11 Increasing frequency of extreme weather events, driven by climate change , has resulted in 12 an increasing demand for coastal structures to protect and stabilize shorelines. Concrete seawalls 13 are a common category of coastal protection structures, designed with the primary objectives of 14 absorbing wave action, preventing coastline erosion, and alleviating flooding. Much research has 15 been carried out on improving the seawall performance . This work is a review of the current state - 16 of-the-art in concrete seawalls focusing on design aspects including wave loading and innovative 17 seawall designs , ecological considerations, and durability aspects. Wave loads on seawalls have 18 received significant attention; however, their quantification remains a challenging task especially 19 for novel designs . Drawing inspiration from natural shorelines , modification of surface complexity 20 at a multitude of scales can improve the otherwise poor ecological performance of seawalls. The 21 corrosion of the steel is a major durability concern, and the use of non -corrosive reinforcement can 22 increase seawall durability towards corrosion. Examples of innovative seawall designs and 23 2 systems which have the capability to outperform conventional seawalls are discussed. Advances 24 in structural design, ecological engineering, and infrastructure materials s cience will drive the 25 development of multi -functional seawalls which are sustainable, durable, and resilient. 26 27 Keywords: Seawall; coastal protection; design; ecology; sustainability; durability 28 29 1. Introduction 30 The management of coastal resources is a matter of utmost importance for the global 31 economy, considering the high percentage of the global population that lives close to the coast [1- 32 2]. Increasing population s in coastal urban communities has meant that coastal protection has 33 become more widespre ad than in the past. Improper selection, design , and construction of coastal 34 protection systems can result in damage to individual properties, loss of land, and environmental 35 degradation , when structures are exposed to extreme weather events [3]. The damag e that occurs 36 is often not repaired due to high costs of repair, leaving the aging infrastructure vulnerable to 37 further damage and potentially catastrophic failure. This alarming situation has been exacerbated 38 by sea level rise and the increased frequency of severe storms and storm surges [4]. Therefore, 39 adequate performance of coastal structures is key to the safety of our coasts and coastal 40 communities. While ‘adequate performance ’ is subjective, an ideal coastal structure would provide 41 protection from extreme weather events, demonstrate ecosystem services, and last for its intended 42 service life without the need for excessive repair. 43 Coastal structures are built on the coastline or at a small distance from the coastline and 44 comprise a variety of structures such as seawalls, jetties, breakwaters, etc. They are generally 45 divided into “soft” and “hard” coastal structures, with soft structures being erodible by the sea, 46 3 while hard struc tures are generally not (substantially) erodible [3]. Coastal structures serve many 47 purposes, including reducing coastline erosion, flooding, and wave impact . The y can also protect 48 boat traffic and stabiliz e navigation channels, in addition to serving aesthetic and limited 49 ecological purposes [5]. General reviews of coastal protection structures can be found in [6-10]. 50 This paper focuses on concrete seawalls, specifically, loading considerations , innovative systems, 51 ecological performance, and material dura bility. Challenges that limit the performance of concrete 52 seawalls and innovations that enhance performance are highlighted. 53 Concrete is the second most used material in the wor ld (after water) as it is a cost -effective, 54 highly versatile , moldable material , made with ingredients that are widely available [11]. Concrete 55 is attractive for marine applications for these reasons and because of its inherent mechanical 56 properties and durability, including in aggressive conditions [11]. Recent advances in material 57 science, civil engineering, and construction technology have the potential to make concrete a 58 sustainable, durable, and resilient material for seawall construction [12,13] . 59 The primary purposes of a concrete seawall are reducing wave impacts, coastline ero sion, 60 and flooding [14,15] . Concrete seawalls are generally massive, long structures made of steel - 61 reinforced concrete. While a very significant amount of research has been performed on seawall 62 structures over the last century, there are still numerous concerns regarding seawalls. T hree of the 63 most important of these concerns are : a) the ecological performance of concrete seawalls is 64 typically poor, b) the steel in the seawalls tends to corrode over time, leading to cracking and 65 damage , and c) repair of damaged seawalls is not always carried out in a timely fashion . 66 Seawalls are subject to high magnitude forces due to wave action, especially during storm 67 surges due to wave breaking and overtopping [16]. In addition, the waves reflected off the se awalls 68 can cause significant amount s of sediments to move away from the seawall into the sea, which can 69 4 result in undesirable scouring at the seawall toe [17]. Figure 1 shows three common seawall designs 70 are typically constructed to protect shorelines [18]. 71 72 Figure 1: Typical concrete seawall designs for coastal protection: a) Vertical seawall, b) 73 Curved seawall, c) Riprap (redrawn based on [18]). 74 75 Vertical seawalls (Figure 1a ): Vertical seawalls are particularly attractive in locations where there 76 is small footprint availability and/or harsh wave conditions and high surges. The stability of a 77 vertical seawall against horizontal forces and sliding is provided by its weight and th e friction with 78 the underlying surface [19]. Such seawalls are subjected to high wave reflection in front of the 79 structure with a reflection coefficient close to unity [20], which leads to greater forces and scouring 80 at the toe. 81 Curved seawalls (Figure 1b ): The curved design of these seawalls reduces wave overtopping and 82 redirects wave energy due to waves breaking back in a seaward direction [21]. The curved -face of 83 the structure attenuates wave energy resulting in lower stress magnitudes [22]. Failure ana lysis 84 reveals that curved seawalls are more stable and perform better during dynamic loading when 85 compared to vertical walls [23]. While they show better performance, curved seawalls are typically 86 more complex to design and construct compared to vertical s eawalls. 87 5 Riprap or mound seawalls (Figure 1c) : These structures are simply mounds of various materials 88 including concrete fill [24] and are relatively inexpensive when compared to vertical and curved 89 walls. As riprap distributes weight over a wide surface, these structures can prevent foundation 90 failure for locations with poor soil characteristics. One of the benefits of riprap is tha t the loss or 91 failure of some of the armor stone will not result in total collapse and the system can continue to 92 function [25]. Repair or restoration of riprap systems is also relatively easier. The porous geometry 93 of riprap results in greater wave energy dissipation and reduced wave reflection. Therefore, riprap 94 is also utilized in front of seawalls to prevent scouring. Special types of riprap configurations 95 consisting of interlocking concrete blocks can also be employed to reduce wave run -up, 96 overtopping, and scouring at the base of existing seawalls in cases of high tidal flows [26]. 97 98 2. Scope, Overview, and Methodology 99 Three major aspects of concrete seawalls are covered in this article: design loads, 100 ecolo gical considerations, and material durability. Common seawall designs, such as those shown 101 in Figure 1, often do not satisfy the expected level of shoreline protection, especially during 102 extreme conditions [27-31]. The design wave forces borne by these str uctures represent a complex 103 topic that has received significant attention by researchers as discussed in Section 3. In many cases, 104 negative impacts on marine ecosystems due to concrete seawalls (including loss of biodiversity 105 and the loss of seagrass near the seawall) are significantly high and unrecoverable in the long -term 106 [32-34]. However, this loss of ecosystem services can be alleviated through the integration of 107 ecological engineering considerations such as the use of multi -scale complexity [35-38] (Section 108 4). Seawalls often show cracking, degradation, and damage, commonly due to the corrosion of the 109 steel reinforcement. Material degradation processes and means to mitigate damage are detailed in 110 6 Section 5. Finally, Section 6 discusses examples of inno vative designs that have the capability to 111 perform better than the traditional seawall structures. 112 The review was carried out in an objective manner, by searching several keywords: 113 “concrete seawall ”, “concrete seawall design ”, “concrete seawall design loads ”, “concrete 114 seawall ecological performance ”, “concrete seawall durability ”, “concrete seawall non -corrosive 115 reinforcement ”, and “ concrete seawall design advances ”. The first 30 results from each search 116 were considered to be relevant publicatio ns and used as a starting point. There was overlap in some 117 of the returned results; indeed only 155 out of the total 210 possible publications were distinct. 118 The abstracts of these 155 publications were then reviewed; 60 of these publications were deemed 119 not relevant or out of scope based on the abstract. The remaining 95 publications were reviewed 120 in depth. Relevant publications that were cited in these papers and were not revealed in our search 121 were also identified and reviewed. In total, more than 200 publications were reviewed. Sections 3 122 to 6 present a compilation of our findings for each topic of interest. 123 124 3. Load Considerations 125 Common design guidelines and standards for seawalls include Coastal Engineering 126 Manual (CEM), Shore Protection Manual (SPH), ASCE standards, and FEMA publication 127 manuals. The predominant forces for designing seawalls include hydrostatic, hydrodynamic, and 128 wave loads [39,40]. These forces should be added to other loads such as dead, wind, and impact 129 loads with a relevant combination factor depending on the coastal condition of the area [27,41]. 130 However, accurate estimation of related design loads is not explicitly prescribed for different built 131 environments. Hydrostatic loads oc cur as a result of static water pressure on the structure; 132 however, they do not usually yield a governing loading condition for seawall design [42]. 133 7 Hydrodynamic loads are caused by the dynamic effect of moving water on the surface of the 134 structure and can be determined based on fluid mechanics. The impact loads result from debris, 135 ice, and other objects striking the structure are typically modelled as concentrated loads [39]. The 136 wave loads, which are usually critical for the design, represent the dynamic pressure induced by 137 waves on structures [43]. The wave forces depend on both wave kinematics and dynamic 138 characteristics of the structure and typically exhibit large variations [44]. Depending on the region, 139 seawalls might be also subjected to lateral and vertical seismic forces as well as soil liquefaction 140 [45,46], however, such loads are out of the scope of this paper. 141 Wave loads are challenging to analyze, as the wave -structure interaction is complex and 142 depends on factors such as site conditions (i.e. , water depth, wave characteristics) and the 143 geometry/form of the structur e [42,47-49]. A proper understanding of wave forces is critical for 144 avoiding failure, i.e., structural collapse and overturning in seawalls, which can occur when 145 seaw alls are exposed to large wave breaking forces [50]. Although existing guidelines and 146 standards propose practical methods to determine the design wave forces for vertical seawalls, to 147 the best knowledge of the authors, there is no explicit method that can provide an accurate 148 estimation of wave loads for all applications and conditions. As the other loads are relatively 149 simple to quantify or do not govern loading conditions for design, the following section is 150 dedicated to critically reviewing existing appro aches for quantifying wave loads. 151 Generally, wave loads on coastal structures can be divided into slowly acting pulsating 152 loads, and impulsive or wave breaking loads. Pulsating loads are non -breaking loads that can be 153 determined with acceptable accuracy b y existing empirical expressions, for example, as an 154 approximately hydrostatic pressure produced by the highest water level reached by the design 155 wave [51]. Sainflou [52] presented an analytical solution for non -breaking waves approaching at 156 8 a right angle to the seawall that is still applicable to current designs [53,54]. On the other hand, 157 impulsive wave loads due to wave breaking on structures are more complicated to calculate and 158 have always been a challenge for designers [55]. There is not a unique univ ersally accepted method 159 accounting for the wave breaking force, also known as the impulsive horizontal force. A method 160 was developed by Minikin (based on the Bagnold model [56]) [57], which takes into account wave 161 impact force due to breaking waves onto a vertical coastal structure (Eq. 1). Although the Minikin 162 formula gives a good approximation of wave breaking force for many cases, it is not dimensionally 163 consistent [58]. 164 𝐹ℎ,𝑖𝑚𝑝 =101 3𝜌𝑔𝐻𝑑2ℎ 𝐿𝑑𝐷(ℎ+𝐷) (1) 165 where 𝐹ℎ,𝑖𝑚𝑝 (𝑁 𝑚) is the wave breaking force, 𝜌 (𝑘𝑔 𝑚3) is the water density, 𝑔 (𝑚 𝑠2) is the gravitational 166 acceleration, 𝐻𝑑 (m) is the design breaker height, 𝐿𝑑 (m) is the design wavelength, 𝐷 (m) is the 167 water depth at distance 𝐿𝑑 in front of the structure, and ℎ (m) is the water depth at the toe of the 168 structure. Additional information regarding the Minikin method and the distribution of dynamic 169 wave pressure can be found in [57,59,60]. 170 Experimental work conducted by Goda [59] resulted in a new expression for predicting 171 wave breaking force on seawalls which was later expanded to consider other parameters such as 172 the ef fect of a berm, slopping top, and incident wave angle [61-63]. Further laboratory testing by 173 Kortenhaus et al. [64] and Klammer et al. [65] led to a new prediction method for wave impact 174 loads that is widely utilized in building and structural manuals such as the British Standards 175 (BS6349 -1, 2000) [66] (Eq. 2). 176 𝐹ℎ,𝑖𝑚𝑝 =15𝜌𝑔𝑑2(𝐻𝑠𝑖 𝑑)3.134 (2) 177 9 where 𝐻𝑠𝑖 (𝑚) is the design significant w ave height at the toe of the structure and 𝑑 (𝑚) the water 178 depth. 179 Further developments were discussed in detail by Allsop et al. [67] and Oumeraci et al. 180 [68] who presented a new formula for determining maximum impulsive horizontal force (N/m) 181 given in Eq. 3. 182 𝐹ℎ,𝑖𝑚𝑝 =𝐹ℎ,𝑖𝑚𝑝∗𝜌𝑔𝐻𝑏2 (3) 183 where 𝐻𝑏 (𝑚) is the wave height at breaking and 𝐹ℎ,𝑖𝑚𝑝∗(𝑁 𝑚) is defined as relative maximum wave 184 breaking force following a Generalized Extreme Value (GEV) distribution, expressed using Eq. 4. 185 𝐹ℎ,𝑖𝑚𝑝∗=𝜃 𝜉(1−𝜉.ln(𝑝)%)+𝜇 (4) 186 where (𝑝)% is the probability of zero breaking forces, 𝜃 is the scale parameter, 𝜉 is the shape 187 parameter, 𝜇 is the location parameters of the GEV probability density function as a function of 188 the bed slopes [65]. 189 More recently, Cuomo et al. [55] conducted a series of physical tests where they measured 190 both pulsating (quasi -static) and impulsive wave forces at vertical structures. They sh owed that 191 the impulsive forces recorded were larger than the values obtained using existing prediction 192 methods. As a result, a new set of physically rational prediction formulae were proposed both for 193 impulsive and pulsating forces (Eq. 5 and Eq. 6). 194 𝐼𝑚𝑝𝑢𝑙𝑠𝑖𝑣𝑒 ℎ𝑜𝑟𝑖𝑧𝑜𝑛𝑡𝑎𝑙 𝑓𝑜𝑟𝑐𝑒 : 𝐹ℎ,𝑖𝑚𝑝(1 250)=𝐶𝑟1.65𝜌𝑔𝐻𝑚0𝐿ℎ𝑠(1−|ℎ𝑏−𝑑| 𝑑) (5) 195 𝑃𝑢𝑙𝑠𝑎𝑡𝑖𝑛𝑔 ℎ𝑜𝑟𝑖𝑧𝑜𝑛𝑡𝑎𝑙 𝑓𝑜𝑟𝑐𝑒 : 𝐹ℎ,𝑝𝑢𝑙(1 250)=4.8𝜌𝑔𝐻𝑚02 (6) 196 10 where 𝐿ℎ𝑠(m) is wavelength at the toe of the wall, 𝐶𝑟 is the dimensionless reflection coefficient, 197 𝐻𝑚0 (m) is significant wave height, ℎ𝑏 (m) is water depth at breaking, and 𝑑 (m) is water depth 198 in front of the wall. The term (1 250) refers to force at 1 250 exceedance level. 199 Although wave loads on seawalls have received significant attention, their quantification 200 remains challenging due to their dependence on a multitude of parameters and their stochastic 201 nature [55,69]. For instance, current prediction formulas for impulsive wave forces are applicable 202 to structures that behave as single degree of freedom systems. More detailed models are required 203 for irregular structures with complex dynamic responses [70]. For innovati ve seawall designs 204 (Section 6), including when seawall s have complex geometry and/or features such as perforations, 205 the wave loads and the performance of the structure in terms of wave reflection has to be quantified 206 through physical testing in wave tanks [10]. In wave tank testing, rigid scaled models of the 207 structure are tested under different waves with loads with performance being defined based on 208 water level, pressure and/or force measurements. It is important to note that physical testing has to 209 maint ain the similarity between the prototype structure and the model. The similarity is typically 210 expressed through the Froude criterion, i.e., the Froude number in the prototype structure and the 211 model have to be the same [71,72] . The design of the physical m odel depends thus on the prototype 212 conditions as well as the wave tank capabilities. Exploring experimentally defined loads along 213 with analytical and numerical models can lead to better seawall designs that use material in a more 214 efficient way while promot ing biocompatibility through multi -scale complexity. 215 216 4. Ecological Considerations 217 Marine diversity decreases with the loss of coastal wetlands and nearshore communities 218 with seawall construction . The loss of horizontal extent of coastal communities occurs with 219 11 seawalls, contributing to their p oor ecological performance. A major reason for the poor ecological 220 performance of seawalls is that seawalls truncate the interti dal zone and are less structurally 221 complex than the natural communities replaced [73-75]. Reduced surface complexity results in 222 overall decrease of microscale diversity of fouling organisms [76-83]. In addition to showing low er 223 levels of benthic and ichthyofaunal diversity, as seawalls are limited in extent both vertically and 224 horizontally compared to natural systems, they can cause habitat fragmentation and disturbance. 225 Fragmentat ion of biotic communities and disturbance is exacerbated by scouring and repetitive 226 maintenance along seawall structures [36,84,85]. Low structural complexity can lead to reduction 227 of available habitat for recruitment and growth of common nearshore marine organisms [86,87]. 228 Despite these shortcomings, if designed considering ecological features, seawalls have the 229 potential to replicate natural marine habitats and support some species of marine life [39]. 230 An effective solution for restoring the habitat of marine organisms and to potentially 231 increase abundance and diversity populations is to design structures using natural materials or 232 structures that emulate the physical properties of natural structures [88,89]. Fishes and benthic 233 invertebrat es populate and interact in greater numbers on materials with rugose and irregular 234 surfaces rather than a smooth, flat wall [90]. In addition to rugose surfaces, the use of 235 holes/crevices also helps increase benthic diversity [35,36,91-93]. Innovative seawall designs 236 using precast concrete steps to introduce shallow water structures can attract fish and increase 237 submerged aquatic vegetation [94]. Introducing multi -scale complexity/texture is critical for 238 enhancing benthic biodiversity as it increases the availability of food [83,95] while providing 239 shelter from predators for various nearshore marine species [92]. An effective example of such 240 design would be to use precast concrete steps, accompanied by surface texture modifications (at 241 small er scales) using cobbled surfaces, grooves, hollow or cast ledges to further enhance 242 12 microhabitats [35,41,96-98]. The importance of surface texture has also been confirmed in 243 concrete breakwaters. Proprietary concrete mixtures with substantial texture show ed greater 244 abundance of fish, invertebrates, submerged aquatic vegetation , along with increases in diversity 245 of marine organisms , when compared to mixtures with conventional concrete and no surface 246 texture [99]. 247 Whether the chemistry of the concrete itself has an effect on the benthic diversity is not 248 clear. In one study, concrete mixture designs including alumina -rich cement blends and slag -based 249 cement with different pozzolans were tested and monitored after 3, 6, 12 and 24 months for their 250 biodiversity [99,100]. The tested matrices had about 10 – 20% higher marine fouling communities 251 cover than standard Portland cement matrices [99]. The higher diversity in these alternative 252 mixtures has tentatively been suggest ed to be caused by lower pH [88,99,101]. On the other hand, 253 a recent study concluded that carbonated (lowered pH) concrete substrates did not support greater 254 species richness and abundance, and/or alter community composition, in both temperate and 255 tropical intertidal habitats [102] . It is possible that the use of ‘natural’ materials in the concrete 256 mixture design could help increase biodiversity. Concrete mixtures using hemp fibers and recycled 257 shell material supported significantly more live cover than con trol materials [103] . The direct 258 effects of concrete composition on biodiversity do not appear to have been adequately investigated. 259 In addition, it is unclear why exactly lower pH or the use of natural materials in the concrete 260 mixture would increase biod iversity and how long such effects would persist . 261 Studies have also compared concrete with other materials. A case study in Australia 262 compared sandstone and concrete used in construction of seawalls in terms of their effects on the 263 marine life (specifical ly grazing gastropod molluscs) [104] . The two habitats supported a similar 264 suite of benthic species, although in different numbers and frequencies of occurrence; tidal height 265 13 also had an effect on the observed species. When comparing materials, the effects of ocean 266 acidification also become important. The effects of ocean warming, acidification, and type of 267 coastal infrastructure material on algal turfs has been investigated [105,106]. Seawater 268 acidification resulted in greater covers of turf, though this e ffect was counteracted by elevated 269 temperatures. Concrete supported a greater cover of turf than granite or high -density polyethylene 270 (HDPE) under all temperature and pH treatments, with the greatest covers occurring under 271 simulated ocean acidification [106]. 272 Solutions to enhanc e the benthic diversity of fouling communities on already existing 273 seawall structures have also been studied [107] . Four 1.5 m x 0.8 m seawall panels made of ‘bio - 274 enhancing’ concrete with high structural complexity were deployed in an active marina. The panels 275 were surveyed 2, 7, 12, 18, and 22 months post deployment using 0.3 × 0.3 m quadrats in both 276 intertidal and s ublittoral zones of each panel and compared to fixed control quadrats comprised of 277 scraped sections of the original concrete marina seawall. Examined parameters included live cover, 278 richness, biodiversity; all were significantly higher on bio -enhanced pane ls compared to control. 279 Moreover, mobile invertebrates and resident fish species were clearly enhanced through design 280 aspects (holes and crevices) of the bio -enhanced panels [107] . While modification of existing 281 concrete should be done with caution, it sta nds to reason that careful modification of the concrete 282 surface would result in increasing the benthic diversity of seawalls [100,107,108]. 283 Multiple studies have focused on the combined effects of physical complexity and biotic 284 complexity of the surface on the benthic diversity of seawalls [92,109]. To provide biotic 285 complexity, the surface was manipulated by seeding with two common habitat -forming species 286 and naturally recruited fouling. Physical complexity was compared for flat surfaces and surfaces 287 with structured crevices [109] . Results showed that physical complexity, regardless of biotic 288 14 complexity (through seeding or natural fouling), increases the amount of time that fishes interact 289 with the substrate (2000 seconds on average for the structured and 1000 seconds on average for 290 flat tiles). Biotic complexity also increased the interaction time – 1100 seconds for tiles seeded 291 with algae, 1600 seconds for naturally fouled tiles, and 1800 seconds for tiles seeds with oysters. 292 The average interaction time for unfouled flat tiles was close to 0 seconds while it was on average 293 2300 seconds for na turally fouled structured tiles, demonstrating the importance of both types of 294 complexity [109,110]. 295 It has been suggested that ecoengineering projects , which seek to maximize the biodiversity 296 of multiple functional (taxa) groups on seawalls , should apply a variety of different microhabitats 297 to alter the environmental conditions available to organisms [92]. Novel artificial structures such 298 as rock pools or pits, planted vegetation, mangrove prop -roots, or reef blocks can be incorporated 299 into new seawalls du ring construction to provide an alternate habitat for fish, invertebrates, reptiles 300 and other marine organisms [12,111-113]. In addition to seawall design considerations which 301 enhance their performance in regard to marine habitat, there are also proposed d esign concepts 302 that provide nesting places for coastal wildlife by integrating hollow sections on areas of the 303 seawall above the sea level [114] . 304 Negative impacts on marine ecosystems due to concrete seawalls occur due to a variety of 305 reasons: truncation of the intertidal zone, r educed surface complexity , reduced vertical and 306 horizontal extent, increased turbidity, coastal eutrophication [115,116 ]. Limiting these negative 307 issues can be challenging and a one size fits all approach is unlikely to work. However, as 308 discussed in this Section, recent research has shown promising results when using approaches 309 inspired from nature including increasing surface rugosity and biotic complexity. 310 311 15 5. Durability of Concrete Seawalls 312 A durable seawall structure will meet the desired level of performance while maintaining 313 acceptable appearance throughout its service life [117]. Since concrete is weak in tension, steel 314 reinforcement is often used to provide the needed tensile strength . However, since steel is 315 vulnerable to corrosion from chlorides in the seawater, in practice, it is challenging to design 316 concrete seawalls that have a long service life with minimal maintenance in the harsh marine 317 environment. Durability concerns for concrete seawalls are di scussed in this Section . 318 5.1. Steel Rebar Corrosion 319 Deterioration of the reinforcement in any concrete structure, including in concrete seawalls , 320 affects structural safety and serviceability in an adverse manner [118]. When steel is embedded in 321 concrete without sufficient protection (cover) or when the cover ing concrete is cracked, chloride 322 ingress initiates. Over time, the chloride ingress leads to the onset of pitting corrosion when the 323 chloride concentration near the steel surface exceeds a cr itical threshold [119-121]. This threshold 324 is not easy to calculate or predict, since it is affected by several factors related to the steel, the 325 concrete, and the exposure environment [122,123]. Much research has been done on chloride 326 ingress and corrosio n in marine conditions, including the development of a theoretical model to 327 predict the service life of reinforced concrete seawalls which shows that the service life increases 328 with cover thickness, concrete tensile strength, threshold chloride concentrati on, burial depth and 329 surface load, but decreases with an increase in rebar diameter and water -cement ratio [124]. 330 Performance of seawalls predicted in theory and practice is often different. This is partly 331 due to concrete cracking, which is challenging to model and can greatly modify the rate of chloride 332 ingress. In seawalls, where the concrete cover is often cracked due to abrasion, storm effects, and 333 poor maintenance, it is rather probable that chloride -induced corrosion will occur at some point 334 16 [125]. Premature corrosion reduces the performance of the structure under normal conditions. 335 Under hurricane loading, such seawalls may fail in a catastrophic manner, causing substantial lo ss 336 of life and economic damage. Therefore, it is critical to minimize chloride -induced corrosion in 337 seawalls. Figure 2 show s examples of damage d seawalls with evident corrosion . 338 Corrosion requires the presence of both oxygen and moisture [126-128]. Therefo re, 339 corrosion does not typically initiate in reinforced concrete that is permanently immersed in 340 seawater. Even if corrosion does initiate, the corrosion rate is negligible because of the small 341 amount of oxygen that reaches the steel surface [129]. 342 343 344 Figure 2: Seawalls showing evident corrosion damage . 345 346 Considering the long service life of seawalls (sometimes up to 100 years), eliminating steel 347 corrosion is challenging. Numerous measures can be considered to mitigate steel corrosion: 348 Concrete mixture design : Concrete mixture design can be modified to increase the time before 349 corrosion initiates. Such modifications include the use of supplementary cementitious materials 350 (SCMs), reducing the water -cement ratio, and decreasing the porosity and per meability of the 351 concrete [130]. Such changes slow down the ingress of chloride into the concrete, though they may 352 17 be less effective once the chloride reaches a critical concentration near the steel surface. The use 353 of SCMs often improves concrete sustaina bility and durability, and most current marine concretes 354 include such materials in their mixture designs. Increases in service life that result with the use of 355 SCMs have been demonstrated, including in a recent laboratory and case study of concrete exposed 356 to chloride environments at the University of Toronto [131]. Chloride transport model s have 357 shown that incorporation of fly ash and slag in concrete slowed chloride ingress to a much - 358 decreased rate over the long -term, leading to corrosion delay/prevention and dramatic increases in 359 the predicted service life [131,132]. 360 Coatings and anti -corrosion admixtures : Implementing waterproofing coatings and anti -corrosion 361 admixtures in concrete, either as topical applications or throughout the material can lead into 362 excellent anti -corrosion performance of the structure [133]. Topical applications need to be re- 363 applied periodically. However, neither solution may be effective if the concrete cover is cracked. 364 The use of such admixtures can also increase initial costs, although they can reduce costs over the 365 life-cycle in addition to providing environmental ben efits. 366 Allowable crack width : Reducing the maximum allowable crack widths to smaller values increases 367 the time needed for chloride ions to reach the steel reinforcement and cause corrosion; however, 368 such changes might also result in over -design [14]. 369 Steel reinforcement alternatives : As long as conventional steel reinforcement is used in seawalls, 370 there is always a probability of corrosion initiation, even when appropriate corrosion mitigation 371 measures are taken. Therefore, the use of non -corrosive rei nforcement, such as stainless steel or 372 fiber -reinforced polymer (FRP) is an attractive alternative to the use of conventional steel rebar. 373 Such reinforcement s do not corrode and are starting to be used more commonly in marine 374 18 applications [134]. While thei r use will increase initial costs, significant cost -savings have been 375 demonstrated through the service life [135]. 376 Quality assurance and maintenance : Implementing appropriate quality assurance protocols and 377 conducting appropriate tests during the construct ion process can reduce the risk of corrosion. 378 These tests include rapid chloride permeability , bulk resistivity , or compressive strength testing of 379 cubes and in -situ cores, water absorption tests, water permeability tests, and reinforcement cover 380 surveys u sing cover meter [136]. Similarly, regular maintenance of seawalls can identify issues 381 before significant repair or reconstruction is required. 382 5.2. Alkali –Silica Reaction 383 Alkali -silica reaction (ASR) is a reaction between reactive silica in concrete aggregates and 384 the alkaline pore solution leading to a gel product, which expands on imbibing moisture and leads 385 to cracking and spalling. Deterioration of coastal structures du e to ASR has been reported in 386 multiple studies in different regions around the world [137,138]. Among these cases, there was a 387 1.4 km long precast concrete armor unit in the United States which was designed for a service life 388 of 50 years, but had to be ent irely replaced after 22 years due to excessive damage [137,138]. 389 Multiple cases of damage in coastal structures attributed to ASR, have been reported in Japan 390 [139]. In addition to seawalls, affected structures included T -type piers, partial shelters at th e 391 seaside, and precast concrete armor units. Reported crack widths ranged from 0.2 to 15 mm [139]. 392 Prevention measures to limit ASR damage have been discussed in detail in literature [140]. 393 The most obvious strategy is to use non -reactive aggregates, but t his is sometimes not possible due 394 to cost restrictions. If reactive aggregates are used, then ASR can be mitigated as follows: 395 - Use of SCMs, especially fly ash, in sufficient quantities; these reduce the pore solution 396 alkalinity and the resulting expansion [141]. 397 19 - Use of adequate dosages of lithium admixtures to suppress the swelling. These 398 materials do not reduce the formation of gel, but reduce its expansive nature [142]. 399 - Reducing the alkali contribution from the Portland cement component of the concrete, 400 either b y reducing the amount of cement or by reducing the amount of alkali in the 401 cement reduces the pore solution alkalinity and the resulting expansion [143]. 402 5.3. Sulfate Attack 403 Sulfate attack is caused by external/internal sources of sulfate reacting with aluminate 404 phases in the concrete. These reactions lead to the formation of expansive phases such as ettringite, 405 which cause damage. Alternatively, sulfate attack could be a physical damage mechanism, where 406 damage is caused by crystallization pressure exerted by certain sulfate phases in the presence of 407 water. Sulfate attack in marine environments occurs when seawater containing sulfate ingresses 408 into the concrete. Sulfate attack in concrete seawall structures can cause deterioration in the form 409 of expa nsion, cracking, loss of strength and stiffness, and disintegration. Sulfate attack, and other 410 damage mechanisms which cause cracking , can eventually lead to greater chloride ingress and 411 steel reinforcement corrosion, compounding deterioration [144,145]. 412 Evidence of external sulfate attack was found in a polished section of a damaged concrete 413 seawall in Wood Island (Kittery, ME) [146]. This attack occurred on the seawall surface exposed 414 to the seawater. Other cases have also been reported; however, it i s noted that sulfate attack may 415 be suppressed in the presence of chloride, which is found in higher concentrations in seawater. 416 There are multiple measures that can be implemented to limit the deterioration by sulfate attack in 417 concrete seawall structures, including replacing a portion of the cement with SCMs such as fly 418 ash, slag, silica fume, and the use of high -quality concrete with low water -cement ratio, porosity 419 and permeability [147-149]. 420 20 6. Innovative Seawall Designs 421 In the following section, recent ly developed innovative seawall structures for protecting 422 shorelines are discussed. 423 6.1. Use of FRP as Reinforcement 424 Considering the pervasive issue of corrosion, the use of FRP and stainless -steel 425 reinforcement can be valuable in eliminating corrosion. Much research has been done on these 426 materials, as they are corrosion resistant and provide sufficient mechanical performance [150,151]. 427 FRP is especially attractive, as these materials have higher specific strength, higher specific 428 stiffness, and lower specific weight relative to conventional steel rebars. FRP has already been 429 used in construction of real -life seawalls in South Florida [150]. 430 In 2019, quality assurance and code compliance evaluation of glass FRP (GFRP) rebars for 431 use in seawalls were carried out for a project at the seaside community of Flagler Beach, FL which 432 was severely impacted by Hurricane Matthew in 2016. The protection system was a GFRP - 433 reinforced concrete secant -pile seawall/bulkhead designed to provide support for the highway in 434 the event the adjacent sand dunes were lost during a major storm [152]. The use of GFRP should 435 eliminat e the possibility of corrosion, result ing in minimal repair and maintenance costs of the 436 structure over its life cycle [152,153]. 437 The feasibility of using basalt FRP (BFRP) as reinforcement has been investigated in the 438 lab [154]. An alternative design of a seawall system using BFRP bars was te sted and the results of 439 the seawall slab specimens demonstrated that the system design achieve d the required performance 440 [154]. Overall, there is a growing awareness of the properties of FRP as a structural material within 441 the construction industry and the re is ongoing research to further investigate where and how it can 442 be used in concrete structures in pursuit of improved durability, performance, and resilience [155]. 443 21 6.2. Permeable and Perforated Structures 444 The use of pervious and perforated concrete seawall structures can mitigate the wave load 445 on the structure and reduce wave reflection in front of the structure [156-161]. Such structures will 446 also result in improved water quality and have lesser negative e ffects on coastal ecosystems due 447 to exchange of flow between open sea and the enclosed regions [8,162-165]. 448 Jarlan [166] proposed a perforated vertical seawall structure to reduce wave reflection, and 449 thus, wave loading ( Figure 3 Error! Reference source not found. ). The system consists of two 450 vertical walls with a chamber between them. The wall that is positioned toward the sea is 451 perforated while the wall behind it is solid. As incident waves pass thro ugh the perforated wall, 452 they are partly reflected dissipating a large portion of their energy through vortices and friction. 453 The hydraulic performance of the system is highly dependent on the porosity of the front wall and 454 the ratio of the chamber width (𝐵) over the wavelength (𝐿) of the incident waves (𝐵𝐿⁄) [167]. 455 Experiments conducted over a full range of incident wave periods showed that the traditional 456 Jarlan -type system has its optimal performance when 𝐵𝐿⁄ is ~2 [168]. A more complex Multi - 457 Chamber System (MCS) has also been designed and experimentally investigated in the Large 458 Wave Flume of Hannover [169]. The MCS seawall design decreases wave reflection, wave run - 459 up, and overtopping. Therefore, it also significantly reduces the resulting horizonta l wave forces 460 applied to a seawall. This design can be an ideal alternative for new concrete seawalls over a wide 461 range of practical 𝐵𝐿⁄ ratios (𝐵𝐿⁄>0.25). 462 A new type of permeable vertical seawall consisting of prefabricated rectangular cells with 463 two perforated walls supported on four circular piles (Figure 4) was investigated by Somervell et 464 al. [170]. The number of cells can be adjusted to reach optimum hy drodynamic performance based 465 on the existing water and wave conditions. This flexible structural system can be modified even 466 22 after construction to meet changing conditions. The upper part of the vertical walls performs like 467 a wave energy attenuator and the space between the piles provides the required sediment 468 transportation through the structure and acceptable water circulation [170]. The proposed system 469 has enhanced wave energy dissipation and reduced wave reflection coefficient especially in 470 shallow wate r (h/L < 0.4) and to a lesser extent for deeper water (h/L < 0.5). Both the wave energy 471 attenuation and the wave reflection effects varied depending on the porosity of the upper -portion, 472 with the changes being more dependent on the seaward -side wall porosi ty rather than shoreward - 473 side. Minimal effects on shoreline morphology and marine habitats are expected with such 474 structures as the space between the piles provides passage for sediment transportation [70]. 475 476 Figure 3 : Illustration of a perforated vertical breakwater (redrawn based on [166]). 477 478 Figure 4: Schematic view of vertical cellular walls (redrawn from [170]). 479 23 6.3. Prefabricated Thin-Walled Hollow Seawall 480 The massive weight of seawalls can be a disadvantage due to high construction costs and 481 large deformation of soil layers underneath resulting in high ground pressure and settlement 482 failures [172]. To reduce the weight and the associated cost, an innovative seawall system 483 consisting of thin -walled concrete blocks (Figure 5Error! Reference source not found. ) has been 484 developed by BUSADCO [172,173]. The structure is composed of two thin -walled hollow 485 concrete blocks (HCB) at the lower levels, a solid concrete curved wall (SCCW) at the top level 486 for reflecting incident waves, concrete tie beams (CTB) for connecting the HCBs, and concrete 487 berm blocks (CBB) for preventing erosion at the toe of the seawall (Figure 6) [172]. This novel 488 design reduces the amount of concrete used compared to regular seawalls and is consequently 489 suggested to lead to more cost -effective design. The results from a numerical study showed that 490 this lightweight concrete seawall can successfully withstand destructive wave -induced forces 491 without soil failures beneath the structure and the entire displacement of the structure can be 492 significantly reduced (up to 5 4% in lateral displacement and 6 4% in vertical displacement) [172- 493 174]. 494 495 Figure 5: Illustration of a thin -walled hollow seawall (redrawn from [172]). 496 497 24 498 Figure 6: Different concrete components of the seawall: a) HCB -1, b) HCB -2, c) SCCW, d) 499 CBB, and e) CTB (redrawn from [172]). 500 501 6.4. New Methods for Seawall Construction and Production 502 Existing seawalls typically suffer from sliding due to soil erosion at the toe and strong 503 hydraulic pressures caused by storm surges and wave impacts [175]. Recently, a method was 504 developed to protect seawalls against sliding through constructing a seawall reinforcement 505 structure [176]. In this novel method, one bottom layer of concrete blocks is laid horizontally over 506 the sediment covering fabric and abutting the seawall base and multiple vertical blocks are applied 507 over the first layer (Figure 7a). Each b lock has a four -prong star -like geometry in an integral paired 508 relationship (Figure 7b). Concrete piles are drilled or cast in place through the bores to the bottom 509 sediment to hold the structure in place in case of severe storm surges to withstand outward 510 movement of the seawall (Figure 7c). This new method and resulting structure can dissipate the 511 incident wave forces and prevent erosion along shorelines and consequently enhance the sliding 512 stability of seawalls. In addition, a variation of this method ca n be employed to construct a jetty 513 by interconnecting vertical and horizontal interior spacers within the blocks (Figure 7d). 514 25 515 Figure 7: Illustration of permeable fabric blocks for seawall reinforcement: a) An existing 516 seawall reinforced with this metho d, b) A perspective view of the concrete block, c) A side 517 elevation of reinforced seawall, d) A section view of a jetty configuration (redrawn from [176]). 518 519 Material composition and structural complexity effects have been investigated in the 520 development of an innovative seawall system called “SEAHIVETM”, which is a modular marine 521 and estuarine protection system [177] (Figure 8)Error! Reference source not found. . The 522 SEAHIVETM system is comprised of a series of perforated hollow prismatic elements. 523 Perforations on the side faces of the elements provide passage for water flow under surging or 524 breaking waves dissipating the wave energy within the elements adding also structural complexity 525 to the structure and thus improving their potential for biocompatibility. Biocompatibility can be 526 further enhanced through the use of textured surfaces at multiple sca les. The modularity of the 527 SEAHIVETM system has adaptive features allowing elements to be oriented vertically or 528 26 horizontally paving the way for various applications and topographies. The proposed system can 529 be employed as an independent shoreline protecti on system or an auxiliary system enhancing 530 existing shoreline protection structures. The design of the SEAHIVETM system is being developed 531 through a series of experimental tests under the direct effect of wind and wave actions in a wave 532 tank facility. Prel iminary results show that the presence of perforations along the elements affects 533 the pressure profile both in distribution and in magnitude with the pressure decreasing for elements 534 with more perforations however this decrease varies for elements with dif ferent cross -sectional 535 profiles. 536 537 Figure 8: Illustration of a SEAHIVETM system configuration (redrawn from [177]). 538 539 Modular seawalls can also be constructed by simply pilling concrete blocks. Concrete 540 block seawalls produced by natural casting methods are typically accompanied by critical 541 disadvantages such as their high production costs, time -consuming production due t o the use of 542 multiple molds, and difficulties in casting process because of intensive material, and low durability 543 in usage conditions. An innovative production method was proposed by Tran in which seawall 544 concrete blocks can be produced with high efficien cy, short production cycle, and lower cost [178]. 545 The method uses semi -dry pressing with high pressure and can use high amounts of waste SCMs 546 27 [178]. Common shapes of concrete blocks and armors can be produced using this method are 547 shown in Figure 9 [178]. 548 549 550 Figure 9: Common shapes of seawall concrete blocks used for shoreline protection [178]. 551 552 It is not clear if land-based sources of pollution increase or decrease with several of these 553 novel designs and further study is needed . Incorporating microbial component s that can absorb 554 Nitrogen and reduce nutrient loading/sediments adjacent to the seawalls could provide furthe r 555 ecological advantages for these designs. In order to truly take advantage of these innovative 556 designs, they must be constructed using durable concrete (which avoids the issues discussed in 557 Section 5) . Using sustainable concrete will also reduce the overa ll environmental impact of 558 construction, and r ecent advances in concrete technology , including the use of ultra -high 559 performance concrete , seawater -mixed concrete, and concrete 3D printing [179-181] can be 560 leveraged to make seawalls (more) sustainable and durable. Additional material and structural 561 optimization can be performed through careful study and analysis of design loads (Section 3), 562 which may need testing in a wave tank. Finally, following the principles identified in Section 4, 563 the designs can prov ide promising ecological performance, leading to next -generation multi - 564 functional seawalls . Only a few studies have addressed the need for multi -functionality of seawalls 565 28 and coastal structures [182,183 ] and it is hoped that fu rther interdisciplinary resea rch will lead to 566 advances in this challenging topic. 567 568 7. Conclusions 569 Coastal protection including through the use of concrete seawalls is becoming increasingly 570 important in light of increasing coastal population, sea level rise, and the increased frequency of 571 extreme weather events. Properly constructed and maintained concrete seawalls are crucial for the 572 resilience of coastal communities, and will protect these communities from erosion , coastal 573 pollution, and flooding. This paper provide d an inter disciplinary overview of important challenges 574 facing concrete seawalls to pave the way for more durable and ecologically engineered coastal 575 infrastructure. The following are the major conclusions of this work: 576 • The absence of a universally accepted methodol ogy for calculating wave forces in different 577 hydrodynamic conditions complicates the research , development , and performance 578 optimization of novel seawall systems . Laboratory testing and pilot installation s are thus 579 critical for the deployment of new techno logies. 580 • Traditional seawalls can cause fragmentation and loss of natural habitats, and generally show 581 poor ecological performance, often as they are less structurally complex than their natural 582 counterparts. Seawalls can be ecologically engineered by incorporating natural materials or 583 using structures that emulate the physical properties of natural nearshore communities . Multi- 584 scale complexity , through the use of rough surfaces, holes/crevices, and texture helps increase 585 biodiversity of concrete seawalls. 586 • Using non -corrosive reinforcement including GFRP and BFRP can significantly increase the 587 durability of the seawalls and reduce life -cycle costs because of the elimination of the 588 29 possibility of corrosion . Seawall concrete durability can also be en hanced through the use of 589 SCMs , low permeable /porosity concrete , and good quality control . 590 • The use of pervious, perforated, or hollow concrete seawall structures can mitigate the wave 591 load, reduce wave reflection, reduce overall mass, improve water quality , and improve 592 nearshore marine diversity . 593 594 8. Acknowledgements 595 This research was supported by the University of Miami U -LINK Phase 2 funding to the 596 authors for the project Next generation of coastal structures: Feasibility, quantification, and 597 optimization. The following people individuals are acknowledged for helpful review and 598 comments: Prof. David Kelly, Prof. Billie Lynn, Prof. Renato Molina . 599 600 9.