Lecture 8 Hyporheic Flashcards
What is the hyporheic layer
Area below a river bed
What characteristic of bedforms makes the hyporheic zone possible
Porus bedload
What pressure regime causes water movement to the hyporheic zone
High bed pressures, often when flow constricts
Why is residence time important between grains in the HZ
Reactions with carbon dioxide and microbial activity governed by movement and replacement of water
What is the name for how flow gets into the river bed
Advective exchange
How does water get into the bed via advective exchange
flow pathways and channels
Which side of a ripple formation does a flow pathway enter the bed
stoss side
In meander channels, how can water move in the hyporheic zone
From one channel to another, same in braided rivers
What are OFG
Open framework gravels, high porosity
What does mixing of different grain sizes lead to
Less porosity as small grains fill in gaps
What is the role of darcy’s law
Way of describing subsurface water movement
When do we see a drop in permeability with sand fraction percentage
Over 18%
What is wrong about conventional velocity profiles with the addition of hyporheic flow
Inconsistency of bed flow understanding, gap
What flow is seen from the bed to the main flow channel
Return flow
What do interconnected pores lead to
Creation of flow networks and channels
What force is adjective exchange said to solve
Lift force
What areas of study does the hyporheic zone link to
Ecology and biogeochemisty
What species lay there eggs within gravel beds
Salmon, trout species
What cycle is the hypirheic zone important in
Carbon cycle
What is the name for the amount of time water spends in pores
Residence time
What is Greg’s view on the size of hyporheic sizes
Just several grains in diameter below bed
How quickly can chemical composition change over the bed boundary
Very significant contrast, quick
How does oxygen differ in the HZ
Much less
Where does flow come out of the bed
Low pressure zones
How has HZ movement been visualized
Tracer tests
As well as bed forms, where can adjective exchange be found
Pool riffle
Why may water move in the HZ across a meander
Pressure and height difference
What is hydraulic conductivity
Ease at which water can move through a substrate
What does dh stand for in darcys law
Change in head
What is strange about Hyporheic flow
Occur across scales with incredible similarities
What does grain size, particle sorting and packing in the river bed change
Heterogeneity of hydraulic conductivity
In the natural world, How is ease of movement determined
The mosaic of different bed types
How does sediment transport rates effect hydraulic conductivity
Sediment sorting, slow transport leads to avalanching and movement together leading to more mixed substrate
What sand fractions are OFGs
Less than 18%
What is the assumpted new layer between hyporheic and normal freeflow
Transition layer
What is the name of the point where flow speed changes to go to the bed
Inflection point
How think is the transition layer
2 grains
What is the porosity of gravel bed rivers
Over 40%
What does larger porosity give us
Larger amount of space which water can move through
What can the lift force lead to
Sediment transport
What is the dominant mechanism for HZ exchange
Advenctive exchange (pressure)
Where are large shear stress events that move down towards the bed
Sweep events
What happens when sweep pushes water into the bed
Forces water outof porespaces into the flow, ejection
What does permeanke beds do to horseshoe ejection structure
Gets rid of legs, changes boil structure
How can pollutants be effected by HF flow
Fine scale pollutants can be filtered out of the flow, pollutants in the bed
Where do we get scour
In the trough
What does dune erosion mean with pollutants at HZ
Larger flood events will take pollutants far down, if flow reduces scour will not be as deep and pollutants will stay there
What does the Ren and Packman paper show
Chemical (pH) water composition can effect repulsion due to electrostatic nature of substrates, can allow more or less HZ flow, mostly with clay
What is the Reaction significance factor
Determines what is more factors within the bed, what is more important, dune forms and bed forms, vs larger scales such as meander bend movement
What is the RSF reaction
(Transit / reaction)×(reach / turnover)
Time × length
In headwaters reaches, what rsf is more important
Small bed forms and dunes
What was found with efficiency with HZ flow
More efficient round meanders and on larger scale than with smaller scale1
How does the classic flow model change with added porosity
Recirc becomes squished into the bank, with zone coming back up
What substrate is still comparable with impermeable beds
Sand beds
What is RIM
Refractive index matched, allows objects to become see through
What can be said about pores within substrates
Flow is turbulent
How far were fine materials in hyporheic flow paths stores compared to solutes (Harvey et al)
two-thirds as deep as solutes
Are fines or solutes more easily mobilized by bedform migration (Harvey et al)
Fines
What did floods do to solute storage
drove them deeper below the bed, beneath 8cm
What three timescales are important in hyporheic reactions
flood duration, relaxation time of bedforms and hyporheic residence times.
