Freshwater Flashcards
Drainage basin
The total area drained by a river and its tributaries
Why is the drainage basin an open system?
Because water can enter the system via precipitation and also leave, moving across the watersheds of multiple drainage basins. Matter can leave the drainage basin.
Inputs, outputs and flows of drainage basin
Input: precipitation
Flows/transfers = infiltration, throughflow, groundwater flow, percolation, base flow
Outputs: Evaporation, transpiration
Stores
Vegetation Aquifers Soil Cyrosphere groundwater
Definition of precipitation
Transfer of moisture from the Earth’s atmosphere to the earth’s surface in the form of rain, hail, sleet, snow.
Definition of interception
The capture of raindrops by plant cover that prevents direct contact with the soil. If rain is prolonged, retaining capacity of leaves will be exceeded and water will drop to the ground (through fall)
Evaporation is a function of…
- Vapour pressure
- Air temp
- wind
- rock surface - bare rocks have greater evap rates
Potentail evapotranspiration
The water loss that would occur if there was an unlimited supply of water in the soil for use by vegetation.
Discharge definition + units?
The total volume of water flowing past a certain point per unit of time (cumecs)
Bradshaw model characteristics
Discharge increases Load size decreases Load angularity decreases Load amount increases Roughness decreases gradient decreases
5 river transport ways
Floatation - leaves and twigs carried on the surface of the river
Saltation - Heavier particles bounce or bump along the river bed
Attrition - particles collide with each other, becoming smaler and rounder
Solution - chemical ions are removed, dissolving particles in water
Traction - boulders are rolled along river bed
Factors affecting erosion
Rock type Vegetation Precipitation intensity Gradient Channel density Discharge Load angularity pH Urbanization
Explain the 4 types of erosion
Attrition - particles collide with each other, becoming smaller and rounder
Abrasion - particles scrape against river bed and banks and wear them away due to the friction
Hydraulic action - force of water directly weakens the rock or gets into cracks.
Solution - removal of chemical ions which dissolves rock
‘Capacity of a stream’ definition
The largest amount of debris a river can carry
‘Competence of a river’ definition
The diameter of the largest particle that can be carried.
Critical erosion velocity
The lowest velocity at which grains of a given size can move
Hjulstrom curve shows the relationship between…..
Debris diameter and velocity
3 main points about Hjulstrom curve
- Smallest and largest particles require high velocities to lift them (smallest is clay but requires big velocity because it is cohesive)
- Higher velocities required for entrainment than for transport
- When velocity falls below a certain level, deposition occurs
Factors affecting regime of a river (variation in the flow)
- Intensity of precipitation
- Porosity and permeability of rocks
- Amount of vegetation cover
Waterfall formation
- There are alternating layers of hard and soft rock
- Soft rock is more easily eroded than hard rock due to hydraulic impact
- This leaves the hard rock undercut
- It eventually collapses into a plunge pool due to lack of support by soft rock
- Abrasion occurs here, causing the waterfall to retreat.
- A gorge is formed
3 situations when deposition likely to occur?
- River floods
- River enters sea
- River enters behind a dam
Formation of a floodplain
They are flat areas found in the lower course, comprising of silt, clay and alluvium. Fertile lands.
Formation of levees
- When a river floods, it loses its velocity and thus energy and has to deposit some of its load
- It deposits the coarsest, heaviest material first
- This forms raised banks called levees at the edge of the river
- Build up over time as more deposition occurs
Factors affecting the formation of a delta?
- Salinity (salt makes particles more cohesive)
- size of load
- vegetation
- gradient of coastline
How is a delta formed?
When a stream flows into a standing body of water it may form a delta
For a delta to be formed, a river needs to carry a large volume of sediment
Deposition is increased if the water is salty as this causes salt particles to clump together (flocculation) and become heavier so they are deposited
Vegetation also increases rate of deposition by slowing down the water (bioconstruction)
Deposition occurs due to a rapid drop in stream velocity
There is a regular succession of deposits
Finest deposits are removed furthest (bottom-set beds) and coarsest material at the top
What does a hydrograph show?
How a river changes over a short period of time.
It shows how a river channel responds to the processes of the hydrological cycle
It measures the speed at which rainfall falling in a drainage basin reaches the river channel
Explain the rising limb, the falling limb, the lag time and the baseflow
Rising limb = indicates the amount of discharge and the speed at which it is increasing
Falling limb = How fast the river levels fall - is steeper than rising limb as overland flow has been carried away and it is mainly throughflow moving into channel
Lag time = The difference in time between when the rainfall peaks and when the river’s discharge peaks at.
Baseflow = movement of groundwater into the river channel
What is the recessional limb influenced by?
- Permeability of rocks
- Local aquifers
- Basin size
- Larger catchments
- Flatter gradients
Factors affecting flood hydrographs
- rock type
- climate
- urbanization
- vegetation
- gradient
- drainage density
- drainage size
- soils
How does a urban hydrograph differ from a rural one?
- shorter lag time
- steeper recessional limb
- steeper rising limb
- higher peak discharge
- discharge may peak earlier
4 urbanizing influences on flooding
- Deforestation: less interception, more saturated soils, less EVP
- Building of houses: more impermeable surfaces, decreased infiltration, more overland flow and surface run off, shorter lag time
- Channelization: straightening of channels may help carry water away quickly but may increase risk downstream
How can flood warnings be improved?
- improved rainfall and snow pack estimates; better and longer forecasts of rainfall
- better gauging of rivers, collection of meteorological information and mapping of channels
- Better and more current information about human populations and infrastructure
- More complete and timely sharing of info needed of meteorological and hydrological within international drainage basins
- Technology should be shared among all agencies involved in flood forecasting
Ways of reducing flood peaks:
- reforestation
- Treatment of slopes such as contour ploughing to decrease them, reducing run off coefficient
- Construction of natural water storage zones e.g: lakes
- vegetation protection
- clearance of sediment
2 examples of Flood Diversion
- levees
- reservoirs