The Obigarm–Nurobod road section of the existing M41 highway, which carries about 3000 vehicles per day, will be inundated once the Rogun HPP reservoir has filled to operating levels. The government of Tajikistan has requested the assistance of the Asian Development Bank (ADB) to construct a 72 km long road section that will bypass the HPP reservoir through mountainous terrain. FutureWater has undertaken a Climate Risk and Vulnerability Assessment (CRVA) in which (i) the sensitivity of the project components to climate and/or weather conditions has been assessed, and (ii) climate risks and adequacy of proposed technical solutions have been evaluated.

ADB is providing a technical assistance grant to the government of Tajikistan (the government) for the preparation of the CAREC corridors 2, 3, and 5 (Obigarm–Nurobod) Road Project. The project road, about 72 km long, will replace a section of the existing M41 highway that will be inundated due to the construction of the Rogun Hydropower (HPP) project. The project road passes through mountainous terrain and includes 3 tunnels of total length about 6 km, several substantial bridges, and a high level 700 m long bridge over the future hydropower reservoir. The bypass road must be completed and opened to traffic by latest November 2023, the date by which the rising water in the HPP reservoir will have inundated several critical sections of the M41 highway. No other part of Tajikistan’s national highway network can provide for this traffic, and the only alternative route would represent a deviation of about 500 km.

The executing agency for implementing the project is the Ministry of Transport (MOT), represented by its Project Implementation Unit for Roads Rehabilitation (PIURR). The detailed design of the road has been completed by a national design consultant appointed by Tajikitan’s Ministry of Transport (MOT). This climate risk and vulnerability assessment (CRVA) has examined the proposed components for CAREC corridors 2, 3, and 5. A detailed climate risk assessment was conducted for the project road for the period to 2050 to ensure the design specifications are adequate for future climatic conditions. The climate model analysis yields following conclusions:

  • Temperature increases by about 2.4 °C (RCP4.5) to 3.1 °C (RCP8.5) are to be expected.
  • Minimum and maximum temperature are likely to change inconsistently, with maximum air temperatures increasing more than minimum air temperatures.
  • Extremes related to temperatures (e.g. warm spells, extremely warm days) are likely to increase in frequency and intensity.
  • Precipitation totals are likely to increase slightly but a large spread in precipitation projections has to be noted.
  • Precipitation extremes are likely to increase in frequency and intensity. For example, maximum 1-day precipitation volumes with return periods of 50 and 100 years are expected to increase by about 20% according to the 75th percentile values in the distribution of change projections of the entire climate model ensemble.

The increase in extreme precipitation events is considered as the most important climate risk for the project road. This not only leads to higher extreme discharge events but can also lead to more frequent and more powerful mudflows, landslides, and/or avalanches. The increase in temperature can pose additional loadings from thermal expansion to bridge joints and bearings as well as the road pavement asphalt, but it is unlikely that these would be significant.

The project design consultant team recalculated the expected flow characteristics for bridge sections for 1:100 years discharge events using a foreseen 20% increase in daily maximum precipitation. The recalculations reveal that bridges have sufficient capacity in the current design to cope with higher discharge levels in the future, although it would be prudent to check the bridge substructure designs to withstand higher flow velocities and increased debris content in the flow. Heavier scour protection works may be required if structural deterioration of bridge components is observed. The project design consultant team similarly recalculated the expected flow characteristics for culvert and roadside drains, but now for 1:50 years discharge events considering a 20% precipitation increase. The recalculations reveal that the drainage capacity of the culverts is well in excess of foreseen increases in flow, whether it be precipitation, mudflow, or avalanche.

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