Temple University's new precision floodplain mapping techniques reveal that more homes and people are at risk across the Philadelphia region than the federal government previously believed.

Most of the federal government's more than 100,000 floodplain maps were drawn from data collected with primitive technology more than two decades ago. The Federal Emergency Management Agency (FEMA) has begun revising the maps, but is several years away from finishing. So FEMA jumped at the chance when in 2002 a team of Temple University scientists offered to take on a piece of that job in one of the Philadelphia region's most flood-imperiled sectors, the Pennypack Creek watershed. The remapping was completed this summer. Temple's scientists found that high-risk floodplains had grown by 24 percent, and encompassed 708 mostly residential buildings, an increase of 131 buildings. Some previously mapped plains had shifted by as much as 400 feet. The fresh portrait also included 47 miles of newly plotted tributaries.

The biggest surprise, however, came from FEMA. After seeing the finished product, the agency told Temple that the maps were too precise to be adopted by the federal government. The agency has asked Temple to redo its calculations, bringing the charts into line with FEMA's longtime standards for floodplain mapping. The maps' quality must be uniform across the country because they are the foundation of the National Flood Insurance Program, which FEMA administers.

FEMA maps, for instance, typically do not show small floodplains or tiny tributaries given to only minor flooding. FEMA maps are also based on the assumption that most culverts and storm drains are clear of debris and free-flowing. In walking the length of the Pennypack, Temple researchers found most culverts clogged or undersized, and counted them risk factors. "We're clearly on the more protective side," said Jeffrey Featherstone, the university hydrologist and planner who headed the project. He said he recognized that such pinpoint mapping would have vast implications for the shaky National Flood Insurance Program, created by Congress in 1968 for residents of floodplains. FEMA officials "don't want to deal with the consequences of... exponentially increasing the number of people" who could take out policies and submit claims, Featherstone said.

He and his colleagues are complying with FEMA's request to redo the maps. But they are not shelving their original work. At an upcoming meeting, they will present the unaltered versions to officials from Pennypack watershed townships and boroughs, which had chipped in $70,000 to the project. "We'll encourage everybody to adopt the maps," Featherstone said, even if FEMA doesn't. http://www.philly.com/mld/inquirer/15592472.htm