Every time it rains, Wilkins Manager Rebecca Bradley worries.

Heavy rains from Hurricane Ivan in 2004 and another storm last year left parts of the borough flooded and residents wary of every storm.

"We're back on our feet. But when we have a severe rain, we all hold our breath," she said. "It's not a question of if, it's a question of when."

Some parts of Wilkins are in the lower-lying sections of the Turtle Creek Watershed, the area where storm water flows into Turtle Creek and its tributaries.

The watershed stretches from Salem and Greensburg westward to Wilkinsburg and North Versailles and also spreads north. It encompasses 33 municipalities and two counties.

In recent decades, residents in the watershed have noticed an increasing frequency of floods and a rise in their severity.

Now, for the first time in five decades, the Army Corps of Engineers is going to conduct a comprehensive study of the watershed to try to figure out why flooding has increased and what can be done to halt the increase and mitigate current flooding problems.

The study was authorized by the U.S. House of Representatives' Committee on Transportation and Infrastructure in late September at the request of U.S. Rep. Jason Altmire, D-McCandless. Money for the study has not yet been allocated.

Many believe that urbanization upstream has increased the amount of storm water runoff downstream, creating more flooding.

"Changes with development and urbanization in the watershed ... may be accelerating storm water runoff," said Curt Meeder, chief of the planning and environmental branch in the Pittsburgh area division of the corps.

The creation of more impervious surfaces, such as concrete and asphalt, means less of the water is being absorbed into the landscape and more is being diverted to creeks and streams.

Jim Pillsbury, a hydraulic engineer with the Westmoreland Conservation District, said development could triple the amount of storm water sent downstream.

The region gets about 40 inches of rain a year. In an undeveloped area, such as a field or a forest, about 10 inches of it will run off and the rest will be absorbed.

"In a heavily developed area, where there's lots of houses, streets ... you might get as much as 30 inches running off," he said.

The Turtle Creek Watershed Association, the group that advocated for the corps to do the study, wants to change the thinking about storm water management.

Rather than treat that water as waste, the association would like to see it used as a resource.

Currently, storm water is diverted to streams and channels, sometimes lined with concrete to speed delivery. This traditional storm water management approach is part of the "structural" method.

"The structural approach is part of the traditional-rainfall-as-a-waste-product attitude and that's not the way we really should be looking at it," said Diane Selvaggio, executive director of the organization. "It's been demonstrated that it hasn't worked quite as well as we had hoped."

Today, thinking has shifted to support a "non-structural" approach. Rather than trying to control and divert runoff, a non-structural solution aims to create less runoff by filtering more of the water back into the water table.

Significantly, Congress last year authorized the Army Corps of Engineers to consider non-structural solutions to flooding problems.

The corps has traditionally focused on structural solutions, such as levees, flood walls and concrete-bottomed channels, to deal with the runoff downstream. But in the study, it will be able to explore more preventative solutions.

"The bigger question is ... are there ways of controlling the flows and the runoff in the upstream areas of the watershed?" he said.

The Turtle Creek Watershed Association advocates for sustainable methods that put water back into the ground. These include green roofs, which are roofs containing a layer of soil and a small garden to absorb most rainfall, eliminating runoff from rooftops altogether.

Also, forgoing impervious asphalt for porous concrete also can significantly reduce runoff and allow for more storm water to enter the water table.

The most powerful mechanism, though, said Larry Larson, executive director of the Association of State Floodplain Managers, are the political ones. Ordinances can limit where development occurs and mandate a maintenance of green space as well as prohibiting developers from dumping all of their storm water into streams, creating problems downstream.

"Let's not make the problem worse," Mr. Larson said. "That means throughout the watershed you develop policies ... so that whatever adverse impact that development would create -- including runoff, erosion -- are mitigated."

In 1990, the state mandated all municipalities in the watershed develop storm water ordinances. But a lack of continued enforcement and maintenance weakens their effectiveness, said Mr. Pillsbury, and because they were enacted only about 18 years ago, they do not address development that occurred before the 1990s.

He said it's time for them to be re-examined, which the corps could explore in the study.

It can be difficult to get the 33 communities in the watershed to work together, said Ms. Bradley, the Wilkins manager. Though new ordinances have been discussed within the Turtle Creek Valley Council of Governments, which includes several watershed communities, municipalities are not mandated to change their ordinances.

"But at the end of the day, all of the municipalities would have to enact their own ordinances," she said.