Scour and Riprap Protection in a Bottomless Arch Culvert

Bottomless arch culverts are employed as ecological passageways at road crossings. Such culverts are commonly used for fish passage as the adjacent natural channel reaches can be simulated and habitat barriers minimized (i.e. high base flow velocities, perched culvert outlets, etc). Though they have been developed to be environmentally sensitive, there is limited understanding of the hydraulic characteristics of bottomless arch culverts. To provide some design guidance specific to streambed stability in culverts, a streambed stability study was conducted using a smooth walled, 0.61-m (2-ft) diameter circular bottomless arch culvert and featured four different streambed armoring materials with the d₅₀ ranging in size from 6.75-mm to 37.3-mm. Several culvert entrance configurations were tested with headwater depths ranging from unsubmerged to submerged inlet conditions. Experimental data were used to evaluate eight published riprap stone-sizing stability relationships for potential applicability to streambed stability design in bottomless arch culverts.