AUGUSTA — Tall grass, weeds and a few small trees have been allowed to grow back on a prominent riverfront granite wharf and parcel of land between the historic Arsenal property and the Kennebec River, prompting the city manager to raise concerns about a highly visible part of the city. As part of a nearly $600,000 project in 2005, the 1,000-foot-long, 17-foot-high rock retaining wall had been reclaimed from overgrowth that all but hid it from view.