Milroy1812 is the original mapper and might have more insight. It looks like he has mapped a number of spillways at local dams.

I don't think this is/was the primary spillway but perhaps the auxiliary / emergency spillway? Milroy1812 also mapped a spillway for Site 7, which appears to be an auxiliary spillway, though it isn't explicitly labeled as such. It's worth noting that their mapping of the other spillway doesn't seem to entirely match the designed flow from what I can determine via media reports from when it was constructed but I'm no expert.

For this site, I found a reference in Upper Brushy Creek WCID's "Dam Portfolio Prioritization Summary Report" that reads "the City of Cedar Park constructed the Buttercup Wastewater Interceptor and Reclaimed Water Line through the auxiliary spillway. The line was installed within the auxiliary spillway and the top of the excavation was covered with a 2 foot concrete cap." I reviewed GIS data from Cedar Park and found the engineering plans for the project. It suggests this feature as mapped is at least somewhat or in part correct as it intersects with the aforementioned lines. 

Perhaps we can reach out to the city and/or Upper Brushy Creek WCID. 

Also, see diagrams at https://www.ubcdams.org/193/Typical-Dam-Design for what we should be looking for typically.