Overview
This lecture covers the primary causes and influencing factors of floods, focusing on definitions, mechanisms, human impacts, and the consequences of flooding events.
Key Flood Terms
- Flooding occurs when river discharge exceeds the channel’s capacity, leading to overflow.
- "Stage" refers to the water level height in a river at a given location and time.
- "Lag time" is the time between peak rainfall and the start of flooding; it is short in flash floods.
Causes of Floods
- Heavy or prolonged precipitation is the most common cause of floods.
- Storm surges from hurricanes or typhoons especially affect coastal and delta areas.
- Land subsidence (sinking) or sea level rise can cause coastal flooding.
- Natural dams, formed by landslides, ice blockages, or lava flows, can block rivers and cause floods.
- Human-made dams create reservoirs and can cause floods if they fail.
Factors Influencing Floods
- The magnitude, timing, and type of precipitation affect flood frequency and size.
- Characteristics of the drainage basin, including size, shape, and slope, influence flooding.
- Soil and bedrock properties (porosity and permeability) determine water infiltration and runoff.
- Human modifications, such as land use changes, can both reduce or worsen flooding.
Impact of Land Use Changes
- Converting forests to farmland increases soil erosion, sediment in rivers, and river channel erosion.
- Urbanization adds impervious surfaces (roads, roofs), increasing runoff and reducing infiltration.
- Storm sewers and drainage systems quickly move water to rivers, reducing lag time and increasing flash flood risk.
- Destroying wetlands removes natural water storage, increasing flood risk.
- Channelization of streams can worsen flooding downstream.
Flood Consequences
- Floods are the most common and costly natural disaster, damaging structures, farmland, and infrastructure.
- Floods can kill livestock, contaminate land and water, deposit sediments, disrupt transportation, and displace people.
- Monitoring stream discharge helps provide early flood warnings.
Key Terms & Definitions
- Flooding — Overbank river flow when discharge exceeds channel capacity.
- Stage — Height of river water at a specific time and place.
- Lag time — Delay between peak rainfall and onset of flooding.
- Discharge — The volume of water flowing in a river.
- Drainage basin — Area where precipitation collects and drains into a river.
- Impervious cover — Surfaces that prevent water infiltration, such as concrete.
Action Items / Next Steps
- Review the five main causes of floods and factors influencing flooding for upcoming quizzes.
- Be prepared to distinguish between causes and influences of floods in assessments.