Earth’s six water reservoirs Reservoir% Earth's Water% Usable Water Oceans97.54%----- Glaciers2.15%----- Shallow Groundwater0.31%96.9% Fresh Lakes/Streams0.009%2.8% Salty Lakes and Streams 0.003% Atmosphere0.001%0.3%

Rivers: Historical Use Americans have lived and worked on floodplains for 200 years Attracted to this area by rich soil, water supply, ease of waste disposal, proximity to river commerce (trade) Building on rivers invites disasters Floodplain 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 20 th 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