The July 19 collapse of a bridge on Interstate 10 in California occurred after nearly 7 inches of rain fell, causing floodwaters that exposed design flaws that led to the failure, according to an engineering expert's study.

The bridge was built in 1967. The Tex Wash where the interstate bridge was located was wide and deltalike, so it demanded a longer, stronger bridge. Instead, construction crews narrowed the wash, then built a short bridge with inadequate foundations on both ends of the span, the study says, according to a report by the Arizona Republic newspaper. Crews also re-shaped the wash into a curve. During the July storm, a flash flood hit the bridge's east foundation until the concrete cracked and the asphalt fell into the water.

Abolhassan Astaneh-Asl, University of California-Berkeley"This was not good engineering,” the newspaper quoted Abolhassan Astaneh-Asl as saying. He is an expert on structure failures and a University of California-Berkeley engineering professor who visited the Tex Wash Bridge site in December.

Astaneh and Maryam Tabbakhha, another structural engineer at Cal-Berkeley, are preparing a research paper on the cause of the collapse. They are spotlighting what they say were four "fatal flaws" in the bridge design. The preliminary findings were reported by the newspaper and include:

• Narrowing the wash. The Tex Wash Bridge was built over a flood plain, shaped like a river delta rather than a stream. Construction crews could have spanned the entire flood plain, which had been done with another bridge in the past, but instead narrowed the wash by filling it with dirt, then built a shorter bridge. This design made construction less costly, but it also forced storm runoff into a channel, creating a bottleneck that increased pressure on the bridge.
• Redirecting the water flow. The path of water, which once flowed straight through the delta wash, was forced into a curve that put additional stress on the eastern side of the bridge. Rushing water carved into the dirt berm around the base of the bridge, digging up its foundation.
• No piles beneath foundation. The foundations on both sides of the bridge span were built with no piles anchoring them into the soil. Instead, the foundation sat in the dirt that had been used to narrow the wash, making it susceptible to waters that flowed through.
• No walls to prevent erosion. The bridge foundations could have been further protected by retaining walls that funneled water inward to prevent erosion on the curved wash. Instead, the wash was lined with protective boulders — called "loose riprap"— which were swept away by the flood.

The California Department of Transportation spent around $8 million to build a replacement bridge, which opened September 24. Astaneh was quoted as saying that the new bridge suffers from many of the same flaws as its predecessor. Reconstruction did not change the configuration of the channel and the curved wash, so future runoff may still bottleneck at the east side of the bridge. Underground piles were added to the foundations of the new bridge, but there is no retaining wall protecting the earth from erosion. And Astaneh says the replacement bridge has introduced a new problem — it will float. The bridge is built from long, hollow concrete boxes and similar bridges floated off their foundations during the flooding after Hurricane Katrina.

“They built a boat,” he was quoted as saying while inspecting the new bridge. “If the 500-year flood comes, the analysis shows that it will very likely lift this and carry it away."