E~uaries
Vol. 23, No. 3, p.400-410
June 2000
Sediment Character in a Micro-Tidal, Harbor-Estuary Environment, G6teborg, Sweden L. T. J O H A N N E S S O N 1 R. L. STEVENS J. H. ALEXANDERSON~
Department of Geology G6teborg University P. O. Box 460 SE405 30 Giiteborg; Sweden ABSTRACT: Harbor-estuaries are c o m p l e x sedimentary environments where both natural and anthropogenic processes influence the distribution and accmnulation of sediments~ which in t u r n is o f importance for maintenance dredging and pollution control, T h i s p a p e r uses sediulent characteristics from the G6ta Mv estuary~ Sweden to evaluate the natural estuarine p r o c e s s e s that persist despite the extensive hmnan impact on the estuary and to separate and characterize depositional sub-environments. T h e m o s t i m p o r t a n t h a r b o r p r o c e s s e s include maintenance dredging and t u r b u l e n c e f r o m ship traffic~ and their influence upon shallow stratigraphy and s e d i m e n t distribution and deposition is significant. Polymodal particle-size distributions are i n t e r p r e t e d to reflect the natural estuarine t r a n s p o r t processes. Suspension t r a n s p o r t is generally predominant~ e x c e p t in the river and inner h a r b o r where bottom transport and selective deposition of sand occur. Particle-size characteristics divide the estuary into f o u r depositional areas: the river and the inner~ middle~ and outer harbor, T h e shallow stratigraphy reveals a hiatus between the firm silty clays and the l o o s e recent sediments. T h e hiatus in the stratigraphy is probably related to d r e d g i n g in UlOSt areas of the harbor, Significant u l a i n t e n a n c e d r e d g i n g and turbulence froul ship traffic also ilffluence the accuululation (0-61 cm) of r e c e n t s e d i m e n t s , during the 20th century,
Introduction T h e G6ta ~ilv estuary is a micro-tidal, essentially non-tidal, salt-wedge estuary ( E k m a n 1875; Pettersson 1923). It is also characterized by the c o m p l e x c o m b i n a t i o n of natural a n d a n t h r o p o g e n i c processes of an u r b a n harbor. T h e estuary is located in a coastal lowland area of southwestern Sweden (Fig. 1), where the geology is characterized by glac i o m a r i n e a n d m a r i n e clays of late Q u a t e r n a r y age a n d e x p o s e d P r e c a m b r i a n bedrock. Erosion of the soils and Q u a t e r n a r y s e d i m e n t s in the s u r r o u n d i n g river valleys is aided by the post-glacial isostatic reb o u n d , c. 2 m m yr -1 ( S u n d b o r g and N o r r m a n 1963), and provides the estuary with mostly finegrained material. T h i s relatively steady supply is occasionally c o m p l e m e n t e d by large, short-term sedi m e n t fluxes associated with u p s t r e a m landslides within unstable clays, whose d e v e l o p m e n t is also related to land uplift a n d increased freshwater leaching of the p o r e w a t e r cations (cf., Fr~Sdin 1919; S u n d b o r g 1961). River influence d o m i n a t e s over tidal a n d wave processes in the estuary. T h e tidal a m p l i t u d e in G~Steborg is only 20 cm at its maxim u m (Pettersson 192S) a n d the m a x i m u m effect
of tidal m o v e m e n t s is 0.1-0.2 m s 1 currents (Jonsson 1973). A l t h o u g h glacially d e e p e n e d , the GSta filv estuary does n o t have a m o d e r n threshold limiting the m a r i n e interaction, b u t a sheltering archipelago limits the fetch a n d influence of s t o r m waves (Fig. 1). T h e surface water in the o u t e r m o s t estuary is seldom of full m a r i n e salinity, b u t ranges f r o m 10-20 psu, whereas the b o t t o m water, originating f r o m the Kattegat surface water, m a y r e a c h salinities up to 34 psu (Pettersson 1923; Selmer a n d Rydberg 1993). T h e high-latitude climate usually results in ice cover in the winter and f r e q u e n t storms with strong westerly winds in the spring a n d fall. Since m a r i n e currents do n o t extensively disturb the river m o u t h processes, the G 6 t a glv estuary provides a suitable b a c k g r o u n d for studying n o n - m a r i n e variability. T h e Port of G 6 t e b o r g is the largest h a r b o r in the N o r d i c countries, and takes u p a large p a r t of the G 6 t a glv estuary. T h e average traffic is c. 14,000 passengers and 35 ships per day. C o n t a i n e r a n d oiltank ships pass t h r o u g h the h a r b o r on their way to a n d fi-om Lake V g n e r n in south-central Sweden, a n d their passage helps to m a i n t a i n a navigable d e p t h in the G 6 t a fin River. Both sides of the estuary are m a n - m a d e shores except for the rock shores on the s o u t h e r n side of the outer estuary. T h e heaviest h a r b o r activity, including docking of large c o n t a i n e r ships, is located on the n o r t h side
~ C o r r e s p o n d i n g a u t h o r ; tele: +46-31-7732819; fax: +46-317732849; e-mail:
[email protected]. P r e s e n t address: D e p a r t m e n t of Q u a t e r n a r y Geology, L u n d University, S61vegatan 13, SE-223 62 L u n d , Sweden. 9 2000 Estuarine Research Federation
40D
Sediment Character in a Harbor-Estuary
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of the outer harbor. T h e i n n e r a n d m i d d l e h a r b o r areas are b e c o m i n g less active, b u t in addition to the large ferries for car and p a s s e n g e r traffic to D e n m a r k a n d G e r m a n y , small ships are still anchoring at the f o r m e r cargo wharves. T h e ferries include high-speed, hydro-jet p r o p u l s i o n ships, the first of which was put into traffic in 1993. T h e extensive a n t h r o p o g e n i c activity, mainly ship traffic a n d dredging, in the h a r b o r disturbs salinity stratification and changes the natural d e p t h of the harbor, which in turn influence the overall sedimentation p a t t e r n . H a r b o r siltation is an evident problem, requiring h a r b o r and c h a n n e l d r e d g i n g every o t h e r year. T h e Coastal Water M o n i t o r i n g P r o g r a m of the G 6 t e b o r g a n d Bohus C o u n t y has b e e n m o n i t o r i n g the Swedish West Coast since 1958. T h e r e are, however, only two stations included in the m o n i t o r i n g p r o g r a m lying within the river-estuary system, a n d b o t t o m sediments are not routinely investigated. O t h e r regional coastal investigations have included a few selected sites in the estuary. T h e s e sites are p r e d o m i n a n t l y characterised by fine-grained material with a relatively high m e t a l c o n t e n t ( A r n b o r g et al. 1968; Cato 1986, 1992, 1997). T h e actual harb o r area of the G o t a alv estuary has n o t previously b e e n t h o r o u g h l y investigated with respect to its sedimentological characteristics. Earlier investigations of the h a r b o r have mainly b e e n in p r e p a r a tion for d r e d g i n g o p e r a t i o n s o r d e r e d by the p o r t authorities, and have mostly c o n c e r n e d the content of PCB, m e r c u r y , a n d c a d m i u m in the sediinents a n d the d e p t h deviations f r o m the nautical charts. G e o t e c h n i c a l m e a s u r e m e n t s have b e e n imp o r t a n t within c o n s t r u c t i o n p r o g r a m s over the last century, motivated in p a r t by the unstable clay deposits that are c o m m o n t h r o u g h o u t southwestern Sweden. T h e r e are n u m e r o u s investigations of s e d i m e n t
401
characteristics in o t h e r estuaries (e.g., P o s t m a 1967; Allen et al. 1976; S u m m e r h a y e s et al. 1985; Eisma et al. 1991a,b; A n t h o n y et al. 1996; Brooks 1998). Investigations of harbor-estuaries are poorly r e p r e s e n t e d in the geologic literature p r e s u m a b l y because of the disturbances of the natural processes, the c o m p l e x s e d i m e n t a t i o n patterns, and the irregular physical g e o m e t r i e s of these environments. T h e n u m b e r of investigated harbor-estuaries is increasing since t h e r e is a n e e d to be able to predict the distribution and a c c u m u l a t i o n of sedim e n t s for p l a n n i n g m a i n t e n a n c e d r e d g i n g a n d pollution control. T h e overall p u r p o s e of this p a p e r is to use sedi m e n t characteristics to evaluate the natural estuarine processes that are still p r e s e n t despite the extensive a r t i f i c i a l / h u m a n i m p a c t on the estuary. T h e h a r b o r processes, including a n t h r o p o g e n i c activities and m a n - i n d u c e d conditions, that affect the natural estuarine processes in this harbor-estuary are addressed t o g e t h e r with a consideration of the g e o g r a p h i c s e d i m e n t distribution, shallow stratigraphy, a n d deposition rates. O u r work is an initial step towards a c o m p a r i s o n of h a r b o r siltation in non-tidal settings of n o r t h e r n E u r o p e . Materials and M e t h o d s
Sampling was carried out m August 1995. Sedim e n t cores, 4 cm in diam, were collected using a simple b o t t o m corer (plexiglas tube c o n n e c t e d to end of iron pipes) at 46 sites t h r o u g h o u t the harb o r p o r t i o n of the estuary (Fig. 1). Water d e p t h in the area of s a m p l i n g varies f r o m 6 (river and i n n e r h a r b o r ) to 1S m (outer h a r b o r ) . T h e 25-80 cm long cores were stored vertically at +4~ until they were taken out of the tubes, described, a n d subsampled. T h e u p p e r m o s t 5 cm of all cores were used for f u r t h e r analyses along with s o m e downcore samples f r o m a few representative cores containing the eight different facies distinguished. Prior to particle-size analysis, the samples were pre-treated with 15% h y d r o g e n p e r o x i d e for 9 - 4 h at 80~ to r e m o v e organic matter. T h e samples were dispersed with 0.05 M s o d i u m p y r o p h o s p h a t e during rotation t u m b l i n g for S-4 h. Wet sieving was carried out on s e d i m e n t larger than 6S I~m a n d pipette analysis was used for the finer fractions (see Kt-umbein and Pettijohn 19S8 for standard p r o c e d u r e s ) . T h e analyses were m a d e with 1/~_(~ intervals between 8 a n d 950 bml and at 1-~ intervals otherwise. T h e organic c a r b o n c o n t e n t was d e t e r m i n e d by oxidation of c. 1 g of dry s e d i m e n t with potassium c h r o m a t e in an acid solution a n d back-titration with f e r r o u s a m m o n i u m sulphate solution (Walkley and Black 19S4; Hesse 1971). T h e result was
402
L.T. Johannesson
et al.
converted to total organic c a r b o n by multiplying with 1.38 (Hesse 1971).
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Results
FACIES DIVISIONS Eight facies were defined in the 46 cores on the basis of particle size ( n o m e n c l a t u r e from Stevens 1984), color, s e d i m e n t a r y features, and total organic c o n t e n t (Table 1). Facies A, B, and C are slightly silty or silty clays with a low sand c o n t e n t (61 cm. Only five cores in the h a r b o r did n o t contain any
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