*4.1. Layout of Physical Model*

On the basis of the results of the numerical simulations that were performed for determining candidate replenishment areas, we selected the replenishment area located on the floodplain just downstream of the afterbay weir (Figure 6). For an undistorted model, the geometrical ratio between the prototype and model was set to 64:1 in both the vertical and horizontal directions due to space constraints at the construction site.

The model layout is sketched in Figure 8. The upstream boundary was set at the cross section of the afterbay weir, and the downstream boundary was set at Section 90 with a relatively straight reach and a narrow cross section further downstream of the replenishment area. Water was recirculated using a pump at various desired steady flow rates and was controlled by the valve of the pipeline, which was connected to the head tank. A sediment-settling basin was constructed at the downstream end to settle part of the outflow sediment, and the inflow sediment concentration was monitored to control the background sediment quantity. Two monitoring points were set at the downstream end for measuring outflow sediment concentrations, from which the average was derived. According to the prototype dimension, the volume of replenished sediment was nearly 100 × 10<sup>3</sup> m3, which was

estimated to be one-fourth of the annual volume of dredged sediment. Accordingly, the thickness of replenished sediment was approximately 1.6 m. To confine the dredged materials inside the selected replenishment area, a permeable barrier with a height of 2 m was constructed. The permeable barrier functioned as a filter to prevent the replenished material containing very fine sediment from polluting the water in the main channel during low-flow periods. Moreover, the barrier was composed of coarse sediment materials with relatively large particle sizes (Figure 2); the materials were obtained from the field dry excavation site near the Lofu gauge station at the upstream end of the Shihmen Reservoir.

**Figure 8.** Layout of the physical model.
