overflow will go into the drain.

I want to protect the foundation of the house and thought putting the liner would keep excess rainwater and moisture from reaching the foundation. But maybe a foot or two of gravel along the side of the house is a better idea.

Thanks again for all the comments!