Search Results (3)

Permeable dams are an important means for river management and ecology protection. Reasonable dam spacing will help regulate sediment transport and reduce sediment load in lakes. Flume experiments were carried out to investigate the effects of hydrological sediment conditions and dam spacing on sediment retention performance and permeability of the cascade permeable dams. The experimental results show that the permeability coefficient of the 1# dam decreased by about 30–40% with a large rate during the initial experiment stage. The decrease amplitude in the permeability coefficient and rising rate of the water level in front of the 1# dam for a large dam spacing (D/L) are positively correlated with the flow rate. At D/L = 5, the water level difference of 1# dam at the end of the experiment was significantly higher than that of other spacing. The sediment mass retained by 1# dam accounts for about 41–65% of the total sediment mass retained, which is about twice that of 2# dam, and plays a major role in cascade permeable dams. A mathematical model for predicting the spatial-temporal sediment concentration inside 1# dam is proposed based on the seepage theory of porous media. The research results are of great guiding significance for the design of the dam parameters. Full article

Sediment deposition is an ecological and environmental problem faced by most water bodies. In view of the poor structural stability and unrepeatable use of existing permeable structures, this paper proposes a cascade permeable dam, which consists of four dam bodies. As the composition of the dam material is the key to sediment interception and discharge capacity, this study sets up two groups of dam material particle sizes for experiments. The results show that the sediment interception performance of the cascade permeable dam is good. When the dam material with a small particle size is selected, the percentage of intercepted sediment mass inside the four dam bodies is 75–89%. The interception sediment rate is much greater than that of the dam material with a large particle size, which tends to decline one by one along the flow direction. The discharge capacity of the dam gradually decreases with time, and there is an obvious decrease in the permeability coefficient of 1# dam. The results of this study provide programmatic support for reducing sediment entering shallow lakes and rivers, which can further optimize the structure design of permeable dams. Full article

In the aftermath of pyroclastic density current-dominated eruptions, lahars are the main geomorphic agent, but at the decadal scale, different sets of processes take place in the volcanic sediment cascade. At Unzen volcano, in the Gokurakudani gully, we investigated the geomorphologic evolution and how the topographic change and the sediment change over time is controlling this transition. For this purpose, a combination of LiDAR data, aerial photography and photogrammetry, ground penetrating radar and sediment grain size analysis was done. The results show choking zones and zones of enlargement of the gully, partly controlled by pre-eruption topography, but also by the overlapping patterns of the pyroclastic flow deposits of 1990–1995. The ground penetrating radar revealed that on top of the typical lahar structure at the bottom of the gully, side wall collapses were trapping finer sandy sediments formed in a relatively low-energy deposition environment. This shows that secondary processes are taking place in the sediment transport process, on top of lahar activity, but also that these temporary dams may be a source of sudden sediment and water release, leading to lahars. Finally, the sediments from the gully walls are being preferentially oozed out of the pyroclastic flow deposit, meaning that over longer period of time, there may be a lack of fines, increasing permeability and reducing internal pore pressure needed for lahar triggering. It also poses the important question of how much of a past event one can understand from outcrops in coarse heterometric material, as the deposit structure can remain, even after losing part of its fine material. Full article