quantification of changes in current intensities induced by wave overtopping around low-crested...

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Coastal Engineering 55

Quantification of changes in current intensities induced by wave overtoppingaround low-crested structures

Iván Cáceres a,⁎, Marcel J.F. Stive a, Agustín Sánchez-Arcilla b, Le Hai Trung a

a Faculty of Civil Engineering and Geosciences, Delft University of Technology, 2600 GA Delft, The Netherlandsb Laboratori d’Enginyeria Marítima, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain

Received 7 March 2007; received in revised form 30 July 2007; accepted 5 September 2007Available online 1 November 2007

Abstract

The phenomenon of overtopping is traditionally studied for well-emerged harbour structures and often focuses on safety and stability. In thispaper laboratory tests are presented and analysed to sharpen the hypothesis that overtopping is capable of changing the horizontal circulationpattern around low-crested structures. A unique data set from laboratory experiments was acquired in the wave basin at Delft University ofTechnology. The experiments were performed using an emerged impermeable low-crested structure (three freeboards and three different waveconditions for each freeboard) and yielded nine different combinations of set-up and overtopping driving forces. Using this information it waspossible to quantify the changes in cross-shore and longshore velocity induced by the overtopping and the set-up changes under the differentfreeboard and wave conditions described. It is found that overtopping enhances the outgoing flows (longshore velocities parallel to the structure)away from the lee side of the structure and dampens the water level gradient driven flow towards the structure.© 2007 Elsevier B.V. All rights reserved.

Keywords: Overtopping; Low-crested structure; Circulation; Laboratory experiments

1. Introduction

Nearshore circulation is a complex phenomenon with anumber of important aspects that are not yet understood, eventhough it has been studied in-depth in recent decades. Theinteraction with coastal structures and, in particular, the effectsassociated with flow over and through normally permeabledetached breakwaters continue to pose a significant challenge toresearchers. There is not yet a universally accepted formulationthat takes into account the physical parameters that control thewater discharge over the structure, which is usually referred toas overtopping. Moreover, overtopping has traditionally beenstudied in well-emerged harbour structures with a focus onsafety and stability. Studies such as Owen (1980), Van der Meerand Janssen (1995) and Hedges and Reis (1998) containformulations that can be used to predict the amount ofovertopping that will occur over a structure depending on itsdesign parameters (freeboard, slope, geometry, roughness,

⁎ Corresponding author.E-mail address: [email protected] (I. Cáceres).

0378-3839/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.coastaleng.2007.09.003

permeability, etc.) and wave characteristics (wave height,period, incidence angle, spectral form, etc.). However, anumber of major uncertainties remain when these formulationsare applied to low-crested coastal structures (LCS).

We assume that overtopping alters the circulation on the leeside of a LCS. In this paper overtopping is evaluated in order toquantify its effect on circulation in the presence of a shore-parallel structure. These structures usually have a permeablecore and a crest elevation only slightly above the mean waterlevel and are frequently overtopped both under normal waveconditions and during severe wave storms. The resultingovertopping and its effect on the associated nearshorecirculation are found to be key elements in determining thecorrect functional design of this type of low-crested structure.However, given the limited field and laboratory observationsavailable there is a clear need for further experimental resultssuch as presented in this paper. The laboratory experimentsperformed in this study yields a unique data set for animpermeable LCS with three different freeboards and threedifferent wave conditions for each freeboard configuration. Thetests therefore provide important information on wave fields,

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http://dx.doi.org/10.1016/j.coastaleng.2007.09.003

Fig. 1. Schematization of wave-driven circulation flows for a longshore uniformbeach with an emerged LCS (revised from Cáceres et al., 2005).

114 I. Cáceres et al. / Coastal Engineering 55 (2008) 113–124

circulation patterns and structural interactions. Furthermore, thetests focus on the phenomena that control the circulation on thelee side of the structure.

The paper discusses the experimental configuration and thebasic physics of the data sets obtained. The text goes on toconsider their suitability for assessing the hydrodynamic impactassociated with detached low-crested structures. This includes

Fig. 2. Wave basin layout in the Fluid Mechanics La

the increase in shore-parallel currents close to the lee side of theLCS due to overtopping and their damping effects on the ripcurrent on the lee side of the structure. This information willalso be used to validate nearshore circulation models and, inturn, to improve the functional design of shore-parallelstructures, whose numerically based analysis still presents anumber of important uncertainties.

2. Physical modelling

There is only limited field and lab data available for low-crested structures that take into account overtopping effects.Recent work carried out by Kramer et al. (2005) in the wavebasin at Aalborg University takes into account the effects ofovertopping on nearshore circulation. Some of the experimentsused a gap configuration between two emerged (0 and 3 cmfreeboard) detached breakwaters at a 1:20 laboratory scale. Thestructures were permeable and built with a rubble-mound cross-section (an armour layer of rock on a core). The velocitymeasurements were taken at the front and the rear of theemerged structures and also in the gap. The simulated waveconditions reproduced regular and irregular wave conditionswith different wave heights (Hs) and over different periods (Tp).Although both of the breakwaters were permeable, the acquireddata revealed a significant increase in the return velocitiesinduced by overtopping in the gap between the structures(Zanuttigh and Lamberti, 2006).

We based our work on the above-mentioned study and on thenumerical modelling of similar conditions in order to verify andquantify the circulation changes induced by overtopping overan impermeable structure (to prevent flow through the structureand focus on the overtopping-induced circulation changes).This is done by comparing the circulation patterns in thepresence and absence of overtopping in the lee side of anemerged LCS. From the inferred circulation patterns weselected the locations in which the highest circulation flow

boratory at the Delft University of Technology.

Table 1Reflection coefficients measured in the wave basin at the Delft University ofTechnology for the three test layouts

Layout 1(Rc=0 cm)

Layout 2(Rc=3.5 cm)

Layout 3(Rc=25 cm)

Hs=6 cm 27% 30% 31%Hs=7 cm 27% 23% 36%Hs=9 cm 30% 34% 43%

115I. Cáceres et al. / Coastal Engineering 55 (2008) 113–124

velocities are expected. This should produce a strong signal-to-noise ratio, which removes the risk of experimental inaccuracycaused by the observation equipment.

As explained by Pilarczyk (2003) and Zyserman andJhonson (2002), there are up to 14 parameters that controlcirculation on the lee side of an emerged/submerged LCS. Themost important of these parameters are the length of thestructure and the distance of the structure from the shoreline.Although many factors have a certain influence, there is ageneral consensus that the main driving forces behind the lee-side circulation of emerged LCSs are the diffraction pattern andthe set-up gradients. Fig. 1 shows that the resulting radiationstress gradients generate water flows close to the shore, whichconverge towards the centre of the sheltered area. Theseconverging flows also show (associated with the resultingeddies) a component directed towards the structure (i.e. flowingtowards the offshore side) and the required diverging flows(which flow parallel to the lee side of the LCS). This traditionalcirculation diagram shows two eddies – one clockwise and theother counter clockwise – to the left and right of the LCS,respectively, as seen from deep water towards the shoreline.These two eddies are clearly defined under perpendicular waveincidence and meander under oblique wave attack.

In this paper we focus on overtopping and normal waveincidence. There is no need to consider all of the 14 controllingparameters to sharpen our hypothesis, i.e. overtopping iscapable of modifying the horizontal circulation on the lee sideof an LCS to a significant degree. The work presented herefocuses on varying the overtopping strength (three freeboardsand three wave conditions for each freeboard) to obtain nine

Fig. 3. Photograph of the second layout configura

different combinations of set-up and overtopping driving forces.Wave height and freeboard were selected from among all of thepossible parameters as they have a significant effect on theovertopping discharge rate (Owen, 1980 and Van der Meer,2002).

2.1. Experimental set-up

The aim of the physical experiments presented in this paperwas to quantify the circulation changes induced by theovertopping of irregular waves over an emerged impermeablebreakwater. The experiments were performed in the 15-m-wideand 30-m-long wave basin of the Fluid Mechanics Laboratory atthe Delft University of Technology. Three piston-type wavegenerators are located at the offshore end of the wave basin.Each wave generator is 5-m-wide, which gives a total span of15 m. The height of the wave board allows for a maximumwater depth of 0.40 m. Opposite to the wave generators, a 1:20plane slope that stretches across the whole width of the wavebasin acts as a dissipative beach. The smooth bottom of thewave basin ensures minimal friction dissipation in the flatsection between the wave generators and the plane slope.

Three different layouts were tested to verify the effects ofovertopping on the circulation on the lee side of the detachedbreakwater. The adjustments between the three layouts weremade to the freeboard of the structure. The first layout had azero freeboard, the second was designed with an emerged,3.5 cm freeboard and the third layout was built by placing anemerged wooden wall of 25 cm on top of the structure toprevent any kind of overtopping. The three layouts providedifferent overtopping discharges and induce different circu-lation patterns depending on the volume of water thatovertops the structure. We anticipated severe overtopping inLayout 1, limited overtopping in Layout 2 and no overtoppingin Layout 3.

The structure was built of concrete with a trapezoidal cross-section. The structure had a total crest length of 5 m and frontand rear slopes of 1:2. The crest of the structure was located5.13 m from the still water level shoreline (Fig. 2).

tion used to test the LCS, looking shoreward.

Table 2Wave conditions used in the laboratory experiments

Test code Hs (m) Tp (s)

I6 0.06 1.22I7 0.07 1.32I9 0.09 1.5

Hs is the significant wave height and Tp is the peak period.


These experiments provide unique information on theinfluence of overtopping on the circulation around an emergedLCS. The reflection in front of the structure disturbed the wavegeneration because the wave paddle generator was not fittedwith an active reflection absorption system. As a result, anarmour layer of rock was placed on the offshore side of the basinin Layout 1 in an attempt to reduce the reflection effects asmuch as possible. The d50 of this armour layer was 2.7 cm andthe width of the constructed layer was approximately 5 cm. Aclear increase in the reflection coefficient was measured forLayouts 2 and 3. To further reduce the wave reflection a newlayer of rock (4-cm-wide) and two nylon nets were placed infront of the structure in Layouts 2 and 3. These adaptations mayhave had an impact on the overtopping but this does not affectthe objective of this paper.

The reflection coefficient according to Mansard and Funke(1980) was obtained in front of the structure from the signals ofthree wave gauges. Table 1 shows the values of this coefficientfor all tested conditions.

2.2. Test conditions

The test conditions were obtained by scaling different wavestates that are commonly found in the Mediterranean. The threeselected wave conditions provide different overtopping rates

Fig. 4. EMF (a) andWHM (b) distribution during the experiments. The exact positionparallel to the structure shows the beginning of the slope (see Fig. 2).

and were combined with the three layouts described above sothat we could study the influence of overtopping on thecirculation around an emerged LCS (Fig. 3). The scale(prototype to model) ratio used is NL=20. The model had tobe geometrically undistorted to ensure the correct reproductionof both refraction and diffraction in a Froude-scaled, short-wavehydrodynamic physical model (Hughes, 1993).

The laboratory experiments were conducted using unidirec-tional irregular waves represented by the JONSWAP spectrum.Second-order wave board control was used to generate second-order Stokes waves, which therefore minimized spurious,second-order free wave generation (Madsen, 1971). Thelaboratory wave conditions used are presented in Table 2.

2.3. Instrumentation

The wave field in all tests was measured by five resistance-type wave gauges (WHM) and five electromagnetic flow meters(EMF). The EMFs and WHMs were moved along the basin toobtain measurements at all selected points for every layout. Theresolution was close to 1 mm for the wave gauges and 1 cm/s forthe EMFs. The exact location of the different instruments isgiven in Table A-1 in the Appendix and is schematized in Fig. 4.The tests were performed three times for every measurementpoint and for each of the simulated wave conditions to check therepeatability of the measurements. The results presented hereare the averaged measured velocities from the three tests; wediscarded a few data measurements that were significantoutliers. The EMFs were placed at 1/3 of the local waterdepth from the bottom, which corresponds to the depth-averaged flow velocity in the case of a logarithmic verticalvelocity. This depth was selected because we expect close tologarithmic longshore velocities and a return flow behind the

of the different devices can be found in Table A-1 (see Appendix). The black line


structure at the lower 2/3 of the water column (which istherefore well represented at the selected level).

Due to the expected circulation and the selected EMF andWHM configuration, we expected the main circulation changesinduced by wave overtopping to be found in the current behindthe structure (V9-13 in Fig. 4-a) and in the currents flowingaway from the protected area on either side of the structure (V3,4, 18 and 19).

Since the freeboard is kept constant in a particular layout, thelargest overtopping should occur under the 9 cm waveconditions. Consequently, we expected to observe the clearestcirculation changes induced by overtopping when comparingLayouts 1 and 3 for I9 conditions (irregular waves withHs=9 cm, see Table 2) and mainly in the regions (EMF sensors)described in the previous paragraph.

2.4. Experimental results

In this section we present the data acquired from the differentexperiments. The numerical information obtained from thedifferent EMFs is included in the Appendix (Tables A-2, A-3and A-4). The measured velocities are plotted in Figs. 5, 6 and 7for all of the simulated wave conditions according to eachlayout. In these figures, the X and Y axis are presented in Fig. 2,i.e. positive towards the shoreline and to the left (when theshoreline is seen from the wave paddle).

From previous experiments carried out in the wave basin ofthe Fluid Mechanics Laboratory at the Delft University ofTechnology, it is known that with the present condition of thewave generators there is a certain asymmetry in the wavegeneration system. This asymmetry was reported by Over(2006) and Cáceres et al. (submitted for publication) and causes

Fig. 5. Measured circulation fields for test Layout 1 (z

greater wave heights on the left (looking to the shoreline fromthe wave paddle) of the basin (5–9% higher than the wavesgenerated on the right of the basin). Although the non-homogeneity of the wave field has an effect on the inducedcirculation, the flow field data are qualitatively consistent withthe expected hydrodynamic behaviour based on the previousmodelling of similar cases (Cáceres et al., 2005). Some specificresults obtained in these tests will be quantitatively affected bythe wave asymmetry, but the general tendencies are consistentwith current knowledge and basic physical principles.

All incoming waves were able to overtop the structure duringthe tests carried out using Layout 1 (Rc=0 m). The maximumcross-shore velocities were measured at the V9 EMF for all ofthe different wave conditions (−0.102 m/s for I6, −0.118 m/sfor I7 and −0.156 m/s for I9). The maximum longshorevelocities were measured at V4 for all of the different simulatedwave conditions (−0.065 m/s for I6, −0.085 m/s for I7 and−0.111 for I9).

The maximum cross-shore velocities for Layout 2(Rc=0.035 m) were measured at the V9 EMF for all waveconditions (−0.090m/s for I6,−0.104m/s for I7 and−0.170m/sfor I9). The maximum longshore velocities were measured at V4for all of the simulated wave conditions (−0.041 m/s for I6,−0.052 m/s for I7 and −0.074 m/s for I9).

The maximum cross-shore velocities for Layout 3 (Rc=0.25 m) were also measured at the V9 EMF for all waveconditions (−0.081 m/s for I6, −0.096 m/s for I7 and −0.148 m/sfor I9). The maximum longshore velocities were measured at theV5EMF (−0.037m/s) for I6wave conditions, at V4 (−0.042m/s)for I7 and at V15 (0.077 m/s) for I9.

From the data presented in this study, it is important tohighlight that all results show the two expected eddies. As

ero freeboard) and wave heights of 6, 7 and 9 cm.

Fig. 6. Measured circulation fields for test Layout 2 (3.5 cm freeboard) and wave heights of 6, 7 and 9 cm.


explained above, both eddies are distorted by small geometricaldifferences and the wave height gradient present in the basin,but this distortion is not large enough to invalidate theexperiments from a qualitative point of view.

As expected, the highest velocities recorded at the fieldcontrol points were observed under the most energetic waveconditions (I9). The velocities are highest at the first measuringpoint in the rip current (cross-shore velocity) and in the

Fig. 7. Measured circulation fields for test Layout 3 (25

outgoing flows parallel to the breakwater (longshore velocity).Further analyses of the acquired data are given in the nextchapter.

3. Analysis and discussion

It can be seen from the data acquired that the studied cases(with and without the influence of overtopping at a LCS) exhibit

cm freeboard) and wave heights of 6, 7 and 9 cm.

Fig. 8. Cross-shore velocity measurements at EMFs V9, V10, V11, V12 and V13 (see Fig. 2) under I6 wave conditions.


the characteristic circulation patterns for this type of structure.The two lateral eddies appear in all cases and overtopping seemsto influence the intensity of the induced circulation pattern.

The main cross-shore velocities in all cases were recorded bythe EMF sensors (V9–V13) in the central transect perpendicularto the shoreline. The main cross-shore velocity was alwaysrecorded at V9, which is the closest EMF to the shoreline. Themaximum cross-shore velocity was recorded under the I9 waveconditions and with test Layout 2. The maximum gradient set-up occurred in the middle cross-section of the sheltered area andwas greater under more energetic wave conditions. Thisgradient set-up is the driving force behind the rip current, soit is logical that the maximum cross-shore velocities wererecorded at the V9 EMF under the most energetic conditions.

The highest longshore velocities were usually observed closeto the inner side of the LCS associated with the outgoing flowfrom the sheltered area (V1, V2, V3, V4, V19 and V20). Themeasured values tended to be higher on the right-hand side (asseen from the wave generator to the shoreline). This wasexpected because of the greater wave heights observed on theleft-hand side of the basin. In contrast with the cross-shorevelocity, the highest longshore velocity was recorded at V4 for

Fig. 9. Cross-shore velocity measurements at EMFs V9, V10,

Layout 1 and under I9 wave conditions (−0.111 m/s). We willdiscuss this result later in the paper.

The greatest changes between the non-overtopping config-uration (Layout 3) and the other two layouts were recordedunder the I9 wave conditions. The highest degree of over-topping was observed under these wave conditions and it had agreater impact on the circulation patterns. When analyzing thecross-shore velocity behaviour in the cross-shore transects V9,V10, V11, V12 and V13 (behind the LCS, see Figs. 8 for I6, 9for I7 and 10 for I9), the greatest differences between measuredvelocities were observed at the V12 EMF. There is nosignificant change between Layouts 1, 2 or 3 under the I6wave conditions, which demonstrates that overtopping underthese conditions is negligible except for the change observed inV13 (x=3.55 m, the closest point to the LCS). This will beexplained in more detail later in the paper.

Overtopping began to have a noticeable effect on cross-shoreflow towards the structure under the I7 wave conditions (Fig. 9).While the results for Layouts 2 and 3 are very similar, Layout 1was affected by a high degree of overtopping and the flowvelocity towards the structure decreased significantly at EMFs11 (x= 4.9 m) and 12 (x=4.05 m).

V11, V12 and V13 (see Fig. 2) under I7 wave conditions.

Fig. 10. Cross-shore velocity measurements at EMFs V9, V10, V11, V12 and V13 (see Fig. 2) under I9 wave conditions.


Overtopping has a very clear effect on cross-shore flowunder the I9 wave conditions (Fig. 10). There is a constantreduction of the velocities going towards the structure whilethere is an increase of the water overtopping the structure. Theinfluence of overtopping on flow velocity reached a maximumat EMF V12, which recorded a decrease of 0.1 m/s betweenLayouts 1 and 3.

The results observed at V13 EMF were surprising; thevelocity measurements at this point tended to be higher forLayout 1 than for Layouts 2 and 3, while we expected to seegreater changes in flow velocity in the opposite direction (i.e.higher velocities for Layouts 2 and 3). V13 was the first EMFthat was placed behind the LCS and we had anticipated that thissensor would record a decrease in flow velocity in this area dueto overtopping. However, the V13 EMF appeared to show anincrease in flow velocity towards the LCS. We concluded thatthis may have been due to the fact that the EMF was placed toodeep within the water column (i.e. at a depth of 15 cm from thebottom, 1/3 of the local depth of 22.13 cm). This depth levelwas close to the toe of the structure (10 cm, as can be seen inFig. 11), in an area with intense 3D circulation.

Different features (related to intensity) of the nearshorecirculating system can be deduced from the apparent increase inflow velocity towards the structure (shown in Figs. 8, 9 and 10).These features were induced by the lower freeboard and thehigher associated overtopping. They can be summarized asfollows: the effects of wave height on the 3D circulation close tothe structure were non-linear and sometimes counter intuitive;the turbulence level generated by overtopping was difficult to

Fig. 11. Qualitative sketch of the 2DV circulation pa

predict and model; overtopping should alter the normal scouringpatterns around an LCS. In order to explain these features wewould require further, in-depth analyses and a combination ofobservations and numerical simulations, which are beyond thescope of this paper.

It is also apparent from the data that cross-shore velocities intransects V9–13, induced by the I9 wave conditions and sortedby layout, as can be seen in Fig. 12, were always highest foreach EMF position. Furthermore, this figure also shows that thechanges in flow velocity measured under different waveconditions decreased when there was greater overtopping.While the average difference between I9 and I6 in Layout 3 (noovertopping) was approximately −0.067 m/s, the equivalentvalue in Layout 2 fell to −0.053 m/s and a minimum of−0.025 m/s was recorded in Layout 1.

As mentioned above, the greatest changes in longshorevelocity were recorded at points on either side of the LCS whereEMFs V1, V2, V3,V4 (on the right-hand side) and V18 and V19(on the left-hand side) were located (see Fig. 2). The circulationpattern in these areas was controlled by the two eddies thatformed on the lee side of the structure. The velocity measure-ments show that the outgoing flows moved away from the rearside of the LCS in parallel to the structure.

In contrast to its effects on cross-shore velocities, over-topping tended to increase the differences in longshorevelocities under the various wave conditions tested when thedata are sorted according to layout. This can be seen in Fig. 13in which where the data collected from EMFs 1, 2, 3 and 4 wereplotted. The difference in recorded velocities between I6 and I9

ttern observed close to the low-crested structure.

Fig. 12. Cross-shore velocities sorted by layout. The highest values correspond to the greatest incident wave height (case I9).


wave conditions for Layout 1 was approximately 0.053 m/s,while the difference between the two wave conditions forLayout 3 was approximately 0.022 m/s.

The contrasting behaviour of longshore and cross-shorevelocities can be explained by the enhanced hypothesis that thevolume of overtopping water reduces the flow velocity towardsthe structure and increases the outgoing flows (longshorevelocities) away from the lee side of the LCS. By reducing thecross-shore velocities, overtopping is homogenizing the

velocities measured under the different wave conditions, andby increasing the longshore velocities it is increasing thevelocity differences between the tested wave conditions bothwith and without overtopping.

Fig. 13 also reveals a sharp increase in the longshore currentat roundheads (EMFs 1 and 3). Wave height increases as a resultand almost every wave overtops the structure: the currentincreases on average by a factor of four, whereas this increase islimited to a factor of two when there is no overtopping.

Fig. 13. Longshore velocities sorted by layout. The continuous lines show the velocities at EMFs V1 and V2 and the dashed lines at positions V3 and V4.


When sorting the longshore velocity according to waveconditions, the most noticeable effect of overtopping is the non-homogeneous increase in velocity at V1–V2, V3–V4 and V18–V19 (V1, V3 and V18 are closest to the LCS). The increase inlongshore velocity at these points is caused by higherovertopping and tends to be more apparent at points locatedfurther from the structure. The average increase in longshorevelocity at the points located closest to the structure (V1, V3 andV18) is approximately 0.008 m/s, while the corresponding

increase at the EMFs located further away (V2, V4 and V19) isapproximately 0.038 m/s. This non-homogenous increase invelocity shows that the gradient of measured velocitiesincreases significantly as we move from the structure to theshoreline.

From the data presented it can be also verified that thegreatest changes in longshore velocity due to overtopping areobserved under the I9 wave conditions and when Layouts 1 and3 are compared, as was the case of cross-shore velocities.

Table A-2Velocities (in m/s) measured by the different EMF (V1–V20) sensors for Layout 1and under different wave conditions (irregular waves of 6, 7 and 9 cm denoted byI6, I7 and I9, respectively)

I6 I7 I9

EMF Vel X Vel Y Vel X Vel Y Vel X Vel Y

V1 −0.007 −0.016 −0.003 −0.028 0.008 −0.074V2 0.010 −0.061 0.019 −0.074 0.029 −0.095V3 −0.003 −0.008 0.004 −0.021 0.030 −0.080V4 0.025 −0.065 0.044 −0.085 0.049 −0.111V5 0.023 −0.038 0.014 −0.055 −0.006 −0.074V6 −0.010 0.003 −0.021 −0.006 −0.048 −0.010

(continued on next page)


4. Conclusions and future work

In this paper we present and discuss a unique experimentaldata set. The data correspond to an emerged impermeable LCSunder three different wave conditions and with three differentfreeboards. This experimental evidence is used to study theinfluence of different overtopping discharges on the circulationpattern on the lee side of the structure.

The main results of the experimental analysis are thatovertopping enhances the divergent longshore flows anddampens the central (offshore directed) return flows. Thesecharacteristics are consistent with basic mass/momentumconservation principles and were quantified in the tests. Theresults also cover a range of freeboard conditions, including thecase of a freeboard that was large enough to completely preventovertopping. This information enables us to assess thedifferences between varying levels of overtopping and thecase in which this flow phenomenon does not occur. The lattercase is relevant to the study of emerged structures whereovertopping is hardly ever considered.

The rip current in the middle of the protected area is the mainflow type affected by overtopping. The velocity of this cross-shore flow decreases considerably when there is an increase inovertopping water volume. The maximum change in velocitywas observed under the most energetic wave conditions(Hs=9 cm) in which the V12 EMF recorded a decrease of0.1 m/s in the presence/absence of overtopping (Layout 1 versusLayout 3). It is hypothesized that the amount of water able toovertop a structure has a homogenizing effect on this type ofvelocity measurement. The rip-current flow under the threedifferent wave conditions was more homogeneous when therewas greater overtopping (Layout 1) than when there was noovertopping at all (Layout 2).

The divergent flow on both sides of the structure is thesecond flow type that was significantly affected. The usualcirculation pattern at these points is a water flow toward theprotected area close to the shoreline and a shore-parallel currentaway from the sheltered area close to the structure. If we focuson the measured flow velocities away from the sheltered area,we can see that the longshore current close to the structureclearly increased at higher overtopping rates. The maximumdifferences in flow velocity (with/without overtopping) wereobserved when comparing Layouts 1 and 2 for positions V1–V4; the values recorded were approximately 0.06 m/s for waveheights of 9 cm and 0.04 m/s for wave heights of 6 and 7 cm.

It is clear from the data presented in this paper that thecirculation pattern on the lee side of a low-crested structure isaffected by overtopping under particular wave conditions andLCS configurations. Therefore, it is advisable to consider theeffects of overtopping in order to improve the functional designof such structures.

Acknowledgements

The authors of this paper would like to thank all of thelaboratory staff who assisted with the experiments. We are alsothankful for the received inputs from the reviewers aiming to

improve this paper. The study was conducted under theframework of the EU Marie Curie-TOK project MARIE(contract number MTKD-CT-2004-014509).

Appendix A

Table A-1Location of sensors – WHM and EMF – throughout the studied domain(corresponding to the Delft University wave basin)

Location
X (m) Y (m) Depth (cm)
WHM
S1 −12 7.5 40 S2 2.16 7.5 29.2 S3 2.88 3.42 25.6 S4 2.88 11.58 25.6 S5 3.75 5 21.3 S6 3.75 6.5 21.3 S7 4.6 6.5 17 S8 5.35 6.5 13.3 S9 4.05 7.5 19.8 S10 4.9 7.5 15.5 S11 5.6 7.5 12 S12 3.75 8.5 21.3 S13 4.6 8.5 17 S14 5.35 8.5 13.3 S15 3.75 10 21.3 S16 4.6 10 17 S17 5.35 10 13.3
EMF
V1 4.1 4 19.5 V2 5.2 4 14 V3 3.75 5 21.3 V4 4.6 5 17 V5 4.4 6 18 V6 3.55 6.5 22.3 V7 6 6.5 10 V8 4.4 7 18 V9 6 7.5 10 V10 5.6 7.5 12 V11 4.9 7.5 15.5 V12 4.05 7.5 19.8 V13 3.55 7.5 22.3 V14 4.4 8 18 V15 3.55 8.5 22.3 V16 6 8.5 10 V17 4.4 9 18 V18 3.75 10 21.3 V19 4.6 10 17 V20 4.1 11 19.5

Table A-2 (continued)

I6 I7 I9


V7 −0.025 0.006 −0.048 0.015 −0.095 0.027V8 −0.039 −0.012 −0.061 −0.023 −0.123 −0.033V9 −0.102 −0.016 −0.118 −0.017 −0.156 −0.036V10 −0.071 −0.034 −0.079 −0.035 −0.109 −0.052V11 −0.043 −0.017 −0.046 −0.018 −0.062 −0.033V12 −0.002 0.013 −0.004 0.014 −0.003 0.020V13 −0.013 0.021 −0.016 0.029 −0.026 0.051V14 0.031 0.011 0.034 0.017 0.038 0.024V15 0.007 −0.007 0.003 0.011 −0.002 0.029V16 0.006 −0.012 0.006 −0.012 −0.002 −0.014V17 0.047 0.030 0.054 0.037 0.057 0.052V18 −0.005 −0.020 −0.010 −0.014 −0.005 −0.020V19 0.012 0.032 0.014 0.060 0.030 0.104

Table A-4 (continued)

I6 I7 I9


V13 0.011 0.014 0.004 0.026 −0.022 0.063V14 0.010 0.022 0.006 0.015 −0.025 −0.002V15 0.017 −0.008 0.024 0.023 0.018 0.077V16 0.000 −0.016 0.007 −0.012 −0.008 −0.038V17 0.029 0.035 0.052 0.029 0.079 0.033V18 −0.006 −0.009 −0.008 −0.012 −0.004 −0.004V19 0.003 0.021 0.007 0.024 0.048 0.055V20 −0.007 −0.004 −0.011 −0.005 −0.014 −0.001

Table A-4 (continued )Table A-2 (continued )


Table A-3Velocities (in m/s) measured by the different EMF (V1–V20) sensors for Layout 2and under different wave conditions (irregular waves of 6, 7 and 9 cm denoted byI6, I7 and I9, respectively)

V20 −0.006 −0.006 −0.006 0.003 −0.015 0.024

Table A-4Velocitiesand underI6, I7 and

EMF

V1V2V3V4V5V6V7V8V9V10V11V12

I6

(in m/s) measured by thedifferent wave conditionI9, respectively)

I6

Vel X Vel Y

−0.002 −0.0170.006 −0.019

−0.003 −0.0150.015 −0.0320.011 −0.0370.025 −0.005

−0.027 0.015−0.026 −0.010−0.081 −0.009−0.072 −0.001−0.047 −0.004−0.003 0.009

I7

different EMF (V1–V20)s (irregular waves of 6, 7

I7

Vel X Vel Y

0.000 −0.0160.017 −0.0370.001 −0.0180.035 −0.0420.017 −0.0320.013 −0.024

−0.040 0.017−0.066 −0.011−0.096 −0.008−0.075 −0.012−0.067 −0.012−0.026 0.012

I9

EMF
Vel X Vel Y Vel X Vel Y Vel X
sensors for Land 9 cm den

I9

Vel X

−0.0020.0410.0440.086

−0.001−0.003−0.063−0.140−0.148−0.130−0.128−0.101

Vel Y

V1
−0.002 −0.008 −0.001 −0.017 0.010 −0.038 V2 0.005 −0.034 0.018 −0.046 0.039 −0.080 V3 −0.004 −0.007 0.002 −0.016 0.036 −0.059 V4 0.022 −0.041 0.051 −0.052 0.079 −0.074 V5 0.022 −0.033 0.023 −0.030 0.014 −0.037 V6 0.011 −0.002 0.002 −0.022 −0.007 −0.053 V7 −0.021 0.008 −0.035 0.015 −0.055 0.029 V8 −0.041 −0.011 −0.077 −0.018 −0.123 −0.027 V9 −0.090 −0.007 −0.104 −0.012 −0.170 −0.014 V10 −0.070 −0.011 −0.081 −0.017 −0.137 −0.022 V11 −0.052 −0.009 −0.068 −0.018 −0.114 −0.025 V12 −0.012 0.010 −0.037 0.008 −0.052 0.000 V13 0.009 0.013 0.009 0.033 −0.006 0.044 V14 0.020 0.014 0.016 0.005 −0.006 0.006 V15 0.023 0.003 0.021 0.026 0.012 0.052 V16 0.007 −0.019 0.006 −0.019 −0.013 −0.035 V17 0.048 0.029 0.070 0.029 0.081 0.045 V18 −0.010 −0.012 −0.010 −0.012 −0.009 −0.006 V19 0.000 0.017 0.009 0.030 0.044 0.068 V20 −0.010 −0.008 −0.011 −0.007 −0.013 0.000
ayout 3oted by

Vel Y

−0.015−0.064−0.044−0.049−0.024−0.0750.033

−0.018−0.019−0.019−0.0170.004

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quantification of changes in current intensities induced by wave overtopping around low-crested...

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