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Ground is scheduled to be broken today on a project to add a 90,000-square-foot Walmart store in North Portland, and company and city officials are excited about what will be on top.

鈥淚t’s going to be the biggest green roof in the state,鈥� said Rachel Wall, senior manager of community affairs for Wal-Mart. 鈥淲e’re proud to incorporate (it) as we continue to strive to create a more sustainable world.鈥�

The 40,600-square-foot green roof will be nearly 10,000 square feet bigger than the city’s largest 鈥� atop the Ramona apartment complex. Additionally, it will have only three parts around 13,000 square feet each; comparatively, the Ramona roof has seven sections around 4,000 square feet each.

The difference is significant in regard to measurement of water runoff, according to officials with the Bureau of Environmental Services; it will use the roof to conduct a three-year performance study. Tom Liptan, a landscape architect with the BES and an eco-roof expert, admitted that he was surprised when the company contacted the city about the project. However, he believes it presents a big opportunity.

鈥淚f it is indeed this big, it would give us an opportunity to measure stormwater management performance on something we really don’t have,鈥� he said. 鈥淲e don’t have anything near it.鈥�

Each of the green roof’s three sections will have different soil levels to let the city determine which level is most effective in reducing stormwater runoff. The results of the study could affect construction costs, Liptan said.

鈥淭he thicker the soil, the heavier the weight and the more structural upgrading to hold the weight,鈥� he said. 鈥淭hat means more money for building.鈥�

A number of organizations, including the Audubon Society of Portland and the Urban Greenspaces Institute, were involved in the roof design process. Because of its size, the roof has potential to generate more data than for just water runoff, said Mike Houck, executive director of the institute.

One of the sections will feature a rougher soil type, which will provide information on what sort of plant life can survive in it. Additionally, certain sections will have different habitat features, such as water basins or compost piles, which could help attract animals.

鈥淚’m personally hopeful at some point we might be able to provide some habitat for some wildlife, certainly for invertebrates,鈥� Houck said. 鈥淵ou might get some meaningful value above and beyond the stormwater.鈥�

Wal-Mart has done only one project like this before, in Chicago. The company chose Portland as the next location for a green roof because of its moderate climate, Wall said.

Wal-Mart earlier this year officially announced it would join the Hayden Meadows office-retail complex, near Delta Park. Two vacant buildings 鈥� one that had never been occupied 鈥� will be demolished to create space for the new building, Wall said.

Area residents support the project because it will provide jobs and convenience, according to Maryhelen Kincaid, land use chairwoman of the East Columbia Neighborhood Association.

Kincaid was somewhat surprised that the project encountered little resistance from residents; she had noticed hostile reactions to other Wal-Mart proposals in Portland. Some people voiced concerns over the traffic implications of this development 鈥� no public transportation is provided to the complex 鈥� but overall the process has been smooth, she added.

鈥淭here’s not the huge outcry that you see in other neighborhoods,鈥� she said. 鈥淚 believe that most (neighbors) have just said, 鈥業f you don’t like Wal-Mart, then don’t shop there.鈥�

The green roof played a factor in residents’ support, Kincaid added.

鈥淲e heard it’s state of the art and will provide some environmental benefits,鈥� she said. 鈥淥verall, we’re excited for the project to be happening.鈥�

The store is expected to open shortly after construction wraps up 鈥� probably in fall 2013.