morphological modeling: a tool for ... engineering today, gainesville, florida, october 2003 6...

Coastal Engineering Today,Gainesville, Florida, October 2003

Brøker, Zyserman, Madsen, Mangor & Jensen1

MORPHOLOGICAL MODELLING: A TOOL FOR OPTIMISATION OFCOASTAL STRUCTURES

Ida Brøker1, Julio Zyserman2, Erik Østergaard Madsen3,Karsten Mangor4, John Jensen5

Abstract: This paper discusses the problems of sedimentation in afishery port and its impact on the morphology of a sandy, very exposedcoastline. An improved layout of the main breakwaters has beendeveloped. The results obtained from testing the new layout in amorphological modelling complex are discussed. The new layout isexpected to lead to improved bypass, decreased sedimentation andcoastal impact, to a greater natural depth at the entrance, and to providesafer navigation conditions.

INTRODUCTION AND BACKGROUNDThe West Coast of Denmark is exposed to the severe North Sea wave climate. The

west coast, from the Wadden Sea in the south to the Skaw in the north, is severalhundred kilometres long. The net sediment transport along the coastline ranges from500,000 to 1,000,000 m3/year. Natural shoreline retreats of up to 5-8 m/year takeplace at some stretches. The sandy coastline is undergoing constant reshaping mainlydue to gradients in the longshore sediment transport. This coastal landscape wasbasically formed following the latest glacial period. An equilibrium shape has not

1 Ida Brøker, M.Sc., PhD. Head of Coastal and Estuarine Dynamics, DHI Water & Environment,Agern Allé 5, 2970 Hørsholm, Denmark, [email protected] Julio Zyserman, PhD. Chief Engineer, Coastal and Estuarine Dynamics, DHI Water & Environment,Agern Allé 5, 2970 Hørsholm, Denmark, [email protected] Erik Østergaard Madsen, M.Sc., PhD. Research Engineer, Coastal and Estuarine Dynamics, DHIWater & Environment, Agern Allé 5, 2970 Hørsholm, Denmark, [email protected] Karsten Mangor, M.Sc. Chief Engineer, Coastal and Estuarine Dynamics, DHI Water &Environment, Agern Allé 5, 2970 Hørsholm, Denmark, [email protected] John Jensen, M.Sc. Senior coastal engineer, Danish Coastal Authority, Højbovej 1, 7620 Lemvig ,Denmark, [email protected]



been reached and even though till cliffs, forming semi-hard points, are present, theentire coastline is slowly retreating in its natural condition.

Thorsminde fishery port is located at a tidal inlet on this coastline, on one of thenarrow barriers which divides coastal lagoons from the sea. The port is located at theentrance to the coastal lagoon. Sluices regulate the water exchange between thelagoon and the sea. Figure 1 shows a location map and a close-up of Thorsminde.

Fig. 1 Location map and close-up of Thorsminde Port.

Up until the 1980's, i.e. for about 100 years, critical parts of the coast wereprotected by traditional structures such as groynes. A comprehensive nourishmentscheme was established in the mid-eighties. Nourishment of about 3 million m3/yearalong a stretch of approximately 115 km has now stabilised the beach at criticalstretches, while other stretches have been left to retreat as part of an overall shorelinemanagement plan prepared and controlled by The Danish Coastal Authority.

The left panel in Figure 2 shows the central part of the coastal stretch, where themost critical retreat takes place. It also shows the shoreline retreat in two periods,before and after nourishment, and indicates the distribution of nourishment. It can beclearly seen how the shoreline retreat has been alleviated by intensive nourishment,Laustrup et al. (1998).

The littoral drift and corresponding shoreline retreat have been simulated by DHIusing the model complex, LITPACK, see Kerper et al. (2002) for details. The middleand right panels of Figure 2 show the simulated yearly littoral drift and shorelineretreat based on the time series of wave parameters from 1991-1996. The littoraldrift was calculated along the entire coastline. Blocking structures, variability inwave conditions, current conditions, sediment properties and the shape of the coastalprofiles were taken into account. Calculations of the littoral drift were carried outevery hour.



LITPACK is a complex of modules for the simulation of wave transformation,longshore wave-driven currents, longshore and cross-shore sediment transport,shoreline evolution and coastal profile evolution. The bed contours are assumed to bequasi-uniform in the longshore direction and the waves and currents are consideredto be quasi-stationary. These two basic assumptions limit the use of the tool to casesof long and uniform sandy beaches and cases where the shoreline evolution is theresult of the overall gradients in the longshore sediment transport capacity. However,total or partial blockage of the littoral drift by structures as well as the effect ofrevetments is included in the shoreline evolution model. Due to the assumptionsdescribed above, long coastal stretches can be investigated over long time spans.Various elements in LITPACK are described in Deigaard et al. (1986), Deigaard etal. (1991), Deigaard et al. (1993) and Elfrink et al. (1996).

Fig. 2 Left: Observed erosion, 1977-1986. Middle and right: simulated yearlylittoral drift and shoreline retreat.

Thorsminde Port is located in the central part of this very exposed stretch, wherethe net littoral drift is southward with an order of magnitude of 0.4 million m3/year,but where the gross transport is several times larger.

The sedimentation and shoaling problems at present affecting the harbour entranceare illustrated in Figure 3. The upper and lower panels show the results of the



simulation of nearshore waves and wave-driven currents for northwesterly wavesand southwesterly waves, respectively.

Fig. 3 Examples of wave and flow fields around the existing harbourUpper Hs = 3.0 m, 315°°°° , Lower Hs = 2.5 m, 235°°°°



The simulations were performed using a spectral wind wave model and a depth-integrated hydrodynamic model. Both modules are included in the morphologicalmodelling system, MIKE 21 CAMS, which will be discussed below.

For northwesterly waves, it is clearly seen how the wave-driven currents convergeat the end of the northernmost groyne and diverge south of the inlet jetty. Thesouthward sediment transported along the coast will thus be pushed around the tipof the groyne and will settle in the large eddy south of the inlet jetty. Forsouthwesterly waves, the northward littoral transport will be pushed directly into theharbour entrance.

The natural (equilibrium) water depth at the entrance to the harbour is 2-3 m, if nomaintenance dredging is carried out. The harbour entrance is at present dredged to3.5-4 m. Sedimentation of the entrance typically occurs during conditions with windspeeds between 8 and 15 m/s. The harbour is �small� compared to the width of thelittoral zone and there is a need for maintenance dredging after almost every storm,partly due to sedimentation in the entrance channel and partly due to shoaling infront of the entrance area. On average, 0.10 million m3 are dredged every year. Thedowndrift coastline suffers from erosion as some of the sediment migrates past theharbour and continues to the south at some distance from the shore. The transportcapacity in the inner nearshore zone immediately south of the harbour is, therefore,bigger than the sediment supply, which leads to the erosion of the beach. At present,this southern beach is protected by beach breakwaters and some of the dredgedmaterial is placed artificially on the eroding beach.

The goal of a new harbour layout can be summarised as follows:

• To increase the natural depth in front of the entrance, thereby decreasing thedowntime for access to the port and decreasing maintenance dredging

• To improve navigation conditions to the port, especially during storms fromSW, where opposing waves and currents co-exist in the present layout

It may be a positive side effect of a new layout that the downdrift erosion isreduced.

EXAMPLES OF BYPASS HARBOURSThe concept of bypass harbours has been used at several locations in Denmark.

Figure 4 shows Hanstholm harbour, a large fishery and ferry port located 80 kmnorth of Thorsminde, see Figure 1. This harbour was built in the 60�s at a criticallocation with about 0.4 million m3/year net northward transport and a grosstransport of around 1.5 million m3/year.

The symmetrical and streamlined layout creates a convergence of the flow pastthe harbour entrance and has resulted in a very small sedimentation, localised in the



outer harbour immediately inside the entrance, and a natural depth in the entrancearea of 9 m. Flows around this harbour are mainly driven by meteorologicalforcing, variations in wind and pressure, and, to a less extent, by wave breaking.

Fig. 4 Hansholm Harbour, west coast of Denmark

Figure 5 shows a small harbour on the northern coast of the Danish island ofSealand, see Figure 1. The old fishery port now constitutes the inner basin. Theupdrift deposition is clearly seen to the right of the photograph. The fishery port ismore than 100 years old.

Fig. 5 Hornbæk harbour, north coast of Sealand, Denmark



The main updrift breakwater was extended several times following its constructionuntil the nineties, when the downdrift breakwater was added. The symmetrical andstreamlined layout has reduced the sedimentation problem by a factor of three. Itcan even be seen from the photo how the natural bypass occurred and how, in yearsto come, it will feed the downdrift coast.

The expected development of the shoreline adjacent to a bypass harbour afterconstruction is sketched in Figure 6.

Fig. 6 Sketch of bypass harbour, from Mangor (2001)

IMPROVED LAYOUT FOR THORSMINDEA new layout has been developed for Thorsminde harbour using the principles of

natural bypass. The new layout includes a downdrift breakwater, a streamlining ofthe entrance by a small extension of the existing main breakwater to the southwestand a shortening of the updrift groyne.

It is expected that the contraction of the wave-driven currents around the harbourentrance will be enough to maintain an equilibrium depth in front of the harbour,which will be suitable for navigation. Strong, but well-defined currents will bepresent in front of the entrance during storms. The large outer harbour basin makesthis current pattern acceptable for navigation in rough weather. The presentsedimentation problem related to waves from a southwesterly direction will be



alleviated. The naturally bypassed sediment from north to south, will, with time,develop a bypass shoal and start to feed the downdrift beach. The new layout can beseen in Figure 9.

Analysis method: the Coastal Area Modelling System, MIKE 21 CAMSThe critical parameter for the new harbour layout is the equilibrium depth at its

entrance. This equilibrium depth is reached when the sediment transport capacity infront of the harbour is similar to the updrift littoral transport. The equilibrium depthhas been evaluated using the numerical modelling system, MIKE 21 CAMS. MIKE21 CAMS consists of modules for the simulation of waves, currents, sedimenttransport and bed level changes, with continuous updating of the bed levels and thesubsequent re-calculation of waves, currents and sediment transport. The use of a(2D) morphological area model paves the way for the possibility of following thebed development with time during storm conditions. This is a large step forward inmodelling procedure. Previously, coastal studies often relied on time series ofsediment transport on a fixed bed. The use of a morphological model allows for abetter comparison of the effectiveness of structures of different layouts.

The applied modules are described briefly in the following.

Wave modules. Two wave modules have been used for this study (i) MIKE 21PMS and (ii) MIKE 21 NSW. MIKE 21 PMS is based on the parabolicapproximation to the mild-slope equation (Kirby, 1986) and accounts for the effectsof shoaling, refraction, diffraction, wave breaking, directional spreading, forwardscattering and bed friction. MIKE 21 NSW is a spectral wind-wave model, whichdescribes the propagation, growth and decay of short-period waves in nearshoreareas by solving the equations for the conservation of wave action (Holthuijsen et al.,1989). The model includes the effects of refraction and shoaling, wave generationdue to wind, and energy dissipation due to bottom friction and wave breaking. Theeffects of current on these phenomena may be included. In both wave modules, thedissipation of wave energy due to breaking is calculated according to the model ofBattjes and Janssen (1978).

The hydrodynamic module, MIKE 21 HD, calculates the flow field from thesolution of the depth-integrated continuity and momentum equations, Abbott (1979).In addition to wind and tide, the forcing terms may include the gradients in theradiation stress field as calculated by the wave module. The currents and the meanwater level are calculated on a bed evolving at a rate equal to ∂z/∂t, as calculated bythe sediment transport module.

The non-cohesive sediment transport module, MIKE 21 ST, is used to calculatethe transport rates of graded sediment and the rates of bed level change ∂z/∂t due tothe combined action of waves and current. MIKE 21 ST uses DHI�s deterministicintra-wave sediment transport model STP to calculate the total (bed load + suspendedload) transport rates of non-cohesive sediment.



The sediment transport model, STP, is identical in LITPACK and MIKE 21 ST.The model has been described in detail in a series of papers, see e.g. Fredsøe (1984)and Deigaard et al. (1986). STP includes a quasi-3-dimensional description of theflow and the sediment transport, as described in Elfrink et al. (1996, 2000). Use ofthis approach allows simultaneous calculation of net sediment transport rates both inthe longshore and cross-shore directions.

The MIKE 21 CAMS, Coastal Area Modelling System, Johnson et al. (1994), is acomplex in which the wave, flow and sediment transport fields and rates of bed levelchanges are simulated in sequence and which includes full feedback from thedeveloping bathymetry to all modules. The updating technique is described in detailin Johnson and Zyserman (2002). The modelling system is sketched in Figure 7.

Model calibration and validationThe morphological modelling complex has been calibrated and validated against

measured pre-storm and post-storm bathymetries of the harbour entrance for twostorms from the northwest and southwest respectively. The Danish Coastal Authoritymaintains a directional wave meter 3 km northwest of the site. The water level isrecorded every 15 minutes inside the harbour. These recordings have been used asboundary conditions for the morphological modelling complex. General surveys areavailable for each summer, but detailed surveys are carried out regularly, especiallyafter a severe event has taken place around the harbour entrance. The pre-storm andpost-storm bathymetries measured around the entrance, as well as the modelbathymetry prior to the storm (lower left panel), are shown in Figure 8.

The grid spacing of the model bathymetry is 6 m. The bathymetry outside the areawhere pre-storm measurements are available is constructed from the generalsurveys. The lower right panel shows the modelled post-storm bathymetry. It can beseen that the sand shoal off the main breakwater was pushed to the south during thestorm and that the model is able to reproduce this process.

The calibration of the morphological modelling complex comprised the tuning ofbed roughness, wave breaking parameters, the selection of a wave model (parabolicmodel versus spectral wind wave model) and the testing of the influence of the startbathymetry outside the area, where pre-storm measurements are available. Thesediment properties are constant all over the model area. The median grain size is0.3 mm is this area and the sediment is well sorted. The best calibration is obtainedwith the following parameters/choices:

• breaking parameter γ2 = 1.0• bed roughness: M = 48 m1/3 /s• the spectral wind wave model



Fig. 7 Flow diagram of the morphological modelling system MIKE 21 CAMS

The adopted breaking parameters result in a narrower surf zone compared to thestandard value of γ2 = 0.8 in Battjes and Janssen(1978). The forward scatter of waveson the structures has shown, in this case, to lead to too high waves, which preventsthe deposition of sand in the entrance when using the parabolic mild slope model.More realistic morphological changes were obtained using the spectral wind wavemodel. The initial bathymetry was changed to include a larger sand bar stretching100 m to the north and with the same shape as the observed pre-storm bar. Thesimulated post-storm bathymetry was not sensitive to this change. The simulation ofa historical storm from the southwest using the above-mentioned model settingshowed acceptable results.



Fig. 8 Boundary conditions, observed pre-storm and post-storm bathymetries,simulated pre-storm and post-storm bathymetries, 16 and 27 October 1997



TESTING AND DOCUMENTATION OF THE NEW LAYOUTThe proposed new layout is seen in Figure 9, which shows the result of a

repetition of the simulation of the October 1997 storm with the new breakwater,including the redesigned existing structures. The left panel is the initial modelbathymetry, the right panel is the bathymetry after the storm. It appears that a waterdepth of about 3.5 m can be maintained in front of the entrance after the storm withthe modified layout. Furthermore, no sedimentation seems to have taken place in theentrance area.

The �equilibrium� depth in front of the harbour was investigated through sixthirty-day morphological simulations with persistent wave conditions. Thebathymetries after 15 and 30 days of constant wave conditions, Hs = 3.5 m, from thenorthwest applied at the offshore boundary of the model area, are shown in Figure10 together with the initial bathymetry.

The initial bathymetry included in all six cases a deep area in front of the harbour,down to �6 m. The simulation results show that this deep area is migrating to thesouth and is filling in, see also Figure 11.

After about 20 days of constant wave action, the water depth at the entrancestabilises at approximately 3.3 m. The lower right panel shows the bed levelchanges with time at two points, A and B, just off the existing main breakwater andat the entrance. The locations of points A and B are shown in Figure 10. It appearsthat large-scale bed forms migrate towards the south in the morphological modeland that the �equilibrium bed level� at the entrance is dynamic, with a minimumdepth of around 3.2 m.

Fig. 9 New layout. Morphological modelling of the October 1997 storm



The results seem to indicate that the 3 m depth contour never reaches the tip of thenorthern breakwater. A shoal develops immediately downdrift of the harbour, in thearea where the contracted current expands after flowing across the entrance. Thisshoal keeps growing until the depth decreases to a level where the sedimenttransport capacity, due to wave breaking and wave-driven currents, corresponds tothe amount which bypasses the harbour. The simulation indicates that the presentcoastal erosion on the downdrift side will decrease with time following theconstruction of the new structures.

This is illustrated further in Figure 11, which shows the sediment transport fieldafter 15 and 30 days of morphological simulation. The figure indicates thatlongshore gradients in transport capacity decrease with time as the downdrift shoaldevelops.

CONCLUSIONSMaintenance of a minimum depth in front of the entrance to a small fishery port

on an exposed sandy coast is the key issue for the success of an improved harbourlayout.

A new streamlined layout, which optimises natural sand bypass, has beendeveloped and investigated. A morphological modelling complex, which simulateswaves, currents, sediment transport and corresponding bed level changes, hasproven to be a useful tool in supporting the understanding of the processes aroundthe harbour and in estimating the minimum depth at the entrance during and afterstorm events.



Fig. 10 Modelled morphological evolution for constant waves,Hs = 3.5 m and 315°°°°. Bathymetries after 0 days, 15 days, 30 days.

Time series of bed level in points A and B.



Fig. 11 Sediment transport capacity after 15 and 30 days, respectively,with constant waves, Hs = 3.5 m and 315°°°°

REFERENCESAbbott M.B. 1979. Computational hydraulics, elements of the theory of free surface

flows. Pitman, London.Battjes, J.A. and Janssen J.P.F.M. 1978. Energy loss and Set-up due to breaking of

random Waves. Procs. of the 16th Int. Conf. On Coastal engineering, ASCE, 569-587.

Deigaard, R. 1993. A note on the 3-dimensional shear stress distribution in a surfzone�. Coastal Engineering, 20, 157-171.

Deigaard, R., Fredsøe, J. and Brøker Hedegaard, I. 1986. Suspended sediment in thesurf zone. J. Waterway, Port, Coastal and Ocean Engineering, 112 (1), ASCE,115-128.

Deigaard, R., Justesen, P. and Fredsøe, J. 1991. Modelling of the undertow by a oneequation turbulence model. Coastal Engineering, 15, 431-458.

Elfrink, B., Brøker, I. and Deigaard, R. 2000. Beach Profile Evolution due to ObliqueWave Attack. Procs. of the 27th Int. Conf. on Coastal Eng., ASCE, 3021-3034.

Elfrink, B., Brøker, I., Deigaard, R., Hansen, E.A. and Justesen, P. 1996. Modellingof 3D sediment transport in the surf zone, Procs. of the 25th Int. Conf. on CoastalEng., ASCE, 3805-3817.

Fredsøe J. 1984. The turbulent boundary layer in combined wave-current motion.Journal of Hydr. Eng., 110(8), ASCE, 1103-1120.



Holthuijsen, L.H., Booij, N. and Herbers, T.H.C. 1989. A prediction model forstationary, short-crested waves in shallow water with ambient currents. CoastalEngineering, 13, 23-54.

Johnson, H.K. and Zyserman, J.A. 2002. Controlling spatial oscillations in bed levelupdate schemes. Coastal Engineering, 46, 109-126.

Kerper R.D., Brøker I., Damgaard Christensen, E. and Zyserman J.A. 2002.Application of coastal modelling systems in support of integrated coastal zonemanagement. Proceeding of Coastal Disasters conference, San Diego, California.

Kirby, J.T. 1986. Rational approximations in the parabolic equation method for waterwaves. Coastal Engineering, 10, 355-378.

Laustrup, C. and Toxvig, H. 1998. Evaluation of the Effect of 20 Years ofNourishment, Procs. of the 26th Int. Conf. on Coastal Eng., ASCE, 3074-3085.

Mangor, K. 2001. Shoreline Management Guidelines, DHI Water & Environment,232p.

morphological modeling: a tool for ... engineering today, gainesville, florida, october 2003 6...

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