Gravel Bed-load Transport And Its Modelling

The compositions of gravel–pebble river beds vary with the alternate erosion and deposition in flood season and non-flood season. As a result, bed materials are often encountered in scouring computation in gravel–pebble shoal's training engineering. During the fluvial processes of the alternate deposition and erosion, one of the key technologies predicting the bed's deformation is how the location characteristics of bed material affects incipient motion for non-uniform sediment and transport rate for bed load. The influences of armoring in gravel-pebble shoals to sediment incipient and the bed-load transport capacity are serious.Armoring degrees in various rivers are different; in certain river, it varies with season. So for different rivers or during different periods for certain river, the carrying sediment capacities are different. The main reason why computed bed-load transport discharge and gravel-pebble incipient motion disagree with the actual ones lies in the foregoing fact.By the analysis of theory, the research elaborates that the threshold condition for incipient motion of non-uniform sediment with a specific size, is directly related to flow condition, sediment size distribution, location of bed material, and income sediment from upstream. Through flume experiments and field observations, the relationship between the exposure degree and grain size has been established under the conditions of sparse motion and strong motion. A new formula reflecting the influence of armoring and bed material's location on incipient motion of uniform gravel and pebble, is developed. On the basis of experiment study on bed material's location characteristics, a concept, exposure height, is introduced and the influence of it on probability of saltation and exchange time is considered, and the non-uniform gravel-pebble bed load transport rate formula is put forward. The formula indicates that armoring and getting finer have great influence on carrying sediment capacity. Armoring degree of different river are not equal, therefore transport capacities are different obviously. Even in a same river reach, bed-load transport capacities are differences when armoring degrees in the river beds are differences in different season periods. This is one of the main reason causing large discrepancies between calculated bed-load yield and field observed values. In this thesis, an important parameter reflecting bed material's location cnaractenstics, A, is introduced. That hiding factors vary with the different A reflects the fact that transport sediment laws in different rivers are different.The present abnormal model design methods for gravel–pebble beds cannot content incipient shear stress similarity and incipient velocity similarity at the same time. Resistance similarity of mobile bed is more difficulty. By the analysis of limitation of current model design approaches, A new method that contents the foregoing similarities at the same time, according to the principle of harmony of slope.A river sediment numerical model for gravel–pebble streams, CRS-1 model, is developed. The computed results by the model agree quite well with the data of flume experiment, model experiment and field.The present bed-load computational methods and formulae that are based on the experimental data or are obtained by regressing on the basis of field data, are applied to river engineering and encounter the situations that conflict with the actual ones. There is a distinct difference between the complex behavior of water and sediment movement in indoor flumes and that in natural rivers. Therefore, some issues on the laws of gravel–pebble bed load movement are explored primarily. Key problems should be further studied in this area are suggested.