LEACOAST

Effect of shore-parallel breakwaters in tidal conditions on coastal morphology

This proposal represents a collaborative venture between the University of Liverpool (UL) and the University East Anglia. The main objective is to evaluate the storm-scale effect of shore-parallel breakwaters in tidal conditions on coastal morphology using a combination of Q3D morphological computer modelling and new hydrodynamic and morphological data from the Sea Palling site in Norfolk as well as some existing data from UKCRF tests on the Elmer breakwaters

1. Incorporate wave-overtopping, porosity and reflection into the Q3D model and test against existing data from the UKCRF collected in the LUPY project

2. Model the morphodynamic changes in an embayment of the Sea Palling breakwater during individual winter storms using Q3D, a combined hydrodynamic and morphodynamic model.

3. Measure hydrodynamics (waves, tides and currents) in an embayment during two winter seasons to capture storm events

4. Survey the beach and bathymetry around the breakwaters immediately following storms events

5. Evaluate the efficacy of the Q3D and evaluate its usefulness in modelling storm-scale changes around breakwaters.

Investigators

Prof. B.A. O’Connor, Department of Civil Engineering, University of Liverpool

Dr S. Pan, Department of Civil Engineering, University of Liverpool

Prof. C.E. Vincent, School of Environmental Sciences, University of East Anglia

Previous Research

The University of Liverpool (LU) The Coastal Engineering Research Group including two of the authors has been involved for many years in national and international research projects concerning sediment transport and morphodynamic modelling. The Research Group has successfully completed several recent EPSRC and European Commission MAST projects involving use of the UK Coastal Research Facility (UKCRF) at HR Wallingford to study wave/current interaction and turbulence over complex bathymetry with the presence of river outflow and tidal currents (O’Connor et al, 1998), and hydrodynamics and morphodynamics with a model of the Elmer breakwaters and different bed materials (Pan et al, 1998; Ilic et al, 1999). In the recently completed MAST III Project SCARCOST [SCour ARound COastal STructures], effects of enhanced turbulence and liquefaction on the sediment transport were studied. The UL co-ordinated EPSRC project COSMOD [Quantification of sediment entrainment processes in the coastal zone] has also recently been completed and involved the development and testing of a new 3D micro-model and the parameterisation of turbulence and sediment transport models over bedforms, which in turn was used in larger scale morphodynamic modelling. The Research Group also provided the lead institute and Co-ordinator of the EU funded MAST III research project INDIA [INlet Dynamics Initiative: Algarve] and was a major contributor to the COAST3D [Coastal Study of Three-Dimensional Sand Transport Processes and Morphodynamics] Project, which concentrated on extensive computer modelling of the hydrodynamics, sediment transport and morphodynamics in a wide range of open coast applications, such as a 2D beach (Egmond, Netherlands), a tidal inlet (Barra Nova, Algarve, Portugal) and an estuary (Teignmouth, UK). The present authors are also involved in a number of European funded programmes using large-scale laboratory facilities in Europe, for example, use of the 300 m Delta flume and the large oscillatory water tunnel (LOWT) in the Netherlands to study sediment transport mechanisms in oscillatory flow in the ripple and sheet flow regimes, and two EPSRC-funded projects, (HYDRO) involving hydrodynamic conditions around nearshore breakwates, and (LUBA) involving collaborative work with the Universities of Bangor (Davis, Oceanography) and Aberdeen (O'Donoghue, Eng) to study sediment entrainment by bursting processes under wave action.

University of East Anglia (UEA) The School of Environmental Sciences at UEA is one of only two 5-Star environmental sciences departments in the country. It currently has more than 50 faculty and 120 research staff and an annual research income of £5M. In the last 5 years it has taken part in 38 EC projects of which it co-ordinated 12. It has a wide range of interests in coastal problems, ranging from the Holocene evolution of coastlines (particularly the East Anglian region in the East Anglian Coastal Research Programme and the NERC-funded LOIS programme) through to studies of small-scale processes of sediment transport and has a large group (2 Professor, 3 Readers and 10 Post-doctoral Research Associates) working on coastal and sedimentary processes. The School has taken part in many field programmes in North America and Europe. The School took part in C2S2 and C-COAST which were two Canadian process studies of beach and coastal evolution (Vincent et al, 1991; Vincent & Downing, 1994), and contributed to DUCK ’94 and SANDYDUCK (1997) at the US Army Corps of Engineers facility on the outer banks of N. Carolina (Hanes et al, 1998). In Europe UEA was a partner in the EC MAST projects RECUSET and STARFISH, which were studies of the short and long-term evolution of the Middelkerke Bank (Vincent et al, 1998) and co-ordinated TRIDISMA a project that designed and built a prototype acoustic instrument for measuring sand flux profiles near the sea bed (Vincent & Taylor, 1999; Vincent et al, 2000).

The Investigators

Professor Chris Vincent has been working on sediment transport problems for more than 25 years and has more than 60 fully-refereed papers. He was involved in the East Anglian Coastal Research Programme, a 4-year study of the state of the Norfolk and Suffolk coast which ended in 1977 (still a major source of information on sediment transport around the East Anglian coast) and has since supervised several PhD, MPhil and MSc projects related to the Norfolk and Suffolk coasts. He has been a Principle Investigator in many international coastal experiments e.g. C2S2, C-COAST, Duck ‘94 and Sandy Duck . He has been the UEA partner in 4 EC Marine Science and Technology (MAST) projects, and co-ordinated the MAST III Marine Technology project TRIDISMA, a consortium which is designing an instrument to measure sediment transport profiles, using a combination of acoustic backscatter, Doppler and correlation techniques. Currently he is part of the SANDPIT project to investigate the rates of sea-bed recovery from sea-bed mining. He is also Co-ordinator of a 4-year $600K NICOP project that is funded by the US Office of Naval Research to support collaboration and interaction between coastal-process oceanographers investigating sediment transport processes. He is a Visiting Scientist at the National Institute for Atmosphere and Ocean (NIWA) in New Zealand and is an advisor to the UKAEA on sediment transport.

Brian O’Connor is the Professor and head of the maritime civil engineering group at the University of Liverpool and has been involved in coastal engineering for thirty-eight years. His combination of practical and research experience is unique in the UK and his expertise is recognised nationally and internationally. Results from his past EPSRC, EU and industrially supported research are currently helping UK industry to compete in both national and international markets. He is the author of over 110 papers and co-author of an international textbook. He has recently co-ordinated the major multi-institute projects COSMOD and INDIA.

Shunqi Pan is a lecturer at the University of Liverpool and has 10 years experience of physical and numerical modelling of hydrodynamics and morphodynamics in coastal area. His research activities include numerical modelling of hydrodynamics and morphodynmics associated with offshore sandbank, beach, inlet/lagoon systems and estuary for MAST projects CSTAB, COAST3D and INDIA. He has also been involved in a series of physical experiments using the UK Coastal Research Facility at HR Wallingford, studying wave-current interactions, turbulence and sediment transport in situations of a river outflow and offshore breakwaters. He has wide experience of using flow-measuring instrumentation such as ADV and LDA, and also data acquisition and data analysis. He has also engaged in consultancy work in numerical modelling for industry both in the UK and China.

Description of the Proposed Research

1. Introduction and Background

The last decade or so has seen the increasing use of artificial reefs and nearshore shore-parallel breakwaters to solve coastal erosion problems. Typical examples can be found in the USA, (Sonu and Warwar, 1987), Denmark, (Zyserman et al, 1998), the UK at Kings Parade, Wirral on Merseyside, (Davies (1989), Elmer in Sussex (Davidson et al, 1996) and Sea Palling in Norfolk (Thomalla, 1999; Fleming & Hamer, 2001). However, while the construction of such structures is straightforward, their costs are high (the Sea Palling reefs cost £2M each in 1995) and their impact on coastal morphology is not fully understood particularly in tidal conditions when groups of structures are used. Impacts on the beaches, both up- and down-drift can be considerable (Thomalla and Vincent, 2001), and require the use of well validated prediction methods on order to avoid future problems.

Present-day conceptual design is largely based on the use of empirical rules for tombolo or salient formation at a single breakwater; such rules usually being obtained from field observations in mainly low tidal energy environments where longshore transport rates are small, Pope and Dean (1986), Herbich (1989), CERC (1993), CIRIA (1996). The problems of using empirical formulae for UK conditions has been highlighted by Axe et al (1996) at the Elmer site and Fleming & Hamer (2001) at Sea Palling site, while the use of sophisticated morphological computer models by the Coastal Engineering Research Group at Liverpool, O'Connor et al (1995), has shown that not all the empirical formulae available are useful, even for non-tidal conditions with non-porous structures. The design philosophy of producing either a tombolo or salient is also questionable for macro-tidal situations, since both may exist at different tidal stages.

Clearly, design rules based on the use of morphological computer models would be an improvement over the use of empirical formulae, as suggested by Zyserman et al (1998), Hamer et al (1998) and Fleming & Hamer (2001). Such models are also able to produce more reliable results than existing hydraulic models with mobile beds, where scale effects can be important, Ilic et al (1999). Computer models have also been shown to give realistic predictions for medium-term (1-2 year) morphological development at the Sea Palling site, Hamer et al (1998), Fleming & Hamer (2001) and Hall & Damgaard (2000), although differences were found in the latter case with field data due to the neglect of tidal range effects. However, other neglected features in the latter case model such as breakwater transmission, wave over-topping, 3D flow effects, the use of a single representative grain size and the indirect simulation of wave diffraction may also have played a part.

Given the future importance of a changing wave climate and rising mean sea level, it is essential that appropriate design tools are available to deal with both short term (storm scale, 2-3 days), medium term (1-10 years) and long term (100 years) term morphological changes at coastal sites protected by reef and breakwater structures. Complex computer models now exist and have recently been tested by the Liverpool Coastal Group, HR Wallingford and others at open coast field sites in Holland (Egmond), Portugal (Barra Nova, Faro) and the UK (Teignmouth) as part of major EU-sponsored (MAST) research programmes, COAST3D and INDIA, Soulsby (2000), O'Connor et al (2000). These programmes confirm the realistic nature of model predictions for open coast situations and the importance of having good bed friction representation as well as the inclusion of bed sediment grading. Unfortunately, only limited model testing has been done for coastal situations with breakwaters. Hypothetical situations without tides have been studied as part of the EU-sponsored (MAST) SASME project, Zyserman et al (1998) and shown the importance of degree of submergence for single breakwaters and the need to represent diffraction processes properly while application of models to a failed breakwater at Sooyung, Korea (Kim et al (1998), Kim et al (1999) as part of the EU-sponsored INDIA/SCARCOST Projects has showed the potential importance of reflected waves in producing both seabed liquefaction and scour on the seaward side of breakwaters. The Sea Palling study, Hall and Damgaard (2000) has looked at groups of breakwaters but has ignored a large number of features, including the important elements of tidal currents and water level variation; the spatial variation of tidal currents across the nearshore zone is considered to be an important element at many UK sites in determining net sediment movements and, therefore, in understanding how groups of structures modify nearshore processes.

A clear need exists, therefore, to obtain a better understanding of the complex morphological changes produced by reef/breakwater systems in UK tidal conditions for a range of temporal scales. Such understanding will need to be obtained by the combined use of both field data and morphological computer models. A two phase approach is currently envisaged with the first thirty-six month phase (I) studying changes over the timescale of individual storms and a second later phase (II) dealing with medium to long-term changes. The present proposal deals only with Phase I and will make use of existing large-scale laboratory data (UKCRF) from the EPSRC-sponsored LUPY Project (Pan et al 1998), a special three-six month field measurement campaign to be done at the Sea Palling site, Figure 1, as part of the present Project as well as existing site data, and state-of-the-art micro- and macro-scale morphological computer models.

The range of technology and information needed means that collaborative working is required between specialist modelling and field data teams, site managers and industrial designers. The various Project partners are detailed below:-

(i) Liverpool University, Department of Civil Engineering (LU). Professor O'Connor and Dr Pan will provide the micro- and macro-models as well as the LUPY UKCRF data and general Project co-ordination.

(ii) The University of East Anglia, School of Environmental Sciences (UEA). Professor Vincent will provide existing site data from previous Sea Palling site studies as well as deploying instrumentation to collect new data on waves, water levels, currents and storm beach level changes for LU model calibration/validation and process interpretation.

(iii) The Environment Agency (EA). Mr Hayman, senior engineer for the East Anglian region will provide beach profile data, access to previous and ongoing hydrodynamic and sediment studies around the Sea palling breakwaters and contribute to project results and interpretation.

(iv) HR Wallingford (HR). Professor Huntington/Dr Tim Chesher will provide details of previous model studies as well as results from new model applications using newly acquired project data and contributions to Project discussions particularly on site and regional processes.

(v) Halcrow (H). Mr Hamer will provide details of previous site data and model operations used for site design as well as site appraisal (Hamer, 1996, 2001) and contribution to Project discussions.

The Project will not only aid understanding of complex processes but will also help to demonstrate the potential of modern morphological models for future coastal zone management.

2.1 Methodology/Programme

Phase I attempts to look at the short time scale (2-3 day) response of a coastline protected by nearshore reefs/breakwaters. Phase II will use the results and models from Phase I to look at the medium to long-term development of the coastal zone using storm and tide aggregation ideas arising from recent EU-sponsored (MAST) Projects, PACE, COAST3D and INDIA, where present approaches based on operating storm duration models with a mean tide to equilibrium conditions have been found to be on over-estimate of actual conditions (Nicholson et al 2002).

The Phase I work involves three distinct activities of 1) model use, 2) fieldwork collection and processing and 3) site process interpretation. Two collaborating research groups will be used to provide the necessary specialist skills. In addition, data and expertise from the Environment Agency and Halcrow, the designers of the Sea Palling structures and HR Wallingford, who have used simplified computer models at the Sea Palling site, will be drawn upon for model calibration, process interpretation, and industrial relevance. Details of the work to be done and the role of the individual research teams is given below.

2.1 Model Activity (Lead Role: LU)

The Liverpool Group, led by Professor O'Connor, has played a leading role in the 21 institute 2M euro EU Project INDIA, in which use has been made of the Liverpool Coastal Group's Q3D morphological modelling system, O'Connor et al (1995), O'Connor et al (2001a), Pan et al (2001) and Nicholson et al (2002) to determine open-coast, storm-scale morphological changes of the beach/sea-bed in the vicinity of the Barra Nova Inlet, Faro, Portugal. The research model system allows for full-wave-current interaction between tide, wind, wave-induced currents through the tidal cycle as well as morphological feedback due to bed level changes over time. The sediment module allows for unsteady spatial concentration gradients tidal lags and bed grading effects. For the present project, the model's hydrodynamic module will be up-dated from an on-going LU EPSRC-supported project (HYDRO, Pan (2001)) to include reflected waves from the structures, wave over-topping, wave transmission and multi-directional seas. In addition, a Liverpool micro-model of 3D flow and sediment movement over bed forms, which has already been developed under the completed EPSRC-supported COSMOD Project (Li et al (1999), Li and O'Connor (2000), O'Connor et al (2001b)), will be used to provide correct bed friction and seabed entrainment conditions for use by the macro Q3D model.

2.1.1 UKCRF Elmer Analysis/Testing

The updated morphological system will be used to study the time dependent development of coastal morphology from initial installation of structures to the development of an equilibrium condition under constant wave action. Such testing is considered essential both to the validation of the model system but also to the future Phase II work in which aggregation of storm effects will be used. Since collection of within storm scale morphological changes are difficult to measure at field scale, use will be made of both hydrodynamic and bathymetric changes observed in one embayment produced in a mobile bed model of the Elmer breakwater scheme, which formed part of an earlier UKCRF study of the effect of groups of breakwaters on nearshore processes (the LUPY Project), Pan et al (1998), Ilic et al (1999). The hydrodynamic and morphological data to test the Q3D model already exists but needs further processing to enable its use in the present project (this operation was not done in the original LUPY Project due to EPSRC-induced delays arising from the re-scheduling of other EPSRC-funded projects in the UKCRF). The LUPY data not only includes spatial details of nearshore currents and wave motions, including turbulence levels, but also details of bed ripples formed in both sand and coal beds of sediment.

2.1.2 Sea Palling Site

An existing Liverpool 2DH morphological model system, O'Connor et al (1995), Nicholson et al (1997), will be set up for the Sea Palling site using bathymetric and hydrodynamic data from the earlier Hall and Damgaard (2000) study and operated for particular individual storm events so as to provide detailed wave and current information for a number of the site embayments, both nearshore and offshore. This data will assist with the best location of the field instrumentation and the planning of the new field data to be collected by the East Anglia Group, see Section 2.2 below.

2.2 Field Data Collection and Analyses (Lead Role: UEA)

The School of Environmental Sciences at UEA has been studying the processes controlling sand transport around the East Anglian for more than 30 years (Clayton, 1982, 1989); a computer model for longshore sand transport in this area (based on wave refraction) already exists (Vincent, 1979). Since the construction of the breakwaters at Sea Palling increasing attention has been paid to the impacts of these structures on the local beaches (Thomalla, 1999; Thomalla and Vincent, 2001) UEA also has considerable experience in processed-base field studies, having taken part in the US Duck’94 and SandyDuck field campaigns (Hanes et al, 1998), the European RESECUS and STARFISH programmes (Vincent et al, 1998) (both related to the morphological evolution of a Flemish sand bank) and more recently the controlled sand transport studies in the Hannover wave channel (Ribberink et al, 2000, Vincent et al 2001).

The primary objective of the field-study is to determine the storm-scale morphological changes in one or two typical embayments (Sea Palling has two distinct sets of breakwaters, differing in size, separation and height) as well as the salients/tombolos on either side and to relate these to the hydrodynamic forcing and sediment transport patterns through direct current and wave measurements. Field-work will take place over two winter seasons, one season at each breakwater set. Two short process-based studies (currents, waves and turbidity) around the Sea Palling reefs were conducted in 1997 and 2000 but neither captured a significant storm event. However these data will be made available to the project (EA/Halcrow) and will be used during the early months of the programme for planning the field campaigns. The field work period will overlap with the Environment Agency’s on-going surveys in the area, see Section 2.2.1 below.

2.2.1 Bathymetry and Beach Morphology

Previous studies at Sea Palling have concentrated on the systematic (typically 3-monthly) profiling of beaches at Low Water Springs but have had little information on the important changes below this level. It is not know what happens to the sand that is transported along the beaches towards the Sea Palling breakwaters. A result of the original modelling studies for these breakwaters (conducted a decade ago) was that sand would by-pass the breakwaters, moving along their seaward side, and back onto the beaches further down drift but this path has yet to be observed or confirmed to be important to the sand budget. This project will make use of the recent advances in GPS position fixing (RTK) in conjunction with a digital echosounder to make detailed bathymetric surveys around the breakwaters with +/-5cm vertical accuracy. Surveys will be obtained at regular intervals during the field campaigns and, most-importantly, as soon as practical following a storm event. This survey technique was used successfully from a jet-ski during the SandyDuck experiment but can also be used from a small fishing boat or rib. The same RTK-GPS will be used to measure the beach topography. This methodology should reveal the storm-scale changes with unrivalled detail.

The Environment Agency has been conducted beach profile surveys (to LW) at 50m spacing since 1995 and will continue to do so during the course of present study. These data will be made available to this programme. These surveys cover all 9 breakwaters at Sea Palling and form an important longer-term data-base for trends in the beaches up- and down-drift. As part of the regional monitoring programme, the Agency is intending to deploy 3 - 5 bottom-mounted wave and tide gauges in the nearshore zone of the Norfolk coast (ie below MLWS), plus one wave-rider buoy at a site deemed most suitable to gather representative data that will inform further studies (the precise deployment locations are not yet known but at least one should be in the vicinity of the Sea Palling breakwaters and will be a very useful data set for the modelling. The Agency have agreed to give us access to these data as their contribution to the project.

2.2.2 Wave and current measurements

Wave and current measurements are relatively expensive and instruments can easily be lost (destroyed by storm waves or trawled by fishermen) or become buried in the highly mobile sand around the breakwaters. Nevertheless it is essential to be able to make sufficient measurements to identify the important physical processes and to verify the models’ predictions.

The boundary conditions for the LU Q3D Modelling System will be obtained from the POL North Sea numerical model, (output will be purchased for storm periods). Wave data will be purchased from the Met Office Wave Model for the offshore point nearest to the breakwaters and supplemented by data from the Environment Agency wave-rider in 2002-3 for calibration adjustment of the POL output. An ADCP running WAVE-software will be similarly deployed at the location of a Q3D-model corner to obtain direct data on the wave and currents for a 2-4 week period during the first winter field season. Prior to deployment shore-normal current transects will be run close to maximum flood and ebb flows.

Close to the breakwaters and within the embayment, 2 wave/tide recorders and 2 2-D wave-current meters will be deployed throughout the field campaigns. These instruments will be relatively easily deployed and maintained from one of the small fishing-boats. This combination of instruments will give considerable flexibility. The two wave/tide gauges will enable set-up and water levels within and across the embayment to be measured, while the 2D wave-current meters will measure the wave and tide-driven currents at sensitive points around the breakwaters.

In addition, fluorescent tracers will be used in a parallel PhD programme at UEA, to study bed load measurement and sediment pathways. All data collected during the Project will be archived for future use.

2.3 Sea Palling Model Application (LU/UEA/HR Wallingford)

The Q3D model will be used to determine wave and current conditions as well as enhanced bathymetric changes and sediment movements in the test embayment for the series of storm events captured during the intensive field campaign. Model boundary conditions will be derived from the measured data and the use of the LU micro-model. HR Wallingford will also use the some data in their latest model system so as to provide cross-validation of model outputs.

2.4 Interpretation (All)

Both the intensive field campaign results and the Q3D model runs will be used to determine the transient behaviour of the test embayment during the observed storms. The Q3D model will then be used to study the effect of breakwater groups and different locations offshore for the groups as well as upstream and downstream effects, sediment patterns around and landward of the reef system, scour around the reefs and the effects of changing offshore bathymetric conditions . Changes in both storm wave severity, and direction as well as surge and mean sea level change will also be studied. The precise number of combinations will depend upon the success of Section 2.3.