A bridge too far out

The road to Dzongu — some 74 km north of Sikkim's capital Gangtok — caved in after a glacial lake outburst flood (GOLF) breached the Chungthang Dam near the Teesta Stage III hydropower project in October 2023. Mountain-ringed Dzongu, the remote abode of the ancient indigenous Lepcha community, was more isolated than ever. "When the dam broke, we were left stranded from the rest of the world for two weeks," says Dzongu resident Mayalmit Lepcha. "So, we used our indigenous knowledge to make ru-soams (cane-bamboo bridges) to connect Dzongu with different parts of Sikkim."

The people of Dzongu typically depend on Mangan, the town closest to the Dzongu block, and Gangtok for daily essentials. However, frequent natural calamities often cut Dzongu off from the rest of the world. So, Mayalmit explains, after a disaster, the local people first construct these footbridges to access food and water.

A ru-soam is an engineering marvel that underlines the significance of traditional practices. It consists of two parallel canes called soamgyang. The soamgyang are tied to trees on the two ends, ensuring that the bridge remains suspended. A ru-soam contains swinging cane loops called ahool. Bamboos, or soamgur, are used to make the deck of the bridge.

The bridges can be 100 metres long, and two or three people can cross them at a time. "Although bamboo and cane bridges are loosely termed as suspended bridges, these are only footbridges… and are not meant to transport vehicles," says Hemant Kaushik, Professor in the Department of Civil Engineering and the Centre for Disaster Management and Research (CDMR) at the Indian Institute of Technology (IIT) Guwahati.

The soamgyangs function as the main cables — similar to the ones used in modern-day suspension bridges — and bear the vertical loads. Such cables help keep the bridge suspended. The trees at the two ends work as a tower, where a part of the vertical load is transferred. The ahool is the suspender cable, which connects the main cable and the deck. Ru-soams are made of horizontal bamboo braces and rails, which run in a parallel pattern along the length of the bridge, providing longitudinal flexibility to the deck. Intermediate vertical components, known as vertical posts or struts, connect the braces that run horizontally. "The struts not only offer additional flexural strength to the overall suspended bridge system, they also prevent the braces from sagging," adds Kaushik's colleague Arindam Dey, Associate Professor in the Department of Civil Engineering and the CDMR at IIT Guwahati.

There is growing interest in ru-soams, as they are environmentally sustainable. Bamboo, also known as 'green steel', is hailed for its durability, corrosion-withstanding abilities, and tensile and compressive strength. While Dzongu has at least 31 bamboo species growing in its tropical and sub-tropical forests, ru-soams are mainly made of mahlu (Bambusa nutans) and podiyang (Bambusa pallida).

Bamboo has a hollow culm, interspersed by diaphragms at each node. In mature bamboo, the culm wall is often thick. The species with greater diameters are used in heavy construction. When observed under a microscope, a transversely sectioned bamboo culm shows cellulose fibres with vessels that increase in number towards the outer periphery. Cellulose, a complex, branched carbohydrate, helps keep the bamboo upright and stiff by acting as a reinforcement. These flexible fibres, which often mirror steel bars in reinforced concrete, help bamboo withstand high-velocity winds, hailstorms, and earthquakes.

Density is the most crucial mechanical property of a material used for sustainable civil engineering and construction purposes. For most bamboo species, used for bridges and architecture, the density is 700-800 kilogram per cubic metre (kg/m3).

The cellulose-surrounded vessels offer tensile strength, or stress, and the ability to withstand bending stress. Tensile strength refers to the maximum stress a material can bear without breaking. Bending stress is defined by the resistance that comes from within a material when an external force is applied. Bending stress helps determine how a material or structure will behave when faced with an impending load, and how long it will last. A dense material has a relatively higher tensile and bending stress.

"Bamboo is lightweight and has a high tensile strength-to-weight ratio… the average tensile strength of bamboo is nearly 225 MPa (megapascal)," points out Satheeskumar Navaratnam, Lecturer in the Department of Civil & Infrastructure Engineering at RMIT University, Australia. Bamboos with a 225 MPa can bear about 225 kilograms of weight.

The high tensile strength-to-weight ratio of bamboo implies that its strength is higher than its mass. If loads are placed on bamboo and then lifted, the bamboo returns to its original form because of its tensile and compression strength. "The bamboo canes in the deck, braces, and struts together impart substantial flexural strength to the suspended bridge structure," Dey adds.

When calamities such as earthquakes strike, the tensile strength in steel stoops, unlike that in bamboo. So, steel structures encounter more damage than bamboo ones. Bamboo is also more resilient than concrete and timber against deformation. An experiment (bit.ly/bamboo-steel) shows that when it comes to strength, steel is the best, followed by bamboo, and then concrete and wood. In terms of stiffness, bamboo is the best, followed by steel. However, bamboo's compression and tensile strengths fluctuate based on the species, habitat, weather, altitude, and soil health.

Ru-soams can sustain several cycles of heat, rain and snow without showing any remarkable deterioration. "Even though in comparison to concrete and steel bridges, their life period is short, it is sufficient for serving the local community," Kaushik says.

However, he points out that these are non-engineered structures and, thus, are not adequately tested. "The risk associated with such a structure remains very high when afflicted by natural disasters." An engineered structure, unlike a bamboo one, undergoes several rounds of assessment to determine the possible damaging effects of seismic waves, flash floods, or high-velocity wind on it. Kaushik says that narrow walkways are ideal for communicatinig in hilly areas at dire times, but "severe disasters" might lead to their collapse. "However, the local community knows where to build a bridge so that it is not ravaged by natural disasters."

Ru-soams have other advantages. Navaratnam adds that they require "little time and fewer qualified labourers" to build. According to Mayalmit, it takes only three days to build a ru-soam. Navaratnam points out that bamboo-cane suspended bridges made with local and community knowledge are also appropriate for earthquake-, landslide-, and hailstorm-prone areas in other Himalayan regions where bamboo and cane are abundantly available. Bamboo and cane are also environment-friendly and take 3-5 years to regrow. Unlike steel or concrete, they do not leave a massive carbon footprint.

The Lepcha community's in-depth understanding of nature and environment, agriculture, indigenous technology, and medicine is carried forward from generation to generation through folklore, lullabies, community songs and taboos.

"An effective and sustainable disaster prevention and response require a good understanding of the place and community," says environmental scientist Henry P. Huntington, who works on indigenous knowledge and disaster management, and is the Arctic Science Director of the U.S.-based Ocean Conservancy. Indigenous and local communities — including the Lepchas — deeply understand the local environment and develop time-tested knowledge for sustainable disaster response, adaptation, and mitigation.

The subduction of the Indian Plate and its descension below the Tibetan Plate are making the Himalayan terrain highly fragile and earthquake-prone, as the severe earthquake in early January 2025 bears out. Additionally, as a consequence of the Southwest monsoon, the region frequently encounters cloud bursts, landslides, flash floods, and debris flow. There are 463 glacial and high-altitude lakes in Sikkim, of which, it is estimated, 40 are at risk of bursting. Rising global temperatures, coupled with glacial melting, are increasing the number and size of such lakes, further exposing Sikkim and neighbouring regions to the risk of GOLF.

Huntington, Dey and Kaushik stress the need for modern-day disaster management strategies across the world to highlight indigenous knowledge. When weather patterns are erratic and the impacts of climate change are becoming punishingly severe, local and indigenous knowledge is "valuable and constantly evolving", Huntington stresses. However, Kaushik and Dey add that indigenous structures must incorporate modern-day engineering principles to reduce risk.

Madhurima Pattanayak, a Kolkata-based science journalist, covers different aspects relating to climate change.