## 2 Methodology
### Determine Crash Risk Factors Using Fault Tree Analysis (FTA)
#### 2.1.3 Hydrological and Geological Factors  
Hydrological dynamics, including rainfall patterns and flood hazards, pose significant risks to road infrastructure by impacting visibility, road stability, and overall driving conditions. Geological considerations, such as rock properties and terrain characteristics, play a role in determining road stability and susceptibility to geological hazards, such as landslides and rockfalls. When integrated into fault tree analysis, these environmental factors help identify potential root causes of road crashes, enabling a better design of roads. Hydrological Investigations for Flood Hazard Mapping  
Flood hazard maps help develop improved routes that prioritize safety from a hydrological perspective. This process involves several key steps: \begin{table}
\begin{tabular}{c c c} \hline
**Linear Density** & **Properties** & **Score** \\ \hline
0–0.000460876 & Very low & 9–10 \\
0.000460876 -0.000938212 & Low & 8 \\
0.000938212-0.001399088 & Low-Medium & 7 \\
0.001399088-0.001859964 & Medium-Low & 6 \\
0.001859964-0.0023373 & Medium & 5 \\
0.0023373-0.002798176 & Medium to High & 4 \\
0.002798176-0.003259052 & High & 3 \\
0.003259052-0.003736388 & High & 1–2 \\
0.003736388-0.004197264 & Very High & 1 \\ \hline \end{tabular}
\end{table} 
Table 4: Linearment Density Index. * **Rainfall Analysis**: A comprehensive statistical analysis of rainfall data is conducted to develop Intensity-Duration-Frequency (IDF) curves and design storms, recognizing rainfall as the primary driver of streamflow and vital for flood protection projects. * **Catchment Delineation and Modeling**: Digital Elevation Models (DEMs) with 30 \(\times\) 30 m resolution are used to delineate streams and their corresponding external catchment areas impacting the proposed road alignments. A hydrological model is then constructed using HEC-HMS software to estimate runoff flows and hyteographs for the delineated catchments. * **2D Floodplain Modeling**: Leveraging outputs from the hydrological model and high-resolution DEMs, a 2D model is developed in HEC-RAS to accurately map floodplains, water depths, and velocities. This information serves as the foundation for flood hazard assessment, with the rain-on-grid approach adopted to capture the nuances of flood characteristics within each stream segment. * **Existing Flood Protection Structures**: To comprehensively assess the flood risk for both the existing and proposed highway alignments, a thorough survey of existing flood protection structures was conducted. This evaluation helps in understanding the effectiveness of the current infrastructure against flooding events. Additionally, the survey will guide the integration of these structures into the design of the proposed alignments, ensuring they capitalize on available flood mitigation measures while minimizing potential disruptions during construction. A factor measuring this aspect will be considered in the proposed technique. * **Flood Hazard Assessment**: Recognizing the multifaceted nature of flood hazards, beyond flow characteristics, a comprehensive approach is implemented. While water depth and velocity remain the primary indicators, additional social, economic, and environmental factors are considered, albeit challenging to quantify. A bespoke code, utilizing the WRL formula and water depth/velocity data at each HEC-RAS time step, is developed to calculate the flood hazard index. This index, ranging from 1 (low hazard) to 6 (high hazard), is subsequently normalized to a scale of 1-10 for integrated analysis, as shown in Table 2.  
Geological Criteria (Lithology and Lineaments Elements)  
Rock properties play a significant role in environmental assessment and site selection for construction rather than the lineament elements of tectonic origin, such as faults, joints, contacts, and fractures [24]. In this study, rock types identified in the constructed geological map were classified into soft and hard types based on their origin.