Differences between Open-Channel and Culvert Hydraulics: Implications for Design

Sediment transport through culverts is poorly understood. Current design methods for culverts are generally based on the characteristics of open channel flow. A careful analysis of previous studies reveals significant differences between open channel flow and flow through a culvert. The purpose of this paper is to identify and categorize the main differences between open channel flow conditions and flow through a culvert embedded in native substrate. The differences in flow characteristics are most easily identified when separated into four categories: geometry, sediment/debris, bed integrity and aquatic life. The geometry of a culvert creates conditions unlike the natural channel in high discharge conditions. A culvert could experience pressurized flow or orifice flow and often needs an entrance loss coefficient to accurately predict flow. There are also differences in shear stress when a streams aspect ratio is decreased by a culvert. All culverts will eventually constrict flow to a smaller area than found in non-incised open channels. Under these conditions, an embedded culvert will often be submerged at the inlet and unsubmerged at the outlet, exacerbating the already constricted flow conditions and resulting in more scour at the entrance and exit. This also can create deposition or floating debris issues at a culvert entrance. The bed of an open channel is much different than in an embedded culvert due to the integrity of the bed structure. A bed placed in a culvert cannot mimic the actual structure of a stream bed because of the sediment sizes and configuration. This causes the bed to be more susceptible to interstitial scour and in some cases can cause the entire fish passage design flow to travel through the bottom of the bed seemingly disappearing while in the culvert. Because culverts are placed in streams, they can potentially cause biological longitudinal continuity issues. These issues are very complex and need to be studied from multiple disciplines. The lack of light in long culverts could prevent certain vegetation from growing and the shade of a culvert has been shown to be a deterrent to some fish species. Knowing the differences between open channel flow and flow through a culvert is crucial as a starting point in design and as a background for further research. By compiling and categorizing a current list of differences, we can begin to understand the complexities of creating functioning culverts as well as researching solutions to minimize these differences. This is especially important in the quickly emerging field of culvert design for aquatic organism passage, where creating conditions within the culvert barrel that mimic those in the open channel is critical to the design procedure.