STUDYING WAVE PENETRATION INSIDE OOSTENDE-HARBOUR AFTER … · Proceedings of COPEDEC 2012, 20-24 February 2012 Studying wave penetration inside Oostende-harbour after the construction

COPEDEC 2012, IIT Madras, Chennai, INDIA.

20-24 Feb. 2012

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STUDYING WAVE PENETRATION INSIDE OOSTENDE-HARBOUR AFTER THE CONSTRUCTION

OF NEW BREAKWATERS

W. Hassan1, M. Willems

2 and T. Verwaest

3

Abstract: This study is part of the ongoing research regarding wave penetration inside the harbour of

Oostende (at the Belgian coast on the North Sea) after construction of two new rubble-mound

breakwaters at the harbour entrance. For the purpose of integrated research wave heights in the inner

harbour were studied by field measurements (using pressure gauges in the harbour, in situ) and by

numerical and physical models. The principle idea is to compare these approaches at specific locations

for the “present and new harbour situations”. A selected set of measured wave conditions from

representative storms at the North Sea was reproduced using a physical scale model at Flanders

Hydraulics Research (FHR) Antwerp, Belgium. This study aims mainly to investigate the future wave

conditions inside the harbour after the construction of the new breakwaters. Particular attention was

paid to the influence of different hydraulic conditions (water level, wave height, wave period and storm

direction) during severe storm conditions on the wave penetration inside the harbour, at different

locations.

Keywords: waves penetration; harbour; breakwater; diffraction; physical models.

INTRODUCTION

Oostende city is one of Belgium's most popular seaside resorts on the North Sea and its port provides

easy access to the city. The harbour is situated inside the city. Different construction works were

executed inside the harbour of Oostende to enlarge the capacity of the harbour for receiving bigger ships

with lengths up to 200 m. To receive these ships inside the harbour important modifications to the

access channel are necessary. Therefore, the existing harbour entrance needs to be broadened in

combination with construction of two new breakwaters. These new breakwaters are also needed to

improve the safety of the city against extreme storm conditions. More information can be found in

Verhaeghe et al (2010) and Gysens et al (2010).

The present study is part of ongoing research regarding extreme waves inside the harbour of Oostende

during super storm conditions, carried out with numerical and physical models, see Gruwez et al (2011).

This study presents: the results of the physical model simulations carried out for the entire harbour of

Oostende for the final situation; the computed wave penetration coefficients in the inner port during

different super storm conditions/directions after the construction of the two breakwaters.

The present research contains and presents all measured and derived parameters in the new series of

tests in the physical model of Oostende harbour, at Flanders Hydraulics Research (WL Antwerp).

1 Senior researcher, Department of Ocean Engineering, Flanders Hydraulics Research, Antwerp, Belgium,

Email: [email protected] and [email protected].


Email: [email protected].


Email: [email protected].

Proceedings of COPEDEC 2012, 20-24 February 2012

Studying wave penetration inside Oostende-harbour after the construction of new breakwaters

Main objectives

The main objectives of the new physical model studies/experiments are:

• To gain more insight into wave characteristics at different locations inside the harbour of Oostende

after the construction of the two breakwaters;

• To investigate the effect of changing storm direction on waves inside the harbour from NNW direction

to the directions of -37o and NW, which were considered to be the most critical directions in this region;

• To obtain a new data set with various storm conditions (water levels and wave heights) for

comparison with numerical wave models;

• To provide the designers of the measures to strength the sea defenses inside the harbour with reliable

information about wave heights, at different locations inside the harbour of Oostende.

THE WAVE BASIN AND THE PHYSICAL SCALE MODEL

The physical model of Oostende harbour was built in the wave basin at FHR at scale 1:100 to study the

different design scenarios of the harbour. Figure 1 shows the schematization of the wave basin. The

wave basin is 18.4 m long, 19.3 m wide and 1.0 m deep. To create more space for the model of Oostende

harbour, the basin was extended with an extra area of about 60 m2; the physical scale model of the

harbour was built in area of 20 m long, 14.0 m wide and 0.49 m deep.

The basin is equipped with a piston type wave generator and 1 pump (60 l/s) to fill/empty the basin. Two

pumps (2 x 200 l/s) are provided to the wave basin to simulate tidal currents. The wave paddle is 12 m

long and can generate regular and irregular long crested waves. The wave paddle has a total stroke

length of 0.3 m. It is also possible to rotate the wave paddle to about 45° (22.5° left and right of the

central axis of the wave basin) to vary the wave direction.

Figures 1 and 2 present general layout of the two new breakwaters outside the harbour as being selected

for the final design at the harbour entrance. The general layout of the physical scale model is introduced

also in Figures 1 and 3. All details about the physical model design can be found in Hassan et al (2011)

and Willems (2005). In total 20 wave gauges were located in the physical-model at different locations to

measure temporal water level changes (waves). Four wave gauges are located outside the harbour (G01,

G02, G03 and G18). G01 and G03 are used in the same locations during all experiments with different

storm directions (NW, -37 and NNW). The other 16 wave gauges are located at different locations inside

the harbour. Figure 3 shows the locations of these wave gauges inside the physical model. Wave gauges

are also used very close to the quay walls to measure the maximum reflected waves at these specific

locations (e.g. G09 and G13).



Fig. 1. General layout of the physical model of the harbour of Oostende.



Fig. 2. General overview of the new breakwaters at the harbour of Oostende, as constructed in the

physical model.

Hydraulic conditions

The new experiments were carried out using various hydraulic conditions according to the natural super

storm conditions in the region of Oostende on the North Sea. These conditions include various water

levels (between +6.00 m TAW and +8.00 m TAW), different wave heights Hm0 (between 0.8 m and 5.0 m)

and wave periods Tp (6.0 s and 12.0 s). All wave conditions were selected to simulate super storms for all

relevant storm directions. An overview of the different conditions in Table 1. This table shows the series

name, the storm direction and storm conditions both in natural and in model scale. In total 120 model

runs were executed in the wave basin to collect all the required wave data, inside and outside the

harbour of Oostende. All tests were performed with a constant water level, using JONSWAP spectrum

with ϒ = 3.3 and duration of 1000 s-model so the wave train is approximately 1000 waves.

Measurement programme

The test programme in this study focuses mainly on:

• Simulations of severe storms at Oostende;

• Variation of the wave parameters (heights and periods) for the model tests;

• Variation of storm directions (NNW, -37o and NW) with different water levels, at the entrance of the

harbour.

Three different storm directions were used in these new series of model runs. The wave paddle was

adjusted to generate waves in the NNW, -37o and NW directions. The breakwaters were reconstructed

after each change in the paddle direction to ensure that the shape and the levels of the breakwaters are

similar to the required design in the physical model. Most tests were repeated three or four times to

show the variations in the measured parameters, related to small differences due to model effects and

to ensure the accuracy of the measurements.



Table 1. An overview of the hydraulic boundary conditions of all experiments, with different storm

directions.

Series

name

Storm conditions

direction

Water levels (m) TAW H m0 (m) T p (s)

prototype model prototype model prototype model

HOG91

NNW

+7.0 +0.31 3.5 to 5.0 0.035 to

0.050 10.0 to

12.0

1.0 to

1.2 HOG92

+6.0 to +8.0 +0.30 to +0.32

0.8 to 4.4 0.008 to

0.044

HOG93 -37o

6.0 to 12.0 0.6 to

1.2 HOG94 NW 3.6 to 4.4

0.036 to

0.044

MAIN RESULTS

All data was analysed using WaveLab software (developed by the University of Aalborg, Denmark) to

determine wave characteristics measured by each wave gauge. The results in this study are based only

on frequency domain analysis to calculate the significant wave height (Hm0) and the peak period (Tp). A

band pass filter is applied between 0.034 Hz-model and 0.500 Hz-model to remove the disturbing

frequencies. Only the reliable part of the measured data is used for wave analysis.

In this study the wave penetration inside the harbour is evaluated using a wave penetration coefficient

Kd. This Kd coefficient is a ratio of wave height inside the harbour (at any selected location) to the wave

height outside the harbour in front of the harbour entrance. Therefore, Kd can be defined as follows:

01_0

_0

Gm

Gxxm

d

H

HK = (1)

Where:

Hm0_Gxx = significant wave height at any wave gauge inside the harbour [m].

Hm0_01 = significant wave height at wave gauge G01 [m], outside the harbour.

The Kd coefficient includes the influence of many different important hydrodynamic processes on wave

propagation inside the harbour, such as diffraction, refraction, shoaling, reflection and sea bottom

friction.



2.5m model

(+6.50)

(+6.82)

(+7.00)

(+7.00)

(+7.00)

(+6.82)

(+6.82)(+6.80)

(+6.80)

(+6.70)(+6.70)

(+6.70)

(+6.70)

(+6.85)

(+6.30)

Fig. 3. Locations of the wave gauges in the physical model of Oostende harbour.



It is important to state that wave overtopping was observed over the two breakwaters, during model

runs with water levels h > +6.50 m TAW. This wave overtopping increases with increasing water level,

resulting in a corresponding increase in wave energy entering the harbour.

The measured wave heights at different locations inside the harbour are used in combination with the

wave height at G01 to compute the Kd coefficient for each wave gauge (location), using Equation 1. These

Kd values computed for all tests are used to study the relation between various hydraulic parameters and

wave conditions inside the harbour. These new data and the computed Kd values with the three storm

directions (NNW, -37o and NW) give the opportunity to investigate the effect of changing storm

directions on wave conditions at various locations inside the harbour. Figure 2 shows an example of the

computed Kd values versus water level with different storm directions at two different locations inside

the harbour (G05 and G08).

Fig. 4. Computed Kd coefficient as a function of water level of the three storm directions, NNW, -

37o and NW (G05 and G08).

From the computed Kd values with different storm directions at different locations inside the harbour of

Oostende we can conclude the following remarks:

• The computed Kd values of different tests, with the same hydraulic conditions, are more or less

the same. Differences between results of the different tests are less than 3%. This means that

the physical scale model is very capable of reproducing the same test conditions;

• Generally, changing the storm wave directions has a limited effect on the wave penetration

coefficient Kd inside the harbour. Most of the effects are taking place at locations on the western

side of the harbour;

• Kd values during the NNW storms are smaller than the other two storm directions, at the

location of G20. At this specific location changing storm direction has clear effect on Kd values,

due to the fact that G20 is located very close to the quay wall which is facing the incoming waves

inside the harbour. Waves reflection close to the quay wall can be maximum with perpendicular



waves attack and can be significantly reduced with changing the angel of waves attack. G20

location is a good example of these effects of reflected waves;

• Increasing water levels results in smaller values of Kd for the three storm directions at most of

locations inside the harbour.

Figure 5 presents an example (G05 and G08) of the computed Kd coefficients as a function of Tp (s) of all

tests with the three different storm directions. G01 is used as a reference for computing Kd coefficients,

which are plotted on x-axis. The measured Tp (s) values with G01 is used for comparisons and plotted on

y-axis. The results show that Tp (s) have no clear fixed relation with the computed Kd values. It was found

that the relation between Kd and Tp is changing from location to another inside the harbour. For example

increasing Tp can results in increasing Kd as for locations of G06, G07 and G09. The opposite behaviour

can happen with G12, larger Tp values lead to smaller Kd values. For some other locations inside the

harbour Tp values have almost no influence on the computed Kd values e.g. G12.

Fig. 5. Computed Kd coefficient as a function of wave period (Tp) of the three storm directions,

NNW, -37o and NW (G05 and G08).

Similar plots are reproduced to declare the relation between computed Kd values and Hm0 (m) inside the

harbour of Oostende. These relations between the two parameters are presents in Figure 6 at the

locations of G05 and G08. These results show a similar relation as observed before between the water

level and Kd coefficient. It was observed also that increasing wave heights Hm0 leads to smaller values of

Kd at most of the locations. Only the locations of G18 and G19 show that Kd values are independent of

Hm0 values.



Fig. 6. Computed Kd coefficient as a function of wave height (Hm0) of the three storm directions,

NNW, -37o and NW (G05 and G08).

CONCLUSIONS

New experimental research was carried out in a physical scale model (1:100) at the wave basin of

Flanders Hydraulics Research. This study aims mainly to investigate the wave penetration inside the

harbour of Oostende under severe storm conditions, after the construction of two new breakwaters. A

wide range of hydraulic conditions have been used in the experiments. Different wave heights, peak

periods and water levels were used in combination with three storm directions (NW, -37o and NNW).

Wave characteristics/heights were measured at different locations inside the harbour using 20 wave

gauges. A large dataset was obtained and can be used for numerical model studies and for design

purposes. Detailed analysis of the data showed that storm direction has a limited effect on the computed

wave penetration coefficients Kd. Most of these effects are taking place on the western side of the

harbour. Increasing water levels results in smaller values of Kd for the three storm directions at most of

locations. Also, the experimental results show also that the peak period Tp have no clear fixed relation

with the computed Kd values. At most of the locations increasing wave height Hm0 leads to smaller values

of the wave penetration coefficient Kd.

ACKNOWLEDGEMENTS

We appreciate the financial support of the Flemish government (Coast Division, Oostende).

REFERENCES

Hassan, W.; Willems, M.; Verwaest, T.; Mostaert, F. (2011). “Extreme wave characteristics inside the

harbour of Oostende: physical modelling with different storm directions (T2).” Versie 2_0. WL

Rapporten, 627_05. Waterbouwkundig Laboratorium: Borgerhout, België.

Gruwez, V.; Bolle, A.; Hassan, W.; Verwaest, T.; Mostaert, F. (2011). Numerieke modellering van het



extreem golfklimaat in de Belgische havens: Deel 1: Haven van Oostende. Versie 2_0. WL Rapporten,

769_03. Waterbouwkundig Laboratorium & IMDC: Antwerpen, België.

Gysens, S., De Rouck, J., Trouw, K., Bolle, A. and Willems, M. 2010. Integrated coastal and maritime plan

for Oostende-design of soft and hard coastal protection measures during the EIA procedures, Proc. of

32nd

ICCE 2010, ASCE.

Verhaeghe, H., Van Damme, L., Goemaere, J., De Rouck, J. and Van Alboom, W. 2010. Construction of

two new breakwaters at Ostend leading to an improved harbour access. Proc. of 32nd

ICCE 2010, ASCE.

Willems, M. 2005. Haven Oostende Golfindringing, Data report of Flanders Hydraulics Reserach, Antwerp.

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STUDYING WAVE PENETRATION INSIDE OOSTENDE-HARBOUR AFTER … · Proceedings of COPEDEC 2012, 20-24 February 2012 Studying wave penetration inside Oostende-harbour after the construction