Flood Dynamics: Hazards, Risks, and Prevention
Understanding Flood Risks and Causes
Floods are a destructive geological phenomenon within the geosphere. Their causes can be geological, climatic, and anthropogenic, originating from events like hurricanes, heavy rainfall leading to river overflow, channel obstructions, and more.
Types of Flood Events
Flooding in continental watercourses (avenidas) can be categorized into two main types:
Torrential Floods
These originate in streams or dry riverbeds carved by water on steep slopes. Water flows sporadically but at high speed due to the steep gradient, eventually reaching a larger main channel, often a flat-bottomed gully or ravine. The speed of water flow in these streams can cause sudden and very dangerous inundations.
River (Fluvial) Floods
These originate in rivers and perennial streams flowing through less steep terrain compared to torrents. The floodwaters are regulated by the river basin itself, often spreading out into floodplains or broad, flat-bottomed valleys known as vegas, which are occupied by alluvial sediments. The risk of river flooding is often increased by human activities, such as extensive urbanization in these areas.
The path of a river within its floodplain can take varied forms:
- Straight: Forming a single channel.
- Anastomosed: Transported materials are deposited in bars, splitting the channel into several branches.
- Meandering: Forming sinuous curves.
Fluvial Terraces
These are flat steps located at different heights parallel to the riverbed. They form when a river excavates vertically into its channel floor and becomes embedded in its own sediments, creating a new, lower floodplain and leaving the former one at a higher level.
Factors Determining Flood Hazard
The hazard posed by floods depends on the energy of the streams and rivers, which in turn depends on the following variables:
- Speed: Increases with the slope of the terrain.
- Flow Volume (Discharge): The volume of water passing through a cross-section of the current per unit of time. Flow depends on rainfall (varying seasonally), and infiltration capacity. Reduced infiltration (due to lack of vegetation, impermeable rock types, or urbanization) increases surface runoff, river flow, and thus, the danger.
Flood Forecasting Techniques
Key steps for predicting major floods (inundaciones) include:
- Analyzing weather reports.
- Monitoring flow variation diagrams (hydrographs).
- Developing risk maps.
Flood Prevention and Mitigation
Prevention measures are broadly classified into two types:
Structural Measures
- Construction of levees on both sides of the river to prevent overflow (desbordamiento).
- Increasing channel capacity through widening and dredging the bottom and sides.
- Diversion of rivers or parts of their flow.
- Reforestation and soil conservation: Considered highly effective as forests retain water, increasing infiltration and decreasing runoff.
- Flood Lamination Measures: Construction of dams upstream to lower the peak flow during flood events.
- Monitoring Stations: Located along river channels and reservoirs to track water levels and flow.
Non-Structural Measures
These aim to reduce vulnerability to flood damage. The main measures include:
Land Use Planning (Ordenación del Territorio)
Laws exist to limit or prohibit certain uses in designated risk areas. These areas typically include:
- Easement Zone (Zona de Servidumbre): A strip (e.g., 5 meters wide) on each side of the channel where all construction, farming, etc., is prohibited.
- Police Zone (Zona de Policía): Extending up to 100 meters from the channel on both sides. Agricultural uses may be permitted, although alteration of the terrain relief is typically prohibited.
- Flood Hazard Zone (Zona Inundable): Areas along the channel margins where there is a significant probability of flooding (e.g., a 1-in-500-year flood event). Regulations here are often less strict than in the previous zones but aim to guarantee the safety of people and property.
Insurance and Assistance Policies
Insurance is often mandatory for construction and other activities located in areas prone to flooding.
Civil Protection Plans
Studying flood risks allows for the creation of warning systems to protect people and property effectively.
Flood Simulation Models
These are developed using Geographic Information Systems (GIS) and data about the territory that could be affected. They serve to delineate the areas likely to be impacted by floods of varying magnitudes.