What Causes Floods?
- local thunderstorms can cause flash floods
- extended rainfall can cause weeklong regional floods
- tropical cyclone causes storm surges
- snow melt
- break-up of winter ice clogs water flow, then fails and floods
- dam break of short-lived natural dams
- failing of human-built dam and levees
- (excessive) sedimentation in lakes and reservoirs
- tsunami
Weather-Related Floods
- Flash Floods
- typically the type of flood with high fatalities
- local short thunderstorms in areas of steep topography; most flood-related deaths by flash-floods; about 50% car-related
- flash floods occur in deserts many miles away from storm; narrow canyons (slot canyons) obstruct
view
(e.g. 12 hikers died in Antelope Canyon, 1997)
NB: more people drown in deserts than die of thirst! - unusually strong thunderstorms can lead to cloudbursts
- habitation of floodplain increases risk of death and loss
- Regional Floods
- typically the type of flood with few deaths but extensive damage
- high water inundating extensive region for weeks
- large river valleys with low topography
- prolonged (heavy) rain from extended cyclonic systems
- 2.5% of land and 6.5% of population in floodplains
- can also be caused by snow melt and ice blockage
Why Do We Need to Study Floods?
- people like to live near rivers thereby increasing the risk of being a flood victim
- rivers provide an effective transport system and water for industry thereby attracting people
- floodplain provide fertile ground for agriculture thereby attracting people
- hazards of flood seem to be increasing despite our efforts of protection
- costs of floods are increasing
- we control river flow at will
- dams and levees have limited lifetime thereby posing a flood risk
How Rivers Work
- Run-off and the Hydrologic Cycle
- water stored in atmosphere (water vapor): 3,100mi3
- water stored as groundwater: 1,920,000mi3
- water stored on land: lakes and rivers (48,000mi3); ice and snow (6,960,000mi3)
- precipitation in balance with evaporation
- evaporation: sea (103,200mi3); land (16,800mi3)
- precipitation: sea (93,600mi3); land (26,400mi3)
- run-off from land that potentially cause floods: (9,600mi3)
- Discharge
- the amount of water discharged through the stream's cross-section per second
- discharge=average flow velocity * height of water column*width of channel
- latter two multiply to be the area of the cross-section
- Amazon: 200,000mi3/s (15% of world's run-off!)
- Congo: 40,000mi3/s
- Mississippi: 17,000mi3/s
- Flow Velocity
- the flow velocity is not constant throughout the stream
- the highest kinetic energy is associated with the highest flow velocity
(EKIN=0.5 * m * v2, where v=velocity, m=mass of water) - this energy can be used to carry a load (i.e. sediments) or to erode the stream bed
- high friction (causing erosion!) at the bottom and the sides slow the flow down
- in a cross-section of a symmetric stream, the highest velocities are found near the top center
- Stream Profile (Longitudinal Profile)
- longitudinal elevation vs. distance profile of a rivers or stream from its spring to its end ( another stream, river, a lake or the ocean ... or the point at which its water disappears into the ground)
- longitudinal profiles of rivers and streams are concave with steeper slope/higher gradient near the top;
- gradient: ratio of change in height vs. change in horizontal distance travelled; expresses the steepness along a stream bed; determines how fast a river flows and cuts into its bed;
- near top of stream profile: origin/spring; highest potential energy, high gradient, quickest gain in kinetic energy, high erosion rate (energy is spent on erosion)
- near bottom of stream profile: end of stream (flowing into other stream/river, lake or ocean) lowest potential energy, low gradient, lowest gain in kinetic energy, high sedimentation rate (energy is gained by dumping sediment load)
- base level: level below which a stream cannot erode (i.e.. ocean, lake or river the stream flows into)
- after scaling, all rivers have the same profile, independent of environment and size
- Stream Gradient and Stream Length
- fast flowing streams are down a steeper slope and straight (symmetric V-shaped river bed)
- slow flowing streams are down a more gentle slope and start to meander
(asymmetric river bed with an erosion and a deposition bank)
 
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Drainage Basins
- also, drainage area, watershed
- area where surface water from rain, melting snow, or ice converges to a single point at a lower elevation (typically feeding into a single stream or river, or a lake or ocean)
- within small drainage basins, short heavy downpour can cause short-lasting flash
floods
(e.g. Guadalupe River in central TX rose from 0.9m to 9m in just 2h) - within large drainage basins, widespread rains lasting for weeks can cause
long-lasting floods
(e.g. Mississippi River, Missouri; 1993 Jefferson Missouri)
Flood Plains
- floor of streams during floods
- subject to seasonal change
- usually flat but have same gradient as river in normal condition
- can to lead terraces when river cut deeper into its bed and its ability to flood the entire floodplain
Meanders
- form on gentle slopes, plains with low gradients
- river may change course seemingly randomly, depending on small perturbations to river bed, discharge, load and other conditions
- unlike rivers on steeper slopes, the cross section of a meandering river is not symmetric; it has an outer and an inner bank
- outer bank: high flow velocities, high kinetic energy -> erosion; undercutting can cause cliff collapse
- inner bank: low flow velocities, low kinetic energy, river unable to carry its load -> deposition of sediments
- over time growing curvature of meanders as a result of prolonged erosion on outer banks
- in a full loop of a meander, there are three places with outer banks, one near the outer top and one on each side
- over time, the loop is enhanced (positive feedback enhances erosion)
- eventually, river cuts off its loops forming an oxbow lake
- new path is shorter and has a steeper gradient, increasing the flow speed and discharge at the downstream end of the meander, increasing the risk of flooding
Some Rivers by Comparison(*)
| River | Length [km] | Elevation of Spring [m] | ave. Discharge [ft3/s] | ave. Discharge [m3/s] | max. Discharge [ft3/s] | max. Discharge [m3/s] |
| San Diego | 84 | 1143 | 38.3 | 1 | 94,500 | 2,676 |
| Tijuana | 193 | 187 | 42.5 | 1 | 33,500 | 950 |
| Sacramento | 719 | 1120 | 23,490 | 665 | 650,000 | 18,400 |
| Columbia | 2000 | 820 | 265,000 | 7500 | 1,240,000 | 35,100 |
| Colorado | 2334 | 3104 | 22,500 | 640 | 384,000 | 10,875 |
| Rio Grande | 3051 | 3658 | 2,400 | 68 | 964,000 | 27,300 |
| Mississippi | 3766 | 450 | 593,000 | 16,790 | 3,065,000 | 86,790 |
| Volga | 3692 | 225(**) | 284,600 | 8060 | - | |
| Danube | 2860 | 1078 | 247,200 | 7000 | - | |
| Rhine | 1230 | 2345 | 102,400 | 2900 | 459,100 | 13,000 |
| Elbe | 1091 | 1386 | 25,100 | 710 | 43,500 | 1230 |
| Loire | 1012 | 1408 | 29,500 | 835 | 146,600 | 4150 |
The Mightiest Rivers in the World
| RANK BY LENGTH | River | Length (km) | Drainage area (km2) | Average Discharge (m3/s) | Outflow | Country/Continent |
| 1 | Nile/Kagera | 6,650 | 3,349,000 | 5100 | Mediterranean Sea | Africa |
| 2 | Amazon/Ucayali Apurimac | 6,400 | 6,915,000 | 219,000 | Atlantic Ocean | South America |
| 3 | Yangtze (Chang Jiang) | 6,300 | 1,800,000 | 31,900 | East China Sea | China |
| 4 | Mississippi/Missouri Jefferson | 6,275 | 2,980,000 | 16,200 | Gulf of Mexico | mainly U.S. |
| 5 | Yenisei/Angara Selenge | 5,539 | 2,580,000 | 19,600 | Kara Sea | mainly Russia |
| 6 | Yellow River (Huang He) | 5,464 | 745,000 | 2,110 | Bohai Sea | China |
| 7 | Ob/Irtysh | 5,410 | 2,990,000 | 12,800 | Gulf of Ob | Russia/Asia |
| 8 | Parana/Rio de la Plata | 4,880 | 2,582,672 | 18,000 | Rio de la Plata | South America |
| 9 | Congo/Chambeshi | 4,700 | 3,680,000 | 41,800 | Atlantic Ocean | Africa |
| 10 | Amur/Argun (Heilong Jiang) | 4,444 | 1,855,000 | 11,400 | Sea of Okhotsk | Russia/Asia |
| 11 | Lena | 4,400 | 2,490,000 | 17,100 | Laptev Sea | Russia |
| 12 | Mekong (Lancang Jiang) | 4,350 | 810,000 | 16,000 | South China Sea | SE Asia |
| 13 | Mackenzie/Peace Finlay | 4,241 | 1,790,000 | 10,300 | Beaufort Sea | Canada |
| 14 | Niger | 4,200 | 2,090,000 | 9,570 | Gulf of Guinea | Africa |
| 15 | Murray/Darling | 3,672 | 1,061,000 | 767 | Southern Ocean | Australia |
| 16 | Tocantins/Araguaia | 3,650 | 950,000 | 13,598 | Atlantic Ocean Amazon | Brazil |
| 17 | Volga | 3,645 | 1,380,000 | 8,080 | Caspian Sea | Russia |
| 18 | Shatt al-Arab Euphrates | 3,596 | 884,000 | 856 | Persian Gulf | Iraq, Turkey Syria |
| 19 | Madeira/Mamore/Grande Caine/Rocha | 3,380 | 1,485,200 | 31,200 | Amazon | South America |
| 20 | Purus | 3,211 | 63,166 | 8,400 | Amazon | Brazil, Peru |
| RANK BY DISCHARGE | River | Length (km) | Drainage area (km2) | Average Discharge (m3/s) | Outflow | Country/Continent |
| 1 | Amazon/Ucayali Apurimac | 6,400 | 6,915,000 | 219,000 | Atlantic Ocean | South America |
| 2 | Congo/Chambeshi | 4,700 | 3,680,000 | 41,800 | Atlantic Ocean | Africa |
| 3 | Orinoco | 2,101 | 1,380,000 | 33,000 | Atlantic Ocean | N. South America |
| 4 | Yangtze (Chang Jiang) | 6,300 | 1,800,000 | 31,900 | East China Sea | China |
| 5 | Madeira/Mamore/Grande Caine/Rocha | 3,380 | 1,485,200 | 31,200 | Amazon | South America |
| 6 | Negro | 2,250 | 720,114 | 26,700 | Amazon | Brazil, Venezuela, Colombia |
| 7 | Yenisei/Angara Selenge | 5,539 | 2,580,000 | 19,600 | Kara Sea | mainly Russia |
| 8 | Brahmaputra/Tsangpo | 2,948 | 1,730,000 | 19,200 | Bay of Bengal | South Central Asia |
| 9 | Parana/Rio de la Plata | 4,880 | 2,582,672 | 18,000 | Rio de la Plata | South America |
| 10 | Lena | 4,400 | 2,490,000 | 17,100 | Laptev Sea | Russia |
| 11 | Mississippi/Missouri Jefferson | 6,275 | 2,980,000 | 16,200 | Gulf of Mexico | mainly U.S. |
| 12 | Mekong (Lancang Jiang) | 4,350 | 810,000 | 16,000 | South China Sea | SE Asia |
| 13 | Pearl (Zhu Jiang) | 2,200 | 437,000 | 13,600 | South China Sea mainly China | Brazil |
| 14 | Tocantins/Araguaia | 3,650 | 950,000 | 13,598 | Atlantic Ocean Amazon | Brazil |
| 15 | Ayeyarwady (Irrawaddy) | 2,170 | 411,000 | 13,000 | Andaman Sea Myanmar | Brazil |
| 16 | Ob/Irtysh | 5,410 | 2,990,000 | 12,800 | Gulf of Ob | Russia/Asia |
| 17 | Ganges/Hooghly Padma | 2,620 | 907,000 | 12,037 | Bay of Bengal | India/Bangladesh |
| 18 | Amur/Argun (Heilong Jiang) | 4,444 | 1,855,000 | 11,400 | Sea of Okhotsk | Russia/Asia |
| 19 | Mackenzie/Peace Finlay | 4,241 | 1,790,000 | 10,300 | Beaufort Sea | Canada |
| 20 | St. Lawrence/Niagara/Detroit St. Clair/St. Marys/St. Louis | 3,058 | 1,030,000 | 10,000 | Gulf of St. Lawrence | North America |
| 20 | Kasai | 2,101 | 880,200 | 10,000 | Congo | Central Africa |
| RANK BY DRAINAGE AREA | River | Length (km) | Drainage area (km2) | Average Discharge (m3/s) | Outflow | Country/Continent |
| 1 | Amazon/Ucayali Apurimac | 6,400 | 6,915,000 | 219,000 | Atlantic Ocean | South America |
| 2 | Congo/Chambeshi | 4,700 | 3,680,000 | 41,800 | Atlantic Ocean | Africa |
| 3 | Nile/Kagera | 6,650 | 3,349,000 | 5100 | Mediterranean Sea | Africa |
| 4 | Ob/Irtysh | 5,410 | 2,990,000 | 12,800 | Gulf of Ob | Russia/Asia |
| 5 | Mississippi/Missouri Jefferson | 6,275 | 2,980,000 | 16,200 | Gulf of Mexico | mainly U.S. |
| 6 | Parana/Rio de la Plata | 4,880 | 2,582,672 | 18,000 | Rio de la Plata | South America |
| 7 | Yenisei/Angara Selenge | 5,539 | 2,580,000 | 19,600 | Kara Sea | mainly Russia |
| 8 | Lena | 4,400 | 2,490,000 | 17,100 | Laptev Sea | Russia |
| 9 | Niger | 4,200 | 2,090,000 | 9,570 | Gulf of Guinea | Africa |
| 10 | Amur/Argun (Heilong Jiang) | 4,444 | 1,855,000 | 11,400 | Sea of Okhotsk | Russia/Asia |
| 11 | Yangtze (Chang Jiang) | 6,300 | 1,800,000 | 31,900 | East China Sea | China |
| 12 | Mackenzie/Peace Finlay | 4,241 | 1,790,000 | 10,300 | Beaufort Sea | Canada |
| 13 | Brahmaputra/Tsangpo | 2,948 | 1,730,000 | 19,200 | Bay of Bengal | South Central Asia |
| 14 | Madeira/Mamore/Grande Caine/Rocha | 3,380 | 1,485,200 | 31,200 | Amazon | South America |
| 15 | Volga | 3,645 | 1,380,000 | 8,080 | Caspian Sea | Russia |
| 16 | Orinoco | 2,101 | 1,380,000 | 33,000 | Atlantic Ocean | N. South America |
| 17 | Zambesi | 2,693 | 1,330,000 | 4,880 | Mozambique Channel | East Africa |
| 18 | Nelson/Saskatchewan | 2,570 | 1,093,000 | 2,575 | Hudson Bay | North America |
| 19 | Murray/Darling | 3,672 | 1,061,000 | 767 | Southern Ocean | Australia |
| 20 | St. Lawrence/Niagara/Detroit St. Clair/St. Marys/St. Louis | 3,058 | 1,030,000 | 10,000 | Gulf of St. Lawrence | North America |
References and Recommended Reading:
- (1)Wikipedia page on
- Johnstown Flood: "The Johnstown Flood" by David McCullough; Touchstone Book by Simon & Schuster Publ., 2. ed, 1987, ISBN: 0-671-20714-8
- A possible location of the biblical/Gilgamesh flood "Noah's Flood" by W. Ryan and W. Pitman, Touchstone Book by Simon & Schuster Publ., 2000, ISBN: 0-684-85920-3
Five Random Review Questions (not a complete review!)
- the highest point where the spring is
- the lowest point to which erosion can occur
- the average discharge
- the lowest possible discharge
- the amount of sediments carried to its mouth
- the amount of erosion in the upper third
- the amount of water sent through its cross-section per second
- the amount of pollutants it carries
- near the top
- near the bottom
- on the inner bank
- on the outer bank
- near the bottom
- near the top center
- near the shores
- heat
- latent heat
- potential energy
- kinetic energy
Answers: 1b; 2c; 3d; 4b; 5d
APPENDIX A: CASE STUDIES
CASE 1: The Galveston Flood (TX Sep 8, 1900); A Hurricane
- deadliest natural disasters in U.S. history
- more than 6000 (might be as high as 7200) of the 38,000 residents perished
- more than 3600 houses destroyed
- caused by a surge from a category 4 hurricane:
- high winds pile up water for 60-80km and push it onshore
- low pressure raises water level
- particularly dangerous at times of high tides
NB: hurricanes can rage through a coastal town for a significant fraction of a tidal cycle; 12h - used to be Texas' wealthiest town at the time, thanks to its importance as a port for trading ships (more than 1000 ships per year)
- factors that made Galveston an easy victim
- Galveston is located near the end of a barrier island
- the surge reached one foot above Galveston's highest point
- winds were clocked at 120mph
- waves up to 30ft high
- at the time, meteorologists were unable to forecast hurricanes
- morning of Sep6, U.S. weather bureau sent telegraph about a tropical disturbance near Cuba
- long ocean swell by afternoon of Sep7; tide higher than usual
- morning of Sep8, air pressure dropped dramatically and winds picked up
- head of Galveston Weather Bureau, Isaac Cline, recommended to evacuate city
- few left; most sought shelter in higher-lying brick houses
- 4pm: tides surging from ocean and bay
- 6:15pm: anemometer ripped off at 84mph (gusts topping 100mph)
- 7pm: 4-ft wave surged through city, then a 20-ft wave
- storm waves on top of high tides and winds > 120mph for rest of evening
- 8:30pm: Cline's house destroyed; 32 of 50 people perished, including his wife
- Cline and his family (three children, brother and a woman) drifted on wreckage for three hours
- 9pm: eye of the storm passed town; barometric pressure: 27.91inches
- 10 inches of rain
- tide reached records high of 15.2ft above normal
- residents then built a sea wall 3mi long, 17ft high and 16ft wide (at base)
- a following category 4 hurricane in 1915 caused minor damage
- since then, city has grown into unprotected areas
- residents rely on satellite technology to forecast hurricanes (from: "The Weather Book" by Jack Williams; Vintage Books)
CASE 2: The Johnstown Flood (PA May 31, 1889); Aggressive Development
- worst dam burst disaster in U.S. history
- 2200 people perished
- town used to be booming coal-and-steel town in the mountains about 60mi east of Pittsburgh
- an old earth dam above the town has been hastily rebuilt to create lake for exclusive summer resort
- repeated warning of a disaster were ignored
- dam burst on May 31
- a storm had started out of Kansas and Nebraska on May 28
- hard rains in Kansas, Missouri, Illinois, Michigan, Indiana, Kentucky, Tennessee on May29
- train delayed; roads washed out
- farms by Cottonwood River/Kansas flattened by tornadoes
- sudden snow squalls in Michigan and Indiana
- nigh of May 29, U.S. Signal Service issued notices the Middle Atlantic states were in for severe storms
- May 30 storm struck western Pennsylvania; worst downpour ever recorded locally (6-8in in 24h)
- noon, May 31, dam started to fail; messages of approaching disaster ignored
- 3:15pm, lake "was running toward the town"
CASE 3: Gilgamesh and the Great Deluge
- nearly all cultures in the world have tales of ancient deluges far greater than seen in modern times
- interesting case is the deluge that is shared in many epics in the Mediterranean Sea/ Near Middle East area
- e.g. tales of Noah in Bible and Homer's Odyssey thought to be based on epics of Gilgamesh where Utnapishtim built an ark
- Gilgamesh was a Sumerian king (B.C. 2,500) in the area of Mesopotamia that is now Iraq (earliest written language)
- deluge may have occurred before written language and tales been handed down through songs
- for such a deluge to be topic of an epic, flood must have occurred while people were not living a nomadic life anymore but in communities; only a mass exodus would make a story; so deluge was likely not before B.C. 8000
- the lower Tigris and Euphrates River floodplains are thought to be one of the possible locations of a 1000-year flood (B.C. 6000 - B.C. 1000) (river system then thought similarly powerful as that of Mississippi or Yellow today)
- some think that flooding of the flood plain would have occurred too often to make an epic; also people would not abandon area to seek new land
- other scenarios include the opening of the Southern Red Sea and the Black Sea
- Black Sea was fresh water late during last ice age (likely attractive to people settling down in communities); while sea level was 150m lower than today
- end of ice age would fill Mediterranean and then spill over narrow Bosporus Straight to cause an immense water fall (200 times as large as Niagara Falls)
- the coastal area flooded after only a short time, forcing people to migrate away
- evidence supporting this idea would be to find remains of these town in the sediments that now accumulate on the shelves of the Black Sea (evidence has recently been found by geophysicists)
APPENDIX B: Equilibrium Stream
- equilibrium: state of balance where a change causes compensating actions
(negative feedback mechanism);
a stream tries to maintain equilibrium state - controlling factors: 1) discharge of water; 2) amount of sediments to be carried away (stream load); 3) slope gradient; 4) channel pattern
- Feedback Systems
- positive feedback: enhances changes; could end in runaway process; e.g. snow falls in cold climate; snow reflects more sunlight making climate colder
- negative feedback: system strives to obtain equilibrium state; counteracts changes
- Graded Stream Theory
- every stream adjusts stream bottom to changes in condition, thus sustaining graded stream (equilibrium state); a graded stream has a concave longitudinal profile and it tries to maintain this profile; a stream tries to use up all the energy that it has at the top by the time it reaches the bottom of the longitudinal profile (incl. sedimentation at the end); it can use only the energy it had at the beginning and not more; any disturbance to this balance results in compensating actions;
- Case 1: Too much discharge
- more water causes stream to flow more rapidly (has more energy)
- erodes stream bed (near top) (energy consumed)
- carries more sediments away (stream load increases; energy consumed)
- sediments deposited near bottom
- process eventually decreases overall slope steepness
- could eventually lead to meandering
- Case 2: Too much load
- more sediments choke stream
- sediments are soon deposited (near top) increasing stream gradient
- water then flows faster
- thus having more load carrying capacity
- could eventually lead to braided streams when load changes significantly
- Case 3: Stream flows into lake
- carries sediments into the lake until water overflows dam
- stream then flows down the steeper dam gradient being more erosive