Mound Breakwater: Types, Construction and Protection

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Breakwater is the artificial protective barrier constructed to enclose harbors and to keep the harbor waters undisturbed by the effect of heavy and strong sea waves. Breakwater enables the use of the enclosed area as a safe anchorage for ships and facilitates cargo loading in comparatively calm waters.

Heap or Mound Breakwater

It is a conglomerate assembling natural rubble, undressed stone blocks, supplemented in many cases by artificial blocks of huge bulk and weight, the whole rubble/block being deposited without regard to bond or bedding.

This is the simplest type and is constructed by pitching or dumping rubble stones into the sea till the heap or mound emerges out of the water, the mound being composited, and the action of waves regulates its side slopes.

The quantity of rubble depends upon the depth and height of tides or waves. The waves gradually drag down the mound on exposed sites, giving it a flat slope on the sea face. Such flattening has to be protected as far as possible.

The most disturbing action of the waves is in between the high and low water levels. Accordingly, all protective methods are adopted above the low water level.

Types of Heaps or Mound Breakwater

1. Mound or Heap Formation

Rubble mounds are formed using the rubble of different weights, placed according to sizes—the smaller and lighter materials constituting the core. The sizes are increased gradually outwards. This arrangement is coherent, exposing the bigger sizes to the action of the waves, while the smaller sizes forming the core are protected.

If rubble is difficult to find, concrete blocks are used to form the mound. Concrete blocks have the advantage that they can be heaped at a steeper slope than rubble, economizing in space and material. Also, the size of concrete blocks could be controlled to suit the exposure condition.

2. Mound with Superstructure at Low Water

A solid superstructure that consists of a Quay protected by a parapet on the sea face is constructed on top of the rubble mound. Such a construction is founded on the low water level.

The advantages of such a construction are:

1. It gives a platform for handling cargo. 
2. It protects the top of the mound.
3. It decreases the mass of rubble required for the mound in proportion to the depth at which it is founded.

Unlike the ordinary or plain mound breakwater, this type of construction makes it possible for ships to come close to the breakwater wall on the inner or harbor side for loading and unloading cargo.

Huge concrete blocks are used on the sea face for protection. In addition, the front batter changes from 2.8 to 1, abruptly to 1:1 to provide a sharp edge to cut the waves on impact.

3. Mound with Superstructure Founded below Low Water Level

This type of construction affords the advantage of founding the superstructure well below the level of disturbance, the waves having practically no disturbing effect at such low levels. Moreover, in deep waters, this type is very economical in mound material.

Stability of mounds with superstructure founded below low water level

The mounds lack variability in shape and section, especially the upper portions. They stand in composure, below levels of wave effect at slopes of 45° to 50°. The wave effect and disruption of the mound are felt maximum between high water level and low water level. Hence, large and bigger blocks of 30 tons each or more are deposited at a slope of 1 to 1 in this region. The concrete blocks, made of large rectangular blocks, are laid as headers, offering minimum face area and maximum resistance to overturning.

Construction of Mound Breakwaters

1. Size of material and arrangement

The mounds are formed in assorted layers, the smaller sized material being disposed at the base and the larger at the top and sides, particularly in between the low and high water levels, the region that is largely affected.

2. Methods of construction

The mound construction can be carried out by any one of the following three methods.

1. Barge method
2. Staging method
3. Low-level method

The types of construction procedures shall be explained in detail in a separate article. 

Methods of Protection for Heap or Mound Breakwater

Dumping heavy concrete blocks on top and the front face. This, to a great extent, resists the flattening action of the waves by sheer weight. These blocks weigh 25 tons to 30 tons and are either deposited at random or laid in courses. These blocks are prepared as rectangular solids and laid with ends towards the waves. This provides maximum mass and minimum area against impact or overturning.

Paving

The upper part up to the low water level placed with deep granite blocks is another method to protect the top and face. Granite paving blocks set in cement mortar reduce the eroding action of the waves. 

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