Effect of rigid, bank vegetation on velocity distribution and water surface profile in open channel

Y

Water depth (m);

Y_o

Water depth in the unvegetated case (m);

Y_in

Average water depth in the vegetated reach (m);

h

Vertical height of the measured velocity point above the bed (m);

V

Water velocity in the vegetated case (m/s);

u

Water velocity in the unvegetated case (m/s);

V_in

Water velocity in the middle of the vegetated reach (m/s);

Average flow velocity in the unvegetated case (m/s);

Fr_o

Froude number in the unvegetated case.

g

Gravitational acceleration (m/s²);

λ

Vegetation density (λ=П N⁰ d²/4, where N⁰ is the number of stems per unit bank area and d the stem diameter (m), and

Q

Discharge (l/s).

Ditch bank vegetation grows on an open channel̀s side slopes. It has both positive and negative effects, depending on the purpose of the hydraulic conduit. For example, it lowers conveyance capacity by obstructing flow – reducing the flow cross-section area and increasing resistance to flow. On the other hand, it increases bank stability, reduces erosion, provides habitat for aquatic and terrestrial wildlife, and filters pollutants (Nepf et al. 1997; Kemp et al. 2000; Tang et al. 2008).

The impact of ditch bank vegetation on the hydraulic parameters of an open channel (velocity distribution, water surface profile, friction coefficient, etc.) changes in relation to the flow discharge, bank slope, vegetation density, etc.

Several laboratory studies (Afzalimehr & Dey 2009; Hirschowitz & James 2009; Hopkinson & Wynn 2009; Afzalimehr et al. 2010; Bledsoe et al. 2011; Czarnomski et al. 2012; Masouminia 2015; Mohammadzade et al. 2016; Liu et al. 2017) on the effect of ditch bank vegetation, evaluating and analyzing vegetation effects on velocity distribution, turbulence intensity and kinetic energy, and Reynold's shear stresses. Table 1 is a review summary of the impact of ditch bank vegetation on flow characteristics.

In fact, the impact of ditch bank vegetation depends on many complex, interacting factors, including flow conditions, distance between the ditch bank vegetation and the measurement point, and vegetation spacing.

This study's primary aim was to investigate the effects of rigid ditch bank vegetation on water's surface profile and velocity under subcritical flow, at different discharge rates and vegetation densities in a trapezoidal open channel.

Experiments were conducted in a 0.6 m wide, 0.42 m deep, 16 m long, horizontal bed, recirculating trapezoidal flume, at the Channel Maintenance Research Institute's hydraulics laboratory. The water level was controlled with a tail-gate at the end of the flume. For all tests, with and without vegetation, a fixed set of 4 discharge rates each with 3 water depths was used.

Simulation of rigid vegetation is common – e.g., in Stone & Shen 2002; James et al. 2004; Meftah et al. 2006; Kothyari et al. 2009; Cheng & Nguyen 2011; Panigrahi 2015; Ahmed & Hady 2017; Chakraborty & Sarkar 2018; Wang et al. 2018; and Tong et al. 2019. (See Table 2).

In recent research, the rigid vegetation stems have been represented by 3 mm diameter steel rods set in a staggered grid pattern with 25, 50, and 75 mm center spacings, both longitudinally and transversely, and secured above a drilled-hole steel panel. Three vegetation densities – 1,600, 400, and 178 stems/m² – were used, with a fixed reach length of 4.00 m at the flume center. Figure 1 shows the experimental channel with vegetation on its sides and Table 3 summarizes the flow conditions of the experiment.

Water depths were measured every 0.50 m along the canal centerline using an ultrasonic level meter (Sondar) in all runs – Figure 2(a). Three velocity profiles were measured – upstream and downstream of, and within the vegetated reach – using a Vectrino (3-D water velocity sensor Lab Probe) – Figures 2(b) and 3.

The water surface profile along the flume centerline was surveyed with an ultrasonic level meter, to understand the influence of vegetation density on it. The water profiles arising at different vegetation densities are shown in Figure 5.

As can be seen in Figure 5, the water depth increased upstream of the vegetated reach, but fell within the reach, compared to the unvegetated case. The relationship between water depth reduction in the vegetated reach, at different vegetation densities, and Fr_o (unvegetated), is illustrated in Figure 6 

Figure 6 shows a positive relationship between Fr_o and the proportional water depth reduction (%). The proportional reduction within the vegetated reach increased with both increasing Fr_o and vegetation density_.

Maintenance programs should be applied to manage ditch bank vegetation in open channels, because water level changes upstream of and within the vegetated reach which affects water distribution on branches as well as the calibration of opening gates. Drainage collectors will also be affected.

The water velocity sensor was mounted on a carriage to measure flow velocity in different vertical sections along the flume centerline. This was done upstream, within, and downstream of the vegetated reach. Figures 7–9 show the velocity profiles for different vegetation densities as ratios of the velocity in the unvegetated cases. In the figures, the term h/Y is the ratio between the vertical measuring distance (h) and the total water depth (Y).

The velocity profile as a ratio of the unvegetated case (V/u) is sigmoid, i.e. the maximum velocity (V/u_max) occurs in the lower half of the water column. The reason for increasing velocity near the channel bed may be the secondary current that occurs due to the presence of the side vegetation. This result accords with the work of (Afzalimehr & Dey 2009; Afzalimehr et al. 2010; Masouminia 2015; Mohammadzade et al. 2016; Liu et al. 2017), where it is concluded that the level of maximum velocity in the presence of vegetation on the channel walls is below the water surface and the maximum shear stress occurs near the channel bed.

Channel-side vegetation increases flow velocity near the bed – i.e., shear stress near the bed increases – which, in turn, increases the possibility of bed erosion on the vegetated channel's centerline. It is also noted that V/u increases with increasing vegetation density and correlates directly with the change in Fr_o in the unvegetated case at the same discharge – i.e., V/u is influenced by channel geometry. The maximum velocity occurred within the vegetated reach and is close to that measured just downstream of the vegetation. Figure 10 shows the relationship between V/u _max within the vegetated reach and Fr_o at different vegetation densities.

DataFit 9.0 statistical software packages were used for both statistical analysis and deriving empirical relationships (Oakdale Engineering 2008). A multiple regression analysis was performed at 95% confidence level. R² was used as a measure of goodness of fit, where the predicted value indicates how well the model predicts responses for new observations.

All contributing factors were shown to be significant in prediction, i.e., all factors had p-values <0.0001. Tables 4 and 5 show the regression analysis results from Equation (3) and the significance of each variable. Figure 11 is a plot of the predicted and measured water depth decreases in the vegetated reach.

All contributing factors were shown to be significant in prediction, i.e., all had p-values <0.0001. Tables 6 and 7 show the results of regression analysis of Equation (4) and the significance of each variable. Figure 12 is a plot of the predicted and measured velocity ratios within the vegetated reach.

The effect of rigid ditch bank vegetation on water surface profile and velocity distribution under sub-critical flow conditions at different discharge rates and vegetation densities in a trapezoidal open channel was studied. It was noted that:

• 1.

  Increasing the ditch bank vegetation density increases the water depth upstream of the vegetated reach significantly and reduces it within the vegetated reach.

• 2.

  The decrease in water depth within the vegetated reach increased with increasing Fr_o.

• 3.

  The velocity profile as a ratio of that of the unvegetated case (V/u) is sigmoid (S-shaped).

• 4.

  V/u increased with increasing vegetation density and is also correlated directly with the change in Fr_o.

• 5.

  V/u is influenced by channel geometry.

• 6.

  The maximum velocity occurred within the vegetated reach and is close to that measured just downstream of the vegetated reach.

• 7.

  V/u_max occurs in the lower half of the water column, which increases shear stress near the bed and increases the possibility of bed erosion on the vegetation channel's centerline.

• 8.

  Multiple regression analysis and the development of empirical equations were used to assess the impact of vegetation density on the reduction in water depth within the vegetated reach and its effect on flow velocity there.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.