Experimental Study of Vertical velocity profiles in compound channels with vegetation on floodplains

Introduction:
Vegetation has traditionally been viewed as a nuisance and obstruction to channel flow by increasing flow resistance and water depth. However, in recent years, vegetation has become a major component of erosion control and stream restoration.
Most of research efforts focus on describing vegetation roughness , determining drag coefficients and empirical formulas for resistance under various vegetation configurations. While the development of experimental solutions for vegetative resistance is important, it is also important to understand the detailed characteristics of flow through vegetation.
Yang et al.(2007) conducted flume experiments with different types of vegetation, and found that, in the cases of non vegetated floodplains, all measured streamwise velocity distributions followed the logarithmic distribution, but for vegetated floodplains, they followed an S-shaped profile
Nezu and Sanju(2008) have investigated turbulence structures and coherent motion in vegetated canopy open-channel flows. They divided the whole flow region into three sub-zones, i.e., the emergent zone, the mixing-layer zone and the log-law zone.
In the present study, some experiments were undertaken herein under different conditions to elucidate the flow structure. The main focus is to examine how the vertical velocities, are affected by simulated vegetation arranged in emergent and submerged conditions. In addition, the effect of dowel density, configuration, and relative depth are examined.

Methodology:
The experiments were conducted in a fixed bed rectangular flume, 9 m long and 0.6 m high and 0.8 m wide. The slope of bed flume was 12 ×10-5. The main channel and floodplain had widths of 24 and 28 cm, respectively, and the main channel had a side slope, s, of 0. The bankfull height, h, was 6 cm. Vegetation were simulated by wooden dowels. The wooden dowels were 140 mm tall and 7 mm in diameter. The dowels were attached to a PVC sheet bolted to the bottom of the flood plain in linear and staggered arrangement. The spacing of the dowels varies from 2.5-10 cm in both lateral and streamwise directions forming stem density of 0.41, 1.64%, 6.04%. The flume was operated under a uniform flow condition, and measurements of discharge, point velocity and flow depth were taken. Flow depths were measured by means of a pointer gauge, discharges were measured by a digital flowmeter, installed upstream of the channel, and a micro propeller current meter were used to velocity measurements. Within the measurement cross section, located at 5.6 m, the authors arranged ten verticals, where the lateral values of y from the first vertical to the last were 0, 4, 8, 12, 12.2, 26 and 34 cm. When the vertical distance from the measurement point to the bed was less than 175 mm, the measurement interval was 10 mm and 5mm in the main channel and floodplain, respectively.

Results and discussion:
The experimental results are presented in three parts, flow through non-vegetated floodplain first, flow through emergent vegetation second, followed by the submerged case. The effects of density and dowel configuration are included in each of the sections. Each section ends with a discussion on the effects of rigid dowels on logarithmic profile.
In the cases of nonvegetated floodplains, all measured streamwise velocity distributions followed the logarithmic distribution, but for vegetated floodplains, they followed an S-shaped profile.
It is seen that after implanting the vegetation over the floodplain, the velocity over the floodplain decreases whereas it increases in the main channel. Also, as the vegetation density, λ, increases, velocity increases in the main channel and decreases in the floodplain.
In the presence of emergent vegetation on floodplain, logarithmic profile does not exist even in the main channel, however it seems that the formation of the S-shaped profile in the main channel is under the bankfull height and above the bankfull height the vertical velocity profile takes on a logarithmic profile again. Under Submerged flow conditions, the velocity characteristics in all locations above the dowel array are well illustrated by the semi-logarithmic expression that has a slip velocity initially near the inflection point.
On the basis of the present experimental results, the whole flow region is divided into the following three sub-zones: (1) Emergent zone (0 ≤ z ≤ hp), (2) Mixing-layer zone (hp < z ≤ hlog), (3) Log-law zone (hlog < z ≤ H). In the present study, hp was equal to 0.2 H and hlog was equal to 0.5 H. In the emergent zone (0 ≤ z ≤ hp) the velocity is almost constant due to strong wake effects of vegetation stems although it may behave slightly in a counter-gradient fashion. In the second zone (hp ≤ z ≤ hlog), the vertical velocity profile are similar in both submerged and emergent conditions, and the effect of bed roughness is completely eliminated and the velocity gradients are reduced and almost fixed.
The velocity in the third zone (hlog < z ≤ H) is significantly higher than the velocity in the second zone. In the log-law zone (hlog < z ≤ H), the log-law of velocity distribution for rough beds is reasonably applied even to vegetated flows.
Comparison the longitudinal velocity profiles for linear and staggered dowel arrangements indicates an increase in the resistance due to the linear arrangement compared to the staggered arrangement.

Conclusion:
In the cases of non vegetated floodplains, all measured streamwise velocity distributions followed the logarithmic distribution, but for vegetated floodplains, they followed an S-shaped profile. However, in the main channel, higher than the bankfull height the velocity profile is logarithmic. The results shows that as the vegetation density, λ, increases, the velocity increases in the main channel and decreases in the floodplain. Linear arrangement resulted higher resistance compared to staggered vegetation arrangement. The velocity profile at all locations above the dowel array are very well represented by the following semi logarithmic expression. In fully submerged vegetation, the whole flow region was divided into three sub-zones, i.e., the emergent zone, (0≤z≤hp) the mixing-layer zone (hp < z≤hlog), and the log-law zone(hlog<z≤H). In the present study, hp was equal to 0.2 H and hlog was equal to 0.5 H.