In 1932, he staged the first recorded East-to-West crossing of the Libyan Desert. His work in the field of Aeolian processes was the basis for the book The Physics of Blown Sand and Desert Dunes, establishing the discipline of aeolian geomorphology, combining field work observations, experiments and physical equations. His work has been used by United States' space agency NASA in its study of the terrain of the planet Mars, the Bagnold Dunes on Mars' surface being named after him by the organization.
During the Second World War, he was a soldier in the British Army, in which he founded the behind-the-lines reconnaissance, espionage and raiding unit the "Long Range Desert Group", serving as its first commanding officer in the North Africa Campaign.
Bagnold was born in Devonport, England. His father, Colonel Arthur Henry Bagnold (1854–1943) (Royal Engineers), participated in the rescue expedition of 1884–85 to rescue General Gordon in Khartoum. His sister was the novelist and playwright Enid Bagnold, who wrote the 1935 novel National Velvet.
After Malvern College, he attended the Royal Military Academy, Woolwich. In 1915, Ralph Bagnold followed in his father's footsteps and was commissioned into the Royal Engineers. He spent three years in the trenches in France, being Mentioned in Despatches in 1917 and receiving the Belgian Order of Leopold in 1919.
After the war Bagnold studied engineering at Gonville and Caius College, Cambridge University, obtaining an MA before returning to active duty with the British Army in 1920 with the Royal Corps of Signals. He served in Cairo and the North West Frontier, India, where he was again mentioned in dispatches. In both of these locations he spent much of his leave exploring the local deserts. After having read Ahmed Hassanein's "Lost Oasis" he spent one such expedition in 1929 using a Ford Model A automobile and two Ford lorries exploring the vast swathe of desert from Cairo to Ain Dalla which was an area reputed to contain the mythical city of Zerzura. After a brief period of half-pay, he left the Army in 1935 but rejoined upon the outbreak of the Second World War II.
Bagnold and his travelling companions were early pioneers in the use of motor vehicles to explore the desert. In 1932, Bagnold explored the Mourdi Depression, now in Chad, and found implements dated to the Palaeolithic period in the valley. Bagnold wrote of his travels in the book Libyan Sands: Travel in a Dead World (first published 1935; reprinted by Eland in 2010). He is credited with developing a sun compass, which is not affected by the large iron ore deposits found in the desert areas or by metal vehicles as a magnetic compass might be. During the 1930s his group also began the practice of reducing tyre pressure when driving over loose sand.
In addition, Bagnold is credited with devising a method of driving over the large sand dunes found in the "sand seas" of the Libyan Desert. He wrote, "I increased speed. ... A huge glaring wall of yellow shot up high into the sky. The lorry tipped violently backwards—and we rose as in a lift, smoothly without vibration. We floated up on a yellow cloud. All the accustomed car movements had ceased; only the speedometer told us we were still moving fast. It was incredible ..." However, noted Fitzroy Maclean, "too much dash had its penalties. Many of the dunes fell away sharply at the far side and if you arrived at the top at full speed, you were likely to plunge headlong over the precipice. ... and end up with your truck upside down on top of you."
A recently discovered Silent film documents Bagnold's explorations and is available via the British Film Institute.
Bagnold wrote, "Never in our peacetime travels had we imagined that war could ever reach the enormous empty solitudes of the inner desert, walled off by sheer distance, lack of water, and impassable seas of sand dunes. Little did we dream that any of the special equipment and techniques we had evolved for very long-distance travel, and for navigation, would ever be put to serious use."
On 10 June 1940, Italy declared war on the United Kingdom in alliance with Germany while Bagnold was in Cairo due to an accident involving a troopship collision that he was on interrupting his journey elsewhere. Upon hearing the news and realizing that North Africa was about to become a theatre of war, he requested an interview with General General Wavell, Commander-in-Chief Middle East. Having secured it, Bagnold suggested that Wavell use his knowledge of the terrain in North Africa to establish a mobile scouting force for desert operations against the Italian Armed Forces in Libya, which Wavell was charged with defeating in the field. During the conversation, Wavell asked Bagnold what he would do if he found that the Italians were not doing anything beyond the Libyan coast in the desert interior. Bagnold replied that the new unit that he had in mind might be able to commit "acts of piracy". Wavell granted Bagnold authority to form a unit along these lines, with it being constituted in July 1940 with the name Long Range Desert Group (L.R.D.G.). After assembling its first formation, Bagnold was the L.R.D.G.'s Commanding Officer until August 1941, when he handed over command to Guy Prendergast on being promoted to the post of Inspector of Desert Troops. Later in the war he was promoted to the post of Deputy Signal Officer-in-Chief Middle East, with the temporary rank of Brigadier.
On 7 June 1944 Bagnold retired from the British Army with the end of military operations in North Africa after the Axis powers' defeat in that theatre. and returned to his scientific interests, being elected to a Fellowship of the Royal Society in the same year.
After the war Bagnold continued to work in the field of the geological science, and he published academic papers into his nineties. He made significant contributions to the understanding of desert terrain such as sand dunes, ripples and sheets. He developed the dimensionless "Bagnold number" and "Bagnold formula" for characterising sand flow. He gave a constitutive relation for a suspension of neutrally buoyant particles in a Newtonian fluid. He also proposed a model for "singing sands". and made contributions to the science of Sedimentology. His work received a number of awards. He was the 1969 recipient of the G. K. Warren Prize from the National Academy of Sciences. In 1971 he received the Wollaston Medal, the highest award granted by the Geological Society of London, and in 1981 the David Linton Award of the British Geomorphological Research Group. He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1974. Other awards included the 1970 Penrose Medal by the Geological Society of America; and the Sorby Medal from the International Association of Sedimentologists. He also received honorary D.Sc. degrees from both the University of East Anglia and the Danish University of Aarhus.
In his final years Bagnold lived in Edenbridge in the county of Kent in Englandd. He died at Hither Green on 28 May 1990 at the age of 94.
Bagnold married Dorothy on 8 May 1946 at Rottingdean in East Sussex, and had a son and a daughter.