Buoyancy Induced Plunging Flow Into Reservoirs and Costal Regions

Abstract

The water in a river flowing into a reservoir, lake or coastal region is rarely of exactly the same density as the ambient water in the waterbody. The density difference may be due to a difference in temperature or in concentration of dissolved or suspended substances. Small density differences can have dramatic effects on the flow patterns that develop in the waterbody. In particular, when the river water is denser than the ambient, I the incoming flow dips beneath the ambient water and flows along the reservoir bottom or beach as a density current. Such flows are termed plunging flows. Figure 1-1 shows a plunging flow situation over a sloping bottom with various aspects of the flow illustrated. The position on the water surface where the flow actually plunges is known as the plunge point or plunge line. It will frequently be delineated by a collection of floating debris held by the reverse current generated in the ambient water by the plunged flow. After plunging, the flow becomes a density current underflow. The dynamics of such currents are reasonably well understood [Ellison and Turner, 1959]. The region surrounding the plunge point and encompassing the transition region between the river inflow and the density current is termed the plunge region. This region can be characterized by its location in the reservoir, as expressed in the case illustrated by the depth at the plunge line H ,and by the amount of mixing that occurs in the region between the inflow and ambient waters. A study of flow in the plunge region with particular reference to these two characteristics forms the subject matter of this report. An understanding of the plunging phenomenon is important from the point of view of water quality modelling, reservoir sedimentation studies, and effluent mixing analyses. Ford and Johnson [1983] review a number of cases in which plunging flow had large water quality implications. The hydraulics of reservoir sedimentation are reviewed by Graf [1983a, b]. Stefan et a1. [1984] describe some effects of plunging on effluent mixing characteristics.