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Importance of Risk Analysis

Awareness of underlying assumptions and making informed decisions regarding safety are crucial components of engineering design. In structural engineering, we meticulously consider the material’s properties, compute various parameters through structural analysis principles, and assess structural safety across different scenarios, adhering to safety standards outlined in the code of practice. In such scenarios, the structural behavior, even under varying conditions, tends to be more predictable.

However, when dealing with natural phenomena like hydrology and hydraulics, uncertainties abound. Despite these challenges, progress must continue. Yet, it’s imperative to make decisions informed by a thorough understanding of the assumptions made, ensuring decision-makers are well-informed about associated risks. In this blog, we explore the significance of informed decision-making in hydrology and hydraulics, drawing from firsthand experiences in these fields.

Design Flood for Bridges and Small storage structures

In 1979-80, immediately following the completion of my Post Graduate Diploma in Hydrology, I was assigned to the Hydrology (Small Catchments) Directorate. Our primary focus was on preparing flood estimation reports for small and medium-sized catchments. These reports aimed to estimate the design flood, often with a 50-year return period, for structures situated in ungauged or inadequately gauged catchments. Through a probabilistic risk analysis, utilizing the complementary probability concept, we identified a significant risk – more than 22% – of a flood of this magnitude occurring within the next 25 years. Conversely, opting for a flood with a 100-year return period reduced this risk to slightly over four percent. Consequently, the Khosla Committee, led by the esteemed Civil Engineer Dr. A.N. Khosla, made an informed decision to recommend a design flood with a 50-year return period for railway and highway bridges, balancing engineering safety and affordability as of 1965. However, the question arises: can we still justify undertaking the same risk in 2024, given our considerably more robust economy, faster trains, and significant increase in traffic? This warrants careful consideration. While I trust that those overseeing our railways and roads are cognizant of this issue and may already be contemplating revisions, if this matter remains overlooked, prompt action is imperative, potentially in consultation with the Central Water Commission (CWC)

Making Informed Decisions

In a recent assignment, I was tasked with estimating the design flood for floating solar platforms in a reservoir equipped with a gated spillway. Considering the average lifespan of a solar panel, which is around 25 years, I conducted a risk assessment to evaluate the implications of recommending a design flood for anchoring the solar panels. Due to insufficient documentation from recognized hydrological organizations, I deemed it too risky to proceed with a design flood 50-year return period, as recommended by some of the documents as it carried more than a 22 percent chance of being equalled or exceeded during the lifetime of the solar panels. Such an eventuality could potentially obstruct the free passage of floodwaters or disrupt spillway operations. Consequently, based on this assessment, I recommended a design flood with a 100-year return period, thereby reducing the risk to less than 5%. Through this process, I made an informed design decision and communicated my findings to the client accordingly.

The above examples illustrate the importance of taking informed decisions in Hydrological Design and making the decision-maker aware about the involved assumptions and risks.

Examples from Hydraulics

Now, let’s delve into a common example from the field of Hydraulic Engineering: the initial step in designing a bridge or barrage, which involves determining the waterway to which we can constrict a bridge or hydraulic structure on a river or canal. This process often relies on Lacey’s formula. Let’s explore this further, drawing from the experiences gained by me and my colleagues.

Limitations of Applying Lacey’s Formula

Drawing from my experience working for DHI (Danish Hydraulic Institute), where we were tasked with preparing a master plan for the Kosi River for flood and sedimentation management, several instances arose where the waterway provided for bridges in the basin appeared inadequate. Bridge engineers often rely on Lacey’s formula for estimating waterway as an indisputable truth. However, particularly for rivers like the Kosi that originate in the young and active formations of the Himalayas, indiscriminate application of Lacey’s formula may lead to various limitations:

Inaccurate Velocity Estimates

Lacey’s formula assumes uniform flow, steady flow, and constant channel slope, which may not hold true for meandering rivers prone to sediment deposition. This can result in inaccurate estimates of water flow velocity, potentially leading to underestimation of the hydraulic forces acting on the bridge structure.

Silt Deposition

Meandering rivers like the Kosi tend to deposit sediment along their banks and in their channels. Lacey’s formula does not account for the impact of sediment deposition on flow velocity or channel capacity. Consequently, there is a risk that the design may not adequately consider the potential for silt buildup around bridge piers or abutments, which could compromise the stability and integrity of the structure.

Channel Migration

Meandering rivers are also prone to lateral channel migration over time. Lacey’s formula does not account for changes in channel alignment, which could affect flow patterns and hydraulic forces on the bridge. This poses a risk of bridge scour and erosion, particularly if the bridge is not designed to accommodate potential changes in channel location. An instance I recall, while working for the Danish International Development Agency, involved intake structures designed for river Amochu or Torsa in Phuentsholing. The structures became nonfunctional almost immediately after construction as the river channel changed course. The location of the intakes could have been better decided by projecting the anticipated changes in river channel using flood and sediment modelling.

Variability in Manning’s Roughness Coefficient

The Manning’s roughness coefficient, used in Lacey’s formula to account for frictional resistance, may vary significantly in silt-prone rivers due to changes in sediment characteristics and bed morphology. Using a constant Manning’s coefficient may lead to inaccurate velocity estimates and hydraulic calculations.

Given these risks, it’s imperative to conduct a thorough hydraulic analysis and consider site-specific factors when designing a bridge for a highly silt-prone and meandering river. This may involve utilizing advanced hydraulic models and incorporating additional factors such as sediment transport, channel stability, and potential changes in channel morphology over time.

Therefore, when designing a bridge for such rivers, a comprehensive and site-specific approach is essential. This includes conducting detailed hydraulic analysis, evaluating sediment transport characteristics, assessing channel stability, considering bridge design considerations, addressing environmental impacts, and implementing monitoring and maintenance programs to ensure the safety and functionality of the bridge over time.

This highlights that collaboration among hydraulic engineers, geotechnical experts, environmental scientists, and other stakeholders is crucial to ensuring a successful outcome in addressing the challenges posed by highly silt-prone and meandering rivers.

Based upon my experience, I would recommend undertaking a review of hydrological and hydraulic designs for important bridges on silt-prone and meandering rivers if not already realized by those responsible.

Conclusion

In conclusion, it is evident that:

1. Informed decisions regarding design floods necessitate careful risk evaluation and transparent communication of impending risks to decision-makers. This ensures that the chosen flood management strategies are well-informed and aligned with safety priorities.
2. All critical structures must undergo comprehensive evaluation, considering not only engineering aspects but also social and environmental factors. This holistic approach ensures that infrastructure projects are sustainable, resilient, and beneficial for communities and ecosystems.
3. There is a pressing need to revisit the hydrological and hydraulic designs of bridges and other structures situated on meandering and silt-prone rivers. This review process, supported by hydraulic and sediment transport modeling, is essential for identifying vulnerabilities and implementing targeted improvements to enhance safety and functionality.
4. Regular review of design practices and the implementation of revised designs, informed by current conditions and advancements in technology, are crucial steps in ensuring the ongoing safety and effectiveness of our infrastructure. This proactive approach helps mitigate risks, adapt to changing environmental conditions, and uphold the integrity of our built environment.

Incorporating these measures will contribute to the resilience and sustainability of our infrastructure systems, safeguarding communities and ecosystems for generations to come.