LECTURE #19: Floods & Flooding Hazards

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1 GEOL 0820 Ramsey Natural Disasters Spring, 2018 LECTURE #19: Floods & Flooding Hazards Date: 29 March 2018 I. Flooding Hazards: what is a flood? o damaging floods result when the volume of river flow exceeds levels of flood preparedness because flow is greater or longer than expected because of incomplete understanding of local hazards o natural causes: heavy rain dam failure rapid snowmelt deforestation steep slopes storm surges during tropical storms flooding can have different effects on different parts of a stream/river: o upstream: rapid rise and fall of water level o downstream: slower rise and fall, but larger area affected Important hazard to understand: o most chronic and costly natural hazard in the United States o causing an average of 140 fatalities per year in the US and $5 billion damage estimated to rise to $50 billion per year by 2050! o United States flood plains: ~ 3,800 towns and cities of more than 2,500 inhabitants Costliest U.S. Flood Disasters (paid by: Nat l Flood Insurance Program)* 2005 Hurricane Katrina $16, Superstorm Sandy $8, Hurricane Ike $2, Louisiana storms/floods $2, Hurricane Ivan $1, Hurricane Irene $1, Tropical Storm Allison $1, Hurricane Matthew $ Louisiana floods $ Tropical Storm Isaac $558 * adjusted to 2016 dollars (in millions) ** note that Hurricanes Maria, Harvey and Irma (2017) will likely end up on this list! Page 1/5

2 Deadliest Floods in the US deaths name state year 2,209 Johnstown Flood PA St. Francis Dam failure CA Ohio River multiple Dayton Flood OH Mississippi Flood multiple Black Hills Flood SD Los Angeles Flood CA Columbus OH Laurel Run Dam failure PA Austin Dam failure TX New Orleans LA 2005 Flooding and Flood Severity Factors: Natural Anthropogenic 1. rainfall 1. urbanization 2. infiltration rate (soil types) 2. slope modification 3. climate 3. timber industry 4. season 4. flood control measures 5. vegetation 5. agriculture II. Streams/Rivers use of River Valleys: o housing o transportation routes highways, railroads, waterways o agriculture fertile soils o water supply drinking, irrigation, manufacturing III. Stream Flow Parameters width: bank-to-bank distance (m or ft) depth: distance from water surface to stream bed (m or ft) o usually averaged from several measurements length: distance from stream head to entrance into a larger water body o largest - 6,693 km (Nile) discharge: volume / time (m 3 /sec or ft 3 /sec) o can vary from < 1 to 200,000 m 3 /sec (Amazon) Page 2/5

3 drainage basin: basic unit of surface water hydrology o outlined by topographic divides o all land area sloping toward a particular discharge point divide: highest point between stream drainage basins relationship between precipitation rate and infiltration rate determines: o how much water remains on surface infiltration capacity: capacity of a soil to absorb water o varies with soil type, condition, time o is a function of the degree of impervious surfaces in an urban environment where precipitation rate > infiltration capacity o increased run off o flooding potential increases o high infiltration rates: coarse soil well-vegetated land low soil moisture porous topsoil o poor infiltration rates: impermeable crusts salt layers cold weather (frozen soil has poor infiltration) compaction paved surfaces natural hazards: o erosion o flooding Page 3/5

4 human-induced hazards: o urbanization and modification of river o valleys increase effects of natural hazards Hydrograph o discharge rate (Q) in m 3 /s versus time o measured by stream gauge o varies with infiltration capacity Frequency o larger floods are more rare o e.g., 100-year flood statistical probability of a large flood on a given river 1% chance in any year can happen 2 years in a row! IV. Case Study: Historical Flooding in the Pittsburgh Region 1936 Pittsburgh Flood o primary causes prolonged precipitation (snow & rain) high intensity, shorter duration rains certain areas had large flash floods steep slopes, thin soil cover, low vegetation o summary: March widespread flooding in many northeastern states Pennsylvania: heavy snows throughout the winter with very little melting o March 17 th - 24 hours of heavy rains o March 18 th - large ice jams on Monongahela and Allegheny o aftermath Ohio River crested at 46 feet (flood stage = 25 feet) initial estimates stated ~ 33 feet therefore, many people returned to homes or never left power and water cut off water contamination by sewage damage: 108 dead in PA (153 dead total) 500 injured 135,000 homeless $250,000,000 in property damage ($3.9 billion in 2010 dollars!) Page 4/5

5 The Point Stanwick St. o consequences of the 1936 Flood Flood Control Act of 1936 full federal funding for large civil works projects dams, levees, etc. recognized flood control was the federal government s responsibility Flood Control Act of 1938 Army Corps of Engineers (ACE) had the right to acquire land for flood control later amendments stipulated that the ACE limit construction incorporate recreational functionality 2004 Pittsburgh Flood o due to the remnants of Hurricane Ivan 5-9 inches of rain fell on southwestern PA 16 dams control about 40% of the drainage area two 7-million-gallon storm drainage ponds captured much of the rainfall from the Smithfield St. Bridge (2004) approximately the same view (2008) Page 5/5