Why Deck-Type Steel Truss Bridge Is Preferred for Upper-Trishuli-3A Post-Disaster Reconstruction？

1. Introduction to Upper-Trishuli-3A Disaster and Site Conditions

1.1 Recent Debris Flow and Flood Disaster Background

On August 26, 2026, a severe glacial rock collapse and catastrophic debris flow broke out in Rasuwa District, Nepal, severely striking the Upper-Trishuli-3A hydropower valley. The sudden flash flood carried massive boulders, tree trunks, and sediment downstream, destroying multiple rural river crossings, temporary access bridges, and key lifeline highways connecting the hydropower station. Critical bridge positions including Devighat and Syabrubesi suffered complete structural failure, cutting off rescue transportation, mechanical access, and daily community passages. Under such harsh post-disaster reconstruction conditions, selecting a suitable modular steel truss bridge becomes the core factor to ensure safe, fast, and long-term effective traffic recovery.

1.2 Unique Geographic and Climatic Characteristics of the Valley

The Upper-Trishuli valley features typical high-altitude mountain terrain with steep slopes, narrow V-shaped river channels, and extremely unstable river hydrology. During monsoon seasons from June to September, concentrated heavy rainfall triggers frequent flash floods and secondary mudslides. The local river water level rises sharply within several hours, accompanied by a large number of floating impact debris. In addition, the mountainous area is humid all year round with strong fog and high salinity in the air, bringing high corrosion risks to steel structures. Construction conditions are extremely limited: narrow river banks leave no space for large hoisting equipment, and most emergency bridge projects can only adopt cantilever pushing and dragging construction methods. All of these harsh environmental factors put forward strict requirements for bridge structural form, impact resistance, traffic adaptability, and construction convenience.

2. Structural and Applicability Comparison: Deck-Type vs Through-Type Truss Bridge

In mountain flood and debris flow disaster areas, deck-type (upper-bearing) steel truss bridges and through-type (lower-bearing) steel truss bridges show completely different adaptation performances. The following table comprehensively compares the two bridge types based on the actual reconstruction demands of the Upper-Trishuli-3A hydropower zone.

Evaluation Item

Deck-Type (Upper-Bearing) Truss Bridge

Through-Type (Lower-Bearing) Truss Bridge

Flood   & Debris Impact Resistance

All truss   members are under the deck; no exposed vertical truss above the road surface.   Effectively avoids collision damage from boulders and floating driftwood   during flood surges.

Vertical   truss frames stand on both sides of the roadway, directly exposed to the   flood water level. Vulnerable to impact deformation and structural damage by   high-speed floating debris.

Construction   Machinery Passage

Full open   deck without width and height restrictions, suitable for heavy excavators,   dump trucks and oversized hydropower rehabilitation equipment.

Restricted   clear width and overhead clearance, easily causing passage obstacles for   large engineering vehicles.

Field   Erection Adaptability

Perfectly   fits narrow valley conditions, suitable for unilateral dragging and   cantilever pushing construction without large cranes.

Complicated   temporary stability control for side frames, higher difficulty in narrow-site   emergency erection.

Corrosion   Maintenance Performance

Under-deck   truss structure is concentrated and easy to inspect, repair and maintain,   adapting to humid mountain climate.

Top   bracing and upper chord components are difficult to maintain, increasing   long-term failure risks.

Applicable   Scenarios

Post-disaster   lifeline roads, hydropower heavy-duty temporary bridges, high debris-flow   risk river crossings

Semi-permanent   rural roads with low flood level and few floating debris

3. Core Advantages of Deck-Type Truss Bridge in Post-Disaster Reconstruction

3.1 Superior Anti-Impact Safety for Flood-Debris Environment

The biggest hidden danger of secondary damage in the Trishuli valley after the disaster is repeated flash floods and rolling boulder impacts. The through-type truss bridge has exposed vertical trusses on both sides, which are directly facing the flood surge. Once impacted by heavy driftwood or boulders, local truss deformation or bolt loosening will occur, forcing traffic interruption and affecting the progress of hydropower station restoration. In contrast, the deck-type structure places all load-bearing trusses under the bridge deck. The integral steel deck completely shields the lower truss system, realizing zero exposure of vulnerable structural components to flood impact. This is the most critical safety advantage for long-term stable operation in Nepal’s disaster-prone valleys.

3.2 Fully Adaptable to Hydropower Heavy-Duty Transportation

The post-disaster reconstruction of Upper-Trishuli-3A requires continuous entry of heavy engineering machinery, including large excavators, loaders, muck transport vehicles and power station maintenance equipment. The unobstructed wide deck of the deck-type steel truss bridge eliminates the traffic limitation of side trusses, ensuring smooth and efficient passage of all heavy-duty vehicles. It provides reliable heavy-load traffic guarantee for site cleaning, slope reinforcement, equipment transportation and subsequent hydropower commissioning work.

3.3 Fit for Narrow Valley Emergency Construction

Affected by the overall valley road collapse, large lifting machinery cannot enter most reconstruction sites. The deck-type modular steel bridge adopts standardized prefabricated components and mature unilateral pushing construction technology, which can complete rapid assembly under extreme narrow working conditions. It matches the emergency rescue and rapid road-opening requirements of the Nepal Army and the Department of Roads (DoR), greatly shortening the reconstruction cycle.

4. EVERCROSS Technical Strength and Local Project Experience

As an experienced modular bridge supplier deeply rooted in the Nepalese market, EVERCROSS has successfully completed two HD200 reinforced steel bailey bridge projects in Nepal in 2026. The 51.816-meter three-row single-layer reinforced steel bailey bridge adopts full hot-dip galvanizing anti-corrosion treatment, which is highly adaptable to local humid and rainy mountain environments. All products comply with international standards including AASHTO HL-93, Eurocode 3 and BS 5400. With mature local logistics, erection guidance and inspection experience, EVERCROSS’s HD200 type bailey bridges have become the most reliable preferred solution for post-disaster emergency reconstruction and hydropower temporary traffic facilities in the Trishuli river basin.

5. Conclusion

Combined with the special geographical environment, frequent flood and debris flow risks, and heavy-duty traffic demands of the Upper-Trishuli-3A hydropower valley, the deck-type upper-bearing steel truss bridge has absolute advantages in impact resistance, construction adaptability, traffic efficiency and later maintenance. Compared with the through-type bridge, it can effectively avoid secondary flood damage, ensure the continuous progress of post-disaster reconstruction, and provide stable and safe traffic support for hydropower station recovery and local lifeline road restoration. It is undoubtedly the optimal bridge type for current and future mountain disaster reconstruction projects in Nepal.