A new parameterisation for the threshold shear velocity to initiate deflation of dry and wet particles is presented. It is based on the balance of moments acting on particles at the instant of particle motion. The model hence includes a term for the aerodynamic forces, including the drag force, the lift force and the aerodynamic-moment force, and a term for the interparticle forces. The effect of gravitation is incorporated in both terms. Rather than using an implicit function for the effect of the aerodynamic forces as reported earlier in literature, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals force between two particles, it was further shown that the effect of the interparticle cohesion force between two dry particles on the deflation threshold should be inversely proportional to the particle diameter squared. The interparticle force was further extended to include wet bonding forces. The latter were considered as the sum of capillary forces and adhesive forces. A model that expresses the capillary force as a function of particle diameter squared and the inverse of capillary potential was deduced from consideration of the well-known model of Fisher and the Young–Laplace equation. The adhesive force was assumed to be equal to tensile strength, and a function which is proportional to particle diameter squared and the inverse of the potential due to adhesive forces was derived. By combining the capillary-force model and the adhesive force model, the interparticle force due to wet bonding was simplified and written as a function of particle diameter squared and the inverse of matric potential. The latter was loglinearly related to the gravimetric moisture content, a relationship that is valid in the low-moisture content range that is important in the light of deflation of sediment by wind. By introducing a correction to force the relationship to converge to zero moisture content at oven dryness, the matric potential–moisture content relationship contained only one unknown model parameter, viz. moisture content at −1.5 MPa. Working out the model led to a rather simple parameterisation containing only three coefficients. Two parameters were incorporated in the term that applies to dry sediment and were determined by using experimental data as reported by Iversen and White [Sedimentology 29 (1982) 111]. The third parameter for the wet-sediment part of the model was determined from wind-tunnel experiments on prewetted sand and sandy loam aggregates. The model was validated using data from wind-tunnel experiments on the same but dry sediment, and on data obtained from simulations with the model of Chepil [Soil Sci. Soc. Am. Proc. 20 (1956) 288]. The experiments showed that soil aggregates should be treated as individual particles with a density equal to their bulk density. Furthermore, it was shown that the surface had to dry to a moisture content of about 75% of the moisture content at −1.5 MPa before deflation became sustained. The threshold shear velocities simulated with our model were found to be in good agreement with own observations and with simulations using Chepil's model. 相似文献
In the study of soil erosion, specifically on detachment of soil particles by raindrop impact, kinetic energy is a commonly suggested indicator of the raindrop's ability to detach soil particles from the soil mass. Since direct measurement of kinetic energy requires sophisticated and costly instruments, the alternative approach is to estimate it from rainfall intensity. The present study aims at establishing a relationship between rainfall intensity and kinetic energy for rainfalls in Central Cebu, Philippines as a preface of a wider regional investigation.
Drop size distributions of rainfalls were measured using the disdrometer RD-80. There are two forms of kinetic energy considered here. One is kinetic energy per unit area per unit time (KER, J m−2 h−1) and the other is kinetic energy per unit area per unit depth (KE, J m−2 mm−1). Relationships between kinetic energy per unit area per unit time (KER) and rainfall intensity (I) were obtained using linear and power relations. The exponential model and the logarithmic model were fitted to the KE–I data to obtain corresponding relationships between kinetic energy per unit area per unit depth of rainfall (KE) and rainfall intensity (I). The equation obtained from the exponential model produced smaller standard error of estimates than the logarithmic model. 相似文献
The Boao coastal system along the eastern coast of Hainan Island is a dynamic delta-tidal inlet-barrier formed during the late Holocene. The delta developed inside a shallow lagoon barred by a sandy barrier with a narrow, shallow tidal inlet opening. Two major distributary channels separated by small islands characterize the delta. The lagoon is silting up receiving and trapping sediments from both the river and, in minor measure during storms, through the tidal inlet opening and barrier washovers. The barrier at the tidal inlet is highly dynamic and changes its form, accreting (migrating spit) against the inlet during fair-weather conditions and being eroded during storms and river floods.The delta has almost completely filled the lagoon and major concerns exist on the effect that ongoing large development plans may have on the environment. These concerns include the effect on floods and rate of siltation once banks of the islands have been stabilized and floodwater and sediment load are impeded from spreading over the lowlands, and the effect of increasing pollutant loads from the new facilities on the ecosystems of the increasingly restricting lagoon water and on the seashores. 相似文献