Modeling Steep Channels with Lateral Inflow

Abstract

The main concern in the design of high-velocity channels is the depth of flow in the channel for the design discharge. The depth must be known to determine sidewall heights and minimum bridge soffit elevations. Determining the depth of flow is complicated by side inflows and boundary features such as contractions, expansions, curves, and obstructions to the flow such as bridge piers, and vehicle access ramps. These boundary features in a supercritical channel, cause flow disturbances, which can result in a significant increase in the local flow depth. An accurate prediction of the water surface shape (i.e. variations in local depth) is essential in the successful design of a high-velocity channel. Confluences of supercritical flow are complicated by the fact that standing waves are generated at any and all boundary alignment changes. Peak flows in the side channel and the main channel may not occur at the same time during a storm event. Therefore, analysis and design must be conducted at all probable combinations as well as at the design flow conditions. The design flows may not produce the largest water-surface elevations because large standing waves that result from unequal water-surface elevations can result in locally high water-surface elevations. The confluence addressed in this paper is the case where culvert flow is introduced into the main channel in a lateral. These storm-water drains are usually located near roadway intersections where the right-of-way is limited and the utilities are numerous. These place constraints on geometric flexibility in routing the culverts. The culvert discharge is only 10 percent or less than the main channel flow, but the momentum effects can produce significant bulking of the main channel flow. That is, locally, the main channel water-surface elevation can be raised due to the introduction of lateral culvert flow. An understanding of the flow conditions in the vicinity of laterals is essential in the economic design of these structures. This paper presents a two-dimensional (2D) model applied to supercritical flow with lateral inflow from a pressurized culvert. Model results are compared with laboratory data.