Earth’s six water reservoirs

Reservoir % Earth's Water % Usable WaterOceans 97.54% -----

Glaciers 2.15% -----

Shallow Groundwater 0.31% 96.9%

Fresh Lakes/Streams 0.009% 2.8%

Salty Lakes and Streams

0.003% -------

Atmosphere 0.001% 0.3%

Rivers: Historical Use

Americans have lived and worked on floodplains for 200 yearsAttracted to this area by rich soil, water supply,

ease of waste disposal, proximity to river commerce (trade)

Building on rivers invites disastersFloodplain

Defined as a flat surface adjacent to the river that is periodically inundated by flood water

Streams and rivers

• Part of the hydrologic cycle• Hydrology—study of water transport in a

natural system– Runoff – Drainage basins

• River slope or gradient– Drop of channel over horizontal distance– Steepest in the headwaters– Higher velocity, greater erosion

• Base level– Theoretical lowest level to which a river may

erode– Usually sea level– Base level may be temporary, as with a lake

Flooding

• Natural process of overbank flow– Function of the amount and distribution of

precipitation, rate of infiltration, and topography• Snow melt, failure of a dam

• Stage—height of water in a river• Floodstage—high water level, damage to

personal property

Upstream and downstream floods

• Upstream– Headwaters– Intense rainfalls of short duration– Hazard: autos

• Downstream– Cover wide area– Long duration storms

Flood alerts

• Flood watch– High flow or overflow is possible

• Flood warning– Flood is actually occurring or is imminent

Dangers of floods

80% of flood deaths occur in vehicles

The mistake: driving on flooded roads

Just 2 feet of water can float a car

1/3 of all flooded roads are so damaged that a car

has only a 50% chance of making it

Just 6” of rapid flood waters can knock you down

Flood waters are usually very cold

Staying safe during floods

Know if you live in a flood-hazard area

Pay attention to signs of possible flooding

Evacuate if flood waters threaten

Don’t walk in flood waters

Don’t drive on flooded roads

If you get stuck in your car, wade out the way you

drove in

Closer look: Magnitude and Frequency of Floods

• Related to amount and intensity of precipitation and runoff

• Large floods from infrequent, intense storms• Described by magnitude (intensity) of

discharge, units of cms, cfs• 10 yr flood, 1/10 or 10% chance of happening in

any one year• 100 yr flood, 1/100 or 1% chance of happening

in any one year

Nature and extent of flood hazards

• #1 disaster in 20th century• Factors that cause damage:– Land use– Depth and velocity of water– Rate of rise and how long the flooding lasts– Season– Effectiveness of forecasting

Effects of flooding

• Primary: directly by flood– Life, homes, bridges

• Secondary: by disruptions– Pollution, hunger, disease