A couple of days after getting back to Seattle, I was throwing out some old stuff in my cubicle and ran across a 2" thick report I had obtained in the mid-1990s that described in detail the geology and engineering of the Lake Michigan shoreline of Milwaukee County. Aerial photos show that this beach was constructed in 1988 (Photos from Southeastern Wisconsin Regional Planning Commission, 1989).
Like other Great Lakes cities, much of Milwaukee's shoreline was built out into the lake with fill, the better to create marinas, public facilities, and recreational space. As a result, beaches tend to be highly engineered pocket beaches, often shaped by groins, adjacent jetties, and offshore breakwaters. McKinley Beach is part of a segmented headland breakwater system - there's a longer beach a short distance farther north (see aerial below).
The gap in the riprap constrains the direction that waves can approach the beach and the breakwater prevents sand and gravel from escaping the confined pocket. Regardless of the direction with which waves approach (the big waves come mainly from the north on Lake Michigan), they pass through the narrow opening and break in regular curves on the crescent-shaped beach.