Question

The following passage is adapted from an essay on nineteenth-century urban infrastructure:

In the early decades of the nineteenth century, New York City faced a severe water crisis triggered by rapid population growth and frequent cholera outbreaks. By the mid-1830s, the municipal government committed to constructing the Croton Aqueduct, an ambitious engineering project designed to channel fresh water over forty miles from Westchester County to Manhattan. Chief Engineer John B. Jervis oversaw the construction of the masonry conduit, which relied entirely on gravity flow to transport water.

To maintain the necessary gradient of approximately thirteen inches per mile, workers excavated extensive tunnels through solid rock and built elevated stone bridges over low-lying valleys. The interior of the horseshoe-shaped brick conduit was lined with hydraulic cement to prevent leakage and soil infiltration. At its terminus in midtown Manhattan, the water emptied into two massive reservoirs: the Receiving Reservoir at 79th Street and the Distributing Reservoir at 42nd Street. The Distributing Reservoir, styled after Egyptian revival architecture with granite walls forty feet high and four feet thick at the top, held over twenty million gallons of water.

A critical yet less frequently documented component of the aqueduct's operations was the installation of intermediate waste-weirs positioned at key points along the route. These heavy iron gates, controlled manually by stationed keepers, served to divert excess water into adjacent natural streams during periods of torrential rainfall or scheduled maintenance. According to Jervis’s official 1842 operational guidelines, keepers were instructed to open the waste-weirs only when water levels within the main conduit exceeded five feet and seven inches, a threshold calculated to prevent hydraulic pressure from breaching the brick lining. Furthermore, routine inspections of the conduit required keepers to record interior temperature readings twice daily, at 6:00 AM and 6:00 PM, using brass-encased thermometers suspended from inspection hatches.

While popular accounts often attribute the aqueduct's success solely to its impressive stone arch bridges, such as High Bridge over the Harlem River, architectural logs reveal that the iron pipe siphon across the Harlem valley was actually the component that delayed the system's formal opening from May to October of 1842. The initial design called for a low-level bridge supporting two 36-inch iron pipes, but state legislative amendments passed in 1839 mandated that the crossing accommodate river navigation, forcing Jervis to revise the plans to construct a high-level bridge 114 feet above high tide. Until High Bridge was completed years later, a temporary 90-inch iron pipe laid across a temporary timber trestle supplied Manhattan with its first continuous flow of Croton water.

According to the passage, waste-weir keepers along the Croton Aqueduct were explicitly instructed to open the diversion gates under which of the following circumstances?