Shoreline movement and vertical textural patterns in salt marsh deposits: implications of a simple model for flow and sedimentation over tidal marshes

Shoreline movement and vertical textural patterns in salt marsh deposits: implications of a simple model for flow and sedimentation over tidal marshes J. R. L. Allen ALLEN, J. R. L. 1996. Shoreline movement and vertical textural patterns in salt marsh deposits: implications of a simple model for flow and sedimentation over tidal marshes. Proceedings o( the Geologists' Association, 107, 15-23. A recently described analytical-numerical model for flow and sedimentation over tidal salt marshes affords a new interpretative tool which, since the shoreline is the effective source of the suspended fine sediment advected over the marshes, permits movements of the muddy shores of estuaries, tidal embayments, and open coasts to be inferred from vertical textural patterns in the accumulated sediments. The use of the model is illustrated by vertical patterns in two closely sampled salt marsh deposits from the Severn Estuary. In each case, something of the development of the marsh is known from historical maps and air photographs. Particularly over the last 50-100 years, there is good agreement between the inferences from the model and from the documentary sources.

Postgraduate Research Institute for Sedimentology. The Universitv of Reading. Whiteknights. Reading RG6 MB.

1. INTRODUCTION Tidal salt marshes contribute substantially to the landscapes and sediment sequences of the lowland coastal zone in the United Kingdom (Burd, 1989; Davidson, Laffoley, Doody, Way, Gordon, Key, Drake, Pienkowski, Mitchell & Duff, 1991), mainland Europe (Chapman, 1960; Verger, 1968; Dijkema, 1984; Tooley & Jelgersma, 1992), and North America (Johnson, 1925; Stevenson, Ward & Kearney, 1986). Some understanding of the morphodynamics of these marshes has emerged from field studies of flow (Myrick & Leopold, 1963; Pestrong, 1965; Boon, 1975; Settlemeyre & Gardner, 1977; Bayliss-Smith, Healey, Lailey, Spencer & Stoddart, 1979; Healey, Pye, Stoddart & Bayliss-Smith, 1981; Reed, Stoddart & Bayliss-Smith, 1985; Stoddart, French, Bayliss-Smith & Raper, 1987; Stoddart, Reed & French, 1989; Reed, 1988; French & Stoddart, 1992; Leopold, Collins & Collins, 1993) and turbulence (French & Clifford, 1992a, b) in tidal creeks, the two-dimensional variation in small marshes of the sediment deposition rate (Hartnall, 1984; French & Spencer, 1993), and the spatial variation, mainly along what amount to transects, in the grain size of the deposited sediment (Pestrong, 1972; Stumpf, 1983; Collins, Collins & Leopold, 1987; Allen, 1992). Fundamental insights into the modus operandi of tidal marshes have come from a recently described, simple, one-dimensional model for fluid flow (Allen, 1994) and sedimentation (Woolnough, Allen & Wood, 1995) over them. This analytical-numerical model, while oversimplified in some respects, identifies a number of the basic parameters that govern the flow and sedimentation processes, and predicts spatial patterns of flow velocity, sedimentation rate and grain size for prescribed tidal and boundary conditions. The purpose of the present Proceedings of the Geologists' Association. 107, 15-23.

paper is to show that the model also has important implications for the development and interpretation of secular sedimentary patterns in salt marsh deposits, that is, vertical stratigraphic patterns such as might be observed on a marsh cliff or from a borehole. Field work in many estuaries and tidal embayments (e.g. Van Straaten, 1954; Verger, 1968; Redfield, 1972; Allen & Rae, 1987; Allen, 1993) has shown that active tidal marshes typically are complex, consisting of a number of erosively-related and off-lapping morphostratigraphic elements generated as the result of a series of coastal advances and regressions with a number of possible causes, none of which need operate alone. The model offers further criteria for elucidating the behaviour of the coast during the build-up of each morphostratigraphic unit or series of units. These indicators, calling for the application of no more than routine laboratory techniques, complement the field criteria for the identification of shoreline movements already comprehensively described (Allen, 1993).

2. THE MODEL AND ITS IMPLICATIONS Outline of model The model and its implementation are fully described elsewhere (Allen, 1994; Woolnough et ai., 1995), but a brief review is appropriate here. In the model (Fig. I), a salt marsh is idealized as a horizontal platform bounded on one side by a body of deep water (e.g. channel or major creek), the source of the water and sediment advected to the marsh during a tidal cycle. On the other side, the marsh is limited by either a physical barrier (e.g. barrier island, bed-rock uplands) or a water confluence-parting separating two or more creek networks, depending on the scale and context 0016-7878/96 $07·00 © 1996 Geologists' Association

16

J. R. L. ALLEN

high velocity, compet..ce, capacity

low velocity. competence, capacity _tide marsh platform

barrier

"channel

~nL

J:l==::::::::::==~~~. .-:': distance landward

I:l ~--_..:~.i i:l :...t-------~~.j distance landward

distance landward

~~ 1

-.:

~------~~!

distance landward

Fig. 1. Qualitative summary of an analytical-numerical model for flow and sedimentation over tidal salt marshes.

point on the marsh, the flow velocity declines as the flood tide rises to high water but, after reversing in direction, increases with the development of the ebb. Whether particles of a single settling velocity are modelled, or populations unimodal in terms of settling are employed, the amount of sediment deposited on the marsh over a tidal cycle declines with increasing distance from the waterbody. In the case of a hydraulically unimodal population, the mean settling velocity of the sediment deposited over a tidal cycle decreases with increasing distance across the marsh from the water-body. Other things being equal, populations of particles with a relatively high settling velocity fall out over shorter horizontal distances than 'finer' populations. Populations also settle out over shorter distances as the height of the marsh increases relative to high water, that is, as the marsh grows increasingly mature. The precise forms of the deposition rate-distance and texture-distance curves, however, vary with the position of the marsh in the tidal frame and the general calibre of the sediment available for advection across its surface. In extending this 'one-tide' model to a secular time-scale, no account has been taken of the small, high-frequency fluctuations of grain size which may be expected to arise from the multi-periodic character of real tidal regimes, which are commonly registered in sediments as complex patterns of lamination. Where sedimentation rates are relatively high, for example, spring-neap, textural variations can be registered in a conventionally-spaced vertical sequence of textural samples. Similarly, the 4-5 year modulations of the lunar nodal cycle could find expression in suites of samples collected from more mature marshes that were building slowly.

Implications of the model for vertical textural patterns of the marsh as a whole. The low-momentum, lowcompetence, low-capacity environment of the tidally submerged marsh platform may be contrasted sharply with the high-momentum, high-competence, high-capacity condition of the waters feeding it from the adjoining channel or major creek. Suspended fine sediment carried over the marsh edge toward the interior should at once begin to settle out. Ignoring both friction and turbulence, the model therefore explores, analytically and numerically in a time-stepping mode over the relevant portion of a tidal cycle, simply the implications of the equations of fluid and sediment continuity. The fine sediment supplied to the marsh is assumed to (a) be suspended at a vertically uniform, steady concentration in the water-body adjoining the marsh, (b) travel horizontally over the marsh at the speed of the ambient fluid, and (c) sink vertically toward the surface of the marsh at the speed of a solitary particle in an unbounded fluid. The chief results from the model may be summarized as follows (Fig. I). At any instant while the marsh is submerged, the local flow velocity varies linearly with distance across the marsh, decreasing away from the water-body during the flood but increasing toward it on the ebb. At any

Although the calculations so far made under the model were executed for single tides, the results obtained have important implications for the evolution of texture in sediments accumulating at a fixed station on a marsh that is building up while the shoreline varies in horizontal position. The following five cases suffice to illustrate these implications.

Case I Case I represents neutrality, for the marsh, originating as a mudflat, builds up within the tidal frame while its seaward edge maintains a constant horizontal position (Fig. 2a). The model predicts that, as the marsh grows upward toward the level of the highest astronomical tide, the sediment deposited at the position of the profile station becomes progressively finer grained. Hence the sequence of states defines a 'shoreline-neutral' trend. The precise form of this trend will vary with the tidal and sediment-supply regimes and with position on the marsh, that is, it will be system- and, to some extent, site-specific. Confirmation of this general prediction of the model comes from the Severn Estuary, which is a muddy system in southwest Britain with an extreme tidal range of 14.8 m (Avonmouth). A recently

"-'4

17

MODELLING FLOW IN TIDAL MARSHES

HAT groin size 4

sampled profile and the marsh-edge and, in terms of the model, defines a textural profile with a more rapid rate of upward grain-size decline than the neutral curve.

h 2

HAT

Case III In Case III (Fig. 2c) we see the build-up of a marsh in parallel with the monotonic, landward retreat of the marsh-edge at a gradually developing, erosional cliff, a form recorded from many salt marshes (Allen & Rae, 1987; Allen, 1989, 1993). The marsh-edge retreats progressively nearer to the profile site in this case, so that the sequence of states (l-t4 now defines, according to the model as outlined, a textural profile that lies largely to the right of the neutral curve. Grain size changes monotonically, and may either decrease upward more gradually than the neutral trend or increase upward if the rate of shoreline retreat is sufficiently large.

HAT sampled profile odvonee-.

HAT ; / marsh groin size . - retreol

/ HAT (el

(b)

Fig.2. Model illustrating the development of the textural profile in a mudflat-marsh under (a) Case I (neutral shoreline), (b) Case II (simple shoreline advance), and (c) Case III (simple shoreline retreat).

concluded two-year survey of mudflats and salt marshes in the middle and outer estuary has shown that the marsh deposits, formed within the uppermost few metres of the tidal frame, have average sand and clay contents of respectively 3.6% and 37.0% by weight. Average sand and clay contents of respectively 10.5% and 32.1 % were measured on samples from the higher mudflats. occurring a few metres lower in the intertidal zone. Hence the neutral curve, representative of temperate-zone tidal flat-salt marsh environments generally, appears to be a very weak trend.

Case /I Beginning with a mudflat, Case II involves the build-up of a marsh, the edge of which advances monotonically seaward (Fig. 2b). The sequence of states at times t l -t4 involves an increasing distance between the position of the

~retreat

/ HAT advanee--.

Fig. 3. Model for the development of the textural profile in a mudflat-marsh displaying net seaward advance (Case IV).

18

J. R. L. ALLEN

Case IV Case IV (Fig. 3), representing a marsh showing a net seaward advance over a period, is made by combining an element from Case III (Fig. 2c) with a dominant element from Case II (Fig. 2b). Overall, the model predicts that the upward-fining textural profile (Fig. 3c) lies to the left of the neutral curve, but consists of three sub-trends, the second of which is upward-coarsening, signifying the temporary phase of retreat.

profile lying to the right of the local neutral curve (Fig. 4c). In the example given, the marsh deposits fine upward in grain size over the intermediate sub-trend, denoting the temporary phase of shoreline advance.

Case V Finally, when there is a net landward retreat of the marshedge over a period (Fig. 4), so that one or more morphostratigraphic elements become partly or wholly eroded away, the model predicts a textural profile for the surviving marsh which, although upward-coarsening overall, combines an element of Case II (Fig. 2b) with a dominant element of Case III (Fig. 2c). This is Case V, the grain-size

Background

HAT (0)

som pled prof i Ie

j1/ ""

,--,

l

morsh (

1\ - 1

,. .. ,. .... " ... ,/

/ ... / .... / '

\

I .... / .... /' , - I .... I

14

" - /3

/',~~ /, '1"'1., . . ,. I / .... " 1_'''' , . . . . . '

'2

-., -\ .... / /,"" /' / 1-"'''''' - I / - ,.. .....' , / . -1\ ...... 1 I-I ' I,... \-, /: 1<"'.\ /-\J \:;/\ ..-.I..:l\~l(/-

1_/::1:,:::

l'\,J_ll~I)' / I ....~

'I

..-retreot

~

~

,-;J_'-;'\y<..J.!.,-/,. -11-:~\";,/~~:;-,;~

"V'\ _' I, \""'-; 1:\\ J 'f 1\ '..-1 j " I ::.. I I :... l ... ..:. f ',.. I ~ ) ; J .." 11""''''\' ..... \1/11 1 _ - ' / \ 1 \ , ... .1'_1 / , / ' ' 1 ' / 1 1

=-

/ ' .... \ I \ - ,

_

7::"""7

/'',-\-1 ... ',.--

HAT

3. COMPARISON WITH TEXTURAL PROFILES FROM SALT MARSHES IN THE SEVERN ESTUARY, SOUTHWEST BRITAIN Do the inferences which, using the model, can be drawn from observed vertical textural patterns in marshes accord with the known history of those marshes? An answer to this question can be given by reference to two salt marshes in the middle Severn Estuary - Frampton on Severn and Pill House (Tidenham) (Fig. Sa) - where an intensive study of heavy-metal and particulate pollutants recently took place (Allen, 1988), but for reasons unconnected with the present interest in shoreline development. The cliff which formed the seaward edge of each marsh was closely sampled in 1985, the amounts of zinc, copper, lead, detrital coal and coal-burning residues in each sample of silt being determined in the laboratory using described procedures (Allen, 1987a,b; Allen & Rae, 1987). Measurements were also made of the amount of rubidium, which is an excellent grain-size proxy (Ackermann, 1980; Allen, 1987c), varying inversely with the physical grain size measured as the percentage by weight of particles greater than 10 llm in diameter (pipette method, equivalent sphere). In silts from the Severn estuary this element can be determined by X-ray fluorescence techniques to a precision (one standard deviation) of approximately 2%, that is, 2 ppm in 100 ppm, equivalent to about 1.3% of grains greater in diameter than 10 llm. At both Frampton on Severn and Pill House the historical behaviour of the edge of the salt marsh can be broadly defined from map and air-photographic records at a sampling interval of 13-86 years. There is also at each site independent artefactual and geochemical dating evidence for the age of the marsh deposits.

Pill House, Tidenham

HAT (e)

+- retreot

Fig.4. Model illustrating the development of the textural profile in a mudflat-marsh displaying net landward retreat (Case V).

The profile sampled on the marsh cliff near Pill House (Fig. Sa, b) begins with an erosion surface cut into pale brown silts overlain by a strew of domestic rubbish that includes pottery and other artefacts showing that deposition began not before the last quarter of the eighteenth century and probably not earlier than the early nineteenth century (Allen, 1988). Trends and sub-trends within the textural profile were resolved by first identifying 'breaks' in the vertical pattern by eye, followed by regression analysis. but without having analysed any evidence for the dating of shoreline movements. As the choice of breaks is to some degree subjecti ve, the sub-trends and trends presented below are those which collectively yielded the greatest average level of significance. An upward-fining trend. significant at the 0.1 % probability level, is revealed by the

19

MODELLING FLOW IN TIDAL MARSHES

rubidium (moss ppm) 0

120

100

(b)....... 0·2

140

rubidium (moss ppm)

~m

100

't

•

n

t. --I

.,

." 0·6

(c)

...

•

.t=

Q.

0·2

E

!-.

- - --

• I•

Severn 0·8

.1· ,~

I

It.

1·0

.•/'•

1·2

104

.f:'

10 km 1·6

•I

1·8

• . / occup;-on debris

2·0

.~V-· ~.I

,,@f

erosional surface

2·2

140

0

-----.----

~ 004

l.---..-J

160

• • m •

--~y

.\.

004

II

.t=

Q.

.r••

~ 0'6 0·8

1·0

1·2

160

'f

•I

'/,

.(

1·4

1·6

1·8

;.

}

•

2·0

Fig. 5. Textural profiles (rubidium proxy) in salt marsh deposits of the Severn Estuary: (a) the Severn Estuary and the location of the sampled profiles; (b) profile sampled in 1985 on the marsh cliff at Pill House, Tidenham; (c) profile sampled in 1985 on the marsh cliff at Frampton on Severn. The (implicit) error bar in each rubidium determination is approximately ± 3 ppm.

rubidium proxy over the lowermost 1.87 m of the overlying sequence, the regression equation reading Rb = 148.9 - 0.08572d, (r = - 0.6223) Here and in the formulae to follow, Rb is the rubidium content in ppm by mass and d the depth in centimetres below the marsh surface. Using the additional dating evidence advanced below, the deposition rate over this trend averages at about 19 mm a-I. Three sub-trends - upwardfining, then upward-coarsening, and finally upward-finingwere recognized within the overall trend, as follows Rb = 170.3 - 0.2127d, (215 > d> 128), (r =0.2127) significant at the 0.1 % level; Rb =0.3000d + 108.6, (218) d > 93), (r =0.6763)

which is not significant at the conventional 5% level; and Rb

= 147.8 - 0.079 I 2d, (93 > d > 28), (r =- 0.2459)

which is also not significant at the conventional 5% level. However, note the small sizes of the samples which are not significant to this standard. The trend in the topmost 0.28 m of the profile, described by Rb

= 1.620d = 108.7, (r = 0.9429)

is an upward-coarsening one significant at the 5% probability level. The average deposition rate over this interval was about 5 mm a-I. The horizontal movements of the edge of the salt marsh at Pill House can be broadly defined (Fig. 6a) from a sequence of estate (Gloucestershire Record Office D2700), tithe (Gloucestershire Record Office GDRff11l82) and

20

1. R. L. ALLEN

2000.---"T---r--.----,---,---r----r--.---r----. (a)

1950

d 1900


o

•...

?deposilion beQins

1850

?deposilion beQins

1800

1750

.~

/

- - - -

gleyed soil associated over most of its lateral extent with medieval arable strip fields (Allen, 1988). The character of ceramic field drains let into this surface, combined with map evidence, indicates that the deposition of tidal silt recommenced some time in the third quarter of the last century, apparently, a response to the breaching and permanent abandonment of flood defences in the area, buried portions of which can be seen on the modern marsh cliff. The average deposition rate in this area of set-back was about 15 mm a-I. In terms of the rubidium proxy, the profile displays two upward-coarsening trends (Fig. 5c), but there is no associated field evidence to explain either the break between these patterns or the marked discontinuity within the lower trend. The lower trend, involving grey silts between 0.80 and 1.79 m below the surface of the marsh, is defined by Rb = 0.07879d + 115.6, (r= 004102)

'-----"_-'-_-!-_.L.----'_--'-_........._-'-_'--~

50

a

landward

50

100 150

seaward

50

0

landward

50 100 150 200 seaward

morsh-edQe relative to profile (m)

Fig. 6. Position on a normal to the coast of the marsh-edge at (a) Pill House, Tidenham, and (b) Frampton on Severn relative to the respective sampled profile stations, as shown by historical maps and air photographs.

Ordnance Survey (Gloucestershire Sheet 54) maps and an air photograph (RAF 1946 RPIUKJCPE 1828 Print 3161). These sources point to a seaward migration of the edge of the marsh up to a date approaching the tum of the nineteenth century, followed by a rapid retreat at c. 4 m a-I. This pattern fits well with the long upward-fining followed by short upward-coarsening trend indicated by rubidium and with the presence (Allen, 1988), within the upwardcoarsening trend, of the base of Allen & Rae's (1986) Chemozone III, dated to c. 1945 (see a refinement in French, Allen & Appleby, 1994). Over the period of this trend, the level of rubidium experiences a loss amounting to about 0.27 ppm m- 1 of retreat. Unfortunately, the historical maps are too far apart in time to allow the sub-trends to be identified and dated. The base of Chemozone II (c. 1845), however, was placed (Allen, 1988) within the topmost subtrend of the upward-fining portion of the sequence (Fig. 5b). Taking this evidence at face value, early deposition at Pill House could have been substantially greater (c. 30 mm a-I) than the average rate quoted above, and accompanied by two short-lived advances and a brief intervening retreat of the shoreline.

Frampton on Severn The profile sampled on the marsh cliff west of the village of Frampton on Severn (Fig. 5a,c) commences at a buried agricultural surface marked by a structureless, locally

which is not significant at the conventional 5% probability level. Two upward-fining sub-trends appear to make up this lower trend, as follows Rb

= 177.5 - 0.3100d, (179) d > 130), (r =- 0.9296)

significant at the 5% level, and Rb

= 125.6 - 0.0400d, (130) d> 80), (r = -

0.2195)

which is not statistically significant at the standard level. The concluding upward-coarsening trend, affecting silts in the topmost 0.80 m of the profile, is much stronger than the lower one and starts from a slightly finer base. It is described by Rb

=0.2170d + 120.5, (r =0.7826)

significant at the 0.1 % level. As at Pill House, a series of inclosure award (Gloucestershire Record Office Q/RI68), estate (DI491F54) and Ordnance Survey (Gloucestershire Sheet 40) maps, together with an air photograph (RAF 1947 RPIUKJCPE 2098 Print 3077), gives some impression of the movement of the shoreline at Frampton on Severn over the period represented by the profile (Fig. 6b). A substantial retreat is in evidence since at least about 1900, at a rate close to 2 m a-I, which matches well with the upward-coarsening trend (Fig. 5c) and with the presence (Allen, 1988) within this trend of the base of Chernozone III (c. 1945) at about 0.22 m below the surface of the marsh. Deposition over Chemozone III averaged about 5.5 mm a-I, in agreement with findings at Pill House. The decline of rubidium in the deposited sediment amounts to about 0.1 ppm m- 1 of retreat. The dating of earlier events is unclear from the few chronological points available, but the sharp retreat of the shoreline suggested by the replacement of the lower by the upper upward-fining sub-trend possibly occurred about 1880. The stratigraphy of the marsh cliff at Frampton on Severn is incompatible with the placing of the cliff in 1815 some 25 m landward of the site sampled in 1985. The map

MODELLING FLOW IN TIDAL MARSHES

accompanying the inclosure award (Gloucestershire Record Office Q1R168) is therefore not wholly to be relied upon, although it may be taken to show that the shoreline in the early nineteenth century was not far to seaward of the sampled site. Deposition of the profile began too late for the boundary between Chemozones 1 and II to be registered within the sediments which buried the agricultural surface.

4. DISCUSSION There are many possible reasons for the retreat or advance of the edge of a muddy salt marsh in an estuary or tidal embayment or on an open coast, including channel wandering (Marshall, 1962; Gray, 1972), fluctuations in sediment supply, variations in tidal regime and relative sea-level, changes in the frequency and intensity of strong winds (Allen, 1987d), and climatic changes which lead to variations in the occurrence of sea ice, freeze-thaw or wetting and drying. Moreover, these factors, some local and others regional in character, operate at different timescales and are likely to function in combinations that vary spatially, particularly in large tidal systems. Whatever the particular factors that controlled shoreline movements at Pill House and Frampton on Severn, the vertical trends of grain size (Fig. 5b,c) measured from these sites, when interpreted in terms of a model for flow and sedimentation over salt marshes (Allen, 1994; Woolnough et al., 1995), point to the same general patterns of shoreline movement as are demonstrated by the sequences of historical maps and air photographs that are available for these sites (Fig. 6). Quite modest shoreline movements, of the order of a few tens of metres to lOO m or so, are clearly encoded within the sequence of deposits. Moreover, the textural patterns seem to contain a faithful record of coastal change that, at times, occurred on a scale less than the separation, typically a few to several decades, of the historical 'snapshots'. The agreement between the model and the documentary evidence is particularly good over the last 50-100 years, when there was strong retreat at Pill House (c. 50 years) and Frampton on Severn (c. 100 years), and the independent chronological control is at its densest. The two profiles described unexpectedly reveal what seem to be two contrasted styles of shoreline movement. At Pill House (Fig. 5b) the edge of the marsh appears to have advanced and retreated relatively smoothly, with one textural pattern leading into the next (allowing for the fact that two curves fully describe a classical regression). By contrast, at Frampton on Severn the sub-trends and trends are clearly discontinuous (Fig. 5c), as if periods when the effective source of the sediment reaching the marsh varied gradually in position alternated with episodes of much more rapid change, if not sudden switches. However, there is no break or change in the sequence of silts visible in the field at the levels where new patterns appear. It is possible that at Frampton on Severn, a purely local factor - sudden shifts in the position of the looping channel encircling the wide shoals called The Noose - chiefly determined the behaviour of the mudflats and marshes.

21

In summary, the model for flow and sedimentation on tidal marshes (Allen, 1994; Woolnough et al., 1995) seems to provide a sound basis for the interpretation of vertical textural patterns in salt marsh sequences in terms of shoreline advance and retreat. The use of the model calls for sediment sampling at an appropriate density - a vertical separation equivalent to the order of 5-15 years of deposition was used at the sites described from the Severn Estuary - combined with a routine method of grain-size analysis. A rubidium proxy measured by X-ray fluorescence was used at Frampton on Severn and Pill House, and would be applicable where there was an interest in the geochemistry of the sediments as well as in shoreline movements, but a conventional index of grain size obtained either by the pipette method or laser granulometry would be equally appropriate. It should be noted as a limiting factor, however, that all laboratory methods for the grain-size analysis of cohesive sediments operate on a dispersion of the sediment, in which the particles are present only as individuals, as compared to the complex mixture of grains and aggregates to be found in the water-body from which deposition occurred (e.g. Bryant & Williams, 1983). The methodology based on the model affords results which can be expected to complement what can be learned from the application to areas of active or reclaimed salt marsh of field criteria for shoreline movement (Allen, 1993). Indeed, because vertical deposition on active salt marshes is a process virtually unbroken by erosion, the method has the potential to reveal continuous patterns of change, including movements beyond the range of documentary sources and others of which direct morpho-stratigraphic evidence no longer survives. Where sufficient map and photographic evidence is available, and the grain-size trends are strong enough, it may be possible to establish a local calibration allowing the amount of shoreline movement to be estimated from the extent of the grain-size change within a trend. The methodology is in principle applicable to all except microtidal, storm-dominated marshes, provided that there is a minerogenic component of tidal origin sufficient for separation from organic matter and reliable textural analysis.

5. CONCLUSIONS On the basis of textural and documentary studies relating to two active salt marshes in the Severn Estuary, interpreted in the light of a model for the morphodynamics of tidal marshes, the following conclusions are advanced. (l) An important implication of an analytical-numerical

model of flow and sedimentation over tidal salt marshes is that the grain size of the silt deposited at a fixed station on the marsh will vary over time with movements of the seaward edge of the marsh (i.e. with distance), the effective source of the fine sediment supplied. Grain size decreases if the shoreline is advancing seaward but increases when there is landward retreat. These patterns will be expressed

22

1. R. L. ALLEN

stratigraphically as respectively upward-fining and upward-coarsening trends. (2) Vertical sequences of samples recovered from two salt marshes in the Severn Estuary, the behaviour of which over time is known from map and air-photographic evidence, display textural patterns consistent with the recorded movements of the local shoreline. (3) Because salt marshes are traps for tidal sediment, generally experiencing little or no erosional losses away from the marsh edge, the method of vertical textural analysis should prove to be a sensitive and

quantifiable tool in the unravelling of the response of muddy shorelines to forcing factors, complementing and extending the insights obtainable from direct field evidence.

ACKNOWLEDGEMENTS I am indebted to landowners at Pill House and Frampton on Severn for their interest and the opportunity to work on the marshes, and to Franz Street for geochemical analyses. This paper is Reading University PRIS Contribution No. 389.

REFERENCES ACKERMANN, F. 1980. A procedure for correcting the grain size effect in heavy metal analyses of estuarine and coastal sediments. Environmental Technology Letters, I, 518-527. ALLEN, J. R. L. 1987a. Microscopic coal-burning residues in the Severn Estuary, southwestern UK. Marine Pollution Bulletin, 18, 13-18. - - 1987b. Coal dust in the Severn Estuary, southwestern UK. Marine Pollution Bulletin, 18,169-174. - - 1987c. Towards a quantitative chemostratigraphic model for sediments of late Flandrian age in the Severn Estuary, UK. Sedimentary Geology, 53, 73-100. - - 1987d. Late Flandrian shoreline oscillations in the Severn Estuary: the Rumney Formation at its typesite (Cardiff area). Philosophical Transactions of the Royal Society, 8315, 157-184. - - 1988. Modern-period muddy sediments in the Severn Estuary (south western UK): a pollutant-based model for dating and correlation. Sedimentary Geology, 58, 1-21. - - 1989. Evolution of salt-marsh cliffs in muddy and sandy systems: a qualitative comparison of British west-coast estuaries. Earth Surface Processes and Landforms, 14, 85-92. - - 1992. Large-scale textural patterns and sedimentary processes on salt marshes in the Severn Estuary, southwest Britain. Sedimentary Geology, 81, 299-318. - - 1993. Muddy alluvial coasts of Britain: field criteria for shoreline position and movement in the recent past. Proceedings of the Geologists' Association, 104, 241-262. - - 1994. A continuity-based sedimentological model for temperate-zone tidal salt marshes. Journal of the Geological Society. London, 151,41-49. - - & RAE, J. E. 1986. Time sequence of metal pollution, Severn Estuary, southwestern UK. Marine Pollution Bulletin, 17, 427-431. - - & - - 1987. late Flandrian shoreline oscillations in the Severn Estuary: a geomorphological and stratigraphical reconnaissance. Philosophical Transactions of the Royal Society, 8315, 185-230. BAYLISS-SMITH, T. P., HEALEY, R., LAlLEY, R., SPENCER, T. & STODDART, D. R. 1979. Tidal flow in salt marsh creeks. Estuarine Coastal and Marine Science, 9, 235-255. BOON, J. D. 1975. Tidal discharge asymmetry in a salt marsh drainage system. Limnology and Oceanography, 20, 71-80. BRYANT, R. & WILLIAMS, D. 1. A. 1983. Characteristics of suspended cohesive sediment in the Severn Estuary, UK. Canadian Journal of Fisheries and Aquatic Science, 40 (Suppl.), 96-\01. BURD, F. 1989. The Saltmarsh Survey of Great Britain. Nature Conservancy Council, Peterborough.

CHAPMAN, V. J. 1960. Salt Marshes and Salt Deserts of the World. Leonard Hill, London. COLLINS, L. M., COLLINS, J. N. & LEOPOLD, L. B. 1987. Geomorphic processes of an estuarine marsh: preliminary hypotheses. In (Gardiner, V. J.; ed.) International Geomorphology, Part I. Wiley, London, pp. 1049-1072. DAVIDSON, N. C., LAFFOLEY, D.d' A., DOODY, J. P., WAY, L. S., GORDON, J., KEY, R., DRAKE, C. M., PIENKOWSKI, M. w., MITCHELL, R. & DUFF, K. L. 1991. Nature Conservation and Estuaries in Great Britain. Nature Conservancy Council, Peterborough. DIJKEMA, K. J. 1984. Salt Marshes in Europe. Council of Europe, Strasbourg. FRENCH, J. R. & CLIFFORD, N. J. I 992a. Estimation of turbulence parameters within intertidal saltmarsh channels. In (Falconer, R. A., Shiono, K. & Matthew, R. G. S.; eds) Hydraulic and Environmental Modelling: Estuarine and River Waters. Ashgate, Aldershot, pp. 41-52. - - & - - 1992b. Characteristics and 'event structure' of nearbed turbulence in a macrotidal saltmarsh channel. Estuarine Coastal and Shelf Science, 34, 49-69. - - & SPENCER, T. 1993. Dynamics of sedimentation in a tidedominated backbarrier salt marsh, Norfolk, UK. Marine Geology, 110, 315-331. - - & STODDART, D. R. 1992. Hydrodynamics of salt marsh creek systems: implications for marsh development and material exchange. Earth Surface Processes and Landforms, 17, 235-252. FRENCH, P. w., ALLEN, J. R. L. & APPLEBY, P. G. 1994.210lead dating of a modern period saltmarsh deposit from the Severn Estuary (S.W. Britain) and its implications. Marine Geology, 118, 327-334. GRAY, A. J. 1972. The ecology of Morecambe Bay V. The salt marshes of Morecambe Bay. Journal of Applied Ecology, 9, 207-220. HARTNALL, T. J. 1984. Salt-marsh vegetation and micro-relief development on the New Marsh at Gibralter Point, Lincolnshire. In (Clark, M. W.; ed) Coastal Research: u.K. Perspectives. Geo Books, Norwich, pp. 37-58. HEALEY, R. G., PYE, K., STODDART, D. R. & BAYLISSSMITH, T. P. 198\. Velocity variations in salt-marsh creeks, Norfolk, England. Estuarine Coastal and Shelf Science, 13, 535-545. JOHNSON, D. W. 1925. The New England-Acadian shoreline. Wiley, New York. LEOPOLD, L. 8., COLLINS, J. N. & COLLINS, L. M. 1993. Hydrology of some tidal channels in estuarine marshland near San Francisco. Catena, 20, 469-493.

MODELLING FLOW IN TIDAL MARSHES

MARSHALL, J. R. 1962. The morphology of the upper Solway salt marshes. Scottish Geographical Magazine, 78,81-99. MYRICK, R. M. & LEOPOLD, L. B. 1963. Hydraulic geometry of a small tidal estuary. Professional Papers of the United States Geological Survey, 422-B, 1-18. PESTRONG, R. 1965. The development of drainage patterns on tidal marshes. Stanford University Publications in Earth Science, 10(2), 1-87. - - 1972. Tidal-flat sedimentation at Cooley Landing, southwest San Francisco Bay. Sedimentary Geology, 8, 251-288. REDFIELD, A. C. 1972. Development of a New England salt marsh. Ecological Monographs, 42, 201-237. REED, D. J. 1988. Sediment dynamics and deposition in a retreating coastal salt marsh. Estuarine Coastal and Shelf Science, 26, 67-79. - , STODDART, D. R. & BAYLISS-SMITH, T. P. 1985. Tidal flows and sediment budget for a salt marsh system, Essex, England. Vegetatio, 62, 375-380. SETTLEMEYRE, J. L. & GARDNER, L. R. 1977. Suspended sediment flux through a salt marsh drainage system. Estuarine Coastal and Marine Science, 5, 653-663. STEVENSON, J. c., WARD, L. G. & KEARNEY, M. S. 1986. Vertical accretion in marshes with varying rates of sea level rise.

23

In (Wolfe, D. A., ed.) Estuarine variability. Academic Press, Orlando, pp. 241-259. STODDART, D. R., REED, D. 1. & FRENCH, J. R. 1989. Understanding salt-marsh accretion, Scolt Head Island, Norfolk, England. Estuaries, 12, 228-236. - - , FRENCH, J. R., BAYLISS-SMITH, T. P. & RAPER, J. 1987. Physical processes on Wash salt marshes. In (Doody, P. & Barnett, B.; eds) The Wash and its Environment. Nature Conservancy Council, Peterborough, pp. 64-76. STUMPF, R. P. 1983. The process of sedimentation on the surface of a salt marsh. Estuarine Coastal and Shelf Science, 17, 495-508. TOOLEY, M. J. & JELGERSMA, S. 1992. Impacts of Sea Level Rise on European Coastal Lowlands. Blackwell, Oxford. VAN STRAATEN, L. M. J. U. 1954. Composition and structure of recent marine sediments in The Netherlands. Leidse Geologische Mededelingen, 19, 1-110. VERGER, F. 1968. Marais et wadden du littoral Francais. Biscaye Frere, Bordeaux. WOOLNOUGH, S. J., ALLEN, J. R. L. & WOOD, W. L. 1995. An exploratory numerical model of sediment deposition over tidal salt marshes. Estuarine Coastal and Shelf Science (in press).

Received 7 November 1994; revised typescript accepted 1 May 1995