Jury Verdict: Frequency Versus Risk‐based Culvert Design

Abstract

A federal district court jury in Cheyenne, Wyoming, ruled that an Act-of-God defense could not be used when a railroad culvert backed water onto residential properties during a catastrophic “10,000-year” rainstorm. The culvert capacity not only met by exceeded industry design standards, yet the jury concluded that the railroad was negligent for not installing a structure large enough to accommodate the “monster” storm. The 5-ft-diameter culvert, installed to replace a larger, deteriorating trestle, is sized by frequency-based methods to discharge the 50-year peak flow rate, with a nominal surcharge anticipated during the 100-year event. It is agreed by both sides that the selected structure could have safely passed more than a 500-year event, but is not able to discharge all the flows from the freak storm. Backwater ponded 9 ft over the top of the culvert, flooding more than 20 basements in one subdivision. The defendant is held liable for the damages and cited as negligent for not anticipating the storm and for failing to consider the risks to the homeowners. Under court order, a larger structure is installed. This paper is a review of several matters debated during the trial and in subsequent analyses, presented as a case study from the writer’s perspective. The writer participated as an expert witness who performed the analysis for the defense, but was called to the stand as a witness for the plaintiff. The jury verdict was in favor of the plaintiff. Implications for improvements in drainage design and expert testimony are discussed.