S. M. Gouloumis1,
C. I. Moutzouris2,
G. Liokalos3
1Research Associate, ² Professor
Laboratory of Harbour Works, Civil Engineering Department, National Technical
University of Athens (N.T.U.A.), 5, Iroon Polytechniou Str., 157 73 Zografou, Greece
Tel: ++ 30 1 7722367, Fax: ++ 30 1 7722368, E-mail: spirosg@central.ntua.gr
3Civil Engineer, Port Authority of Heraklion Harbor, Crete island
Abstract: Inspection and maintenance of harbor works are essential in order to reassure port¡¯s operation, most significant for any island.
The port of
Heraklion city located at northern Crete Island, South Aegean Sea, Greece, is
one of the greatest harbors of Greece, servicing much of the island¡¯s sea
transportation. It has been formed in stages during the 20th century by a
breakwater, partly armored with tetrapodes experiencing extreme waves of about
6.00 m. high, not complying with the design wave of 5.00 m. high. Ground survey
as well as underwater inspection using a Remote Operated Vehicle prove damages
of tetrapodes and sections¡¯ deformation of the armor.
Since Heraklion harbor nowadays has to service bigger ships than ever
before, there were severe indications that undermining of the seawalls
foundation was going on. Thus, the port authorities¡¯ engineers decided to have
underwater investigation of the seawall between Dock I and Dock II, where a
sewage pipe was flown. Even though the sea was very dirty, with the help of the
spotlights of the Remote Operated Vechicle (R.O.V.) system, the image
transmitted from the camera was clear enough and very helpful to investigate the
situation. The seawall foundation was found to be extensively eroded to the
point that some columns of concrete blocks just stood in place because of the
friction between them.
The underwater inspection was proved to be most helpful to the authorities¡¯ engineers since they can guide the surveillance in real time as they were diving themselves, recording everything for later investigation.
The performance of the armor¡¯s tetrapodes seems to be very much alike stones under the same situation.
Keywords: inspection and maintenance of harbor works, underestimated wave height, deformation of the armor, undermining of the seawall¡¯s foundation, underwater investigation
The harbor of Heraklion city, on the northern coast of Crete Island, is protected by a 2300 m long vertical wall breakwater, partly armored with a stone revetment protected by a top layer of tetrapods. The first part of the breakwater is 990 m long reaching a depth of -11.60 m and was built during the period 1923-31. The harbor layout and the stability of the breakwater section were tested in the Hydraulic Laboratory of the University of Berlin in 1964. An extension 1016 m long of the existing breakwater at a depth reaching -20 m was constructed during the period 1966-73, using the same construction method.
In order to protect the first part of the breakwater, which is under the action of breaking waves, a rock embankment with a 2 to 3 slope was built seaward, armored with two layers of 15.12 ton tetrapods. The design wave height for the breakwater was taken equal to Hs = 5.00 m corresponding to characteristic period Tz = 8 to 12 sec, while the effective fetch was estimated to be of the order of 130 nautical miles.
Gouloumis, 1996, has noted that peculiar weather conditions in conjunction with the specific topography of the Aegean Sea, which has a length of 610 km from North to South, generate high waves propagating along the Aegean basin reaching the northern coast of Crete, not complying with the S.M.B. prediction model. A characteristic case is shown in Fig. 1, where several storm events of developing sea are presented.
Fig. 1 Dimensionless significant wave heights versus the fetch coefficient.
The data had been collected during a research program when a Waverider buoy was moored about a mile off Heraklion harbor, at a water depth of 23 m, recording wave characteristics from October 1, 1992 until September 22, 1993. A total of 5014 records were processed, interrelating significant wave heights Hs with the corresponding wind speeds blowing from the North and recorded onshore.
During the three squall episodes, caused by similar peculiar weather conditions where a barometric pressure wave was propagating from north to south over the Aegean basin, high waves occur not complying to the S.M.B. prediction model, widely used in harbor works design.
During wave measurements conducted a few years
later at 90 m depth in the same area, the maximum significant wave height
recorded was 5.44 m with a corresponding Tz of 8.49 sec, caused by a severe but
not extreme storm, demonstrating that the harbor¡¯s defense works were designed
using a rather underestimated wave height. The measurements lasted over one year
only, considered not to be enough to extrapolate data, but statistical analysis
of wind data indicates that the maximum characteristic wave height may reach as
much as 6 m.
An underwater investigation was carried out in order to check the response of the tetrapods to the action of waves higher than the designed ones (Photo 1).
Major sections¡¯ deformations have been also surveyed, while damaged tetrapods were found at many locations. The deformations of the armor are typically of the ¡°S¡± type (Fig. 2) coincide with experimental results of the modeled rubble mound deformations at the wave flume of the Laboratory of Harbor Works, N.T.U.A., as well as with former experimental approaches (Moutzouris, 1978).
Photo 1 View of the breakwater¡¯s armor tetrapodes. The tetrapode in front has been broken.
Fig. 2 Deformed versus initial slope of the armor.
Further investigation of the breakwater both of the armored and the unprotected part was conducted in order to estimate the overall stability of the structure since many tetrapods have been swept away. The Remote Operated Vehicle (R.O.V.) was used to accomplish video recordings of the underwater region of the breakwater and to define damages of the tetrapodes, the breakwater wall and its foundation due to the wave forces experienced.
The overall inspection showed that the breakwater of Heraklion was designed using an underestimated wave height. The original armor slope has been deformed to an ¡°S¡± type equilibrium profile complying with experimental results. Tetrapods seem to experience wave forces much alike stone armor seeking an equilibrium position to settle. Some of the tetrapods placed over and under the sea level were found to be broken.
Vertical seawalls are formed in the interior of harbors used for the mooring of ships and they are usually made of huge concrete blocks. Once these blocks of concrete are deposited they form vertical columns with horizontal joints. Seawalls are severely affected by water jets produced by the ships¡¯ propellers during mooring maneuvers. As a result, water under pressure penetrates inside the concrete blocks¡¯ joints of seawalls and widens them by constantly eroding the material behind the seawall face and the material of the seawall foundation. This problem has become worse the past years due to a marked increase in the number and size of ships. Consequently, seawalls in many harbors around the country have been subject to significant damage and immediate repair works must be undertaken to ensure the safety of the installations and the ships as well as the overall proper function of the harbor. To be able to perform repair works of a particular seawall, an accurate investigation of its condition is needed by the authorities¡¯ engineers.
The best method for underwater inspection of seawalls is with the use of a special device, mentioned before, known as Remote Operated Vehicle (R.O.V.). This consists of a versatile remotely operated vehicle that can be fitted with a wide range of video color cameras. The standard system is equipped with a number of features and controls that have been designed to make the operation of the vehicle as straight forward as possible. These include automatic control of heading and depth, lighting intensity control, camera position sensing and display and depth and heading indication. The device is connected via an umbilical cable to the surface unit. The splash proof surface unit houses all the equipment needed to control the vehicle underwater and to display through the monitor the picture that is recorded by the camera. A video recorder records all this information so that a proper description of the whole process can be made. All this information is invaluable in order to do an in depth diagnosis of the current condition of seawalls.
The procedure in the underwater inspection program is following: the R.O.V. device is immersed facing the under-investigation seawall. The colored camera of the device transmits the picture that is seen on the monitor of the surface unit. This way the pilot in cooperation with the engineers can guide the vehicle to perform the necessary maneuvers in order to continuously record the important features of the seawall.
This type of inspection was done at the Heraklio Harbor of Crete and it was deduced that the dock between Pier I and II where passenger and ferry ships are moored, had been seriously damaged. In particular, the base of the seawall had been considerably undermined both in terms of width and length and depth behind the seawall face. The water depth at the Dock was designed to be -8.0 m, lied between Piers I and II, founded at a water depth of -10.0 m. (Figure 3)
Fig. 3 Seawall inspection site between Pier I and II, of Heraklion port.
Photo 2 Undermining of the seawall between Pier I and II at the contact with the seawall of Pier I on the right, founded at a depth of ¨C10.00 m.
At the point of contact between the dock and the seawall, undermining of the foundation had increased the water depth down to -10.0m throughout considerable length of the concrete columns. The latter stayed in place due to action of friction that was experienced between the columns, since the foundation had been significantly eroded. It was clearly seen that for safety reasons ships should not allowed to use this seawall while severe maintenance works should undertaken in order to reconstruct the eroded foundation.
The underwater inspection of marine works has proved to be an invaluable method in order to achieve proper understanding of the condition of these works and to determine ways to perform repairs. The most important advantage of this method is that no diver needs to risk, while the engineers can watch and guide in real time and continuously the inspection process and therefore can focus on the most interesting clues (e.g. undermining, widened joints). The simultaneous recording of these events, helps in the investigation and analyses that takes place at a later stage in order to decide on appropriate maintenance works.
It has been proved that underestimated waves can cause damage to the tetrapodes of the breakwater armor, deforming the sections to a similar way as for a stone armor.
Also, the berthing of nowadays big ships could erode the foundation of the seawalls in case they are not properly protected against the propellers¡¯ water jets.
References
Gouloumis S. M. (1996): Wind / wave interrelation and the pressure jump effect, Proceedings of the ICCE¡¯96 conf., Orlando, Fl., Publ. by ASCE, NY, USA.
Moutzouris
C. I. (1978): A
profile of a sloping breakwater based on recent results concerning wave
propagation and breaking, 7th Int. Harbor Congress, Antwerp,
2.04/1-2.04/7.
Laboratory data experiments at the channel of the Laboratory of Harbor Works, N.T.U.A., several years.
M. Daskalakis, J. Theocharis (1997) : Seawalls¡¯ undermining due to waves or ships¡¯ proppelers ¨C treatment ways, 1st Pan Hellenic Conference on Harbor Works , Athens 1997.