flooding and streams! Flashcards
what is the hydrosphere?
all water at or near the surface of the earth
what steps are in the hydrologic cycle
evaporation
condensation
precipitation
runoff
infiltration
percolation
what is the largest river?
amazon! 175,000 meters cubed per second water flow
what is a stream?
a body of water confined to a channel
what is a river?
major branches of a stream system
what is a drainage basin
region from which a stream draws its water supply
isolates water
the size of a stream is related in part to the size of the drainage basin, which determines how much water can flow into the stream
what is the drainage basin defined by?
topographically determined divides
what is stream discharge?
volume of water that passes a given point in a given time through a channel of a certain width and depth
how would you calculate discharge?
multiple cross-sectional area (width x depth) by velocity of the flow (distance/time)
what will change the shape of the stream / land?
faster moving water
what is laminar flow?
straight or gently curved streamlines run parallel to one another without mixing (slow moving rivers)
what is turbulent flow?
streams mix, forming eddies (fast flowing rivers)
what is a streams capacity?
total sediment load carried by a stream
what is a streams competence?
ability to carry material of a given size
how are capacity and discharge related?
the faster the stream flows, more water present, more material moved, larger material moved
what does stream velocity impact?
sediment sorting
- slow moving water: fine-grain
- swift currents: wider range of grain
what is a graded stream?
various factors combine to yield an average of neither erosion or sedimentation over the course of the stream
at equilibrium!
low spots filled in, high spot sediment grabbed
what can effect the equilibrium of sediment transport?
discharge, base level, tectonics, resistance of weathering, human intervention
what is a streams gradient?
velocity of stream largely dependent on steepness of the stream
gradient decreases as the stream approaches its baselevel
what is the base level of a stream?
lowest elevation to which a stream can flow
longitudinal stream profile
approaching base level (potentially sea level)
slide shaped line on graph
a change in the base level disrupts?
equilibrium! tries to go back
steps of changes to a graded stream
- normal stream profile
- change in bas level changes profile
- erosion moves material toward sea level. sediment moving to river delta is deposited in lake. previous delta (where sediment is deposited) abandoned
- eventually stream profile restored
how can humans change base level?
by building a dam, which also changes longitudinal profiles
what are the two major stream types?
meandering and braided
what is a meandering stream?
gentle gradients, generally erode unconsolitaed sediment and weak bedrock. carry fine sediment, path may shift significantly with erosion and deposition
edmonton! straight line
what is a braided stream?
many channels which diverge and merge, steep gradients, high sediment loads, variations in discharge
jasper / banff !
how is a braided stream created?
high velocity / high sediment streams flow over nearly flat, easily eroded terrain, the fast-moving sediment-laden water cuts across soft sediments at the end of existing channels
what is a point bar?
a place where sediment is left behind
how are meandering streams formed?
shift from side to side in a snaking motion
current is faster on outside banks: erode
current is slower on the inside banks: deposited (forms point bar)
where do you want to build you house?
point bar!
how is a floodplain formed?
deposit and erosion in meandering streams
what are natural levees?
ridges of course material that confine the stream within its banks between floods
how are natural levees formed?
before flood: sediment deposit by flooding of stream channel build up low levees
after flood: water leaves channel, rapidly lose velocity, drop sediment along channel borders
what is a hydrograph
a plot of steam discharge at a point over time
- records fluctuations in discharge or stream height over time
- discharge / stage on vertical axis; time on horizontal axis
when the _____ exceeds the _______, a flood is caused
input; output
what are the factors that can cause flooding?
excessive rainfall
snowmelt in mountains
severe storms
hazardous blockage of stream channel (trees, rock avalanches)
what are the characteristics of a flood?
stage and crest
what is a stage
elevation / height of the water
what is a crest?
maximum stage reached (bulge of water coming downstream)
where do upstream floods occur?
small, localized upper part of a basin
where do downstream floods occur?
larger, lower part of a drainage basin
what is a flash flood?
upstream flood characterized by a rapid rise of steam stage
what events cause upstream floods?
locally intense rainstorms or dam bursts (flash floods)
what events cause downstream floods?
heavy rain or snowmelts
what would a hydrograph look like for an upstream flood?
sharp peak, short duration
what would a hydrograph look like for an downstream flood?
lower peaks, broader duration
what is the rate of surface runoff influenced by?
- ground cover: different rates in infiltration in soils, rocks and manmade structures
- topography: steepness of terrain governs how much is infiltrated into soil vs surface runoff
3.presence of vegetation: plants may provide physical barrier +increase soil infiltration (absorb water) - climate: determines level of rainfall and periodicity of storms
how to calculate the flood frequency to plot curve?
R = (N+1)/M
R: recurrence
N: years
M: annual maximum
reasons why people live in floodplain
ignorance of hazards + inexpensive land that is often beuatiful
what impact does climate change have on floods?
faster hydrological cycle = more frequent floods
plus raise in sea level
what are some flood hazard reduction strategies?
restrictive zoning
retention pond
diversion channel
channelization
levees
flood control dams and reservoirs
do not build in a floodplain
how do levees prevent flooding?
allowing waters to reach higher stage without spilling onto the floodplain. sometimes artificially enlarged or constructed to constrict flooding
what are flood control dams?
retain excess runoff and release in a controlled manner to not overwhelm the stream
what was the worlds most catastrophic flood?
breaching of the ice dams and retreating of continental ice sheets