Question

You have been asked to work, as an adviser, on a major infrastructure project. It involves a response to the prospect of coastal flooding. The investment suggested is to build a seawall. Critically assess this suggestion. [We are particularly interested in how your advice would be structured, based on whether the situation we are dealing with (the prospect of coastal flooding) represents a “risky”, “uncertain”, or “fundamentally uncertain” situation.]

Answer 1

Coastal flooding occurs when normally dry, low-lying land is flooded by seawater. The extent of coastal flooding is a function of the elevation inland flood waters penetrate which is controlled by the topography of the coastal land exposed to flooding.The seawater can flood the land via from several different paths:

• Direct flooding — where the sea height exceeds the elevation of the land, often where waves have not built up a natural barrier such as a dune
• Overtopping of a barrier — the barrier may be natural or human engineered and overtopping occurs due to swell conditions during storm or high tides often on open stretches of the coast. The height of the waves exceeds the height of the barrier and water flows over the top of the barrier to flood the land behind it. Overtopping can result in high velocity flows that can erode significant amounts of the land surface which can undermine defense structures.
• Breaching of a barrier — again the barrier may be natural (sand dune) or human engineered (sea wall), and breaching occurs on open coasts exposed to large waves. Breaching is where the barrier is broken down or destroyed by waves allowing the seawater to extend inland and flood the areas.

Coastal flooding is largely a natural event, however human influence on the coastal environment can exacerbate coastal flooding. Extraction of water from groundwater reservoirs in the coastal zone can enhance subsidence of the land increasing the risk of flooding. Engineered protection structures along the coast such as sea walls alter the natural processes of the beach, often leading to erosion on adjacent stretches of the coast which also increases the risk of flooding.

Causes

Coastal flooding can result from a variety of different causes including storm surges created by storms like hurricanes and tropical cyclones, rising sea levels due to climate change and by tsunamis.

Storms and storm surges

Storms, including hurricanes and tropical cyclones can cause flooding through storm surges which are waves significantly larger than normal. If a storm event coincides with the high astronomical tide extensive flooding can occur. Storm surges involve three processes:

1. wind setup
2. barometric setup
3. wave setup

Winds blowing in an onshore direction (from the sea towards the land) can cause the water to 'pile up' against the coast; this is known as wind setup. Low atmospheric pressure is associated with storm systems and this tends to increase the surface sea level; this is barometric setup. Finally increased wave breaking height results in a higher water level in the surf zone which is wave setup. These three processes interact to create waves that can overtop natural and engineered coastal protection structures thus penetrating seawater further inland than normal.

Sea level rise

The Intergovernmental Panel on Climate Change (IPCC) estimate global mean sea-level rise from 1990 to 2100 to be between nine and eighty eight centimeters. It is also predicted that with climate change there will be an increase in the intensity and frequency of storm events such as hurricanes. This suggests that coastal flooding from storm surges will become more frequent with sea level rise. A rise in sea level alone threatens increased levels of flooding and permanent inundation of low-lying land as sea level simply may exceed the land elevation. This therefore indicates that coastal flooding associated with sea level rise will become a significant issue into the next 100 years especially as human populations continue to grow and occupy the coastal zone.

Tsunami

Coastal areas can be significantly flooded as the result of tsunami waveswhich propagate through the ocean as the result of the displacement of a significant body of water through earthquakes, landslides, volcanic eruptions and glacier calvings. There is also evidence to suggest that significant tsunami have been caused in the past by meteor impact into the ocean.^[Tsunami waves are so destructive due to the velocity of the approaching waves, the height of the waves when they reach land and the debris the water entrains as it flows over land can cause further damage.

Mitigation

It has been said that one way to prevent significant flooding of coastal areas now and into the future is by reducing global sea level rise. This could be minimized by further reducing greenhouse gas emissions. However, even if significant emission decreases are achieved, there is already a substantial commitment to sea level rise into the future. International climate change policies like the Kyoto Protocol are seeking to mitigate the future effects of climate change, including sea level rise.

In addition, more immediate measures of engineered and natural defenses are put in place to prevent coastal flooding.

Engineered defenses

There are a variety of ways in which humans are trying to prevent the flooding of coastal environments, typically through so-called hard engineering structures such as seawalls and levees.That armoring of the coast is typical to protect towns and cities which have developed right up to the beachfront. Enhancing depositional processes along the coast can also help prevent coastal flooding. Structures such as groynes breakwaters and artificial headlands promote the deposition of sediment on the beach thus helping to buffer against storm waves and surges as the wave energy is spent on moving the sediments in the beach than on moving water inland.

Natural defenses

The coast does provide natural protective structures to guard against coastal flooding. These include physical features like gravel bars and sand dune systems, but also ecosystems such as salt marshes and mangrove forests have a buffering function. Mangroves and wetlands are often considered to provide significant protection against storm waves, tsunamis and shoreline erosion through their ability to attenuate wave energy.To protect the coastal zone from flooding, the natural defenses should, therefore, be protected and maintained

Responses

As coastal flooding is typically a natural process, it is inherently difficult to prevent flood occurrence. If human systems are affected by flooding, an adaption to how that system operates on the coast through behavioral and institutional changes is required; these changes are the so-called non-structural mechanisms of coastal flooding response. Building regulations, coastal hazard zoning, urban development planning, spreading the risk through insurance and enhancing public awareness are some ways of achieving this. Adapting to the risk of flood occurrence, can be the best option if the cost of building defense structures outweighs any benefits or if the natural processes in that stretch of coastline add to its natural character and attractiveness. A more extreme and often difficult to accept response to coastal flooding is abandoning the area (also known as managed retreat) prone to flooding. This however raises issues for where the people and infrastructure affected would go and what sort of compensation should/could be paid.

Social and economic impacts

The coastal zone (the area both within 100 kilometers distance of the coast and 100 meters elevation of sea level) is home to a large and growing proportion of the global population. Over 50 percent of the global population and 65 percent of cities with populations over five million people are in the coastal zone. In addition to the significant number of people at risk of coastal flooding, these coastal urban centres are producing a considerable amount of the global Gross Domestic Product (GDP).^[People's lives, homes, businesses and city infrastructure like roads, railways and industrial plants are all at risk of coastal flooding with massive potential social and economic costs. The recent earthquakes and tsunami in Indonesia in 2004 and in Japan in March 2011 clearly illustrate the devastation coastal flooding can produce. Indirect economic costs can be incurred if economically important sandy beaches are eroded away resulting in a loss of tourism in areas dependent on the attractiveness of those beaches

Environmental impacts

Coastal flooding can result in a wide variety of environmental impacts on different spatial and temporal scales. Flooding can destroy coastal habitats such as coastal wetlands and estuaries and can erode dune systems. These places are characterized by their high biological diversity therefore coastal flooding can cause significant biodiversity loss and potentially species extinctions. In addition to this, these coastal features are the coasts natural buffering system against storm waves; consistent coastal flooding and sea level rise can cause this natural protection to be reduced allowing waves to penetrate greater distances inland exacerbating erosion and furthering coastal flooding. Prolonged inundation of seawater after flooding can also cause salination of agriculturally productive soils thus resulting in a loss of productivity for long periods of time. Food crops and forests can be completely killed off by salination of soils or wiped out by the movement of flood waters. Coastal freshwater bodies including lakes, lagoons and coastal freshwater aquifers can also be affected by saltwater intrusion. This can destroy these water bodies as habitats for freshwater organisms and sources of drinking water for towns and cities.^[

                   SOLUTION

Construction of seawall is th solution to overcome coastal flooding. A structure separating land and water areas. It is designed to prevent coastal erosion and other damage due to wave action and storm surge, such as flooding. Seawalls are normally very massive structures because they are designed to resist the full force of waves and storm surge. It is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a sea wall is to protect areas of human habitation, conservation, and leisure activities from the action of tides, waves, or tsunamis. Seawalls are generally massive concrete structures emplaced along a considerable stretch of shoreline at urban beaches.

Method

A seawall is constructed at the coastline, at the foot of possible cliffs or dunes. A seawall is typically a sloping concrete structure; it can be smooth, stepped-faced, or curved-faced. A seawall can also be built as a rubble-mound structure, as a block seawall, steel or wooden structure. The common characteristic is that the structure is designed to withstand severe wave action and storm surge. A rubble-mound revetment often protects the foot of such non-flexible seawalls. A rubble-mound seawall bears a great similarity to a rubble-mound revetment; however a revetment is often used as a supplement to a seawall or as a stand-alone structure at less exposed locations. An exposed dike, which has been strengthened to resist wave action, is sometimes referred to as a seawall.

Functional characteristic

The nearly vertical seawall, which was mainly used in the past, had the unfortunate function of reflecting some of the wave energy, whereby the erosion was aggravated, resulting in accelerated disappearance of the beach. However, all kinds of seawalls involve beach degradation as they are used at locations where the coast is exposed to erosion. The seawall will fix the location of the coastline, but it will not arrest the ongoing erosion in the coastal profile. On the contrary, it will to a varying degree, accelerate the erosion. It is quite normal that the beach disappears in front of a seawall, and it will most often be necessary, after some years, to strengthen the foot of the seawall with a rubble revetment.

A seawall will decrease the release of sediments from the section it protects and will have a negative impact on the sediment budget along adjacent shorelines.

Applicability

A seawall is a passive structure, which protects the coast against erosion and flooding. Seawalls were often used at locations off exposed city fronts, where good protection was needed and where space was scarce. Promenades have often been constructed on top of these seawalls. They are also used along other less inhabited coasts, where combined coast protection and sea defence is urgently needed. Seawalls are primarily used at exposed coasts, but they are also used at moderately exposed coasts.

Solving coastal engineering problems

1.Clear transition beach-mainland

Especially in sandy coastal areas with a lot of human (recreational) activities, a clear and fixed distinction between beach and mainland is desirable. A seawall will serve that aim. At the sea side of the seawall a more or less normal beach is assumed to be present; at the land side a road or a boulevard is present. Staircases facilitate the access to the beach.

The coast is assumed to be stable. The beaches in front of the seawall do not erode, or in case of a structural eroding coast an essentially (time-averaged) stable situation has been achieved with e.g. regular artificial beach nourishments. So a normal beach is assumed to be present in front of the seawall (and can be used for recreational purposes).

While in a situation without a seawall even a moderate storm (surge) will attack and erode the mainland, in the situation with a seawall this is prevented. Some scour in front of the seawall during a storm (surge) must be taken into account in the design. (A part of) the 'denied' erosion volume from the mainland, is now eroded just in front of the seawall. The scour hole might undermine the seawall. With e.g. the DUROSTA computation model an estimate of expected scour depths can be made

The design conditions for the seawall have to be properly chosen. The heavier the design conditions, the heavier the seawall must be constructed and especially the 'safe' foundation depth will increase accordingly. To build a seawall which will be safe under 'all' conditions might be an unrealistic option.

Although achieving a clear transition between beach and mainland was the primary goal in the discussion so far, automatically some protection of the (infrastructure at the) mainland is achieved. The design conditions as selected, determine the rate of provided protection.

The crest height of a seawall determines (together with the boundary conditions at sea) to a great extent the rate of overtopping (water reaching the mainland by wave run-up and breaking waves and splash water transported by landward directed wind). With an additional wall and/or a slightly curved front, rates of overtopping may be reduced.

Decrease risks of valuable infrastructure and buildings

Infrastructure and buildings situated close to the edge of mainland or dunes have a chance to be destroyed during a severe storm surge .

The risk (risk = chance x consequence) is felt to be too large in an existing situation. Or for example by extension and improvements of an existing hotel the 'consequence' has been increased and so the risk would increase to a too high level. Reducing the 'chance' might result in an acceptable 'risk' level (again).

With a robust seawall or revetment the required aim can be achieved. Aspects like proper design conditions and scour holes are in this case similar to the discussion in the previous case.

Let us consider a given a stretch of sandy coast. A very severe storm surge will cause a rate of mainland erosion of say 40 m in case the stretch of coast is unprotected. With a seawall which is able to withstand these conditions the erosion of the mainland will be zero. (In front of the seawall a deep scour hole will be formed.) When the entire seawall keeps its integrity; no further problems arise. (The scour hole will be re-filled again after some time with ordinary boundary conditions.) If, however, the seawall partly collapses and locally a gap in the seawall is formed during the severe storm surge, a rather dangerous situation will occur. Large volumes of sediment from the mainland are able to disappear through the gap and will flow along the sections of the seawall which are still in good condition in both longshore directions adjacent to the gap, filling the scour hole. It is expected that the ultimate rate of erosion of the mainland behind the gap will be larger than the 40 m as mentioned for the unprotected case.

Similar phenomena will occur at the two transitions between seawall and the adjacent, unprotected parts of the coast. Especially just adjacent to an abrupt end of a seawall, relatively much erosion is expected during a severe storm surge.

Existing row of dunes does not meet safety requirements

A row of dunes is apparently too weak (too slender) to guarantee the safety requirements. Under design conditions a break-through is expected; the low-lying hinterland behind the slender row of dunes will be flooded. A seawall or revetment might be chosen as a solution, provided that the seawall or revetment will keep its integrity during the design conditions. A risky alternative would be that the structure is 'allowed' to collapse in a latter stage of the storm surge. The time left to the end of the storm surge (with lower water levels) is then thought to be too short to cause yet a break-through.

If a revetment is selected as protection tool, two aspects call for some remarks, viz.:

• slope of revetment in relation to depth of scour hole;
• Level of upper end of revetment.

At least for rather smooth revetments it has (also experimentally) been proven that the depth of the scour hole depends on the slope characteristics. During tests in the Delta Flume of WL|Delft Hydraulics it turned out that with a slope of 1:3.6 a deeper scour hole was found than with a slope of 1:1.8 (other test conditions the same). The deepest point of a scour hole is not always found at the intersection point between revetment and cross-shore profile . It is conceivable that the depth of a scour hole is less for a rough slope than for a smooth slope.

The level of the upper end of a revetment (above that level the normal front slope of the dunes is assumed to be present up to the top of the dunes) determines the still occurring (remaining) dune erosion above that level, but also to some extent the depth of the scour hole. The higher that level the less remaining erosion, but also the deeper the scour hole. It is remarkable that if the level of the upper end of a revetment is equal to the storm surge level (or lower than that level), no reduction in dune erosion is found compared to a situation without any protection.

Structural erosion problems

The former section showed why and how seawalls or revetments may be used to resolve a coastal engineering problem or to achieve to goal. It is important to realize that seawalls and revetments are no solution for structural erosion problems. We pay attention to this issue in this article, because in coastal engineering practice too often this principally 'wrong' combination is applied. Many bad examples can be found all over the world.

Structural erosion caused by a gradient in the longshore sediment transport, means that volumes of sediment are lost out of the control volume area in a cross-shore profile (for a more detailed article on this, see Dealing with coastal erosion). This loss process takes mainly place under ordinary conditions; the contribution of storm conditions to this loss process is often rather small. The initial losses of sediments out of a cross-shore profile take place where water and waves are; where actual longshore sediment transports do occur; so in the 'wet' part of a cross-shore profile. The 'dry' parts of a cross-shore profile are not involved in the longshore sediment transports; it looks like that the 'dry' parts do not form an integral part of the cross-shore profile during ordinary conditions.

During high tides and/or modest storms (storm surges) all parts of a cross-shore profile participate in the coastal processes. By offshore directed cross-shore sediment transports, sediments from the higher parts of the profile ('dry' beach; even mainland under the more severe conditions) are transported to deeper water, filling the 'gap' that has been developed because of the gradient in the longshore sediment transport . This sequence of processes causes a permanent loss of material out of the upper parts of a cross-shore profile.

By 'protecting' the mainland in this case with a seawall, one indeed prevents that sediments from the mainland are transported in seaward direction (less filling of the 'gap'). The losses, however, continue; the 'dry' beach disappears; it becomes deeper and deeper in front of the seawall. Initially, right after the construction of the seawall, still a more or less normal beach was present. The beach did 'protect' the seawall to some extent; only moderate storms could reach the seawall. When the beach had disappeared, much more frequent wave attacks directly to the seawall will occur. (Most likely in the design of the seawall this was not taken into account.) Damage occurs; reinforcements have to take place.

A somewhat confusing element is the time-scale of the developments as have been discussed so far. Local people (their houses are at stake) have noticed in the past that every storm surge has taken some square metres of their gardens. The edge of the mainland is coming closer and closer to their houses. Not seldom the responsible coastal zone manager is 'forced' by the local people 'to do something'. Building locally a seawall or revetment (e.g. in front of the properties which are situated closest to the sea) indeed seems to resolve the problem. During the next storm surge, the just 'protected' parts of the coast do not show any further erosion; in the un-protected parts the erosion of the mainland continued. Local people believe that this solution 'works' (own experience). The coastal zone manager is forced to build seawalls along the other parts of the coast. However, when time elapses, it will be quite clear that a quite wrong solution has been chosen. Only with huge costs the situation can be redressed.