From an email from a City staffer explaining it:
The work near the Lesner Bridge is indeed maintenance dredging of the Crab Creek channel that serves the Lynnhaven Inlet Boat Ramp and Beach Use Facility.

This channel system is subject to constant shoaling and requires dredging on an annual basis to maintain access to the boat ramp.  Dredging will be complete by February 28, site restoration and full demobilization of the equipment will be complete by mid-March.