Hard Defences Research Articles

Short- to mid-term shoreline changes along the southeastern coast of Gran Canaria Island (Spain)

The paper focuses on the mid-term shoreline evolution and recent short-term shoreline trends of five sandy beaches located along the southeastern coast of Gran Canaria. Using the Digital Shoreline Analysis System (DSAS), shoreline changes during the period 1961–2016 and sub-periods 1961–1998, 1998–2010 and 2010–2016 were assessed. Major results highlight a mid-term shoreline retreat of the southern coast stretch (Maspalomas Beach) and shoreline advance/stability of the eastern coast stretch (El Ingles, El Veril, Las Burras and San Agustin beaches), conversely positive and negative shoreline trends of Maspalomas and the eastern beaches, respectively, during the short-term period 2010–2016. Shoreline data and wind/wave data covering the period 1958–2004 highlight for Maspalomas a probable relationship between its negative mid-term shoreline evolution and a relative major severity of wave climate along the southern coast stretch. Moreover, regarding the negative short-term shoreline trend assessed for eastern beaches, an increase of wave dynamics is hypothesized, to be investigated in the near future. For the possible influence of hard defence structures on shoreline changes, data suggest that the groins placed along El Ingles, Las Burras and El Veril beaches have positively influenced shoreline trends in a first period, but did not contribute to overall beach stabilization or progradation in later periods. Results and data acquired during this study, concerning not only shoreline changes and related rates, but also possible causal aspects, can validly support both future research on sandy beaches in southeastern Gran Canaria and management interventions aimed at the development of coastal sustainable tourism in the studied coastal area.

Coastal flood alleviation through management interventions under changing climate conditions

PurposeCoastal flooding has disastrous consequences on people, infrastructure, properties and the environment. Increasing flood risk as a result of global climate change is a significant concern both within the UK and globally. To counter any potential increase in future flooding, a range of potential management options are being considered. This study aims to explore future coastal management practice for flood alleviation, incorporating the influence of climate change.Design/methodology/approachThe Taf estuary in South West Wales, a macro-tidal estuary which has a history of coastal flooding, was chosen as the case study in this paper to investigate the impact of coastal management interventions such as construction of hard defences, managed realignment or altering land use of affiliated ecosystems such as salt marshes on the complex hydrodynamics and hence flooding of the surrounding areas of the estuary. The study was carried out using a numerical hydrodynamic model of the Taf estuary, developed using the process-based Delft3D modelling software.FindingsThe role of the selected management interventions on coastal flooding was investigated using an extreme storm condition, both with and without the impact of future sea level rise. The results highlight the scale of the effect of sea level rise, with the selected management interventions revealing that minimising the increase in flooding in future requires careful consideration of the available options.Originality/valueThis paper explores the highlighted role of coastal management practice in future with the influence of climate change to study how effective alternative methods can be for flood alleviation.

Lessons learned from the past for a better resilience to contemporary risks

PurposeThe purpose of this paper is to provide an overview of the potentialities offered by a historical approach by addressing its scientific and societal issues as well as its opportunities at the scale of different continents and cultural areas. The authors then show the major role played by traditional societies and indigenous peoples in preserving and transmitting a culture of risk which today is threatened by an unprecedented memory break resulting from the process of globalization. Finally, the authors present two concrete examples of projects aiming to use historical lessons learned to reduce the vulnerability of local communities.Design/methodology/approachHistorical documentation provides a series of very varied archives, voluminous and geographically scattered. Several types of series will be studied. Besides the written archives, the authors shall also realize an inventory of all the elements of the cultural heritage and the memory evoking the risks and the vulnerabilities.FindingsThis study shows how forgetting past disasters has contributed to increasing the vulnerability of the modern societies and building a “society of risk.” Paradoxically, industrialization and the era of the engineer opposed “pre-modern” societies to so-called “modern” societies. In this way, ancestral knowledge and strategies have often been despised in favor of hard defense works whose limits are now being measured after the recent disasters. On the other hand, the paper promotes a different model combining both engineering and local historical/cultural knowledge in order to design a more sustainable and applicable strategy.Originality/valueThe authors show the major role played by traditional societies and indigenous peoples in preserving and transmitting a culture of risk which today is threatened by an unprecedented memory break resulting from the process of globalization.

Shelving the Coast With Vertipools: Retrofitting Artificial Rock Pools on Coastal Structures as Mitigation for Coastal Squeeze

Coastal squeeze caused by sea level rise threatens the size, type and quality of intertidal habitats. Along coastlines protected by hard defences, there is a risk that natural rocky shore habitat will be lost, with remaining assemblages characteristic of hard substrata confined to sea walls and breakwaters. These assemblages are likely to be less diverse and different to those found on natural shores as these structures lack features that provide moist refugia required by many organisms at low tide, such as pools and crevices. Engineering solutions can help mitigate the impacts of sea level rise by creating habitats that retain water on existing structures. However, as experimental trials are strongly affected by local conditions and motivations, the development of new techniques and solutions are important to meet the needs of local communities and developers. Following a small scale community project, a feasibility study retrofitted five concrete-cast artificial rock pools (‘Vertipools’) on to a vertical seawall on the south coast of England. After 5 years, the artificial pools increased the species richness of the sea wall and attracted mobile fauna previously absent, including fish and crabs. The Vertipools had assemblages which supported several functional groups including predators and grazers. Although disturbance of algal assemblages on the seawall from the retrofitting process was still evident after 3 years, succession to full canopy cover was underway. Collaboration between policy makers, ecologists, children and artists produced an ecologically sensitive design that delivered substantial benefits for biodiversity and that could be adapted and scaled-up to both mitigate habitat loss and enhance coastal recreational amenity.

Building global change resilience: Concrete has the potential to ameliorate the negative effects of climate-driven ocean change on a newly-settled calcifying invertebrate

Global climate change is driving sea level rise and increasingly frequent storm events, which are negatively impacting rapidly-growing coastal communities. To mitigate these impacts, coastal infrastructure must be further protected by upgrading hard defences. We propose that incorporating pH-buffering materials into these upgrades could safeguard marine organisms from the adverse effects of ocean acidification and ocean warming during the vulnerable transition from planktonic larvae to benthic juveniles. To test this, we examined the effects of ocean warming (24 or 27 °C), ocean acidification (pH 8.1, 7.9, 7.7), and substratum (concrete, greywacke, granite) in all combinations on the settlement success of an ecologically and commercially important sea urchin, Tripneustes gratilla. Low pH (7.9, 7.7) generally reduced the quantity and size of juveniles four weeks post-settlement, although this was partially ameliorated by increased temperature (24 vs. 27 °C). In the warmed and acidified treatments, settlement rates were lower on concrete than granite or greywacke, but two weeks post-settlement, juveniles on concrete were larger, and had longer spines and higher survival rates than on greywacke or granite, respectively. The benefits provided by concrete to newly-settled juveniles may be related to alkali chemicals leaching from concrete buffering low pH conditions in surrounding seawater and/or increased availability of bicarbonate in the boundary layers around its surface. Our results highlight the potential for pH-buffering materials to assist marine organisms in coping with the effects of changing ocean conditions, but further research is required to understand the generality and mechanism(s) driving the beneficial effects of concrete and to test pH-buffering materials in the field.

Dune dynamics safeguard ecosystem services

Intensively used coastal zones often know a history of hard defense structures to prevent erosion and protect infrastructure against floods. The interruption of sand transport between sea, beach and dunes however causes a domination of late successional stages such as dune shrub. With the decline of young, dynamic vegetation types, a change occurs in the provision of ecosystem services. In spite of the growing awareness on the role of dune dynamics to support human well-being and biodiversity, redynamisation of dunes is rarely implemented in coastal zone management. It has been argued in research documents that this may be caused by a failure to make those benefits tangible and specific. This study aims to underpin the added value of dynamic versus fixed dunes. Five different ecosystem services in a case-study in Belgium were quantified based on (compound) indicators and expressed in monetary units. The value of a natural, dynamic dune system covering the entire gradient of dune succession and dominated by young successional stages was compared with the value of a fixed dune system dominated by late successional stages. The results indicate that a dynamic dune complex may create up to ∼50% higher economic benefits, and that the main benefits are on account of recreation and coastal safety maintenance. The results underpin the statement that we can only continue benefitting from the services dunes provide if we accept their mobile nature, but that redynamisation requires a site-specific feasibility analysis.

Medium-term dynamics of a middle Adriatic barred beach

Abstract. In recent years, attention has been paid to beach protection by means of soft and hard defenses. Along the Italian coast of the Adriatic Sea, sandy beaches are the most common landscape feature and around 70 % of the Marche region's coast (central Adriatic) is protected by defense structures. The longest free-from-obstacle nearshore area in the region includes the beach of Senigallia, frequently monitored in the last decades and characterized by a multiple bar system, which represents a natural beach defense. The bathymetries surveyed in 2006, 2010, 2011, 2012 and 2013 show long-term stability, confirmed by a good adaptation of an analyzed stretch of the beach to the Dean-type equilibrium profile, though a strong short- to medium-term variability of the wave climate has been observed during the monitored periods. The medium-term dynamics of the beach, which deal with the evolution of submerged bars and are of the order of years or seasons, have been related to the wave climate collected, during the analyzed temporal windows, by a wave buoy located about 40 km off Senigallia. An overall interpretation of the hydrodynamics, sediment characteristics and seabed morphology suggests that the wave climate is fundamental for the morphodynamic changes of the beach in the medium term. These medium-term time ranges during which waves mainly come from NNE/ESE are characterized by a larger/smaller steepness and by a larger/smaller relative wave height, and seem to induce seaward/shoreward bar migration as well as bar smoothing/steepening. Moving southeastward, the bar dimension increases, while the equilibrium profile shape suggests the adaptation to a decreasing sediment size in the submerged beach. This is probably due to the presence of both the harbor jetty and river mouth north of the investigated area.

Benefits of coastal managed realignment for society: Evidence from ecosystem service assessments in two UK regions

Abstract Coastal managed realignment has the potential to deliver both nature conservation and other benefits to people, but these are rarely quantified. We used an ecosystem services assessment toolkit, TESSA, at two locations in the UK; Hesketh Outmarsh West (northwest England), where realignment has already been carried out, and the Inner Firth of Forth (central Scotland), where realignment is proposed for multiple sites. At the Inner Forth we focus on one site, Inch of Ferryton, in Clackmannanshire but also assess ecosystem services across several sites. Using dedicated data collection where possible, and site-appropriate existing data, we estimate the value of these sites to people in the realigned state compared with the most realistic alternative: continued agricultural production behind hard flood defences (agricultural state). Services assessed were climate change mitigation, agricultural production, nature-based recreation, and flood risk protection. At both sites agricultural production was estimated to be greater in the agricultural state, while other services were estimated to be greater in the realigned state. We are cautious about assigning overall monetary values based on biophysical attributes, particularly considering that climate change mitigation is highly sensitive to carbon prices, and that by necessity we were unable to quantify all services. Nevertheless, using a price for carbon that incorporates the societal cost of emissions, we estimate that the net annual provision of services is £262,935 (£1460.75/ha) at Hesketh Outmarsh West and £93,216 (£574.70/ha) at Inch of Ferryton. At both sites, sequestered carbon in accreting sediments outweighs greenhouse gas emissions from intertidal habitats, and the net value of this in turn outweighs the income foregone from crops and grazing. At Hesketh Outmarsh West the value of ecosystem services is increased by the reduction to flood risk arising from the coastal managed realignment action. Nature-based recreation is estimated to increase at both sites under realignment, with the visitor profile expected to be strongly local. Decisions about coastal management, including realignment, should incorporate information about the ecosystem services provided under different scenarios. This applies beyond coastal ecosystems, and the site-scale is often the most appropriate scale to carry out such assessments, as this is the scale at which decisions are often taken.

Protection by hard defence structures or relocation of assets exposed to coastal risks: Contributions and drawbacks of cost-benefit analysis for long-term adaptation choices to climate change

Adapting to sea-level rise due to climate change involves new public policies that aim to relocate those assets most at risk from coastline erosion or flooding. It is no longer solely a question of studying the merits of a defence infrastructure project designed to prevent risks but of looking instead into a broader and longer-term project implying a whole new logic of land-use management for the areas concerned. In this context, the aim of the present article is to compare different adaptation scenarios and to show the need for evolving economic assessment and decision-making tools to include multidimensional and long-term aspects of adaptation policies. It is important to show the limitations of traditional Cost-Benefit Analysis (CBA) by integrating economic impact and non-market factors which are currently only assessed in multi-criteria approaches. Such assessments enable comparison of the Net Present Value (NPV) of a protection scenario using hard defence structures with various relocation scenarios, depending on whether the CBA includes only the direct damages avoided (classic CBA) or integrates the long-term tourist economy and environmental impacts (enhanced CBA). As costs of property purchasing are high, CBA may initially favour the protection scenario over relocations despite unfavourable tourist and environmental consequences. However, if one takes into consideration innovative land-purchase mechanisms which enable reduced investment costs in relocation scenarios, the latter measures may have a positive NPV. We therefore conclude that, in the long term, taking into account the local tourist economy and environmental benefits, the likely fall in prices of real estate at risk and the implementation of anticipatory schemes could enable relocation policies to become economically viable.

The effectiveness of beach mega-nourishment, assessed over three management epochs

Resilient coastal protection requires adaptive management strategies that build with nature to maintain long-term sustainability. With increasing pressures on shorelines from urbanisation, industrial growth, sea-level rise and changing storm climates soft approaches to coastal management are implemented to support natural habitats and maintain healthy coastal ecosystems. The impact of a beach mega-nourishment along a frontage of interactive natural and engineered systems that incorporate soft and hard defences is explored. A coastal evolution model is applied to simulate the impact of different hypothetical mega-nourishment interventions to assess their impacts’ over 3 shoreline management planning epochs: present-day (0–20 years), medium-term (20–50 years) and long-term (50–100 years). The impacts of the smaller interventions when appropriately positioned are found to be as effective as larger schemes, thus making them more cost-effective for present-day management. Over time the benefit from larger interventions becomes more noticeable, with multi-location schemes requiring a smaller initial nourishment to achieve at least the same benefit as that of a single-location scheme. While the longer-term impact of larger schemes reduces erosion across a frontage the short-term impact down drift of the scheme can lead to an increase in erosion as the natural sediment drift becomes interrupted. This research presents a transferable modelling tool to assess the impact of nourishment schemes for a variety of sedimentary shorelines and highlights both the positive and negative impact of beach mega-nourishment.

A novel approach to flood risk assessment: the Exposure-Vulnerability matrices

The classical approach to flood defence, focused on reducing the probability of flooding through hard defences, has been gradually substituted by flood risk management approach, which accepts the idea of coping with floods, and aims at reducing both probability and the consequences of flooding. In this view, the concept of vulnerability becomes central, such as the (non-structural) measures for its increment. However, the evaluations for the effectiveness and methods of non-structural measure and the vulnerability are less studied, compared to the structural solutions. In this paper, we adopted the Longano catchment in Sicily, Italy, as the case study. The methodology developed in the work enabled a qualitative evaluation of the consequences of floods, based on a crisscross analysis of vulnerability curves and classes of exposure for assets at risk. A GIS-based tool was used to evaluate each element at risk inside an Exposure-Vulnerability matrix. The construction of an E-V matrix allowed a better understanding of the actual situation within a catchment and the effectiveness of non-structural measures for a site. Referring directly to vulnerability can also estimate the possible consequences of an event even in those catchments where the damage data are absent. The instrument proposed can be useful for authorities responsible for development and periodical review of adaptive flood risk management plans.

Briefing: Abrasion performance of concrete in coastal structures

Abrasive wear of concrete in coastal structures can be a significant issue in areas where coarse or gravel sediments abound. In such environments, abrasion may be the governing factor for concrete specification in new construction. In the case of hard defences, concrete will constitute a major project cost, thus necessitating optimisation of mix design. This briefing paper explores some of the background to the problem and existing approaches in design codes. A case study of a major coast protection scheme in the UK is discussed, focussing on the approach to concrete specification through abrasion testing.

宜宾县山洪灾害调查实践

山洪灾害具有突发性强、预测预报预警难度大、破坏性强、成灾快、防御难等特点，历来是防汛抗灾工作中的一道难题。根据统一要求与安排，组织有关单位对本县山洪灾害情况进行全面调查，取得了系列调查成果。这些成果将成为进一步加强山洪灾害防治建设的基础数据和技术支撑。建议做好行政区域基础统计，有利于“防、放、走、进”，有的放矢地实施山洪灾害防治工作。 The mountain flood disaster has always been a difficult problem in the practice of flood prevention and fighting against natural disasters, which has the characteristics of bursting strongly, forecasting and warning difficulty, strong destruction, fast disaster forming, hard defense etc. According to the unified requirements and arrangements of the state, a comprehensive investigation of the mountain flood dis-aster was carried out, which achieved a series of investigation results. These results will provide a basic data and technical support for strengthening the construction of mountain flood disaster prevention. Doing more regional statistics work is suggested in order to control mountain flood disaster much better in the Yibin country.

Shoreline response of eroding soft cliffs due to hard defences

Soft cliff retreat has often triggered a hard adaptation response by the building of seawalls and groynes. On adjacent undefended coasts erosion continues, resulting in set-backs. This paper examines the impact of coastal defences on the adjacent coast from historic records and present practices, and explores possible future response. Continued set-back often leads to outflanking of defences, making them ineffective at their extremities, particularly at the down-drift end where it is most severe. Solutions to outflanking usually involve extending defences, initiating a cycle of set-back, outflanking and further extensions. Multiple defence extensions and continued retreat of the unprotected adjacent coast results in artificial headland formation. Over several decades, headlands experience foreshore steepening and reduced sediment availability, making them more difficult and expensive to defend. Shoreline management plan policies of managed retreat advocate selective defence abandonment, which may change the nature of artificial headland formation. Defence abandonment and new engineering works must be planned, anticipating the processes described in this paper. This will reduce unexpected changes and reduce maintenance and emergency work costs.

Shoreline response of eroding soft cliffs due to hard defences

Shoreline response of eroding soft cliffs due to hard defences

Managing industrialised coastal fine sediment systems

A high level of concern surrounds the management of industrialised muddy coasts and estuaries. Many centuries of apparently desirable alterations, such as hard defences, training walls and groynes, aided by traditional dredging and disposal, are today recognised as having associated disadvantages. The port and shipping industry is the principal facilitator of world trade, but the trends in its development place ever-increasing strains on conventional methods of management. In particular, inadequacies in “water-side” technologies are constraining the demands of modern societies. The present paper reviews a number of emerging self-sediment engineering technologies to manipulate intertidal zones, many of which are finding wide acceptance by practitioners, although many of these were first used some time ago: salt marsh inducement, set-back, tidal flat regeneration, etc. all have their place. A far greater challenge, and one less readily accepted, is attached to generic Sediment Management Systems for the subtidal zone. A range of more economic, more environmentally desirable methodologies to manage fine sediment for the ports and shipping industry are now well-proven: Auto-flushing, Entrance Flow Optimisation, and various manifestations of the Nautical Depth concept. Although for the moment these apply narrowly to ports, it is shown that they readily have a wider application to the management of whole systems. The technologies and their costs and benefits are herein set out. Bearing in mind that these technologies are well-proven, their wider adoption has now become a “people issue”. The environmental benefits of these methods are also discussed. A challenge facing our highly specialised society is that, in the optimal case, aspects of marine physics, chemistry and microbiology must all be carefully manipulated at the same time. Notably, no port in the world currently has a sediment management plan suited to the modern era.

Human Impact on Shoreline Evolution Along the Follonica Gulf (Southern Tuscany): How Tourism May Kill the Goose that Lays the Golden Eggs

CIPRIANI, L.E., FERRI, S., LAMI, G and PRANZINI, E., 2011. Human Impact on Shoreline Evolution Along the Follonica Gulf (Southern Tuscany): How Tourism May Kill the Goose That Lays the Golden Eggs. In: Micallef, A. (ed.), MCRR3-2010 Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 290–294. Grosseto, Tuscany, Italy, ISSN 0749-0208.Follonica Gulf beach is subject to widespread low-rate erosion, which started in the 19th Century due to river diversion for coastal marsh reclamation. Once rivers were re-directed to the coast; land subsidence induced by water extraction sustained erosion. From 1954 to 1984 mean shoreline retreat was approximately 9 m.In the 1960's, development of coastal tourism villages increased beach use and the need for house protection from storms. Detached breakwaters were built, trapping sand from adjacent coastal sectors. This induced shoreline progradation in protected sectors creating a wider beach, but unprotected coastal sectors experienced severe erosion. Stakeholders requested additional protection: the most developed area was gradually protected by detached breakwaters up to complete closure. Comparing the 2005 and 1954 shoreline positions, we see that out of the total length of approximately 21 km of coastline 8 km are accreting; less than 1 km is stable and over 12 km are eroding. Recently, a marina was built on the eastern margin of the study area; however wave diffraction at breakwater tip induced longshore transport and entrance siltation at the cost of western neighbouring beaches.Tourism industry stakeholders, whose economy is based on beach width and quality, were responsible for such coastal degradation, but now cooperate with local and regional administrations, accepting reduction in the use of hard defences - although shoreline realignment will end up penalising someone. Some detached breakwaters have been lowered to 50 cm below mean sea level, whereas others will undergo similar reshaping in the near future. The two oldest sets of coastal defence are now under study, in order to find solutions for a gradual return to more natural conditions. Changes to the marina configuration are being designed to prevent siltation, reaching a new equilibrium for the adjacent beach. We now realise that money spent for shore protection in some sectors would have maintained the 1950's shoreline position along the whole Gulf using beach nourishment, with no landscape deterioration.

Residents' attitude towards possible adaptation measures to the sea coast erosion in Latvia

PurposeIn addition to climate changes, human activities in the sea coast area are also important in the context of coastal erosion. There is a need to consider possible measures of adaptation at a national level to solve this issue. The purpose of this paper is to analyze the findings of the interviews made by the author and come to conclusions about residents' attitude towards possible measures of adaptation to the coastal erosion in Latvia.Design/methodology/approachThe research was made by interviewing those seashore residents whose households are most influenced by sea coast erosion along the entire coastline of Latvia in the areas where erosion rate is projected to be at least 6 m until the year of 2025.FindingsHalf of the interviewees consider hard coast defense structures as an effective adaptation measure to the sea coast erosion in households concerned. The other half would not support that because of either long‐term ineffectiveness or other unwilling consequences. Retreat as a strategy is acceptable only for quite a small part of interviewees.Originality/valueThe results of the research can be taken into consideration when making political decisions concerning adaptation to climate change.

Coastal retreat and/or advance adjacent to defences in England and Wales

Retreat and advance of shoreline position occurs naturally, and also as a result of defences which are constructed to prevent erosion and flooding. Retreat more commonly manifests itself down-drift of defences due to a sediment deficit causing the coast to become ‘set-back’. Advance normally develops due to sediment accumulation up-drift of a barrier inhibiting longshore drift, resulting in the coast becoming ‘set-forward’. Many examples of set-backs and set-forwards are recorded, but their location, number and cause is not known on a national scale. Using the Futurecoast aerial photographs, approximately 200 localities were identified as set-back or set-forward in England and Wales, with half situated in the Eastern and South East regions of England. Half of the total set-backs or set-forwards were on cliffed coasts, and half on low-lying coasts. Without local knowledge it is difficult to distinguish between set-backs and set-forwards. Set-backs often indicate higher retreat rates, thus threatening cliff-top infrastructure which requires defence upgrade and extensions, as well as raising maintenance costs. Monitoring set-backs is important for shoreline management, because as retreat continues, set-backs evolve and artificial headlands form and grow. This is reinforced by the shift from hard defence policies towards softer engineering approaches, managed realignment and limited intervention.

Development of reproductive structures in the introduced green alga, Codium fragile ssp. tomentosoides , in the northern Adriatic Sea

Biological invasions are among the most important drivers of global change. The introduced green alga, Codium fragile ssp. tomentosoides, has become an important component of low-shore assemblages on hard coastal defence structures along the north-west coast of the Adriatic Sea. We investigated the temporal dynamics, the spatial scales of variation and the effects of wave exposure on the abundance of reproductive structures of C. fragile. Furthermore, we assessed whether the reproductive output of C. fragile was correlated with the density of thalli. On the landward side of breakwaters, gametangia were found on thalli of C. fragile from June to September in both 2003 and 2004. Although there was some variation among breakwaters, the abundance of gametangia of C. fragile was greater on the landward than on the seaward side of breakwaters. Scales of variation in the number of gametangia and density of thalli of C. fragile were similar, with most of the variability occurring at a scale of tens of cm. Little variation emerged at larger (tens or hundreds of m) or smaller scales (within thalli, cm). The results of this study indicate that the provision of sheltered rocky habitats has been crucial for the establishment of C. fragile in the northern Adriatic Sea, enabling the alga to grow and reproduce. Processes operating at small spatial scales are responsible for patterns of variation in the number of gametangia and density of thalli of C. fragile. The tendency for a positive correlation between density of thalli and reproductive output in C. fragile in August suggests that greater densities of plants could ameliorate harsh environmental conditions during daytime low tides, fostering reproduction.