Dynamics of sedimentation in a tide-dominated backbarrier salt marsh, Norfolk, UK

Marine Geology, 110 (1993) 315-331 Elsevier Science Publishers B.V., Amsterdam

315

Dynamics of sedimentation in a tide-dominated backbarrier salt marsh, Norfolk, UK Jonathan

R. French a and Tom Spencer b

aDepartment of Geography, University College London, UK bDepartment of Geography, University of Cambridge, UK (Received August 25, 1992; revision accepted November 19, 1992)

ABSTRACT French, J.R. and Spencer, T., 1993. Dynamics of sedimentation in a tide-dominated backbarrier salt marsh, Norfolk, UK. Mar. Geol.. I10: 315-331. Progressive burial of artificial markers over a 5-year period is used to determine the rate and pattern of vertical accretion within a large backbarrier salt marsh on the UK east coast. Over this period, annual accretion varies spatially from 1 to 8 mm yr-1. The arithmetic mean rate for the whole marsh is 3.9 mm yr -1. Spatial variability in accretion is a joint function of(l) elevation-dependent inundation frequency and (2) progressive sediment removal from water masses advected across channel margins. Accretion is, therefore, inadequately represented by simple averaging of point measurements. Numerical integration of the 'accretion surface" results in a spatial average rate of around 3 mm yr- 1, well below the arithmetic mean rate. Short-term sediment trap deployments show that local and long-range meteorological effects, and remobilisation of sediment deposited within tidal creeks, often mask the expected link between tidal height and sedimentation rate. Retention of sediment on plant surfaces is minimal, with direct settling accounting for approximately 95% of total deposition. Time-extrapolation of weekly sediment trap data, and comparison with the 5-year marker horizon burial, shows that processes associated with ordinary tides can account for long-term accretion over most of the marsh. However, the highest surfaces receive appreciable sediment input only during aperiodic storm events.

Introduction A n extensive literature describes the a c c u m u l a tion o f fine-grained sediments within c o a s t a l a n d estuarine wetlands. In p a r t i c u l a r , researchers have o b t a i n e d direct m e a s u r e m e n t s o f the rate o f sedim e n t a c c r e t i o n ( R i c h a r d s , 1934; Steers, 1938, 1946; Ranwell, 1964; Letzsch a n d F r e y , 1980; Reed, 1988; S t o d d a r t et al., 1989), o r have utilised wellpreserved s t r a t i g r a p h i c sequences (Redfield, 1972; M c C a f f r e y a n d T h o m p s o n , 1980; K e a r n e y a n d W a r d , 1986; Allen a n d Rae, 1988) to infer the m e d i u m - to l o n g - t e r m d y n a m i c s o f w e t l a n d growth. I m p r o v e d u n d e r s t a n d i n g o f tidal h y d r o d y n a m i c s

Correspondence to: J.R. French, Department of Geography, University College London, 26 Bedford Way, London WCIH 0AP, UK. 0025-3227/93/$06.00

has t e n d e d to displace the n o t i o n o f coastal wetlands as simple i n p u t - o u t p u t systems (Boon, 1974; Settlemyre a n d G a r d n e r , 1977; R e e d et al., 1985). M o r e recently, a t t e n t i o n has focused on internal w a t e r a n d m a t e r i a l fluxes, a n d the time-scales over which these are o p e r a t i v e (Stumpf, 1983; D a n k e r s et al., 1984; Reed, 1988; F r e n c h , 1989; F r e n c h a n d S t o d d a r t , 1992). Also, c o n c e r n over the fate o f w e t l a n d s u n d e r c o n t e m p o r a r y rates o f relative sealevel rise ( R S L R ; B a u m a n n et al., 1984; O r s o n et al., 1985; Stevenson et al., 1986, 1988) has highlighted the need for i m p r o v e d u n d e r s t a n d i n g o f the n a t u r e a n d rate o f s e d i m e n t a t i o n in these systems. C o a s t a l w e t l a n d s range from the highly o r g a n i c ( K a y e a n d B a r g h o o r n , 1964; H a t t o n et al., 1983) to the p r e d o m i n a n t l y i n o r g a n i c (Evans, 1965; Allen a n d Rae, 1988), are subject to v a r y i n g degrees o f tidal exchange ( H a r r i s o n a n d Bloom, 1977; Steven-

© 1993 - - Elsevier Science Publishers B.V. All rights reserved.

316

son et al., 1988; Wood et al., 1989) and have experienced different sea-level histories (Stevenson et al., 1986; Allen, 1990; French, 1991). Interpretation of marsh 'sedimentary status' (i.e. accretion minus local RSLR) is critically dependent upon the basis of measurement. Thus, analysis of shortterm (< 101 years) vertical accretion, whether by (1) monitoring the burial of artificial markers (Stoddart et al., 1989) or (2) repeated levelling (Dalby, 1970; Carr and Blackley, 1987) affords a different perspective to the interpretation of longerterm (101-103 years) sedimentary sequences based upon dated horizons (Hubbard and Stebbings, 1968; Flessa et al., 1977; Allen and Rae, 1988) or radionuclide profiles (Kearney and Ward, 1986). The former may overestimate net accretion where substrate autocompaction is significant (Stevenson et al., 1986). The latter may be inappropriate in highly inorganic settings where external sediment input is determined by marsh elevation and is strongly time-dependent (Pethick, 1981; French, in press). Although several studies examine the link between sedimentation and specific physical and biological factors (Ranwell, 1964; Richard, 1978; Harper, 1979; Letzsch and Frey, 1980; Stumpf, 1983; Stoddart et al., 1989; Wood et al., 1989), few cover the full range of temporal and spatial scales that are relevant for marsh adjustment to contemporary sea-level trends. Meso- and macro-tidal salt marshes with highly inorganic (i.e. allochthonous) substrates are particularly important within a European context (Dijkema, 1987). This paper presents results from an investigation of short- to medium-term salt marsh sedimentation in a macro-tidal backbarrier setting on the UK east coast. Buried marker horizons and surface sediment traps are used to (1) characterise spatial variability in the rate of sedimentation; and (2) examine the hypothesis that surfaces of different elevation are adjusted variously to ordinary tidal flooding or storm-induced sedimentation.

Geomorphic and hydrographic context The north Norfolk coast extends west to east over a distance of approximately 40 km from Holme-next-the-Sea to Salthouse (Fig. la). Intertidal sand and mud flats and an assemblage of

J.R. F R E N C H A N D T. S P E N C E R

semi-discrete salt marshes exist in conjunction with a series of recurved shingle and dune barriers (Steers, 1960; Funnell and Pearson, 1989; French et al., 1990; French, 1992). Whilst the outer marshes (notably at Scolt Head Island) remain in a 'near-natural' state (ungrazed and unditched), considerable areas of mainland marsh have been reclaimed for agriculture. Compared to other barrier-marsh coasts (Hayes, 1979; Fitzgerald and Penland, 1987), north Norfolk is unusual in that it is subject to a macrotidal semi-diurnal tidal influence (mean range 3.2 m at neaps; 6.4 m at springs (Hydrographer of the Navy, 1992). Salt marsh surfaces lie 2.0-3.2 m above Ordnance Datum (O.D.), with the larger dune-covered barriers reaching a height of 15 20 m O.D. Highest astronomical tides (HAT) reach 3.8-4.0 m O.D., although the North Sea is susceptible to storm surges, the largest of which occasionally produce water levels in excess of 5.0 m O.D. (Pugh, 1987). Backbarrier marsh sites typically exhibit a stratigraphic succession from intertidal muds through alternating salt marsh and brackish peat facies associated with minor variations in the magnitude and direction of relative sea-level tendency. The most recent phase of marine alluviation commenced around 2750 yr B.P., although some sites have clearly experienced continuous marsh sedimentation since before 6500 yr B.P. (Funnell and Pearson, 1989). Barrier-connected marshes vary greatly in age (Pethick, 1980), reflecting discontinuous evolution of the protective barriers. All are highly inorganic. The source of this material has not been firmly established, but erosion of Quaternary cliffs to the southeast in Norfolk and Suffolk, and to the north along the Yorkshire Holderness coast, presently yields a large volume of fine sediment (McCave, 1987). Marsh sedimentation is initially rapid but decelerates as the tidal prism is infilled. Ultimately surfaces tend towards equilibrium with the twin constraints of sediment supply and local relative sea-level rise (Pethick, 1981; French, 1991 and in press). Regional subsidence (quasi-linear) has occurred at approximately l mm yr 1 over the last few thousands of years (Woodworth, 1987; Shen-

TIDE-DOMINATED BACKBARRIER SALT MARSH: NORFOLK, UK

317

ScoI! Head Island ~

/The ~___Wash

Hut Marsh

Brancaster km

m

Barrier

[

I Salt Marsh

~

~'~Burnham Overy o Reclaimed n

.....

km

NoRT.

B

: SEA:"

o

0I. 200 j ~ " ~ ~ ~r~metres

oB7

Vegetated shingle Sand dune

oB6\

Bs\

NORTON. CREEK:-II

• :i ~ "

'

•

Sites 1-83

Fig. 1. (a) Locality map showing north Norfolk coast; (b) Hut Marsh. Note integrated creek drainage networks, partial enclosure of marsh surface by dune-covered shingle barriers, and location of the main accretion sites (1-83); (c) Inset of the northwestern corner of Hut Marsh, showing the location of 9 sediment trap sites (BI-B9) and array of 18 closely-spaced accretion monitoring sites (A1-A18). nan, 1989) c o m p a r e d to a eustatic rise o f approximately 0.9 m m y r - 1 this century (Pirazzoli, 1989). Overall relative sea-level rise at present is, therefore, approximately 2 m m y r - a. D a t a presented here refer to H u t Marsh, Scolt H e a d Island (Fig. l a and b). This is approximately 0.54 km 2 in extent and is enclosed by an outer dune-covered barrier and by low shingle/dune laterals. The marsh surface lies between 2.6 and 3.2 m O.D. (mean 2.8 m O.D), and supports a diverse halophytic plant cover. L o w and intermediate sur-

faces are d o m i n a t e d by Aster tripolium, with Salicornia spp., Puccinellia maritima, Triglochin maritima, Spergularia marginata and (locally) Spartina angliea; whilst the highest surfaces support a mixed marsh c o m m u n i t y comprising Limon-

ium vulgare, Armeria maritima, P. maritima, S. marginata and T. maritima (the General Salt Marsh c o m m u n i t y o f C h a p m a n , 1974). A n extensive zone o f Halimione portulacoides, a shrubby perennial, fringes the entire channel network but extends also to form a continuous cover across large areas o f

3t8

J R F R E N C H A N D T. S P E N C E R

the middle marsh. Suaeda fruticosa, rooted in shingle, defines the marsh perimeter. Early Ordnance Survey map editions and historical evidence suggest a transition from intertidal flat to vegetated salt marsh in the late 19th century (Allison, 1985; French, 1989). Essential characteristics of the muddy marsh sediments are given in Table 1. Values for loss on ignition are low in comparison with many meso- and micro-tidal sites (Kolb and Van Lopik, 1966; Frey and Basan, 1985), indicating the importance of externallyderived clastic material in the maintenance of this system. Tidal exchange takes place via two integrated creek systems connecting with Norton Creek, and the Brancaster harbour inlet. However, sheetflow across the marsh front accounts for a significant proportion of the total tidal prism during high spring tides (French and Stoddart, 1992). Along most of their length, the creeks are incised into the underlying tidal flat deposits. These are comprised of clean to muddy medium sands (median 1.95 1.60qb) or medium to coarse shingle with localised shell debris (largely Scrobicularia and Mytilus spp). Marsh surface sediments often exhibit a fine sand mode (3.2-3.0~; Table 1). This may be partly derived from channel sediments entrained during high intensity flow ~transients" associated with the initial stages of overbank flow and marsh inundation (French and Stoddart, 1992: French and Clifford, 1992a,b). Upper marsh deposits formed adjacent to the enclosing dunes typically contain a significant fraction of aeolian sand (French, 1989).

Research design Accretion measurement utilised marker horizons of medium sand (median 1.7qb), laid in 1 m 2

patches along intersecting transects. 83 markers were installed between 1983 and 1986 (Fig. lb). In 1986, a second array of 18 closely-spaced sites (AI AI8) was installed within a large meander of Hut Creek (Fig. lc). The rate of burial, determined from annual micro-cores, yields a measure of net accretion that incorporates short-range compaction due to sediment desiccation and synaeresis but neglects long-range autocompaction. Accuracy is dependent upon both the rate of deposition and the averaging period, and is adequate to permit the resolution of sediment input over time-scales in excess of 1 year. Preliminary results were reported by Stoddart et al. (1989). Here, data averaged over a longer period (1986-1991) are used to characterise spatial variation in sediment input and to calculate the total annual sediment input to the marsh surface. Sedimentation within the northwestern corner of the marsh was measured over single tides and spring tide sequences using surface-mounted ~traps'. These were deployed at 9 sites along a 280 m creek-normal transect (BI B9) and at sites A1 AI8 adjacent to Hut Creek (Fig. lc). The transect sites group naturally as follows: (1) A1 AI8 and B I - B 3 within the creek-margin Halimione zone: (2) B4 B6 interior sites characterised by an Aster-dominated community; and (3) B7 B9 within the high 'mixed marsh" community. The intertidal location precludes the use of cylindrical traps, since the trap must be free to drain at low tide whilst retaining sediment deposited in it. Flat plates, however, have a low sediment-trapping efficiency when constructed of smooth materials (Gardner, 1980). Here, pre-weighed ashless filter papers (9 cm diameter W h a t m a n 542) were laid on plastic petri dish lids, held flush with the marsh

TABLE I Summary characteristics of Hut Marsh muddy sediments, based upon analysis of 82 near-surface samples, classified according to vegetation zone. Figures in parenthesis represent sample standard deviation

'Mixed marsh" n = 19 Aster-dominated n=23 Halimione n=40

Mean size (05)

Sorting (05 s.d.)

Skewness (fl~)

% sand (>63 ~tm)

Ignitionloss (%)

6.6 (0.4) 6.8 (0.2) 6.7 (0.3)

3.9 (0.5) 3.9 (0.4) 4.2 (0.4)

0.14 0.16 0.13

4.2 (4.3) 3.8 (3.t) 5.8 (6.5)

I4.5 (7.6) 13.7 (5.1) 11.1 (4.1)

TIDE-DOMINATED BACKBARRIER SALT MARSH: NORFOLK, UK

surface using metal pins. Experiments with 3 different trap diameters (5, 9 and 11 cm) revealed no significant variation in sedimentation per unit area, indicating minimal 'edge' effects. Traps were deployed in August, October (1987), January and March (1988), according to the scheme in Table 2. After recovery, filters were flushed with distilled water (3 separate 100 ml washes) to remove excess salt, oven-dried at 50°C and re-weighed to 10 -4 g precision. Organic content was estimated by ashing (400°C for 3 hours). Settling flux per unit area can be converted to equivalent vertical accretion rates using dry bulk densities obtained from near-surface core samples. However, since this paper is also concerned with spatial patterns in deposition, trap data are presented graphically in their raw form; this minimises the effect of any errors in the bulk density measurements (which also vary spatially). Conversions to equivalent vertical accretion are made only as necessary. Although traps cannot record net erosion, none of the sand markers were eroded over the measurement period. Also, measurements within the creek systems invariably indicate suspended sediment import into the marsh (French and Stoddart, 1992). Several researchers have observed a film of sediment on the leaves and stems of marsh plants. Alizai and McManus (1980) measured up to 2.6 kg m 2 of sediment stored at any one time within broken reed stems (Phragmites communis) in the Tay Estuary, Scotland. This mechanism may account for 0.6 mm accretion annually, although total accretion was not recorded in their study.

319

Stumpf (1983) determined, however, that sediment retention on Spartina alterniflora could account for up to 50% of the material lost from suspension in a small Delaware marsh. To evaluate the importance of this mechanism, samples of plant material (approximately 200 g) were taken before and after each sequence of over-marsh tides. These were airdried for 24 h and subjected to ultrasonic cleaning in a distilled water/Calgon mixture for 10 minutes, the residue being collected on Whatman 542 ashless filters. Filters were ashed (400°C for 3 h) and the data reduced to weight of sediment per unit area using above-ground biomass derived from 3 replicate harvests (0.25 m 2) at each site. Tidal data were obtained from gauges in Hut Creek and a staff situated on the marsh adjacent to site B9. Elevations and water levels were surveyed to O.D. via local benchmarks (closure < 1 0 m m for accretion sites; < 2 m m for gauge datums).

Results and analysis

Annual sediment input The sand markers proved to be stable references for the estimation of accretion, with a final recovery rate of 91% for sites 1-83, and 100% for sites A1-A18. Interannual comparison of sedimentation is complicated by the existence of a poorly consolidated surface layer (millimetre scale) representing part of the preceding year's deposition (cf. Ranwell, 1964; Letzsch and Frey, 1980). This limits

TABLE 2 Summary of sediment trap sampling scheme Trap deployment

Arrays of 3 traps over complete sequence of over-marsh tides; sites Bl-B9 (2) Singletraps at each of sites AI-A18 for above sequenceof tides (3) Arraysof 3 traps at sites B1-B9 for eachinundation during the above periods (1)

individual tides not sampled due to limited site access

1987

1988

August 12-17 October2-11

January1-7

March 2-10

(12 tides) ••• •

(12 tides)

(9 tides)

(10 tides)

000

000

000

• ••

000

1

000

320

J R. F R E N C H A N D T. S P E N C E R

resolution to __+i mm, unacceptably close to the annual accretion rate in some parts of the marsh. The present analysis is, therefore, restricted to mean annual accretion over the whole 5 year period. Five year mean accretion varied spatially by an order of magnitude, from approximately 1 mm y r - 1 at the back of the marsh to nearly 8 mm y r - 1 mid-marsh, in the vicinity of major channel bifurcations. Accretion isolines (Fig. 2a) indicate a dependence of accretion rate upon both proximity to the major creek systems and elevation. The results differ in detail to those reported by Stoddart et al. (1989). Higher accretion rates on the highest surfaces are attributed to the longer averaging interval. Elsewhere, the data recovery rate was higher, apart from some missing sites between the two creek mouths. The significance of sedimentation gradients lateral to major creeks is more clearly illustrated by the results from sites A I - A 1 8 (Fig. 2b). Here, within an area of essentially uniform vegetation

characteristics (continuous H. portulacoides) and minimal topography (elevation range ~ 0.1 m), the rate of sedimentation varies almost as much over 25 m (from 3 to 8 mm yr -1) as over the entire marsh. This indicates rapid removal of suspended particulates from water masses advected laterally across the creek margins, in contrast to the more spatially uniform settling expected under the classic 'slack water' model (Frey and Basan, 1985). Previous work at this site (French, 1989; French and Stoddart, 1992) has shown that creek suspended sediment concentrations may be as high as 1000 mg 1- 1. By comparison, concentrations over the marsh decline rapidly from 50-150 mg 1 1 close to major creeks to 10-20 mg 1-1 at the back of the marsh. Data for all the marker sites are overlain in Fig. 3a. Sites more remote from the creek network exhibit even lower accretion. Moderately rapid accretion along the marsh edge, adjacent to Norton Creek, is consistent with some sediment transfer occurring independently of the major creeks (French and Stoddart, 1992). The highest rates of

"m...

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25 I metres

Fig. 2. (a) Five year mean accretion isolines (1986 1991: m m yr-1) simplified from computer interpolation of marker horizon results from sites 1-83: (b) 5 year mean accretion isolines for sites A I - A I 8 adjacent to Hut Creek.

TIDE-DOMINATED

BACKBARRIER

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NORFOLK.

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The former explanation is intuitively valid and, as a powerful form-process feedback, accounts well for the observed decline in sedimentation rate with marsh age (French, 1991, in press). It is instructive to consider the distribution of recorded accretion rates over the marsh as a whole, and within the principal vegetation communities (Fig. 4a). Overall, the mean accretion rate for sites 1-83 over the study period is 3.9 mm yr-1. The distribution exhibits a slight positive skew (1~1= 0.2) due to very high rates of deposition at creekmargin sites. Application of the Kruskall-Wallis test (Hollander and Wolfe, 1973)--a non-parametric equivalent of the analysis of variance and F test--shows variation in sedimentation between the three major vegetation communities to be statistically signi~cant at p=0.005 ( H = 16.54). However, the same

o •

o o

6

gs g ~4

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321

UK

10

• Sites A 1 - A 1 8 o o

o

o

o

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2 o

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2.4

2.5

2.6

2.7 2.8 2.9 3 Elevation (m O.D.)

3.1

3.2

•

Fig. 3.

Five year mean annual accretion plotted against (a) distance to nearest creek (order /> 3); and (b) surface elevation. Fitted linear regressions illustrate general trends in the data, although non-linearity is evident in the larger dataset.

....h~Li~"""..o......................................... o ........ ~ " "

o

Sites 1-83

Halimione

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'1

'mixed marsh'

lO

marginal accretion (within the 6 mm yr- 1 contour on Fig. 2a) occur adjacent to the only cliffed portion of the marsh edge. Although proximity to the creek system is the main local control of sedimentation, the broader pattern is strongly related to topography. These controls are scale-dependent, as revealed by Fig. 3b. The expected negative relationship between accretion and elevation is evident at a marsh-wide scale (sites 1-83), but is completely reversed locally (i.e. sites A l-A18). Within similar tide-dominated systems, the link between elevation and sedimentation has previously been attributed to the combined influence of spatial contrasts in annual inundation frequency and in the duration of any given inundation period (Pethick, 1981).

9

8 7

3.4

B

Accretion

Elevation 3.2

o

o°

o

3

m

<¢

.~

s 3 2

2.8

2.6 .o

o o

o

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o ,k.,

8 o

1 0

Creek-margin

Fig. 4. Schematic

Marsh 'interior' Creek-margin

Marsh 'interior'

'box-whisker' plots of mean annual accretion data for: (a) the marsh as a whole and major vegetation zones; and (b) creek-margin and mid-marsh sites, with comparable elevation data. Plots show (1) 10th and 90th percentiles; (2) arithmetic mean; and (3) median or 50th percentile. Horizontal broken line in (a) represents the estimated relative sea-level rise (RSLR).

322

test applied to both elevation and distance to the nearest creek resulted in H = 25.0 and 26.8, respectively. In other words, both variables are themselves highly correlated with vegetation type. Multivariate statistical analyses are, not, therefore, the most appropriate tool for conceptualising the important physical and biological determinants of marsh sedimentation--at least under field conditions. As in Ranwell's (1964) exhaustive regression analysis of sedimentation within a UK Spartina anglica marsh, the empirical constants derived are difficult to interpret in physical terms. The effect of rapid channel-margin accretion on the overall distribution of accretion measurements is shown in Fig. 4b. Here, sites are classified according to those within and beyond 15 m of a 3rd order (Shreve, 1967) creek. Not surprisingly given the high network density, 25 of sites fall into the former category. The mean creek-margin accretion rate of 4.5 mm yr-1 compares with the marsh 'interior' mean of 3.4 mm yr -1. The range of values within both zones is large. Rather intriguingly, the mean elevation of creek margin sites is lower than that of 'mid-marsh' sites. Possible reasons for this are discussed in more detail below, but it further illustrates how the introduction of suspended material via the creek system fundamentally determines the spatial pattern of accretion and may override broader topographic controls. Spatial variation in accretion has important implications for the estimation of marsh sediment budgets from a finite number of point measurements. Volumetric sediment input can be calculated from measurements of vertical accretion, and this can be converted to a mass budget where the nearsurface bulk density at each site is known (French, 1989). Simply extrapolating the arithmetic mean accretion rate of 3.96 mm y r - 1 over the 0.543 km 2 area of the marsh surface gives a volumetric input of 2150 m 3 yr 1 (equivalent to 1011 tonnes yr-1). However, the spatial information content of these data can be better utilised by numerically integrating the computer-interpolated 'accretion surface' used in the production of Fig. 3a. This method, using a 20 m x 20 m mesh size, yields a volumetric input of 1613 m 3 yr -1. This is equivalent to an areal mean rate of 2.97mm yr 1,

J.R. FRENCH AND T. SPENCER

significantly lower than the arithmetic mean rate. If this is taken as a 'best estimate', areal extrapolation of the arithmetic mean rate overestimates the volume flux by approximately 33%. It is argued that the lower areal average rate is a more appropriate indicator of marsh sedimentary status, and one that suggests a much closer adjustment to RSLR (estimated at 2 mm yr-1) than might otherwise be supposed. Both methods slightly overestimate the true budget since the averaging includes no correction for the area occupied by channels (about 6% of the total).

Seasonal variabilitv The sediment trap deployments allow separation of annual accretion into component inputs occurring over shorter time-scales. For convenience, these data are presented as mass deposition per unit area. Total sedimentation during each of the 4 monitoring periods (Fig. 5a-d) is dominated by non-combustibles. Examination by light microscope and SEM showed this to be largely comprised of a varied clay mineralogy with some fine quartz grains. Composite particles were abundant, ranging from simple clay doublets and triplets only a few microns in size, to more complex composites (many containing organic detritus) several hundred millimetres across. The largest composites (up to 1000 lam projected diameter) are readily transported by creek flows, yet settle rapidly over the marsh. Numerous cylindrical to elliptical faecal pellets (100-350 jam in length) are attributed to the gastropod Hydrobia ulvae, which occur at high densities (order 103 organisms m-2) on the marsh. These graze microbial populations on both sediment and plant surfaces (Anderson, 1971). Overall deposition is lowest in August (Fig. 5a), and is associated with low suspended sediment concentrations within the tidal inlet and in the creek systems (French, 1989). August is the only period in which organics account for more than 10% of the material deposited. The largest fraction of identifiable macro-detritus was always found towards the back of the marsh. At times (e.g. during strong westerlies), large quantities of detritus are deposited within the S. fruticosa and dune-

323

TIDE-DOMINATED BACKBARRIER SALT MARSH: NORFOLK, UK

200-

700-

August 1987 (12 bdes)

October1987

600-

150-

5004O0-

Inorganic Organic

100-

3002O0-

50-

lo0E v

~ a

0

0

v 200

100

300- ~, January1988 (9 tides)

300

0

o

~6o

c

400- ~ 200-

3

260

500- ~, March1988

D

~

300200-

100-

1000

~ ° m m

0

u

--..m

....

F

1O0 200 Distancefrom Hut Creek(m)

300

0-o

~oo

~o

~6o

Fig. 5. C u m u l a t i v e i n o r g a n i c a n d o r g a n i c d e p o s i t i o n d u r i n g each of the m o n i t o r i n g periods. N o t e different scale o f vertical axes.

marginal zones (cf. Dankers et al., 1984). Organic deposition exhibits surprisingly little seasonality and is low throughout the year. This is consistent with observations in the Netherlands (Dankers et al., 1984; Oenema and DeLaune, 1988) of a low rate of incorporation of above-ground production into similarly inorganic marsh substrates. However, further observations are needed before the implied detrital fluxes can be properly quantified. All datasets exhibit an rapid distance-decay of sedimentation rate relative to Hut Creek and a secondary sedimentation peak adjacent to a smaller tributary. This confirms the previous observation of Stoddart et al. (1989) that higher order creeks are more effective conduits for suspended sediment transport, and is also consistent with an observed decrease in flow competence during flood-tide inundation of a rapidly bifurcating channel network. Temporal variability in inorganic sedimentation is similar to that observed on the northeastern margin of the Wash (Harper, 1979), but differs from that elsewhere in the UK. In south coast Spartina marshes, for example, Ranwell (1964) correlated rapid sedimentation during the summer

months with substrate stabilisation by bacteria and microalgae (cf. Frostick and McCave, 1979). Seasonal variation in the supply of fine sediment from 'feeder' cliffs (Cambers, 1976) and in the 'background' concentration of fine sediment offshore (Evans and Collins, 1987) appears to be particularly important on the east coast. Prior to a sequence of over-marsh tides, sediment retention on plant surfaces was always extremely low (within the probable experimental error). Sediment adhering to plant surfaces at the end of each period can, therefore, be considered to represent net input. Low sediment retention at the start of each period implies effective removal to the marsh surface, probably largely due to rainwash, but possibly also via grazing and sediment pelletisation by Hydrobia ulvae. Average retention within each vegetation zone is given in Table 3. Although this was easily measurable--the maximum recorded at any one site was 18 g m 2 (site B1; Halimione; March 1988)--it accounts for only 2 to 5% of total deposition. Higher maximum values within the Aster-dominated and 'mixed marsh' zones should be treated with caution in view of the low absolute values. All figures are probably overesti-

324

J R F R E N C H A N D T. S P E N C E R

TABLE 3 Inorganic sediment retained on above-ground plant surfaces within each vegetation zone after sequences of 9-12 over-marsh tides. Data are averages of 3 replicate measurements at each of the 3 sites within each zone. Figures in parenthesis represent the standard deviation

Halimione

August October January March Fraction of total deposition

Aster-dominated*

General Salt Marsh*

Biomass (g m -z)

Sediment(g m -2)

Biomass

Sediment

Biomass

Sediment

2163 (100) 1674 (213) 1651 (191) 1678 (194)

3.3 (0.49) 4.9 (0.81) 10.1 (0.32) 13.8 (3.6) 4.7% 5.8%

525 (125) 473 (189) 199 (33) t95 (27)

0.58 (0.75) 0.51 (0.55) 1.97 (0.26) 2.24 (0.15) 3.8% 8.5%

162 (32) 143 (15) 73 (25) 67 (20)

0.083 (0.015) 0.11 (0.012) 0.093 (0.012) 0.17 (0.017) 2.2% 5.7%

Mean: Max:

*All species.

mates, since small fragments of plant material inevitably contaminated the sediment samples. In these marshes, therefore, direct settling is the principal sedimentation mechanism and the significance of vegetational retention should not be overstated.

Sedimentation during individual tidal cycles Traps deployed over individual tides reveal remarkable consistency in the spatial pattern of sedimentation relative to the creek system (Fig. 6). More interesting are plots of mean single tide deposition against tidal height (Fig. 7). The August

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1987 sequence of over-marsh tides, characterised by calm sea conditions throughout, shows a strong positive relationship between sedimentation and tidal height over the length of the transect (Fig, 7a). Close to Hut Creek, anomalously large sediment input occurs over the first few tides. Tides 2 and 3 (Fig. 7a) thus plot above the general trend in the data. This may be due to resuspension (under peak flood velocities) of poorly consolidated muddy sediments deposited within the creeks during preceding neap tides. Although evident from visual observation, this is difficult to confirm with creek sediment flux measurements, partly because gauging errors are large in relation to residual sediment movement (French and Stoddart, 1992). A similar effect is evident within the March 1988 data, where the first 2 tides that inundate the marsh also plot well above the underlying trend (Fig. 7c). Given a near-surface bulk density of 610 kg m 3, sedimentation during each of these tides is equivalent to 0 . 1 4 m m vertical accretion. In comparison, the annual accretion recorded by the nearest sand marker is 6.1 mm. Approximately 240 tides flood this site annually. Choppy sea conditions during October (Fig. 7b) result in a more confused pattern. Several relatively small tides introduced disproportionate amounts of material. Notably, near-gale conditions on the evening of October 8 (westerlies gusting to 60 km h - ' ) resulted in enhanced nearshore sediment mobilisation and high rates of deposition within

325

T I D E - D O M I N A T E D B A C K B A R R I E R SALT M A R S H : N O R F O L K , U K

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Fig. 7. Single tide deposition (mean for each vegetation zone) plotted against tidal height. Vertical bars represent standard error of data; very low errors omitted for clarity. Note different scale of vertical axes. In (a) and (c), numbers indicate order of tides within the time-sequence of individual inundation cycles.

the Halimione zone. Southwesterly and westerly winds coincide with the longest fetch within the backbarrier inlet and can result in appreciable wave action over the marsh surfaces-sufficient for ripple formation in the sandy sediments of high marsh salt pans. Wave resuspension of finer material transported beyond the creek-marginal zone may account for the low deposition rates observed at high marsh sites. The expected link between sedimentation and tidal height can, therefore, be obscured by (1) spring-neap deposition and resuspension cycles within the creek systems; (2) sediment mobilisation by local wind waves; and (3) high background sediment concentrations offshore (which effectively

represent a lagged input from more distant meteorological disturbances). Discussion Annual accretion measurement

Vertical marsh growth in North Norfolk is controlled by the accumulation of externally-derived fine clastic sediment. Most tidal exchange of water and materials occurs via creek systems. However, recent gauging studies indicate that sheetflow across the marsh edge accounts for up to 40% of flood-tidal exchange during the highest spring tides (French and Stoddart, 1992). This greatly compli-

326

cates the interpretation of residual transports calculated for creek mouth sections (cf. Stevenson et al., 1988). Direct measurement of surface deposition offers a more appropriate methodology for the assessment of marsh sedimentary status. In highly inorganic marshes, sediment dewatering is relatively rapid and reduced rates of autocompaction (French, 1989) render artificial marker horizons a reliable indicator of contemporary accretion. The arithmetic mean accretion rate for Hut Marsh is approximately 3.9mm yr -1, well in excess of the estimated local RSLR of 2 mm yr 1 This accretion rate is below transect-average rates of 5-14 mm yr 1 recorded in an Essex open coast marsh (Reed, 1988) and 4-15 mm yr-1 in estuarine marshes within the Eastern Scheldt, in the southwest of the Netherlands (Oenema and DeLaune, 1988). Both those sites are influenced by higher background suspended sediment concentrations offshore and over the adjacent tidal flats, and both are subject to more rapid sea-level rise. Within Hut Marsh, the rate of accretion varies over nearly an order of magnitude ( < 1 to > 8 mm y r - 1). This is related to two fundamental controls: (1) elevation-dependent frequency of inundation; and (2) proximity to the creek network as an intermediate sediment source. Accretion and marsh elevation

The relationship between accretion and surface elevation was emphasized by Steers (1938, 1946) following early marker horizon experiments on the Norfolk salt marshes. Pethick (1981) and French (1989, 1991, in press) have also demonstrated the importance of the elevation-inundation-sedimentation linkage in determining long-term marsh growth trajectories. Both local and areallyaveraged accretion rates are strongly timedependent and, in the absence of complex eustatic forcing, decline asymptotically as marsh infilling of the tidal prism proceeds (see also Allen, 1990). Consequently, the relation between historical and contemporary accretion rates may be rather different to that observed in more organic substrates. In the latter case, dewatering of marsh peats contributes to a significant disparity between

J.R,

FRENCH

AND

T. SPENCER

more rapid accretion rates observed in near-surface sediments (e.g. by marker burial) and lower depthaveraged rates based upon 14C and 2X°pb geochronology (Redfield, 1972; Orson et al., 1985; Stevenson et al., 1985). North American estuarine marshes, for example, often exhibit an apparent acceleration in accretion towards more recent times, although the effects of dewatering and compaction may be conflated with those of enhanced sediment runoff following European settlement (Brush, 1984; Kearney and Ward, 1986). In contrast, the asymptotic reduction of the tidal prism that characterises inorganic systems may result in historical accretion rates that are comparable with or even exceed present accumulation. Historical evidence shows that Hut Marsh is 100 (+20) years old (French, 1989). Augerholes sunk to the marsh-tidal flat contact at the 83 principal accretion sites show that 0.1-1.5 m of muddy sediments have accumulated over this period (Fig. 8). As with contemporary accretion the arithmetic mean deposit thickness is a biased estimate of historical deposition. However, a simple comparison of the creek-margin sites (classified as before) shows that the historical rate of 5.8mm yr I (range assuming +20 year age, error=0.49 to 0.73 mm yr-1) exceeds the present creek-margin accretion rate of 0.45 mm yr ~. Despite rapid accretion along channel margins, these areas remain, on average, lower than the interior of the marsh (Fig. 5b). This reflects the 4

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300

400

500

600

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700

800

900

1000

(rn)

Fig. 8. Comparison of landward gradients of present marsh surface and of pre-existing muddy sand flat. Based upon augerholes sunk at sites 1-83, aligned onto single N-S transect.

327

TIDE-DOMINATED BACKBARRIER SALT MARSH: NORFOLK. UK

inheritance of a seaward-sloping tidal flat topography and its levelling as the tidal prism is progressively infilled. Sedimentation proceeds from a low initial starting elevation along the margins of the tidal flat channels, but ultimately results in the stabilisation of these into channels having a much lower width:depth ratio. Levee formation, and the transition from broadly convex to concave topography is a feature of the older marsh surfaces (French et al., 1990). The relative gradients of the basal tidal flat and present marsh surface (Fig. 8) indicate that Hut Marsh exists at an intermediate stage. Clearly, not just the mean rate but also the spatial pattern of accretion will vary between marshes existing at different developmental stages.

Hydrodynamic controls on sedimentation Whilst elevation accounts for a substantial proportion of the variance in accretion at a marshwide scale, its effect can be counteracted at a local scale due to progressive particle settling as water masses are advected away from feeder creeks (Fig. 3b). In contrast to fluvial systems, where channel-floodplain sediment exchange may be dominated by diffusion along lateral concentration gradients (Pizzuto, 1987), horizontal advective processes are more important within the highly unsteady flows which inundate these marshes. Over the marsh surface, a quasi-exponential decline in the vertical settling flux away from the creek margin might be expected as a result of rapid settling of composite particles ('flocs'), with finer material experiencing progressively shallower settling trajectories as a given water parcel moves across the marsh. French (1989) estimated median settling velocities of overmarsh suspended particulates adjacent to Hut Creek to be of the order 3-8 x 10 -4 m s -~. Thus, in contrast to the findings of Stumpf (1983), direct settling removes a large proportion of the total load within a short distance of a source creek (Figs. 5 and 6). Though the detailed pattern of sedimentation will vary between sites, observations of tidal flows and sediment transport within marshes in The Wash (Stoddart et al., 1987) and Essex (Reed et al., 1985) suggest that this model

of creek-marsh interaction is also applicable to open-coast marshes. Marsh sediment budgets Large variations in accretion over comparatively short distances necessitate caution in estimating areal sediment budgets from a finite sample of point measurements. This study has shown that the arithmetic mean accretion rate may be significantly different from an areal average based upon an interpolated accretion 'surface'. Significantly, the areally averaged accretion for Hut Marsh is much closer to estimated regional RSLR than the arithmetic mean of the accretion measurements would suggest. The October 1987 conditions notwithstanding, none of the sediment trap deployments coincided with full gale or storm surge conditions. It is thus possible that, whereas most of the marsh exists in equilibrium with spring tidal flooding, the highest surfaces receive appreciable amounts of sediment only during storm conditions. Comparison of short-term sediment trap deposition and annual accretion at the nearest marker sites supports this hypothesis. Extrapolation of sediment trap deposition at sites A1-A18 reveals a close correspondence with annual accretion at these sites (Table 4a), confirming the reliability of the sediment trap technique, but also suggesting that 'normal' tidal flooding is sufficient to account for the observed accretion. However, the same exercise applied to sites along the creek-normal transect yields very different results (Table 4b). Sedimentation within the creek-margin Halimione is more than adequate to account for the estimated annual accretion of 6.1 mm, yet elsewhere along the transect there is a large mismatch between actual shortterm deposition and that implied by the annual rates. Within the high 'mixed marsh' zone, extrapolation of sedimentation occurring during normal tidal conditions accounts for only 11% of the 5 year mean accretion. Whilst some of this deficiency may be accounted for by aeolian input from the nearby dunes, there is some evidence that enhanced sedimentation during occasional storms is important in maintaining these sites. In February 1990, several days of northerly gales

328

J R. FRENCH AND T. SPENCER

TABLE 4 (a) Comparison of annual accretion and time-extrapolated sediment trap data for sites AI AI8. The mean near-surface dry bulk density for the 18 sites is 610 kg m -3 Trap deposition

Equivalent accretion (mm)

August 1987 October 1987 January 1988 March 1988

0.8 2.5 1.8 3.4 (total 43 tides) ~ = 8 . 5 Approximately 270 tides y r - ~ inundate this area of marsh to a depth of at least 0.1 m 270 Predicted accretion (mm yr 2) × 8.5 = 5.3 43 Measured accretion 1986-1991 (mm y r - 1) = 5. t (b) Comparison of annual accretion rates at nearest marker horizon sites with extrapolated sediment trap data Zone Mean elevation (m O.D.):

Halimione 2.8

Aster-dominated 2.9

GSM 3.1

Mean annual accretion (mm yr -1) Dry bulk density (kg m -3) Equivalent mass flux (g m -2)

610 610 3721

2.86 490 1401

1.49 410 611

Monitored tides as percentage of annual inundations Extrapolated trap mass flux (g m -2)

18% 4337

21% 724

34% 68

followed a succession of deep depressions in the North Atlantic. Storm surge conditions on February 26, resulted in maximum still water levels of 4.65 m O.D. (J. French and T. Spencer, unpubl. data)--sufficient to inundate Hut Marsh to a depth of 1.4-2.0 m. Upper marsh areas received a discontinuous drape of fine mud with a maximum thickness of 0.7-0.9 mm after desiccation. Under a strong northerly gale, significant waves ( > 0.5 m height) may be generated over the marsh, especially during the flood, when wind and tide are in opposition. This may inhibit the normally rapid sedimentation in the vicinity of the feeder creeks. Some of this material may settle on the highest surfaces, where the protection of the outer barrier dunes results in greatly reduced wind stress over a narrow strip of marsh. Events of this magnitude appear to be important in maintaining the sedimentary balance of high marsh surfaces over a 101 year time-scale. Obviously, comparisons of marsh accretion with independently estimated trends in relative sea level, require careful consideration of the relevant time-

averaging interval. This needs to allow for (l) short-term sediment dewatering and compaction: (2) 'nonstationarity' in the accretion rate due to changes in marsh elevation relative to the tidal prism; and (3) the effects of infrequent storm events. Hut Marsh is the subject of a continuing programme of sedimentation monitoring aimed at further elucidating these controls. Conclusions

(1) Over a 5 year period, vertical accretion in Hut Marsh varies from 8 mm yr -1 adjacent to larger channels to less than 1 mm yr-1 on the highest surfaces, remote from the creek network. Accretion measurements provide a more secure basis for estimating marsh sediment budgets than a limited number of creek fluxes, but careful attention must be paid to problem of spatial averaging. In the case of Hut Marsh, areal extrapolation of the arithmetic mean accretion rate leads to overestimation of total annual deposition by more than 30%.

TIDE-DOMINATEDBACKBARRIERSALTMARSH:NORFOLK, UK

(2) Complex spatial variation in sedimentation results from the interaction between (a) elevational control on inundation frequency; and (b) overmarsh hydraulic gradients, with suspended sediment 'exhaustion' occurring along pathways of water movement. (3) The underlying link between tidal height and sediment deposition is occasionally disrupted by meteorological forcing, and by resuspension of muddy sediments stored within the creek system. (4) Sedimentation under 'normal' tidal conditions can account for the maintenance of marsh elevations. However, aperiodic storm events account for a significant fraction of long-term sedimentation on the highest surfaces. Sediment budget calculations should, therefore, utilise a time-averaging interval that (a) incorporates a representative sample of storm events; and (b) is short enough to minimise the effects of progressive trends in marsh elevation.

Acknowledgements The first 76 accretion markers were deployed under the direction of Prof. D.R. Stoddart (now of Univ. California, Berkeley), and in collaboration with Dr D.J. Reed (now of Louisiana Universities Marine Center). Ms A. Murray (Univ. Cambridge) helped collect the 1991 accretion data. This project is part of the Cambridge-UCL Coastal Research Programme.

References Alizai, S.A.K. and McManus, J., 1980. The significance of reed beds on siltation in the Tay Estuary. Proc. R. Soc. Edinburgh, 78B: sl-sl3. Allen, J.R.L. and Rae, J.E., 1988. Vertical salt-marsh accretion since the Roman period in the Severn Estuary, southwest Britain. Mar. Geol., 83: 225-235. Allen, J.R.L., 1990. Saltmarsh growth and stratification: a numerical model with special reference to the Severn Estuary, southwest Britain. Mar. Geol., 95: 77-96. Allison, H.M., 1985. The Holocene evolution of Scolt Head Island. Ph.D. Thesis, Univ. Cambridge (unpubl.). Anderson, A., 1971. Intertidal activity, breeding and the floating habit of Hydrobia ulvae in the Ythan valley. J. Mar. Biol. Assoc. UK, 51: 423-437. Baumann, R.H., Day, J.W. and Miller, C.A., 1984. Mississippi deltaic wetland survival: sedimentation vs. coastal submergence. Science, 224: 1093-1095. Boon, J.D., 1974. Suspended sediment transport in a salt marsh

329 creek--an analysis of errors. In: B. Kjerfve (Editor), Estuarine Transport Processes. Univ. S. Carolina Press, Colombia, pp. 147-159. Brush, G.S., 1984. Patterns of recent sediment accretion in Chesapeake Bay (Virginia-Maryland, USA) tributaries. Chem. Geol., 44: 227-242. Cambers, G., 1976. Temporal scales in coastal erosion systems. Trans. Inst. Br. Geogr., 1: 246-256. Carr, A.P. and Blackley, M.W.L., 1987. Further data on elevational changes and water circulation in a Cumbrian salt marsh. Estuarine Coastal Shelf Sci., 13: 267-275. Chapman, V.J., 1974. Salt Marshes and Salt Deserts of the World. Cramer, Lehre, Germany, 2nd ed., 392 pp. Dalby, D.H., 1970. The salt marshes of Milford Haven, Pembrokeshire. Field Stud., 3: 297-330. Dankers, N., Binsberger, M., Zegers, K., Laane, R. and Van de Loeff, M.R., 1984. Transportation of water, particulate and dissolved organic and inorganic matter between a salt marsh and the Ems-Dollard estuary, The Netherlands. Estuarine Coastal Shelf Sci., 19: 143-165. Dijkema, K.S., 1987. Geography of salt marshes in Europe. Z. Geomorphol. N.F., 31: 489-499. Evans, G., 1965. Intertidal fiat sediments and their environments of deposition in The Wash. Q. J. Geol. Soc. London, 121: 209-241. Evans, G. and Collins, M.B., 1987. Sediment supply and deposition in The Wash. In: P. Doody and B. Barnett (Editors), The Wash and its Environment. (Research and Survey in Nature Conservation Series, 7.) Nature Conservancy Council, Peterborough, pp. 48-63. Fitzgerald, D.M. and Penland, S., 1987. Backbarrier dynamics of the east Friesian Islands. J. Sediment. Petrol., 57: 746-754. Flessa, K.W., Constantine, K.J. and Cushman, M.K., 1977. Sedimentation rates in coastal marshes determined from historical records. Chesapeake Sci., 18: 172-176. French, J.R., 1989. Hydrodynamics and sedimentation in a macro-tidal salt marsh, Norfolk, England. Ph.D. Thesis, Univ. Cambridge (unpubl.). French, J.R., 1991. Eustatic and neotectonic controls on salt marsh sedimentation, north Norfolk, England. In: N.C. Kraus, K.J. Gingerich and D.L. Kriebel (Editors), Coastal Sediments '91. Am. Soc. Civ. Eng., New York, pp. 1223-1236. French, J.R., 1992. North Norfolk Coastal Wetlands Scientific Research Bibliography. Dep. Geogr., Univ. College London. French, J.R., in press. Numerical simulation of vertical marsh growth and adjustment to accelerated sea-level rise, North Norfolk, UK. Earth Surface Processes Landforms. French, J.R. and Clifford, N.J., 1992a. Characteristics and "event-structure' of near-bed turbulence in a macrotidal saltmarsh channel. Estuarine Coastal Shelf Sci., 34: 49-69. French, J.R. and Stoddart, D.R., 1992. Hydrodynamics of salt marsh creek systems: implications for marsh morphological development and material exchange. Earth Surface Processes Landforms, 17: 235-252. French, J.R. and Clifford, N.J., 1992b. Estimation of turbulence parameters in intertidal saltmarsh channels. In: R.A. Falconer, K. Shiono and R.G.S. Matthew (Editors), Hydraulic and Environmental Modelling: Estuarine and River Waters. Ashgate, Aldershot, pp. 41-52.

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