The 25-year-old came from behind in the torridly wet sprint finale, passing Hannah Barnes (Canyon-SRAM) in the final hundred metres. Sara Mustonen (Experza-Footlogix) held off Mitchelton-Scott's Georgia Williams for the final spot on the podium.
It was Sierra's fifth victory of the season.
"I'm incredibly happy for this victory," she said. "It's for sure the best way to end the season.
"Non-stop rain made the race quite hard, I had a puncture few kilometres before the first climb but I managed to come back to the bunch. In the last kilometre I chose to follow Barnes' wheel, I did a good sprint and I got this wonderful victory."
The final event of the UCI Women's WorldTour featured two short bumps between 55 and 40 kilometres to go where gaps formed in the main bunch, but a group of 34 riders joined back together for the long flat run into the finish.
