Hydraulic Stability

What is 'hydraulic stability' of a breakwater?

Hydraulic stability is the resistance of the breakwater in general and of the armor layer in particular against the forces generated by incoming waves.

How has the hydraulic stability of the Xbloc been determined?

The hydraulic stability of the Xbloc® has been determined by means of small scale hydraulic model tests. Both 2D and 3D hydraulic model tests were performed at Delft Hydraulics and additional 2D tests were performed at DHI in Denmark.

How does the hydraulic stability of the Xbloc® compare to other units?

From the first 2D tests at Delft Hydraulics it was found that the hydraulic stability is at least comparable with the hydraulic stability of other single layer armor units. The Hudson equation [Kd = 16] or the Van der Meer's equation [stability number = 2.8] can be used for preliminary design, based on a slope angle of 3:4. In further 2D model testing even better stability results were obtained, as a result of an improved unit placement method. This results in additional safety of the design.

What is the safety factor of the Xbloc® unit?

The design values stated above are based on a safety factor of 1.25. This means that a design wave [based on the wave height that may only occur once in the lifetime of the structure] could be exceeded by 25% before the first damage to the slope would occur. The safety on unit weight is therefore 1.95  [=(1.25)³]. The Hudson formula may be applied to determine the hydraulic stability of the Xbloc®, however this formula does not include the wave period or the wave steepness.

How are such parameters accounted for in the stability assessment?

During hydraulic model tests, the effects of different wave steepness were analysed. Moreover the crest height was varied and deepwater and shallow water waves were analysed. The tests indicate that the governing factor for the stability of the Xbloc® is the wave height and not the wave steepness or the wave period. The Hudson formula can therefore be applied for each wave condition.

On what slope angles can the Xbloc® be laid?

The Xbloc® has been tested on slope angles of 3 [V]: 4 [H] and 1 [V]: 1.5 [H] as such slopes are commonly used for single layer armor units.  Also tests have been carried out for a slope of 1 [V]: 2 [H], as such a slope may be required in severe seismic areas for geotechnical reasons. Xbloc has been applied on slopes of 3:4 to 1:2. Note that for slopes flatter than 3:4 no reduction in weight is advised. For Xbloc® application at different slope angles the stability should be investigated in the model tests.

What is the influence of wave period on the stability?

Tests have been performed with various wave heights and various wave periods. The wave steepness was varied between 2% and 6%. For all values of wave steepness the hydraulic stability is excellent.

What is the influence of breaking and non-breaking waves on stability?

Tests have been performed with both breaking and non-breaking wave conditions. No significant influence of either breaking or non-breaking waves on the hydraulic stability was observed.

What is the influence of a steep foreshore on the stability?

Based on experience with other armor units it is known that the effect of a steep foreshore may be expected to reduce the stability of the armor layer. Therefore where the foreshore is steeper than 1:30 the armour mass in conceptual design should be increased.

Are larger units required for the roundhead of a breakwater?

In common with other single layer armor units the roundhead may normally be expected to be constructed of armour units with a mass that is about 25% heavier than the units at the trunk. The slope angle of the roundhead may be kept equal to the slope angle of the trunk, which is a more economical and technically sound approach than the option of using the same sized armor on the trunk but laid on a flatter slope.

Are model tests always required?

In general model tests are an essential part of the design scope of breakwaters. This is considered to be best practice.

Where can we get model units for model testing?

Xbloc® armour units of various sizes can be hired through Delta Marine Consultants. DMC will also provide instructions on placement of the units.

How does self-repairing work?

A self-healing process can take place when an Xbloc® unit is taken out of the slope by wave action during an extreme overload situation [> 125% of design wave conditions]. The result would be that under wave attack one or two Xbloc® above the hole so caused would settle slightly towards the hole and would automatically interlock with its neighbouring units. This would partly re- establish the stability of the armour layer. Due to such rearrangement of the armour units the slope would be able to withstand such conditions and probably even more extreme wave conditions before failure of the armour layer would occur.

Has the effect of overtopping on the stability of the armor layer been analysed?

In the model tests both low crested situations [with high overtopping rates] as well as high crested situations [with little or no overtopping] have been created. No clear influence on the amount of overtopping on the stability of the armor layer could be observed.

When do the armor units start to 'rock'?

In wave conditions that exceed the design wave conditions, rocking of some armor units was observed. An increasing number of rocking units occur with increasing wave overload. Under normal wave conditions and during storms with wave heights below the design wave conditions no rocking of the units may be expected.

What is the effect of a rocking unit on a slope?

Severe rocking of armor units will cause high forces in the armor units. Under wave conditions that may typically be expected during the lifetime of the structure little or no rocking of units may be expected. Furthermore, from extensive calculations and prototype drop tests it was found in contrast with some other armor units, that the Xbloc® unit can deal with forces related to rocking. Therefore rocking of a few units is not regarded to be a serious threat for a structure built using Xbloc® armor.

How much overtopping occurs over an Xbloc® armoured breakwater compared to other units?

Results of the CLASH tests indicated that the overtopping of an Xbloc® armour layer is approximately the same as it would be with other single layer armour units.

What is the influence of oblique wave attack on the stability of the armor units?

In 3D tests it was found that the stability of the armor layer increases for oblique wave attack. The number of oblique tests undertaken to date is insufficient to enable determination of a reduction coefficient for design purposes.

What is the size of the under layer between the units and the core?

The size of the under layer depends on the size of the units. In the design table the required under layer in a standard rock grading is displayed for each Xbloc® unit size.

What kind of supporting toe does an armor layer of Xbloc® units require?

A stable toe solution is made with the Xbloc® special toe unit. Read more about this special toe unit at this site.