Dunes

Figure 6-32, taken from Govier and Aziz, schematically indicates four flow pattern regions superimposed on a plot of pressure gradient vs. mixture velocity = Vl -t- V5 = Qj + ( s)/A where and Vs are the superficial liquid and solid velocities, Qi, and ( 5 are liquid and solid volumetric flow rates, and A is the pipe cross-sectional area. is the transition velocity above which a bed exists in the bottom of the pipe, part of which is stationary and part of which moves by saltation, with the upper particles tumbling and bouncing over one another, often with Formation of dunes. With a broad particle-size distribution, the finer particles may be fully suspended. Near V 4, the pressure gra-... 

Finally, inadequate hole cleaning results in an overloading of the annulus with cuttings, especially in very high penetration rate, poor mud properties, and insufficient annular velocity or circulation time. Inadequate hole cleaning can also be experienced in deviated wells with the formation of cutting beds on the low side migrating in a sand dune fashion. 

Sand Dunes in California" ( Ansel Adams Publishing Rights Trust/CORBIS)... 

As the particles enter the conveying line, they tend to settle out before they are fully accelerated. They form dunes which are then swept bodily downstream giving an uneven longitudinal distribution of particles along the pipeline. 

The particles settle out as in slug flow but the dunes remain stationary with particles being conveyed above the dunes and also being swept from one dune to the next. 

Following slug flow, the particles, instead of forming stationary dunes, gradually build up over the cross-section until they eventually cause a blockage. This type of flow is less common than dune flow. 

H. Gryta, J. C. Debaud, A. Effosse, G. Gay, and R. Marmeisse, Fine-scale structure of populations of the ectomycorrhizal fungus HeheUnna cylindrosponim in coastal sand dune forest ecosystems. Mol. Ecol. 6 353 (1997). 

K. Old and T. Nicol.son, Electron microscopical. studies of the microflora of roots of sand dune grasses. New Phytol. 74 51-58 (1975). 

G. A. Kowalchuk, J. R. Stephen, W. DeBoer, J. 1. Prosser, T. M. Embley, and J. W. Woldendorp, Analysis of ammonia-oxidizing bacteria of the beta subdivision of the class Proteobacteria in coastal sand dunes by denaturing gradient gel electrophoresis and sequencing of PCR-amplified I6S ribosomal DNA fragments. Appl. Environ. Microbiol. 6.1 1489 (1997). 

The outline of Victoria Crater is serrated, with sharp and steep promontories separated by rounded alcoves (Fig. 8.36). The crater formed in sulfate-rich sedimentary rocks, and is surrounded by a smooth terrain that extends about one crater diameter from the rim. On the crater floor is a dune field. There are no perched ejecta blocks preserved on the smooth terrain around the crater rim, probably planed off by Aeohan abrasion. The Mossbauer mineralogy of the sedimentary rocks at the crater rim and inside the crater itself is nearly the same as at Eagle crater landing site and Endurance crater, both about 6-8 km away [335]. 

Aeolian sands are deposited by wind action, either in dunes or in extensive sheets ( cover sand areas ) Wind action is particularly effective in hot and dry regions such as deserts but sand dunes are also common in (sub)humid regions with sparse vegetation, notably in overgrazed areas and along beaches and fluvial braid plains . The (weathering) history of the parent materials in the source area determines whether the sands are rich in quartz and/or carbonates. 

Free dunes have no fixed position, but migrate downwind by erosion on the gently inclined windward side and deposition on the leeward side (slip face) in the same way as described for fixed dunes. The smallest free dunes are common wind ripples that measure only a few centimeters in height. Large dunes are found in extensive dune areas in deserts, in sand seas known as ergs . 

Coastal dunes occur along beaches or sand-flats that form part of a non-erosional sandy or deltaic coast. The source areas of the sand will eventually lose all sand, silt and clay particles some become wet (groundwater) depressions whereas others acquire a rocky or boulder-strewn surface known as a desert pavement . 

Two main types of free dunes are distinguished, viz. crescentic dunes and linear dunes. 

Crescentic dunes are typically wider than long. They assume a crescent shape ( barchan dunes ) where winds are predominantly uni-directional. The rate of advancement of the sand is roughly... 

The area of actual erg and dune formation is delimited by the 150 mm/yr isohyet. This precipitation boundary appears to have shifted strongly in the recent past. Between 20,000 and 13,000 yr BP, the southern limit of active dune formation in the Sahara desert was 800 km south of its present position and most of the now sparely vegetated Sahelian zone was an area of active dune formation at that time. These dunes, mostly of the longitudinal type, are now fixed by vegetation, but their aeolian parentage is still obvious from their well-sorted material. A similar story can be told for the Kalahari sands. Overgrazing in recent times has reactivated aeolian transport in many regions with sands. 

Dunes and sand plains form where strong winds carry sand grains in saltation over short distances. Particles finer than sand are transported in suspension and over greater distances until they settle as loess , predominantly in the steppe regions adjacent to the desert zone. 

Clearly, aeolian processes were much more important at that time than at present. Large parts of the present temperate zone, from the cover sands of the Netherlands to the sand dunes in north-east Siberia are Ice Age (aeolian) sands. South and east of this cover sand belt lies a belt of loess deposits, extending from France, across Belgium, the southern Netherlands, Germany and large parts of Eastern Europe into the vast steppes of Russia, and further east to Siberia and China. A similar east-west loess belt exists in the USA and less extensive areas occur on the Southern Hemisphere, e.g. in the Argentinean pampas. 

Arid and semi-arid lands are quite widespread in Asia, where deserts make up some 11% of this continent (Table 1.1) in Central Asia, north and east of the Caspian to Aral Sea area. In the Turkestan desert, large dunes form huge sand seas (Nettleton and Peterson, 1983). West and North of China, the Takla-Makan and Gobi deserts dominate the landscapes. More to the south, the Great Indian Desert extends up to the Himalayan foothills. More to the west are the hyperarid areas of the Dasht-i-Lat and Dasht-i-Kavri deserts in Iran and Syrian deserts. Immense areas in the north-central part of the Arabian Peninsula and in the Rub al Khali deserts in its southeastern part are hyperarid or arid. 

The semi-arid areas receiving somewhat higher rainfall are covered by a steppe-like fairly continuous vegetative cover of Xerophilous shrubs and grasses. In the Central Asian Plains, east of the Caspian Sea, wind erosion and transport are the dominant features. Sand dunes dominate the landscape for hundreds of kilometers. 

Wind transport. Wind-blown components are carried away over a more or less important distance as a function of wind velocity and particle size of the material. Wind speeds up till 6.5 m/sec transport dust and fine sand with a diameter of less than 0.25 mm sand grains up to 1 mm diameter are uplifted at wind speeds of 10 m/sec. At 20 m/sec also particles of 4-5 mm may be removed. Based on these physical laws, the transportation of coarse fragments, in casu the sand fraction, occurs over-relatively short distances from the deflation zones. These sand grains settle then in more or less continuous layers and either become progressively mixed with the underlying soil layers, or concentrate in dune formations. 

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