Floods and Hurricanes

Stream

Water that flows in a channel.

Drainage basin (watershed)

Area that collects and channels precipitation to streams and rivers.

River discharge (Q)

Volume of water moving past a point per unit time; Q = width × depth × velocity.

Units for discharge

Cubic meters per second (cms) or cubic feet per second (cfs).

Controls river erosion and deposition

Stream velocity and discharge.

Sediment deposition

Occurs when stream flow slows; forms alluvial fans or deltas.

Flood

Overflow of water beyond a river's channel caused by excessive runoff.

Factors influencing flooding

Precipitation, infiltration, soil moisture, topography, and land use.

Floodplain

Flat land next to a river that floods periodically.

Flood recurrence interval

Average time between floods of a certain size (used for risk prediction).

1% annual chance flood

The 100-year flood.

Primary flood effects

Injury, death, property damage, sediment movement.

Secondary flood effects

Water pollution, disease, hunger, homelessness.

Factors affecting flood damage

Land use, flood depth/velocity, rate of rise, sediment, warning systems.

Urbanization effect on flooding

Increases runoff and reduces infiltration → higher flood peaks.

Flood control methods

Levees, floodwalls, and dams.

Problems with levees

Increase downstream flooding, destroy wetlands, can fail catastrophically.

Problems with dams

Loss of habitat, disrupts sediment and temperature, can fail and flood downstream.

Flood-proofing examples

Raising foundations, flood walls, improved drainage.

Types of floods

Upstream/Flash Floods: Short, intense, local; Downstream Floods: Longer duration, larger area.

1976 Big Thompson River Flood

8-12 inches rain in 3 hours; 145 deaths, 418 homes destroyed; Peak flow: 29,000 cfs.

1927 Mississippi River Flood

Heavy rains + levee failures (225 breaks); 27,000 mi² flooded; up to 70 miles wide; 637,000 homeless; $1 billion in losses.

Hurricane formation

Hot, dry air from Africa rises over warm ocean → pulls moist air upward → creates circulation.

Stages of tropical storms

Tropical Depression: 23-38 mph winds; Tropical Storm: ≥39 mph; Hurricane: ≥74 mph.

Hurricane structure

Eye (calm center), eyewall (strongest winds), spiral rain bands.

Environmental conditions for hurricanes

Warm ocean water, steep temperature gradient, weak vertical wind shear.

Controls hurricane movement

Coriolis effect and steering winds.

Why hurricanes lose strength over land

They lose their warm ocean energy source.

Regions at risk for hurricanes

East Coast USA, Caribbean, eastern Pacific, and Indian Ocean coasts.

Storm probabilities

High likelihood of storm formation in the Atlantic's warm tropical zone.

Cyclone hazards

Storm Surge: Wind-driven water pushed onto coast (worst in NE quadrant); High Winds: Categorized by Saffir-Simpson Scale; Heavy Rain: Causes inland flooding.

Saffir-Simpson Scale

Cat 1: 74-95 mph; Cat 2: 96-110 mph; Cat 3: 111-129 mph; Cat 4: 130-156 mph; Cat 5: ≥157 mph.

Flooding factors

Storm speed, other weather systems, and pre-storm soil moisture.

Hurricane Watch

Possible in 36 hours.

Hurricane Warning

Likely within 24 hours.

Tools for forecasting

Satellites, aircraft, Doppler radar, computer models.

Improvements reducing deaths from hurricanes

Better forecasting, education, awareness, and infrastructure.

Hurricane Maria (2017)

Struck Dominica, St. Croix, and Puerto Rico; Winds up to 155 mph (Cat 4-5); 80% power lines destroyed; 3.4 million without electricity; $90 billion damage; ~2,975 deaths in PR.