Large Scale Experiments Coastal Engineering

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Large scale experiments on gravel and mixed beaches:

Experimental procedure, data documentation and initial

results

Belen López de San Román-Blanco , Tom T. Coates1, Pat Holmes2, Andrew J.

Chadwick3*, Andrew Bradbury4, Tom E. Baldock5, Adrián Pedrozo-Acuña3, John

Lawrence3, Joachim Grüne6

1 HR Wallingford, Howbery Park Wallingford, OX10 8BA, Oxon, United Kingdom

2 Department of Civil and Environmental Engineering, Imperial College of Science, Technology and Medicine,

London, SW7 2BU, United Kingdom

3 University of Plymouth, Centre for Coastal Dynamics and Engineering, School of Engineering, Drake Circus,

PL4 8AA, Plymouth, United Kingdom 4 Channel Coastal Observatory, Southhampton, United Kingdom

5 Department of Civil Engineering, University of Queensland, Brisbane, Queensland, Australia 6 Coastal Research Centre (FZK), University Hannover and Technical University Braunschweig, Merkurstrasse

11, 304019, Hannover, Germany

Abstract

This paper provides information on the experimental set-up, data collection

methods and results to date for the project �“Large scale modelling of coarse

grained beaches�”, undertaken at the Large Wave Channel (GWK) of FZK in

Hannover by an international group of researchers in Spring 2002. The main

objective of the experiments was to provide full scale measurements of cross-

shore processes on gravel and mixed beaches for the verification and further

development of cross-shore numerical models of gravel and mixed sediment

beaches. Identical random and regular wave tests were undertaken for a

gravel beach and a mixed sand/gravel beach set up in the flume.

Formerly HR Wallingford, Howbery Park Wallingford, Oxon, United Kingdom * Corresponding author:

Email: [email protected]

Fax: + 44 1752 23 26 38

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Measurements included profile development, water surface elevation along

the flume, internal pressures in the swash zone, piezometric head levels

within the beach, run-up, flow velocities in the surf-zone and sediment size

distributions.

The purpose of the paper is to present to the scientific community the

experimental procedure, a summary of the data collected, some initial results,

as well as a brief outline of the on-going research being carried out with the

data by different research groups. The experimental data is available to all the

scientific community following submission of a statement of objectives,

specification of data requirements and an agreement to abide with the GWK

and EU protocols.

Keywords: Shingle beach, gravel, mixed beaches; large-scale experiments, GWK,

physical model, morphology

1 Rationale

The majority of existing research into coastal morphodynamics in the last

twenty years has been concerned with sandy beaches. Little research has been

devoted to gravel beaches and even less to mixed beaches, which results in

this research field being in a stage of early development, or for some issues no

development. Management issues such as storm response or the long-term

stability of gravel and mixed beaches demand new observations and new

methodologies to predict morphological behaviour. Mixed beaches containing

sediment sizes ranging over several orders of magnitude (sand to gravel) are

increasingly seen as being important to coastal engineers and managers

around the world. These practitioners have expressed their concerns about

the difficulties in dealing with such beaches with available predictive tools.

Such beaches are often barrier beaches protecting important backshore

infrastructure or agricultural areas.

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In order to develop a management strategy for a section of coastline, it is

important to consider the likely response of beaches both under long term

and storm conditions. The prediction of the profile development during

storms can be carried out using either physical models or numerical models.

Although acceptable approaches to scaling narrow graded gravel beaches

have been used for over twenty-five years (see Powell, 1990), small scale

modelling of beaches with a mixture of sand and gravel is limited because of

the incompatibility of having both fractions within the same model.

Numerical approaches are broadly divided into parametric models, often

derived from observed results in the laboratory or from field measurements,

and physics based models which attempt to account explicitly for the main

physical processes active across the foreshore. Each of these approaches has

weaknesses and strengths, but none is able to simulate all of the important

and complex processes influencing mixed beaches. Most numerical models

have been derived for sand beaches and have then been extended to include a

wider range of grain sizes. The main problems with respect to extending

these models to mixed beaches are set out in Mason and Coates (2001) and

López de San Román-Blanco et al. (2000) and arise from the assumptions of a

simplistic description of beach sediment, usually defining the complete beach

by a single D50 value and ignoring flows within the beach face.

For longshore sediment transport both process models (Damgaard et al., 1996)

and empirical models, such as the CERC formula, Brampton and Motyka

(1984) or Damgaard and Soulsby (1996), provide reasonable results. Van

Wellen et al. (2000) provides a comprehensive review of the different

longshore transport formulae available and their applicability to coarse-

grained sediments, as well as an inter-comparison of the different formulae.

They found that the most accurate predictions were from formulae previously

validated at sites similar to that used for the subsequent comparison, and

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therefore required further testing against field data from dissimilar sites

before their accuracy could be assessed properly.

In contrast, cross-shore numerical models have had limited success �– mainly

due to a lack of knowledge of the governing physical processes and/or an

inability to model the processes adequately. Hence no process-based model is

available to predict the morphological response of a coarse grained or mixed

beach to a given hydrodynamic forcing. Research work aimed at filling this

gap had been started prior to the experiments by some members of the

research team associated with this study. HR Wallingford had been working

on the extension of OTTP-1D (one-dimensional swash zone model with a

porous layer) towards a morphological capability (Clarke and Damgaard,

2002). The University of Plymouth had been working on the coupling of a 1-D

phase resolving numerical wave model (based on the weakly non-linear

Boussinesq equations) with a sediment transport module and a

morphodynamic module (Lawrence et al., 2003). The sediment transport

module includes a hiding function to estimate the sediment transport for

different sediment size fractions and tracks the changing sediment

composition over the profile with time.

At present, the coarse grained profile empirical or parametric models

available are those of Powell (1990) known as SHINGLE, and BREAKWAT,

by van der Meer (1988), based on extensive scaled laboratory flume tests

(small scale with anthracite in Powell�’s and large and small scale with gravel

in van der Meer�’s). The main factors influencing gravel beach profiles are,

according to Powell (1990) and van der Meer (1988), wave height, wave

period, wave duration, beach material and angle of wave attack. Parametric

models for mixed-grained beaches do not exist at the moment.

Several major field measurement programmes, including the UK Shingle

Beach Project (e.g. Van Wellen et al., 1997), beach field work by Imperial

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College, (e.g. Blewett et al., 2001) and the EU Coast 3D project (Soulsby, 2001)

have provided a range of useful data. In common with all field research, the

results of these programmes are limited by the specific site characteristics and

the uncontrolled sea conditions. Holmes et al. (1996) presented small scale

tests of the profile evolution of fine, coarse and mixed sand beaches, including

beaches with different mix ratios. They also examined the post response

distribution of sediment and found that the finer sediment tended to be

deposited or exposed in the most energetic regions, e.g. at the crest of the bar

and in swash zone. However, there are scaling uncertainties associated with

small scale mobile bed models. These can be avoided in large-scale (1:1) flume

tests while at the same time retaining the advantages of controlled wave and

water level conditions.

In order to address this issue, the EU project �“Large Scale Modelling of

Coarse Grained Beaches�” was undertaken between March-May 2002, with the

main objective to provide data for the verification and further development of

numerical models describing the profile evolution of mixed sediment and

coarse grained beaches. A secondary aim was to obtain more detailed data on

nearshore hydrodynamics and beach groundwater, both difficult to achieve in

the field for these types of beach.

The project team was composed of members of the Steering Group (SG) and

Research Team (RT). The project was led by HR Wallingford (Tom Coates,

project manager; Belén Blanco, senior researcher and Jesper Damgaard, SG)

and included members of Imperial College, UK, (Prof. Pat Holmes, SG and Dr.

Tom Baldock, SG), University of Plymouth, UK, (Prof. Andrew Chadwick, SG,

John Lawrence, RT and Adrián Pedrozo Acuña, RT), University of

Southampton, UK, (Prof. Andrew Bradbury, SG and Maurice Mc Cabe, RT),

New Forest District Council, UK, (Prof. Andrew Bradbury, RT), University of

Caen, France, (Prof. Frank Levoy, SG) and University of Firenze, Italy, (Prof.

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Enzo Pranzini, SG). During the experiments in the GWK the Research Team

was assisted by the FZK staff.

The test program was designed to provide the following data sets:

�• beach profile change for different sediment types under a range of

wave conditions

�• hydraulic gradients within the beach face, both above and below the

still water line

�• cross shore and vertical velocity distributions within breaking waves

and the swash zone

�• cross shore and vertical changes in sediment distributions following

wave exposure.

More detailed information about the experiments and the data can be found

in López de San Román-Blanco (2002).

The data derived from the test programme will be useful to the many

European researchers interested in beach response modelling. The future

developments resulting from this work have the potential to raise coarse-

grained and mixed beach transport modelling to a new level of confidence,

with benefits to coastal engineers and coastal zone managers throughout the

world.

The remainder of the paper is organised as follows: Section 2 of the paper

presents the experimental procedure and set-up, where a description of the

flume, beaches and measuring techniques is given. Section 3 describes the test

programme undertaken for each beach and the characteristics of each test.

Information about how the data was collected and stored is given in section 4,

while section 5 presents representative results of the experiments. Section 6

gives a summary of the analysis and research carried out to date with the

experimental data. Section 7 presents the main conclusions to date.

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2 Experimental procedure and set-up

2.1 Introduction

The experiments were carried out at the GWK (Grosser Wellenkanal �– Large

Wave Channel), which is described in Section 2.2 (see Plate 1).

The experiments ran for nearly 3 months during Spring 2002 and consisted of

2 phases:

�• Beach I - gravel only; consisting of construction of Beach I,

instrumentation set up and calibration and Beach I testing

�• Beach II �– mixed; consisting of demobilisation of Beach I, construction

of Beach II, instrumentation set up and calibration and Beach II testing

Identical random and regular wave tests were undertaken for the Gravel

Beach and the Mixed Beach. Measurements included: profile development,

water surface elevation along the flume, internal pressures in the swash zone,

internal set-up, run-up, velocities in the surf-zone and sediment distributions.

This experimental exercise was complemented with a series of full-scale beach

deployments at sand, gravel and mixed sediment beaches at three locations in

the UK (see Holmes et al., 2002).

2.2 The GWK flume

The GWK is a 309m long, 7m deep and 5m wide flume in the FZK (Coastal

Research Centre), which is a joint central institution of the University of

Hanover and the Technical University Braunschweig and situated in Hanover,

Germany. The flume has a permanent 1:6 asphalt permanent slope (see Figure

1), over which the sediment was placed. The facility is equipped with a

mobile instrument carriage (shown in Plate 1), overhead lifting gantry and a

2m wave capability wave paddle plus a range of instrumentation. Further

details can be found in Dette et al. (1998).

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2.3 Beach construction

For both beaches (Beach I- Gravel only and Beach II-Mixed) an initial profile

of 1:8 was placed over the asphalt permanent slope of 1:6, the minimum depth

of the beach being 2m (see Figure 1). At the toe of the beach three sloped

concrete structures were placed to minimise loses of sediment (especially

sand) towards the wavemaker.

Although the two beaches were initially constructed at a 1:8 slope over the

asphalt permanent slope, they were not reshaped during the experimental

procedure, so that the initial condition for each test was the final profile from

the previous test. Although it is acknowledged that this procedure can

introduce some uncertainty due to an extra variable, the initial condition for

each test, it is commonly assumed to be acceptable. Reshaping the beach in

such a large facility would have been very time consuming and therefore not

practical, and there are also uncertainties as to an appropriate initial condition

in any event.

Beach I - Gravel only

The gravel used had a size between 16 and 32mm, with a mean diameter of

D50gravel=21mm. Although the gravel was not as rounded as that found on

natural beaches (it was rounded material from interglacial rivers), it was

considered to be within acceptable limits of angularity. The beach porosity

was approximately 0.44.

Beach II �– Mixed beach

The sediment used consisted of a bimodal mix between the 21mm gravel and

sand with a D50sand=300µm. The desired mix of the sediment was 30% sand

and 70% gravel, representative of natural mixed beaches (López de San

Román-Blanco, 2003). When demobilising Beach I, the gravel was thoroughly

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mixed with the sand outside the flume. When placing the mixture back in the

flume it was then further mixed with the bucket of the machine in order to

achieve a uniform mixture before being shaped.

Pre and post test sediment sampling was carried out to verify the mix ratio

(see section 2.6). For the initial mixed beach, the porosity was lower than for

the gravel beach, at around 0.2. However, as the beach sediment was

redistributed under wave attack, the porosity also changed, adopting values

between 0.4 and 0.2, depending on the amount of sand present in the sample.

Figure 2 shows the initial sediment size distribution for both beaches.

Construction issues

For the construction of the beaches a number of factors had to be considered.

Whilst it was important to construct the beach in the most realistic manner,

the logistical difficulties of transporting large quantities of gravel material had

to be accounted for. The compaction of material by the machinery carrying

the sediment raised issues such as the uniformity of beach construction across

the length and breadth of the flume. These and other issues are discussed in

more detail in this section:

�• Quantity of materials used: It is estimated that a total of 860 tonnes of

gravel were placed to cover a volume of approximately 600m3 (see Figure 1)

for Beach I �–gravel only. For Beach II (mixed) it was anticipated that the entire

quantity of gravel from Beach I would be used to form the equivalent slope

for Beach II (i.e. the sand would permeate into the gravel pores). However, it

is important to note that not all the gravel was used to form Beach II.

Unfortunately, it was not possible to estimate the quantity of Beach I gravel

that remained due to complicated storage issues.

�• Compaction: For both beaches, visual observation showed the

sediment to be compacted due to the machinery used in the construction.

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Also, due to the instrumentation on the right side of the flume, the machinery

drove over the centre and left side. This resulted in a different compaction of

the sediment across the flume. It is considered likely that this compaction was

the cause of some of the irregularities across the flume seen during the

experiments, especially in Beach II.

�• Settlement: Beach II (mixed) appeared to be quite compacted at the end

of the construction. However, the flume was filled with water over 1 day and

left 1 day before carrying out the probe calibration. During this time, it was

apparent that some settlement had taken place as evidenced by previously

covered instrument becoming exposed.

2.4 Instrumentation

The following instrumentation was used in order to measure the

hydrodynamics:

�• Wave gauges: A total of 25 capacitance wire wave gauges (wg) were

placed along the flume to record water surface elevations. The locations of the

fixed wg (16 to 24) are shown in Figure 1. The exact position of all gauges as

well as their vertical range (upper and lower recording range) is given in

Table 1. The position of wg 25 was variable during the experiments as it was

mounted on the mobile carriage.

�• Pressure Transducers: A total of 24 Druck pressure transducers (pts)

were deployed in the beach. Figure 1 shows the position of both sets of pts

and Table 2 gives the characteristics of each probe and their position. Their

deployment was divided in 2 sets:

Arrays of pts. These consisted of 5 arrays of 4 pts, each deployed

in the beach within the swash zone. Each array was mounted of

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a 100mm wide x 500mm long plastic board with special fittings

to hold the pts every 100mm. At the bottom end of each board

two metal spikes were used to position and fix the board in

place in the sediment. At the top end, a 500mm long spike was

attached in order to be able to see the position of the array once

covered with sediment. These arrays were deployed in order to

measure pressure propagation within the swash zone due to

flows over, in and out of the beach. Plate 2 shows the pt arrays

in place prior to final burial in the beach.

Buried pts. These consisted of 4 pts mounted on the permanent

flume asphalt floor (impermeable slope in the figures) further

up in the beach. They were set up in order to see the variation of

the piezometric head in that area. These pressure transducers

were deployed in special filtering devices used at the GWK to

avoid any influence from pressures directly between the sand

particles (using the pressure cells as real �“pore-pressure

sensors�”). Plate 3 shows the buried pts in place before being

covered.

�• Mobile array: ADVs and Hydrophone: A mobile instrument array was

mounted on the carriage (see Plate 4), enabling this station to be moved

within the surf and swash zone (chainages 250m-262m) during the tests, with

a total of 4 to 7 different positions investigated for each test. The instruments

consisted of:

An array of 3 acoustic Doppler velocimeters (ADV), vertically

separated by 200mm. The bottom-most ADV head was placed at

300mm from the bottom of the array. These ADVs had a

±2.5m/sec velocity range, which proved not to be sufficient in

several cases.

A wave gauge (wg 25).

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A hydrophone, situated in between the two lower ADVs, at

400mm from the bottom of the array.

�• Run-up: Maximum run-up elevation was recorded with a video for all

the tests. A grid had been painted in the wall of the flume in order to facilitate

the readings. The grid can be seen in Plate 1.

Calibration

In order to calibrate the wave gauges and pressure transducers, the water

level at the flume was varied in stages. For each water level, voltage readings

were taken for a total of 180s at 30Hz frequency for the pts and wgs. At the

same time, the water level oscillations were monitored. At the end of each test

the mean water level and the mean voltage reading over the 180s were

calculated. A total of 13 different water levels were recorded, at the end of

which regression line between average voltage readings and average water

levels was drawn. The calibration parameter for each probe is taken from this

regression line.

2.5 Wave conditions

The majority of tests comprised a total of 3000 waves. These waves however,

were not run as a continuous time series but as a series of batches. The length

of the batches varied in response to the rate of morphodynamic changes (see

section 2.5).

Wave generation

An important aspect of the experiments was the wave generation, as

disturbances and primary reflections should be excluded in order to have

accurate generated waves. The paddle (pusher type motion) of the wave

generator at GWK is equipped with an online absorption control system,

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which enables the compensation of all kinds of reflections directly at the wave

paddle (Schmidt-Koppenhagen et.al. 1997). The JONSWAP spectral shape

was chosen as the reference spectrum for all the irregular test series.

Usually, long duration random wave simulations were used to ensure that a

wide spectrum of wave heights and periods were generated for a given test

duration. However, for extreme wave conditions, the largest waves for a

specified Hs would have exceeded the wavemaker capability. Consequently,

for certain tests, the generated waves consisted of the repetition of the shorter

time series with equivalent Hs but smaller maximum wave height. Note that

the total generation time included a small period for ramping the wavemaker

signal up and down at the beginning and the end of each test. This period was

set to four times the peak period.

2.6 Morphology

In order to see the morphodynamic response of the beach, profile

measurements were taken after each wave test. Sediment samples at different

positions along the flume were taken while the profile was observed to be

changing significantly and at the end of each wave test, when the profile was

assumed to have reached an equilibrium position.

Profile measurements

Profile measurements were carried out with the GWK mobile carriage. The

profiler consists of a 7.5m long beam equipped with three parallel mounted

plastic rollers. The angular position of the beam is recorded and converted

into position and elevation information. (Details of its mechanical system can

be found in Berend et al., 1997). Plate 1 shows the profiler beam (not in

operation) while the experiments were running.

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This device allows a quick (around 10m/minute) profiling after a single test,

without having to drain the flume or have disturbances due to changing

water level. These profiles were always taken down the centre of the flume.

The profiler measurement error in the vertical is estimated to be 50mm

(approximate 95% confidence interval). Noting that the mean size of the

gravel was 21mm, such accuracy was deemed sufficient.

Sediment sampling

Surface sediment samples were collected at early stages of the profile

development (normally, after 500 waves) and at the end of each test. Samples

were taken with a shovel at three different positions; at the beach step, at the

SWL and at the crest of the berm formed by each wave condition. The location

of the beach step was deduced from the profile taken, visual examinations of

the breaking zone and the interpretation of the person performing the

sampling.

For beach II (mixed sediment), samples at different depths were also collected

for the SWL and crest positions at the end of each wave test. These samples

were taken from the base of the hole left by the removal of sediment with the

large grab suspended from the mobile gantry. The base of the hole was

levelled to determine the sample depth. Although disturbance of the

sediment in the area is high, it is believed to be reasonable, due to the fact that

in-depth samples are taken at least every 3000 waves.

Samples weighed an average of 7kg; they were dried for 24 hours in an oven

and then sieved. Sieves covered the gravel mode: 31.5mm, 24mm, 16mm,

8mm and 2mm. The sand was treated as only one fraction and not sieved.

Although it is understood that the sand may have also sorted out during the

experiments, the effort of sieving it into different fractions was not considered

necessary for the purposes of our experiments.

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Post-experiment sampling

At the end of the tests for Beach II (mixed), additional sampling was carried

out to record the sub-surface sediment distribution across the beach. Firstly,

the surface layer of gravel was scraped with a mini-digger from the top of the

beach for approximately 20m of the profile length. As can be seen from Plate 5,

the interface between the superficial layer of gravel and the mixed sediment

was notably sharp and easily identified, also a feature of natural beaches. The

interface was then profiled to obtain the depth of the mixed layer.

Subsequently, a trench of approximately 300mm depth was dug to investigate

the variation in sediment distribution along the flume. Finally, two sampling

pits were dug at two different chainages and three samples were taken at

different depths.

3 Test Programme

A matrix of tests was developed that enabled a full range of parameters to be

compared. The first entrance of the matrix, target wave steepness (H/L),

adopted three different values: 0.05 (series 1), 0.03 (series 2) and 0.015 (series

3). The second entrance, target significant wave height (Hs), took values of

0.6m, 1.0m or 1.2m. However, there were uncertainties regarding the total

number of tests that could be completed in the allocated flume time.

Core Programme

To ensure that a good range of parameters was covered, a core programme

was developed. The core programme consisted of 5 different random wave

tests and 2 regular waves tests (shown in Table 3). Wave Test 1 was also

repeated in order to investigate the effects of the initial plane shape on overall

profile development. All these tests had a constant water level of 4.7m. This

core programme was completed for Beach I and II.

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Additional tests for Beach I- gravel only

The core tests for Beach I were completed in good time, which enabled 3

additional tests (Tests 6, 7 and 8) from the matrix to be carried out. (Test 9 was

not carried out, as the beach would have grown well over the top of the

flume). These additional tests are shown in Table 3 in Italics.

Additional tests for Beach II- mixed

The core tests for Beach II were also completed in good time. However,

additional tests for this beach were not the same as for the gravel only beach.

The reasons for this were the incorporation of some of the comments from the

experts group meeting and the fact that the beach composition was partially

changed at the end of the core testing for Beach II .

For Beach II (mixed) the additional tests were the following:

�• Test 10 �– �“Erosive�” condition. Wave Test 2 target conditions (Hs=1m,

Tp=4.14s) were run for a lower water level (SWL=3.4m). The reason behind

this experiment is that only �“accretion�” profiles had been seen during the core

testing. As the tests were run from milder to more severe conditions, the

profiles developing to get in equilibrium with the new condition mostly

meant accretion at the berm.

�• Test 11 �– �“Tidal�” condition. In order to see the effects of varying tidal

levels and as a first approximation of tide modelling, the water level was

varied in stages for Wave Test 2 target conditions (Hs=1m, Tp=4.14s). A total

of 9 steps were investigated as an approximation of a tidal cycle from SWL to

high tide and back to SWL.

�• Test 12 �– �“Tidal�” condition. A different wave condition (Wave Test 4

with target conditions of Hs=1m and Tp=5.29s) was investigated in the same

way as Test 11.

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3.1 Sequencing

Each wave test was run in batches in order to enable measurement of the

morphodynamic response over time. At the beginning of the wave test,

batches of small numbers of waves were run, as important changes in the

profile occur at the beginning. The number of waves in the batches increased

as the profile developed towards equilibrium and changes reduce in

magnitude. A usual sequence for a wave test was: 50, 450, 500, 1000, 1000

waves; total number of waves in this case being of 3000. For beach II (mixed)

the total number of waves was usually bigger as the profile took longer to

develop. In Table 4 the batches used in each wave test have been indicated.

Note that the table is in chronological order so that the sequencing of the

different wave tests can also be extracted from it. Also, Table 4 contains

information on the time intervals at which profile measurement and sediment

sampling were carried out.

4 Data collection and Storage

Collected data can be divided into two categories:

�• PRESTON data. This comprises the data directly logged into the

AC/DC board and collected with the GWK PRESTON data acquisition

program. This data was logged at 60Hz with 57 channels having been used in

this experiment. It includes the wave gauges, pressure transducers and ADV

data. Each channel data are kept in a file named after the date, the experiment

number and the channel number. Each file contains a digit for each 1/60th of

a second

�• Other data. This comprises all the other data that was not collected with

PRESTON and therefore not synchronised. It includes the hydrophone,

profile, video and sampling data.

Profile data: The measured profile is given in a series of

spreadsheets. The data are stored as a couple of columns, the

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first one representing the position in the flume whereas the

second one describes the profile height above the channel floor

in meters.

Sampling information: Information on location of the samples

and the samples sediment distribution can be found in a

spreadsheet containing the results from the sieving exercises.

5 Results

Representative results of the experiments are presented in this section in

order to provide the scientific and engineering community with an example.

Beach I �– Gravel Beach wave test 4 (with the characteristics shown in Table 4)

has been selected as �“representative�” and time series of the measurements

during this test are given in this section. For representation purposes,

sequence 4a (the shortest in terms of number of waves) has been selected for

most of the cases.

Figure 3 shows the profile evolution for Test 4 (Beach I �– Gravel) after each

batch of waves, so that the morphological development of the beach can be

appreciated. Also, a small plot containing the initial and final profile has been

included (upper-left corner) in order to provide the total balance of material

moved for that test.

Figure 4 shows the water surface elevation time series as recorded by one of

the wave gauges (wg 5) during the duration of Test 4a (Beach I �– Gravel). In

this figure the ramping up and down of the waves, as well as their random

characteristic is easily seen.

Figure 5 shows a time series of the currents as measured by the ADVs for Test

4b (Beach I �– Gravel).

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Figure 6 shows the subsurface pressure time series for Test 4a (Beach I �–

Gravel). For representation purposes, only the pressures measured by the first

array of pressure transducers is shown. These pressures are represented in

terms of absolute head of water. These absolute heads measured at the same

horizontal position but different vertical positions are separated by 0.1m,

which accounts for the hydrostatic pressure change among them.

Figure 7 shows the internal set-up time series for Test 4a (Beach I �– Gravel) as

measured by the buried pressure transducers. In this case the 4th buried

pressure transducer did not record any signal. The internal set-up is

represented as head of water measured.

In Figure 8, a summary of the measurements carried out during Test 4c

(Beach I �– Gravel) is shown. They are represented imagining that the reader is

looking into the flume through a window; being able to see the measurement

devices as well as the measurements taken. These measurements include:

�• Wave envelope: The envelope of the mean wave height (Hm0) as

calculated for each wave gauge.

�• Set-down: measured mean sea level for each wave gauge

�• Sub-surface head: mean sub-surface heads of water (measured relative

to a datum taken at the bottom of the flume) as calculated for each rows of pts.

The difference between measurements at different rows gives the potential for

hydraulic gradients and therefore infiltration / exfiltration.

�• Internal set-up: mean calculated internal elevation of the water table as

calculated from the buried pressure transducers, assuming piezometric head

levels correspond to the water surface elevation.

�• Morphological changes: initial and final profile for test 4, as measured

by the profiler.

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6 Analyses

In this section, a summary of the analysis and research already undertaken

with the experimental data is presented with reference to publications

containing detailed explanations. Also, a summary of ongoing investigations

is also given. Currently, amongst the work that is being carried out by

different partners in the project, we can mention the following:

A comparative study of the dynamics of coarse-grained (gravel and mixed,

sand and gravel) beaches has been undertaken with the analysis of the GWK

physical model experiments (López de San Román-Blanco, 2003). The main

deliverable from this study is a conceptual model, in which the physical

processes involved in the morphological response of sand, mixed and gravel

beaches are given, in a qualitative manner. This implies a significant step

forward in understanding the key cross-shore processes involved, and their

interaction, and identifies further gaps in knowledge. The performance of

currently available empirical predictive tools for the morphological response

of coarse-grained beaches was examined, reassuring the engineering

community in the use of such tools for gravel beaches, as well as providing an

idea of the applicability, in a qualitative manner, for mixed beaches. New

empirical formulas for the crest and step elevation of coarse-grained beaches

were also given (López de San Román-Blanco et al.,(in prep)). The analysis of

the internal pressures in the beach provides a remarkable step forward in

understanding the possible flows within the beach and associated water table

and new formulas for the coefficient of reflection, set-up at the beach and

water table �“over-height�” were also given for coarse-grained beaches (López

de San Román-Blanco et al.,(in prep)).

Pedrozo-Acuña (2005) developed a time-domain model to discuss the key

processes controlling the cross-shore profile development of coarse grained

beaches. The modelling approach uses a time-domain model based on the

highly non-linear Boussinesq equations coupled with a sediment transport

21

formulation and a morphology module. Sediment transport rates were

estimated by using a bed load formulation and by solving the equation for

conservation of sediment for bed evolution. Simulations are discussed in the

context of the measurements from the GWK experiments. The results related

to the observed morphological changes in the gravel beach were presented in

Pedrozo-Acuña et al. (in review), where infiltration in the beachface was

assumed to be one of the main processes that determines the dominant

direction of sediment transport. Pedrozo-Acuña et al. (2005, in press)

extended this study by two means, firstly the effect of acceleration in the

sediment transport formulation was included and secondly the modelling

approach was also applied to the mixed beach case from the GWK

experiments.

In addition, analysis of wave reflection from the GWK gravel and mixed

beaches, Pedrozo-Acuña et al. (in prep) showed a marked difference in the

reflectivity of the two beach types. This is discernible when Kr is correlated

with a surf-similarity number based on the swash beach slope. Support for

this comes form field observations by Mason (1997) and this study

complements previous studies of sand beaches and of structures.

Baldock et al. (2005) have developed a swash model to estimate overtopping

and sediment overwash at the crest of beach berms. The latter is a necessary

condition for berm growth. This model will be compared to the growth in

berm height between different tests from the GWK. The GWK data will also

be used by a collaborative Anglo-Australian project modelling beach

groundwater on sand, shingle and mixed beaches. In particular, the GWK

data will be compared with that from recent small scale physical model tests

(Ang et al., 2004).

In an addition to the main experiments, instrumentation was deployed to

further the technique of using acoustic energy to detect particles in motion,

22

and to extend the method into the surf zone. Collisions between moving

sediment particles generate acoustic energy, referred to as Self Generated

Noise (SGN) which is transmitted into the water and can be measured by a

hydrophone. A Bruel & Kjaer Type 8105 passive hydrophone was co-located

with the ADV's on the mobile gantry, at a height of 0.4m from the bed. The

hydrophone is omni-directional with a dynamic range of 50 to 20,000 Hz.

Data from the hydrophone were digitized at 48 kHz and stored

simultaneously with video recordings on a digital video recorder. In this way,

the SGN record is tied instantaneously to the video recording of waves

passing over the hydrophone. The mobile gantry was moved to different

locations whilst the wave conditions were constant and, accordingly, acoustic

measurements could be made at different distances from the beach, both

inside and outside the surf zone. From the SGN record, it was possible to

identify discrete transport phases, swash and backwash, even within a wave

group. Further results from the hydrophone experiments are given in Mason

et al. (2004).

7 Conclusions

This major large scale experimental study of gravel and mixed beaches was

successfully completed in 2002 in the GWK (large wave channel of FZK in

Hanover) by an international team. A unique data set of measurements

including profile development, water surface elevation along the flume,

internal pressures in the swash zone, piezometric head levels within the

beach, run-up, flow velocities in the surf-zone and sediment size distributions

for identical wave forcing conditions for a gravel and mixed, sand and gravel,

beach are available to other research groups.

The conclusions of the analysis to date of the experimental results are:

�• A new conceptual model of gravel and mixed beach processes has been

developed.

23

�• The applicability of existing parametric profile models has been

evaluated

�• The reflection characteristics of gravel and mixed beaches have been

parameterised

�• New morphological models have been developed which are showing

promise as an important tool to identify key processes involved in profile

evolution for coarse-grained beaches, with the aim of improving our

knowledge of beach profile response.

8 Acknowledgments

The authors would like to acknowledge the assistance and support provided

by staff of the FZK (GWK flume) in Hanover. In particular thanks are due to

Dr Uwe Sparboom, Reinold Schmidt-Koppenhagen, Wolfgang Malewski,

Dieter Junge, Kai Jürgensen, Günter Bergmann and Kai Irschik. Thanks are

also due to the rest of the Research Team and Steering Group.

The large scale tests in the Large Wave Channel (GWK) of the Coastal

Research Centre (FZK) were supported by the European Community under

the Access to Research Infrastructures action of the Human Potential

Programme (contract HPRI-CT-1999-00101). The work was also supported by

the Department of Environment, Food and Rural Affairs (DEFRA - formerly

MAFF) under Commission FD1901.

9 References

Ang, L.S. C.H-Y. Sum, T.E. Baldock, L. Li, and P. Nielsen, (2004).

Measurement and modelling of controlled beach groundwater levels under

24

wave action. Proceedings of 15th Australian Fluid Mechanics Conf., Sydney,

December 2004, pp. 1-4.

Baldock, T. E., M. G. Hughes, K. Day and J. Louys, (2005). Swash overtopping

and sediment overwash on a truncated beach, Coastal Engineering, Vol.52,

No.7, pp. 633-645.

Berend O, Schmidt-Koppenhagen, R and Dursthoff, W (1997). Measurement

of sand beach profiles in the Large Wave Flume.7. International Offshore and

Polar Engineering Conference, Honolulu, Hawaii, USA.

Blewett J C, Holmes P and Horn D P (2001) Field Measurements of swash

hydrodynamics on sand and shingle beaches. Implications for sediment

transport. Coastal Dynamics 2001.

Brampton A H and Motyka J M (1984) Modelling the plan shape of shingle

beaches. Lecture Notes in Coastal & Estuarine Studies, 12 Offshore & Coastal

Modelling'85; 219-234.

Clarke S and Damgaard J S (2002) Applications of a numerical model of

swash zone flow on gravel beaches. Proceedings of 28th Intl. Conference on

Coastal Engineering. Cardiff, UK.

Coates T T, and Mason T (1998) Development of Predictive Tools and Design

Guidance for Mixed Beaches. Scoping Study. HR Wallingford Report TR 56

Damgaard J S and Soulsby R L (1996) Longshore bed-load transport.

Proceedings of the 25th International Conference on Coastal Engineering,

American Society of Civil Engineers.

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Damgaard J S, Stripling S and Soulsby R L (1996) Numerical Modelling of

Coastal Shingle Transport. HR Wallingford report TR4, April 1996.

Dette H-H, Peters K and J Newe (1998) Large wave flume experiments�’96/97-

Experiments on beach and dune stability. Technical University of

Braunschweig report N0 830.

Holmes, P., Baldock, T.E., Chan, R.T.C. and Neshaei, M.A.L., 1996. Beach

evolution under random waves. Proc.25th ICCE, Orlando, Florida, 3006-3018.

Holmes P, Blanco B, Blewett J, Horn D, Peel-Yates T and Shanehsaz-zadeh A

(2002) Hydraulic gradients and bed level changes in the swash zone on sand

and gravel beaches. Proceedings of 28th International Conference on Coastal

Engineering. Cardiff, Wales, UK.

Lawrence, J, Karunarathna, H, Chadwick, A J and Fleming, C A (2003) Cross-

shore sediment transport on mixed coarse grain sized beaches: modelling and

measurements. Proceedings of the International Conference on Coastal

Engineering 2002 (ICCE 2002).

Lopez de San Román-Blanco, B (2002) Data report Experimental Procedure

and Data Documentation. GWK Large wave flume experiments on gravel and

mixed beaches HR Wallingford Report TR 130.

López de San Román-Blanco B, (2003) Dynamics of gravel and mixed, sand

and gravel, beaches. Unpublished PhD Thesis Imperial College, University of

London.

López de San Román-Blanco B, JS Damgaard, TT Coates and P Holmes (2000)

�“Management of Mixed Sediment Beaches�”. Proceedings of the 1st

International Conference on Soft Shore Protection. Patras, Greece.

26

López de San Román-Blanco B, Holmes P and Whitehouse R (in prep)

Conceptual model on coarse-grained beaches dynamics (ii): infiltration /

exfiltration and groundwater response

López de San Román-Blanco B, Whitehouse R and Holmes P (in prep)

Conceptual model on coarse-grained beaches dynamics (i): morphodynamics.

Mason T (1997). Hydrodynamics and sediment transport on a macro-tidal,

mixed (sand and shingle) beach. PhD thesis University of Southampton

Mason T and Coates TT (2001). Measuring and modelling sediment transport

on mixed beaches: A review. Journal of Coastal Research, 17(3); 645-657.

Mason T, Priestley A D, Blanco B, Bradbury A P, McCabe M, Reeve D, Coates

T T & Smith N D (2004). Acoustic characterisation of shingle movement in the

surf zone (to be presented at ICCE, Lisbon, September 2004)

Pedrozo-Acuña, A., 2005. Concerning swash on steep beaches. PhD Thesis,

University of Plymouth, Plymouth, UK, 225 pp.

Pedrozo-Acuña, A., Chadwick, A.J., Simmonds, D.J., López de San Román-

Blanco B, (in preparation), �“The reflectivity of coarse and mixed sediment

beaches�” Coastal Engineering.

Pedrozo-Acuña, A., Simmonds, D.J., Otta, A.K., Chadwick, A.J., (in review),

�“On the cross-shore profile change of gravel beaches�” Coastal Engineering.

Pedrozo-Acuña, A., Simmonds, D.J., Otta, A.K. and Chadwick, A.J., 2005, in

press. A numerical study of coarse-grained beach dynamics, 5th International

Conference on Coastal Dynamics 2005, Barcelona, Spain.

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Powell K A (1990) Predicting short term profile response for shingle beaches.

HR Wallingford SR report 219.

Schmidt-Koppenhagen, R.; Gerdes, M.; Tautenhain, E.; Grüne, J. (1997):

Online absorption control system for wave generation. Proc. 3rd Intern. Symp.

on Ocean Wave Measurements and Analysis (WAVES´97). Virginia Beach,

USA.

Soulsby R.L (2001). Sediment transport and morphodynamics on complex

coastlines - the COAST3D project. Proc.. Coastal Dynamics 2001, Lund,

Sweden, Eds: H Hanson and M. Larson, ASCE,. pp 92-101, 2001.

Van der Meer J (1988) Rock slopes and gravel beaches under wave attack.

Delft Hydraulics Publications n 396.

Van Wellen, E, Chadwick, A J and Mason, T (2000) A review and assessment

of longshore sediment transport equations for coarse grained beaches. Coastal

Engineering, 40, 3, 243-275.

Van Wellen, E, Chadwick, A J, Bird, P A D, Bray, M, Lee, M and Morfett, J.

(1997). Coastal sediment transport on shingle beaches. In: D A Huntley and E

Thornton (eds), Coastal Dynamics 97, International Conference on the Role of

Large Scale Experiments in Coastal Research, 38-47. American Society of Civil

Engineers.

28

Tables

Wg Number

x-pos (m)

Lower y-pos (m)

Upper y-pos (m)

Wg Number

x-pos (m)

Lower y-pos (m)

Upper y-pos (m)

1 79.05 2.2 7.0 13 197.00 2.8 7.0 2 81.15 2.2 7.0 14 205.30 2.8 7.0 3 84.85 2.2 7.0 15 220.00 2.8 7.0 4 90.29 2.2 7.0 16 226.00 2.8 7.0 5 115.00 2.2 7.0 17 232.00 3.1 7.0 6 126.22 2.2 7.0 18 236.00 3.1 7.0 7 151.20 2.2 7.0 19 240.00 3.1 7.0 8 162.40 2.2 7.0 20 244.00 3.1 7.0 9 176.30 2.5 7.0 21 248.00 3.1 7.0 10 177.45 2.5 7.0 22 252.00 3.1 7.0 11 180.00 2.5 7.0 23 256.00 3.5 7.0 12 185.30 2.5 7.0

24 260.00 4.0 7.0

Table 1. Wave gauge positions along the flume and vertical range.

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Pt set x-pos (m)

y-pos (m)

Pt number

Range (bar) Pt set x-pos

(m) y-pos (m)

Pt number

Range (bar)

264.7 4.51 20 0.15 269.2 5.08 23 0.15 264.7 4.41 19 0.15 269.2 4.98 9 0.70 264.7 4.31 1 0.70 269.2 4.88 10 0.70 pt array 1

264.7 4.21 2 0.70

pt array 4

269.2 4.78 11 0.70 266.2 4.70 21 0.15 270.7 5.26 24 0.15 266.2 4.60 3 0.70 270.7 5.16 12 0.70 266.2 4.50 4 0.70 270.7 5.06 13 0.70 pt array 2

266.2 4.40 5 0.70

pt array 5

270.7 4.96 14 0.70 267.7 4.89 22 0.15 276.1 4.45 15 0.70 267.7 4.79 6 0.70 277.6 4.70 16 0.70 267.7 4.69 7 0.70 279.1 4.95 17 0.70 pt array 3

267.7 4.59 8 0.70

buried pts

280.6 5.20 18 0.70

Table 2. Pressure transducer locations and characteristics.

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SWL=4.7m

Random Tests Regular Tests

Hs Series 1 H/L=0.05

Series 2 H/L=0.03

Series 3 H/L=0.015 H Series 1

H/L=0.05 Series 3 H/L=0.015

0.6m Test 1& 1r

Tp=3.22s

Test 6

Tp=4.14s

Test 8

Tp=5.75s

1.0m Test 2

Tp=4.14s

Test 4

Tp=5.29s

Test 5

Tp=7.47s 1.0m

Test Reg1

Tm=3.6s

Test Reg2

Tm=6.5s

1.2m Test 3

Tp=4.48s

Test 7

Tp=5.86s

Table 3. Core test programme for Beach I (gravel only) and Beach II

(mixed). Additional tests for Beach I (gravel only) shown in Italics.

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Sequencing of Batches (cumulative number of waves) Profile data and sediment sampling2 Beach Test File name1 Target

H/L (-) A B C D E F G 50 100 500 1000 1500 3000 Gravel Test 1 J473206 0.05 PS PS PS PS PS PS 50 500 1000 2000 Gravel Test 2 J474110 0.05 PS PS PS PS 50 500 1000 2000 Gravel Test 3 J474412 0.05 P PS P PS 50 500 1000 2000 3000 Gravel Test 4 J475210 0.03 P PS P P PS 50 500 1000 2000 3000 Gravel Test 5 J477410 0.015 P PS P P PS 50 100 200 300 400 500

Gravel Test Reg1 R473610 0.05 P P 50 100 200 300 400 500 Gravel Test Reg 2 R476510 0.015 P P 50 500 1000 2000 3000 Gravel Test 6 J474106 0.03 P PS P P PS 50 500 1000 2000 3000 Gravel Test 7 J475812 0.03 P PS P P PS 50 100 500 1000 1500 3000 Gravel Test 1R J473206 0.05 P PS P P PS 50 500 1000 2000 3000 Gravel Test 8 J475706 0.015 P PS P P PS 50 100 500 1000 1500 3000 4500 Mixed Test 1 J473206 0.05 P PS P P P PS 50 500 1000 2000 3000 Mixed Test 2 J474110 0.05 P PS P P PS 50 500 1000 2000 3000 Mixed Test 3 J474412 0.05 P PS P P PS 50 500 1000 2000 3000 Mixed Test 4 J475210 0.03 P PS P P PS 50 500 1000 2000 3000 Mixed Test 5 J477410 0.015 P PS P P PS 50 100 200 300 400 500 Mixed Test Reg1 R473610 0.05 P P 50 100 200 300 400 500 Mixed Test Reg 2 R476510 0.015 P P 50 150 250 500 1500 3000 Mixed Test 10 J344110 0.05 P P P P P 8 steps with SWL= 3.7,4.0,4.4,4.7,4.4,4.0,3.7,3.4 (m) Mixed Test 11 J**4110 0.05 All P 50 100 500 1000 2000 3000 4500 Mixed Test 1R J473206 0.05 P P P P P P 7 steps with SWL= 3.4,3.7,4.0,4.4,4.7,4.4,4.0 (m) Mixed Test 12 J**5210 0.03 All P

1 File name contains characteristics of the tests and �“target values�”, so that for example J473206 accounts

for: J or R: J for JONSWAP spectrum, R for regular waves; 47 for water level, in this case d=4.7m; 32 for

peak period, in this case Tp=3.2s; 06 for wave height, in this case Hs=0.6m

32

Table 4. Experimental programme and sequencing batches information

for tests.

Figure captions

Figure 1. Experimental set-up.

Figure 2. Initial sediment particle size distributions for both beaches.

Figure 3. Profile evolution for Test 4 Beach I �– Gravel.

Figure 4. Wave time series for Test 4a Beach I �– Gravel.

Figure 5. Currents time series for Test 4b Beach I �– Gravel. Top panel ADV 1,

medium panel ADV 2, Bottom panel ADV 3

Figure 6. Subsurface pressures time series (Array 1) for Test 4a Beach I �–

Gravel.

Figure 7. Internal set-up time series for Test 4a Beach I �– Gravel.

Figure 8. The whole picture for Test 4 Beach I �– Gravel. Filled diamonds

represent the position of the pressure transducers.

Plate captions

Plate 1. Example of waves in Beach II (mixed).

Plate 2. Pressure transducers array (prior to be buried).

Plate 3. Buried pts (prior to be buried).

Plate 4. Mobile array.

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Plate 5. Trench.

Figure 1. López de San Román Blanco et al.

34


0102030405060708090

100

0 5 10 15 20 25 30 35Sediment size (mm)

Perc

enta

ge p

assi

ng (%

)

Gravel BeachMixed Beach

D50 D16 D84(mm) (mm) (mm)

GravelBeach 21 17 26

MixedBeach 17 1 23

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36


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1550 1600 1650 1700 1750

-1

0

1

Vel

ocity

(m/s

)

1550 1600 1650 1700 1750

-1

0

1

Vel

ocity

(m/s

)

1550 1600 1650 1700 1750

-1

0

1

time (s)

Vel

ocity

(m/s

)


38


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Plates

Plate 1. López de San Román Blanco et al.

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