Bridge building process needs significant improvement

By Marv Rops

----

The Gazette's recent and past articles about bridges have been interesting and informative. These structures represent a significant part of vehicle transportation infrastructure costs.

The focus of articles is usually on their structural ability to each day get vehicles safely across waterways. There is often little mention of their safely getting less frequent floodwaters through them without their causing damming and flooding. Experience shows that a too-small bridge can quickly cause millions of dollars of flood water damage in just a few blocks of residential or business area.

All bridges in this area cause concern about whether that process is rigorous enough. There are many related details that could be mentioned but here are just a few of the major items:

1. The Cottage Grove/Indian Creek bridge was designed for 10,900 cfs. (cubic feet per second), but the 2002 flood was 24,400 cfs. There is stonewall resistance to discussing this. If that is not a problem, then perhaps the Cedar River flood protection can be designed for 58,100 cfs. rather than the reported 130,000 cfs. that occurred. The proportion is the same.

2. Why does the new East Post Road/Indian Creek bridge have 24 feet (41 percent) of its 58-foot width devoted to extremely costly sidewalk? There is no sidewalk south of this bridge. The old bridge had about 6 feet of sidewalk. What has changed in this unchanged neighborhood that requires four times greater sidewalk width? The bridge width may be justified for some hidden unspecified future need, but it is unjustified as sidewalk for the estimated one pedestrian per hour who crosses the bridge.

3. The Otis Road/Indian Creek bridge is well designed and appears very cost effective compared with the others. It is what might be called a rural design because it has no sidewalks. There have been groups of six to eight people, including children, either sitting in lawn chairs or standing by the bridge while fishing. Otis Road probably has a low traffic count and there is a trail bridge upstream.

The adjacent Nature Center has reported 41,000 annual visitors, including 13,000 school children. There would also be late-afternoon, evening and weekend visitors that are not counted. It appears that there is need for a sidewalk on the south side of this bridge and if not now, then certainly within the future 50-year life of this bridge.

The preceding few examples show the need for changes in the management of how bridges get designed and approved for construction.

Marv Rops is a retired consulting civil engineer with many decades of engineering design and project management experience. Comments: (319) 362-4695.