Step-Pool Morphology in High-Gradient Countersunk Culverts

Abstract

Bed stability and morphology in countersunk culverts on steep slopes were examined to improve understanding of the parameters governing sediment flow characteristics in mountain streams. The knowledge gained was used to provide preliminary construction guidelines for the stream simulation approach of countersunk culverts, an approach that is under consideration in Washington State. Prototype conditions were evaluated in a flume for a 30 percent countersunk culvert with slopes ranging from 3 percent to 7 percent and particle relative submergence varying from 0.5 to 2.0 for three bed size distributions. The experiments were designed to satisfy the conditions of dynamic similarity, and they are preferred to field measurements because they allow a high degree of control over testing conditions. It was found that step-pool bedforms are the most ubiquitous features along the culvert gravel bed. A new formula was developed that correlates step height with the gravel-bed size distribution, relative submergence of the particles, and the Froude number. The step spacing was found to be related to step height and streambed longitudinal slope. This information was combined into a generic design method for streambed simulation of high-gradient countersunk culverts. An example case that illustrates the application of the newly derived formulas to the construction of a gravel bed for a countersunk culvert is provided.