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Current INDOT policy requires that culvert-like structures with spans greater than 20 feet be treated for purposes of hydraulic analysis as a bridge, and hence mandates the use of software such as HEC-RAS for predicting the headwater, rather than the culvert-specific software, HY-8. In this context, culvert-like structures are assumed to have a standard inlet geometry (e.g., such as those already modeled in HY8) and a constant barrel geometry. As larger-span culverts with spans greater than 20 feet have become more readily available, they may provide a cost-effective alternative to traditional bridges, and the technical basis of the current INDOT policy needs to be re-examined, and more flexibility in allowing conventional culvert hydraulic analysis to be applied to structures with spans larger than 20 feet may be warranted. The main aim of the present study was a comparative assessment of conventional culvert hydraulics, specifically as implemented in HY-8, and bridge-hydraulics modeling, as implemented in the HEC-RAS bridge models, in applications where the water-encapsulating structure has a large span relative to its streamwise length. Laboratory experiments were performed with model box culverts of span 1.5 feet and two streamwise lengths, 2.1 feet and 8 feet, and performance curves describing the variation of headwater with discharge were obtained. The effects of bed roughness, the presence or absence of a cover (if present, the rise was 0.5 feet), and a range of tailwater levels were investigated. The laboratory observations were compared with predictions by HY-8 and HEC-RAS models, and the model performance was assessed.

Assessment of HY-8 and HEC-RAS Bridge Models for Large-Span Water-Encapsulating Structures