Castles Made of Sand
The date is September 13, 1940. A quarter-million Italian troops cross the Libyan border into Egypt to attack the British. The plan? Break through the weak defenses and seize control of the Suez Canal, a vital link between England and her overseas colonies. Just as the plan got underway, the Italian commander, Marshal Graziani, heard his supply lines were under attack, depots were being raided, convoys were captured and airfields well behind the frontlines were destroyed.
The attacks all came from the generalís southern flank, which he thought was safe because it was buffered by an impassable sand sea, the Libyan Desert. He halted his attack while he decided how to face this new threat, which gave the British time to get reinforcements and counterattack. So what happened?
Major Ralph Bagnold and a small band of raiders, cobbled together only six weeks prior, happened. Bagnold, a World War I veteran, had spent the time between 1932 and 1938 exploring the desert and his knowledge of the Libyan Desert in particular was unmatched in his time. He had studied sand and sand dunes more closely than anyone before him. He offered his service to General Sir Archibald Wavell who quickly took him up on it. With a few New Zealanders, unarmored or lightly armored vehicles, and supplies for two weeks, he sprinted out into the desert.
Over the next few months, he led his men in dangerous reconnaissance missions and made daring raids through the inhospitable desert terrain. He was so successful, the group was expanded to include the 3rd Coldstream Guards and the 2nd Scots Guard, grew into three teams, and was formally named the Long Range Desert Group (LRDG). Through the end of the North Africa campaign in 1943, the “Desert Rats,” as they were sometimes known, harried the Germans and Italians so successfully, they had a significant role in shortening the campaign against Rommel. When Rommelís deputy commander, von Thoma, was captured, he discovered, to his chagrin, that the British knew more about his troop strength and deployment than he did, largely thanks to the LRDG.
Just as the LRDG kept the British North Africa campaign alive in 1940, they also helped end it in 1943. When General Freyberg attacked Rommel in his last stand at Mareth, he brought his troops through “impassable” terrain over a route scouted for him by the LRDG. The LRDG and their American counterparts, famously depicted in the movie and television show “Rat Patrol,” altered the course of World War II in North Africa.
Fascinating, of course, but why does this matter to weather folks? In his day, Bagnold was an expert on sand dunes and sand movement. He has yet to be found wrong in any essential facts, sixty plus years after he published his book on the subject, The Physics of Blown Sand and Desert Dunes, in 1941. Sand dunes are formed, shaped and shifted by something weather forecasters and their customers find interesting: wind. Understanding how and why dunes form, and being able to predict sand movement based on wind patterns can give you tools for helping your customers conduct successful desert operations.
Sand dunes have two habitats, coastal and riverine beaches and the barren, waterless deserts. Vegetation near water modifies the shape and extent of dunes on beaches, but the desert dunes occur in areas of free interplay between sand and wind and have a simplicity of form unknown elsewhere in nature.
Because of this, desert dunes are the ones this article will discuss. Although there are several sub-types, sand dunes come in two basic varieties, barcans and transverse dunes. Within these two basic types, subsets include dunes that are modified by transitory or multidirectional wind regimes. Barcan dunes develop under unidirectional wind regimes and transverse dunes develop under bi- or multi-directional wind regimes.
A classic barcan is shaped like a smoothly rising hill that drops off suddenly at its highest point into a scalloped cliff-face called the slipface. (The slipface looks a bit like someone came along and took a bite out of the dune.) The wind comes up over the smooth back of the dune and keeps moving sand up the slope until the angle of repose (a maximum of 34 degrees) is exceeded and sand avalanches down the slipface.
Barchan size varies a lot and depends on wind strength and persistence as well as available sand. The maximum length and width is about 1,310 feet (400 meters) and the maximum height is about 100 feet (30 meters). They can occur singly where sand is limited, or in belts.
The same overall rule applies to transverse (or longitudinal dunes) but itís a bit more complicated. Transverse dunes develop in lines perpendicular (at right angles) to the wind. They are most clearly seen in areas where the wind changes direction seasonally, such as a place with a monsoon wind shift. Transverse dunes are also called seifs and have two or more slipfaces depending on how many wind regimes occur around them.
Seifs (swords) have sharp central peaks and slip faces on both sides. They are often irregular because one regime is stronger than another and can take on a long, serpentine appearance. Transverse dunes line up in symmetrical lines and, where there is enough sand, barcans form in the troughs between dune lines (because the wind is steered down the troughs). Seif dunes and their orderly chains may be enormous.
Many dunes in the Egyptian Sand Sea reach 330 feet (100 meters) in height. The highest known dunes are in southern Iran, where they rise to 690 feet (210 meters) in height and are six times as wide as they are tall. Seif chains may extend unbroken for 30-55 NM (55-100 km).
Dunes move by saltation and surface creep. Saltation is basically when sand grains are lifted and carried downrange by the wind. The sand drops out to bounce periodically because it actually puts a drag into the wind and slows it down. This process is also known as sand driving.
Surface creep occurs when, on impact with the ground, sand grains collide with others on the surface. The impact of the falling sand grain causes a “splash” and some impacted grains hop forward while other are sent up into the wind to begin the process all over again. Surface creep is responsible for roughly a quarter of dune movement and saltation takes care of the rest.
A dune can move surprisingly quickly and easily buries roads, buildings and equipment. Understanding how dunes build and move allows you to plan for this movement, predict its direction and even stop it. For instance, a barcan can be stopped, shrunk or even made to disappear entirely by the simple technique of sprinkling pebbles across its top.
Lifted sand moves mostly within 3.3 feet or 1 meter of the surface (maximum 6.5 feet or 2 meters) and turbulence in the wind flow is what gets the ball going. A hard surface will produce a thicker cloud (greater bounce effect) and a sand surface will produce a thinner one (bouncing is muffled). A true sandstorm will produce a sand cloud that “boils” along the surface and leaves people wading through it with head and shoulders above the cloud. Over a fine sand surface, the cloud may only be a few inches above the ground.
Where the surface is alluvial (silty), with little or no sand on it, such as Iraq or the vicinity of Khartoum, the dust rises in dense clouds to a height of several thousand feet and obscures the sun for days at a time. True sand storms do not do that. In the initial minutes, loose fine grains will blow off, producing a dusty period, but then the sky clears above the sand cloud and visibility above it is excellent once again through the remaining life of the sand storm. This is not well known because at the desert fringe, where most human activity occurs, dust from cultivated areas mixes in with the sand and obscures the actual source of visibility restrictions.
When wind regimes change, dunes of one type build up over dunes of another. By studying these compound dunes in cross section, climate change can be dated with some degree of accuracy. They arenít the only ones that can tell a story either. Barcan belts currently at the eastern rim of the Libyan Desert were actually the first dunes to develop when the regional climate changed 7,000 years ago.
When they first formed in southern Egypt and northern Sudan, they were at the upwind extreme of the sand field but moved steadily downwind over time and eventually ended up hundreds of miles from where they began. Today, the prevailing wind regime in this region is clearly imprinted in the sand. The whole dune system of northeastern Africa makes a vast wheel round from a north-northwest to south-southeast axis in the north, through true north to south, to an axis of east-northeast to west-southwest in the south.
One last bit of sand trivia to round things out for you. Did you know that sand can sing and boom? Although no one is entirely sure why it occurs, piled sand can set up a uniform resonant vibration that is acoustically enhanced deep in the dunes and can come out quite loud at times.
Ranging from squeaks and squeals to a booming so loud you have to shout to be heard above it, sand song chiefly occurs in dry sand and is often triggered by movement of the sand. A slipface avalanche could start it or something as minor as a sweep of a hand through the sand could do the trick. A legend Bagnold heard said the sound was the bells of a long-buried church. The Singing Sands of the American Southwest are reputed to be the sound of a dinosaur calling for its lost mate. Bagnold himself reported it often sounded much like the roar of a multi-prop airplane.
Sand, in the vast quantities seen in deserts, is beautiful, fascinating, and mysterious stuff. With research and insight, the successors of R.A. Bagnold continue to unlock some of those mysteries yet still find they conceal a thousand more!
Written by Melody L. Higdon, 14 WS/DOPA