estuarys

Estuaries

boundaries of estuaries

these can not always be fixed boundaries

head

- marks the limit of tidal influence

-freshwater-seawater changes with tides

mouth

-maybe defined by line joining end of river banks

-not always defined

- river plumes extend further

types

drowned river vally - glacier deposit

fjord - craved out by glacier

bar built-sand deposit

tectonic - drowned faults with crust

classification by circulation

most are positive.

positive

surface water is directed downstream

more saline and therefore dense pushes into the estuary = bottom layer

Negative

surface water movement directed inwards - this is driven by intense evaporation cause sinking of highly saline water near the head - so only in aired hot regions

there are several drivers

River flow

- freshwater flowing into head - some may recieve large amounts of river input

Tidal currents

-Reversing direction over one tidal current

-current velocities vary with time and depth

Gravitational circulation

-driven by density

-varies with stratification

coriolis deflection

-only effects wide estuaries

cause lateral movement

classification

classification by stratification - salinity layering which can be clear in others

stratification and circulation

these are interrelated

-vertically mixed estuaries - surface and bottom layers always move in the same direction - only one that has water moving to river to sea at all depths

-stratified and salt wedge -surface and bottom layers move in opposite direction

affected by

-riverflow

-tidal currents

-morphology

-bathymetry

what controls stratification?

if the tides are not strong enough =less tidal turbulence =layers arn't mixed well

if the estuary is narrower there is more mixing=more friction=less statification

Flushing times

these are important to understand - water exchange, flushing times of dissolved particles and ecology and management

the longer the estuarine flushing time the more time avialable for processing - larger estauries with little river in put have long times

residence time of freshwater is defined

T=V/R

where r is the discharge calculated by gauging

where v volume already in the estuary taken from salinity data as its a conservative property changes due to mixing

Tidal prism method

based on the idea that

- incoming coastal seawater during flood mixes with estaurine water

-estuary is well mixed at high tide

- resulting mix is exposed during ebb

- water exchange is soley due to tidal exchange

Tf=(T x Vm)/(Vhw - Vlw)

T is tidal period

Vm is volume at mean sealevel

Vh/l is the volume at high and low water respectively

Fraction method

this is calculated form average salinity in the estuary and ave. salinity outside the mouth

ff= (Sout+Sin)/Sout

then you times this by the volume given total of freshwater

V = ff x Vt

then you can substitute this into the original equation.

Requirements

-rate of river discharge

- volume of estuary from bathymetric survey

- average salinity from detailed survey

Advantages

- most data readily avilable, except salinity distribution

- implicitly accounts for all mixing processes

disadvantages

-requires detailed surveys and relys on it

- this varies with river flow - requires repeats

Requirments

-tidal hieghts

-surface area of estuary

-mean depth of the estuary

advantages

-requires little input of data

disadvantages

- usually underestimates flushing times due to assuming complete vertical mixing, neglects role of freshwater and gravitaional circulation

neglects returning plume waters

best from tidal dominated, well mixed estuaries

these vary with

-tides-weaker neap times will cause longer times

-river discharge - Tf decrease with river input how they arnt exactly inverse because the freshwater fraction also increase

mixing

salinity is a conservative property

in positive estuaries, salinity only changes due to mixing between freshwater and coastal water

Freshwater fraction is inversely related to salinity

therefore salinity can be used to trace estuarine mixing

yhis is done by using the 2 component mixing diagrams

calculation

the salinity from any mixture of two water masses of different salinities is given by:

Sab = Fa*Sa + Fb*Sb

Sab - salinity of mix

Fa/b are the fractions of water masses a and b

Sa for estuaries is always o

Fa and Fb alwats adds up to one

2 component mixing diagram

Estuarine mixing diagram

useful tool for identifying estuarine processes

2 components mixing diagrams assume the following

-2 end member mixing - freshwater input dominant by one river mixes with the water from seawater

three possible scenarios

- conservative mixing

-non-conservative removal or addition of water constituent

Non-conservative mixing

removal and addition must happen during transport - addition is above and removal is below

removal processes

dissolved substances - exists as individual ions moles or atoms so basically anything that can pass through a 0.45 nano meter sieve

particular matter - denotes suspended particles retain by a 0.45 mm filter

flocculation - some riverine, dissolved organic matter flocculates = suspended particles - also trace metals do this

significant trace metals removal from the dissolved phase -estuarine removal is partly resposible for low trace metal concentrations

mixing and point sources

inputs of material or contaminants can be classed as diffuse

- urban runoff

- inputs from stretches of intertidal areas or wetlands

point source

- individual sewage outfalls

- discharge of tributes with contrasting water properties

Estuarine Flocculation

•occurs mainly

in upper reaches near seawater-freshwater interface

•Riverine

material first exposed to saline water

•Upper reaches

often have high suspended clay mineral load

•This suspended

particulate matter (=SPM) promotes flocculation

•High SPM loads

in upper estuaries result from trapping and resuspension of particulate matter

•Estuarine

removal often strongest near turbidity maximum - increased turbidity