How do submerged plants affect the velocity of streams?

Posted on September 24, 2012

A knowledge of the vertical distribution of velocities in flowing streams is essential if flow patterns and discharge rates are to estimated accurately.  However, complexities caused by the presence of submerged aquatic vegetation in channels represent a significant challenge when attempting to predict velocities at different depths in the water column.  Circulating flumes have been employed to study the influence of plants on velocity distributions, but in most cases non-aquatic plant species, or artificial plants with unrealistically low flexibility and streamlining properties, have been used.  Also, the equation systems traditionally used to model velocity don’t accommodate the fact that the maximum stream velocity is commonly reached at a point below, rather than at, the water surface.   Taiwanese civil engineers have dealt with these problems by measuring velocities in a flume planted with a real aquatic plant species (Egeria densa, Dense Waterweed) and by developing a new velocity equation system based on probability theory.  In their flume experiments, as  plant density increased the vertical velocity distribution became more S-shaped and velocities within the weed bed near the bottom of the flume approached zero.  The new method predicted the measured vertical velocity distribution very accurately and reliably.  In contrast to previous approaches, the new equation can be applied to all depth zones, rather than having to use different equations for the plant layer, the transition zone and the  upper layer of free water.

Reference:  Yen-Chang Chen and Su-Pai Kao  2011.    Velocity distribution in open channels with submerged aquatic plants.  Hydrological Processes 25, 2009–2017.