Question

The small yachting company is constructing a breakwater that extends out from the beach at an angle (sketch below). Recall how bays create a low-energy wave environment (right figure above). This is the shape of the beach/breakwater west (left) of the breakwater. 2. If we assume that we have created a low-wave-energy environment west of the breakwater, what would happen to the sand in the longshore current? 3. If sand is deposited west of the breakwater, what happens to the sand amount in the longshore current and how would that affect beaches east of the harbor? 4. As more sand is deposited on the west side of the new breakwater, what will eventually happen? 5. Where will the new closest low-wave-energy area be located and what effect will that have on the harbor?

Answer 1

1. Movement of water, in the form of waves, tides and currents, plays an important role in our oceans and impacts on all marine organisms.Wave action is one of the most significant environmental conditions affecting life in the sea. Water in the ocean is constantly moving due to wind, waves, the tide and underwater currents. The impact of waves generally reduces with depth.

• Waves are caused by wind acting on the surface of the sea. However, they impact on life beneath the surface, too. For example, a wave that is 2m high above the surface may be felt up to 6m below the surface. Waves can be very destructive; however, they also cleanse organisms and help to increase oxygen availability.
• Depending on the location, coastlines can be categorised as high energy or low energy.
  • High energy coasts are usually characterised by:- Strong wave action, typically during the winter months.The rate of erosion usually exceeds the amount of material that is depositedMain features usually are big cliffs and headlands.
  • Low energy coasts are generally found where:- Wave energy is less powerful and more sediment deposited than lost-Landforms include beaches and spits.

This is why bays that are found creates a low wave action as it is partially surrounded by land. as the shape of bay is too narrow, possible flow of currents will be low but followed by mass deposits.

2. An aqueous sedimentary environment in which there is standing water with a general lack of wave or current action, permitting accumulation of very fine-grained sediments. An attached or headland breakwater, as the name suggests, is connected to the mostly sandy beach shoreline, often with beach fill. The headland breakwater system is composed of a series of pocket beach and breakwater units designed to maintain the adjacent beach in a predictable shore planform for shore protection or to create a recreational beach. Recreational beaches are often larger than what might be needed for a shore protection application. Both the desired width and length of a recreational beach may exceed what is necessary for shore protection. If there is sufficient sand in the littoral system, beach fill requirements may be minimal if any. On sheltered coasts, a scarcity of sand often leads to bank erosion. Therefore, a breakwater system will usually include a beach-fill component.

3. As in this case, the sand trapped behind breakwaters causes a shortage of sediment downdrift in the directions of dominant longshore transport, leading to additional shoreline retreat (e.g. beach and dune loss, scarping of the fastland, accelerated mass wasting).

Coastal breakwaters are located within a distance from the shoreline of half the width of the surf-zone, up to twice the width of the surf-zone, 2 > x > 0.5. Such breakwaters trap sand within the part of the littoral zone they cover, thus securing that part of the coastal profile against erosion.

Beach breakwaters are located within less than half the width of the surf-zone from the shoreline, x>0.5. Beach breakwaters trap sand on the foreshore without interfering significantly with the overall transport pattern.

Coastal and beach breakwaters are designed:

• to partly provide wave shelter for a certain part of the shore and the coast
• to modify the littoral transport in a predictable way, so that the combined shore-restoration and coastal protection function can be properly designed.

4. The extent of the sand trapping, or the ability to establish the desired sand accumulation pattern, is mainly regulated by choosing the length of the breakwater, the distance of the breakwater to the shore and the number of single breakwaters in a segmented breakwater and, finally, the length of the gaps.

The advantage of a breakwater in comparison to a groyne is that it is possible to modify the littoral transport in a smoother manner than for a groyne. In this way there will be less lee side erosion on the downstream shoreline. This applies especially to breakwaters, which are so short that a permanent tombolo does not develop.

Sometimes coastal breakwaters are planned so as to provide shelter for the beach landing of small fishing boats or sheltered water for swimming. This is difficult as sufficient shelter for the boats and the swimmers requires such a long breakwater that a tombolo develops. When the tombolo has developed there is no sheltered water area left for boat landing and swimming, only a semi-protected bay downstream of the tombolo is left. This bay may be suitable for landing small boats, but it is certainly not suitable for swimming during rough wave conditions due to the eddy, which will always be present during such conditions.

Sand will be trapped behind a breakwater if initial sand fill is not performed. The trapped sand comes from the adjacent beaches, which means that both the upstream and downstream beach will suffer from erosion during the development of a salient or a tombolo. When a tombolo has been formed, the adjacent beaches are influenced in a way similar to that of a groyne with upstream accretion and lee side erosion. The influence of a salient will be smoother.

5. The term low energy be used in locations where:

(1) non-storm significant wave heights are minimal (e.g. < 0.25 m)

(2) significant wave heights during strong onshore winds are low (e.g. < 0.50 m)

(3) beachface widths are narrow (e.g. < 20 m in microtidal environments)

(4) morphologic features include those inherited from higher energy events.

Micro-topographic features can persist in the swash zone of low energy beaches under non-storm wave conditions. There is little evidence of cyclic cross-shore sediment exchange. Bars, excepting transverse forms, located seaward of low still-water level do not appear to be part of the sediment exchange system with the foreshore.

The effect of tides is to move different hydrodynamic zones (swash, surf and shoaling waves) up and down the intertidal profile, thereby reducing the amount of time waves can rework a given portion of the beach (Masselink, 1993) and inhibit the formation of nearshore bar morphology. The tidal translation rate is an important factor and depends on tidal range and the intertidal beach gradient.Small-scale sedimentary and biogenic features on low energy beaches are sensitive to small variations in wave energy conditions compared to overall profile shape.