The problem with the concept of the 'line in the sand' is that nobody appears to be able to guarantee where that line runs. Should building therefore be prohibited from all dunes? Another problem is that beaches appear to be eroding past all conceivable 'lines in the sand' and that permanent erosion appears to have set in. As we have seen before, this has identifyable (human-induced) reasons.

Even if the dunes were marked in risk zones of say, 25, 50, 100, 200 years life expectancy, businesses would still want to build close to the water because they can simply mark their prices up, in order to write buildings off in a much shorter time. The free market system is thus not able to save beaches.

Engineers pump sand from offshore or transport it in by barge. They prefer coarse sand because it stays on top of the old fine sand, stopping it from being blown away and giving the impression that it also remains there longer. But it does not feel as pleasant as the old sand and it disappears rapidly seaward. Renourishment needs to be done every five to ten years, involving high costs. It puts more sand near the beach, creating sand bars and causing the beach to lay flatter, eventually no longer able to dry. No attention is paid to remove the causes of the beach erosion.

The process of beach renourishment is well known and has been documented extensively. But it can never offer a lasting solution. We have to ask ourselves now, how to design coastal settlements and resorts in such a way that their dunes and beaches remain natural and healthy. The free market does not appear capable to do so.
 
 
 

This aerial view shows Miami Beach on left in 1982 when 14 million cubic metres of sand were applied and on right on 6 Nov 1993 showing that most of the sand had disappeared. Groynes have been built in an attempt to slow the erosion but the beach has no dunes left and the tall buildings deprive it of the necessary sea wind to accumulate a buffer of dry sand. No amount of wet sand can protect property in the long run. Note that by 1993 nearly half the shoreline was protected by sea walls. Note also how property has expanded seaward. Pictures: Cyril Zaneski, Florida. courtesy of Mr Burgess, Omaha, NZ. [A note from James Houston, 16 Dec 2012:"The photographs you have of beach nourishment performance at Miami Beach are reversed.  The photo that says it was taken in 1993 is Miami Beach in the late 1970s prior to beach nourishment.  The Miami Beach nourishment looks today about like it did right after the beach nourishment and is considered one of the most successful beach nourishment projects in history.  It completely rejuvenated Miami Beach.  There are all kinds of articles and pictures on the internet that verify what I am saying." ]

Beach drainage A very recent development is that of draining beaches. The idea is to place a drainage pipe under the sand below the high tide level. The drainage pipe conducts water to a collection point from where it is pumped out. The water is then led back to the sea. This method recognises that many beaches are sick because their sand cannot dry. These beaches usually lie too flat. By draining the high tide region, the water table is lowered and the sand has a better chance to dry. The sea wind completes the renourishment.  The method is quite simple to implement, requires no visible structures or mechanical transportation of sand, and the sand that is already in the sea is used. Energy can be saved by pumping only during the low tide and by day when the sand can dry. For beaches with other problems such as sea wind obstruction or crusting, this method may be less successful.

What is perhaps surprising of this beach restoration method, is that it works while almost completely invisible. Although it does not address the causes of beach degradation such as tall buildings and trees, sand contamination and more, it does change the balance of the physical factors that maintain the beach. But when pumping is stopped, the beach degrades back to its previous state. Storms of course, cause the beach to recede temporarily.
Consider the three main components contributing to beach maintenance: draining, drying and blowing. When either becomes weakened, it can be compensated for by increasing the functioning of one of the remaining components. Of these, drainage is the easiest to enhance. Surprisingly, a previously 'sick' beach can be coaxed to produce over 30m3 of sand per metre of beach per year!

Another interesting aspect of the success of beach draining is that it refutes the theory that beaches are degrading because of rising sea levels. Sea levels may rise, but beach draining can always produce sand to form a beach higher up. A healthy beach can do likewise, without technological assistance.

Where people go wrong is that they cannot imagine the strength of a hurricane compared to every day exposure. (See also Oceanography/extreme events). For example, when fine weather waves of about 1m height pound on the beach, they dissipate an average of 10kW per metre of beach or the power of a small car at full throttle, every five metres. (Ref Douglas L Inman in Oceanography, the last frontier, 1974)
A three metre wave, by no means abnormal, dissipates about ten times that energy. As the energy in the waves increases with the fourth power of wind speed, a storm of 60 knots can produce waves of 15-20m (250 times the sample energy) and hurricanes of category four, with winds of 250 km/h, is again over ten times stronger. So an extreme event can dissipate hundreds, even thousands of times more energy than everyday events. Engineers, used to safety factors of three, have difficulty calculating structures that will stand the anger of the sea or these become unaffordable.

When Omaha Beach, north of Auckland, New Zealand eroded suddenly after a storm in 1975, engineers decided to build a seawall hugging what remained of the shore. They assured that it would be strong enough to last all storms and that it would provide permanent protection to worried property owners. Hardly one year later, another storm carved out  10m behind it and once again 30m and the whole structure  was destroyed by waves. With hindsight, the exercise was rather naive, considering how much experience engineers world wide have had in constructing seawalls (but at a very much higher cost). The Dutch would certainly smile about this story. Pictures: H E Bridges, Omaha

Another commonly overlooked design shortcoming is that extreme events do not only arrive with bigger waves but also with a storm surge that raises the water level. Thus at some time, it is unavoidable that the sea crashes over the wall and the water has to go somewhere, usually back to the sea. During its return trip, the wall is attacked from behind, resulting in deep gouging of the soil placed behind it. In the top drawing one can see the waves, riding on a storm surge, crashing over the wall and returning behind the wall, under-cutting it at places. To prevent the water from running up the wall and over the top, engineers make them rather steep. It causes the waves to bounce back. On their return trip they interfere with the oncoming waves, creating waves twice the size and much more powerful. These eat out all the beach sand, and require more reinforcement to protect the foot of the wall (second drawing). Whereas waves moving towards the beach drag sand towards it, bounce waves are moving away from the beach, dragging sand towards the sea. Sea walls always constitute compromises, not only between cost and efficacy but also because people do not wish to be locked behind tall sea walls. As a result, many have been unsuccessful.

Groynes Beaches strung between natural headlands are very stable and they do not experience long-shore erosion as much. Groynes (French word for pig's snout) are artificial headlands, running out into the sea. Between them the wet sand is trapped. Although groynes increase the quantity of wet sand close to the beach, they do not improve the beach's self repair mechanism. Not only are they ugly, they can even cause problems. Groynes can be thought of as 'leaking' sand boxes with one open side from which the sand can enter but where it can also be lost.

Most long-shore sand transport is caused by wind-induced currents. If a wind blows perpendicular to the coast, there are no wind currents. But when it arrives on an angle, a wind induced current occurs. Where the sand is continuously held in suspension, the breaker zone, small currents are able to transport much sand easily. Deeper down, where the sand fails to be stirred by waves, currents have very little effect. The groyne thus prevents the sand from leaving its 'cell'. It works so well that groynes are able to starve un-groyned down-wind beaches from the sand they need to survive.

Groynes accumulate wet sand (up to about 2.5 times their length) so that they have the tendency to lay beaches flatter. Although exposing more sand during low tide, they could decrease the amount of sand able to dry in the wet zone. Groynes do not protect against extreme events and are likely to lose all their sand when these occur.

One would be tempted to make groynes as long as one could, extending far into the sea but that could cause other problems like jetties do.

Note that groynes do give extra protection but the dunes behind them never seem to accrete and move seaward. In almost all cases the beach needs to be renourished regularly, which proves that groynes do not fix the original cause of erosion.
Note also that groynes used to deflect currents from the shoreline, such as for deflecting tidal currents in current channels, work very well.

Jetties Jetties are long dams or groynes extending into the sea for hundreds to thousands of metres. They are designed to make harbour entrances navigable for large ships. The idea is that the current of the outgoing tide becomes strong and focussed enough (a jet) to gouge a deep entrance channel between the jetties. The problems these structures cause are well documented. In case of a substantial coastal sand drift, the jetties act as a sand sink, starving beaches down-drift from sand. By far, most of the sand in littoral sand drift is moved in the shallow breaker zone. As it encounters the jetty, rip currents and outgoing tide currents suck the sand down into deep water where it remains for a long time until storms slowly move it back towards the shore. The down-drift beaches are then starved from the sand they need to survive. Sand banks will develop from the accumulated sand, hindering shipping. Is it possible to design structures that will provide all the benefits without the draw-backs?

The loss of Assateague Island south of Ocean City, USA (top right corner of map) is a classic example of sand starvation caused by jetties. Assateague Island is a long and narrow barrier island that was once inhabited by holidaymakers. Behind the island lies a large lagoon or natural harbour. Barrier Islands usually lack a healthy dune structure because they are usually created by overwash: during large storms, waves wash over the island, depositing sand which stays behind. Wind-borne accretion is also present but less prominent. Ocean City was able to grow after it was linked to the mainland by a railroad bridge in 1880.  During a large storm in 1933, an inlet appeared south of Ocean Beach at the weakest point. The city then decided to convert it into an important harbour entrance by constructing jetties. The jetties further starved Assateague Island, which, in the meantime had become uninhabitable. Even though the groynes and jetties trap so much sand in favour of Ocean City, its northern beaches still need renourishing. Photo Stephen P Leatherman, from Cornelia Dean's book Against the Tide.

Breakwaters Breakwaters are groynes with a sea wall or a sea wall with openings. They are very effective at retaining sand to form wet beaches. Particularly popular for lakeside resorts, they are able to retain man-made beaches where none existed before. Lakes do not have tides nor do they have large waves. So dunes cannot develop and natural beaches remain small. Sand brought in to create an artificial beach is easily lost to deeper water but breakwaters help to retain it. In the ocean, breakwaters are easily destroyed by storms. They need to be large and sturdy and would be enormously expensive when constructed to last.

Two pictures of lake-side breakwaters. Although ugly in appearance and not at all natural, they retain sand and provide for safe swimming. By allowing enough wave action in, waves are able to wash the sand from fine sediment (mud). Otherwise they could trap mud like the little harbour in the foreground would. It is not uncommon that such harbours silt up in less than 50 years, requiring periodic dredging.

Artifical surfing reefs Using computer models, scientists are now able to design structures in the sea that pile the sand up in such a way that it creates the ideal conditions for surfing. In the accompanying diagram a hypothetical structure is placed in front of the beach to show the ideas behind it. The good surfing beaches of the world have a number of things in common, described in surfers jargon. The parallel wave fronts arriving at the coast need to be deflected on an angle over a promontory or focus. This is located exactly where the wave starts to build up height because it is entering shallow water. Running along the side of the promontory, which has a slope underneath (the wedge), the wave is pushed up further than it normally would. This rising causes the wave to peel at only one place but as it proceeds towards shallower water over the ramp, the wave peels further and further, while keeping its height. Finally it comes to grief along the wedge provided by the beach. Surfers follow a track on an angle towards the beach. They can swim back towards the wedge in sheltered water and much fun is to be had by all.

A breakthrough in its design is that the entire structure remains invisible, about a metre below the low tide. Rather than transporting masses of beach sand with fossil fuel, the relatively small artificial reef uses the energy of waves to transport the sand.

Although such structures may attract business while protecting only a small part of the beach, they do not take away the underlying causes of beach erosion: wind broken by high-rise buildings and pollution of the water. They could also be shredded by the first major storm, covering the beach with millions of strands of black, undegradable polypropylene.

20 May 2000: view of waves rising over the first artificial surfing reef at Narrowneck, Surfers Paradise, Australia. About 40 surfers can be seen in the water. At this point in time, the reef was only 80% complete, lacking its third storey. It appears to have survived the recent storm, which halted progress. Resident surfies say that nothing much has changed. The beach shows no signs of widening. Estimated cost 7 million A$. In the right column some conversation overheard on the Net.

Sorry guys, I hate to be the one to grouch on this whole topic but I don't think I am the only one who thinks this way. I live on the beach at Surfers and have been eagerly awaiting the arrival of supposed perfect waves but have been severely disapointed. Every time I look at the progress report about the artificial reef I see the same $#@*'n 80 percent complete. When is the damn thing going to be finished and what is holding up the progress?? Surely it can't be because of too much swell activity because I wouldn't be the only one saying that the size has been consistently small this summer. The only solace I've recieved is from the perfectly placed sand pump in front of my units which filled the gutter and produced some nice waves for a while. I just hope that the gentlemen constructing this reef actually know what they are doing because if they fail, it doesn't take a genius to figure out that the council will never fund any more of these projects. Mr Reef please produce the goods that you know we want!  ----------------- We had a sick session on the reef. It was pumping, the waves were doubling up on the bags and barrelling on the first section just because of the reef. I reckon that when the sand settles around the bags, the place will be the the bomb (actually, I'd say it was the bomb yesterday already). Keep up with the good work.

For further information, visit:
www.burleighcam.com.au - A surfers information site with live camera pictures.
www.onthenet.com.au/~jackson/ - Good background information on the reef technology and motivation.

From our new insights we have learnt that the amount of sand has little bearing on the health of the dune/beach system. But a minimal quantity of sand should always remain. We've seen that in two places, excessive amounts of sand cause damage to the system:

• High (rear) dunes: When dunes become too high, they lift the sea wind from the beach, leading to permanent erosion. Where such is the case, the dunes should be mined and a better wind profile established.
• Sand banks out in sea: Sand banks out in sea (not the ones attached to some beaches) break oncoming waves and shelter the beach. They make the beach lay flat and less able to dry. These sand banks must be mined.
• Seasand: Although the sand in the sea near the beach belongs to the beach-dune system, there is a depth beyond which the sand can no longer move towards the beach. It is called the closure boundary, usually at around 20m depth, depending on wave exposure (in sheltered waters it is shallower). Visit the extensive chapter on seasand mining.

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