METHOD OF STUDY AND ITS RESULT

The Experiments

The experiments were undertaken in moveable bed flume : Long = 16 m Wide = 0,3 m High = 0,4 m Location = at the State Key Hydraulics Laboratory (SKHL) of Sichuan University

Location of the measurement : 1. The main channel 2. The inner edge of the main channel 3. Base at the upper edge of the main channel/beginning of the floodplain 4. Near the central line of the floodplain

Model vegetation was planted on the floodplain. To represent vegetation on the floodplain, it was chosen plastic grass, duck feathers, and plastic straws as model grass, shrubs, and model trees, as shown in Figure 2. The measurement cross section located several point, as shown in previous slide. An asymmetric compound channel was molded, using a non-uniform sediment with a median diameter of 0.4 mm

VELOCITY DISTRIBUTIONLaw of The Wall, for fully developed 2D flow near the bed :

1 ZpU * U ! ln c U* k v Where : U = Streamwise point velocity U* = Local shear velocity

U * ! a y A VK Zp v c = von Karman constant = Distance from the measurement point to bed = kinematic viscosity = constant

1 ZpU * U ! ln c U* k v

U * ! ay A V

U Za V 1 y A H y c ! ln a y A k H y v V U a V 1 Zp 1 y A H y ay c ! ay ln ay ln VA k y H k v H(y) = flow depth at the location y

1 C ! ay k

a V 1 y A D ! ay ln H y ay c k v

U 1 ZpU * ! ln c...........................(1) U* k v where : U * ! ay A .............(2) VSubstitute equation (2) into equation (1) :

U 1 y A Za V ! ln H y c...................(3) a y A k H y v V or

U 1 Zp 1 y A a V ! ay ln H y ay c....(4) a y ln VA k y H k v Where : H(y) = flow depth at the location y

If we split equation (4) into two, we get :

1 C ! ay ..............................(5) k 1 y A a V D ! a y ln H y a y c.....................(6) k v relative depth : Zp Hr ! ................................(7) H y

Substitute equation (5), (6), and (7) into equation (4) :

U ! C ln( Hr ) D.....................(8) VA

For the main channel side-slope region :

relative depth : Z Hr ! H

z y b s Hr ! H y b s

For the floodplain:

zh Hr ! H h

From figure 4-6 : Once the floodplain is planted with vegetation, the velocity in the main channel increases and that on the floodplain decreases, reflecting the different resistance to the flow offered by the different types of vegetation. For all types of vegetation, the velocity distributions are no longer logarithmic but follow approximately S-shaped distributions. The velocity distribution is affected by two boundary layers, one at the bed and another at the top of the vegetation.

FLUCTUATING VELOCITIES

Figure 9 illustrates the temporal variation of turbulence data for u and v at the main channel/floodplain for four types of vegetation. The turbulence intensity clearly increases as a result of the presence of the vegetation

REYNOLDS SHEAR STRESSES The Reynolds shear stresses were calculated from the raw data using the equation :

X ij ! Vuiu j Because the water in the main channel generally moves faster than on the vegetated floodplains, a shear layer is created in the interaction region between the main channel and the floodplain. For vegetated cases, the vertical gradients of yx are approximately equal to zero in the main channel, while the vertical gradients of zx vary a great deal, especially in the near-bed flow region. This is probably because the main channel behaves more like a single channel after its floodplain is vegetated.

Result of the experiments1. These small-scale experiments indicate that the distribution of streamwise timeaveraged velocity for non-vegetated floodplains may be described by the logarithmic equation. For vegetated floodplains, the velocity distribution on the floodplain follows an S-shaped curve. The resistance to flow varies with the vegetation type, with long grass retarding the flow the most. The lateral gradient of velocity increases after the floodplain is vegetated, thereby increasing the apparent shear stress on the vertical interface between the main channel and the floodplain. The vertical distribution of streamwise and lateral turbulence intensities are Sshaped for vegetated floodplains, similar to the distributions of streamwise point velocity.

2. 3. 4.

5.

Result of the experiments6. The vertical turbulence intensity does not follow the same kind of distribution. The streamwise and lateral turbulence intensities are approximately equal. The vegetation on the floodplain affects the spatial distributions of Reynolds stresses, especially near the main channel/floodplain boundary. For vegetated cases, the vertical gradients of yx are approximately equal to zero in the main channel, while the vertical gradients of zx vary a great deal, especially in the near-bed flow region. On the floodplain, the low velocity between the first and second boundary layers causes the lateral and vertical shear stresses to approach zero.

7. 8.

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RELATED WORK

OPEN CHANNEL FLOW RESISTANCE BEN CHIE YEN, F.ASCE

WALL SURFACE RESISTANCE AND BOUNDARY LAYER THEORYAmong the four component types of channel flow resistance classified by Rouse (1965), the wall surface or skin friction resistance (hereafter simply referred to as wall resistance) always exists and can readily be linked to the boundary layer theory in fluid mechanics. It has long been suspected that flow resistance is related to the velocity distribution. Stokes (1845) suggested the internal tangential shear stress i j proportional to the molecular dynamic viscosity Q and the velocity gradient, i.e.,

Hui Hu j X ij ! Q Hx Hx i j

In accordance with the boundary layer theory, the distribution of u along the wall-normal y direction is adequately described by two universal laws, namely, the inner law or law of the wall where the viscous effect dominates, and the outer law or velocity defect law (Rouse 1959; Hinze 1975; Schlichting 1979) U ! F ( y*, k *) U*

RESISTANCE OF COMPOSITE OR COMPOUND CHANNELSPhysically, the composite/compound roughness on the wall as well as the shape of the channel modifies the velocity distribution across the cross section, and hence alters the resistance coefficient. Traditionally, the compound/ composite roughness resistance coefficient of a cross section is conventionally expressed in the Manning n form, with the cross sectional value, nc , being a weighted sum of the local resistance factor, ni

nc ! wi ni dpP

OPINION

OPINION Vegetated floodplains will decreases the velocity in the floodplains, but it will increases the velocity in the main channel. The presence of vegetated in the floodplains will modifies the velocity distributions. It was shown in the figure 4 6. For non-vegetated floodplain velocity distribution usually follow logarithmic distribution, but for vegetated floodplain will get the Sshape distribution. The non-vegetated floodplains will cause the increasing of turbulence intensity and it is affected to increase the Reynolds shear stress. Grow the vegetated floodplains will overcome the flood problem, because the floodplain will be more retard to the flow, furthermore will decrease the velocity and the discharge of the flow in the floodplains .

REFERENCES1. Yang, Kejun., Cao, Shuyou., and Knight, Donald W., (2007). Flow Patterns in Compound Channels With Vegetated Floodplains . Journal of Hydraulic Engineering., 2(148), 148-159. 2. Yen, Beni Chie. (2002). Open Channel Flow Resistance. Journal of Hydraulic Engineering., 1(20), 20-39.