Northwest Austin residents worry a particular phase of the Main Street flood mitigation project may negatively affect them in the event of another flood.

However, City Engineer Jon Erichson and two engineering firms think otherwise.

Phase six of the mitigation project aims to put a three-foot wall on the southwest side of the Cedar River near Pizza Hut, with a protected area behind it. That empty space behind the wall is the result of Austin’s acquisition of buildings for the flood project. It will allow for spillage and no damage to homes.

But residents wonder if installing a wall would cause the water to rise faster and cause a bottle-neck effect with the adjacent bridge. They think the water could back up and still flood their homes.

Erichson showed information from the engineers, which detailed possible effects to water height caused by the project. Two different hydrolic models both show water levels rising less than one inch in the event of a 100-year flood (equal to the 2004 flood).

“The worst case scenario, that water will be six hundredths of a foot higher,” Erichson said.

Also, the walls allow for another three feet of water rise above the 100-year event.

“This type of project is the well-accepted standard in the flood protection business,” Erichson added. He mentioned how projects in other cities mirror what Austin plans to do.

One hitch in the proposal is a house the city wishes to buy and remove. Without removing the house, the city can’t connect the wall to a high point. However, Erichson is confident the city will obtain the house.

Of all 12 phases of the project, phase eight is the only completion, which is an earthen berm near riverside arena. Other phases look to raise the road and bridge near Mill Pond, add an “invisible wall” at that location and install other walls in areas along the Cedar River.

The invisible wall is a short wall with tall pillars that allow view of Mill Pond. During a flood, panels can be installed between the pillars to block water.

Funding for the projects is largely in place, as the city of Austin received $8 million in outside funding for the entire project in the last 6 months. The local option sales tax has also been adding to the funds.

“It has exceeded all expectations,” Erichson said. “Those projects are moving forward as we speak.”

The city plans to bid on some portions of the project as early as late April. Others will be bid on in the fall, and the city hopes to complete the entire project by late 2012.

To get an idea what most of the project will look like, phase eight near Riverside Arena is a good place to look, Erichson said. According to him the protection that area has offered thus far has been excellent. “Basically, I would say it was an A plus.”

The Cedar River Watershed District is also eyeing the project proposals closely. It has also hired an engineering firm to model the flooding scenarios, which will add some assurance if its results come back similar to the other engineering firms.