Hydrology And Fluvial Flashcards
What are the natural systems composed of?
- Outputs; matter or energy leaves the system
- Stores; matter or energy builds up and kept in system
- Flows; matter or energy moves in system
What is a drainage basin?
- A natural system which includes inputs, flows and stores of water and sediment. Refers to the area drained by a river and its tributaries. It is also an open system as it allows the movement of energy and matter across its boundaries.
What is the hydrological cycle?
- Refers to the cycle of water between the atmosphere, lithosphere and biosphere. The movement of water between air, land sea.
Define evaporation (output)
- The process of water transforming into water vapour (gas) when heat is applied.
Define evapotranspiration
- The combined loss of water to the atmosphere through transpiration and evaporation.
What are the stores?
- Interception
- Soil water
- Surface water
- Groundwater
- Channel Storage
What are the two main types of flows?
- Above ground flows
- Below ground flows
Define transpiration
Where water is drawn up from the soil by the plant and leaves the plant as water vapour through tiny pores under the leaf - stomata.
What is discharge?
What is the formula?
The volume of water in a river at a given point.
Q= A*V
(Discharge= cross sectional area * velocity)
M^3/s
What is interception
Precipitation is caught and stored by vegetation before it reaches the ground
What is surface water.
Temporary & permanent store examples.
When infiltration capacity is exceeded, water builds upon the surface.
T - puddles
P - lakes
What is ground water
Water that has percolated into bedrock, it is a store of freshwater.
What is channel storage
All water which is stored in river streams and drainage channels.
What is soil moisture
Water held sub-surface in soil pores. Sandy soils have many large poles so is permeable & has quick infiltration rates. Clay minerals swell when in contact with water making it impermeable and unstable.
What is soil moisture surplus?
Soil is saturated so water cannot enter and it flows over the surface.
What is throughfall?
Leaves and twigs become saturated so water drips from them. Precipitation can also fall through gaps in vegetation cover.
What is stem flow?
Precipitation intercepted by vegetation which runs down the branches and main trunk.
What is overland flow?
When soil is saturated, or precipitation exceeds infiltration rate, surface runoff occurs where water flows over the surface.
Channel flow
Movement of water through established channels such as streams or rivers. Channel flow is fed by 2 sources; overland flow, groundwater flow.
What is infiltration
The movement of water from above ground into the soil.
Factors that affect infiltration
- Permeability of soil
- Relief
- Rate of precipitation
- Saturation levels of soil
What is percolation
Where water flows from the ground or soil into porous rock or rock fractures. Dependent on the fractures that may be present in the rock and the permeability of the rock.
What is throughflow
Water flows through the soil into streams or rivers. Speed of flow is dependent on the types of soil; e.g clay soils slower flow rate than sandy soils.
What is groundwater flow
Where water flows through permeable rocks, below the water table and also may flow in springs. Groundwater maintains water levels elsewhere.
What is base flow
The level of channel flow when there is no overland flow
Underground water
Makes up a large amount of the Earth’s water stores.
It can be held in soils or in between rock spaces.
Aquifers are underground water stores.
Shallow groundwater aquifers can store water for up to 200 years.
What is the water table
The water table is the level at which the pore spaces and fractures in the ground become saturated, meaning above the water table is unsaturated soil and below is saturated soil.
What happens when the surface dips below the water table
Groundwater will fill the surface space to become surface water
What happens when the surface water dries up in the water table
Groundwater will continue to replenish the area that is underneath the water table.
When the water table rises and falls what happens int eh water table?
The surface water will rise/fall accordingly.
Groundwater Recharge
When groundwater levels deplete (by human extraction/ groundwater replenishing surface water levels) groundwater can be charged.
What does groundwater recharge occur with?
- infiltration (from precipitation)
- irrigation, reservoirs
What does groundwater loss occur with?
- Evapotranspiration
- Natural discharge
- Leakage and outflow
- Artificial abstraction
What is an aquifer?
Permeable rocks (clay, limestone) that contain significant quantities of water. Water inside moves slowly and maintains streamflow.
What are springs?
Water flow reaches the surface, making a spring which might be substantial enough to become a source.
What does an annual hydrograph study?
Studies responses of the river to its environment; highlights seasonal characteristics therefore biggest influencer is climate.
What does a storm hydrograph show
Shows variations of river discharge over a short time period. Includes both discharge and rainfall on the y axis.
What are aspects within a hydrograph
- rising limb
- peak discharge
- lag time
- receding limb
- storm flow
- quick flow
What are drainage basin characteristics
- Size and shape
- Drainage density
- Soil porosity and permeability
- Rock type
- Slopes
- Vegetation type
- Land use
What are the erosion processes
- Hydraulic action
- Abrasion
- Attrition
- Solution
- Cavitation
Define erosion
The process of the wearing away of soil and rock. This could be due to force of water and as well as fragments of rock.
Define hydraulic action
When the sheer force of water causes cracks and crevices in the river bank which gradually breaks away.
Define attrition
Where rocks and pebbles are knocked into each other and break down into smaller, rounder pieces of sediment.
What are the forms of transport within a river
- Traction
- Saltation
- Suspension
- Solution
Define abrasion
Rocks and sediment carried by the water grind away on the river bed in a sand-paper like action, weakening it and gradually wearing away.
Define corrasion
The erosive action of particles carried by the river.
Define solution
The dissolving of rocks by removing chemical irons.
Define traction
The movement of larger rocks and pebbles through water rolling them along the river bed. As the rocks are heavier, they cannot be carried by water within the channel since river doesn’t have enough energy.
What is drainage density
- The total length of all rivers and streams in a drainage basin divided by the total area of the drainage basin.
What are the characteristics of a high drainage density
There are more streams and rivers carrying water so peak discharge is higher so short lag time.Water is drained quickly so a steep falling limb.
Characteristics of low drainage density
Overland flow is more likely to infiltrate so lag time is longer and peak discharge is lower.
What types of particles have a more subdued hydrograph (sandy soils)
Ones with larger pores as infiltration will occur faster since there is more space for water to enter. Periods of intense & heavy rainfall sandy soils will infiltrate more water than clay so drainage basins with sandy soils have subdued hydrograph since overland flow is less.
How does rock type affect drainage
Permeable rocks infiltrate more water than impermeable ones as there are spaces for water to seep into. So drainage basins with permeable rock will have less overland flow so less discharge. Soil would become less saturated as the water stored in the soil on top of rocks can percolate into the rocks below.
How does the topography and relief of land affect drainage
Rough/jaggered topography reduces the velocity of water as the flow is interrupted with rocks and bumps (debris). So peak discharge is lower due to slower overland flow. A smoother topography allows water to move quickly.
A steep slope allows water to travel faster since force of gravity is stronger. Water too has less time to infiltrate so more flows to overland flow. Lag time is quicker and a higher peak discharge. (Flashy hydrograph)
How does vegetation affect drainage
A well vegetated drainage basin encourages surface water to drain into ground rather than overland flow. Tree and plant roots create passages for water to infiltrate into lower layers of soil so less water flows as overland flow and less water goes into river channel. Vegetation decreases soil moisture content since it takes up water and releases it through transpiration.
Types of land use affecting drainage
- Deforestation
- Agriculture
- Urbanisation
How does deforestation affect drainage and hydrograph
Less interception so surface runoff increases. Soil no longer held together by roots so soil water storage decreases. Fewer plants so transpiration decreases. Results in a flashier hydrograph shorter lag time and higher peak discharge.
How does agriculture affect drainage and hydrograph
Livestock can trample ground reducing infiltration since pore spaces are lost. Ploughing increases infiltration by creating looser soil decreasing overland flow. However, digging drainage ditches increases surface water storage so decreases runoff. Irrigation can lead to groundwater depletion allowing more water to enter channels before the river is in flood.
How does urbanisation affect drainage and hydrographs
Roads and buildings have impermeable surfaces reducing infiltration and increasing overland flow. Reduces lag time and increases peak discharge. Urban drainage reroutes large amounts of water to different areas so water outsides natural drainage basin can flow into a river, increasing peak discharge.
Define saltation
Where pebbles are bounced along the river bed
Define suspension
Small pebbles and materials are carried within the water instead of rolling/bouncing
Define solution
Soluble materials are carried within the water
What is deposition
Deposition is where the river drops its load when the river no longer has enough energy to carry the load. May occur when a river becomes shallower or when volume of water decreases.
What is the Hjulström Curve
A graph that shows how the velocity of a river affects the rivers material. Graph shows where the material will be eroded, transported or deposited based on its size and river velocity.
What is laminar flow
- flow of water in parallel streams
- in one direction
What is turbulent flow
- a disorderly flow
- changes in velocity
- caused by friction; when river channels are irregular
What is helicoidal flow
- corkscrew movement of water
- occurs in meanders
- responsible for the erosion and deposition in a meander
What is the thalweg
- the line of fastest flow within a river
- also the deepest point within the river channel
Straight channel characteristics
- upper course of river
- single channel
- vertical erosion is prevalent
- thalweg moves from side to side
Meandering channel characteristics
- middle and lower course of river
- series of bends
- deposition on inside bend
- erosion on outside bend
Braided channel characteristics
- large deposits of sediment; causes multiple channels separated by islands of sediment
- usually not in upper course
- river deposits when it loses energy
- usually occurs where discharge fluctuates
Waterfall process
- occurs where a river flows over an area of hard and soft rock, soft rock erodes quicker creating a step
- over time soft rock continues to erode further undercutting the hard rock
- hard rock isn’t eroded as quick leaving it suspended in air as an overhang
- the rotational movement of the water quicken erosion creating a deep plunge pool
- eventually the unsupported over hang collapses
- broken rocks fall into plunge pool
- erosion continues to undercut the hard rock creating another overhang
- the repeated process of overhang collapsing causes waterfall to retreat upstream overtime
- plunge pool continues to deepen and the hard rock continues to be undercut creating an overhang
What is a gorge and how is it formed
- a steep sided, narrow valley with a river flowing through it
- forms when a waterfall continually retreats over time
- the back of a waterfall wall and the channel floor is eroded, whereas the valley sides are not; creates a deep gorge when waterfall retreats upstream
How does a meander form
- thalweg is located on the outside bend of a river
- erosion is stronger on the outside of a bend and the river channel is at its deepest meaning there is less friction
- deposition is more prominent on the inside of a bend where the rivers velocity is at its lowest
- so it has less energy to carry the material; also shallowest causing water to be slow due to friction
- overtime erosion causes outside bends to get closer, until only a small area of land remains (neck)
What is a river cliff (landform of a meander)
- a cliff created when the bank is undercut by erosion on the outside of the bend
What is a point bar ( landform of a meander)
- a deposit of fine sediment (sand) on the river bank inside of a meander bend
- usually changes position leaving a mark where it once was
How is an oxbow lake formed
- when a river is in flood and breaks its banks, the river will flow along the shortest available course;
- usually across the neck cutting off the meander
- eventually the meander bend becomes fully cut off due to deposition;
- creates an oxbow lake
What are riffle & pool sequences and how are they formed
- alternating patterns of shallow and deeper water in a river channel, common in rivers with gentle slopes
- formed as a river tends to adjust its course of flow to efficiently transport its load downstream
- Riffle: higher, shallower area of the sequence, characterised by deposits of material
- Pool: the lower, deeper area of the sequence
What is a floodplain
- a large flat area of land near a river that forms due to erosion and deposition
Floodplain process
- overtime meanders can erode a v-shaped valley into a wider, flat valley
- when a river is in flood the sediment that is transported within the river leaves the Chanel and spreads out onto the river bank
- the water loses energy when it is no longer flowing through it’s channel meaning the sediment is deposited onto the flat plain
- over time this deposition builds up, making it higher; soil is very fertile
What are levees
- natural embankments along a river that build up due to repeated flooding
- create raised edges on the river bank making it less likely for the river to break its banks
What is the levee process
- when a river floods the sediment being transported in river floods onto floodplain
- heavier, coarse material is deposited first as it requires the most energy to be transported
- finer, light sediment (sand) require less energy so it can be transported and deposited further away from the channel; creates a tapered shape
- over time levee becomes larger and well-formed
- river levels ca be higher without breaking the bank and flooding; act as natural flood defences
How do deltas form
- they form at the mouth (end) of a river where it carries large amounts of sediment
- when a river meets a body of water there is a larger difference in velocity between high velocity river and low velocity water body
- causes the river to mass deposit sediment causing a series of sediment islands with smaller streams and rivers flowing through them
How can deforestation affect the drainage basin
- evapotranspiration decreases; less trees = less water evaporated into atmosphere via transpiration so more water on/in ground that can flow into river
- infiltration rates decrease; despite there being less plant material blocking water from infiltrating into soil tree roots play a vital role in infiltration; provide natural gaps within soil for water to enter and percolate into groundwater stores: ground becomes less permeable without trees increasing overland flow
- interception decreases; less trees to intercept precipitation= higher infiltration or flows as overland flow
- channel flow increases as overland flow increases (less transpiration, infiltration,interception) more water flows into river channel increasing rivers discharge
How does afforestation affect the drainage basin
- increased interception
- less overland flow
- higher transpiration rates
- decrease channel flow; lower river levels
How does urbanisation affect the drainage basin
- causes larger flows into the river channel:
- roads & buildings have impermeable surfaces and are likely to have drains which reduces infiltration but increases overland flow
What is water abstraction
- the removal of water from a water body
- is completed in order to meet water demands for drinking water, sanitation
How can water abstraction affect the drainage basin
- may be unstable causing changes in a river in its catchment area
- water levels can deplete both on surface and groundwater levels affecting channel flow and discharge potentially causing more deposition
What are examples of water storages
- above ground by damns & reservoirs
How do water storages affect the flows and catchment areas
- damns directly stop and control channel flow as they provide a barrier
- reservoirs behind damns can raise water table underground and affect soil saturation if not managed which could lead to lowering of water table in other areas
- reservoirs are typically large so a lot of water can enter through the atmosphere through evaporation
Physical causes of flooding
- heavy rainfall; overland flow
- prolonged rainfall; saturated soils no infiltration
- impermeable surfaces
- melting snow/glaciers
Human causes of flooding
- urbanisation
- mechanised farming
What are the impacts of flooding
- waterlog plants and put wildlife in danger due to excess water and sediments on floodplains
- transport unwanted/harmful materials and deposit them in clean water supplies affecting aquatic life
- damage buildings & infrastructure
- homelessness
- death/injury
- negative toll on economy as business may be forced to close and money is spent on repairmen’s insurance outs
How can we predict floods
- probability of a certain sized flood can be predicted on past flooding records known as a flood recurrence interval
- can be modelled using software and other equipment to predict how different circumstances would affect flooding; slope, topography, precipitation rates, soil moisture content
How do forecasts and warning help manage floods
- forecasts use precipitation forecasts to estimate level of flooding risk within an area
- streamflow data and models of streamflow routes can calculate areas most at risk so warnings can be issued in advance; allows evacuation, sandbagging
What is hard engineering
- physically altering a river and its channel through building structures or using machinery
How do damns restrict flooding
- they physically obstruct a rivers natural course leading to build up of a reservoir
- effective as river can no longer flow as there is an obstruction in the way
- however they are expensive, displace settlements and disrupt wildlife
How does channel straightening prevent floods
- its where bends and irregularities in river channel are removed using machinery leaving a straight channel (cutting off meanders)
- straightened so water can flow faster through the channel so less water is likely to overflow onto floodplain and moves downstream instead
- however as water is just sent further downstream, it can cause flooding further downstream and enhance erosion (as it has high velocity)
How do levees prevent flooding
- as well as occurring naturally they can be reinforced or constructed for flood prevention
- similarly to natural levees artificial ones provide a raised embankment so water within river channel has further to rise before it can spill out of channel onto floodplain
How do diversion spillways prevent floods
- its a constructed channel that allows excess water to flow into it when channel is overflowing
- may direct water to further downstream or direct water to a different river
- tend to be floodgates that control when they are opened/closes
- however may direct water to areas that do not flood naturally which can cause environmental damage if the spillway floods
What is soft engineering
- the use of natural, sustainable means to reduce flooding rather than building structures
How can afforestation and tree management prevent floods
- decrease surface runoff in drainage basin area where trees are planted
- however if not managed properly it can lead to further problems :
- unstable trees on floodplains can lead to more flooding as they can block rivers if they fall over
Ways in which floodplain management can prevent floods
- they provide space for excess water to infiltrate into the ground as well as naturally blocking and diverting flood water
