Floods

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Great Boston Molasses Flood
2+ million gallons of fermented molasses exploded
Hydrologic Cycle (Water Cycle)
the cycle that moves water and water vapor among the oceans, land and atmosphere through evaporation, condensation, precipitation transpiration and respiration
Earth's water
2-3% fresh water
70% in ice
30% groundwater
~0.01% rivers and lakes
Stream
any flowing body of water, large or small
US has 2 million estimated
River
Major branches of a large stream system
Rivers carry 16 billion tons of sediment and 2 - 4 billion tons of dissolved material/year
other names for streams
River, bayou, creek, run, brook, kill
How we make streams
At first, water flows across the ground surface as overland or sheet flow by stream's headwaters
Eventually, it erodes depressions, and flows in a channel
Channels form a network of tributaries, which collect water and feed it into the main trunk stream.
Drainage Basins
Extent or an area of land where all surface water from rain, melting snow, or ice converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another body of water
Boundary between basins is a drainage divide
What do streams do?
Move water, either in laminar or turbulent flows
Weather the material (bedrock or sediment) that they flow over.
Transport sediment (erosion).
Laminar flow
water moves in parallel lines
individual paths maintain their velocities (gently turning a faucet on)
Turbulent flow
water does NOT move in parallel lines
changes in velocity in individual streams (turning on faucet all the way)
Abrasion (how rivers weather and erode)
water-borne particles physically break off pieces of other particles they come into contact with, moving clasts colliding with other clasts
Turbulent flow (how rivers weather and erode)
loosens and lifts material from stream bed using water pressure
Dissolution (how rivers weather and erode)
chemical weathering of soluble material (will dissolve in water)
Bed load (how rivers transport sediment)
Sediment is bounced and rolled across the stream bed (large clasts)
Suspended load (how rivers transport sediment)
Sediment is suspended by the water (medium/small clasts) Silt and clay particles that are carried in the water
- Accounts for nearly 90% of the total load of most rivers
Dissolved load (how rivers transport sediment)
Stuff that is dissolved in the water (sodium, calcium, etc.)
Ions that are carried in solution in the water
Hjulstrom Diagram
At what velocity different sized particles will get deposited from the bed
Gradient
How fast is the stream dropping?
Vertical drop in elevation over a given horizontal distance
Deltas
Base level: the level to which a stream can erode
Ultimate base level: sea level
how dams affect rivers
1. Dams reduce river levels
2. Dams block rivers
3. Dams slow rivers
4. Dams alter water temperatures
5. Dams alter timing of flows
6. Dams fluctuate reservoir levels
7. Dams decrease oxygen levels in reservoir waters
8. Dams hold back silt, debris, and nutrients
9. Dam turbines hurt fish
10. Dams increase predator risk
how many rivers are dammed?
60% of rivers
removal of dam
Reservoir drained in 2 hours
New base level for upstream
Sediment accumulation
Cold, sediment free water
Meandering streams
Low sediment load
Low velocity
One main channel
Common in streams flowing on low slopes in plains or lowlands, cutting through fine sand/silt/mud
Flow on flat floodplains, create meanders
Braided streams
High sediment load
High velocity
Many channels
May have large variations in flow volume, easily eroded banks
High-velocity, high sediment load flows over nearly flat, easily eroded terrain (mouth of canyon, end of glacier...)
Fast-sediment moving laden water cuts across sediments at edges of existing channels, creating shallow, braided channels
Aggradation: Sediment load exceeds transport capacity, results in deposition and builds multiple channels
Oxbow lakes and meander scars
As erosion and deposition continue, bend grows more extreme.
During higher water flow, river takes shorter course, cutting across loop.
Abandoned loop remains as oxbow lake. When this dries, it becomes a meander scar.
Floodplains
flat, low-lying area along channel subject to floods. Fine grained sediment.
Discharge
how much water is actually passing a point at any given time (cfs-cubic feet/second).
Stage
measures stream height. Once water height is high enough to jeopardize human structures/safety, flood stage is reached.
Flash floods
intense local rainfall causes stream to suddenly flood small area, usually local watershed
Ice-jam floods
melting ice-floes dam a portion of a river, normally in spring, causing it to flood upstream
Dam failure floods
more often than not result of faulty engineering (NOT natural disasters).
Regional floods
river overflows banks on a large scale, flooding entire regions
Johnson, PA Dam Failure flash flood
Worst flash flood in US history: 2,209 dead
Worst downpour ever recorded locally: 6 to 10 inches of rain fell in 24 hours.
Dam (1650 ft elevation) breached by heavy rain
20 million tons of lake drained downstream at 40 mph and up to 60 feet high
Wave reaches meander, separates and part follows river, part cuts across channel. Debris momentarily dams at Conemaugh Viaduct before it collapsed.
In Johnstown, 2000+ people drown as 60 foot wall of debris-laden floodwater moving at 40 mph hits town.
1993 Mississippi River floods
Causes:
Wet fall 1992 and heavy snowfall in winter
Saturated soils in U. Mississippi
Missouri catchment area
Entire region had above average rain beginning in April. From June through early August, E. Central Iowa got ~38.5" rain!!
Over 50 fatalities and $15 billion in damages
Flood Facts
Most flood-related deaths are due to flash floods.
50% of all flash flood fatalities are vehicle-related
trapped and drown.
6'' will stall most passenger cars (and can knock you over)
12" will float many vehicles
24'' will carry away most vehicles
2008 Floods
500 year flood
Causes: heavy rain and snow in preceding fall and winter, also wet spring Intense rainfall June 1-13 (record in many places (Decorah - 14.1")
Record flooding in Iowa and Wisconsin
~$2.7 billion damage, 24 killed, crop loss
Recurrence Interval
Statistical average of number of years between flows of a certain peak discharge or streamflow
Probability of an event with X recurrence interval occurring in any given year is P = 1/RI 50 yr flood = 2%; 100 yr flood = 1%; 500 yr flood= 0.2%
Problems with RI
Data must be representative - timescale of records?
Paleoflood data can extend record, may change recurrence interval
Conditions affecting stream should remain constant (urbanization, levees, etc. can change RI)
Do floods originate from similar causes?
Recurrence intervals are statistics of past occurrences. They don't predict the future.
100-year flood does not mean once every 100 years, only 1% chance every year, and...
Each new largest flood makes recurrence interval of now 2nd largest flood drop by ~1/2
Dams
Can stop small floods by regulating flow
Finite capacity
Can't stop large floods
Finite lifespan
Enhance channel erosion downstream due to sediment removal
Flood damage in the USA is increasing over time
Increasing Flood Damage in USA
Increase in population in flood-prone areas (e.g. coasts and low-lying areas - storm surge; flood plains)
Human & natural effects on coastlines and river systems
Permeability
ability of a material to transmit fluids
For every 1° F increase in temperature, the atmosphere can hold 4% more water.
Hydrograph
a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow.
Natural levees
Natural levees result from deposition, accumulation of sediment along banks during flood events
What happens when a stream curves?
Just as the outer edge of a wiffle bat has to travel a longer distance at a higher velocity than the handle, water on the outer edge of a meander moves MUCH faster than the water on the inner curve.
This results in erosion along the outer bank (cutback), and deposition along the inner bank (point-bar)
Bends become more extreme over time
What if we put a dam on a river?
Base level
Sediment transport above the dam
Sediment transport below the dam
Deltas at the mouth of the river system
Recurrence Interval Equation
R=N+1/m
N=number of floods on record
m=rank of flood
R is recurrence interval