The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. Explain the difference between a slow-onset flood and a flash flood, and describe the causes of each. (E)

A slow-onset flood is a type of flood that occurs gradually over a period of days to weeks. The water level in rivers or other water bodies slowly rises, eventually leading to flooding.

A flash flood, on the other hand, is a sudden and unexpected flood that typically occurs within hours or even minutes after a heavy rainfall event. Flash floods are characterized by rapid rises in water levels and fast-flowing water.

The main differences between slow-onset floods and flash floods are: - Time frame: Slow-onset floods occur gradually over days or weeks, while flash floods happen suddenly, usually within hours or minutes. - Water level rise: The water level in slow-onset floods increases slowly, while in flash floods, it rises rapidly. - Flow speed: The speed of water flow is usually slower in slow-onset floods compared to flash floods.

Slow-onset floods can be caused by various factors, including: 1. Prolonged rainfall: When rainfall continues for several days, the soil becomes saturated, and the excess water eventually runs off into rivers, leading to increased water levels and, eventually, flooding. 2. Snowmelt: In colder climates, snow accumulates throughout the winter months. When the temperature rises in spring, the snow melts and flows into rivers and streams, which can lead to slow-onset flooding. 3. Dam failure or controlled release of water: If water is released from a dam gradually, it can raise the water levels downstream and cause slow-onset flooding. 4. Human activities: Land use changes, urbanization, and deforestation can all contribute to an increase in slow-onset floods, as they can cause decreased water infiltration in the soil and increased water runoff.

Flash floods can be caused by several factors, including: 1. Intense rainfall: Heavy rainfall events, especially in a short duration, can overwhelm the soil's ability to absorb water, leading to rapid runoff and a sudden rise in water levels. 2. Thunderstorms and tropical storms: These weather events can result in intense, localized rainfall, causing flash floods in susceptible areas. 3. Urbanization: As land gets covered with impervious surfaces such as concrete and asphalt, the ability of the ground to absorb water decreases, leading to rapid runoff and flash flooding. 4. Blocked or insufficient drainage systems: If drainage systems are blocked or inadequate, water can quickly accumulate and result in flash flooding. 5. Landslides or debris flows: A landslide or debris flow can temporarily block a river or stream, causing the water to accumulate behind the blockage. If the obstruction is suddenly released, a flash flood can occur downstream.