1.1 Background of the Study
Coastal zones are highly dynamic environments where not only interactions between land based, oceanic and atmospheric processes occur over different time scales, but also human related activities come into play inducing additional effects. Rapid increment on industrial, commercial, touristic, urban and recreational activities over the last 50 years have resulted in 1.2 billion people living on the coast. In other words, 23% of the world population live within this narrow fringe of the planet, representing a population density that is three times higher than the global mean; this, just accounting for the reported values until the year 1990 (Small & Nichols, 2003). Moreover, a higher growth rate than the global mean for coastal populations is reported since that time. In addition, coastal environments have been stressed with an increment in sea level exerted by climate change on a global basis. Different estimations exist, the IPCC (Intergovernmental Panel on Climate Change) have derived from considering several tidal gauges and altimetry records an increment rate of 1.8 ± 0.5mm/year for the period 1961-2003 and forecast an increment trend of the sea level rise for upcoming years. A broad range from 0.18 m to 0.59 m is shown among the different SRES (Special Report on Emission Scenarios) projections between the present and the end of this century. Furthermore, if climate change continues, both Greenland and Antarctica could eventually become additional sources of water volumes, contributing significantly to sea level rise (IPCC, 2007).
Coastal areas are often threatened by flooding, particularly urbanized flood-prone areas that are densely populated and high-valued. Floods can cause loss of life and property, disrupt society and economy, and degrade the environment (Escuder-Bueno et al. 2010). In the United States, although flood-prone areas have received federal and local support to mitigate flood risks, flood damage is increasing (Pielke 2002; Smith and Katz 2013). California, with the complexity of its water system and the need for integrated approaches, in particular has to manage frequent and extreme floods (Hanak and Lund 2012).
Hazards such as floods of coastal lowlands, erosion of sandy beaches and destruction of coastal wetlands (depending on their natural ability for adaptation by vertical accretion and/or horizontal migration) are potential straight forward effects, not to mention the impacts from increments in the frequency and intensity of extreme events (Nichols & Lowe, 2004). Hard coastal engineering alternatives (i.e., use of rock, steel or concrete structures) have been classical approaches as protective measures towards those potential hazards and will be still likely employed, in order to extend the present uses of coasts into the future. Possibly providing valuable time, until more conclusive knowledge regarding climate change, its consequences and decisions on coastal human development are generated.
A variety of options are available for flood management. According to their implementation timing, these options are classified as preparatory (before floods), response (during floods) and recovery (after floods) actions. Flood risk is defined as the summed probability of flood events multiplied by the expected consequences of each event (the event’s vulnerability) (Escuder-Bueno et al. 2010), over all events. Options in each category can be further classified as protection actions (protecting the area from the inundation) and vulnerability reduction actions(reducing the susceptibility of a community to flood damage from inundation) (Lund 2012). For example, levee and bypass construction are preparatory protection actions; flood warning and flood insurance are preparatory vulnerability reduction actions; sandbagging and levee monitoring are response protection actions; evacuation and emergency mobilization are response vulnerability reduction actions; reconstruction and repairing flood infrastructure are recovery protection actions; flood damage assessment and flood reinsurance are recovery vulnerability reduction actions. These options can be applied individually or as an integrated portfolio. To protect a floodplain, an optimal integrated flood management system will often combine a range of options. The foundation for developing such systems is that options are optimally designed and complement each other (Zhu, et al. 2007; Patterson and Doyle 2009; Castellarin et al. 2011). In addition to technical designs, economic considerations are needed for optimal design. Economically optimal flood management can be achieved with Risk-based Analysis and Benefit-cost Analysis (Howe 1971; Karlsson and Haimes 1988; Eijgenraam et al. 2014),particularly the probabilistic risk analysis (Lund 2012; Eijgenraam et al. 2014).
The use of hard coastal engineering alternatives has been a common practice since the last 50 years. Wide variety on type and form of coastal structures are found within its domain that account for differences in approaches, causes and magnitudes of investments for their construction throughout different periods of time. When considering all, the typical distinction between perpendicular and parallel oriented structures (respect to the shoreline) can be made. The design of seawalls is distinctly differentiated by the nature of shoreline which usually considers the following variants. The variants could be Coastal structures located on the seaward domain and coastal structures located in the coastal front or landward domain. Coastal structures located on the seaward domain, aimed to modify the local hydrodynamic forces and erosive processes such as groins, jetties and breakwaters. Coastal structures located in the coastal front or landward domain, aimed to prevent inundation and stabilize or set the shoreline into a fix position, preventing the loss of hinterland material into the sea. This group accounts for seawalls, revetments and bulkheads; which are commonly referred as the generic term “seawalls” for simplification. These coastal structures mainly seawalls are of primary concern in this study.
A number of ambiguities have been on the fore on the use of seawalls which has led to a substantial amount of controversy, up to the extent that the use of seawalls has been prohibited or strictly limited in some places, including several ocean bordering states of USA. Particularly for the Mediterranean coasts, some reviews of isolated storm surge effects and drastic failure events on particular beaches with the presence of this type of structures have been realized in the past. Nevertheless, foreseen specific induced morphological changes have not been fully established.
Seawalls which are free standing structure related to flood or shoreline retreat prevention are commonly made by concrete material in order to over stand sliding and overturning moments. Usually, the main element for design is the crest elevation, being wave overtopping and wave run-up as the main driver parameters considered for estimation. The basic function of all seawalls is to support, stabilize, and protect upland property and construction against wave action and erosion. The importance of any specific design used must take into consideration the consequences of seawall failure, as well as the initial cost of seawalls.Sea walls range in type and may include steel sheet-pile walls, very large concrete barriers, rubble mound structures, brick or block walls, or gabions (Kamphuis 2000). Sea walls are typically heavily engineered and inflexible, expensive to construct, and require proper design and construction supervision (UNFCCC 1999). The shape of the seaward face is important when considering incoming waves: smooth surfaces reflect wave energy, while sloping or irregular surfaces can scatter and dissipate it. Waves are likely to hit the structure with high force, moving sand off and along the shore away from it (Kamphuis 2000). Sea walls usually have a deep foundation for stability. Burying earth anchors with connecting rods upland from the shore can further stabilize them. This reduces pressure on the landward side of the structure (Dean and Dalrymple 2002). The considerations for designing seawalls, hence, are vast and any effective design and construction of a seawall must follow laid down procedures and standards.