Hydrology And Fluvial Geomorphology Flashcards
What is a drainage basin?
An area drained by a single river and all its tributaries
Vary in size:
Amazon basin (40% of South America with 1,100 tributaries)
To the micro scale of a single river or stream
Lithosphere
Geological world
Biosphere
Living world
Describe watershed?
The boundary of drainage basin, the contour of land surrounding a river or stream
List transfers in the hydrological cycle?
Stem flow Surface runoff Infiltration Through flow Channel flow Percolation Groundwater flow
List outputs in the hydrological cycle?
Transpiration
Evaporation
River discharge
List stores in the hydrological cycle?
Interception Surface storage Soil moisture storage Vegetation storage Channel storage Groundwater storage
List inputs in the hydrological cycle?
Precipitation
What does precipitation include?
Rainfall
Snowfall
Hail
Dew
Describe precipitation?
The conversion and transfer of moisture from the atmosphere to the land
Varies due to: amount, extent, intensity, type, duration
Why is the hydrological cycle a ‘cycle’?
Continuous ( no start or end)
Why is the global hydrological cycle a closed system?
Water is not entering or leaving
Amount of water stays the same
Descrive storage in the hydrological cycle?
Parts of the system that hold or retain water for periods of time
They can be open stores on the surface of land within vegetation or hidden deep within the rock structure
What is interception?
Refers to water that is caught and stored by vegetation
It is largely affected by the size and coverage of plants with broad leaves trees catching the most water (in summer)
What three main mechanisms is intercepted water transferred?
Interception loss
Theoughfqll
Stem flow
Describe interception loss?
Water that is retained by plant surfaces and that is later evaporated away or absorbed by the plant
Describe stem flow?
Water that trickles along twigs and branches and finally down the main trunk
Describe throughfall?
Water that either falls through gaps in vegetation or drops from leaves or twigs
What type of trees has lots of interception?
With lots of leaves
What type of trees intercept less?
Trees with no leaves
Trees with needles intercept less due to surface tension
Where in the world does interception occur a lot?
Tropical areas as broad leaves
Describe levels of interception in woodland?
30% of rain falls on woodland is intercepted
Light - increased to 60%
Heavy - drop to 15%
How does interception vary with farm crops?
Corn - high
Soy beans - low
What is secondary interception?
Undergrowth intercepts water that has already been intercepted by leaves
What function does interception have?
Delays time taken from water being rain to being water in river
What is soil water?
The subsurface water in soil and subsurface layers above the water table
What is surface storage?
The name given to any parts of the system where water lies above the ground on the earths surface
Within a drainage basin water may naturally accumulate in lakes ponds or puddles through human intervention (e.g. Reservoirs or swimming pools)
What is channel storage?
Refers to water that is contained within a river channel or stream at any given time
Describe variations in channel storage?
Rivers can be seasonal
May disappear underground either naturally (limestone areas)
Or covers in urban areas
Where is surface storage permenant?
Swamp marsh or peat bog
What is vegetation storage?
When vegetation absorbs moisture directly through its root system it comes stores within the organism or plant
How does the amount of water stored in vegetation storage vary?
The size and variety of plants and the local conditions at any given time
A large leafy and thirsty plant will require more than a well watered shrub
Fact about groundwater storage?
97% liquid freshwater is groundwater
When does a marsh or bog occur?
When water table is at the surface level
What does groundwater refer to?
Subsurface water that is stored under the surface in rocks
Difference between soil moisture and ground water?
Groundwater may not be recycled for 20,000 years
Whilst soil moisture may be recycled by evaporation into atmospheric moisture within a matter of days or weeks
What does recharge refer to?
The refilling of water of water in pores where the water has been dried up or extracted due to human activity
When can groundwater be considered non renewable?
When recharge does not take pace
What type of rocks does groundwater occur in?
Porous - chalk or sandstone
Pervious - limestone
What is an aquifer?
An underground layer of water bearing permeable rock that can be found at any depth
What processes cause groundwater recharge?
Percolation Infiltration Precipitation Leakage Seepage
What is throughfall?
Water that falls through gaps in vegetation or drops from leaves or twigs
What is stem flow?
Water that trickles along twigs and branches and finally down the main trunk
What two main ways does surface run off occur?
When precipitation exceeds infiltration rate
When soil is saturated (all the pore spaces filled with water)
What is channel flow?
The movement or water in channels such as streams of rivers
The speed and flow depends on factors such as gradient and efficiency
Describe why infiltration occurs she to precipitation?
When precipitation exceeds infiltration capacity accumulated water will flow downslopensue to the effect of gravity
Define porosity?
The capacity of the rock to hold water eg sandstone has a porosity (pore space) of 5-15% whereas clay may be up to 50%
What is permeability?
The ability to transmit water through a rock via joints and fissures
What is the infiltration capacity?
The maximum rate at which rain can be absorbed by a soil in a given condition
What is infiltration rate measured in?
Mm per hour
What influences infiltration rate?
Water in soil already How loose soil is (sandy) Size of raindrops (large = less) Slope angle (steep = less due to runoff) Amount of vegetation (roots break up soil)
Describe the phreatic zone?
Permenantly saturated
Describe how infiltration rates vary with rock types?
Clays: 0-4 mm / hour
Sands: 3-12 mm / hour
Describe percolation?
Water moves slowly downwards from the soil into the bedrock
Speed depends on the permeability of rock
Describe throughflow?
Refers to water flowing through the soil in natural pipes and percolines (lines of concentrated water flow between horizons)
Define surface storage?2
Water that has not infiltrated the surface and is stored on the surface in the form of puddles or lakes etc
Define groundwater storage?2
Percolated water that is held within the aquifers below the water table
Describe how saturated overland flow occurs?
Water is not infiltrated on a slope due to the saturated nature of the soil, and flows across the surface
Most frequently occurs on the lower parts of slopes that have been saturated by water from above
Define percolation?2
The downward movement of water through the soil underlying bedrock after infiltration has taken place
Define throughfall?2
The movement of precipitation through the branches and leaves of trees and vegetation to the earths surfqce
What governs infiltration?
Amount
Intensity
Of precipitation
How can infiltration and precipitation lead to overland flow?
Low infiltration than precipitation leads overland flow to occur
Once the soil is saturated, saturated overland flow occurs
What is porosity?
The proportion of pore space to solid material
How does the porosity and permeability of clay affect water?
High porosity
Low permeability
Clay soils will inhibit the flow of water
Preventing groundwater flow
What happens if water flows over sandy soils?
It will pass through
Reducing overland flow
Increasing through flow
Descrive pressure release?
The creation of joints and other weakness in rocks by the removal of overburden and the release of pressure
What is chelation?
The chemical weathering of rocks by organic acids
Why might the death rate fall less in some areas of a country than others?
More rapidly URBAN areas - provision of and access to better healthcare
- improvement schemes provide clean water and efficient sewage system
PROSPEROUS POPULATION
- higher standard of living with clean water, better food, high quality medical care
DEMOGRAPHIC STRUCTURE - falls less rapidly with ageing population
What is base flow?
The normal level of water in the channel determined by the groundwater flow prior to a rainfall event
What is lag time?
This is the period between the peak precipitation and the peak discharge
What is peak flow?
This is the maximum river discharge for any given event measured in cubic metres per second
What is the rising limb?
The part of the graph that initially rises
Indicates the increasing level of water as determined by the combined rate of surface runoff
Throughflow and groundwater flow following a precipitation event
What is storm flow?
This is the additional discharge created as a result of precipitation event
What is the falling limb?
This is the part of the graph that shows the discharge decreasing and river levels falling back towards base levels
what is the watershed?
the boundary of a drainage basin
what can hydrography be useful for?
identifying the potential flood risk for an area
How does urbanisation affect a storm hydro gray?
Water can’t infiltrate through tarmac and concrete as they are impermeable
Gutters and drained that channel and direct runoff, water can be carried very quickly to water ways
how can the presence of clay ina. soil change the way water interacts with it?
the greater the clay content the more water retentive the soil is as clay particles bond together tightly restricting the flow of water
how can the presence of sand ina. soil change the way water interacts with it?
free draining as the larger sand particles provide gaps and spaces for water to pass through
how can relief influence infiliteation rates?
less than 5 degrees has a greater chance of infiltration
the greater the gradient the rate of surface runoff as there is less opportunity for infiltrating
example of how vegetation can influence infiltration rate?
80% in tropical areas
10% in arable land
what forms of transportation are common in the upper course of the river?
traction - largest stones boulders and cobbles are rolled along the riverbed by strong turbulent follow
satiation - smaller bedload such as pebbles bounce along the riverbed
Describe the material carried by the river in the upper course?
High level of bedload
Large angular material eg screw
Describe the environment of the upper course?
High energy
High levels of erosion
Turbulent flow
how is the middle course of the river characterised?
decreasing gradient
greater lateral erosion
river becomes more sinuous (winding)
greater number of tributaries mean additional water
high proportion of suspended load
bedload is smaller and less angular than upstream
Characteristics of the lower course?
low lying where the river joins the sea wide flat sweeping flood plains large meander bends depositional small rounded stones
high proportion of suspended material
what are the three types of flow?
turbulent
helicoidal
laminar
describe laminar flow?
horizontal smooth motion
too simplistic and complex for natural river environments (due to steps and gradients)
may be found in carrefuoly managed chanelised sections on a relatively small scale where there are few additional influences
describe turbulent flow?
dominant method of flow in a river
series of erratic horizontal and vertical spiral flows (eddies)
that disturb the smooth appearance of the river
amount depends on the velocity of the flow as well as friction and energy available
the greater the velocity he greater the amount of spare energy after friction so the greater turbulence
describe helicoidal flow?
cork screw like flow mainly found as water travels around river bends
associated with meanders and the formation of point bars and slip off slopes
define the thalweg?
the path of least resistance where the river flows fastest
in a straight channel describe the location of the thalweg?
middle of the channel under the surface of the water furthest from the influence of friction from the riverbanks river bed and the air
in a bent channel describe the location of the thalweg?
will continue in a straight line before hitting the outside bend and being reflected downstream
describe hydraulic action?
the force of the water pushing into cracks and hitting against the rivers banks
this repeated action weakens the riverbank as air in the cracks is compressed and pressure builds up
in rivers it is slow and ineffective
what is cavitation?
when air bubbles collapse and create small shock waves - a form of hydraulic action
describe corrosion?
when sediment in the river is thrown into or scraped along the banks and bed or the riv e
during times of high flow or flood the river has a greater capacity to transport larger material which results in the greatest amount of damage
potholes may form as stones become trapped in depresssions and hollows and are continually swirled around by eddies in turbulent flow
describe attrition?
the process by which stones and sediment within the river become increasingly rounded
as material is transported it collides with other objects in the river
the collisions cause the stones to break off into smaller pieces and the edges and points of the stones break off
describe solution?
a continuous chemical process that occurs independently from river flow
water that has elightly acidic properties (e.g. as a result of decomposing organic material - humid acid - or acid rain - carbonic acid) will chemically dissolve and weaken certain types of rock
limestone id particularly vulnerable
what is a rivers capacity?
the name given to the total load of material actually transported
what is a rivers competence?
the name given for the maximum size of material that a river is capable of transporting
describe the process of traction?
when the largest stones boulders and cobbles are rolled along the riverbed by strong turbulent flow
often these sediments will lie undisturbed on the riverbed until sufficient discharged is reached to displace them
describe the process of saltation?
smaller bedload such as pebbles stones and gravel are lifted and carried temporarily in the flow in a hopping or bouncing motion
as turbulent flow is it constant the river will have varying amoints of energy to carry and lift the load
describe the process of suspension?
very fine particles of sand and silt are carried in the water itself when it is fast flowing
the faster and more turbulent the water the greater the amount and size of material transported
describe the process of solution?
the pr fees by which small dissolved sediments and minerals are transported within the river
they form just a small proportion of the total load but are significant as constantly occurs
when does deposition occur?
when the river no longer has energy to transport material it deposits it
list the conditions when deposition is most likely to occur?
low periods of precipitation where river levels drop
where the river flow meets the sea
in areas of slow flow within a channel such as on meander bends
when the load suddenly increases above the capacity e.g. landslide
i water has carried material outside of the channel such as in flood
what will never be deposited?
material in solution
what does the hjustrom curve show?
the relationship between particle size and velocity
why does clay need a greater velocity to be transported?
the particles stick together
why does gravel need a higher velocity to be transported?
size
weight
how does an interlocking spur form?
when the river flows downstream and is forced to wind through he landscape
this creates protrusions of the riverbank in the valley known as spurs
as the river continues to wind downstream in a zig zag pattern the view along the course of the river may be redirected as the spurs appear to knit together like clasped fingers
how does a v shaped valley form?
the upper reaches of a catchment often experience large seasonal variations and as a result the rate of erosion can vary greatly
large angular boulders often choke the upper channel creating more friction and distrusting flow
during times of peak discharge such as periods of snow melt, vertical erosion will be high as there is tester capacity
what are rapids?
areas of high velocity and turbulent flow
what are waterfalls?
large steps in the river as a result of differential erosion usually attributed to bands of hard and soft rock
explain how a waterfall is created?
water flowing over hard rock had a relatively negligible impact erosively
once it meets a band of soft rock is eroded more
over time the erosion will be so great that a noticeable overhang forms (step in profile)
continu d erosion may cause undercutting or the rock layers leading to the overhang collapsing into the plunge pool
the plunge pool is created as the material that falls is often large and angular and is forced to swirl around scouring out a depression
example of a gorge?
niagra falls
retreats 1m a year
erosionsl features list?
v shaped valley
interlocking spurs
waterfalls
rapids
depositinal features list?
flood plains
levees
braiding
deltas
when does helicoidal flow occur in a meander?
wh m surface water flows towards the outer banks while the bottom flow is toward the inner bank
how do river cliffs form?
on the outside bend
erosion is greatest
combination of hydraulic action and abrasion weakens the riverbank causing it to collapse
over time a steep bank will be formed with some of the collapsed material remaining on the river bed
how does a point bar form?
on the insider bend of a meander
discharge is at a minimum and friction is greatest
deposition is greatest
sediment accumulated to grate a gentle sloping bar
the particles are graded in size with the largest material being found on the upstream side of the bar
describe riffles and pools?
riffle is a shallow area of fast flowing oxygenated water
pools are deep but slow moving water
what is a yahoo tributary?
a tributary that runs parallel to the river within the same valley for some distance
what is a floodplain?
a large area of flat land surrounding a river channel
it’s an area most susceptible to flooding
fertile and used for agriculture
as initially cut by the river it is made up of a large amount of alluvial deposits dropped during times of floods
how do flood plains formed?
when’s. river floods alluvial deposits are dropped
as the river spills over the food plain in time or floor th is an increase in friction
a loss of energy and a resulting deposition of material
repeated flooding causes he deposits to build up in height forming a s for iand s or layers above the bedrock
what is the edge of the flood plain?
a slightly raised line known as a bluff
describe how a leveeforms?
when a river floods its banks the largest and coarsest material is deposited first
creating a ridge along the edge of the riv r channel
over time more sediment is added to the ridge through the same process
creating a natural preventative barrier to flooding
explain how braiding forms?
when there is a high proportion of load in relation to discharge eg result of seasonal snow melt in the alphs
at times or low flow the river may be forced to cut a series or paths that converge and diverse as they weave through large expanses or deposited material
braiding begins with a mid channel bar that grows downstr an as the discharge decreases (after a flood)
the coarse bedload is deposited first
this forms the basis of bars and as the flood is reduced finer sediment is deposited
how does flood hazard mapping work?
using historical data on river stages and the discharge of previous floods
along with topographic data
maps can be constructed to show areas rdpect e to be covered in floodwater for various discharges or stages
they highlight risk but are very inaccurate
describe the differences between laminar and turbulent flow?
laminar is very smooth where channel is deep
turbulent flow is a series of rapid eddies caused by channel roughness
in the formation of a waterfall, how is the back wall and plunge pool eroded?
CAVITATION
implosion if air bubbles
hydraulic action in joints and cracks
how can river erosion produce rapids?
over outcrops of resistant rock that often bring about a steeping ofnchannel slope
increasing turubulencr and hence erosive capacity
describe how river floods occur?
due to an overvsnkful level of discharge leading to the inundation of surrounding areas
notably flood plains
sudden excess of input in terms of precipitation
sudden event e.g. storm or cyclone
or seasonal event such as snowmelt
effects can be exacerbated by human activities of deforestation or urbanisation
why is the drainage basin system an open system
it allows movement of energy and matter across its boundaries
what are the main characteristics that affect local hydrology
intensity mm per hour
type of precipitation e.g. rain or snow
geographical distribution
temporal variability eg seasonality
what is throughfall
water that either falls through gaps in vegetation or drops from leaves twigs or stems
what is interception loss
water retained by plant surfaces and which is later evapoursted away or absorbed by the plant
what conditions increase transpiration
warm and dry
what is the most important factor affecting evapotranspiration
temperature
what is infiltration inversely related to
surface runoff
factors influencing infiltration capacity
antecedent soil moisture
duration of rainfall
soil porosity
vegetation cover
what is soil moisture
subsurface water in soil
what is baseflow
the part of a rivers discharge that is provided by groundwater seeping into the bed of s river
relatively constant
but increased slightly in a wet period
what is recharge
the refilling of water in por s where the water has dried up or been artificially extracted
what effects the character or regime of a river
amount and nature of precipitation local rocks porosity and permeability shape of drainage basin (area and slope) amount and type of vegetation dover amount and type of soil cover
what effect does urban development have on hydrographs
increase peak flow
decrease lag time
what factors vary a storm hydrographic
drainage density
rock type
basin shape
gradient
what are the three causes of deposition
reduction in gradient which decreases velocity and energy
a decrease in the volume of water in the channel
an increase in the friction between water and channel
how does a slope influence a flood hydrograph
more overland flow
shorter lag time
higher peak flow
how can soil influence infiltration
sandy soil allows it
clay is much more impermeable and causes water to pass overland
how can antecedent moisture influence a flood hydrograph
if it has been raining before
and ground is saturated or near saturated
rain produces overland flow quickly
and a high peak flow
and a short time lag
how can low intensity rainfall influence a hydrographic
infiltrate into soil
slowly percolate
increased lag time
reduced peak flow
what does the hydraulic radius measure
stream efficiency
what factors lead to the formation of braided channels
large proportion of coarse material
easily eroded bank material
highly variable discharge
when does braiding occur
when channel is divided by islands or bars
islands are vegetated and long lived
bats are unvegetayed less stable and often short term
what forms pools and riffles
riffles form of coarse gravel
pools form of erosion
reasons for gorge formation
retreat of waterfalls eg niagara
glacial overflow channeling
collapse of underground caverns in carboniferous limestone areas
Gordale Scar
where are alluvial fans found
semi arid areas
where swirly flowing mountain streams enter a main valley or plain at the foot of the mountains
how does an alluvial fan form
a fast flowing mountain stream enters a valley or plain at the door of a mountain
sudden decrease in velocity
deposition occurs
fine material spread out as an alluvial fan
with a slope angle or leee then one degree
what are deltas
river sediments deposited when a river enters a standing body of water with negligible currents
eg Mediterranean
eg lake sea or ocean
define water table
the line seperating the unsaturated zone and the saturated zone
or the upper level of the saturated zone
define recharge
the refilling of the zone of saturation by percolated water
lack of water or water abstraction lowers it
throughfall
precipitation reaching the ground via leaf drop
stem flow
precipitation which has been intercepted by vegetation and subsequently trickles down the stems of the plants
difference between porosity and permeability
priority is amount of pore spaces the soil has
so is ability to hold water
permeability is the ability of soil to transmit fluids
explain the formation of delta
boundary between a river and a reviving body of water e.g. lake or sea
the deposition of sediment due to the loss of energy and competence
clay particles are deposited through flocculation as a result of river water and sea water intervening
deltas are built up as the heaviest material is deposited first and the finest being deposited later
distributaries build up as the channel braids and it becomes less efficient
when does braiding occur
when rivers are carrying a large amount of eroded sediment
why does braiding occur
sediment load too heavy for river to carry
velocity drops
varying dischrages
when is the only time deltas form
when the rate of sediment removal is less than the rate of depositon
when does flocculation occur
in clay particles when fresh water meets sea water
sink due to increased weight
what is a delta composed of in terms of layers
bottom set bed a
clay (via flocculation) fine grained sediments
reaches fair distance from river mouth as fine sediment can be transported further
forset bed
coarser medium sized sediments
doesn’t travel very far from mouth
top set bed
composed of larger and heavier particles
shape depends on river currents
Describe how saltation occurs
the transport of medium sized particles by hopping along the bed of the river
as a result of sufficient energy
how are bluffs formed in river valleys
created by the undercutting of the valley sides on either side of the flood plain
by the migration of meander
swinging in the channel
what does the Hjulstrom curve show
the relationship between sediment size and velocity needed to ERODE TRANSPORT AND DEPOSIT
where does saturated overland flow most commonly occur?
lower parts of slopes
that have been saturated by water from above
what factors lead to high overland flow
impermeable soil
lack of vegetation and interception
steep slopes
heavy precipitation
how does soil type influence flooding?
greater the clay content the more water retentive (clay particles bond tightly together), restricting the flow of water
sandy soil is free draining as the larger sand particles provide gaps and spaces for water to pass through
how can rock type influence flooding
sedimentary rocks are pervious eg limestone with cracks and bedding planes
water can pass through the rock
metamorphic rocks depends on the nature of its transformation
igneous rocks eg basalt and granite do not let water through and are impermeable
what does a circular drainage basin mean for a storm hydrograph?
flash floods
rapid response
as precipitated water more likely to reach river at same time treaclleing an equal distance
what does a steep drainage basin mean for a storm hydrograph?
short lag time
due to gravity
increased the rate of flow to the river
what does a large drainage basin mean for a storm hydrograph?
greater potential discharge
longer lag time as precipitation is caught over a wide area
soft engineering river quaggy
EA sustainable approach
park lowered and shaped creating flood plain where water naturally collects (capacity of 85,000)
quality of park improved wetland environments (redness trees wild flower meadows)
1990
hard engineering jubilee river
artificial channel diverts water
4,800 properties
700 commercial toperties
£110m to construct (£40m protected)
12km long
38 reedbed constructed
250,000 trees
woodpeckers
National cycle route and footpaths
Dam construction in Egypt
55km3 released each year
control floods
supports farmland (cotton)
electricity (12 generators)
100,000 people relocated
archaeological sites relocated
explain the formation of an alluvial fan? i
heavy sediment load
sudden change in gradient
river leaves the confines of s narrow channel and the ability for the flow to spread laterally
outward zina took of sediment size
explain how throughflow occurs
the movement of infiltrated water under gravity downslope towards a streak fhannel
the water can move through soil by a form of piping
describe evapotranspirstion?
evaporation from leaves
release of water from plant stomata
define salt crystal growth
phydcial process
involving the growth of salt crystals in the pores of rock by the evaporation of water rich in salts
define hydration
the physical process whereby water is absorbed by certain minerals leading to stress and possible fracture
reversible
briefly describe what drainage density is
area found by measuring the total length of all the streams with a basin
divided by the area of the whole basin
average length of stream in an area
