Investigations on the morphodynamics of sandy tidal flats ... · Investigations on the morphodynamics of sandy tidal flats: a modeling application Gonzalo Malvareza,*, F. Navasb,

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Coastal Engineering 51

Investigations on the morphodynamics of sandy tidal flats:

a modeling application

Gonzalo Malvareza,*, F. Navasb, D.W.T. Jacksonc

aUniversidad Pablo de Olavide, Sevilla, Ctra. Utrera, km 1, Sevilla, 41013, SpainbUniversidad de Sevilla, Sevilla, Spain

cUniversity of Ulster, Coleraine, Londonderry, Northern Ireland

Available online 11 September 2004

Abstract

Modeling of morphodynamic behavior and evolution of sandy tidal flats commonly comprises the utilization of a sequence

of techniques to characterize and/or quantify the interaction between various hydrodynamic forcing factors and the

geomorphologic response of the receiving environment. An alternative for establishing the geomorphological behavior of sandy

tidal flats could be introduced in the context of coastal classification of morphodynamic regime. This contribution investigates

tidal flat morphodynamics in the context of generic coastal classifications and aims at a medium to long-term characterization of

its evolution, by presenting the methodology and results of a series of investigations conducted on sandy tidal flats in Northern

Ireland.

The active inter-tidal sandy tidal flats of Newtownards vary between 1200 and 500 m wide and can be characterized by a

relative tidal range (RTR) factor of 2.3–7. This situates the inter-tidal zones in the mixed waves and tide domain. The sensitivity

of sediment redistribution to combined water level/wave height makes wave energy dissipation due to bottom friction the

primary shoaling process during relative high-energy events. Tidal currents, a constant feature on the ultra dissipative

environments, are also most acute when combined wave/tide currents are present. Thresholds of sediment entrainment are

exceeded and resuspension initiated when water levels approach 1.0 m OD and significant wave height are at a maxima of 1.5

m. These modeling results when taken into an empirical situation by deployment of instruments illustrated that the relationship

between waves and water levels in these environments is very dependent on wave penetration in the water column.

Morphodynamic characterization of tidal flats could allow for further understanding on their long-term behavior if modeling is

utilized to reproduce morphodynamic scenarios, which help identify wave/water level relationships on fine sands.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Waves; Tidal flat; Sand flat; Orbital velocity; Strangford Lough; Northern Ireland

0378-3839/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.coastaleng.2004.07.006

* Corresponding author.

E-mail address: [email protected] (G. Malvarez).

1. Introduction

Coastal classification schemes have been developed

to synthesize morphodynamics and its rather complex

(2004) 731–747

G. Malvarez et al. / Coastal Engineering 51 (2004) 731–747732

morphological and dynamical interactions. Some

expressions are able to characterize most littoral

settings on beaches ranging from reflective to dis-

sipative environments that behave in a somewhat

predictable manner (if the environment is natural and

sediment supply stable). However, at the very extreme

of the morphodynamic classifications of ultra dissipa-

tive beaches, sandy tidal flats are neglected in the

literature of beach science as the result of the

complexity of investigating wave shoaling, breaking

and swash on a high spatial resolution on such low

energy and flat morphology. While open ocean

beaches have been the subject of a tremendous variety

of empirical and numerical studies and three morpho-

dynamic zones (the swash zone, the surf zone and the

zone of shoaling waves) are considered, tidal flats have

consistently been the focus of separate research efforts.

Dominance of cohesive sediments, extreme flat-

ness of its morphology, higher degree of biological

bioturbation and variable strength of tidal flows as the

main hydrodynamic force built a strong case for the

isolation of tidal flats in morphodynamic approaches,

especially in the case of muddy tidal flats. Sandy tidal

flats, however, are dominated by hydrodynamic

forcing factors and morphologic settings that put

them closer to open ocean beaches and, thus, a

morphodynamic approach seems appropriate since

wave-induced sediment entrainment and combined

wave–current interactions are expressed in complex

sedimentary spatial distributions.

In macro-tidal environments, it is generally accep-

ted that the influence of waves is diminished, mainly

because tidal translation over the inter-tidal region

results in rapid movement of surf and swash zones over

the active area and, thus, tidal currents should play a

greater role both in absolute terms and relative to local

waves (Nordstrom and Jackson, 1992). The role of

waves in tidal flat sedimentation is acknowledged to be

particularly important in the inter-tidal area (e.g.,

Reineck, 1967; Amos, 1995; Ryan and Cooper,

1998). On sandy tidal flats affected by frequent wave

action, waves may be expected to exert a control on

sedimentation patterns particularly because high-fre-

quency wind waves dominate estuaries when sheltered

from open seas. Wind waves exhibit broad spectra of

frequency and direction (Holtuijsen et al., 1989) and,

therefore, may be subject to sharp directional changes

induced by refraction and/or diffraction. The effects on

local morphodynamics are that wave-induced potential

sediment transport may occur in multiple pathways

linked to slight variations in topography of the sea bed.

Inter-tidal flat topography commonly lacks relief (out-

side channels) and, therefore, a highly sensitive

analysis of wave propagation and decay is essential if

energy gradients at different stages of the tide are to be

interpreted in terms of their effect on sediment entrain-

ment and/or transport. Wave energy dissipation, related

to bottom friction and breaking, can be directly linked

to water depth in shallow environments (Collins, 1976)

and thus the expanding/contracting tidal prism affects

wave fetch and water depth. Variations in tidal water

level affect wave sediment interaction but wave

parameters are difficult to assess across a broad spatial

zone, and the study of their effect on tidal flats is further

hampered by difficulties of measurement and simu-

lation under regular tidal variation.

Several studies have demonstrated a broad link

between wave energy and tidal flat morphology (Boyd

et al., 1992; Ryan and Cooper, 1998) and the

distribution of flora (Hammond et al., 2002). At a

local scale, Carling (1982) investigated the role of

waves by the deployment of a wave pole at a single

location and using time-averaged results as a measure

of wave intensity. While the results showed a link

between wave action and tidal flat behavior at the

scale of the whole tidal flat, these results were based

on data from a single point and cannot explain

adequately, variability within a tidal flat.

Rates of wave energy dissipation (due to shoaling

and not breaking) must affect tidal flats in the same

fashion as nearshore regions of beaches despite the

morphological adjustments. Tidal currents are a

constant feature on the ultra dissipative environments

of tidal flats but the hydrodynamic effects of them are

most acute when combined wave/tide currents are

present. For instance, tidal flows of 0.2 m s�1 in the

Foyle estuary may be amplified to 0.93 m s�1 under

the combined action of tide and waves (Nordstrom and

Jackson, 1992). The complex interaction of the two

forcing factors have been analyzed by many authors

but it appears that it is accepted that tidal forces have

little influence on beach morphodynamics (Masselink

and Turner, 1999). However, these authors explain that

as tidal range increases tidal flows and coupling tide/

wave interaction play a significant role in affecting the

directional component of sediment transport.

G. Malvarez et al. / Coastal Engineering 51 (2004) 731–747 733

Inside estuaries waves are the result of filtering

through its seaward entrance and, mostly, as a result of

wind generation inside the estuary. The types of

waves generated in this way are typically short

crested, high frequency and steep, and their geometry

reflects the fetch available given constant wind

conditions. Under steady wind, waves are highest at

high tide because fetch is generally longer (in onshore

wind conditions) but wave-induced stress on the

bottom surface may be small because, under short

period waves, wave orbital motions cannot penetrate

to the bed. Green and Macdonald (2001) illustrated

that at some optimum combination of wave/water

depth, wave penetration under steep waves influences

resuspension of bottom sediments. Changing wind

conditions and stable water levels are responsible for

sediment resuspension in lakes given that wind

generated waves are the only force in play that

induces water column deep stresses (Jin and Ji, 2001).

Wave sensitivity to water level (induced by tidal

effects) appears, therefore, to be a crucial element in

shoaling processes associated with waves acting

across tidal flats. As is the case on macro-tidal

beaches, tidal stage determines how waves affect the

bed sediments and also where. The duration of the

effects of shoaling waves on the mobile bnearshoreQdictates potential morphodynamical stages (Masselink

and Turner, 1999). Whereas beach profiles seem

relatively insensitive to wave height across the

dissipation zone, tidal flats appear to respond to

variations in wave energy dissipation and wave orbital

velocity by relocating sediments according to fine

variations in grain size (Navas et al., 2001). The

combined effect of water level variation, fetch and

tidal prism dynamics and related wave penetration

presents a very complex scenario for characterization

of morphodynamic criteria for tidal flats resulting in

greatly fine-tuned models of waves, currents, mor-

phology and sediment grain sizes interactions.

The aim of this contribution is to illustrate a series

of efforts in characterizing long-term sandy tidal flat

morphodynamics centered on modeling and empirical

investigations conducted over the extensive sandy

tidal flats of Newtownards in Strangford Lough,

Northern Ireland. The structure follows: (i) a numerical

modeling exercise simulating storm waves from the

S.E., the most effective fetch generating greater wave

energy, and establishment of relationship between

wave related parameters and the spatial distribution

of mean grain sizes to identify effects of varying water

levels, (ii) numerical modeling of multiple wave

conditions as generated by local (recorded) winds at

the identified optimum water level, and establishment

of relationship with distribution of mean grain sizes,

and (iii) deployment of an empirical measurement

campaign around a high energy event (characterized

by strong winds) at optimum water level using

morphodynamic indicators to test modeled results.

1.1. Study area

The study area is located at the northern end of

Strangford Lough (Fig. 1), on the eastern coast of

Northern Ireland. The eastern coastline of Ireland, the

Irish Sea, is typically of lower energy levels than the

more exposed Atlantic open coasts of the west. The

Lough is an elongate embayment with a north–south

axis (approximately 30 km long). It is bordered by

bedrock topped by glacial sediments and contains

many drowned drumlins within its margins. Contem-

porary marine conditions in Strangford Lough are

characterized by an average tidal range of 3.0 m

(spring tide range 3.5 m) with little amplification of

the tidal wave in the Lough. It is connected to the Irish

Sea by the Narrows, in which tidal velocities reach 3.5

m s�1.These strong currents are confined to central

deep channels and are significantly weakened approx-

imately 14 km north of the Narrows. The northern end

of the Lough is affected by minor tidal forces confined

to the major tidal channels. Tidal modeling (KKM,

1993) indicates that currents on the Newtownards tidal

flat reach a maximum at mid-tide levels (approx-

imately 0.1 m s�1). Waves develop within the Lough

and propagate following local wind direction. The

wave climate is dominated by these wind waves

within the Lough with limited fetch. Irish Sea swell

does not penetrate the Lough with sufficient energy to

have any significant morphodynamic effects in the

northern end. Wind directions affecting the wave

climate at Newtownards are typically from the south-

east and effective fetch is about 16 km (BBV, 1997).

Central regions of the Lough reach 60 m in depth

whereas the margins of the system are characterized

by a variety of inter-tidal flats, bedrock, and relict

glacial shorelines. The inter-tidal areas at Newtow-

nards exhibit a flat or gently sloping sandy surface

Fig. 1. Location and general bathymetry and modeling arrangement of Strangford Lough, Newtownards tidal flats and site for empirical

experiment.


(siliciclastic sediments on the flats are generally fine

sands, ranging from 0.10 to 0.25 mm) intersected by a

major drainage channel that divides the eastern and

western flats. Examination of tidal flat cross sections

shows that the inter-tidal region is 1000–1200 m wide

along the western shore (including the area fronting an

existing sea wall) and only 300–500 m along the

eastern shore. This means that the flats along the

western margin are of much lower gradient (0.003)

than those on the eastern shore (0.007). Cross-sections

of the tidal flat generally have a marked break in slope

that separates a gently inclined upper tidal flat from a

steeper lower section leading to the low-tide mark at

the margin of the tidal and storm channel that drains

from Newtownards.

2. Methods

To investigate the theoretical relationship of

wave-induced currents and distribution of surficial

sediment grain sizes a data set was generated

comprising the variables considered. Detailed cover-

age of variation of sediment textures required

sampling frequently across the tidal flats. Wave

parameters were also required at the same stations

to obtain geographically matching data sets. Posi-

tioning of sampling and hydrodynamic data was

carefully conducted to minimize errors. Hydrody-

namic parameters were not recorded using empirical

approaches, as simultaneous measuring of wave

height and wave orbital velocity was unfeasible for

over 75 stations across the flats. Numerical modeling

was conducted using a calibrated and a tested wave

generation and propagation model specifically

designed for shallow water wave conditions (more

details follow). Data from both data sets (sediment

and wave-related) were then combined using stat-

istical methods to show the relationships between the

two sets of variables. Input data for the numerical

wave propagation simulator were topography and

bathymetry of the tidal flats, initial wave geometry

(deepwater waves), wind speed and direction, and

tide levels. Water levels and wind speed and

direction were recorded nearby using equipment

deployed for the study.


2.1. Topography and bathymetry (DGPS)

Topographic surveys were carried out using a

Trimble 4400 Ssi Differential Global Positioning

System (DGPS) achieving theoretical sub-centimeter

accuracy. The topographic data from the high-reso-

lution topographic DGPS surveys was appended to

digitized bathymetric data (from published Admiralty

Chart bathymetric charts of the Lough), which

described sea bed topography of the deepwater areas.

All the coordinates were then normalized to Irish Grid

Reference and elevations were reduced to meters

(Ordnance Datum Belfast). The resulting random

points of the synthetic topography of the inter-tidal

flats and sea bed were organized in a grid for use in

the wave propagation model.

2.2. Sediment sampling and analysis

A total of 75 sediment samples were collected in

the area. Samples consisted of about 30 g of surficial

(2–4 cm depth) sediments randomly taken in the area.

The sample grain size was analyzed using an

automated settling column linked to a computer that

provided moment measures and settling velocities for

sediment. The analyses provide the median, mean,

sorting and skewness of the distribution as well as

settling velocities of the different fractions of the

spectrum of grain sizes, which are classified in groups

of very fine sands, fine sands, medium sands and

course sands.

Results from the sediment particle analysis are

directly geo-referenced given sample location

recorded in the field with DGPS during collection.

This facilitated mapping of the spatial distribution of

textural information and guarantees reliability of

subsequent analysis of spatial variability in sediment

particle size and correlations.

2.3. Water level recording

An automated bDobieQ water level recording

system from the National Institute of Water and

Atmospheric Research (New Zealand) was deployed

near the site.

The interrogation period was set to 30 min to

reduce excessive resolution and simplify wave prop-

agation simulation. Data from the water level recorded

were calibrated before deployment and output records

were normalized using DGPS for local elevation

control. The tidal record was input directly into a

wave propagation model, which was modified to input

sequential water levels up to 50 times, at intervals of

30 min thus covering semidiurnal tidal cycle.

2.4. Wind data recording

Wind data were recorded using aWeatherWizard III

meteorological station from Davis. It was installed at

the Ulster Flying Club facilities in Newtownards with

the measuring apparatus at an elevation of approx-

imately 5 m above the ground to reduce turbulence.

Wind velocity and direction were logged at 30-min

intervals. Retrieval of data was carried out using PC-

LINK software via modem and data were input in the

wave model to achieve realistic wave conditions

(wind-generated waves and directional effects).

2.5. Wave propagation modeling

The numerical computer model HISWA (HIndcast-

ing of Shallow Water wAves; Holtuijsen et al., 1989;

Booij and Holthuijsen, 1995) was used to generate

wave energy scenarios, and to analyze spatial dis-

tribution of wave-related hydrodynamic parameters.

The package uses gridded bathymetric information

and parametric deepwater wave data. This enables

calculation of wave characteristics in shallow waters

and at the shoreline. Gridded output of the model can

be plotted and/or tabulated for interpretation.

High-resolution (grid cellsb10 m2) computations

were then enabled by sequential simulation of wave

propagation over the inshore bathymetry at different

water levels.

Because of the restrained inlet morphology, it was

considered necessary to establish whether Irish Sea

waves exerted an influence on wave energy distribu-

tion within the Lough. The results, not discussed

further in this contribution, showed no significant

penetration, and thus waves inside the Lough can be

regarded as locally generated within the Lough.

Deepwater wave parameters for Newtownards tidal

flats were established from wind speed in the fetch

limited environment. Effective fetch calculations were

carried out to establish maximum development of

waves from high wind velocities. Single wave


geometry was generated from fetch limited wind

generation resulting in significant wave height of 1.5

m, zero crossing period of 4.5 s and approach from the

S.E. This wave climate was coupled with a wind field

that extended over the entire study area in the

simulation to reproduce realistic wave action over

the sand flats. Wind speed data were taken from the

recorded series at the Ards Airfield and the mean

high-speed value was 10 m s�1. The wind was set

constant for upper Strangford Lough. For multidirec-

tional simulations, wind data taken from the mete-

orological station were directly used as wave

generator. HISWA was run without initial wave

parameters so that the wave generation and propaga-

tion would correspond entirely to the appropriate fetch

distances and wind force for modal conditions.

Wave propagation, as performed by HISWA, is

stationary (i.e., waves propagate simultaneously over

the area but there is no time evolution in the model).

The results can be interpreted as a dsnap-shotT of thedistribution of wave energies in an area. The water

level is therefore also fixed at one stage of the tidal

cycle. This was modified and the model was run in a

loop, reading sequential water levels and wind speed

and directions from data recorded for this experiment

(see Sections 2.3 and 2.4). Spatial distributions of

different wave energy scenarios across the tidal flats

were generated at 49 stages of the tide (i.e., more than

a full semidiurnal tidal cycle at 30-min interval

readings) and nine wind directions.

Several output parameters of the model (significant

wave height, wave energy dissipation, wave-induced

stress and near bottom orbital velocity) were provided

at fixed geographical points.

2.6. Empirical measurements

An empirical measurement campaign was also

conducted after the optimum location and water level

and most probable optimum exposure had been

identified using the modeling approach described

above (Fig. 1). A morphological and hydrodynamical

experiment was organized to cover three tidal events

during Spring tides. In all, sediment transport was

measured using a specially designed streamer trap and

fluorescent tracers, depth of disturbance and sediment

bed change, and tidal currents. Wave height, period and

orbital velocities were recorded in shallow and deep

water andwater levels in three sites (deep channel, deep

flats and shallow flats). All the measurements were

recorded in high temporal resolution over the three

semidiurnal tidal cycles coinciding with Spring tides.

The rationale behind the experiment was to

measure simultaneously tides, waves and sediment

transport (bedload and suspended load) to establish

relationships between the morphodynamic variables.

Three time slots, coinciding with predicted optimum

water levels and tidal current velocities were targeted

for short duration streamer trap measuring periods.

Divers would open sequentially the streamer trap at

given times for given duration (see Fig. 2A). The rest

of the instrumentation (described below) was

deployed to record continuous time series and/or

depth of disturbance during the three tides under

investigation. An electromagnetic current meter was

deployed across the flood-ebb tidal current path to

record flow velocities during the experiment. The

interrogation period for the data logger was set at 5

min. The measuring probe was fixed to a stainless

steel frame held in position at 1.0 m height. Three

Dobie wave and tide gauges (Green, 1998) were

deployed at different locations to record waves and

water levels. Hydrostatic pressure was recorded as a

time series of 2048 samples at 10-min intervals during

the three events. Hydrostatic pressure was then

processed through the PEDP software, which uses

semi-empirical formulas to establish wave height,

period, significant orbital speed at the bed, wave

penetration through the column of water, etc. Water

levels were recorded for temporal mesoscale tidal

reference as well. One of the wave recorders was

deployed in the center of the instrument array to

establish local wave hydrodynamics on the shallow

(higher) flats. A second unit was deployed at the

deeper end of the tidal flats (barely exposed in Spring

Low Water) and the third wave/water level recorder

was deployed 10 km south of the experimental site to

record deeper water conditions. Data were recorded

simultaneously on the two wave recorders to enable

synchronous post experiment analysis.

A sediment transport Streamer Trap was con-

structed based on a modified design after that of

Kraus (1987) (Fig. 2A). The trap was located in the

instrument array site facing the incoming flood tide in

the first instance and was then rotated after the first

tide to measure ebb related sediment transport. The

Fig. 2. (A) Streamer Trap in position for recording Ebb Tide (with shutters on), and (B) Sediment Activity Meter (SAM) deployed.


trap had 12 inlets that were covered with shutters for

selective release by divers during 5-min recording

periods when expected water level was achieved.

Each column of inlets was coded identifying a water

level within the experiment. The bsocksQ (1.2 m bags

of 63 Am mesh) was placed semi-horizontally to retain

sediments in low velocity flow conditions.

Sea bed position wasmeasured using three methods:

a Sediment Activity Meter (SAM; Jackson and

Malvarez, 2002) deployed in the experiment array site

to measure sea bed motion during the tide at 124 s

intervals (Fig. 2B). Depth of disturbance was also

measured using a 100�100 DGPS referenced grid of

rods and washer combination and relative to this grid,

deployed beside the main experimental site, dyed sand

was injected over a 1 m2 area. Sand, originally

extracted from the experimental site (i.e., native sedi-

ment), was prepared in the laboratory using fluorescent

orange paint. Grain sizes were tested in the settling tube

to check relationship between native (non-colored

sand) and dyed sand and results were satisfactory

(native sand: Median 2.78; Mean 0.154; Sorting 0.76;

Skewness �1.42, and dyed sand: Median 2.75, Mean

0.155, Sorting 0.53, Skewness �1.85).

Wind data were also recorded simultaneously at

30-min intervals. Wind direction and speed were

logged during the 3-day experiment.

3. Results

The results of the surficial sediment grain size

analyses are presented first to help characterize the

breceivingQ environment. These results provide exten-

sive coverage of a range of grain size related

parameters (moment measures) although here only

mean grain size is used for clarity. All 75 samples

were contoured using triangulation and linear inter-

polation functions in Surfer 6.0 software. The grain

size variability across the tidal flats was limited and

ranged from 0.1 to 0.2 mm (i.e., fine and very fine

grained sand).


Finer particles are found near the existing north-

ern end and at the S.W. section of tidal flats. A zone

of larger mean grain size between those two regions

is characterized by an average size of 0.19 mm.

Mean sizes of particles range from 0.12 mm to about

0.20 mm. Finer material is found at higher eleva-

tions, and coarser sediment normally corresponds to

the central areas and the southeastern shore, where

drainage of the flats produces a steeper topography.

The western shore is nearly homogeneous with

Fig. 3. Sequence of wave orbital velocity plots from multi-water level sim

orbital velocities (above entrainment velocities) affect areas of the S.W. o

average grain sizes of about 0.16 mm characterizing

large extensions of the sand flats.

3.1. Modeling a storm from the southeast

Wind generated waves affecting Newtownards’

flats have a typical wide frequency spectrum and the

directional sector of propagation is considerably

wider than that of oceanic swell. The total fetch is

limited and levels of wave energy are generally low.

ulations. Gradients on tidal flats indicate that large patches of high

f the flats during mid tide (exactly at Tide=1.22 m OD).

Fig. 5. Correlation coefficients, obtained between water levels and

coefficients between wave orbital velocities and sediment grain


A sequence of frames obtained from repeated

numerical simulation of wave propagation shows

wave-induced current velocities at the sea floor in

meters per seconds in Fig. 3. At high tide wave

activity, as indicated by near-bed orbital velocity, is

confined to areas to the east, with relatively steep

shores. As water depth decreases, a stronger signa-

ture is registered on the eastern and also western

shores, especially on those reaches facing south.

Note the high relative magnitude of orbital velocity

at the western tidal flats. Finally, wave activity is

confined to shallow regions of the western shores

and the Mean Low Water Mark (MLWM).

The distribution of wave energy and the con-

sequent degree of wave energy dissipation due to

bottom friction and breaking is largely controlled by

water depth in this shallow water environment.

Higher degrees of energy dissipation may maintain

steeper slopes and greater particle sizes at these

eastern shores. Conversely, the reduction in wave

energy toward the western and northern shores may

produce deposition of fine sediment there, given the

Fig. 4. Sediment mean grain size diameter versus simulated wave

orbital velocities over the sand flats and calculated threshold for

sediment motion (Komar and Miller, 1973). Two water levels are

selected to illustrate the point that threshold is exceeded in many

locations across the tidal flats.

sizes in storm conditions. A band between �0.2 and 1.1 meters OD

shows best (significant) correlations.

very gentle slope and corresponding low gradients in

wave energy dissipation.

The threshold of sediment motion was calculated

with simulated and recorded data following Komar

and Miller (1973) and results showed that the sedi-

ments of some areas of the flats are potentially

activated by wave action (Fig. 4) when water levels

fluctuate between 0.8 and 1.22 m OD.

Bivariate correlations were calculated taking as

variables water levels and the relationship coef-

ficient already calculated between modeled wave

orbital velocity and measured sediment texture at

the 75 points on the tidal flat surface (Fig. 5).

Wave orbital velocities at the bed were simulated at

tidal levels between �0.5 and +2.38 m OD and

significant correlations were found in a distinct

water level band within the tidal variation envelope

(further details are presented in Malvarez et al.,

2001). The most significant correlation (0.631) was

at 0.88 m OD and the significant correlations were

restricted to between levels of �0.15 and 1.0 m

OD. At higher water levels, correlation coefficients

were reduced and were not significant, and at

Fig. 6. Average wind speed and direction measured in Newtownards

from 1997 to 1999.


lower levels, no significant correlation could be

found.

3.2. Modeling average wave conditions from multiple

directions

The wind records obtained for the simulation

involved a directional sector of more than 1808 (fromeast-northeast to west-northwest, see Fig. 6). Waves

were simulated (generated and propagated) on grids

from Newtownards tidal flats taking into account only

effective fetch distances, wind speed and direction. A

total of 11 simulations yielded wave parameters

(significant wave height, wavelength, near bottom

orbital velocity, local depth and wave period) that were

correlated with the moment measures extracted from

analysis of scattered sediment samples.

Wave energy direction was analyzed under various

wind approaches over the tidal flats to establish

refraction patterns. Given the limited fetch distance,

wave energy was low and, therefore, refraction intense

particularly on the higher flats where very shallow

water controls wave direction. The upper flats (north-

ern end of Strangford Lough) are affected by refracted

waves under different initial wind conditions. This

implies that, although of varying energies, wave action

reflects directional signatures that are comparable

under a wide range of initial wind/wave directions.

Wave orbital velocity was calculated from the

multidirectional simulations (Fig. 7). Higher wave

velocities are found in shallow water where dissipa-

tion is also more intense with values peaking under

southerly winds from the northwest of the area. Wind-

wave generation occurs also under easterly winds

forming narrow bands of higher velocities near the

northwestern area coinciding with the southerly,

affected primarily by refraction. Even under south-

westerly conditions, higher speeds are shown at the

northwestern region, although values are low. The

areas of potential resuspension (due to wave activity)

concentrate in the middle northern region under

western wind approaches. The spatial distribution

under these conditions (west, west-northwest, and

west-southwest) appears more evenly spread and does

not show high values of orbital velocity at any

location. It appears that the eastern shore is affected

by wave-induced currents under these conditions with

gradients more acute than those seen on the western

side.

Again, following Komar and Miller (1973), the

threshold of sediment motion was calculated based

upon the sediment size and orbital velocity. Modal

conditions used in this part of the study involved

moderate wind speeds and the resulting orbital

velocities did not exceed the threshold for sediment

motion. Only under southerly waves did one

location register a positive result.

The results of the multidirectional simulation

were then tested for correlations between modal

wave conditions and sediment size distributions.

Poor correlations resulted between settling velocity

of sediments and bottom orbital velocities. Other

variables were plotted against each other although

are not included here due to the lack of statistically

significant correlations. These included mean grain

sizes against orbital velocity, skewness of the

distribution and wave height and period, settling

velocities versus wave period and length. Multiple

regression analysis performed on the entire data set

showed no significant trends of relations between

the variables.

3.3. Empirical measurements

Given the results from the modeling experiments,

the precise location and water levels were theoret-

Fig. 7. Simulated spatial distribution of wave orbital velocities over tidal flats under 11 different wind directions and velocities but constant

water levels. Block arrows on the left hand side of each frame indicate wind direction for each of the simulations (data from locally deployed

weather station).


ically identified, and thus an empirical campaign was

designed in which a significant amount of measuring

equipment was deployed within an isolated region on

the tidal flat (see Fig. 1).

During the experiment, which comprised three full

semidiurnal cycles around Spring tide, waves reacted

to the expansion of the tidal prism (and related fetch)

achieving maximum wave height during the high tide

of the third tide of the experiment (Fig. 8A). Values

for orbital velocity (Fig. 8B) 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 under

locally generated waves. The significant short periods

combined with relatively high waves depicts geome-

try of very steep and irregular waves inducing high

orbital velocities but penetrating very little in the

column of water as a result of short wavelengths.

Tidal currents were low during the three tidal

events despite Spring tide conditions (Fig. 8C). The

current meter recorded values outside the main drain-

age channels to portray realistic currents on the flats.

The two vector components of the recorded currents

were added to establish the overall flow. During the

second tide, the data logger was set to off by the

integrated timer thus recording no currents. However,

during the third tide, values for the ebbing tide peaked

under the combined effects of N.W. winds and falling

tide (Fig. 8D and E). Waves recorded during that

period also showed greater values, and coupling of

wind-induced currents plus hydrodynamic forces may

have occurred.

The most significant result of the experiment

was related to the sediment transport elements.

Despite underwater visual evidence (as divers

opened the inlets for the streamer traps) of the

Fig. 8. Data recorded at experimental site during June 2002 empirical campaign.


existence of suspended sediment, no significant

volume of sediment was trapped by the streamer

trap, measured by the Sediment Activity Meter,

recorded as depth of disturbance nor inflicted on

the injected colored sand (Fig. 9a and b). Despite

the high-resolution capability of the SAM, no

variation that could not be considered as record

noise was found (there was also some algae that

Fig. 9. Injected tracers on deployment (a), and after three tidal cycles (b), and depth of disturbance rods before (c), and after (d) three tidal

cycles.


blocked SAMs actions at a given time which

distorted the record).

The streamer traps recorded no input of sediments

during the time that the bgatesQ were opened for

sediments to enter. Although suspended sediment

was visible during diving, there was no sufficient

flow that would achieve effective transport. The

streamer would have captured the sediments at three

water levels but none of those recorded anything

significant. When the streamer was turned to

coincide with the main flow of combined wave-tide

transport (for the ebb tide) the results were the same,

nil. This demonstrated that the wave-induced resus-

pension was not effective on the day of the experi-

ment and that tidal currents were not exceeding the

threshold for entrainment. This notable result was

consistent throughout the three tides recorded in the

experiment.

There was no evidence of any morphological

change along the depth of disturbance grid either (as

measured by DGPS). The 100 m2 grid was re-

measured at sub-centimeter precision and differences

were not appreciable. The rods-washer combination

showed no sedimentation (positive or negative) as

washers appeared at the exact same position as they

were deployed (Fig. 9c and d).

4. Discussion

4.1. Modeling morphodynamics on tidal flats: water

level, tides and waves

The context in which this discussion places

modeling and the morphodynamics of sandy tidal

flats is that of approaching an overall broader picture

of how and when morphodynamic processes occur in

such homogeneous, yet complex geomorphic environ-

ments. The modeling approach presented in the first

half of this paper aims at illustrating that the

distribution of wave energy across the sandy tidal

flats of Newtownards varies according to the state of

the tide and, in particular, that most effective energy

dissipation occurs across the tidal flat at about mid-

tide. At lower tidal levels, high orbital velocities occur

in narrowly defined zones that represent a narrow surf

zone toward the seaward margins of the inter-tidal

flat. At intermediate water levels, wave orbital


velocities are less intense but are above sediment

transport thresholds capable of work across a much

greater area of the tidal flat. At elevations greater than

1 m OD, wave bottom orbital velocities decrease

across most of the tidal flat and a narrow zone of high

orbital velocity develops at the high-tide margin of the

tidal flat. Above this depth, penetration of wave

energy through the water column is impeded and

wave action on the sea bed does not produce sediment

movement.

Examination of results indicates best (and statisti-

cally significant) correlations between sediment

mean grain size and wave orbital velocity at water

levels between �0.15 and 1.0 m OD. At higher and

lower water levels, the correlation between grain size

and wave orbital velocity is not significant. The

significant correlations between grain size and orbital

velocity exist in the presence of other (not assessed)

potential variations in sediment transport related

factors (including spatially variable algal and diatom

concentrations, mucus cementation of grains, dis-

turbances by grazing birds, etc). On the basis of the

wave-energy simulations alone, these results suggest

that variations in wave energy can explain some of

the observed variation in sediment grain size. In

addition, a number of interesting findings indicated

the relationship between wave processes and sed-

imentation. For example, rather than a rapidly

migrating zone of intense activity of breaking waves

(such as exists at the margin of the advancing or

retreating tide) being the main control on sediment

texture on such a sandy tidal flat, prolonged activity

at a slightly lower energy level may have a more

significant role on the distribution of sediment

texture. Thus, when water levels are between

�0.15 and 1.0 m OD and wave shoaling processes

are active across a broad area of the tidal flat, the

strongest relationship is found between wave veloc-

ities and sediment texture. The most effective and

widespread sediment transport under wave action

appears to occur while shoaling waves affect the

tidal flat surface for a prolonged period (i.e., when

water level is rising over large areas after flooding

tidal channels).

The results further suggest that to examine the role

of wave energy on sedimentation on the tidal flats

requires analysis at different water levels. Interest-

ingly, the periods of maximum wave influence at

mid-tide are also likely to be coincident with

maximum tidal current velocities although, in the

case of Strangford Lough, model studies (KMM,

1993) have shown that tidal current velocities are

relatively low on the tidal flats. It is thus possible that

previous workers, who have ascribed sediment trans-

port and sorting on tidal flats solely to tidal currents,

may have disregarded the contribution of wave-

induced forces, which may also exert their maximum

influence at mid-tide.

The results of the multidirectional modeling

effort show that modal waves synthetically propa-

gated over the tidal flats are not significantly related

to spatial distribution of sediment sizes or its

modification. Good correlations between near bot-

tom orbital velocity and elevation of the sample

location validates the technique, and demonstrate

that lack of correlations with spatial distributions of

sediments may be due to the intervention of some

other environmental parameters, or the fact that

forcing factors are not powerful enough to trigger

morphodynamic processes. Parameters characteristi-

cally contemplated in beach morphodynamics, such

as bed slope, may make wave propagation and

morphodynamic adjustment more sensitive. Thus,

although potentially the threshold for entrainment

may be approached, sediment activation and trans-

port is the sole response to high-energy events and/

or coupling of tidal, wave and water level induced

morphodynamics that occur in narrow temporal

bands.

4.2. Tidal flat morphodynamics

The mega dissipative environments found in

tidal flats are not frequently classified under

morphodynamic classification due to expected

dominance of tidal flows over wave-induced mor-

phodynamics. In macro-tidal environments in par-

ticular, the extent of beaches can dictate that the

flat morphology dissipate wave and tidal energy

generating a shift by which the tide may be

regarded as a water level controller rather than a

dynamic forcing factor. Tidal current alone may not

be sufficient in these conditions to exert entrain-

ment velocities on sea bed sediments, and thus

deposition dominates (extremely fine sediments and

muds).


If considered like a beach, the extent of tidal flats

can be characterized by the expression (from Masse-

link and Turner, 1999)

XIT ¼ TR=tanb ð1Þ

where XIT is the lateral extent of the inter-tidal area,

TR is the tidal range and tanb is the beach gradient.

Newtownards tidal flats described in this paper could

be characterized by inter-tidal extents of 1230 and 520

m for the western and eastern shores, respectively. A

better definition of the morphodynamic states are

described by the parametric factor RTR

RTR ¼ TR=H ð2Þ

where RTR is the relative tidal range, TR is the tidal

range, and H is wave height. Using this index for

Newtownards the tidal flats may be characterized as

wave dominated to mixed wave-tide dominated (index

2.5 using local measured H and TR on the inter-tidal,

2.3 using storm H, and 7 on average conditions)

clearly placing the study of these tidal flats in the

context of other macro-tidal beaches.

Given then the classification of these environments

as mixed wave-tide dominated, it follows that the

approach taken to the investigation of such environ-

ments could be marked within empirical and numer-

ical investigations on macro-tidal beaches. Sandy tidal

flats have, however, never been viewed in this way.

There is little research done from this standpoint and,

to our knowledge, has always coincided with under-

stated reflections on the role of wave action on the

sedimentation of tidal flats.

Variations in wave energy can explain much of

the observed variability in sediment grain size.

Rather than a rapidly migrating zone of intense

activity of breaking waves (such as exists at the

margin of the advancing or retreating tide) being the

main control on sediment texture on such a sandy

tidal flat, prolonged activity at a slightly lower

energy level may have a more significant role on

the distribution of sediment texture. Wave shoaling

processes are active across a broad area of the tidal

flat; the strongest relationship is found between wave

velocities and sediment texture.

Previous research shows how wave penetration

through the column of water and the variability of

shoaling and surf zones lead to the sandy tidal flat

sedimentation in the context of generic coastal

morphodynamics. On tidal flats, Green and MacDon-

ald (2001) indicated that some optimum water level

appeared to occur during their experiments in New

Zealand that affected sedimentation under the effects

of waves. This optimum water level was documented

in the first section of this contribution. The encourag-

ing results shown here under higher energy waves

(modeled) helped to identify the location of an

empirical exercise.

The focus of the empirical experiment presented

here was to measure how high tidal flow velocities

(Spring tides in the area) and strong winds could be

used to illustrate the forcing factors in play upon the

macro-tidal beach using a suit of equipment designed

for surf zone investigations. Although wind speeds

and current velocities reached as high values as

expected and relatively high and steep waves were

generated locally under very limited fetch, no

morphological or sedimentological response was

noted at any point (despite measuring at theoretically

optimum water levels).

The tidal prism was blown from the N.W. quadrant

and the effects on tidal current velocities were also

noted during the third ebbing tide. The coupling of

optimum wave penetration (combination of high

orbital velocities and water levels within closure

depth) did not appear to affect the decisive role

indicated in previous research, perhaps because the

limited fetch controlled wave generation and develop-

ment above water depth.

Flocculation during the experiment was not

visible, and suspended sediment was very little.

This was radically documented by the lack of

motion on the dyed sands deployed over the study

area. Ciavola et al. (1997) reported the effect of

breaking wave height on activation of sediments as

documented by measurements of depth of disturb-

ance. Using this framework the SAM apparatus was

tested on open beaches in the North of Ireland

yielding results that were comparable but used

shoaling wave height (local) rather than breaking

(Jackson and Malvarez, 2002). Within the frame-

work of morphodynamic research, measurements of

depth of disturbance should have yielded results

despite the fact that the slope (as a controlling

factor) was much lower than on beaches. Thus, the

lack of sedimentation under waves of significant

potential for activation shows that the temporal


scales under which beaches operate may not be

comparable to that of tidal flats.

The effects of combined wave and tidal action is

the current focus of a variety of research projects, but

further research is needed to document the complex

scenario presented on sandy tidal flat sedimentation.

The decisive role of biological factors in sedimenta-

tion processes may be more significant than antici-

pated, and certainly than it is considered in beach

science. The sensitive balance of tidal and wave

interaction may also need to be reflected in the

manner in which sea bed morphology is described.

5. Conclusions

On tidal flats modeling and empirical studies,

investigating medium term evolution as well as

engineering applications are possibly using the wrong

tools or basing calculations on equations and methods

that were most definitely not designed for these

environments. This shortcoming has an effect in the

way we expect tidal flat morphodynamics to behave

and consequently, this contribution aims at demon-

strating that tidal flats cannot be monitored or

investigated as open beaches.

However, morphodynamic classification schemes

are capable of placing sandy tidal flats within mega

dissipative beach environments. The tidal flats of

Newtownards in Northern Ireland are included in

such classification schemes, and thus it is now a

matter of fine tuning modeling and morphodynamic

research to enable better characterization of sedi-

mentation and long-term evolution of these low

energy, low gradient environments. The extreme low

variation found in both forcing and receiving

environments presents a serious challenge to coastal

modelers, scientist and engineers.

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Investigations on the morphodynamics of sandy tidal flats ... · Investigations on the morphodynamics of sandy tidal flats: a modeling application Gonzalo Malvareza,*, F. Navasb,