Risk analysis
Multihazard risk analysis
Steps:
- Remote sensing and photogrammetry
- Urban model creation
- Vulnerability and exposure modeling
- Environmental hazard analysis
- Flooding modeling
- Risk assessment (economic and social losses)
Remote sensing and urban models
Accurate urban models are required for adequate and precise risk assessment.
Several remote sensing procedures are usually employed for urban modeling, being the use of drones (small Unmanned aircraft systems) the most flexible and reliable methodology nowadays.
Drone missions capture billions of photos and videos.
Data analysis and photogrammetric procedures generate orthomosaics, 2D and 3D models.
Lidar data capturing provides high precision models.
Once the models are available, the urban exposure and the vulnerability can be assessed.
Vulnerability and exposure
Extensive experimental and analytical research and empirical observation and assessment of past disasters support the definition of fragility and vulnerability curves that characterize the behavior of structures subjected to environmental hazards. Each curve is defined as a function of a complex set of variables that condition the structural behavior. The vulnerability curves synthesize the probable losses produced by events of a given magnitude. Some of the variables that define the structural behavior include:
- General Data: Location, Land Use, Occupancy Type, Population, Number of Floors, Built area, Built year, Building and Content Costs
- Specific Data: Building type, Roof shape, Cover and wall connections, Shutters, Type, and Height Foundation, Basements Type.
Hazard
Deterministic and probabilistic approaches can be employed to define the hazard and estimate physical, economic, and social losses.
The deterministic approach is based on historically recorded events, such as hurricane Andrew or El Centro earthquake.
The probabilistic approach is based on the probability of exceedance of a certain magnitude of a given event in a particular period. For example, buildings are usually designed to withstand an earthquake with a certain acceleration (i.e., 0.5g) with a 10% probability of exceedance in 50 years; this acceleration corresponds to the earthquake with a return period of 475 years.
The hazards that are considered in this type of analysis are:
- Earthquakes: ground motion effects measured as acceleration
- Hurricanes: wind pressure effect (peak gust wind speed)
- Flood: riverine or coastal type (flood depth)
- Tsunami: coastal effect (inundation height)
Flood analysis and Sea-level rise
Flood analysis requires modeling the surface and the terrains, drainage, sewers, water pomps, seawalls, docks, breakwaters. Flooding models deliver a depth grid that facilitates the evaluation of the damage. Flooding analysis can also incorporate the effect of the sea-level rise. We usually employ the software RiverFlow2D developed by Hydronia.
Risk assessment: expected losses
The risk analysis delivers the expected values of the physical, economic and social losses:
- Physical damage (probabilistic assessment of the number of affected buildings and debris)
- Direct economic losses ($ in property values due to various levels of damage)
- Direct social losses (casualties, required shelters)
- Loss of functions (probability or days of working inability)
