- Email: [email protected]
Fall-Season Patterns of Turbidity and Sediment Transport in the Korea Strait and Southeastern Yellow Sea
381
9. T u r b i d i t y
FALL-SEASON PATTERNS OF TUKBIDITY AIuD SEDIMENT TRAKSPORT I K THE KOREA STRAIT AND SOUTHEASTERN YELLOW SEA J . T.
WELLS a n d 0. K.
HUH
C o a s t a l S t u d i e s I n s t i t u t e , L o u i s i a n a S t a t e U n i v e r s i t y , B a t o n Rouge , Louisiana
7 0 8 0 3 (U.S.A.)
ABSTRACT S u s p e n d e d - s e d i m e n t d a t a c o l l e c t e d i n mid-November o f 1 9 8 0 a n d 1 9 8 1 a n d c u r r e n t meter m o o r i n g s d e p l o y e d i n S e p t e m b e r a n d e a r l y October o f 1982 s u g g e s t a n e a r l y - f a l l e n t r a i n m e n t and s u b s e q u e n t t r a n s p o r t o f s u s p e n d e d s e d i m e n t i n t o t h e Korea S t r a i t by t h e S o u t h Korean C o a s t a l C u r r e n t . D u r i n g t r a n s i t i o n f r o m summer t o w i n t e r c o n d i t i o n s , suspended-sediment c o n c e n t r a t i o n s i n t h e s o u t h e a s t e r n Y e l l o w S e a r a n g e f r o m 5 t o 1 0 0 mg / l a t t h e s u r f a c e a n d 1 5 t o 250 These concenm g / l a t t h e b o t t o m i n w a t e r s t h a t a r e 20-80 m d e e p . t r a t i o n s , 1 t o 3 o r d e r s of m a g n i t u d e h i g h e r t h a n " t y p i c a l " s h e l f d e p t h w a t e r s i n most o t h e r p a r t s o f t h e w o r l d , p r o v i d e t h e o p p o r t u n i t y f o r enormous s e d i m e n t t r a n s p o r t rates w i t h i n t h i s coastal mudstream, even under r e l a t i v e l y weak c u r r e n t s . The a b r u p t termin a t i o n o f t h i s i n s h o r e b a n d o f c o l d , t u r b i d water a s a t u r b i d i t y f r o n t some 25-50 km o f f s h o r e m a r k s t h e s e a w a r d b o u n d a r y o f t h e h i g h - t r a n s p o r t zone. L an d war d o f t h e t u r b i d i t y f r o n t , s u r f a c e w a v e s a s s o c i a t e d w i t h s t r o n g c o l d - a i r o u t b r e a k s e v e r y 7-10 d a y s may d e s t r a t i f y t h e coastal mudstream and e n t r a i n a d d i t i o n a l bottom sediment. S u s p e n d e d s e d i m e n t s t r a n s p o r t e d i n t o t h e Korea S t r a i t may a c c u m u l a t e i n s o u t h - c o a s t e m b a y m e n t s , r e s i d e i n t h e d e e p s t r a i t , or be c a r r i e d i n t o t h e S e a o f J a e a n . I NT RODUCTION
T h e Korea S t r a i t s e r v e s a s a c o n d u i t f o r t h e t r a n s p o r t of sediment-laden waters from t h e Y e l l o w Sea and E a s t China Sea t o t h e Sea of
Japan.
C h a r a c t e r i z e d b y mesoscale e d d i e s a n d m e a n d e r i n g
f r o n t s , t h e waters o f t h e s t r a i t , p a r t i c u l a r l y t h o s e close t o t h e Korean P e n i n s u l a ,
h a v e b e e n shown f r o m o c e a n o g r a p h i c d a t a t o b e
c o n s i s t e n t l y t u r b i d and h i g h l y complex
(Huh e t a l . ,
1981).
The
c h a r a c t e r i s t i c s a n d d i s t r i b u t i o n of s u s p e n d e d s e d i m e n t i n t h e Korea S t r a i t a n d v i c i n i t y , t h e s u b j e c t of t h i s p a p e r , may p r o v i d e import a n t c l u e s t o t h e h y d r o d y n a m i c processes t h a t o p e r a t e i n t h i s p a r t of n o r t h e a s t A s i a .
388
S e v e r a l f e a t u r e s o f t h e Y e l l o w S e a - E a s t China Sea-Korea S t r a i t r e g i o n promote t h e u s e f u l n e s s of s u s p e n d e d - s e d i m e n t d a t a i n o c e a n o g r a p h i c problems.
First,
t h e dominance of
fine-grained
sediment
f r o m l a r g e r i v e r s s u c h a s t h e Y a n g t z e a n d Hwangho i n C h i n a ,
and
smaller r i v e r s s u c h a s t h e H a n , K u m , a n d Y e o n g s a n i n Korea, p r o v i d e a n e n o r m o u s s o u r c e o f s i l t a n d c l a y f o r e n t r a i n m e n t i n t o t h e water column.
S e c o n d , t h e m i n e r a l o g y a n d g e o c h e m i s t r y of s e d i m e n t s , com-
plex yet d i s t i n c t within drainage basins, tunity
t o d i f f e r e n t i a t e s o u r c e areas.
c o n d i t i o n s of
late-fall
and w i n t e r
may p r o v i d e
Finally,
an oppor-
t h e high-energy
associated with cola-air
out-
breaks provide a d e t e c t a b l e p e r t u r b a t i o n i n p a r t i c u l a t e material t h a t may a i d i n d e t e r m i n i n g t h i s paper
t r a n s p o r t pathways.
The p u r p o s e o f
i s t o u s e s u s p e n d e d s e d i m e n t a n d c u r r e n t meter d a t a ,
c o l l e c t e d d u r i n g f i e l d s e a s o n s 1980-1982 suspended-sediment
d i s t r i b u t i o n during
w i n t e r monsoon w i n d s ,
,
the
t o 1) c h a r a c t e r i z e t h e f a l l - s e a s o n o n s e t of
2) i d e n t i f y t h e n a t u r e and e x t e n t of t u r b i d -
i t y f r o n t s f i r s t n o t e d i n s a t e l l i t e i m a g e r y , a n d 3 ) s u p p o r t a hyp o t h e s i s f o r t r a n s p o r t of suspended sediment from t h e s o u t h e a s t e r n
Y e l l o w S e a i n t o t h e Korea S t r a i t . DATA ACQUISITIOh T h e c o l l e c t i o n of 5 5 7 water samples f o r s u s p e n a e a s o l i d s a n a l y s i s was u n d e r t a k e n a s p a r t o f KEYRSEX I a n d I1 (Korea, E a s t C h i n a S e a , Y e l l o w S e a Remote S e n s i n g E x p e r i m e n t ) , c o n d u c t e d j o i n t l y w i t h Korean u n i v e r s i t y and government s c i e n t i s t s . Three to f i v e depths were s a m p l e d a t e a c h o f 1 5 4 s t a t i o n s d u r i n g mid-November 1 9 8 0 a n d
a minimum,
samples were t a k e n f r o m t h e s u r f a c e , m i d d e p t h , and bottom a l o n g t r a n s e c t l i n e s g i v e n l e t t e r d e s i g n a t i o n s i n 1981.
As
F i g u r e 1.
S a m p l e s p a c i n g was t y p i c a l l y 1 0 km.
To d e t e r m i n e c o n c e n t r a t i o n o f p a r t i c u l a t e m a t e r i a l , samples o f 200-400 m l were f i l t e r e d t h r o u g h p r e w e i g h e d 0.45-pm brane 1977).
f i l t e r s following standard
M i l l i p o r e mem-
(Wells a n d
techniques
Coleman,
I n samples w h e r e s u f f i c i e n t p a r t i c u l a t e m a t e r i a l was a v a i l -
a b l e ( 8 5 samples) , s i z e d e t e r m i n a t i o n s were made u s i n g a m o d e l TA
e l e c i r o n i c C o u l t e r C o u n t e r w i t h a 100-pm a p e r t u r e t u b e . c a t i o n o f c l a y m i n e r a l s was made f r o m X-ray using mounts.
a Norelco w i d e - a n g l e Because of
diffraction analysis
d i f f r a c t o m e t e r on
space c o n s t r a i n t s ,
Identifi-
random b u l k - s a m p l e
presentation
of
sediment
d a t a w i l l be r e s t r i c t e d t o t h a t c o l l e c t e d i n 1981 and from l i n e H i n 1980.
389
F i g . 1. I n d e x map of s o u t h e a s t e r n Yellow S e a a n d k o r e a S t r a i t s h o w i n g t r a n s e c t l i n e s and sampling s t a t i o n s . Current
speed
and
d i r e c t i o n were d e t e r m i n e d f r o m m i d - d e p t h
m o o r i n g s a t t w o s t a t i o n s l o c a t e d a p p r o x i m a t e l y 6 0 km n o r t h of s t u d y area
(36'01'N,
126O02'E;
36'01'N,
126'14'E).
deployed during September and e a r l y October neutrally
buoyant
Endeco 174 i m p e l l o r - t y p e
the
Each mooring
a Data
1 9 8 2 c o n s i s t e d of c u r r e n t meter.
were r e c o r d e d on m a g n e t i c t a p e a t 2 - m i n u t e i n t e r v a l s , t h e n a v e r a g e d upon r e t u r n to t h e l a b o r a t o r y to p r o v i d e a r e a d i n g o n c e e a c h h o u r . S e l e c t i o n o f t h e m o o r i n g s i t e s was made so t h a t b o t h s t a t i o n s w o u l d b e l o c a t e d a l o n g a n east-west l i n e n o r m a l t o t h e s h o r e l i n e a n d t o t h e s o u t h e r l y f l o w , w h i c h h a s b e e n r e p o r t e d t o o c c u r a t t h e o n s e t of win-
t e r monsoons. SUSPENDED SEDIMENT CISTRIBUTION S u s p e n d e d s e d i m e n t s a r e almost e x c l u s i v e l y s i l t a n d c l a y s i z e d , w i t h s a n d s c o m p r i s i n g l e s s t h a n 1%of e v e n t h e c o a r s e s t d i s t r i b u tions.
I n f a c t , o n l y 2 5 % of t h e samples a n a l y z e d c o n t a i n e d a n y s e d i -
m e n t coarser t h a n 40 pm a n d , o n a v e r a g e , 9 0 % o f t h e s e d i m e n t i n e a c h d i s t r i b u t i o n was b r a c k e t e d b e t w e e n 2 pm a n d 40 pm, t h e n o m i n a l s i z i n g
390
r a n g e o f t h e 1 0 0 pm a p e r t u r e t u b e . F i g u r e 2 s h o w s from f o u r r e p r e s e n t a t i v e f r e q u e n c y c u r v e s t h a t n o s y s t e m a t i c v a r i a t i o n i n s i z e i s a p p a r e n t w i t h d i s t a n c e above t h e bottom. E x t r e m e s i n m e d i a n s i z e o f i n d i v i d u a l samples a r e 4 . 2 pm a n d 1 7 . 5 pm, a n d a v e r a g e g r a i n s i z e a l o n g t r a n s e c t l i n e s s h o w s l i n e s I a n d J t o b e t h e c o a r s e s t (11.1 pm a n d 1 1 . 7 pm, r e s p e c t i v e l y ) , f o l lo w e d by l i n e s H i n 1 9 8 1 ( 9 . 5 pm) a n d H i n 1 9 8 0 ( 7 . 1 pm).
The i n t e n -
s i t y of storm w a v e s a n d c o n t i n u o u s t i d a l s t i r r i n g s u g g e s t t h a t t h e l a c k o f s a n d s i n t h e water c o l u m n r e f l e c t s t h e f i n e - g r a i n e d n a t u r e o f
t h e s e d i m e n t s on t h e bottom r a t h e r t h a n a n i n a b i l i t y of c u r r e n t s t o s u s p e n d coarse p a r t i c l e s . The p a t t e r n s and l e v e l s o f t u r b i d i t y a r e r e m a r k a b l y s i m i l a r i n s u r f a c e a n d b o t t o m waters ( F i g s . 3 , 4 ) . T h e h i g h e s t c o n c e n t r a t i o n s occur o f f t h e s o u t h w e s t t i p of t h e Korean P e n i n s u l a i n a n area where b o t t o m s e d i m e n t s a r e r e s u s p e n d e d by storm w a v e s o f maximum p o s s i b l e f e t c h . T u r b i d i t y l e v e l s i n c r e a s e close t o s h o r e , e x c e e d i n g 1 0 0 mg/l a t b o t h t h e s u r f a c e a n d t h e b o t t o m . A t o n g u e o f l o w - t u r b i d i t y water a p p e a r s t o e n t e r t h e s t r a i t from f l o w o f t h e Tsushima C u r r e n t n o r t h o f C h e j u I s l a n d . A t o n g u e o f h i g h - t u r b i d i t y water n o r t h e a s t o f C h e j u e x t e n d s s o u t h e a s t more t h a n h a l f w a y across t h e Korea S t r a i t t o l a t i t u d e of n e a r l y 3 3 O N o v e r a n area o f b r o a d d e p t h c o n t o u r s . T h e n o r t h e a s t e r n p a r t o f t h e s t r a i t i s a g a i n c h a r a c t e r i z e d by c o n c e n t r a t i o n s o n t h e o r d e r o f 10-15 mg/l. T h e d i s t r i b u t i o n o f c l a y m i n e r a l s o b s e r v e d i n water s a m p l e s i n t h i s s t u d y i s close t o t h a t r e p o r t e d f r o m b o t t o m samples by Chough
a n d K i m ( 1 9 8 1 ) , C h en ( 1 9 7 8 ) , a n d Oinuma a n d a n d K o b a y o s h i ( 1 9 6 6 ) . T r a n s e c t l i n e s I a n d J a n d t h e n o r t h e r n p a r t o f l i n e H show a p r e d o m i -
nance of i l l i t e (-70%) w i t h lesser amounts of k a o l i n i t e (-10%) and c h l o r i t e ( - 1 3 % ) . M o n t m o r i l l o n i t e i s v i r t u a l l y a b s e n t . S e v e r a l samp l e s from t h e s o u t h e r n p a r t o f
l i n e H show a s l i g h t d e c r e a s e i n
k a o l i n i t e a n d i n c r e a s e i n i l l i t e , most l i k e l y a r e s u l t o f i n f l u e n c e o f t h e Tsushima C u r r e n t . A c c o r d i n g t o Chough a n d K i m ( 1 9 8 1 ) , t h e i l l i t e - r i c h waters i n a n d j u s t n o r t h o f t h e s t u d y area a r e d i l u t e d by k a o l i n t e a n d c h l o r i t e f r o m l o c a l r i v e r s ;
however, o n c e w i t h i n
t h e main f l o w o f t h e s t r a i t , m i n e r a l o g y o f s e d i m e n t s from t h e E a s t C h i n a S e a sh o ws g r e a t e r i n f l u e n c e . Although i n s u f f i c i e n t sediment
was a v a i l a b l e f o r m i n e r a l o g i c a l a n a l y s i s o f water samples e a s t w a r d t h r o u g h t h e Korea S t r a i t , Han ( 1 9 7 9 ) r e p o r t s t h a t i l l i t e f r o m t h e E a s t China Sea c o n t i n u e s to predominate.
391 Diameter in microns
r
63 loo
-r
31
16
8
4
10 20 30 40
I
-
L\
50-
60 30
-
20
-
10
-
.---.
70
Middaplh
Bottom
80
90
10 Diameter in PHI units
F i g . 2. T y p i c a l s i z e d i s t r i b u t i o n c u r v e s i n d i c a t i n g median p a r t i c l e diameters and sorting values. TURBIDITY FRONTS T h e t u r b i d w a t e r s a l o n g m o s t o f t h e s o u t h a n d w e s t c o a s t s o f Korea e n d a b r u p t l y a s t u r b i d i t y f r o n t s 2 5 t o 50 km o f f s h o r e i n w a t e r s 20 t o 8 0 m deep. A s f i r s t o b s e r v e d i n s a t e l l i t e i m a g e r y , t u r b i d i t y f r o n t s a r e w e l l d e f i n e d by t h e s a t u r a t e d , s e d i m e n t - l a d e n
(white) returns i n
b a n d s 4 a n d 5 o f LANDSAT ( F i g . 5 ) . W h e r e a s t h e t u r b i d i t y f r o n t a t t h e w e s t e r n e n d o f t h e s t r a i t appears t o f o l l o w b o t t o m t o p o g r a p h y , e n d i n g o f f s h o r e a t t h e 40-m d e p t h c o n t o u r ( F i g .
5 ) , i t loses l a t e r a l con-
t i n u i t y midway t h r o u g h t h e s t r a i t a t t h e e a s t e r n m a r g i n o f t h e s o u t h e a s t - t r e n d i n g b a n d o f t u r b i d water shown p r o m i n e n t l y i n F i g u r e s 3 a n d
4 ( l o n g i t u d e 128OE). A l t h o u g h a t u r b i d i t y f r o n t i s p r e s e n t a l o n g t h e e n t i r e west coast o f Korea, i t s w i d t h i s v a r i a b l e , b o t h s p a t i a l l y a n d t e m p o r a l l y , a n d r e f l e c t s g r a d i e n t of t h e b o t t o m , c o n s i s t e n c y o f t h e b o t t o m s e d i m e n t , a n d , most i m p o r t a n t l y , i n t e n s i t y o f wave a n d t i d e p r o c e s s e s . Examin a t i o n o f 2 5 c l e a r - s k y LANDSAT s c e n e s b e t w e e n S e p t e m b e r 1 9 7 2 a n d O c t o b e r 1980 i n d i c a t e s t h a t t h e f r o n t i s b e s t d e v e l o p e d between Sept e m b e r a n d November a n d weakest b e t w e e n J a n u a r y a n d Ma rc h. U n f o r t u n a t e l y , c l o u d c o v e r a f t e r A p r i l p r e v e n t s d e t e c t i o n o f a summer t u r b i d i t y f r o n t . I n t e n s e mixing a s s o c i a t e d with cold-air outbreaks duri n g w i n t e r may e x p l a i n l a c k o f d e f i n i t i o n i n t h e f r o n t s b e t w e e n J a n u a r y and March. M o r e o v e r , f e a t u r e s s u c h a s e d d i e s , w ind rows, a n d
392
Fig. 3. Surface suspended-sediment mid-November 1981.
concentrations
in
Fig. 4. Bottom suspended-sediment concentrations in midNovember 1981.
393
F i g . 5 . LANDSAT b a n d 4 i m a g e o f t h e s o u t h w e s t p a r t of t h e Korean P e n i n s u l a . Note z o n e of h i g h t u r b i d i t y ( w h i t e ) a n d i t s t e r m i n a t i o n as a t u r b i d i t y f r o n t a t t h e 40-m d e p t h c o n t o u r . b r e a k i n g waves o v e r s u b t i d a l s a n d r i d g e s a r e t y p i c a l w i n t e r - s e a s o n occurrences t h a t are a b l e to obscure t h e t u r b i d i t y f r o n t s . F i g u r e 6 shows t h e s t r u c t u r e o f t h e s o u t h coast f r o n t i n 1980 a n d a t t h e same t i m e (mid-November) i n 1 9 8 1 , b u t u n d e r som e w ha t different conditions.
Wind s t r e s s a t C h e j u I s l a n d was 1 2 % h i g h e r
d u r i n g s a m p l i n g and 1 7 % h i g h e r d u r i n g t h e month and a h a l f c e d i n g sampling i n 1 9 8 1 t h a n i n 1980.
pre-
Suspenlied s e d i m e n t c o n t o u r s
i n 1 9 8 1 r e f l e c t t h i s d i f f e r e n c e by s h o w i n g a more n e a r l y v e r t i c a l s t r u c t u r e a s a r e s u l t o f r e s u s p e n s i o n o f b o t t o m s e d i m e n t a n d de s t r a t i f i c a t i o n o f t h e water c o l u m n .
L a y e r s of
t u r b i d water n e a r
t h e b o t t o m , s u c h as t h a t shown by t h e 2 5 0 m g/l c o n t o u r i n 1 9 8 0 , h a v e b e e n d i s p e r s e d i n t o t h e water c o l u m n i n 1 9 8 1 ( F i g . 6 ) .
The t u r -
i s m a r k e d by a s h a r p d e c r e a s e i n s u s p e n d e d to less than 10 mg/l. Although s a t e l l i t e s e n s o r s c a n n o t r e l i a b l y be used to quant i f y s u s p e n d e d - s e d i m e n t c o n c e n t r a t i o n s or t o look b e l o w t h e s u r f a c e w a t e r s , s e d i m e n t s a m p l i n g c o n f i r m s t h a t LAKDSAT i m a g e r y c o n s i s t e n t l y d e l i n e a t e s as t u r b i d i t y f r o n t s suspended-sediment g r a d i e n t s of 1 0 - 2 0 m g / l a n d t h a t t h i c k n e s s of t h e f r o n t s i s t y p i c a l l y e q u a l t o t h e water d e p t h . bidity front itself
s e d i m e n t c o n c e n t r a t i o n from g r e a t e r t h a n 20 mg/l
394 0 10
20
30 40 50
60
T -
f
70 TOTAL SUSPENDED MATTER lrngili
80 90
TRANSECT H
100 110
I20
0
20
40
60
80
100
120
140
160
180
200
220
240
0 10
20 30 40 50 -
-
60 f
a
0"
70 80
TRANSECT H
90 100 110
0
20
40
60
80
100
120
140
160
180
200
220
240
120
260
D i r t o n c e [km)
F i g . 6. Suspended-sediment c o n c e n t r a t i o n s and b i d i t y f r o n t s a l o n g l i n e h' i n 1 9 8 0 a n d 1 9 8 1 .
location
of
tur-
MEAN FLOW AND SEDIMENT TRAEjSPORT T h e west coast t u r b i d i t y f r o n t m a r k s t h e s e a w a r d b o u n d a r y o f a high-transportcoastalmudstream w i t h i n t h e S o u t h K o r e a n C o a s t a l C u r rent. The f a c t t h a t suspended-sediment c o n c e n t r a t i o n s a r e o n e t o t h r e e o r d e r s of magnitude h i g h e r t h a n i n " t y p i c a l " s h e l f waters i n d i -
c a t e s t h a t s e d i m e n t t r a n s p o r t r a t e s w i t h i n t h i s c o a s t a l m u d s t r e a m may be enormous, e v e n under r e l a t i v e l y w e a k n e t c u r r e n t s . S e v e r a l l i n e s of
e v i d e n c e s u g g e s t t h a t n e t flow i s to t h e s o u t h d u r i n g w i n t e r
m o n s o o n s , a n d p e r h a p s t h e y e a r a r o u n d . Uda ( 1 9 6 6 ) r e p o r t e d r e s i d a l c u r r e n t s o f 1 5 cm/sec i n t h e c o a s t a l c u r r e n t o f t h e s o u t h e a s t e r n Y e l l o w Sea. of
1 0 cm/sec
L e e ( 1 9 6 8 ) i n d i c a t e d from d r i f t b o t t l e s t u d i e s f l o w s i n winter,
and t h e U.S.
Navy,
i n Summaries o f
Ship
D r i f t R e p o r t s ( 1 9 6 4 ) , i n d i c a t e d w e a k flow t o t h e s o u t h d u r i n g a l l seasons.
Further, observations of sea-surface temperature p a t t e r n s
f r o m NOAA s e r i e s s a t e l l i t e s ( Hu h , 1 9 8 2 ) f o l l o w c l o s e l y t h o s e f r o m
395 N MEAN CURRENTS
Outer station
1
3
-10
-
-12
-
i
\
, '.-.*..'
5
\
.
*'
F i g . 7 . Low-pass f i l t e r d a t a (mean f l o w ) f r o m m o o r i n g s o f f t h e Kum River e n t r a n c e a t l a t i t u d e 36ON. v i s i b l e b a n d s o f LANDSAT. F i g u r e 7 shows a v e r a g e c u r r e n t s i n September and e a r l y O c t o b e r from moorings deployed as p a r t of t h i s s t u d y . r e s e n t low-pass
Average c u r r e n t s rep-
f i l t e r v a l u e s r u n t h r o u g h a 75-hour Lanczos f i l t e r
w i t h a 50-hour half-power p o i n t .
T h e i n n e r s t a t i o n ( 2 5 km o f f s h o r e )
a n d t h e l a s t h a l f o f t h e d a t a r e c o r d f r o m t h e o u t e r s t a t i o n ( 5 0 km o f f s h o r e ) show mean f l o w t o b e d i r e c t e d t o t h e s o u t h a t s p e e d s o f 7 . 8 a n d 3 . 8 cm/sec, r e s p e c t i v e l y . Wi n d s were l i g h t a n d v a r i a b l e f o r t h e d u r a t i o n of t h e moorings. I t i s r e a s o n a b l e t o assume t h a t t h i s weak
transitional-season
current
w ould
become
substantially
s t r o n g e r under t h e i n f l u e n c e of s t r o n g northwest winds c h a r a c t e r i s t i c of
t h e w i n t e r mo n s o o n s e a s o n t h a t
(Wells e t a l . ,
begins
i n l a t e October
i n press).
Flow o f t h e S o u t h K o r e a n C o a s t a l C u r r e n t p r o v i d e s a m e c h a n i s m f o r t r a n s p o r t i n g s u s p e n d e d s e d i m e n t s o u t h i n t o t h e Korea S t r a i t . T h i s i s p a r t i c u l a r l y s i g n i f i c a n t i n t h a t f l o w may s t r e n g t h e n i n t h e f a l l , p r e c i s e l y a t a t i m e when t u r b i d i t i e s a r e i n c r e a s i n g f r o m t h e o n s e t o f monsoon w i n d s .
The f a t e o f t h i s suspended coastal s e d i m e n t , t r a n s -
p o r t e d s o u t h i n t o t h e Korea S t r a i t a t r a t e s e s t i m a t e d t o b e 25-250 x 106 m 3/ y r , i s n o t f u l l y known. F r a g m e n t a r y e v i d e n c e f r o m s a t e l l i t e a n d sea-floor s e d i m e n t d i s t r i b u t i o n d a t a i n d i c a t e s t h a t some o f i t may a c c u m u l a t e i n s h a l l o w emb ay men t s a l o n g t h e s o u t h c o a s t o f t h e K o r e a n P e n i n s u l a , w h e r e a s t h e r e m a i n d e r may b e t r a n s p o r t e d a l o n g t h e C h u o - G o t o s t r a t h s t o t h e Goto s u b m a r i n e c a n y o n or b e t r a n s p o r t e d i n t o t h e western Sea o f Japan.
3 96
CONCLUSIONS 1) F a l l - s e a s o n
s a m p l i n g shows t h a t h i g h l e v e l s o f t u r b i d i t y a r e
m a i n t a i n e d i n t h e Korea S t r a i t a n d s o u t h e a s t e r n Y e l l o w S e a .
Strong
t i d a l c u r r e n t s t h r o u g h o u t t h e y e a r , t o g e t h e r w i t h storm waves i n l a t e f a l l and w i n t e r ,
keep t u r b i d i t i e s
w a t e r s up t o 8 0 m d e e p .
c o n s i s t e n t l y above 1 0 mg/l
in
D e s t r a t i f i c a t i o n o f t h e water c o l u m n w i t h t h e
o n s e t o f w i n t e r monsoon w i n d s r e s u l t s i n n e a r l y u n i f o r m s u s p e n d e d s e d i m e n t c o n c e n t r a t i o n s from t h e s u r f a c e to t h e bottom. 2 ) T u r b i d i t y f r o n t s , c h a r a c t e r i z e d by t h e a b r u p t d e c r e a s e o f f s h o r e
to less t h a n 10 mg/l, o c c u r a l o n g t h e Korean w e s t coast and w e s t e r n h a l f o f t h e Korea S t r a i t . K h e r e a s b a t h y m e t r y c o r r e l a t e s r e a s o n a b l y w e l l w i t h t u r b i d i t y f r o n t s i n t h e Korea S t r a i t , d e f i n i t i o n o f f r o n t s i n i n suspended-sediment
c o n c e n t r a t i o n s f r o m g r e a t e r t h a n 20 mg/l
t h e s o u t h e a s t e r n Y e l l o w S e a may b e c o n t r o l l e d m a i n l y by s e d i m e n t res u s p e n s i o n f r o m storm w a v e s . 3)
The o n s e t o f w i n t e r monsoon w i n d s ( f r o m t h e n o r t h ) i n l a t e f a l l
i n i t i a t e s a r e s i d u a l flow to t h e s o u t h t h a t p o t e n t i a l l y carries l a r g e volumes of resuspended bottom s e d i m e n t s i n t o t h e s t r a i t .
A tongue of
t u r b i d w a t e r , most p r o n o u n c e d n e a r t h e b o t t o m , s u g g e s t s t h a t s e d i e n t e r t h e s t r a i t , t h e n b e g i n t o move s o u t h e a s t across t h e s t r a i t a t l o n g i t u d e 128O. A l t h o u g h t h e f a t e o f f i n e - g r a i n e d s e d i m e n t
ments
t h a t e n t e r s t h e s t r a i t i s unknown, i t i s r e a s o n a b l e t o a s s u m e t h a t some of i t a c c u m u l a t e s i n t h e s o u t h c o a s t e m b a y m e n t s a n d t h e r e m a i n d e r i s t r a n s p o r t e d t o t h e S e a o f J a p a n or a c c u m u l a t e s i n t h e d e e p
strait. REFERENCES C h e n , P.Y., 1978. M i n e r a l s i n bottom s e d i m e n t s of t h e S o u t h China Sea. B u l l . Geol. SOC. America, 8 9 : 211-222. C h o u g h , S.K. a n d K i m , D.C., 1 9 8 1 . Dispersal of f i n e - g r a i n e d s e d i m e n t s i n t h e s o u t h e a s t e r n Yellow S e a : a s t e a d y - s t a t e m o d e l . Jour. S e d . P e t r o l . , 51: 721-728. Han, S . J . , 1979. C l a y m i n e r a l s i n r e c e n t s e d i m e n t s o f t h e Korea Strait. B u l l . KORDI, 1: 23-37. Huh, O . K . , 1982. S p r i n g s e a s o n f l o w of t h e Tsushima C u r r e n t and i t s s e p a r a t i o n from t h e Kuroshio: s a t e l l i t e e v i d e n c e . Jour. Geophys. R e s . , 87: 9687-9693. Huh, O.K., Rouse, L.J., J r . , HSU, S.A., Wells, J . T . , and Shim, Korea S t r a i t - E a s t C h i n a S e a - Y e l l o w S e a remoteTaebo, 1981. sensing experiment. Data R e p t . t o O f f i c e o f N a v a l R e s e a r c h . L e e , C.K., 1968. The d r i f t b o t t l e e x p e r i m e n t i n t h e east o f t h e Y e l l o w S e a ( t h e w e s t coast o f Korea) d u r i n g t h e y e a r of t h e 1962-1966. Bull. F i s h e r i e s Res. and Devel. A g e n c y , pp. 29-41. O i n u m a , K. a n d K o b a y a s h i , K . , 1 9 6 6 . Quantitative study of clay m i n e r a l s i n some R e c e n t m a r i n e s e d i m e n t s a n d s e d i m e n t a r y rocks f r o m Japan. I n : A. S w i n e f o r d a n d N.V. Plummer ( E d i t o r s ) , C l a y s and Clay Minerals, Proc. 1 4 t h N a t i o n a l Conf., Pergamon P r e s s ,
397
New York, pp. 209-219. Uda, M., 1966. Yellow Sea. In: R.W. Fairbridge (Editor), Encyclopedia of Oceanoqraphy, - - - Van Nostrand and Reinhold, New York, 1021 pp. U.S. Naval OceanoqraDhic Office. 1964. Ocean currents in the vicinity of the Japdnese Islands and China coast. Publ. NO. 237, 14 PP. Wells, J.T., and Coleman, J.M., 1977. Nearshore suspended sediment variations, central Surinam coast. Mar. Geol., 2 4 : M47-M54. Wells, J.T., Huh, O . K . , and Park, Y.A., in press, Dispersal of silts and clays by winter monsoon surges in the southeastern Yellow Sea. Proc. , Sympos. Sediment. Cont. Shelf , Hangzhou, China.
9. T u r b i d i t y
FALL-SEASON PATTERNS OF TUKBIDITY AIuD SEDIMENT TRAKSPORT I K THE KOREA STRAIT AND SOUTHEASTERN YELLOW SEA J . T.
WELLS a n d 0. K.
HUH
C o a s t a l S t u d i e s I n s t i t u t e , L o u i s i a n a S t a t e U n i v e r s i t y , B a t o n Rouge , Louisiana
7 0 8 0 3 (U.S.A.)
ABSTRACT S u s p e n d e d - s e d i m e n t d a t a c o l l e c t e d i n mid-November o f 1 9 8 0 a n d 1 9 8 1 a n d c u r r e n t meter m o o r i n g s d e p l o y e d i n S e p t e m b e r a n d e a r l y October o f 1982 s u g g e s t a n e a r l y - f a l l e n t r a i n m e n t and s u b s e q u e n t t r a n s p o r t o f s u s p e n d e d s e d i m e n t i n t o t h e Korea S t r a i t by t h e S o u t h Korean C o a s t a l C u r r e n t . D u r i n g t r a n s i t i o n f r o m summer t o w i n t e r c o n d i t i o n s , suspended-sediment c o n c e n t r a t i o n s i n t h e s o u t h e a s t e r n Y e l l o w S e a r a n g e f r o m 5 t o 1 0 0 mg / l a t t h e s u r f a c e a n d 1 5 t o 250 These concenm g / l a t t h e b o t t o m i n w a t e r s t h a t a r e 20-80 m d e e p . t r a t i o n s , 1 t o 3 o r d e r s of m a g n i t u d e h i g h e r t h a n " t y p i c a l " s h e l f d e p t h w a t e r s i n most o t h e r p a r t s o f t h e w o r l d , p r o v i d e t h e o p p o r t u n i t y f o r enormous s e d i m e n t t r a n s p o r t rates w i t h i n t h i s coastal mudstream, even under r e l a t i v e l y weak c u r r e n t s . The a b r u p t termin a t i o n o f t h i s i n s h o r e b a n d o f c o l d , t u r b i d water a s a t u r b i d i t y f r o n t some 25-50 km o f f s h o r e m a r k s t h e s e a w a r d b o u n d a r y o f t h e h i g h - t r a n s p o r t zone. L an d war d o f t h e t u r b i d i t y f r o n t , s u r f a c e w a v e s a s s o c i a t e d w i t h s t r o n g c o l d - a i r o u t b r e a k s e v e r y 7-10 d a y s may d e s t r a t i f y t h e coastal mudstream and e n t r a i n a d d i t i o n a l bottom sediment. S u s p e n d e d s e d i m e n t s t r a n s p o r t e d i n t o t h e Korea S t r a i t may a c c u m u l a t e i n s o u t h - c o a s t e m b a y m e n t s , r e s i d e i n t h e d e e p s t r a i t , or be c a r r i e d i n t o t h e S e a o f J a e a n . I NT RODUCTION
T h e Korea S t r a i t s e r v e s a s a c o n d u i t f o r t h e t r a n s p o r t of sediment-laden waters from t h e Y e l l o w Sea and E a s t China Sea t o t h e Sea of
Japan.
C h a r a c t e r i z e d b y mesoscale e d d i e s a n d m e a n d e r i n g
f r o n t s , t h e waters o f t h e s t r a i t , p a r t i c u l a r l y t h o s e close t o t h e Korean P e n i n s u l a ,
h a v e b e e n shown f r o m o c e a n o g r a p h i c d a t a t o b e
c o n s i s t e n t l y t u r b i d and h i g h l y complex
(Huh e t a l . ,
1981).
The
c h a r a c t e r i s t i c s a n d d i s t r i b u t i o n of s u s p e n d e d s e d i m e n t i n t h e Korea S t r a i t a n d v i c i n i t y , t h e s u b j e c t of t h i s p a p e r , may p r o v i d e import a n t c l u e s t o t h e h y d r o d y n a m i c processes t h a t o p e r a t e i n t h i s p a r t of n o r t h e a s t A s i a .
388
S e v e r a l f e a t u r e s o f t h e Y e l l o w S e a - E a s t China Sea-Korea S t r a i t r e g i o n promote t h e u s e f u l n e s s of s u s p e n d e d - s e d i m e n t d a t a i n o c e a n o g r a p h i c problems.
First,
t h e dominance of
fine-grained
sediment
f r o m l a r g e r i v e r s s u c h a s t h e Y a n g t z e a n d Hwangho i n C h i n a ,
and
smaller r i v e r s s u c h a s t h e H a n , K u m , a n d Y e o n g s a n i n Korea, p r o v i d e a n e n o r m o u s s o u r c e o f s i l t a n d c l a y f o r e n t r a i n m e n t i n t o t h e water column.
S e c o n d , t h e m i n e r a l o g y a n d g e o c h e m i s t r y of s e d i m e n t s , com-
plex yet d i s t i n c t within drainage basins, tunity
t o d i f f e r e n t i a t e s o u r c e areas.
c o n d i t i o n s of
late-fall
and w i n t e r
may p r o v i d e
Finally,
an oppor-
t h e high-energy
associated with cola-air
out-
breaks provide a d e t e c t a b l e p e r t u r b a t i o n i n p a r t i c u l a t e material t h a t may a i d i n d e t e r m i n i n g t h i s paper
t r a n s p o r t pathways.
The p u r p o s e o f
i s t o u s e s u s p e n d e d s e d i m e n t a n d c u r r e n t meter d a t a ,
c o l l e c t e d d u r i n g f i e l d s e a s o n s 1980-1982 suspended-sediment
d i s t r i b u t i o n during
w i n t e r monsoon w i n d s ,
,
the
t o 1) c h a r a c t e r i z e t h e f a l l - s e a s o n o n s e t of
2) i d e n t i f y t h e n a t u r e and e x t e n t of t u r b i d -
i t y f r o n t s f i r s t n o t e d i n s a t e l l i t e i m a g e r y , a n d 3 ) s u p p o r t a hyp o t h e s i s f o r t r a n s p o r t of suspended sediment from t h e s o u t h e a s t e r n
Y e l l o w S e a i n t o t h e Korea S t r a i t . DATA ACQUISITIOh T h e c o l l e c t i o n of 5 5 7 water samples f o r s u s p e n a e a s o l i d s a n a l y s i s was u n d e r t a k e n a s p a r t o f KEYRSEX I a n d I1 (Korea, E a s t C h i n a S e a , Y e l l o w S e a Remote S e n s i n g E x p e r i m e n t ) , c o n d u c t e d j o i n t l y w i t h Korean u n i v e r s i t y and government s c i e n t i s t s . Three to f i v e depths were s a m p l e d a t e a c h o f 1 5 4 s t a t i o n s d u r i n g mid-November 1 9 8 0 a n d
a minimum,
samples were t a k e n f r o m t h e s u r f a c e , m i d d e p t h , and bottom a l o n g t r a n s e c t l i n e s g i v e n l e t t e r d e s i g n a t i o n s i n 1981.
As
F i g u r e 1.
S a m p l e s p a c i n g was t y p i c a l l y 1 0 km.
To d e t e r m i n e c o n c e n t r a t i o n o f p a r t i c u l a t e m a t e r i a l , samples o f 200-400 m l were f i l t e r e d t h r o u g h p r e w e i g h e d 0.45-pm brane 1977).
f i l t e r s following standard
M i l l i p o r e mem-
(Wells a n d
techniques
Coleman,
I n samples w h e r e s u f f i c i e n t p a r t i c u l a t e m a t e r i a l was a v a i l -
a b l e ( 8 5 samples) , s i z e d e t e r m i n a t i o n s were made u s i n g a m o d e l TA
e l e c i r o n i c C o u l t e r C o u n t e r w i t h a 100-pm a p e r t u r e t u b e . c a t i o n o f c l a y m i n e r a l s was made f r o m X-ray using mounts.
a Norelco w i d e - a n g l e Because of
diffraction analysis
d i f f r a c t o m e t e r on
space c o n s t r a i n t s ,
Identifi-
random b u l k - s a m p l e
presentation
of
sediment
d a t a w i l l be r e s t r i c t e d t o t h a t c o l l e c t e d i n 1981 and from l i n e H i n 1980.
389
F i g . 1. I n d e x map of s o u t h e a s t e r n Yellow S e a a n d k o r e a S t r a i t s h o w i n g t r a n s e c t l i n e s and sampling s t a t i o n s . Current
speed
and
d i r e c t i o n were d e t e r m i n e d f r o m m i d - d e p t h
m o o r i n g s a t t w o s t a t i o n s l o c a t e d a p p r o x i m a t e l y 6 0 km n o r t h of s t u d y area
(36'01'N,
126O02'E;
36'01'N,
126'14'E).
deployed during September and e a r l y October neutrally
buoyant
Endeco 174 i m p e l l o r - t y p e
the
Each mooring
a Data
1 9 8 2 c o n s i s t e d of c u r r e n t meter.
were r e c o r d e d on m a g n e t i c t a p e a t 2 - m i n u t e i n t e r v a l s , t h e n a v e r a g e d upon r e t u r n to t h e l a b o r a t o r y to p r o v i d e a r e a d i n g o n c e e a c h h o u r . S e l e c t i o n o f t h e m o o r i n g s i t e s was made so t h a t b o t h s t a t i o n s w o u l d b e l o c a t e d a l o n g a n east-west l i n e n o r m a l t o t h e s h o r e l i n e a n d t o t h e s o u t h e r l y f l o w , w h i c h h a s b e e n r e p o r t e d t o o c c u r a t t h e o n s e t of win-
t e r monsoons. SUSPENDED SEDIMENT CISTRIBUTION S u s p e n d e d s e d i m e n t s a r e almost e x c l u s i v e l y s i l t a n d c l a y s i z e d , w i t h s a n d s c o m p r i s i n g l e s s t h a n 1%of e v e n t h e c o a r s e s t d i s t r i b u tions.
I n f a c t , o n l y 2 5 % of t h e samples a n a l y z e d c o n t a i n e d a n y s e d i -
m e n t coarser t h a n 40 pm a n d , o n a v e r a g e , 9 0 % o f t h e s e d i m e n t i n e a c h d i s t r i b u t i o n was b r a c k e t e d b e t w e e n 2 pm a n d 40 pm, t h e n o m i n a l s i z i n g
390
r a n g e o f t h e 1 0 0 pm a p e r t u r e t u b e . F i g u r e 2 s h o w s from f o u r r e p r e s e n t a t i v e f r e q u e n c y c u r v e s t h a t n o s y s t e m a t i c v a r i a t i o n i n s i z e i s a p p a r e n t w i t h d i s t a n c e above t h e bottom. E x t r e m e s i n m e d i a n s i z e o f i n d i v i d u a l samples a r e 4 . 2 pm a n d 1 7 . 5 pm, a n d a v e r a g e g r a i n s i z e a l o n g t r a n s e c t l i n e s s h o w s l i n e s I a n d J t o b e t h e c o a r s e s t (11.1 pm a n d 1 1 . 7 pm, r e s p e c t i v e l y ) , f o l lo w e d by l i n e s H i n 1 9 8 1 ( 9 . 5 pm) a n d H i n 1 9 8 0 ( 7 . 1 pm).
The i n t e n -
s i t y of storm w a v e s a n d c o n t i n u o u s t i d a l s t i r r i n g s u g g e s t t h a t t h e l a c k o f s a n d s i n t h e water c o l u m n r e f l e c t s t h e f i n e - g r a i n e d n a t u r e o f
t h e s e d i m e n t s on t h e bottom r a t h e r t h a n a n i n a b i l i t y of c u r r e n t s t o s u s p e n d coarse p a r t i c l e s . The p a t t e r n s and l e v e l s o f t u r b i d i t y a r e r e m a r k a b l y s i m i l a r i n s u r f a c e a n d b o t t o m waters ( F i g s . 3 , 4 ) . T h e h i g h e s t c o n c e n t r a t i o n s occur o f f t h e s o u t h w e s t t i p of t h e Korean P e n i n s u l a i n a n area where b o t t o m s e d i m e n t s a r e r e s u s p e n d e d by storm w a v e s o f maximum p o s s i b l e f e t c h . T u r b i d i t y l e v e l s i n c r e a s e close t o s h o r e , e x c e e d i n g 1 0 0 mg/l a t b o t h t h e s u r f a c e a n d t h e b o t t o m . A t o n g u e o f l o w - t u r b i d i t y water a p p e a r s t o e n t e r t h e s t r a i t from f l o w o f t h e Tsushima C u r r e n t n o r t h o f C h e j u I s l a n d . A t o n g u e o f h i g h - t u r b i d i t y water n o r t h e a s t o f C h e j u e x t e n d s s o u t h e a s t more t h a n h a l f w a y across t h e Korea S t r a i t t o l a t i t u d e of n e a r l y 3 3 O N o v e r a n area o f b r o a d d e p t h c o n t o u r s . T h e n o r t h e a s t e r n p a r t o f t h e s t r a i t i s a g a i n c h a r a c t e r i z e d by c o n c e n t r a t i o n s o n t h e o r d e r o f 10-15 mg/l. T h e d i s t r i b u t i o n o f c l a y m i n e r a l s o b s e r v e d i n water s a m p l e s i n t h i s s t u d y i s close t o t h a t r e p o r t e d f r o m b o t t o m samples by Chough
a n d K i m ( 1 9 8 1 ) , C h en ( 1 9 7 8 ) , a n d Oinuma a n d a n d K o b a y o s h i ( 1 9 6 6 ) . T r a n s e c t l i n e s I a n d J a n d t h e n o r t h e r n p a r t o f l i n e H show a p r e d o m i -
nance of i l l i t e (-70%) w i t h lesser amounts of k a o l i n i t e (-10%) and c h l o r i t e ( - 1 3 % ) . M o n t m o r i l l o n i t e i s v i r t u a l l y a b s e n t . S e v e r a l samp l e s from t h e s o u t h e r n p a r t o f
l i n e H show a s l i g h t d e c r e a s e i n
k a o l i n i t e a n d i n c r e a s e i n i l l i t e , most l i k e l y a r e s u l t o f i n f l u e n c e o f t h e Tsushima C u r r e n t . A c c o r d i n g t o Chough a n d K i m ( 1 9 8 1 ) , t h e i l l i t e - r i c h waters i n a n d j u s t n o r t h o f t h e s t u d y area a r e d i l u t e d by k a o l i n t e a n d c h l o r i t e f r o m l o c a l r i v e r s ;
however, o n c e w i t h i n
t h e main f l o w o f t h e s t r a i t , m i n e r a l o g y o f s e d i m e n t s from t h e E a s t C h i n a S e a sh o ws g r e a t e r i n f l u e n c e . Although i n s u f f i c i e n t sediment
was a v a i l a b l e f o r m i n e r a l o g i c a l a n a l y s i s o f water samples e a s t w a r d t h r o u g h t h e Korea S t r a i t , Han ( 1 9 7 9 ) r e p o r t s t h a t i l l i t e f r o m t h e E a s t China Sea c o n t i n u e s to predominate.
391 Diameter in microns
r
63 loo
-r
31
16
8
4
10 20 30 40
I
-
L\
50-
60 30
-
20
-
10
-
.---.
70
Middaplh
Bottom
80
90
10 Diameter in PHI units
F i g . 2. T y p i c a l s i z e d i s t r i b u t i o n c u r v e s i n d i c a t i n g median p a r t i c l e diameters and sorting values. TURBIDITY FRONTS T h e t u r b i d w a t e r s a l o n g m o s t o f t h e s o u t h a n d w e s t c o a s t s o f Korea e n d a b r u p t l y a s t u r b i d i t y f r o n t s 2 5 t o 50 km o f f s h o r e i n w a t e r s 20 t o 8 0 m deep. A s f i r s t o b s e r v e d i n s a t e l l i t e i m a g e r y , t u r b i d i t y f r o n t s a r e w e l l d e f i n e d by t h e s a t u r a t e d , s e d i m e n t - l a d e n
(white) returns i n
b a n d s 4 a n d 5 o f LANDSAT ( F i g . 5 ) . W h e r e a s t h e t u r b i d i t y f r o n t a t t h e w e s t e r n e n d o f t h e s t r a i t appears t o f o l l o w b o t t o m t o p o g r a p h y , e n d i n g o f f s h o r e a t t h e 40-m d e p t h c o n t o u r ( F i g .
5 ) , i t loses l a t e r a l con-
t i n u i t y midway t h r o u g h t h e s t r a i t a t t h e e a s t e r n m a r g i n o f t h e s o u t h e a s t - t r e n d i n g b a n d o f t u r b i d water shown p r o m i n e n t l y i n F i g u r e s 3 a n d
4 ( l o n g i t u d e 128OE). A l t h o u g h a t u r b i d i t y f r o n t i s p r e s e n t a l o n g t h e e n t i r e west coast o f Korea, i t s w i d t h i s v a r i a b l e , b o t h s p a t i a l l y a n d t e m p o r a l l y , a n d r e f l e c t s g r a d i e n t of t h e b o t t o m , c o n s i s t e n c y o f t h e b o t t o m s e d i m e n t , a n d , most i m p o r t a n t l y , i n t e n s i t y o f wave a n d t i d e p r o c e s s e s . Examin a t i o n o f 2 5 c l e a r - s k y LANDSAT s c e n e s b e t w e e n S e p t e m b e r 1 9 7 2 a n d O c t o b e r 1980 i n d i c a t e s t h a t t h e f r o n t i s b e s t d e v e l o p e d between Sept e m b e r a n d November a n d weakest b e t w e e n J a n u a r y a n d Ma rc h. U n f o r t u n a t e l y , c l o u d c o v e r a f t e r A p r i l p r e v e n t s d e t e c t i o n o f a summer t u r b i d i t y f r o n t . I n t e n s e mixing a s s o c i a t e d with cold-air outbreaks duri n g w i n t e r may e x p l a i n l a c k o f d e f i n i t i o n i n t h e f r o n t s b e t w e e n J a n u a r y and March. M o r e o v e r , f e a t u r e s s u c h a s e d d i e s , w ind rows, a n d
392
Fig. 3. Surface suspended-sediment mid-November 1981.
concentrations
in
Fig. 4. Bottom suspended-sediment concentrations in midNovember 1981.
393
F i g . 5 . LANDSAT b a n d 4 i m a g e o f t h e s o u t h w e s t p a r t of t h e Korean P e n i n s u l a . Note z o n e of h i g h t u r b i d i t y ( w h i t e ) a n d i t s t e r m i n a t i o n as a t u r b i d i t y f r o n t a t t h e 40-m d e p t h c o n t o u r . b r e a k i n g waves o v e r s u b t i d a l s a n d r i d g e s a r e t y p i c a l w i n t e r - s e a s o n occurrences t h a t are a b l e to obscure t h e t u r b i d i t y f r o n t s . F i g u r e 6 shows t h e s t r u c t u r e o f t h e s o u t h coast f r o n t i n 1980 a n d a t t h e same t i m e (mid-November) i n 1 9 8 1 , b u t u n d e r som e w ha t different conditions.
Wind s t r e s s a t C h e j u I s l a n d was 1 2 % h i g h e r
d u r i n g s a m p l i n g and 1 7 % h i g h e r d u r i n g t h e month and a h a l f c e d i n g sampling i n 1 9 8 1 t h a n i n 1980.
pre-
Suspenlied s e d i m e n t c o n t o u r s
i n 1 9 8 1 r e f l e c t t h i s d i f f e r e n c e by s h o w i n g a more n e a r l y v e r t i c a l s t r u c t u r e a s a r e s u l t o f r e s u s p e n s i o n o f b o t t o m s e d i m e n t a n d de s t r a t i f i c a t i o n o f t h e water c o l u m n .
L a y e r s of
t u r b i d water n e a r
t h e b o t t o m , s u c h as t h a t shown by t h e 2 5 0 m g/l c o n t o u r i n 1 9 8 0 , h a v e b e e n d i s p e r s e d i n t o t h e water c o l u m n i n 1 9 8 1 ( F i g . 6 ) .
The t u r -
i s m a r k e d by a s h a r p d e c r e a s e i n s u s p e n d e d to less than 10 mg/l. Although s a t e l l i t e s e n s o r s c a n n o t r e l i a b l y be used to quant i f y s u s p e n d e d - s e d i m e n t c o n c e n t r a t i o n s or t o look b e l o w t h e s u r f a c e w a t e r s , s e d i m e n t s a m p l i n g c o n f i r m s t h a t LAKDSAT i m a g e r y c o n s i s t e n t l y d e l i n e a t e s as t u r b i d i t y f r o n t s suspended-sediment g r a d i e n t s of 1 0 - 2 0 m g / l a n d t h a t t h i c k n e s s of t h e f r o n t s i s t y p i c a l l y e q u a l t o t h e water d e p t h . bidity front itself
s e d i m e n t c o n c e n t r a t i o n from g r e a t e r t h a n 20 mg/l
394 0 10
20
30 40 50
60
T -
f
70 TOTAL SUSPENDED MATTER lrngili
80 90
TRANSECT H
100 110
I20
0
20
40
60
80
100
120
140
160
180
200
220
240
0 10
20 30 40 50 -
-
60 f
a
0"
70 80
TRANSECT H
90 100 110
0
20
40
60
80
100
120
140
160
180
200
220
240
120
260
D i r t o n c e [km)
F i g . 6. Suspended-sediment c o n c e n t r a t i o n s and b i d i t y f r o n t s a l o n g l i n e h' i n 1 9 8 0 a n d 1 9 8 1 .
location
of
tur-
MEAN FLOW AND SEDIMENT TRAEjSPORT T h e west coast t u r b i d i t y f r o n t m a r k s t h e s e a w a r d b o u n d a r y o f a high-transportcoastalmudstream w i t h i n t h e S o u t h K o r e a n C o a s t a l C u r rent. The f a c t t h a t suspended-sediment c o n c e n t r a t i o n s a r e o n e t o t h r e e o r d e r s of magnitude h i g h e r t h a n i n " t y p i c a l " s h e l f waters i n d i -
c a t e s t h a t s e d i m e n t t r a n s p o r t r a t e s w i t h i n t h i s c o a s t a l m u d s t r e a m may be enormous, e v e n under r e l a t i v e l y w e a k n e t c u r r e n t s . S e v e r a l l i n e s of
e v i d e n c e s u g g e s t t h a t n e t flow i s to t h e s o u t h d u r i n g w i n t e r
m o n s o o n s , a n d p e r h a p s t h e y e a r a r o u n d . Uda ( 1 9 6 6 ) r e p o r t e d r e s i d a l c u r r e n t s o f 1 5 cm/sec i n t h e c o a s t a l c u r r e n t o f t h e s o u t h e a s t e r n Y e l l o w Sea. of
1 0 cm/sec
L e e ( 1 9 6 8 ) i n d i c a t e d from d r i f t b o t t l e s t u d i e s f l o w s i n winter,
and t h e U.S.
Navy,
i n Summaries o f
Ship
D r i f t R e p o r t s ( 1 9 6 4 ) , i n d i c a t e d w e a k flow t o t h e s o u t h d u r i n g a l l seasons.
Further, observations of sea-surface temperature p a t t e r n s
f r o m NOAA s e r i e s s a t e l l i t e s ( Hu h , 1 9 8 2 ) f o l l o w c l o s e l y t h o s e f r o m
395 N MEAN CURRENTS
Outer station
1
3
-10
-
-12
-
i
\
, '.-.*..'
5
\
.
*'
F i g . 7 . Low-pass f i l t e r d a t a (mean f l o w ) f r o m m o o r i n g s o f f t h e Kum River e n t r a n c e a t l a t i t u d e 36ON. v i s i b l e b a n d s o f LANDSAT. F i g u r e 7 shows a v e r a g e c u r r e n t s i n September and e a r l y O c t o b e r from moorings deployed as p a r t of t h i s s t u d y . r e s e n t low-pass
Average c u r r e n t s rep-
f i l t e r v a l u e s r u n t h r o u g h a 75-hour Lanczos f i l t e r
w i t h a 50-hour half-power p o i n t .
T h e i n n e r s t a t i o n ( 2 5 km o f f s h o r e )
a n d t h e l a s t h a l f o f t h e d a t a r e c o r d f r o m t h e o u t e r s t a t i o n ( 5 0 km o f f s h o r e ) show mean f l o w t o b e d i r e c t e d t o t h e s o u t h a t s p e e d s o f 7 . 8 a n d 3 . 8 cm/sec, r e s p e c t i v e l y . Wi n d s were l i g h t a n d v a r i a b l e f o r t h e d u r a t i o n of t h e moorings. I t i s r e a s o n a b l e t o assume t h a t t h i s weak
transitional-season
current
w ould
become
substantially
s t r o n g e r under t h e i n f l u e n c e of s t r o n g northwest winds c h a r a c t e r i s t i c of
t h e w i n t e r mo n s o o n s e a s o n t h a t
(Wells e t a l . ,
begins
i n l a t e October
i n press).
Flow o f t h e S o u t h K o r e a n C o a s t a l C u r r e n t p r o v i d e s a m e c h a n i s m f o r t r a n s p o r t i n g s u s p e n d e d s e d i m e n t s o u t h i n t o t h e Korea S t r a i t . T h i s i s p a r t i c u l a r l y s i g n i f i c a n t i n t h a t f l o w may s t r e n g t h e n i n t h e f a l l , p r e c i s e l y a t a t i m e when t u r b i d i t i e s a r e i n c r e a s i n g f r o m t h e o n s e t o f monsoon w i n d s .
The f a t e o f t h i s suspended coastal s e d i m e n t , t r a n s -
p o r t e d s o u t h i n t o t h e Korea S t r a i t a t r a t e s e s t i m a t e d t o b e 25-250 x 106 m 3/ y r , i s n o t f u l l y known. F r a g m e n t a r y e v i d e n c e f r o m s a t e l l i t e a n d sea-floor s e d i m e n t d i s t r i b u t i o n d a t a i n d i c a t e s t h a t some o f i t may a c c u m u l a t e i n s h a l l o w emb ay men t s a l o n g t h e s o u t h c o a s t o f t h e K o r e a n P e n i n s u l a , w h e r e a s t h e r e m a i n d e r may b e t r a n s p o r t e d a l o n g t h e C h u o - G o t o s t r a t h s t o t h e Goto s u b m a r i n e c a n y o n or b e t r a n s p o r t e d i n t o t h e western Sea o f Japan.
3 96
CONCLUSIONS 1) F a l l - s e a s o n
s a m p l i n g shows t h a t h i g h l e v e l s o f t u r b i d i t y a r e
m a i n t a i n e d i n t h e Korea S t r a i t a n d s o u t h e a s t e r n Y e l l o w S e a .
Strong
t i d a l c u r r e n t s t h r o u g h o u t t h e y e a r , t o g e t h e r w i t h storm waves i n l a t e f a l l and w i n t e r ,
keep t u r b i d i t i e s
w a t e r s up t o 8 0 m d e e p .
c o n s i s t e n t l y above 1 0 mg/l
in
D e s t r a t i f i c a t i o n o f t h e water c o l u m n w i t h t h e
o n s e t o f w i n t e r monsoon w i n d s r e s u l t s i n n e a r l y u n i f o r m s u s p e n d e d s e d i m e n t c o n c e n t r a t i o n s from t h e s u r f a c e to t h e bottom. 2 ) T u r b i d i t y f r o n t s , c h a r a c t e r i z e d by t h e a b r u p t d e c r e a s e o f f s h o r e
to less t h a n 10 mg/l, o c c u r a l o n g t h e Korean w e s t coast and w e s t e r n h a l f o f t h e Korea S t r a i t . K h e r e a s b a t h y m e t r y c o r r e l a t e s r e a s o n a b l y w e l l w i t h t u r b i d i t y f r o n t s i n t h e Korea S t r a i t , d e f i n i t i o n o f f r o n t s i n i n suspended-sediment
c o n c e n t r a t i o n s f r o m g r e a t e r t h a n 20 mg/l
t h e s o u t h e a s t e r n Y e l l o w S e a may b e c o n t r o l l e d m a i n l y by s e d i m e n t res u s p e n s i o n f r o m storm w a v e s . 3)
The o n s e t o f w i n t e r monsoon w i n d s ( f r o m t h e n o r t h ) i n l a t e f a l l
i n i t i a t e s a r e s i d u a l flow to t h e s o u t h t h a t p o t e n t i a l l y carries l a r g e volumes of resuspended bottom s e d i m e n t s i n t o t h e s t r a i t .
A tongue of
t u r b i d w a t e r , most p r o n o u n c e d n e a r t h e b o t t o m , s u g g e s t s t h a t s e d i e n t e r t h e s t r a i t , t h e n b e g i n t o move s o u t h e a s t across t h e s t r a i t a t l o n g i t u d e 128O. A l t h o u g h t h e f a t e o f f i n e - g r a i n e d s e d i m e n t
ments
t h a t e n t e r s t h e s t r a i t i s unknown, i t i s r e a s o n a b l e t o a s s u m e t h a t some of i t a c c u m u l a t e s i n t h e s o u t h c o a s t e m b a y m e n t s a n d t h e r e m a i n d e r i s t r a n s p o r t e d t o t h e S e a o f J a p a n or a c c u m u l a t e s i n t h e d e e p
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