Overview
This lecture covers the main types of floods—regional, flash, and storm surge—using historical case studies to illustrate their causes, impacts, and differences.
Types of Floods
- Floods are categorized into three main types: regional, flash floods, and storm surge.
- Regional floods occur seasonally, often due to rain on snowpack, frozen ground, or saturated soils increasing runoff.
- Flash floods arise suddenly with rapid water rise, high flow velocity, and are common in mountains, canyons, deserts, and urban areas.
- Storm surge floods are caused by onshore winds and low atmospheric pressure pushing seawater inland, often during hurricanes.
Regional Floods Case Studies
- Major Mississippi River floods occurred in 1927, 1937, 1947, 1973, 1979, 1983, 1993, 2008, and 2011.
- Levees can mitigate flooding, but breaches still lead to significant land inundation.
- 2008 and 2011 floods showed levees being overtopped or breached, destroying infrastructure.
- Floodwaters can destroy bridges and transport debris, causing substantial property and infrastructure damage.
Flash Floods Case Studies
- Flash floods often strike with no warning, most deadly in vehicles (around 50% of deaths).
- Desert flash floods are severe due to hard ground and lack of vegetation, leading to high runoff during rare storms.
- In 2015, Death Valley received unusually high rainfall, causing major destructive floods and long-term closure of Scotty’s Castle.
- The 2015 Zion National Park flash flood killed 20 people, including experienced hikers caught in canyons.
- The 1976 Big Thompson Canyon, Colorado flash flood killed 143 people, triggered by a stationary thunderstorm producing 12 inches of rain in less than 4 hours.
Flood Hazards and Mitigation
- Urbanization, impervious surfaces, and landscape changes can increase flash flood risk.
- Effective flood mitigation (e.g., levees, flood warnings) can reduce damages but may not always be sufficient.
Key Terms & Definitions
- Regional Flood — Large-area flooding caused by persistent wet conditions, snowmelt, or saturated ground.
- Flash Flood — Rapid-onset flood often due to high-intensity rainfall and poor soil absorption, common in canyons and urban areas.
- Storm Surge — Coastal flooding from sea level rise due to hurricane winds and low pressure.
- Levee — Man-made embankment built to prevent river overflow.
Action Items / Next Steps
- Review textbook sections on flood types for deeper detail.
- Prepare for the next lecture on strategies for living safely along rivers and streams.