A simulation model of blowout development

Blowouts are the main feature of wind erosion in the dunes along the Dutch coast. A number of characteristics, such as their growth against the prevailing wind and the spontaneous stabilization when certain dimensions are reached, are believed to be caused by a prominent quality of the southwest wind, viz. its gustiness. This hypothesis is tested in a two-dimensional computer model which simulates the effect of wind gusts on a sandy surface along the main axis of the blowout. A number of constraints based on field observations are built into the model. The results closely resemble the longitudinal section of the blowout.     

环坡深耥大垄豆禾混播治理风蚀穴试验

严重风蚀穴（群众称之为风剥板）不均匀分布于流动沙地中，坚硬紧实，单一生物措施难以治理。经采取环坡深耥大垄，运用豆科牧草沙打旺与禾本科谷子混播的方法，使风蚀穴得到了治理，方法简单易行，并取得明显的生态效益和经济效益     

Relationships between deflation and near surface wind velocity in a coastal dune blowout

Detailed field measurements were made of the degree of surface level change in a blowout, situated in the coastal dune area of Meijendel, The Netherlands. A formula was established to express the erosivity of the wind for the corresponding 34 measurement intervals, over a period of about 3 years. Having established, for 12 wind sectors, the relationship between wind velocity at the nearest standard weather station and at five locations in the blowout, correlations were derived between the deflation rate in the blowout and the wind erosivity using standard hourly wind data. The winter season, although the most windy, is by far the least effective season: the threshold shear wind velocity is disproportionally increased because the moisture content of the surface sand is high. The soil moisture conditions are described as a function of the daily precipitation and evaporation rate. The two most relevant upper and lower threshold shear wind velocities are determined empirically. During the summer season, deflation rate is even higher than indicated by the net surface level change because deflation is partly compensated by deposition of sand transported by rainwash from the blowout margin. With regard to spatial variation in the deflation rate within the blowout, it appears that locations with relatively high velocities coincide with higher deflation rates. However, the spatial variation is commonly less than is expected from the horizontal wind velocity distribution. In view of this the role of wind direction, blowout morphology and algal coverage of the soil in the deflation rate is discussed.     

The role of algae in the stabilization of coastal dune blowouts

A number of mechanisms prevent blowouts in the dunes along the Dutch-coast from growing beyond a certain length. Plants play an important role in the stabilization process. Algae are the first plants to colonize the blowouts. Their filaments are surrounded by mucilaginous sheaths which adhere to the sand grains and increase the resistance of the sand to entrainment by wind. This suggests that artificially induced algal growths might be used in cheap, and ecologically justifiable, stabilization measures.     

砾石覆盖对土壤吹蚀的抑制效应

在风洞实验室模拟测定了不同砾石覆盖密度和覆盖方式的土壤吹蚀速率。结果表明,砾石覆盖对吹蚀速率的抑制作用可表达为砾石铺压的密度效应和空间排列效应。在一定的风力条件下,吹蚀速率随砾石覆盖密度的增加呈现指数递减变化;在一定的铺压密度下,不同铺压方式表现出不同的吹蚀抑制效应,从小到大依次是：平行条带铺压＜簇状铺压＜斜交条带铺压＜垂直条带铺压＜随机铺压。     

20 years of active deformation on volcano caldera: Joint analysis of InSAR and AInSAR techniques

InSAR (interferometric synthetic aperture radar) techniques are applied to investigate last two decades of surface deformation of the Cerro Blanco/Robledo Caldera (CBRC). The objective is the identification of deforming patterns that alter the shape of these complex structures when they show low or null activity. The joint analysis between results by using different methods over a long time span, represents a unique opportunity to improve knowledge of volcanic structures located in remote area and, for this, poorly or not monitored.In this work we identify displacement patterns over the volcanic area, by using both classical differential InSAR analysis, and A-InSAR (advanced InSAR) analysis based on SAR data acquired by ERS-1/2 and ENVISAT sensors during the 1996–2010 time interval. The satellite-derived information allows us to characterize the deformation pattern that affected the CBRC and shows that the actively deforming CBRC is subsiding in the observed period. In order to figure out the deformation history of CBRC, we analyzed the four sub-periods 1992–1996, 1996–2000, and 2005–2010 by using standard differential InSAR technique, and the interval 2003–2007 by adopting an A-InSAR technique.Subsidence velocities of the CBRC caldera are about 2.6 cm/yr in the time interval 1992–1996 (measured with ERS descending data), 1.8 cm/yr in 1996–2000 (ERS descending data), 1.2 cm/yr in 2003–2007 (ENVISAT descending data), and finally, 0.87 cm/yr in 2005–2010 (ENVISAT ascending data). Moreover, outside the caldera and in particular in the NW area, we observe the presence of positive velocity values. Results show that: (a) a decreasing subsidence rate might be related to the reduction of volcanic activity in correspondence of the CBRC; (b) positive velocity signal, decreasing with time, might be interpreted as follows: – evidence of volcano structure lateral spreading, according to the velocity pattern distribution in this area and to the relative local flanks topographic convexity of the volcano structure; – uplift signal of this sector of mountain chain; – combination of the two mechanisms above.     

A simple model for the prediction of the deflation threshold shear velocity of dry loose particles

A very important parameter in aeolian equations is the deflation threshold shear velocity, which quantifies the instant of particle motion. In this paper, a simple model is presented for the prediction of the threshold shear velocity of dry loose particles. It has the same functional form as the widely used models of Bagnold (1941) and Greeley & Iversen (1985), but differs in its treatment of the so‐called threshold parameter. As the new expression was based on the moment balance equation used by Greeley & Iversen, it includes a function for the aerodynamic forces, including the drag force, the lift force and the aerodynamic moment force, and a function for the interparticle forces. The effect of gravitation is incorporated in both functions. However, rather than using an implicit function for the effect of the aerodynamic forces as in the Greeley & Iversen model, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals' force between two particles, it was also shown that the function for the interparticle cohesion force is inversely proportional to the particle diameter squared. The model was calibrated on data reported by Iversen & White (1982). The new expression compared, at least for terrestrial conditions, very well with the Greeley & Iversen model, although it is much simpler. It was finally validated with data from wind‐tunnel experiments on different fractions of dune sand and sandy loam soil aggregates. The soil aggregates were treated as individual particles with a density equal to their bulk density. The good agreement between observations and predictions means that, when predicting mass transport of particles above a given soil, minimally dispersed particle‐size distributions should be considered rather than the granulometric composition of the soil.     

敦煌莫高窟岩体风蚀机理及其防护对策的研究

利用风洞模拟实验研究莫高窟岩体的风蚀机理，结果表明，挟沙风风蚀速率明显大于净风。沙岩（层）结构、构造脆弱是产生岩体风蚀差异的主要原因。其中“沙楔”作用加速了风蚀烈度。采用１０％的ＳＳ和１０％的ＰＳ材料加固岩体是防止岩体风蚀的一条重要途径。     

腾格里沙漠东南缘横向沙丘粒度变化及其与坡面形态的关系

腾格里沙漠东南缘的横向沙丘因断面形态不同可分为凸形和凹形两种类型。横向沙丘丘顶至落沙坡为风沙堆积区, 沙粒沿滑落面作重力下滑时, 较粗颗粒先到坡脚, 背风坡的沉积物下粗上细。沙丘迎风坡则因断面形态不同而影响沙丘表面过程。其中, 凸形沙丘迎风坡为高饱和风沙流环境, 下部有大量蚀余粗颗粒, 上部发生加积, 故下粗上细; 而凹形沙丘的迎风坡则为低饱和风沙流环境, 表面普遍风蚀, 前积纹层出露表面, 沉积物下粗上细, 实际上是背风坡沉积物重力分异结果的重现。沙丘高度与平均粒径和分选参数之间具有良好的一致性。风成床面作为一种地形, 对风沙地貌表面过程产生了重要影响, 这是风沙地貌尚须努力开拓的领域之一。     

The effect of surface moisture on the entrainment of dune sand by wind: an evaluation of selected models

Abstract Reliable predictions of wind erosion depend on the accuracy of determining whether erosion occurs or not. Among the several factors that govern the initiation of soil movement by wind, surface moisture is one of the most significant. Some widely used models that predict the threshold shear velocity for particle detachment of wet soils by wind were critically reviewed and evaluated. Wind‐tunnel experiments were conducted on pre‐wetted dune sand with moisture contents ranging from 0·00 to 0·04 kg kg^?1. Sand samples were exposed to different wind speeds for 2 min. Moisture content was determined gravimetrically before and after each experiment, and the saltation of sand particles was recorded electronically with a saltiphone. Shear velocities were deduced from the wind speed profiles. For each moisture content, the experiments were repeated at different shear velocities, with the threshold shear velocity being determined by least‐squares analysis of the relationships between particle number rates and shear velocity. Within the 2‐min test runs, temporal changes in particle number rates and moisture contents were detected. A steep increase in the threshold shear velocity with moisture content was observed. When comparing the models, large differences between the predicted results became apparent. At a moisture content of 0·007 kg kg^?1, which is half the moisture content retained to the soil matrix at a water tension (or matric potential) of ?1·5 MPa, the increase in ‘wet’ threshold shear velocity predicted with the different models relative to the dry threshold shear velocity ranged from 117% to 171%. The highest care should therefore be taken when using current models to predict the threshold shear velocity of wet sediment. Nevertheless, the models of Chepil (1956; Proc. Soil Sci. Soc. Am., 20, 288–292) and Saleh & Fryrear (1995; Soil Sci., 160, 304–309) are the best alternatives available.