OSX-2004 PR Revision Cards

Tidal Range Classification:

• Microtidal (range <2m) [rare in UK]
• Mesotidal (2-4m range) [typical in eastern UK]
• Macrotidal (4-6m range)
• Hypertidal (>6m range)

Hard to define the limits of estuaries due to:

• Tidal influence can exceed saltwater intrusion in river (FW held back by flood tide - backwater effect)
• ROFI, or sediment plume, may extend far offshore

Key Drivers of Estuaries:

• Tidal Flows - Flood and ebb. Impact sediment erosion, transport, deposition & water quality. Drive or contribute to mixing
• Tidal Inundation - Rising and falling tide levels meet coasts and rivers. Affects sediment errosion, inter-tidal habitats and causes flooding. Controls area/size of inter-tidal range
• Density-Driven Flows - Diff. in densities causes circulation & mixing of SW and FW

Other Drivers of Estuaries:

• Biological-Physical Interactions - Aquaculture in estuaries. Introduce organic matter, dissipate energy and filter material in water
  
• Anthropogenic Change - Land use change and land management change
• Climate Change
• Loading - River loading of pathogen, nutrients, metals and other pollution

Fjord - Deep and heavily stratified with sill exchange

Bar-built - Shallow with narrow mouth

Embayment - Shallow with open mouth

Ria - Deep, ocean mouths with NO sill

Barrier Beach - Offshore sandbanks

Flushing/Residence Time -> Average time taken to replace ALL water

- Affects concentrations of dissolved nutrients:

• Promotes phytoplankton blooms
• Removes dissolved O2 from water column
• Promotes pollutants that may impact the health of estuaries

(Estuaries with short residence times usually 'healthier' than ones with longer residence times)

Flushing time changes rapidly with discharge variation at low river flow, but changes slowly at high river flow

Difference high water and low water divided by the average area:

T = V/Qout (V = mean vol. of estuary (m3) Qout = Estuary outflow rate (m3/s) T = Residence time (s))

T = V/Qout(1-b) (With salinity info. 1-b = return flow factor)

Salt Balance:

• Inflow and outflow equal because mass is conserved: Qout = Qin + Qr (Assumes that inflow only due to tide and river)
• Salt also conserved: Qout x Sout = (Qin x Sin) + (Qr x Sr) (Qr x Sr = 0 as assume 100% FW)

Water entering on flood assumed to become fully mixed and vol. of SW and FW introduced equals vol. of the tidal prism

On ebb same vol. of water is removed and fresh water content of it must equal increment of the river flow

Salt-Wedge:

• Residual (net average) circulation = In at bottom, out at top (FW floats on top of SW and gradually thinks as it moves seaward. Denser SW moves along bottom up estuary, forming a wedge-shaped layer
• Sharp densityt interface between upper layer of FW and bottom layer of saline water
• FW dominates - little mixing - small tides - big river discharge
• Flow out = Estuarine Front

Circulation & Structure:

Sharp interface between salt and FW:

• As FW flows over wedge, internal waves formed (May break and transfer SW to upper layer - entrainment - Fresh later becomes slightly salty - brackish - To replace water lost by entrainment, there is flow of SW into wedge
• Seaward end = Fresh or brackish layer spreads out into sea as plume over the SW

Partially-Mixed Estuary:

• Salt pushed out of estuary by river flow and pushed in by 'mixing'  quantified by longitudinal mixing coefficient
• Intermediate stage between salt wedge and well-mixed estuary
• Turbulent mixing - induced by tidal currents - stratification - turbulent eddies then mix
• Two-layer flow - Speed of circulation dependent on strength of vertical mixing - Exists within max. salinity gradient at mid-depth

Well-mixed Estuary:

• Salinity decreases from ocean to river
• Shallow - large tides - strong tidal mixing
• Mix SW throughout shallow estuary - salinity equal all way through water column
• Many shallow estuaries in UK are well-mixed estuaries (e.g. Conwy)
  

Fjords

• Highly stratified estuaries
• Form silled basins
• High inflow, small tides
• Slow import of SW flows over sill. Sinks to bottom (due to it being saltier than ambient water)
• Water in fjords can remain stagnant for long periods such as Clyde Sea, UK.

Inverse or reverse Estuary:

• Found in hot countries - Australia etc.
• FW loss by evaporation exceeds river inflow
• Salinity max. zone formed - water pushed down and spread along bottom in both directions
• Salinity max. zone can reach high values and residence time can be several months
• Salinity max. zone blocks mixing of estuarine and oceanic water
• FW does NOT reach ocean

Physical Processes:

• Estuary circulation through salinity gradients and tidal stirring
• Material dispersal through tidal propagation and tidal asymmetry
• Through secondary flows, such as axial convergent fronts
• Phase difference between velocity and concentration variation produce a residual flux of sediment, even when there is no residual movement of water, provided that the currents are asymmetrical

Tidal Currents:

• In and out in equal proportion between the flood and ebb tide, over a lunar cycle of ~12.5 hours
• Tidal streams penetrate to the seabed, getting weaker towards the bottom

Coastal Dynamics:

• Wave-current interactions (longshore drift)
• Wind and wave

Types of Material Dispersal:

• Sinking - Towards seabed or rising to surface
• Decay - Organic materials have the ability to decay
• Dispersion (mixing and advection):
• Molecular mixing = collision of molecules at nanoscale
• Turbulent mixing = small-scale random movements of 'parcels' within a fluid
• Advection = Transporting substances in direction of flow

Types of Material Dispersed:

• Pollutants: chemical
• Dissolved organic matters (C, N, P)
• Sediment (silts, muds, sands = flocculation)
• Salt
• Particulate inorganic matter (metals, plastics, microplastics, oils)
• Particulate organic matter (POM) (larvae, phytoplankton, detritus, pathogens, bacteria, viruses)

(Tend to have fast flood and slow ebb in normal conditions, where flood currents are shorter and faster)

(Strong tidal flows can occur due to a narrow mouth, e.g. Conwy, or due to a constrained tide. resulting in phase lag, e.g. Menai Strait)

Estuaries:

• Shallow
• Bigger tides in shelf seas produce tides in estuaries start that are asymmetrical
• Large frictional effects on flow in estuaries

Shelf Seas:

• Tidal amplitudes are larger, resulting in larger tides
• In shallow seas, there are also large fricitonal effects on flow

Ebb Tide:

• Where sea is lower than estuary, the water flows out (occurs in shallow water)

Flood Tide:

• Where tide is higher in sea than estuary, the water flows in. Rising water on the shelf (i.e. deeper water) occuring in deep water due to being driven by high tide out at sea
• Time delay between low water at mouth and at the head is greater than the time delay of high water. Combination of these two processes produces short flood phase, and fast flood currents, creating flood dominance

Bottom Friction:

• Flood currents are faster than ebb (due to occurring in deep water, with reduced bottom friction)
• Short flood, long ebb

(Movement or spreading of tides)

• 1. On shelf sea, tidal elevation is symmetrical
• 2. Amplitude increases from 1 (due to constrictions at estuary mouth)
• 3. Friction in estuary dampens tidal amplitude. Estuary tidal elevation then becomes asymmetrical

Standing Waves:

• Normally offshore
• Phase lag between tidal elevation ~2-3 hours, or 90 degrees
• Slack water (i.e. 0 flow) found at HW and LW. Peak flow at mid-tide

Progressive Waves:

• In shallow water (likely to produce asymmetrical tidal flows due to flood and ebb occurring in different water depths
• Phase lag between tidal elevations, and tidal velocities, are < 1 hour
• Slack water found at mid-tide. Peak flow at HW and LW

(Fast flood, slow ebb = normal conditions)

> Tidal currents generate flow asymmetry (due to flood and ebb tides occurring at different water depths)

> Estuarine tidal currents respond to tidal asymmetry:

• Currents transport nearly same volume of water up estuary as down
• Flood currents faster and stronger
• Ebb currents weaker but last longer

In shallow water of estuaries, two processes affect the tidal wave:

• Where tidal variation is large, regardless of frictional effects, the wave crest will move quicker than trough (i.e. shorter flood (wave crest) and slower ebb (trough))
• Bottom friction = greater in shallow waters (slows down movement at low water relative to high water)

Barotropic - Gravity-driven:

• Wind-driven
• Tidal

Baroclinic - Density-driven:

Tidal Advection (Origin of tida currents is tidal advection):

• A floating object will be carried, back and forth, by 14km for each 1 m/s of maximum tidal stream
• Over one tidal cycle (12.5 hours)

(Effect of vertical changes in tidal currents on horizontal density gradients)

Straining:

• Due to shear in the velocity profile, surface water travels faster, and with larger trajectory of motion than the near-bed waters.
• Flood pushes salty water atop of freshwater, resulting in vertical mixing of the water column
• Ebb pushes freshwater atop of salty water, resulting in temporary stratification
• Tidal straining can be the main control on strength of stratification produced by input of buoyancy by the fresh water. (Degree of stratification dependent on horizontal density gradient and on tidal excursion)

> Estuary infills with salt

> Some of water flowing into estuary on flood tide comes back by a slower route on the ebb

> Longitudinal mixing :

(Bend in an estuary, resulting in lateral density gradient and secondary flow near bed)

Recirculating flow at surface

> Can be in the form of a meander etc where a bend will experience centrifugal force towards the outside of the bend

> Deeper, strong flows on outside of bend advects salty water

> Shallow weak flow on inside of bend results in less salty (i.e. less dense) water here

> Presence of secondary circulation can increase the lateral mixing and consequently the value of the horziontal disperson coefficient

Richardson Number - Comparison of stabilising forces to destablising influences of velocity shear

• Ri > 0 = Stable stratification (laminar flow)
• Ri < 0 = Unstable stratification (turbulent flow)

(Simplification of equation: Ri = Acceleration/density x Stratification / Velocity shear squared)

Reynold's Number - Relative importance of inertial and viscous forces in determing resistance to flow

Re = uD / v (u = velocity, D = Depth, v = Kinematic viscosity)

• Calculating kinematic visosity = v = μ/p (μ = molecular viscosity, p = density)

Humber:

• Large/ east coast
• Compound events NOT common, with slow extreme river flows (over days)
• Fluvial surge phasing - Due to the Humber having a large, gentle slope catchment, and large estuary, phasing makes little difference to flooding due to it being a slow response system.
• River floods can last >2 days, over several tidal cycles

Dyfi:

• Small/west coast
• Compound events common
• Flashy extreme river flows
• Fluvial surge phasing - Phasing of drivers makes big difference to flooding, due to it being a fast response system
• River floods last <1 day

(Worst case flood = max. river flows and max. storm surge. Baseline flood = mean river flows and max. river surge)

> Negative correlation between storm rate and catchment size (larger catchment size, reduced storm rate)

> Positive correlation between storm duration and catchment size (larger catchment size, larger duration)

• With a SLR of 2m, the SLR would be the strongest flood driver
• With a storm surge of +0.5m, total flux is important. SLR and surge need to then be modelled separately
• Future increase inf fluvial flood volume of +40% is the least important flooding driver (only occurs in upper estuary)

SLR of 0.5m predictions:

• 10,000km2 of coastal wetlands could be lost, release of nutrients would stimulate productivity, but loss of detritus food webs and of habitat for species likely lead to reduced fish yields. If coastal protection absent, some coastal communities of salt marshes and mangroves may migrate inland with rising waters

(Using example of the Menai Strait)

• Flow along channel with different tidal ranges at the two ends
• Difference in timing of both tiudes
• Flow faster at HW (due to reduced effect of bottom friction)

Residual Currents (Wind-driven):

• 3 parts (Stokes Drift, Ekman surface stress & surface roughness)
• OR you can calculate it as wind current = ~3% of wind speed and in wind direction
• Decays with depth (unlikely to be felt below 100m). Requires downwelling

Density-Driven Currents:

• Seawater denser than freshwater. Flows underneath, resulting in stratification
• Can produce ROFI offshore, forming a halocine (vertical zone in water column where salinity changes rapidly with depth)

The Law of Diffusion (Fick's Law):

When applied to a spreading patch:  D = square root of 32Kt (K = diffusion coefficient m2/s, t = time s, D = size of patch)

Calculating diffusion coefficient, when applied to vertical diffusion, in unstratified tidal flow: K = 0.0025hU (h = water depth m, U = max. value of tidal stream)

> Material introduced to sea tends to spread out and become diluted as it mixes

• Material lighter than water will rise to the surface. (when bubbles rise to surface, they get larger due to reduced pressure
• Material denser than water will sink to the seabed

Stokes' Law:

• Of spherical particles, the sinking speed is proportional to the square of the diameter to the density difference between particle and the surrounding water.
• Simplified equation: v = 2/9(Pp = Pf / μ)gRsquared (μ = dynamic viscosity kg/ms, Pp = mass density of particles kg/m3, Pf = mass density of fluid kg/m3, g = gravitational field strength m/s2, R = radius of spherical partcle m)
• Pp > Pf = Vertically downwards
• Pp < Pf = Upwards

(Particles determined by size, and how well they sink)

> Discharge of faster sinking particles will have shorter settling distances

> Discharge of slower sinking particles will have longer settling distances

(Sinking rate also determined by the tidal current)

> Materials introduced into sea can decay:

• Organic particulate matter eaten
• Bacteria will die
• Radioactive material turns into something else

> In many cases, decay proportional to concentration of material which gives rise to an exponential decay curve

(c = concentration, 0 = concentration at time 0, t = time, H = half-life)

(h/u3)

Criterion for complete vertical mixing:

h/u3 < C

• C = Constant (assumed to be ~80)
• h = water depth (m)
• u3 = maximum tidal currents (m/s)

(If h/u3 < C = vertical mixing)

(If h/u3 > C = temporary stratification)

Bed Shear Stress (T0):

• Proportional to logarithmic vertical gradient (small increase in velocity produces large increase in shear stress. Vertical gradients in velcoity and K)

Shear in Boundary Layer:

• Current Shear:
  

T varies with height above seabed (z). T increases as velocity gradient increases, or viscosity (turbulence) increases.

> Occurs because of friction between moving water and rough seabed, resulting in bed shear stress.

Sediment Transport:

• Amount of sediment transport dependent on turbulence and shear in benthic boundary layer
• Depends on flow velocity and bed roughness
• Benthic boundary layer - Where velocity increases from zero to 'free-stream velcoity' (velocity shear)
• 'Free-stream velocity' - Away from seabed, water is not affected by the presence of the bed

Flows:

• Flat beds = Low flows (Sheet flows over flat beds where ripples are 'washed out')
• Larger bedforms = Stronger flows (With ripples)

• Gravity - Due to weight of particle

• Life Force - Due to fluid flowing around protruding part of particles

• Drag Force - Due to bed shear stress

• Contact Forces - Due to friction and cohesion

How sediment flows in a fluid flow (velocity increased between each step):

• 1. Nothing occurs due to shear stress generated too small to dislodge grains
• 2. When critical shear stress (Tc) reached, grains move (Threshold of Movement)
• 3. Grains roll over bed surface, keeping contact with bed (Surface Creep)
  
• 4. Grains rise above bed, the fall back (Saltation)
• 5. Grains rise higher, eventually not returning to the bed (Suspension)

Simplified:

• 1. No change
• 2. Threshold of movement
• 3. Surface Creep
• 4. Saltation
• 5. Suspension

(Dependent on weight = density & size)

Interparticle Contact Forces:

• Cohesive Forces = Electrochemical attraction between certain particles produces additional resistance to relative motion
• e.g. clay, biogenic particles, fine-grained sands/silts coated with biological secretions
• Cohesionless Forces = Friction between particles in contact on seabed
• e.g. gravels, sands, silts (abiotic)

Relationship between current speed and sediment grain size:

> Fine-grained sand will be most mobile particles

> More difficult to erode cohesive grains

> More difficult to erode larger grains

Q = qb + qs (qb = bed load transport rate, qs = suspended load transport rate)

> Dependent on time, either over a tidal cycle, a season, or in response to river flood

Sand-waves:

• Transverse 
• Occur where there are fast tidal currents and abundance of sands. Up to 18m height wavelength
  

Megaripples (Dunes):

• Up to 1m height wavelength
  

Flatbed:

• No ripples

Ripples:

• Wavelength measured in cms.

qb = U3 (qb = sand transport, U3 = speed)

Waves and Longshore Currents:

• Swash and Surf zones
• Swash zone is the path of sand grains
• Longshore current moves suspended sand in surf zone
• Longshore Drift - Coming in at angle to coast. Inshore wave current, backwash at another angle. Slow creep of sediment along coast
• Bottom flow in shallow water (~2-4 ft deep)

• Small changes in velocities can result in large differences in sediment
• Produces net bedload transport over tidal transport
• More transport on flood tide
• Deeper channels cause ebb asymmetry (net export of sediment)
• Shallow and inter-tidal regions cause flood asymmetry (net import of sediment)

> Currents that go offshore perpendicular (at an angle of 90 degrees) to the coast

> If there are incoming waves, may be oblique to the coast

Suspended Sediment Transport:

• qs = concentration x velocity (velocity profile for water column summed throughout the entire water column)

Settling velocity in quartz spheres in water

• D > 2mm (Use Ws proportional to D1/2 of Impact Law)
• D < 100μm (Use Ws proportional to D2 of Stokes Law)

Threshold of Suspension (Ts):

• Depends on settling velocity of the particle (Ws) which depends on: size, density and shape of particles, Density and molecular viscosity of suspending fluid and upward component of turbulent velocity > W2)
  
• As bed shear stress increases, more particles will temporarily be transported higher into flow
• Threshold may be reached wherre eddies have energy to maintain particles in permanent suspension

Cohesive Sediments:

• Mostly clay-rich sands
• Stickiness and difficult to erode (Clay particles colonised by microbiota, that secrete sticky polysaccharides)
• Always changing overtime

> Cohesive beds may be deposited at low current speeds. Require much faster current speeds to erode once compacted

Non-cohesive sediments:

• Quartz-rich sands
• Most important property is grain size
• Do not change overtime

> When first deposited, contain a lot of water. Water squeezed out overtime due to pressure of deposited grains on top, resulting in compaction/consolidation (Consolidated beds more resistant to erosion)

Seawater:

• Negative charge on clay particles reduced
• Particles attract each other = FLOCS GROW
  
• Cohesive particles contain more than clay particles (such as containing organic matter and water. i.e. larger particles, low densities)

Freshwater:

• Clay particles negatively charged
• Particles repel each other = NO FLOC

• Decrease of charge with increasing salinity = exponential
• Flocculation occurs rapidly at low salinites. However, flocs are fragile and hard to measure due to being destroyed by physical sampling-

In a salt wedge, or partially-muxed estuary there is mixing across salt/fresh interace, resulting in a two-layer flow:

During Flood:

• Suspended sediment carries pollutants and contaminants, resulting in toxicity in turbidity maxima potentially reaching high levels
• Flocs aggregate to form larger flocs that settle to bed. Suspended conc. of maxima can also remain high

• Tidal Propagation: Up-estuary on flood, Down estuary on ebb
• Tidal Mixing: Two-layered structure disappears. Is a tide dominant force. If no landward flow at bottom, no convergence of currents, and no turbidity maximum. However, there will always be sediment transport
• Large tide -> Increased Velocities -> Increased Stress -> Increased Suspended Sediment -> Increased turbidity Maxima