As the river rises, water saturates the sandy soil and brings the water table closer to the surface, Nelson said. As the water table rises above the level of basement floors, groundwater finds its way through foundation cracks resulting in flooded basements, first in low-lying areas near the river, then spreading with the rise in river level and the duration of the flood. The same process led to the Front Street collapse and two similar problems - one behind RTP and one at a residence on East Fourth Street, Nelson said. He said that where cracks or holes occur in a sewer line lying beneath the elevated water table, groundwater flows into the sewer, carrying with it tiny grains of sand creating a cavity around the leak in the pipe. When the cavity reaches a certain size, the overlaying ground or pavement caves in, he said.
Infiltration continues to raise water levels behind the Prairie Island dike.
The rising waters could submerge the airport runway, but buildings should stay dry. Water is seeping through dikes at several locations as well. Along the Prairie Island Road, just beyond the flood gate, clear water can be seen trickling over saturated earth. Dike watchers are keeping a close eye on areas where seepage is occurring. If the seep water becomes muddy or carries sand, city crews will sandbag the area to ease the pressure on that section of the dike.