A portion of the Northbank Riverwalk in front of the Hyatt was damaged over the holiday weekend, and no one seems to know what happened. City spokeswoman Kortney Mosley said the 20-yard portion of the Riverwalk has been closed. “It will require extensive work and we are looking into the problem,” she said. “We are looking for divers to go down and see what happened, but with the holidays that might be impossible. We have blocked access to that portion of the Riverwalk and have checked the whole length of the Riverwalk for other damage.” A 10- to 15-foot wide section of the Riverwalk has cracked and fallen about 6 to 10 inches, creating a dangerous obstacle for walkers and runners. Mosley said engineers will be brought in to help determine what happened and how it can be fixed.