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Hydrology and fluvial geomorphology - Coggle Diagram
Hydrology and fluvial geomorphology
1.1. The drainage basin system
Inputs: Precipitation, antecedent moisture, water vapor in atmosphere
Outputs: Evaporation, evapotranspiration
Evapotranspiration: Water vapor escapes from living plants, evaporation and transpiration combine.
Storages are ways that water can stay in the ground. Types of storage:
Groundwater
Vegetation
Interception: Water that is caught and stored by vegetation.
Interception loss - water held by plant surfaces later evaporated or absorbed
Throughfall - water either falls through gaps in the vegetation or drops from leaves or twigs
Stemflow - water flows along branches and onto stem
Soil moisture - degree to which soil moisture falls below field capacity.
Soil moisture recharge - precipitation exceeds potential evapotranspiration
Soil moisture surplus - period when soil is saturated, water can't enter and flows overland flow
Soil moisture utilization - water is drawn to surface through capillary
Flow - above ground:
Overland, when water flows over land surface.
Infiltration - the amount of water entering the ground via soil or porous rock.
Below ground:
Porosity is the capacity of rock holding water eg. sandstone is less porous but clay is very porous.
Permeability is ability to pass water through rock via joints and faults.
Infiltration - Where water soaks into soil. Infiltration capacity is maximum rate where rain can be absorbed by soil in a certain condition.
Capacity decreases with time through periods of rainfall until more of a constant value is reached.
Infiltration is inversely proportional to overland runoff and affected by things like length of rainfall, antecedent soil moisture, soil porosity, slope angle and vegetation cover.
Percolation - water moves downwards from soil into bedrock.
Throughflow is where water flows through the soil in natural pipes.
Aquifers - provide reservoir of water, permeable rocks like sandstone and limestone.
They can absorb water that otherwise would flood a river channel.
Groundwater recharge:
Infiltration - part of total precipitation at ground surface
Seepage - banks and bed of surface water bodies like ditches, rivers, oceans
Groundwater leakage, inflow - adjacent rocks and aquifers
Artificial recharge - irrigation, reservoirs and so on
Losses groundwater:
Evapotranspiration - particularly in low-lying areas where water table is close to ground surface
Natural discharge - means of spring flow and seepage into surface water bodies like lakes and reservoirs
Groundwater leakage and outflow - along aquicludes and into adjacent aquifers
Artificial abstraction - eg. water table near Lubbock on High Plains Texas, has declined by 30-50m in just 50 years, Saudi Arabia groundwater reserve in 2010 was 42 per cent less than in 1985
1.2. Discharge relationships within drainage basins
Hydrographs
A storm hydrograph shows how the discharge of a river varies over a short time
During a storm some water infiltrates into the soil while some flows over the surface as overland flow
The rising limb shows how quickly the flood waters begin to rise
The recessional limb is the speed at which it declines
The peak is the maximum discharge of the river and the lag time is the time between the height of the storm and the maximum flow in the river
The character or regime of the resulting stream or river is influenced by several variable factors:
the amount and nature of precipitation
the local rocks, especially porosity and permeability
the amount and type of vegetation cover
The amount and type of soil cover
Influences on Hydrographs
Climate
Precipitation type and intensity
Temperature, evaporation, transpiration and evapotranspiration
Antecedent moisture
Drainage basin characteristics
Drainage basin size and shape
Drainage density
Porosity and impermeability of rocks and soil
Rock type
Slopes
Vegetation type
Land use
1.3. River channel processes and landforms
Erosion
Abrasion (Corrasion) is the wearing of the bed and the bank by the load carried in the river.
Attrition is the wearing away of the load carried by a river
Hydraulic Action is the force of air and water on the sides of a river and cracks in the rocks
Caviation is the force of air exploding caused by a pressure drop
Corrosion/Solution is the removal of chemical ions.
Factors Affecting Rates of Erosion:
Load - the heavier and sharper the load, the greater the potential for erosion
Velocity - the greater the velocity the greater the potential for erosion
Gradient - increased gradient increases the rate of erosion
Geology - soft, unconsolidated rocks such as sand and gravel are easily eroded
PH - rates of solutio are increased when the water is more acidic
Human Impact - deforestation, damns and bridges interfere with the natural flow of a river and frequently end up increasing the rate of erosion
Eroded material is carried in the river as its load
Load Transport
The load is transported downstream in a number of ways:
Silt and clays are carried as suspension load
Large particles are transported in a series of 'hops' as saltated load
Pebbles are shunted along the bed as the bed or tracted a load
In areas of calcareous rock, material is carried in solution as the dissolved load
The capactiy of a stream refers to the largest amount of debris that a stream can carry, while the competence refers to the diametere of the largest particle that can be carried
Deposition and Sediment
There a number of causes of deposition, such as:
a shallowing of the 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
The Hjulstrom Curve
The critical erosion velocity is the lowest velocity at which grains of a given szie can be moved
There are three important features on Hjulstrom curves:
The smallest and largest particles high velocities to lift them
Higher velocities are required for entertainment than for transport
When velocity falls below a certain level, those particles particles are deposited
River Flow
Velocity and discharge
River flow and associated features of erosion are complex. The velocity and energy of a stream are controlled by:
the gradient of the channel bed
the volume of water within the channel, which is controlled largely by precipitation in the drainage basin
the shape of the channel
channel roughness, including friction
Patterns of Flow
Laminar flow: a smooth, straight channel with a low velocity is confused
Turbulent flow occurs where there are higher velocities and complex channel morphology such as meandering channels with alternating pools and riffles
Helicoidal flow is a corkscrewing motion. This is associated with the presence of alternating pools and riffles in the channel bed
Channel Types
Braiding occurs when a channel is divided by islands
The factors affecting this are:
a steep channel gradient
a large proportion of coarse material
highly variable discharge
Meanders
Meanders can occur in on a variety of materials and occur in conditions where channel slope, discharge and lead combine to create a situation where meadnering is the only way a channel can use up its energy. The wavelength is affected by the channel width, discharge abd nature of the bed and banks
Meander wavelengths are genrally 6-10 times channel width and discharge
Meander wavelength wavelengths are generally 5 times the radius of curvature
The meander belt (peak-to-peak amplitude) is generally 14-20 times the channel width
Riffles occur at about 6 times the channel width
Sinuosity increases as depth of channel increases in relation to width
Meanderin is more pronounced when the bed load is varied
Meander wavelength increases in streams that carry more coarse debris
Meandering is more likely on shallow slopes
Meandering best develops at or near bankfull state
Causes of meanders
Friction
Sand Bars
Sinuosity
Heliocoidal flow
Change over time
There are a number of possibilities:
Meanders may migrate downstream and erode river cliffs
They may migrate laterally and erode the floodplain
They may become exaggerated and become cut-offs
Under special conditions, they may become intrenched or ingrown
Landforms
Waterfalls and Gorges
Waterfalls occur where the river spills over a sudden change in gradient undercutting rocks by hydraulic impact and abrasion
There are many reason for this sudden change in gradient along the river:
a band of resistant strata such as the resistant limestones at Niagara Falls
a plateau edge such as Victoria Falls on the Zimbabwe-Zambia border
a fault scarp such as Gordale, Yorkshire
a hanging valley such asGlencoyne, Cumbria in the UK
coastal cliffs
Gorges may also be formed as a result of:
antecedent drainage
glacial overflow channelling
the collapse of underground caverns in Carboniferous limestone areas
the retreat of waterfalls
superimposed drainage
Levees, floodplains and bluffs
Formed when a river bursts its banks
Oxbow lakes are formed when a meander is cut off at its neck
Deltas
Bottomset beds - Very fine material and the lower parts of the delta
Foreset beds - over the bottomset beds, inclined/sloping layers of coarse material
Topset beds - composed of fine material
Influenced by:
the rate of river deposition
the rate of stabilisation by vegetation growth
Tidal currents
the presence or lack thereof of longshore drift
Human activity
Arculate delta - Fan Shaped Deltas, longshore drift or other currents keep the seaward edge of the delta trimmed and relatively smooth shape such as the Nile
Cuspate delta - Pointed and tooth-like, for example the Ebro or Tiber deltas
Birds-foot delta - where the river brings down enormous amounts of fine silt