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Hydrology > W6 - Sediments and solutes > Flashcards

W6 - Sediments and solutes Flashcards

1
Q

Why is sediment important in river systems?

A
  • System is in a state of flux
  • Key link between uplands and lowlands
  • Key link between hillslopes and aquatic system
  • Nutrient cycling – uplands are mineral rich, carries them down to the lowlands
  • Heavily managed
    o preventing sediment getting to where you don’t want it is key for engineering
    o Dams can also hold back sediment that needs to travel downstream
  • Highly changeable
    o Need to understand those changes to manage for human use
  • Hazards
  • Pollution
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2
Q

What are sediments?

A
  • Collection of pre-existing grains
    o Something that can be transported by water
  • Generally loose and unconsolidated
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3
Q

What are the 3 major types of sediment:

A

Biological
–> Remains of dead organisms e.g., shells, plants
–> Remains of framework organisms e.g., corals

Chemical
–> Produced from chemical processes
–> Precipitation directly from water body e.g., salt

Clastic
 Particles weathered or eroded from rocks
 Resistance to weathering → implications for transport

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4
Q

How do we classify sediment?

A

Grain size - tells us about transport mechanism and distance
Grain shape - tells us about how long grains have been in fluvial system and how they behave
Rougher sediments have a higher friction coefficient

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5
Q

What are the driving stresses in sediment transport?

A

drag
lift

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6
Q

What are the resisting stresses in sediment transport?

A

friction
gravity

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7
Q

Why does it take a higher velocity to entrain clay particles than sand?

A
  • Clays hold onto water very well so they clump together (flocculate) so need more force to break the bonds
  • Grain size and cohesion are key controls on transport
  • Way of conceptualising how speed of water can impact transport.
  • Entrainment = picking up sediment
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8
Q

What is the sediment continuity concept?

A

the physical transfer or exchange of sediment from one part of the fluvial system to another, and represents the conservation of mass between sediment inputs, stores and outputs.

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9
Q

What are the three sediment continuity systems?

A

Equilibrium –> input = output –> no change in bed level
Degradation –> input < output –> supply limited and bed lowers
Aggradation –> input > output –> transport limited

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10
Q

What is sediment supply controlled by?

A

Source - hillslope or erosion of bank & bed
Rate of production
Connectivity - hillslope –> channel, delivery from upstream

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11
Q

Potential sources of sediment

A
  • Hillslopes mass wasting processes landslides + soil creep (uplands)
  • Bank erosion (lowlands)
  • Overland flow (both)
    Construction sites
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12
Q

Where are inputs greatest in the sediment cascade?

A

Uplands

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13
Q

Controls on sediment transport:

A

Sediment supply
Bed and bank material
Discharge
Stream power
Shear stress
Flow behaviour
Biotic factors
Vegetation
Types of sediment

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14
Q

How do bed and bank materials influence sediment transport?

A

Cohesiveness of material influences bank erosion/bed incision
Material that makes up the source area is important
Bedrock channels = highly cohesive

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15
Q

How does flow velocity influence sediment transport?

A

Manning’s equation
Higher velocity = higher potential for sediment transport

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16
Q

Controls on velocity:

A

slope, flow depth, roughness
increase hydraulic radius –> increase velocity
increase slope –> increase velocity
increase roughness –> decrease velocity
increased cross-sectional area –> increased velocity
increased wetted perimeter –> decreased velocity More wetted perimeter = more friction forces acting on the water –> reduces velocity of flow

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17
Q

How does stream power influence sediment transport?

A

The energy available to transport sediment
The function of discharge and steepness of slope
Higher total stream power = higher potential for transport
But we also need to take channel width into account

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18
Q

How does shear stress influence sediment transport?

A
  • Measure of force of water on the bed
  • Related to sediment mobilisation and transport
  • Shear stress is function of depth and channel slope
  • Deeper the water column, heavier the water,
    increases shear stress,
    increases transport
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19
Q

What is the difference between bankfull discharge and effective discharge?

A

Bankfull discharge is the maximum discharge contained within the channel
Effective discharge is that which transports the largest proportion of sediment load

Bankfull discharge should be our effective discharge
However, in monsoonal systems effective discharge is monsoonal baseflow as lots of fine sediment is transported even in shallow rivers
Arid systems will be very dry, meaning that even a small amount of flow will carry a great deal of sediment

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20
Q

How does flow behaviour influence sediment transport?

A

Laminar flow (completely straight)
–> parallel, little mixing, slow flowing/very shallow waters, extremely rare in natural streams
Turbulent flow -
-> multiple directions (net flow in one direction), well mixed, high energy system, fast/deep waters, almost always the case in natural streams

More turbulent the flow, more energy required to interact with other parts of the water –> less sediment transport

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21
Q

How do biotic factors influence sediment transport?

A

Beavers – dams lower velocity, less sediment transport
Crayfish make the bead coarser as they disturb fine sediment  invasive species in this country

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22
Q

How does vegetation influence sediment transport?

A

Provides roughness and slows flow down
Large woody debris will have impacts on sediment movement –> chaos as it clogs the channel
Can act as sediment traps as they slow water flow

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23
Q

How do types of sediment influence sediment transport?

A

Suspended load
* Low transport velocity
* Low flows important
* Continuous transport
* Supply limited
* Flow paths critical
Bed load
* High transport velocity
* Flood magnitudes critical
* Discontinuous transport
* Transport limited
* Event dominated

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24
Q

What are examples of sediment inputs, sinks, and outputs?

A

Inputs:
–> Tributaries
–> Upland gullies
–> Hillslope mass movements
Sinks
–> upland valleys
–> Tributary valleys
–> Upper main valley
–> Lower main valley
Outputs:
–> sediment yield to river/ocean

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25
Q

Residence times and type of sediment stored on hillslopes:

A
  • Residence times vary
    o minutes – millennia
  • Most sediment stored as colluvium or alluvium
    o disconnected from channel network
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26
Q

Residence times and type of sediment stored on channels:

A

Residence times controlled by… valley morphology (is there a flood plain?)
–> Channel width controls stream power
–> In gorge in Tibet, flow will be relatively uniform
–> As soon as a small floodplain is formed, there will be some form of sediment storage

Residence times controlled by… grain size
–> humid/temperate regions
–> Need powerful flows to transport all available sediment

Residence times controlled by… episodic rainfall
–> arid/monsoonal regions
–> All the sediment is transported and deposited at once

27
Q

Typical units for sediment yield

A

t km-2 yr-1 (tonnes/km2/year)

28
Q

Why is sediment important in river systems?

A
  • Key link between atmosphere and hydrosphere
  • Nutrient cycling
  • Linked to land use
  • Highly changeable
  • Pollution- even nutrients in too high volumes can become pollutants
29
Q

What are solutes?

A

Substances dissolved in water
o Naturally occurring or manmade
o Contaminant – level greater than natural
o Often categorise them as ions

30
Q

what is an anion?

A

(-ve charge)
e.g. sulphate, chloride, phosphate, nitrate

31
Q

what is a cation?

A

(+ve charge)
e.g. calcium, sodium, iron

32
Q

why is the form of solute important?

A

o Toxicity and bioavailability
o e.g. Carbon as organic (e.g. DOC) and inorganic (e.g. HCO3 -)

33
Q

What controls solute form?

A

pH, temperature, DOC, and pressure

34
Q

How does pH control solute form?

A

pH important as some things will be more soluble in acids

35
Q

How does DOC control solute form?

A
  • redox potential is impacted by dissolved oxygen
  • Low oxygen conditions, iron takes the form of iron but increasing the amount of oxygen changes its elemental structure –> Fe3+ is very bioavailable and can be incredibly toxic – dangerous to ecosystem health
36
Q

How does temperature control solute form?

A
  • At high temperatures, a lot of things outgas out of water – end up with lower conc. Of solutes
37
Q

How does pressure control solute form?

A

Opposite of temperature with pressure high pressure high conc

38
Q

What is concentration?

A
  • Mass of sediment in water
  • e.g. mg L-1
39
Q

What is sediment flux?

A
  • Rate of movement
  • Mass of sediment that flows through unit area per unit time e.g. kg ha-1 a-1
40
Q

What are the controls on solute transport?

A

Atmospheric deposition
Surface pickup (runoff)
Land cover and use
Weathering

41
Q

How do Hydrological pathways influence solute transport?

A

runoff processes heavily control solute transport
* Precipitation inputs generally have low solute concentrations
* How long is the water in contact with a solid surface – how much leaching can occur
* Difference with volume though as a less concentrated overland flow could still carry more solutes than a more concentrated through flow
* Interaction and residence times of water will determine solute characteristics
o Short residence time e.g. IEOF
o Longer residence times e.g. through flow

42
Q

How does atmospheric deposition influence solute concentration?

A

Dry deposition
Could be from human or natural sources e.g., heavy metals from combustion
Can see hotspots where it is more of a problem
Natural ones include chloride from the sea or geological  can still be a problem

43
Q

How does rainfall influence solute transport?

A

Wet deposition
Acid rain – clean air acts have reduced acid rain
(Grenfelt et al. 2019)

44
Q

How does surface pickup (runoff) influence solute transport?

A

Intense area of fertiliser use links closely with nitrate vulnerability

45
Q

How does Vegetation and litter leaching influence solute transport?

A

Organic acids leaching out of vegetation
Has implications for water treatment

46
Q

How does geology influence solute type/concentration?

A

Classic example:
Water hardness (hard water containing calcium/magnesium)
Carboniferous bedrock means a lot of calcium leaches into the water

47
Q

How does chemical weathering influence solute transport?

A
  • Rocks exposed at earth’s surface are volatile
  • Breaking of chemical bonds
  • Primary minerals (e.g. quartz, feldspar) → secondary minerals (e.g. clays, oxides)
  • Water is crucial
  • Key processes:
    o Electron exchange
    o Solution
    o Ion exchange
48
Q

How does mineral stability influence solute transport?

A

Different substances are more mobile than others

49
Q

How does solution influence solute transport?

A
  • Different elements are differently soluble in different pHs
  • Water is composed of:
    o H2O molecules
    o H+(protons)
    o OH- (hydroxyl)
    o H3O+ (hydronium)
  • The more H+, the lower the pH…
  • the more OHions to combine with base cations (Ca2+, Na+ , Mg2+, etc) in solution
    o destabilises mineral structures
    o promotes further weathering
  • Calcium more soluble in acidic conditions
  • Silica in alkaline conditions
50
Q

What is cation exchange controlled by?

A
  • Cation exchange capacity of clays/OM
  • Base saturation of clays/OM
  • Composition and pH of soil water
51
Q

An example of temporal variation?

A
  • Phosphorus in the River Frome
  • They vary greatly and fluctuates annually – seasonal variability
  • Seasonal pattern: with the highest concentrations in Jul-Oct – linked to agricultural runoff
  • Increasing trend from early 1960s to early 2000s – linked to population increase and sewage
  • Step change reduction in early 2000s- linked to better sewage management
52
Q

2 types of pollution:

A

Point source pollution
* Defined outlet such as CSO – known point in the landscape that is causing pollution

Non-point source (NPS) pollution (diffuse sources)
* Can’t identify one point in the landscape where pollution is occurring – e.g. agricultural runoff

53
Q

Type of pollution and example of temporal variability

A
  • The urban dimension
    o Phosphate (P) in sewage effluent
    o Point source
    o Mainly in dissolved form
     Sediments are removed during the sewage treatment process
  • The rural dimension
    o P in runoff from agricultural land (fertilizers and manure)
    o Diffuse source
    o Dissolved or particulate
  • Need to take into account different sources of pollution
  • Point sources (STWs) constant input of P to the river over the year
  • Diffuse sources (agricultural runoff) input to river when mobilisation occurs
  • Relationship with discharge is complicated
  • High flow events mobilise diffuse contributions AND dilute point source contributions
54
Q

What are rating curves?

A
  • Discharge against suspended sediment concentration
55
Q

What is the relationship between discharge and suspended sediment concentration

A

higher the discharge, higher the conc. In the River Creedy
o Would need large error bars as it is very scattered
* Link sediment and solute concentrations to discharge
* Useful tool as it’s easier to monitor discharge than sediment
* Empirical relationship between sediment or solute concentration (SSC or C) and discharge (Q)

56
Q

Do rating curves always work?

A

NO
* Discharge (forces) supporting suspended sediment is more complex (C≠f(Q))
o E.g. changes in fluid mechanics associated with changes in bed forms e.g. rapids
* Supply limitations - many rivers transport solutes or sediment at the rate at which they reach the river, rather than the amount limited by flow capacity
(Labadz et al., 1991)

57
Q

why might we get positive and negative hysteresis?

A

lower conc. on rising limb, higher conc. on falling limb = negative hysteresis

Higher conc. on rising limb, lower conc. on falling limb = positive hysteresis

58
Q

Why is hysteresis looped?

A

there is a lag between the two peaks

59
Q

When and where does positive hysteresis occur?

A

Typical of upland environments in UK
A lot of easily erodible material but there has to be a weathering event to make is available
Without weathering event there can’t be sediment transport

60
Q

When and where does negative hysteresis occur?

A

In larger systems connectivity is important – sediment coming from tributaries in the bottom of the catchment and takes a long time to reach the end of the catchment whereas discharge has already peaked
Two different types of flow in the Rhine – in same river system you can see different types of behaviour

61
Q

hysteresis and rating curves

A

Asselman (1999) noted that if you look at successive storm events throughout winter, you need to have different rating curves

61
Q

hysteresis and rating curves

A

Asselman (1999) noted that if you look at successive storm events throughout winter, you need to have different rating curves

62
Q

Hysteresis and point vs diffuse sources

A

Speed of transmission of flow
Upland moors - slow diffuse P deliver
Upland soils haven’t got much phosphorous – get into stream system through throughflow so peak would be much slower
Rapid overland flow will result in a quick peak of solutes
Intensely farmed lowlands - rapid mobilisation of P stored within channel and riverbank

Hydrology (7 decks)

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