The purpose of the Winslow Road – Timber Bridge project in northern Marion, Iowa was to convert a rural county road to a road that will serve a more urban development. The existing gravel surface road crossed Indian Creek at an old pony truss bridge structure.

The one-lane bridge was embargoed for truck traffic, including emergency vehicles, which presented an inconvenience to residents. In addition, the bridge would become inundated during significant storm events, and Linn County would have to close the bridge and road on those occasions.

Over the years, significant storm events caused Indian Creek to inundate the existing road and bridge. These events would close the road for a period of time while gravel was replaced and the bridge was repaired. The design of the new bridge and road needed to take flooding into consideration and provide a bridge and roadway that would handle flooding and remain usable after such an event.

A wooden bridge structure provided the lowest initial cost and a reasonable 30-year ‘ lifespan. It also allowed a smaller profile, which would help keep Indian Creek flowing. A concrete bridge would have had a much thicker profile, which would have caused water to flood upstream.

Besides being more costly, a concrete bridge would have been at a much higher elevation, a factor that was determined to interfere with the objective of continued privacy for the neighboring residences. The new wooden bridge provides an attractive solution that blends into the rural setting.

The most unique factor was that the bridge was flooded by a 500-year storm only one day prior to its completion. A heavy asphalt surface would have kept it in place, but had not been installed. As a result, the wooden frame floated out of position and required repair.

Ament, Inc. acted quickly, turning to the existing contractor to negotiate repair. When an agreement could not be made with this contractor, Ament referred to the list of qualified wooden bridge builders they had gathered from Linn County in the pre-bidding phase.

The former contractor was paid for work to date and relieved, and Ament drew up a new contract with an available, experienced builder. Ament's negotiating skills combined with their design expertise helped them solve this problem cost-effectively and earlier than expected.

Social/Economic Considerations

A wooden bridge on Winslow Road provided a cost-effective structure and minimized the profile of the structure. The small profile was intended to help maintain creek flow. Creek flow was also a factor in selecting a 5-inch asphalt concrete surface, which weighs down the structure when it is inundated. Ament, Inc. also employed mechanical methods to secure the bridge against floating during such an event.

Ament applied hydraulic design principles to the Winslow Road – Timber Bridge project. During a flood, the new bridge allows the waters of Indian Creek to flow over the surfaces of the bridge and road, which will help prevent flooding upstream. The flood surface profile of the bridge provides desirable flow characteristics, and the structure is kept in place by the weight of the asphalt surface and mechanical attachment methods.

Winslow Road – Timber Bridge serves 250 vehicles a day passing to and from Hunter's Ridge residences and a golf course. Now that the road is a clean asphalt surface, more people use the road to drive to and from their homes and the golf course.