Study on porosity of sediment mixtures and a bed-porosity variation model 混合砂礫の空隙率と空隙率の変化を考慮した河床変動モデルに関する研究
Study on porosity of sediment mixtures and a bed-porosity variation model
The sediment movement system in a river basin consists of sediment production process in the mountainous region, sediment supply process to the torrents and sediment deposition process in the lower reach and coastal area. There are human impacts as well as natural impacts in the system. These impacts affect the topographical feature and ecosystem in the basin including the coastal area. Bed variation model is one of the tools for assessing the topographical feature of river. In previous riverbed variation calculations, engineers or researchers conventionally assumed that the porosity in riverbed material is a constant, regardless of whether the grain sizes of the riverbed material was uniform. Since there is no doubt that the porosity depends on the grain sizes distribution, fixing the porosity at a constant value is inadequate for simulating practical sediment movements, such as the removal of fine materials out of the riverbed material or the deposition of fine material into voids between the coarse material. Voids in a riverbed themselves are also important as habitat for aquatic biota. Not only natural sediment transport phenomena, such as floods and debris flows induced by heavy rainstorms, but also artificial impacts, such as the construction of dams or sediment flushing from reservoirs, seriously affect the voids in the riverbed. So far no bed variation model has been available for the analysis of the change in porosity. As the void of bed material plays an important role in fluvial geomorphology, infiltration system in riverbeds and river ecosystem, a structural change of the void with bed variation is one of the concerned issues in river management. Thus, a bed-porosity variation model is strongly required and it is expected that the model contributes the analysis of those problems as a tool for integrated sediment management. The objectives of this work are: 1) to point out recent problems in a volcanic river basin, as well as the impacts on riverbed variation and ecosystem; the problems in Merapi volcano area and Progo River, Indonesia were chosen as case studies; 2) to develop a method for identifying the type of grain size distribution and two methods for obtaining the porosity for the different type of grain size distribution; 3) to develop a framework and a bed variation model available for the analysis of the change in porosity of bed material as well as the bed variation. The report consists of four subjects and organized into six chapters as shown in the diagram below (Figure 1). The following diagram shows the framework of proposed bed-porosity variation model and the correspondence of each chapter of this report. In Chapter 2, the sediment-related problems in volcanic area, particularly in Mt. Merapi and Progo River, Indonesia and the impacts on bed variation and ecosystem were pointed out. The sediment-related problems persist in the upper reach, middle reach, also in lower reach. Some problems are triggered by natural activities such as volcanic activity of Mt. Merapi and heavy rainfall, and many others are occurred due to the human interfere such as deforestation, construction of sabo dam and sand mining. Uncontrolled sand mining is the serious problem in this area. Those problems are increasing the susceptibility in the downstream and deteriorating the watershed. A flume experiment was conducted to realize the impact of mining pit on bed variation. Countermeasures of sediment problems, which have been done in Mt. Merapi area and Progo River, were also presented. Finally, the necessity of a tool for integrated sediment management in consideration of the ecosystem in river was indicated. In Chapter 3, the method for classifying and geometrically identifying the type of grain size distribution was presented. First, grain size distribution was classified into some typical types and those characteristic parameters were found out. Then a method for geometrically identifying the type of grain size distribution by using geometric indices .. and .. was presented. Based on the geometrical analysis of typical grain size distributions, a diagram on classification of grain size distribution type was indicated. The presented identification method was then applied to the natural grain size distribution data and the validity of the method was verified. In Chapter 4, two methods for estimating the porosity of sediment mixtures were presented. One was based on a particle packing simulation model and the other was based on a measurement method. The porosity of particle mixtures depends on not only the grain size distribution but also the compaction degree. However, the compaction degree could not be intentionally controlled in the model. Both of the methods were applied to estimate the porosity of three typical grain size distributions, namely lognormal distribution, modified-Talbot distribution and bimodal distribution. Particularly in the measurement, it was very difficult to mix the sediment evenly. Consequently, the coarser particle lies at higher position than the finer particle. This grading process made the porosity larger, while in the simulation the particles were mixed evenly. Thus, the particles packing in the simulation might be denser than the packing of particles in the measurement. The results showed that the relationship between grain size distribution and porosity could be determined by using the characteristic parameters of typical grain size distribution. This relation could be introduced into the bed variation model. In Chapter 5, a one dimensional bed-porosity variation model was developed for simulating the changes in porosity of bed material as well as the bed variation. Analytical model for binary mixtures with much different grain sizes and the relationship between the characteristic parameters of grain size distribution and porosity presented in Chapter 4 were introduced into the bed variation model. Two numerical methods were employed to solve the governing equations, i.e., standard successive approximation and MacCormack scheme. A flume experiment was conducted to realize the transformation processes of void structure for two conditions; one was the only fine sediment was removed from a sediment mixture and another was the fine sediment deposited into a coarser bed material. After the validity of the presented model was verified using a data set provided by the experiment, the model was applied to the bed and porosity variation process for bed material with binary mixtures and continuous grain size distribution. Its performance was examined in detail for two conditions; (1) no sediment supply condition and (2) sediment supply condition. The simulation results showed the model could produce a reasonable distribution of porosity of the riverbed material in the longitudinal and vertical directions for both conditions. A one-dimensional bed-porosity variation model proposed in this study is different from the previous model from a viewpoint of considering the porosity of bed material. Hence, the proposed model is available for the analysis of the change in porosity of bed material as well as the bed variation. The model contributes in two aspects; from the hydraulics point of view, the model provides an improvement of the accuracy in the riverbed variation calculation and from ecological point of view, the model provides the changes in porosity with the bed variation. In the case of binary mixtures, the validity of the model has been verified using a data set provided by the experiment and the simulation result showed that the model produced a reasonable result on the change in porosity as well as the bed variation. In the case of sediment mixtures with continuous grain size distribution, although the validity of the model has not been verified yet, the simulation result showed the model available for analysis of bed and porosity variation.