Innovative Flood Hazard Classification for Resilient Urban Development

Background and Objective:
Flood zones and hazard classifications are vital tools for assessing flood risk to guide sustainable land development near floodplains. As the Neom project advances its visionary plan for a sustainable urban region in northwest Saudi Arabia, AtkinsRéalis was engaged to evaluate flood risks across this arid region. Given the unique hydrology of the area, with rainfall events characterized by peak intensities within the first 6 hours (Figure 1), warning times are limited. This underscores the need for a comprehensive and robust flood hazard study to support resilient urban planning.

Methodology:
AtkinsRéalis developed flood zone maps for each watershed district, delineating flood extents for 0.2% AEP, 1% AEP, and 10% AEP scenarios. The flood zones were categorized into three key classifications:
- Zone 1: Areas between district boundaries and the 500-year Floodplain.
- Zone 2: Areas between the 500-year and 100-year floodplains.
- Zone 3: Areas between the 100-year and 10-year floodplains, with the Channel Zone (Zone 3a) defined by the 10% AEP floodplain.
These classifications, developed in collaboration with the Neom client, integrate global flood risk methodologies and account for the absence of permanent rivers in the region.

A flood severity grid was also created by integrating Depth, Velocity, and Depth-Velocity (D*V) metrics. This grid utilized benchmarks from studies in Australia (Smith et al. 2014), the U.K. (DEFRA, 2006), and the U.S. (FEMA, 2023). Specific thresholds were established to evaluate impacts on people, vehicles, and buildings (Table 1):
- Low Depth (0.0-0.3 m) with low velocity (0-1.0 m/s): Small motor vehicles become unstable in flowing water.
- Medium Depth (0.3-1.2 m) with Low Velocity (0.0-2.0 m/s): Walking becomes dangerous.
- Any Depth (0.0-4.0 m) with High Velocity (> 2.0 m/s): Unsafe for people, vehicles, and buildings.

This framework provided a detailed hazard classification matrix (Figure 2), offering critical insights for Neom's emergency response planning and infrastructure development (Figure 3 - HNFMSC, 2006).

Result and significance of the Study:
The proposed classifications and recommendations will guide Neom authorities in determining suitable infrastructure placement and emergency response plans during flash floods. The hazard levels are defined as follows:
- Low Risk: Generally safe, with caution for individuals.
- Medium Risk: Unsafe for children and older adults.
- High Risk: Requires shelter-in-place warnings; unsafe for both people and vehicles.
- Very High/Extreme Risk: Evacuation is recommended; buildings may become structurally vulnerable.

AtkinsRéalis also introduced an innovative hazard classification formula-Depth x (Velocity + 0.5)-tailored for the Neom region. This refinement adapts the 'Flood Risk to People' methodology and FEMA's Depth x Velocity approach to suit local conditions. Figure 4 illustrates an example of developed hazard classifications in a proposed masterplan area in the Neom region.

Finally, this presentation will address the challenges of developing flood hazard classifications in arid regions with limited data, highlighting how AtkinsRéalis refined IDF curves and rainfall patterns while adapting methodologies from the U.S. and U.K. The innovative hazard classification approach represents a significant step forward in flood risk assessment. Lessons learned from this project will offer valuable insights for future flood studies in similar environments.