Experimental Study of Flow Through Submerged Artificial Vegetation

The presence of vegetation in rivers, streams and riparian zones affects significantly the flow field and consequently the resistance, pollutant dispersion, sediment transport and ecological habitat. Despite a large amount of previous research, the understanding and quantification of the complex interaction between vegetation and flow field still presents many challenges. For example, there is no systematic assessment of the effect of the vegetation elements geometry, density and submergence on the flow field. In this paper, an experimental study is carried out to investigate the effect of an array of compound rigid or semi-flexible elements, resembling submerged small plants, on key features of the flow field, namely the velocity profiles and turbulent stresses. The experiments were conducted in a laboratory flume 16 m long and 0.50 m wide with slope 1‰. The artificial vegetation elements were placed in the middle part of the flume, of 8 m length, where the flow depth was nearly constant, controlled by a sluice gate. Three types of elements having common parts were used: (a) Thin rods 0.5 cm in diameter and 4 cm high; (b) Compound rigid elements consisting of 3 cm diameter spheres fixed on top of those rods; (c) Compound elements consisting of a set of flexible (plastic) needles adjusted axisymmetrically on top of those rods. For each type of element, two patterns of placement were tested, on a regular and on a staggered 10x10 cm mesh, yielding respective plan densities of 100 and 200 stems/m². Measurements were taken by means of a 3-D ADV instrument at selected locations within the vegetation array and downstream of it. Vertical profiles of the (temporal mean) velocities, turbulent fluctuations and turbulent stresses were obtained and are presented in dimensionless plots. Comparison of the results shows the significant effects of the element geometry and pattern of placement on the flow characteristics.