Nepal is a mountainous country covered by rugged and fragile geological features and has faced many disasters.Until 1950, there was lack of research on estimating sediment load that passes through a hydraulic structure. In the past three decades, research on sediment transport and its management has gained momentum. Thus, most of the hydraulic infrastructures designed 30-40 years ago are facing problems relating to sedimentation, threatening their safety
The increasing trend of average annual rainfall, concentrated storms (cloud burst) resulting in massive flood events, and many more other events such as landslide-triggered floods have threatened the safety of hydraulic structures, including infrastructure of national importance like the Melamchi headworks.
This has motivated designers and engineers to explore such hydraulic structures that can withstand extreme hydrological conditions as well as climate-induced disasters.
In other words, it is necessary to make structures more resilient, that is, they should be able to absorb, recover and adapt to disturbances.
From an engineering perspective, a structure is considered resilient if it retains sufficient usability and stability with minimum damage after an extreme event. Thus, engineers and The social and economic development of Nepal has intensified investments into hydraulic infrastructures, such as dams, barrages, canals and dykes.
Besides, bridges across water channels form another crucial physical asset.
In the past, the hydraulic design of structures involved only the water level/ discharge of the highest flood observed with a certain factor of safety.
Now, a series of discharge values recorded or modeled over a long period of time are used for probabilistic design.
This development has allowed for a risk-based design as well as risk management concerning dams and flood control systems.
The expansion of knowledge on climate change and more available data for statistics point towards the need for re-evaluation of the safety criteria of hydraulic works.
The changes in the design floods would in turn lead to rehabilitation of a large number of existing water structures.
Similarly, until 1950, there was lack of research on estimating the sediment load that passes through a hydraulic structure.
In the past three decades, however, research on sediment transport and its management has gained momentum. Thus, most of the hydraulic infrastructures designed 30-40 years back are facing problems relating to sedimentation, threatening their safety.
The Koshi barrage, for instance, was built between 1958 and1962 for a design discharge of 9.5 lakh cusecs of water. Now it can hardly withstand a 5-6 lakh cusecs flow. A flow that is more than this quantity will likely overtop the barrage deck level. At that time, rise in the bed level due to sediment load could not have been estimated accurately, resulting in decreased hydraulic depth with each passing year.
Moreover, it has crossed its design life period of 50 years and is now operating beyond its design life span without a thorough inspection.
To tackle issues posed by aging structures, most European countries and the USA have already initiated rehabilitation or decommissioning of such infrastructures. Recently, India started a Dam Rehabilitation and Improvement Project (DRIP) to restore aging dams.
Another set of problems includes the glacier lake outburst flood (GLOF) and landslide dam break outburst flood (LDOF). Recent surveys show that GLOF is expanding at a considerable rate due to global warming, so the risk of breaching is high.
Nepal has experienced 24 GLOF events in the past, according to ICIMOD, several of which have caused considerable damage and loss of life. In recent times, floods due to sudden breach of pool formed by landslide sediment deposit, or LDOFs, have become a frequent event. The LDOF in the Seti River (2012) and LDOF in the Melamchi (2021) are the two most recent episodes.
The re-analysis of hydrological and hydraulic loads and consideration for additional discharge due to events like GLOF have prompted interest in modern hydraulic structures.
One such example is a newly developed type of non-linear weir (low dam to raise water level upstream) called Piano Key Weir (PKW).
A PKW was first constructed in France to rehabilitate the existing dams in order to increase their discharge capacity. As these are non-gated weirs which have high discharging ability at low heads, they are more resilient than other conventional weirs.
Moreover, we need to think of more robust structures that can withstand LDOF,GLOF-like disasters.
The robustness of these hydraulic structures can be improved if we protect the low head dams with a highly abrasion-resistant cover layer of fibre reinforced concrete (FRC) mixed with silica, which will give them a much better chance to resist and survive against such destructive hydrodynamic forces unleashed by a debris flood event.
A sudden flood due to GLOF/LDOF may result in very high stream power.
Such flow conditions can be minimised up to a certain level using drops.
Block ramps are another nature-based solution to prevent possible damage to downstream structures due to elevated stream power.
Similarly, additional flow due to sudden events like LDOF/GOLF and cloud burst event has made us think about the provision of enough space for a river in terms of a bypass channel upstream of the headworks or river crossing. Frequent overtopping/ breaching of levee has become a common problem in the Tarai region of Nepal during a flood.
The levee can be made more resilient by introducing regular overflow sections to avoid the undesired overtopping at random or problematic spots, although this leads to overflow and the loss of the flood protection function.
While the design flood may be exceeded, the levee as a structure will hopefully be protected from failing.
Thus, there are a number of ways to make a structure more resilient against the increasing number of disasters.
Chaudhary and Mishra are engineers trained at IIT, Roorkee, India
A version of this article appears in the print on August 27 2021, of The Himalayan Times.
