Effects of Pile Density and Arrangement on Flow Characteristics Around a Pile-group Dike

Pile-group dikes as river training structures can be used for velocity control along the bank and protection of the bank from severe erosion. Yet no detailed explanation has been found in the literature on how these structures function. In order to study the effects of different pile-groups on the flow, an experimental and numerical study was conducted. A different number of piles per group which was defined as pile-group density was considered. In addition, two types of piles arrangement namely in-line and staggered were applied for each pile-group density. Flow structure around pile-groups was measured experimentally by PIV method.

The results indicate that for all the cases, the flow penetrated to the pile-group and the penetrated flow discharged from the structure with reduced velocity. This function of pile-group reduced the flow velocity for downstream bank protection purpose. The strength of the velocity downstream of the pile-groups is inversely proportional to the pile-group density, while it is the opposite in the mainstream. By changing the arrangement of the piles from in-line to staggered arrays, significant desirable change in the flow structure occurred. Staggered arrangement cases generated a suitable velocity reduction pattern downstream of the pile-group. In a lateral section in the downstream of the structure, it minimized the velocity near the bank and then the velocity was increasing gradually to the mainstream, however, for the in-line arrays it was the opposite, as shown in Figure 1. Regarding the turbulence in the channel, the in-line cases generated a strong turbulence region downstream of the structure, on the other hand, staggered cases did not show such strong turbulence. Furthermore, a lower pile-group density in staggered arrays can reduce the velocity as a higher pile-group density of in-line arrangement. For this reason, to obtain a certain velocity near the bank, staggered arrays had the advantages of using less number of piles and hence less effect on the mainstream flow. The small effect on the mainstream can refer to the less resistance of the structure against the flow. Consequently, reduced local scour is expected for these pile-groups. The flow structures around the pile-groups were well simulated by a 2D numerical model. A comparison of the measured and calculated results is shown in Figure 2.

Figure 1: Contours of longitudinal velocity normalized by the mean velocity, case 8x8_in-line (left), and case 8x8_staggered (right).

Figure 2: Longitudinal velocity just behind structure: (a) Measured result, (b) Calculated result