Hydro-morphological study
Tagus Estuary
Mltfiiofaiidiiin on i f m salinity on tfio
innar ooiuary
ADMINISTRACAO-GERAL DO PORTO DE LISBOA
Hydro-morphological study
Tagus Estuary
Memorandum on the salinity on the
inner estuary
August 1983 / P 6 4 2
PORT AND WATERWAY ENGINEERS
^^^^^^
hyöRonAmiG
INTRODUCTION
T h i s memorandum has been p r e p a r e d on r e q u e s t
mr.
Perestnello;
general
the aim o f
interpretation
t h e Tagus e s t u a r y
in
relation
to
this
the
findings
t h e most
recent
influence
o f works
in the o u t e r
upstream s e c t i o n o f
s t u d y was t h a t
estuary
that
studies
consequently
tidal
distribution
are
i t was n o t s t u d i e d ,
P o n t e de 25 A b r i I may have an
it
the
from
in
this
inner
the Corredor,
and
e n g i n e e r i n g works west
of
negligible.
became o b v i o u s
t h a t works east of
the
distribution.
considerable.
aspects
a r e d i s c u s s e d , as w e l l
s t u d i e d by AGPL b e f o r e and a f t e r
important works
of
especially
in the
i m p a c t on on t h e s a l i n i t y
memorandum some g e n e r a l
distribution
studies
in the p a s t ,
d e t e r m i n e d by t h e g e o m e t r y o f
Belem on t h e s a l i n i t y
In t h i s
the previous
prism, etc.
civil
i m p a c t may be
t o g i v e a more
The main c o n c l u s i o n
influence of
This
and
b a r on t h e s a l i n i t y - d i s t r i b u t i o n
the
Although
Castro
f o r AGPL, H y d r o n a m i c s t u d i e d
the e s t u a r y .
current-patterns,
are - f u l l y
of
mr.
problems.
In one o f
the
memorandum i s
as p e r f o r m e d by H y d r o n a m i c
to s a l i n i t y
of
in the
area'.
of
i m p a c t s on t h e
salinity
as t h o s e a s p e c t s w h i c h s h o u l d
permission
is g i v e n t o
construct
be
THE EFFECT OF SALINITY AND SALINITY
Both
for
salinity
the physical
i s an
as f o r
important
beyond t h e scope o f
From a p h y s i c a l
this
point
of
DIFFERENCES
the b i o l o g i c a l
item.
balances
in the
Because t h e b i o l o g i c a l
memorandum, t h e y w i l l
view
two a s p e c t s
of
estuary
aspects
are
n o t be d i s c u s s e d
s a l t water
are
here.
very
important;
-
s a l t water
-
chloride
is
of
pattern
flat
increase
small,
and i n
of
the clay
forces
particles
particles.
can s t r o n g l y
Suspended c l a y
Because o f
structure
clay
influence
in f r e s h
water
h a v i n g a maximum
form,
large
surface
the clay m i n e r a l s ,
these
particles
rather
their
dimension
Since
than
and w o r k
the p a r t i c l e s
the g r a v i t y
are
control
t o keep t h e p a r t i c l e s
so
data
seperated
suspension.
present
the clay
3 o/oo
process
c h a r g e d on t h e s u r f a c e .
As t h e s a l i n i t y
etc.)
coagulation of
in the e s t u a r y .
the e l e c t r o s t a t i c
behaviour of
in
this
t h a n a few m i c r o m e t e r s .
are negatively
water
or needle-shaped p a r t i c l e s ,
a r e a and t h e c r y s t a l
of
the water
this
increase
tend t o n e u t r a l i s e
particles
in
important
effect
35 km u p s t r e a m o f
this
is
important
Pa^o
and s e t t l e .
process.
for s a l i n i t y
ions
(Na"^, Mg"*""*", Ca"^"*",
forces,
A salinity
The p h y s i c a l
varitations
for
d'Arcos.
the p o s i t i v e
the e l e c t r o s t a t i c
to f l o c c u l a t e ,
is c r i t i c a l
are only
that
fresh
t h e second a s p e c t
the s i l t a t i o n
less
than
i o n s c a u s e an
Because o f
consists
heavier
below
thus
of
chemical
this
upper s e c t i o n o f
value.
allowing
about
influences
This
the e s t u a r y ,
means
more
than
The f l o c c u l a t i o n
partially
c a u s e d by an
reversable.
decreases,
the floes
"explode" dispersing
When, l a t e r
of
clay
the
This
measurements
in a r e a ' s where
found.
implies
35 km l i m i t
process
that
i n t h e downstream
for
the
estuary.
sea.
An i m p o r t
than
value
to
From t h e a b o v e f o l l o w s
currents
various
the bottom
that
is
mixed,
as t h e Tagus
considerably
for.the
for
due t o
(both
Into
partially
the
rate
transport
detailed
of
is mainly
inner estuary
transport
type of
the t i d e
o f an e s t u a r y
in the
is
investigation
the degree o f
locations,
might
determined
magnitude).
differences
such a d e t a i l e d
Because
stratification
research
due t o d e n s i t y
an
(I966).
d i r e c t i o n and
the sediment
(1953)
mixed
may
from the depthraveraged c u r r e n t
various
the
£ Schönfeld
theoretical.
this
the s t r a t i f i c a t i o n
deviations
is w e l l
seasons,
the
applied
flow over
discharging
is q u i t e
Sediment
öhe d e t e r m i n a t i o n • i f
transport
may d i f f e r
Schijf
channel
i n s u c h an e s t u a r y
determined
For b i g e s t u a r i e s
upstream
intervals
it will
o c c i i r , making a s o - c a l l e d
importance.
near
transport
the estuary
can be
Such a . s a I t - w e d g e can be
this
from the
If
the
freshwater
rectangular
induced c u r r e n t s ,
sediment
sediment
concentrations
than
can
in
measurements
t h e method o f
Due t o t h e d e n s i t y
for
salinity
once a g a i n
salt
transport
s a l t wedge.
the d e n s i t y
considerable
important
the
least
the fresh water
, as d e f i n e d by H a r l e m a n and Abraham
For s e d i m e n t a t i o n
by t h e c u r r e n t
variable
In f a c t ,
mixing w i l l
stratification
be o f
is h e a v i e r
horizontal
non-tidal
tides
is a t
d i s t u r b i n g on s u s p e n d e d
intervals
quite accurately with
a prismatic,
infinite,
of
longer
cycle,
exposed t o
particles
siediment
forming a so-called
calculated
low,
tide
salinity
area.
Because s a l F n e w a t e r
bottom,
particles
can p r o v i d e
have t o c o v e r
in water
In t h e
individual
suspension.
This
increase
and may a l s o
(spring-tide,
devide
value
only.
is
field
of
required.
Is n o t
required
stratification
differ
neap-tide).
for
PRACTICAL ELABORATION
The o n l y
t r u s t w o r t h y way t o d e t e r m i n e
do m e a s u r e m e n t s .
be done q u i t e
As w i l l
from the
important
of
sets
is
is o f
56 measurements
each p o i n t
Effects
of
1ateron,
is
river
to determine
the water
rate which are already
course
the set of
i n 21 d i f f e r e n t
measurements a r e f u l l y
for
be d i s c u s s e d
rate of
stratification
such
worked o u t ,
It
is
After
worked o u t ,
available.
data
etc,
One o f
and
the
in f a c t
can o n l y
and c a n n o t
be used a t
advisable
having worked out
e s t u a r y were not y e t
groups o f
river-discharge)
AGPL, h o w e v e r
this
to s t a r t
with
for
these
stratification
tide.
two
points
days.
these data are
the. e l . a b o r a t i o n o f
these
t h e s e d a t a one can see w h i c h a r e a s o f
c o v e r e d by m e a s u r e m e n t s .
environmental
data are
most
not
moment.
situations
(like
season,
data.
the
A l s o one can see
from
tide,
missing.
Knowing w h i c h d a t a a r e m i s s i n g one can make p l a n s
campa i g n s .
from
one neap
be s t u d i e d
the
consisting
rate of
and f o r
for
the
f r o m '\31h/\S']S,
The d a t a
known f o r one s p r i n g t i d e
discharge
stratification
locations.
data are a v a i l a b l e w i t h i n
therefore
which o t h e r
to
measurements
GMontana and J u s a n t ë . ) ' w f t i c h were, m e a s u r e d d u r i n g more
More f i e l d
is
easily.
The b e s t way t o s t a r t
estuary
the
for
additional
measuring
Especially
expected
in areas where
in the near
Cabo R u i v o ;
if
permission
future
Gala do N o r t è )
H a v i n g done s a l i n i t y
on f a c t s
larger
infrastructural
(e.g.
t o proceed w i t h
area,
Canal
s h o u l d be c o v e r e d by t h e s e
measurements
a more d e t a i l e d
Siderurgia
worl<s may be
stady^'
measurements.
i n t h é s e a r e a s one may
is
t h e worl<s.
required,
OP n o t
do
decide
before
giving
PERFORMANCE OF MEASUREMENTS
The b e s t way t o do t h e s e m e a s u r e m e n t s
the v e l o c i t y
of
this
and s a l i n i t y - p r o f i l e
method
is
that
than 5 s t a t i o n / w e e k
because
it
week a t
approx.
But
necessary
type n r .
two m i n u t e s .
c a m p a i g n can be
With
one b o a t
the s t a t i o n ,
of
of
the p r o f i l e s
every
20 s t a t i o n s
days,
shifts)
If
measuring,
can he a p p r o x one
in f o u r
available.
s u c h a week
the s a l i n i t y ,
the v e l o c i t i e s .
used
(like
And
is a
it
Provided that
is
a
not
fast,
the salimometer o f Yellow
a salinity
profile
such a f a s t
a small
the s h i p ) .
and 2 m i n u t e s
is
hours.
can be m e a s u r e d
salinometer
speed-boat w i t h
a dynamic
Springs
in
only
measuring
an o u t b o a r d e n g i n e
( t h e m e t e r does n o t need a power s u p p l y ,
motions
two
33)
The'dlsadvanta
set-up.
As m e a s u r i n g v e s s e l
So i f
if
t o g e t a good i m p r e s s i o n
is
locations.
13 h o u r s
a v e r y e x p e n s i v e m e t h o d , b e c a u s e no more
t o go t o
75 w o r k i n g
salinometer
Instruments,
used
time
t h e measure a l s o
electronic
one o r
is
t o measure d u r i n g
at èeveral
can be m e a s u r e d ,
takes a l s o
in order
it
is
half
one uses a scheme o f
5 stations
hour.
and
is
k minutes
can be m e a s u r e d ,
The d i s t a n c e
not
between
if
the
can be
sensitive
sailing
one
measures
stations
kilometer.
are
required,
(For p r a c t i c a l
all
reasons
these s t a t i o n s
it
can be m e a s u r e d
is a d v i s a b l e
to
t o work
in
For
comparison:
normal
way
15 h o u r s / d a y
4 man
1200hours
20 days
dynami c way
15
hours/day
2 x 3
k
man
days
A second advantage o f
5 stations
tidal
. 360 tnours
t h e d y n a m i c way o f m e a s u r i n g
have t h e same e n v i r o n m e n t a l
condition), while with
environmental
different
conditions,
days.
conditions,
is
that
(especially
the s t a t i c way, each s t a t i o n
b e c a u s e t h e measurements
groups
has
a r e made on
of
same
different
SALINE ENGINEERING'
It
might
be a d v a n t a g e o u s
an u p s t r e e m
location,
and a s t r a t i f i c a t i o n
In s u c h p o i n t s
water
t o l<eep t h e s a l t
especially
as p o s s i b l e
i n a r e a s w h e r e channe1s a r e
improve
f1ow t h rough t h a t
dredged,
the s i t u a t i o n
by
i n c r e a s i ng t h e
fresh-
channeI.
As an e x a m p l e , one may i n c r e a s e
C a l a do N o r t e by c l o s i n g
the
fresh-water
t h e gaps b e t w e e n t h e
fresh-water of
t h e Tagus
flow
through
i sIands
in
the
t h rough t h e
Cala
the
estuary,
do
Norte.
Doing t h i s
one may g u a r a n t e e
have a I w a y s
However,
system o f
that water
intakes
northwest
of
Povoa
fresh-water.
b e f o r e m a k i n g s u c h an
the e s t u a r y ,
important
a thourough
change
study of
in the
salinity
in
hydraulic
that
area
has t o be made.
Of c o u r s e s u c h a change s h o u l d a l s o be t e s t e d
model
of
the
inner
to
occurs.
one can
and f o r c i ng a I I t h e
w a t e r as f a r
Estuary.
in the
mathematical
CONCLUSIONS
From o u r e x p e r i e n c e s
from studies
changes w e s t o f
Belem a r e n o t
changes e a s t o f
Belem may have an
t h e dens i t y
It
distribution
is e x p e c t e d
that
of
only
is not
poss i b l e
t h e Tagus e s t u a r y
affecting
the
in the
Po(;o do B i s p o and A l c o c h e t e )
of
the
influence
inner
follows
i nner e s t u a r y ,
that
but
on t h e h y d r a u l i cs
that
and
estuary.
trans i tion
the e f f e c t s
zone
(i.e.
between
a r e cons i d e r a b l e
t o say on b e f o r e h a n d t h a t
but
changes e l s e w h e r e
it
are
negligible.
Therefore
of
it
is a d v i s a b l e
s a l i n i ty-changes
to get
a general
by d e t e r m i n i n g
the
rate of
a number o f
stations
function
the e n v i ronmenta1 condi t ions
of
in the e s t u a r y ,
i dea o f
if
the
importance
strati fication
possible all
(tide,
for
as a
seasons,
ri ver
discharge).
First
of
alI
t h e measurements
and a n a l y s e d , wi t h
the
al ready
resu1ts of
be d e t e r m i n e d and an a d d i t i o n a l
Additional
measurements
After
in Portugal
the t o t a l
the s a I i n i ty
With
the data
addi t i o n a l
project
th i s analysis
sa-| i n o m e t e r s .
through
analys i s at
distribution
of
and r e l a t i v e l y
specialised hydro-survey
alI
data a general
the whole e s t u a r y
s a l i n i ty e f f e c t s
are
are o f
requi r e d ,
out
can
set-up.
cheap,
These m e t e r s
t h a n a v a i l a b l e one can d e t e r m i n e
salinity-studies
mi ss i ng d a t a
m e a s u r i n g p r o g r a m can be
can be done f a s t
a p p l y i n g modern, e l e c t r o n i c
available
done s h o u l d be w o r k e d
are
companies.
impress ion
of
can be g i v e n .
f o r w h i ch
and a l s o
no i m p o r t a n c e a t
projects
f o r wi t h
all.