Sandy Tidal Flat Morphodynamics? Examples from Strangford Lough in Northern Ireland
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Journal of Coastal Research SI 41 pg - pg STRAEE 2002 Ferrara, Italy ISSN 0749-0208
Sandy Tidal Flat Morphodynamics? Examples from Strangford Lough in
Northern Ireland
Gonzalo C. Malvarez†, Fatima Navas†† Javier Alcántara††† and Derek W. T. Jackson‡
† Geografia Fisica , †† Coastal Management and ††† Facultad de Ciencias ‡ Coastal Studies Research
Universidad Pablo de Olavide. Information Systems Res. Group del Mar. Group.
Ctra. Utrera, Km 1, Sevilla. Spain Universidad de Sevilla, Spain Universidad de Vigo, Spain University of Ulster,
Email: gcmalgar@dhuma.upo.es Coleraine, N. Ireland
ABSTRACT
GONZALO C. MALVAREZ, FATIMA NAVAS, JAVIER ALCÁNTARA and DEREK W. T. JACKSON,
2004. Sandy Tidal Flat Morphodynamics? Examples from Strangford Lough in Northern Ireland. Journal of
Coastal Research, SI 41, pg – pg. Ferrara (Italy), ISSN 0749-0208
In coastal research tidal flats are often classified and investigated in the context of estuarine dynamics due to the
widely accepted view that tidal forces are the main responsible for their geomorphological evolution and that
biological variables need to play a major role in its evolution. On the other hand coastal morphodyanmics deals
mostly with geomorphological evolution of beaches taking a purely geological view on sediment dynamics. In
this paper, a discussion is presented in relation to the adequate treatment of tidal flat research by illustrating
various numerical and empirical methods used to address morphodynamics on the sandy tidal flats of
Newtownards. Common to other sandy tidal flats, the intertidal shores of Newtownards in Strangford Lough
(Northern Ireland) are affected by tidal flows (c 0.1 ms-1) given the average tidal range of 3.5 m and the action of
wind generated waves over the fine grained siliciclastic sediments which frequently shows rippled surfaces.
The results from a wide range of investigations (previously published) that utilized mainly numerical modeling
indicated that the flats of Newtownards characterized by a RTR (relative tidal range) factor of 2.3 to 7, are very
sensitive to sediment redistribution due to combined water level / wave height effects which implies that wave
energy dissipation due to bottom friction the primarily shoaling process during relative high energy events.
Using empirical methods designed for beach morphodynamics research, it was found that (i) waves exceeded
predicted values given short fetch under westerly winds, (ii) wave penetration was a parameter that proved
extremely sensitive for sediment entrainment and was not sufficient to generate bedload sediment transport and
(iii) further numerical simulations need to be conducted to achieve further understanding on the role of shoaling
and spatial distribution of wave energy dissipation on the translating surf and nearshore zones to achieve a
complete morphodynamic classification of these ultra-mega dissipative wave dominated environments.
ADDITIONAL INDEX WORDS: ultra dissipative beaches, surf zone migration, wave height sensitivity.
INTRODUCTION muddy tidal flats. Sandy tidal flats are dominated by
hydrodynamic forcing factors and morphologic settings that get
them closer to open ocean beaches and thus a morphodynamic
Coastal geomorphologists have moved from simplistic
approach seems justified since wave induced sediment
classification schemes to rather complex and fine tuned
entrainment and combined wave-current interactions are expressed
morphological and dynamical expressions that are able to
in complex assemblages of sedimentary spatial distributions.
characterize most littoral settings. However, at the very extreme of
Empirical relationship between the equilibrium gradient of
the morphodynamic classifications of ultra dissipative beaches,
morphodynamic zoning and wave and sediment conditions have
sandy tidal flats are commonly ignored in the literature of beach
been assumed for open ocean beach nearshores, but as zones shift
science as the result of the complexity of investigating wave
up and down the intertidal flat with the tide, relative occurrences
shoaling, breaking and swash on a high spatial resolution. Whilst
of each of these zones (morphodynamic zones) over the profile
open ocean beaches have been the subject of a tremendous variety
during a tidal cycle may be assumed to have a wide and mobile
of empirical and numerical studies and three morphodynamic
morphological imprint on tidal flats evolution. Sandy tidal flats
zones (the swash zone, the surf zone and the zone of shoaling
can be also be included alongside the most extreme of tidal
waves) are considered, tidal flats have consistently been the focus
beaches.
of separate research efforts. Dominance of cohesive sediments,
Although tidal flats generally occur in low energetic settings,
extreme flatness of its morphology, higher degree of biological
features may be encountered that suggest that the relative levels of
bioturbation and variable strength of tidal flows as main
energy could be high. CARTER (1988) approached a preliminary
hydrodynamic force built a strong case for the isolation of tidal
classification of tidal beaches from an energetic stand point and
flats in morphodynamic approaches, especially in the case of
described the features to be expected as energy levels decreased
Journal of Coastal Research, Special Issue 41, 2004Sandy tidal flat morphodynamics
from washed out profiles to mixed current ripple environments seem relatively insensitive to wave height across the dissipation
where sinusoidal forces are sometime superceded by the laminar zone, tidal flats appear to respond sedimentologically to variations
flow associated with tidal currents. The lowest energy in wave energy dissipation and wave orbital velocity. The
environments in macrotidal beaches are supposed to be combined effect of water level variation, fetch and tidal prism
characterized by ridge and runnel morphologies as mobile dynamics and related wave penetration presents a very complex
shoaling, surf and swash zones operated on varying water depths scenario for characterization of morphodynamic criteria for tidal
(KING and WILLIAMS, 1949; MASSELINK and SHORT, 1993; flats resulting in greatly fine-tuned models of waves, current,
NAVAS et al., 2001). Following this argument, most sandy flats morphology and sediment grain sizes interactions.
would be morphologically dominated by ridge and runnel The aim of this paper is to contribute in the discussion on
topographies; this, however, seems not always to be the case and weather tidal flat research should be placed in the context of
instead a variety of wave and tide associated forms develop upon coastal morphodynamics or estuarine research through an example
the extreme low gradients of the mega intertidal extents. from Newtownards in Strangford Lough, Northern Ireland. The
Several studies have demonstrated a broad link between wave flats are studied using concepts applicable to macrotidal beaches
energy and tidal flat morphology (BOYD et al., 1992; RYAN & to establish whether some types of tidal flats could be addressed as
COOPER, 1998). At a local scale, CARLING (1982) investigated extreme dissipative environments in a morphodynamic context.
the role of waves by the deployment of a wave pole at a single Drawing on results from numerical simulations previously
location and using time-averaged results as a measure of wave published by the authors in the same line of work in this paper the
intensity. While the results apparently showed a link between results of an experiment are presented in the context of in situ
wave action and tidal flat behavior at the scale of the whole tidal measurements of hydrodynamic and sediment activation and
flat, these results were based on data from a single point and transport during continuous recording sessions.
cannot explain adequately, variability within a tidal flat.
What seems clear is that the rates of wave energy dissipation ENVIRONMENTAL SETTING
(due to shoaling and not breaking) must affect tidal flats in the
same fashion as nearshore regions of beaches despite the The intertidal sandy flats of Newtownards are located on the
morphological adjustments. Tidal currents are a constant feature northern most confines of Strangford Lough (Figure 1). The
on the ultra dissipative environments of tidal flats but the Lough is an elongate embayment with an N-S traveling axis. It is
hydrodynamic effects of them are most acute when combined bordered by bedrock topped by glacial sediments and contains
wave/tide currents are present. In macrotidal environments, many drowned drumlins within its margins. Average tidal range is
however, it is generally accepted that the influence of waves is 3.5m throughout and little amplification of the tidal wave is
diminished, mainly because tidal translation over the intertidal evident in the Lough. The bathymetric configuration of the Lough
region results in rapid movement of surf and swash zones over the in the central regions exceeds 40m in depth whereas the margins
active area and thus tidal currents should play a greater role both of the system are characterized by a variety of intertidal flats,
in absolute terms and relative to local waves (NORDSTROM, bedrock and glacial shorelines.
1992). The intertidal areas are characterized by a flat or gently sloping
The complex interaction of the two forcing factors have been sandy surface intersected by one major drainage channel and a
analysed by many authors but it appears that it is accepted that number of temporally variable tributary channels. These display a
tidal forces have little influence on beach morphodynamics characteristic morphology around the embayment. Examination
(MASSELINK and TURNER, 1999). However, these authors of tidal flat cross sections shows that the intertidal area between
explain that as tidal range increases tidal flows and coupling high tide and low tide is about 1000-1200m along the western
tide/wave interaction plays a significant role particularly affecting shore and only about 300-500m along the Eastern shore. Cross
the directional component of sediment transport. sections of the tidal flat generally have a marked break in slope
Inside estuaries waves may occur as the result of filtering that separates a gently inclined upper tidal flat from a steeper
through its seaward entrance and, mostly, as result of wind lower section leading to the low tide mark at the margin of the
generation inside the estuary. The types of waves generated this tidal and storm channel that drains from Newtownards.
way are typically short crested, high frequency and steep and their Silici-clastic sediments on the flats are generally fine sands
geometry reflects the fetch available given constant wind ranging from 0.10mm to 0.25mm. The wave climate is dominated
conditions. Under steady wind, waves are highest at high tide by wind waves within the Lough with limited fetch distance and
because fetch is generally longer (in onshore wind conditions) but narrow entrance from the Irish Sea. Wind directions affecting the
wave induced stress on the bottom surface may be small because wave climate at Newtownards are typically from the SE.
under short period waves wave orbital motions cannot penetrate to The extensive intertidal regions for shoaling of wind generated
the bed. GREEN and MACDONALD (2001) illustrated that at waves on Newtownards tidal flats are characterized by extensive
some optimum combination of wave/water depth, wave intertidal profiles with an extension that varies from the western to
penetration under steep choppy waves influences resuspension of eastern shores.
bottom sediments. Changing wind conditions and stable water Methods utilized for the investigation of this kind of
levels are responsible for sediment resuspension in lakes given environments often present shortcomings in addressing wave
that wind generated waves are the only force in play that induces related information. Several authors have taken the empirical
water column deep stresses (JIN and JI, 2001). Wave sensitivity to approach to the limit but in situ measurement is never capable to
water level (induced by tidal effects) appears to be a crucial resolve wave propagation, shoaling etc. on a satisfactory level
element in shoaling processes associated with waves acting across given the impracticability of deployment of an adequate number
tidal flats. As is the case on macrotidal beaches, tidal stage of probes and pressure transducers. As background to the
determines how waves affect the bed sediments and also where. experiment described in this article results from numerical
The duration of the effects of shoaling waves on the mobile simulation from previous research have been used (MALVAREZ
“nearshore” dictates potential morphodynamical stages et al. 2001a). These simulations were conducted to overcome the
(MASSELINK and TURNER, 1999). Whereas beach profiles spatial resolution problem and illustrated a variety of issues.
Journal of Coastal Research, Special Issue 41, 2004Malvarez et al.
Figure 1. Location of sample area
Amongst the main findings of this previous work was that location indicated by significant correlations between wave orbital velocity
of sediment grain sizes appeared to be related to wave action as and mean grain size of sediment (Figure 2).
The combined hydrodynamic and morpho-sedimentary data sets
provided input for statistical analysis yielding significant
correlations to determine optimum water levels that affected wave
penetration on the sandy tidal flats. Multidirectional simulations
upon the optimum water level from MALVAREZ et al. (2001b)
3 also identified that wind records could be added to the high
resolution approach, although results demonstrated that further
2.5
O……...….Significant control was needed on water level under variable wave geometries
X…….Not significant generated by real wind conditions.
2
Water Level (m. OD)
1.5 Single point empirical approach
1 An empirical measurement campaign was planned after the
optimum water level had been identified (MALVAREZ et al
0.5 2001a and b) which suggested that further control was necessary
over the water level wave sensitivity relationships. A single point
0 morphological and hydrodynamic experiment was organized to
-0.5 0 0.5 1 cover three events largest tidal events during the month of June
-0.5 2002. In all, sediment transport was measured by a specially
design streamer trap and fluorescent tracers, depth of disturbance
-1 and sediment bed change, tidal currents, wave height, period and
orbital velocities in two sites and water levels in three sites (Figure
Correlation coefficient (Pearson's)
1). All the measurements were recorded in high temporal
resolution over the three high tidal events.
The rational behind the experiment was to measure
Figure 2. Relationship between recorded water level and simultaneously tides, waves and sediment transport (bed load and
statistical coefficient (relating wave action and sediment grain suspended) to establish relationships between the morphodynamic
size). variables. Three time slots, coinciding with predicted optimum
water levels and tidal current velocities were targeted for short
Journal of Coastal Research, (SI STRAEE Workshop)Sandy tidal flat morphodynamics
duration streamer trap measuring periods. Divers would open The trap was located in the instrument array site facing the
sequentially the streamer trap at given times for given duration. incoming flood tide in the first instance and was then rotated after
The rest of the instrumentation (described below) was to record the first tide to measure ebb related sediment transport. The trap
continuous time series and/or depth of disturbance during the three had 12 inlets that were covered with individual shutters for
tides under investigation. selective release by divers. Each column of inlets was coded
identifying a water level within the experiment. The “socks”
(1.2m bags of 63mm mesh) were placed semi-horizontally to
Hydrodynamics retain sediments in low velocity flow conditions.
Depth of disturbance, sea bed position or active mixing layer
A Valeport Ltd. Electromagnetic current meter (Series 800-2 was measured using three methods: A Sediment Activity Meter
axis) was deployed across the flood-ebb tidal current path to (JACKSON and MALVAREZ, 2002) was deployed in the
record flow velocities during the experiment. A Squirrel Grant experiment array site to measure sea bed motion during the tide at
data logger and battery pack was assembled into a stainless steel 124 seconds intervals (Figure 3). Depth of disturbance was
water-tight casing constructed for immersion deployment beside measured using a 100m X 100m D-GPS referenced grid of rods
the probe. The interrogation period for the data logger was set at 5 and washer combination. Relative to this grid, 50kg of dyed sand
minutes. The measuring probe was fixed to a stainless steel frame were injected beside the main experimental site over. Dyed sand
held in position at 1.0 m height. was prepared in the laboratory, using sediment previously
Three Dobie wave and tide gauges (GREEN, 1998) were extracted from the experimental site and fluorescent orange paint.
deployed at three locations to record waves and water levels. Grain sizes were tested in the settling tube to check relationship
Hydrostatic pressure was recorded as time series of 2048 samples between native (non-colored sand) and dyed sand, and results
at 10 minutes intervals during the three events pressure is then were satisfactory (native sand: Median 2.78, Mean 0.154, Sorting
processed through the PEDP software which uses semi-empirical 0.76, Skewness -1.42; and dyed sand: Median 2.75, Mean 0.155,
formulae to establish wave height, period, significant orbital speed Sorting 0.53, Skewness -1.85).
at the bed, wave penetration through the column of water, etc.
Water levels were recorded at Killinchy for temporal meso scale Wind data was also recorded simultaneously at 30 minutes
tidal reference. One of the wave recorders was deployed in the interval from a Davies Weather Wizard III weather station linked
center of the instrument array to establish local wave to PCLink software. Wind direction and speed were logged during
hydrodynamics and the third wave/water level recorder was the three-day experiment
deployed 1.2 kilometers south of the experimental site to record
deeper water wave conditions. Data was recorded simultaneously METHOD
on all wave recorders to enable synchronous post experiment
analysis. Wave modeling was conducted to reproduce nearshore wave
conditions given the initial deep water wave parameters stated in
table 1. The objective was to generate high resolution wave
Sediment Transport parameters in the nearshore for further interpretation. After
simulation of wave propagation from the deep water boundary,
A stainless steel sediment transport Streamer Trap (Figure 3) co-ordinates of the points where wave energy dissipation occurs
was constructed based on a modified design from KRAUS (1987). were used to isolate wave/sediment interaction zones (then
nearshore circulations can be calculated including sediment
Figure 3. Sediment transport Streamer Trap, current meter data logger (left photograph) and Sediment Activity Meter (S.A.M.) (right
photograph) deployed on the sandy tidal flats.
Journal of Coastal Research, Special Issue 41, 2004Malvarez et al.
transport, surf scaling parameter, etc). The geographical extent modification introduced for the interpretation of circulatory cells
and the morphology of the surf zone have, in fact, been used as an in this study are: firstly, the entire surf zone is observed from
indicator of nearshore and beach state and type in earlier research gridded model results of wave propagation. This enables a
itself (MALVAREZ and COOPER, 2000). characterisation of wave induced stress across the breaker but also
Once the geographical extent of the surf zone has been in all other areas where wave energy dissipation occurs, thus
identified, the appropriate phenomena should be considered to improving (overestimating) the active area in the nearshore. This
investigate the magnitude and nature of wave generated processes. interpretation includes a longshore cross-shore approach.
It is significant to focus in the directional components that drive Secondly, instead of taking wave power as the main driving force,
littoral drift, because of the sensitivity of these to be over or gradients in radiation stress are analysed delineating circulations
underestimated in oversimplified modeling approaches. In the surf that can be potentially derived from both longshore and cross-
zone the reduction in water depth and the number of waves shore fluxes across the surf zone.
present induces an excess flow of momentum (KOMAR, 1976) or Points or areas along the coast where force fluxes converge can
radiation stress. It drives the changes in mean water level (wave be labelled fixed boundary ´a´/´a´. The location of maximum
set-up) at the shoreline and is directly caused by the presence of intensity in the gradient would be the ´c´ point and the end of the
waves (LONGUET-HIGGINS and STEWART, 1964a). cell, understood as the end of potential transport given by a zero
The application of the concept of radiation stress has also crossing point in the flux is the ´e´ point. Fixed points delimit cells
helped in the development of theories of nearshore current which diverge in intensity and direction. These environments tend
generation (KOMAR, 1975). With shoaling waves and no towards reflective modes. Areas of convergence are mainly
externally driven current, circulations are generated in the surf tending to dissipative morphodynamic environments.
zone due to a decrease in radiation stresses (LONGUET- Bathymetric charts need to be digitized from high resolution
HIGGINS, 1970; DEIGAARD et al., 1986). The gradients of the records to enable reasonable output resolution. In this study data
radiation stresses are also present outside the breaking region and sets from the hydrographic office (Instituto Hidrografico de la
are induced by shoaling and refractive processes (LONGUET- Marina) were digitized, georeferenced and interpolated for
HIGGINS and STEWART, 1964b). gridding input to the wave modeling package. The initial high
The directional component of the radiation stress is taken in this resolution of the soundings in the charts (the data was the
method as the driving force generating circulatory cells across the sounding logs from the survey on 1:25000) enabled an
surf zone. “interpolated” resolution of 10 m2 without too much generation of
In the original concept of littoral cells (e.g. MAY, 1974; spurious artificial data.
STAPOR and MAY, 1983; STONE et al, 1992) a quantitative Wave records were taken from the oceanographic stations
interpretation is made from the available wave power at the deployed all around the coastlines of Spain by the port authorities
breaker point. Cells vary in response to changing wave conditions (Puertos del Estado) of the Ministerio de Fomento. The records
and results were particularly susceptible to small variations in extended back to 1985 and a total of 10 years were analysed to
deep water wave approach. Based on the interpretation of the establish significant wave height, zero crossing period and mean
directional component of the wave power boundaries were wave direction.
established including free and fixed boundary types. The main
Figure 4. Wind speed and direction recorded at Newtownards meteorological station during the experiment at 30 minute interval.
Journal of Coastal Research, (SI STRAEE Workshop)Sandy tidal flat morphodynamics
Figure 5. Hydrodynamic data recorded during the experiment: a) water levels, b) wave orbital velocity and c) tidal current velocity.
RESULTS Hydrodynamics
All recorded data was downloaded and entered onto a database Water levels and wave records are shown in Figure 5a for the
for analyses. duration of the experiment. The three tides recorded can be
Wind data (speed and direction) was also recorded during the identified and an asymmetry in the flood/ebb recognized. The
experiment. The partial record over the experimental period relative tidal envelope on the site was small reaching a maximum
(Figure 4) shows an increasing wind velocity towards the third of 2m. during high tide. Waves reacted to the expansion of the
tidal event coinciding with a change in direction to take a full tidal prism (and fetch) achieving maximum wave height during
WNW approach during this high wind period. This resulted in the the high tide of the third tide of the experiment. Values for orbital
generation of steep waves during the latter part of the experiment. velocity (Figure 5b) also peaked during the third high tide. Wave
period remained stable during most of the experiment but with
very low values (high frequency waves), which would be expected
Journal of Coastal Research, Special Issue 41, 2004Malvarez et al.
under locally, generated waves. The significant short periods measurements of depth of disturbance should have yielded results
combined with relatively high waves depicts geometry of very despite the fact that the slope (as a controlling factor) was much
steep and irregular waves inducing high orbital velocities but lower than on beaches. Thus the lack of sedimentation under
penetrating very little in the column of water as a result of short waves of significant potential for activation shows that the
wave lengths. temporal scales under which beaches operate may not be
Tidal currents were low during the three tidal events despite comparable to that of tidal flats.
Spring tide conditions. The current meter recorded values outside
the main drainage channels to portray realistic currents on the
flats. The two vector components of the recorded currents were Table 1. Classification of morphodynamic environments based on
added to establish the overall flow (Figure 5c). During the second the RTR index. After MASSELINK and TURNER, 1999.
tide the data logger was set to off by the integrated timer thus
recording no currents. However, during the third tide, values for Relative Tide Range Group Beach type
the ebbing tide peaked under the combined effects of 1. Reflective
Northwesterly winds and falling tide. Waves recorded during that
RTR < 3 Wave-dominated 2. Barred
period also showed greater values and coupling of wind induced
currents plus hydrodynamic forces may had occurred. 3. Dissipative
3 < RTR < 15 Mixed wave-tide 4. Low tide terrace
Sediment transport 3 RTR < 7 5. Low tide bar/rip
3 RTR < 15 6. Ultra-dissipative
The most significant result of the experiment was related to the
sediment transport elements. Despite underwater evidence (as RTR > 15 Tide-dominated 7. Transitional (beach
divers opened the inlets for the streamer traps) of existence to tidal flat)
suspended sediment no significant volume of sediment was
trapped by the streamer trap, measured by Sediment Activity
Meter (SAM), recorded as depth of disturbance nor inflicted on
the injected dyed sand. This notable result was consistent
DISCUSSION
throughout the three tides recorded in the experiment.
The definitive results of these experiments are interpreted here
There was no evidence of any morphological change along the
in the light that if tidal flats are investigated utilizing equipment
depth of disturbance grid either (as measured by D-GPS which
that is designed to study beaches no significant sediment motion
recorded exact same positions within the error band of the
or topographic change would be measured. If physical processes
apparatus).
affecting tidal flats and beaches are the same, why theoretical
The focus of the experiment presented in this article was to
predictions (such as interpreted from numerical simulations) did
empirically measure how high tidal flow velocities (Spring tides in
not match expected results in real empirical conditions?
the area) and strong winds could be used to illustrate the forcing
Conceptually, however, the mega dissipative environments found
factors in play upon the macrotidal beach using a suit of
in tidal flats could be classified under morphodynamic
equipment designed for surf zone investigations. Although wind
classification due to expected dominance of tidal flows over wave-
speeds and current velocities reached as high values as expected
induced morphodynamics. This approach may be the most
and relatively high and steep waves were generated locally under
effective to go forward in tidal flat research. In macrotidal
very limited fetch, no morphological or sedimentological response
environments in particular, the extent of beaches can dictate that
was noted at any point (despite measuring at theoretically
the flat morphology dissipate wave and tidal energy generating a
optimum water levels).
shift by which the tide may be regarded as a water level controller
Northwesterly winds dominated the recording period which
rather than a dynamic forcing factor. Tidal current alone may not
produced extremely steep waves that grew (developed) offshore
be sufficient in these conditions to exert entrainment velocities on
rather than propagate onshore. The tidal prism was blown from the
sea bed sediments and thus deposition dominates (extremely fine
NW quadrant and the effects on tidal current velocities were also
sediments and muds).
noted during the third ebbing tide. The coupling of optimum wave
Therefore to introduce tidal flat research into beach
penetration (combination of high orbital velocities and water
morphodynamics it is proposed to place the tidal flats of
levels within closure depth) did not appear to affect the decisive
Newtownards along with open ocean macrotidal beach
role indicated in previous research, perhaps because the limited
morphodynamic parameters. First the extent of tidal flats can be
fetch controlled wave generation and development above water
characterized by the expression:
depth.
Resuspension was visible during diving although very clear
XIT = TR / tanb
waters throughout the experiment indicate that sediment
concentration was low, flocculation nil and suspended sediment
where XIT is the lateral extent of the intertidal area, TR is the
was deposited over the source area. This was radically
tidal range and tanb is the beach gradient. For Newtownards the
documented by the lack of motion on the dyed sands deployed
tidal flats can then be calculated as intertidal extents of 1230 and
over the study area. CIAVOLA et al., (1997) documented the
520 m for the western and eastern shores respectively. To refine
effect of breaking wave height on activation of sediments as
this concept and further into a morphodynamic classification a
documented by measurements of depth of disturbance. Using this
definition of the morphodynamic states are described by the
framework the SAM apparatus, which had been tested on open
parametric factor RTR (MASSELINK and TURNER, 1999):
beaches in the North of Ireland yielding results that were
comparable but used shoaling rather than breaking wave height
RTR = TR / H
(JACKSON and MALVAREZ, 2002), proved inefficient on the
tidal flat. Within the framework of morphodynamic research
Journal of Coastal Research, (SI STRAEE Workshop)Sandy tidal flat morphodynamics
where RTR is the relative tidal range, TR is the tidal range and processes as well a the topographic effect that worm mounts may
H is wave height. Table 1 shows a classification of have on wave propagation over extreme shallow environments.
morphodynamic environments based on the above RTR index. It is proposed that given the unpredictability of morphodynamic
Using this index for Newtownards the tidal flats may be conditions to design a more successful empirical experiment and
characterized as wave dominated to mixed wave-tide dominated the large uncertainty that is intrinsic to numerical modeling in
(index 2.5 using local measured H and TR on the intertidal, 2.3 these environments, in situ measurements could be the way to
using storm H and 7 on average conditions) clearly placing the better our knowledge on sandy tidal flat morphodynamics.
study of these tidal flats in the context of other macrotidal Research carried out on tidal flats using in situ flumes deployed on
beaches. the surface of the (exposed) tidal flats has documented the validity
Given then the classification of these environments as mixed of such an approach. An in situ wave simulator could be proved a
wave-tide dominated, it follows that the approach taken to the useful addition to the state of the art in the context of
investigation of such environments could be marked within morphodyncmics of tidal flats.
empirical and numerical investigations on macrotidal beaches.
Sandy tidal flats have, however, never been viewed in this way. CONCLUSIONS
There is little research done from this standpoint and has to our
knowledge has always coincided with understated reflection on In this paper a review of the context of macrotidal beaches has
the role of wave action on tidal flats sedimentation. been presented in view to considering sandy tidal flats as mega-
An analysis of the variables involved can also help macrotidal beaches. Measurements and calculation showed that
understanding tidal flats as macrotidal beaches. Wave energy using existing morphodynamic indicators, the sandy tidal flats of
distribution across sandy tidal flats varies according to the state of Newtownards in Strangford Lough, Northern Ireland should be
the tide and, is most effective across the tidal flat at about mid- classified as wave-tide dominated intermediate environments and
tide. At lower tidal levels high orbital velocities occur in narrowly thus its investigation could be approached from the stand point of
defined zones that represent a narrow surf-zone towards the beach science rather than a more traditional estuarine one.
seaward margins of the intertidal flat. At lower tidal levels Previous research illustrated how water level combined with
(between 0 and 1m OD in Newtownards with storm waves), wave numerically simulated wave energy could be used to document
orbital velocities are less intense but are above sediment transport potential dominance of wave induced processes over tidal currents
thresholds capable of work across a much greater area of the tidal on sandy tidal flats. A new experiment was then carried out using
flat. At elevations greater than 1m OD, wave bottom orbital the knowledge gathered from this previous research. The
velocities decrease across most of the tidal flat and a narrow zone deployment of various apparatus to measure hydrodynamic (tide
of high orbital velocity develops at high-tide margin of the tidal and waves) as well as sedimentation processes (bed load, depth of
flat. Under these conditions penetration of wave energy through disturbance and suspended sediment transport) was carried out on
the water column is impeded by the increased water depth, and a selected portion of the sandy tidal flats of Newtownards. Results
wave action on the sea bed does not produce sediment movement. illustrated that although the hydrodynamic conditions during the
Spatial variations in wave energy levels can explain much of the high energy events recorded in the experiment should have been
observed variability in sediment grain size (MALVAREZ et al., sufficient to entrain sediment, traps and other devised deployed for
2001a). Rather than a rapidly migrating zone of intense activity of the experiment recorded negligible quantities.
breaking waves acting as main control on sediments grain size on These results illustrate that the combination of factors affecting
such a sandy tidal flat (such as exists at the margin of the sedimentation on tidal flats are more complex than measured and
advancing or retreating tide), prolonged activity at a slightly lower that further research needs to be conducted to document further
energy level may have a more significant role on the distribution the relationships between water level, wave orbital velocity and
of sediment texture. Wave shoaling processes are active across a penetration, tidal current velocities and direction and its
broad area of the tidal flat; the strongest relationship is found integration with morphologic change.
between wave velocities and sediment texture. A discussion is presented opening a variety of possibilities for
The issue of water level and wave penetration is however, taking tidal flat research further. A series of recommended lines of
particularly sensitive in these mega dissipative tidal flats. Previous work are proposed including the development of in situ wave
research has also shown how wave penetration through the simulators to develop more controlled experiments in the field
column of water and the variability of shoaling and surf zones lead including oscillatory motions for the first time in the simulation of
to the sandy tidal flat sedimentation in the context of generic hydrodynamic forces.
coastal morphodynamics. GREEN and MCDONALD (2001)
indicated that some optimum water level appeared to occur during
their experiments in New Zealand that affected sedimentation ACKNOWLEDGEMENTS
under the effects of waves. The authors would like to thank the staff of the mechanical
The effects of combined wave and tidal action is being workshop and the personnel of the electronic workshop of the
considered the focus of a variety of research projects currently Faculty of Life and Health Sciences of the University of Ulster at
(e.g. some work published in this volume) but further research is Coleraine, in particular Nigel MacAuley and Terry Griffin, for
needed to document the complex scenario presented on sandy tidal capturing our ideas and transforming them into scientifically
flat sedimentation. The decisive role of biological factors in sound instrumentation.
sedimentation processes may be more significant than anticipated
and certainly than it is considered in beach science. The sensitive LITERATURE CITED
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