Low-crested coastal defence structures as artificial habitats for marine life: Using ecological criteria in design

Hard structures for protection against erosion of shores are some of the most common human-made constructions in coastal areas. Nevertheless, little is known as to how marine organisms respond to their presence. The composition and distribution of intertidal epibiota at different positions around different types of defence structures (groynes and breakwaters) at three stations along the Emilia Romagna coast (Italy) were analysed. Sampling covered a range of scales: meters (distance among replicate plots), hundreds of meters (distance among replicate areas) and tens of kilometres (distance among stations). The colonisation and dynamics of conspicuous species over 1 year, following maintenance works on two structures, were also quantified. Assemblages on defence structures were characterised by a notably low richness of species, by strong spatial dominance of mussels and green ephemeral algae, and by high rates of colonisation. Abundance of mussels, Enteromorpha intestinalis and filamentous algae differed significantly among nearby areas within stations. Mussels were significantly less abundant along the landward side of breakwaters compared with all other positions on both groynes and breakwaters. Overall, however, fewer differences, than expected, were observed in the distribution of species at different positions around groynes and breakwaters, probably as a consequence of the low complexity of the colonising assemblages. Possible factors explaining the patterns of distribution observed are discussed, including the roles of harvesting of mussels and frequent maintenance works on the structures. The implications of the low richness of species observed in terms of management of defence structures and other human-made constructions are discussed.

http://www.ecology.unibo.it/page/Bacchiocchi%20%26%20Airoldi2003.pdf

Distribution and dynamics of epibiota on hard structures for coastal protection

https://vbn.aau.dk/files/5425961/European_Experience_of_Low_Crested_Structures_for_Coastal_Management

European experience of low crested structures for coastal management

This paper presents a newly developed approach for the numerical modelling of wind effects on the generation and dynamics of freak waves. In this approach, the quasi arbitrary Lagrangian-Eulerian finite element method (QALE-FEM) developed by the authors of this paper is combined with a commercial software (StarCD). The former is based on the fully nonlinear potential model, in which the wind-excited pressure is modelled using a modified Jeffreys’ model (C. Kharif, et al. J. Fluid Mech. 594:209-247,2008). The latter has a volume of fluid (VOF) solver which can handle violent air-wave interaction problems. The combination can simulate the interaction between freak waves and winds with an improved computational efficiency. The numerical approach is validated by comparing its predictions with experimental data. Satisfactory agreements are achieved. Detailed numerical investigations of the interaction between winds and 2D freak waves are carried out, which not only explore different air flow states but also reveal the wind effects on the change of freak wave profiles. Both breaking and non-breaking freak waves are considered.

/pdf/numerical-simulation-of-interaction-between-wind-and-2d-2bjwglaltm.pdf

Numerical simulation of interaction between wind and 2D freak waves

Near the shore, cross-shore winds strongly affect the location of the break point and the breaking-wave height. From casual observation from the beach, wind direction (onshore or offshore) and speed also appear to affect wave shape (i.e., skewness and asymmetry), although as of yet this effect has not been quantified near the shore. The effect of wind on shoaling wave shape is investigated with laboratory experiments using monochromatic waves and onshore-directed wind. Wind increases the shoaling wave energy at discrete multiples of the primary frequency and has a significant effect on the wave shape at both a deeper and shallower shoaling locations. At the shallower location, the ratio of wave energy at 2 times the primary frequency to the primary frequency is also a function of wind speed, indicating interaction between the wind and the nonlinear wave shoaling process. Nearshore wave models do not account for these wind effects. Incorrect predictions of third-order velocity moments (wave shape), believed to control wave-driven sediment transport, would result in incorrect beach morphological evolution predictions.

/pdf/wind-effects-on-shoaling-wave-shape-3pp7rjr773.pdf

Wind Effects on Shoaling Wave Shape

The use of offshore breakwaters in the UK is relatively uncommon with only 6 schemes constructed to date (early 1998). The use of this technology in the UK is examined via a case study of the S-unit Elmer Beach offshorebreakwater scheme which fronts a shingle beach. Profile line surveying and aerial survey data are used to show the development of the planform during the first 38 months and the development is compared to empirical design guidelines. Also an intensive fieldwork program investigated shingle transport inshore of the breakwaters using sediment tracing techniques. The results show that the scheme exhibited rapid changes to planform during the first 6 months, with shingle salients and sand tombolos developing in the lee of the breakwaters. Changes to planform after this time were less marked, although areas of continued erosion/accretion were identified. The sediment transport experiments identified sediment pathways and rates of transport in the lee of the breakwaters. It is suggested that the breakwaters have reduced shingle transport by a factor of at least 2 compared to similar open beaches.

/pdf/application-of-offshore-breakwaters-to-the-uk-a-case-study-3cfo9vcugd.pdf

Application of Offshore Breakwaters to the UK: A Case Study at Elmer Beach

Wind/wave interactions in the surf zone were studied using a wave tank and environmental wind tunnel. Waves were modelled at approximately 1/50 scale and broke on a model scale beach of 1/20 slope. The wind was simulated using accelerated growth techniques over a relatively short fetch, and a model scale of roughly 1/100 was achieved. Thus there was some scaling mis-match between wind and wave simulations. Results show that the wind has a significant effect on the breaking of the waves. Both breaker location and breaker type are affected, and the results agree with those of Douglass [10 and 11], The main findings, common to both studies, were that offshore winds promote plunging waves and delay the moment of breaking (reducing surf zone width), whereas onshore winds encourage early wave breaking, generally by spilling. Wave heights at breaking were not generally affected, but due to changes in breaker depth, the height-to-depth ratio is shown to be affected by the wind. Additionally, studies of the motion...

Dan King

Papers

Application of Offshore Breakwaters to the UK: A Case Study at Elmer Beach

Changes to wave parameters in the surf zone due to wind effects