A comparison of wood-sediment-water mixture flows at a closed type and an open type of check dams in mountain rivers

Abstract

The debris flow event in Hofu City, Yamaguchi Prefecture, Japan on July 21, 2009 yielded a large amount of sediment transport and woody debris in the mountain river basin, where a closed type of check dam and an open type of check dam were built (Maricar et al., 2011). Based on laboratory tests, the debris flow control by these check dam was studied. This paper compares the deposition process of sediment grains and pieces of wood at the open and closed check dam in a laboratory flume experiment. The experimental results showed that the open check dam trapped the wood pieces and the trapped pieces deposited the sediment grains behind themselves. On the other hand, the closed check dam made most of wood pieces pass through the check dam and most of sediment grains get trapped in the water pool behind the closed check dam. Tsurugi river basin. On the other hand, the open check dams trapped a large amount of driftwood in their opening and as a result a large amount of sediment (Figure 2). This interrupted the outflow of wood and sediment from the Hachimandani River basin. This paper compares the deposition processes of sediment and driftwood in the open and closed check dam with laboratory flume experiments. 2 EXPERIMENTAL METHOD 2.1 Model check dam and laboratory flume The experiments were carried out to compare the deposition processes of sediment and driftwood at the open check dam and the closed check dam (Maricar et al., 2011; Maricar et al., 2012). The open check dam was built in a rectangular flume; it was 12 m long, 30 cm wide, and 32.8 cm high with smooth glass walls. A schematic diagram of this flume is shown in Figure 3. The flume bed was composed of movable and fixed parts; the movable part was 6 m long and the fixed part is 4 m long. The fixed bed was roughened with the same material used for the movable bed. The bed sediment grains and wood were placed on the movable bed part and the check dam model was on the fixed part of the bed. Referring to the steel-pipe check dam in the Hachimandani River, we made the model of the open check dam for the laboratory experiments. Figure 4 (a) shows the situation of the check dam in the Hachimandani River, and Figure 4 (b) shows the check dam in the laboratory flume. Stainless steel tubes with a diameter of 18 mm were used for the model of the open check dam. The reduced size of the prototype was 1/50. The opening size of this check dam model was l0=3.5 cm. The check dam was set 1.5 m from the downstream end. Referring to the closed check dam in the Tsurugi River, we made the model of the closed check dam for the laboratory experiments. The model was made with impermeable plywood, as shown in Figure 5. The reduced size of the prototype was 1/120. It is schematically shown in Figure 6. 2.2 Bed material and model wood The grain classes with a diameter d50 = 3.6 mm, 7.4 mm and 22mm were mixed to produce the movable bed material. The mixing ratio of the gravel, fine gravel and very coarse sand was 1: 2.5: 5. The representative diameter of the mixture was d50 = 4.4 mm. The grain density was 2.65 g/cm 3 . The particle size distribution of the mixture is shown in Figure 7, and the sediment and wood conditions are shown in Table 1. The largest size of the sediment grains almost correspond to the opening size of the model check dam (l0=3.5 cm). Figure 3. Laboratory flume Figure 5. Closed check dam in the flume (view from the righthand and downstream side). (a) side view (b) front view Figure 6. Scale of the closed check dam in the flume. Figure 7. Particle size distribution curve for the mixture in the experiment. Table 1. Characteristics of model sediment and wood. Figure 4(a). The open check dam in the Hachimandani River. Figure 4(b). The open check dam in the laborato-