Chapter 8. Detailed design of preferred scheme. 8.4. Designs mitigation measures

CHAPTER 8

Detailed design of preferred scheme

8.1. OPTIMIZATION OF LAY-OUT AND CROSS SECTIONS OF LCSs BASED ON SHORT-TERM AND LONG-TERM MORPHODYNAMIC SIMULATIONS

(Gonzfilez-Marco, Mrsso, S(mchez-Arcilla, UPC)

From an engineering (~ point of view, the optimization of the lay-out and cross section of LCSs, on the basis of short and long term morphodynamic numerical simulations, should follow these five main steps.

1) Definition of Boundary Conditions for a Refined LCS Design

The optimum structural design (optimization process) must be preceded by a compilation of information/boundary conditions regarding hydrodynamic and morphodynamic pre- existent conditions as a pre-process for numerical modeling. This compilation should include, at least, information regarding average and episodic values of: waves/wind/tide climates, sediment characteristics, sediment transport rates and trends of beach plan and profile dynamics. The accuracy of this pre-existing information will play an important role in the optimization process, since it provides the initial boundary conditions as well as information on the morphodynamic evolution of the affected area. The meta-information of the <<transient stages>~ will also be a useful tool to verify the model performance during this numerical optimization process.

2) Modelling Tools

Depending on the considered temporal and spatial scales as well as the structural/functional parameters to be optimized, it is necessary to make use of different numerical modelling approaches. In this sense, 1-Line morphodynamic models should be used to initially assess structural length, orientation, distance to the coast, functionality of gaps, and other structural parameters within time scales from months to years and spatial scales from hundred meters to kilometers. These models (see e.g. Hanson and Krauss, 1989) have been widely employed to design detached LCSs, mainly emerged. The most important limitation of this kind of models is that they are based on the computation and balance of wave-induced long-shore sediment transport and do not take into account other hydrodynamic processes, which could contribute to sediment transport. This includes the important effect of wave induced currents, overtopping

(!) Ecological and socio-economic impacts are out of the scope of these considerations.

48 Environmental Design Guidelines for Low Crested Coastal Structures

and, sometimes, even transmission, amongst others. In this respect, Hanson and Krauss (1990) and later Jimenez and Sanchez-Arcilla (2002) analyzed the influence of wave transmission and LCS freeboard on the shoreline evolution with a 1-Line (1L) model.

However, in order to assess more accurately the morphodynamics associated to these structural parameters at smaller temporal and spatial scales (of about hours to days and meters to hundred meters, respectively) focusing on the effects of mean, storms or extreme conditions, 2-dimensional Depth Averaged (2 DH) morphodynamic simulations should be performed. This type of numerical models must simulate accurately, in a 3D domain, the most important hydro-morphodynamic processes acting around LCSs, both submerged and emerged. This explicitly includes the diffraction and reflection of waves, currents due to waves, wind and tides, turbulence and sediment transport-distinguishing between bed and suspended loads for the different parts of the domain. The morphodynamic evolution results hence as a function of beach state, driving terms and structural geometry. These <<coastal area morphodynamic models>> allow the modelling of complex hydrodynamic patterns around LCSs, considering the effect of a number of both environmental and design variables (see Figure 8.1) for smaller time and spatial scales in comparison with 1L Models. The applications of 2DH morphodynamic models should be considered within this scope. Figure 8.1 illustrates the most important hydrodynamic fluxes around LCSs which can be simulated by this kind of numerical models within the limits of their application regarding time and spatial scales. Examples of this can be found in Watanabe et al. (1986), Zysermann et al. (1999), Alsina et al. (2003), Alsina (2005) or Sdnchez-Arcilla et al. (2004, 2005).

A l l

p ~

1

Figure 8.1 Main hydrodynamic fluxes around LCSs for both cases, emerged (right) and submerged (left).

For more complex scenarios, for which it is necessary to take into account additional structural parameters such as freeboard, crest width, permeability, and then more intricate hydrodynamic processes, Quasi 3-Dimensional (Q3D) or 3D morphodynamic simulations are required. These models should deal adequately with the overtopping fluxes and the fluxes through the structure via mass and momentum conservation laws, and provide also the profile dynamics with the presence of the structure in a manner consistent with state-of- art 2-Dimensional Vertical (2DV) profile models. Over the past several years, significant efforts have been dedicated to develop advanced 3D computational fluid dynamics tools,

Chapter 8 Detailed design of preferred scheme 49

mainly centred on the solution of the three-dimensional Navier-Stokes equations (see e.g. Mayer et al. 1998). This level of numerical simulations allows an accurate description of the hydrodynamics acting both around and inside (in case of permeable structures) LCSs. Nowadays the applicability of these models is limited due to the complex process of model calibration, as well as the high computational costs required to run them. For this reason, their use is mainly centred on the solution of very specific problems in small computational regions.

In addition, as a complement to numerical simulations, physical modelling both in flumes and wave tanks should be carried out in order to reduce uncertainties in the hydrodynamic and morphodynamic processes simulated around LCSs.

3) Predictions with Error Bounds

The final objective of numerical simulations must be to improve the knowledge of expected shoreline and beach morphodynamic behaviour (both 2DH and Q3D or 3D) with its corresponding error bounds. These morphological changes will be a function of meteo- oceanographic characteristics (waves, tide, wind, currents), sediment characteristics and structural and geometrical aspects (structure length, orientation and distance to coast, gaps, freeboard, crest width and permeability).

The level of uncertainty of hydro-morphodynamic parameters is well known and described

Table 8.1. Estimated uncertainties intervals for some usual variables in coastal engineering projects (From Soulsby, 1997).

Input Parameter Uncertainty

Density of water, p +_ 0.2% Kinematic viscosity of water, v _ 10%

Sediment density Ps __. 2% Grain diameters, dl0, ds0, dg0, etc. __. 20%

Water depth, h _ 5% Current speed, U +_ 10% Current direction __. 1 0 ~

Significant wave height, Hs __. 10% Wave period, T __. 10%

Wave direction, 0 +_. 15 ~

(see e.g. Soulsby, 1997). The most important error typical values are compiled in Table 1. These uncertainties, together with those intrinsic to numerical models, have to be taken

into account in order to evaluate and interpret numerical results. Then, when making predictions, it is prudent to perform a priori a sensitivity analysis of the models in order to estimate differences between prediction methods and errors in the output as a result of the uncertainties in the input parameters. In this respect, in van Rijn et al. (2003) there is an intercomparison exercise in which several models (prediction methods) are evaluated for the same scenarios. In the same way, in Mrsso (2004) there is an exhaustive sensitivity analysis of a hydromorphodynamic suite of models, in which an extensive number of input parameters has been evaluated.


4) Assessment of Predicted Shoreline and Beach Dynamics

The assessment should be carried out for a full sequence of stages, going from initial to a final, through several transient stages. The predicted shoreline and bottom geometry must be compared with acceptability criteria from three standpoints: 1) Morphodynamics, which is related to the beach physical state, 2) Ecology, which takes into account beach ecological state and 3) Socio-Economy, which represents the relation of the construction and maintenance costs of the structure versus the benefit of the resulting protected beach.

5) Corrections of Lay-Out

In this final step, a re-evaluation of the general state must be done by introducing the corrections resulting from the analysis done within previous steps. It is then necessary to evaluate the convenience of starting an iteration process from step 2 onwards.

8.2. STRUCTURAL DESIGN BY THE USE OF FORMULAE AND MODEL TESTS

(Burcharth, AA U)

Detailed structural design contains a detailed examination of the performance of the various parts of the structure and an economical optimization based on amounts and types of materials, methods of construction, and long-term maintenance.

The formulae for armour stability, toe stability and scour protection, given in Section 13.11, will normally be sufficient for the detailed design for LCSs. In case of design of very large structures reference is given to breakwater design tools, for example as given in the Coastal Engineering Manual (CEM) and the Manual on the use of Rock in Hydraulic Engineering.

If these tools are insufficient, maybe because less uncertainty is wanted, it is necessary to perform hydraulic model tests, cf. Section 13.12.

8.3. STATEMENT OF SOCIO-ENVIRONMENTAL IMPACTS

(Moschella, MBA ; Abbiati, Airoldi, Bacchiocchi, Bertasi, B ulleri, Ceccherelli, FF ; Cedha gen, BIAU; Colangelo, FF; De Vries WL-DH; Dinesen; BIAU; Aberg & Granhag, UGOT; Jonsson, UGOT; Gacia, Macpherson, Martin & Satta, CSIC; Sundelgf, UGOT; Frost, Thompson & Hawkins, MBA)

LCSs can cause severe impacts on the surrounding environment at both local and regional scale. Soft-sediments are the most affected by LCSs; their presence always induces a disruption in the normal transition of assemblages from deep waters to the shoreline, due to the physical presence of the structure on the sediments as well as to the modification of the hydrodynamic regime. Marked changes in the water characteristics also occur, particularly on the landward side. The construction of LCSs as well as other man-made structures has some implications for rocky-bottom communities as the structures provide new hard substrate for colonisation of species typical of rocky shores that naturally would not be there.

The modifications induced in water circulation patterns, water quality and assemblage types can strongly affect the social enjoyment of protected beaches and consequently beach value and usage.


8.3.1. Impacts on soft-bottoms (habitats and associated biota)

Unavoidable large scale changes in sedimentation patterns of the coastal cell due to the presence of the LCSs may impact not only immediate sea bottoms but also nearby updrift/ downdrift areas affected by changed erosion/sedimentation processes with major negative consequences for the associated fauna and flora. The construction of one or more LCSs have two direct consequences: habitat loss and habitat fragmentation. The construction of LCSs leads to loss of sandy areas and the associated infaunal communities.

Where coastlines are defended by a series of LCSs, habitat loss becomes important and can lead to severe disruption of soft-bottoms at large scale. Impacts of LCS on infaunal communities, however, are mainly indirect, through modification of the local hydrodynamics and sediment regime including physical and chemical characteristics of the water column and sediment. Changes to the physical environment are particularly evident on the landward side of the LCS and include reduced water movement, increased scour in proximity of the structures, increase of silt/clay fraction, organic matter and anoxic layer in the sediments, and trapping of coarse material (i.e. pebbles, shells, algal detritus). These modifications of the sedimentary habitat surrounding the structures will in turn affect the associated biota. The main effects are:

- changes in the structure (composition and abundance) of the assemblages. Certain species are more sensitive to changes under the new habitat conditions and can decrease in abundance or in some cases disappear. Others will take advantage of the new environmental conditions and from reduced interspecific competition. As a result, the relative abundance of species in the infaunal assemblages could permanently change as well as diversity being altered.

- In extremely altered conditions the composition of the infaunal community can change completely, leading to replacement of all the local species with others typical of other ecosystems (from an open beach to a lagoon).

- Increased risk of spread of non-native species. The modified habitat can also provide an opportunity for non-native, invasive species to expand their range of distribution.

The presence of soft-sediments vegetation should also be taken into account. Seagrass meadows are important engineering species in the coastal zone providing sediment stability and refugee for associated species. Vegetated soft-bottoms are richer in terms of diversity than unvegetated areas; thus, LCSs should not be built in such areas. This is particularly critical when in the area there are endangered species such as Posidonia oceanica in the Mediterranean.

8.3.2. Implications for hard-substrate assemblages

LCSs provide new rocky habitats for colonization by species typical of natural rocky shores. The type of habitat can vary depending on a series of natural factors and processes (see Ecological Tools) but is also influenced by LCSs design features, including the layout of structures and the building material used. Also, the sheltered and exposed side of the structures increases the variety of habitats provided. The main ecological implication is that LCSs can function as <<stepping stones>> in coastal areas lacking of rocky shores, promoting the expansion of hard bottom species beyond the limits set by the availability of suitable natural habitats. For example, in the UK two species of grazers (Gibbula umbilicalis and


Melaraphe neritoides) have extended their distribution along the south east of England by colonizing the LCSs at Elmer. In Italy, the alga Codiumfragile ssp tomentosoides has much of spread along the north Adriatic coast, colonizing the sheltered side of LCSs. This has serious implications for the identity of rocky shore communities, as the composition and dynamics of assemblages can change considerably after the introduction of non-native species and the detrimental effects of invasive species on native assemblages have already been demonstrated (e.g. Sargassum muticum, see review in Rueness, 1989).

8.3.3. Impacts on water quality

Emerged and rarely overtopped structures significantly reduce water movement and mixing on the landward side of the structures, thus oxygen exchange is often minimal and nutrients tend to accumulate. This can lead to hypoxia and increase the risk of algal blooms, particularly in shallow, eutrophic waters such as in the Adriatic Sea. Reduction of water movement on the landward side may also enhance accumulation of algal detritus, leading to anoxic sediments, proliferation of flies and unpleasant odours.

The worsening of water quality, the presence of algae and stagnant enclosed waters will reduce the quality of recreational activities such as swimming and sunbathing.

LCS due to frequent overtopping allow greater water movement and mixing thereby avoiding stagnant conditions. Thus water quality is minimally affected as are recreational activities.

8.3.4. Impacts on safety issues

LCSs partially reduce wave kinetic energy in the protected area and thus increase safety for beach visitors in general. Nevertheless, rip currents at gaps (in case of multiple structures, see Fig. 2.5) and roundheads may occur and be very risky for bathers; moreover, the location of submerged structures has to be marked not to be dangerous for boating and water sports.

8.4. DESIGN MITIGATION MEASURES

(Moschella, MBA ; Abbiati, Airoldi, Bacchiocchi, Bertasi, B ulleri, Colan gelo & Ceccherelli, FF ; Cedhagen, BIAU; De Vries WL-DH; Dinesen; BIAU; Granhag & Jonsson, UGOT; Gacia, Macpherson, Martin & Satta, CSIC; Sundel6f, UGOT; Frost, Thompson & Hawkins, MBA)

LCS are designed to modify hydrodynamics and geo-morphological coastal processes and, inevitably, these changes will have ecological consequences (see Chapter 2). It is therefore important to ensure that adequate measures are considered in the design procedure of LCS to minimise environmental impacts.

The following LCS design features influence the type and magnitude of impacts on the surrounding habitats and associated biota:

a) Extensively defended coastlines

Results of DELOS project have shown that proliferation of LCSs causes broad-scale alteration of the whole coastline, resulting in important changes on habitats and species (see Sub-Section 8.3.2). Along the coasts of the North Adriatic Sea, for example, the proliferation of defence structures has substantially changed the identity and nature of the coastal landscape of this region (see Sub-section 11.4.6 and Chapter 12). Local coastal defence planning should also take into account regional environmental conditions, and avoid any unnecessary overengineering.


b) Spatial arrangement of structures

Spatial arrangement (i.e. location, relative proximity to natural reefs and other artificial structures) of coastal defence structures is of great importance in influencing the type of hard-bottom species that will colonise any novel structure, including the dispersal of invasive species.

c) Distance from the shore

In microtidal systems distance from the shore can be important in determining the degree of impacts on water quality (e.g. sediment suspension, eutrophication, turbidity) on the landward side, especially in shallow waters. In this case, LCSs should not be built too close to the shoreline. In microtidal systems distance from the shore can be important in determining the degree of impacts on water quality (etc., sediment suspension, eutrophication, turbidity) on the landward side, especially in shallow waters.

d) Tombolo and salient formation

Tombolo formation can cause burial of assemblages colonising the lower part of the structures on the landward side. The extent of the zone affected can vary depending on the height of tombolo from the sediment level.

e) Shore connectors, groynes

The addition of perpendicular rock groynes connected or unconnected to the structures significantly decreases water mixing on the landward side, thus worsening impacts on sedimentary habitat and the associated biota and water quality. These additional structures should not be considered in the design of LCSs unless strictly necessary.

f) Length of structures

At a local scale length of structures might affect hydrodynamics, particularly on the landward side. In case of emerged structures, shorter structures should be preferred, as long structures create more sheltered conditions on the landward side to the detriment of water quality and sedimentary habitat. In addition, the very sheltered habitats that are likely to be created by longer structures increase the risk for spread of non-native species such as the invasive species Codiumfragile ssp tomentosoides along the Adriatic coast.

g) Submerged versus emerged barriers

Height of the structure affects the hydrodynamics at the landward side of the structure. This has important consequences for both soft-bottom and hard-bottom assemblages. Reducing the height of structures allows greater water movement on the landward side thus mitigate impacts on soft-bottom habitats and the water column. Greater water movement also reduces the effects of siltation that negatively affect hard-substrate species. Submerged structures should therefore be preferred, recreational value is lower, however, since the structures can be accessed only by diving or snorkelling as they also minimise aesthetic impacts.

h) Distance between structures

In case of high emerged structures, currents at gaps are usually of low intensity and thus gap width is not a critical design parameter. Conversely, for moderately submerged structures, due to the great velocities that rip currents may reach, wide gaps have to be preferred both


for safety issues and ecological reasons. Slower currents will reduce erosion at gaps and hence risk of structure instability and disturbance of colonising organisms.

i) Type of material (see also Section 9.4)

The physical and chemical attributes of materials used to build LCSs will affect the development of the epibiota. In particular, ifLCSs are built with materials that are not typical of the area (e.g., granite in an area of limestone bedrock or concrete blocks) this may affect the local distribution of species, providing suitable substrata for species that would normally be rare or absent in the area, including invasive species. For example certain type of smooth geotextiles may be colonised only be ephemeral algae which can represent a nuisance for the local community. Therefore the same or similar stone materials typical of the area should be used. Carbonate rocks used for construction of LCS are softer and are more easily weathered and bioeroded, leading to a more complex topography (crevices, small pits) which enhance colonisation and growth by algae and marine invertebrates.

j) Porosity

Large pores between blocks allow greater water flow through the structures and increase water mixing on the landward side, thus reducing impacts on sediments and water quality (see Sub-Sections 8.3.1 and 8.3.3). In addition, small pores can be easily filled blocked by growth of marine organisms such as mussels and polychaetes (Sabellaria), which facilitate sediment trapping thus further reducing porosity.

k) Scouring and abrasion

Scour at the base of the structures causes high level of disturbance to communities, leading to increased mortality, especially for filter feeders such as barnacles and algae. This effect can be minimised by building a berm around the structures, particularly on the seaward side or by providing more refugia such as crevices and holes.

l) Maintenance works

Frequent maintenance of LCSs leads to greater disturbance of epibiotic assemblages. These will remain at a permanent pioneer stage, characterised by abundance of ephemeral green algae (Ulva spp.) that are often considered a nuisance for recreational activities. Stability of the structure should be increased to allow development of assemblages and succession of species leading to a more diverse community.

8.5. IDENTIFICATION OF DESIGN OPTIONS THAT MAXIMISE SPECIFIC SECONDARY MANAGEMENT GOALS

(Moschella,MBA ; Abbiati, Airoldi, Bacchiocchi, Bertasi, Bulleri, Colangelo & Ceccherelli, FF; Cedhagen, BIAU; Colangelo, FF; De Vries WL-DH; Dinesen; BIAU; Granhag & Jonsson, UGOT; Gacia, Macpherson, Martin & Satta, CSIC; Aberg; Frost, Thompson & Hawkins, MBA)

8.5.1. Tools to maximise recreational activities

Appropriate LCS design can also provide suitable habitat for living resources for exploitation of food (usually non-commercial or recreational) or act as the focus for recreational


activities, primarily angling but also snorkelling, appreciation of marine wildlife such as ~rock-pooling~ and ornithology. In some cases such activities have been an accidental by- product of the building of LCS and other sea defence structures. For example, in some Mediterranean countries such as Italy, shellfish harvesting (mussels, oysters) is a very popular recreational activity on LCS, particularly during summer. In the UK, where the structures can be easily reached at low tide, many people consider LCS as sites of natural interest for observation of marine life. This effect can also have a potential educational value particularly on coastal areas lacking of natural rocky shores. Some recreational activities can, however, compromise the ecological value of the structures. For example, frequent trampling on the rocks and intense mussel harvesting have a negative effect on the diversity and dynamics of epibiotic communities.

8.5.2. Tools to maximise diversity of species (e.g. for recreational or commercial purposes)

Some species are generally perceived as benefits in coastal environments because they represent a resource to exploit for commercial and recreational activities. Other species can also contribute in ameliorating environmental conditions (e.g. bivalves filtering the water, see Allen et al., 1995; Wilkinson et al., 1996).

1) A general rule is that location of structure is one of the most important factors influencing the species that will colonise the structures. Further, for any new LCS introduced into the marine environment it will take time for the biological assemblage to reach a diverse community that is most likely to resemble that of a natural shores. For mature biological communities to develop, LCSs need to be stable and built in such a way that maintenance will be minimal. Unless LCSs meet these criteria, there is little point in introducing additional features to enhance diversity (for example by enhancing complexity), as attempts to repair the structure will result in considerable degradation of developing communities.

2) Surfaces that are complex on different spatial scales enhance settlement of a wide variety of sessile species. Many larvae and algal propagules prefer to settle in small pits or crevices as they provide protection from desiccation, wave exposure and refuges from grazing. The surface of the blocks can be made rougher by chiselling grooves or drilling small pits and deeper holes. The choice of building material can also significantly contribute to increase diversity of microhabitats. Rough or complex surfaces can be easily cast in concrete units, although similar features can be naturally created by weathering and bioerosion when using limestone blocks. Much more time (5-10 years), however, is needed to obtain complex and heterogeneous surfaces on the natural rock.

3) Rock pools can also be incorporated into design of LCSs to increase diversity on blocks located above mean tidal level and to provide suitable habitats for recruitment and settlement of lower shore species and mobile animals such as limpets, winkles (littorinids) and crabs. Artificial rock pools can be created either by pre-cast units or by modification of drainage patterns on the blocks.

4) On macrotidal systems, location of LCS on the shore is also important to determine the number of epibiotic species that will colonise the structure. Structures built lower on the shore will have greater diversity than those built above mean tidal level.


5) Large mobile species (crabs, lobsters, octopuses) need small-medium size (10-20 cm diameter) refuges and the interstices between boulders/blocks provide them. The design should avoid large crevices and cavities where scouring can be exaggerated.

6) Living resources will regenerate if exploited in a sustainable manner. Therefore fishing and shellfish collection may need to be managed. There are a variety of methods

(closed seasons, licenses, quotas) to limit these activities. Artificial structures are particularly

suitable for management by defining areas open or closed to access to be interspersed along

the structures.

8.5.3. Tools for minimising growth of ephemeral green algae

1) Minimising disturbance. The high macroalgal growth on LCS is generally perceived as negative. Along the shores of the North Adriatic, for example, the banks of ephemeral green algae that are torn off the structures and washed up on the shore is a major problem for beach tourism, and leads to major costs to clean the beach. Green ephemeral algae are opportunistic species that flourish on disturbed habitats and they are the first colonisers when a new bare substrate becomes available. Maintenance of LCSs significantly increases

disturbance to the epibiotic assemblages, and remove later colonisers. Minimal maintenance should be carried out on LCSs. The stability of the structures should also be ameliorated, in

order to minimize translocation and overturning of the blocks, which can provide new

substratum for colonisation by early stage colonisers.

2) Increasing recruitment of grazers. Promoting settlement of limpets can be a very useful, cost-effective and environ-mentally sensitive tool for drastically reducing the abundance of nuisance green on LCSs. Settlement of limpets generally occurs in rock pools. Therefore building blocks should be included features such as artificial pools and small pits

Table 8.2. Design parameters for emerged LCS. Reference is given to the scheme in Fig. 12.9.

Water depth (m) h = 3.0 Crest elevation (m MSL) Rc = + 1.5 Crest width (m) B = 4 Shoreward slope 1:2 Seaward slope 1:2 Armour rock weight ( k g ) 3000-6000 Stones for bedding layer (kg) 0-200 Thickness of bedding layer (m) 1.0

Table 8.3. Design parameters for submerged LCS. Reference is given to the scheme in Fig. 12.7.

Water depth (m) h = 3.5 Crest elevation (m MSL) Rc = - 1.5 Crest width (m) B = 16 Shoreward slope 1:2 Seaward slope 1:2 Armour rock weight ( k g ) 500-1000 Stones for bedding layer (kg) 0-200 Thickness of bedding layer (m) 0.7

which retain water during low tide.

8.6. E V A L U A T I O N OF IN IT IA L AND M A I N T E N A N C E COSTS

(Franco, MOD; Lamberti, UB)

Preliminary analysis of construction costs is

carried outas an example for two typical LCS

geometries, namely emerged and submerged

rubble mounds, assuming unit costs and other

typical constraints (wave climate, foreshore

slope, sediment characteristics, construction material and technology) of the Italian North

East regions.

Table 8.4. Design parameters for gap bed protection.

Water depth (m) h = 3.5 Crown width (m) B = 30 Stones for bedding layer (kg) 0-200 Thickness of bedding layer (m) 0.7

Chapter 8 Deta i led design o f p re f e r red scheme 57

Table 8.5. Unit costs for emerged LCS.

Item

Armour

Bedding

Geotextile

Total

Unit cost

40 ~ / m 3

37 ~/m 3

12 ~/m 2

Amount

38,50 m3/m

28,00 m3/m

34,00 m2/m

Cost

1.540 ~/m

1.036 ~/m

408 ~/m

2.984 ~/m

Table 8.6. Unit costs for submerged LCS.

Item

Armour

Bedding

Geotextile

Unit cost

39 ~ / m 3

37 ~/m 3

12 ~/m 2

Total

Amount

24,18 m3/m

21,42 m3/m

38,00 m2[m

Cost

943 ~/m

792 ~/m

456 ~/m

2.191 ~/m

Table 8.7. Unit costs for gap protection among LCS.

Item

Bedding

Geotextile

Unit cost

37 ~/m 3

12 ~/m 2

Total

Amount

22,00 m3/m

38,00 m2/m

Cost

813 ~/m

456 ~/m

1.269 ~/m

The construction costs include material supply (the material is supposed to be imported

from Croatia) and placement with floating equipment. Geometric-structural characteristics are given in Table 8.2 (emerged LCS), Table 8.3

(submerged LCS), Table 8.4 (gap protection), while corresponding unit costs (per metre

length) are given in Tables 8.5-8.6-8.7. Structure design is provided in Chapter 12, figs. 12.7 and 12.9. It is obvious that construction costs are proportional to the LCS volume. Maintenance costs could be determined with reference to the expected damage

during LCS lifetime as predicted by stability formulae (see Section 13.11), though the total costs will increase due to the higher mobilization costs of the equipment for a small volume

of rock to be placed. LCS maintenance is relatively expensive and causes disturbance to local ecology and

recreational activities and should therefore be reduced to a minimum or avoided with a more conservative and careful design. Significant and rare (every 10 years, once in economic lifetime) maintenance interventions should be preferred to small and frequent ones (twice

or more in economic lifetime).

58 E

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8.7. FORMULATION OF MONITORING PROGRAMMES

(Paphitis, Plomaritis & Collins, UoS; Moschella, Thompson & Hawkins, MBA)

The monitoring programme should incorporate information about beach levels, sediment distributions, tidal information (i.e. tidal currents and levels), wave and wind conditions. The exact techniques used for collection of the data can be decided on the degree of accuracy that each measurement requires and on the monitoring costs. For the case of the beach and intertidal zone the best method is beach profiling that provides both high accuracy and low cost (Serra and Medina, 1997). The spacing between beach profiles (or any beach levelling technique) is very important since it will determine the accuracy of any derived calculation (i.e. sediment budget, beach volume, etc.) (Irish et al., 1997). Where data exist, these can be used for estimating the optimum beach profile sampling interval (Philips, 1985). Beach profiles should extend, in the offshore direction, down to the estimated closure depth for the area. Sediment sampling/analysis should be undertaken following standard techniques (grabs, shallow cores, mechanical sieving, settling towers, microscopy, etc.); care should be taken for the collections of an appropriate number of samples and spatial density for the proper representation of the sedimentary environment. Hydrodynamic information can be collected using various methods (i.e. pressure transducers, current meters, etc.); these will depend upon the required accuracy and frequency of measurements.

When dealing with defence schemes, involving LCSs, the programme for monitoring the structures and assessing the environmental impacts must be comprised of methods and techniques that are referring to different spatial and temporal scales. For an integrated investigation on the performance and impact of the structures, measurements have to be undertaken in the vicinity of individual breakwaters, scheme-wide and on a regional scale (see Table 8.8). Furthermore, especially in the assessment of the impacts, information about the pre-construction environment, together with post-construction information is required. An outline of the methods proposed for the monitoring, is presented in Table 8.8. The different monitoring programmes that can be used will be explored in relation to the timing of the construction.

In the pre-construction period the main task of the monitoring programme should be a desk study; the purpose of this is to identify all the available information which is related to the geological and historical development of the area. Existing monitoring programmes in the area should be evaluated with regards to the collected information. Both on a regional scale and in the area of the future scheme, beach level data and their accuracy should be established. In situations were an ongoing beach level programme is not established by the local authorities, a baseline study must be undertaken before the beginning of the construction works. Superficial sediment samples have to be collected from the area for the determination of seasonal or long-term changes in beach composition and possibly for the identification of sediment transport trends. A combined study of beach profiles and grain parameters can give an indication of beach stability (Mohan and Kana, 1997). Hydrodynamic measurements have to be undertaken to establish the current and wave regime prior to the construction. All the above information can be used to investigate the performance and impact of the proposed scheme by means of numerical and physical models.

During the actual construction of the scheme the monitoring procedures (i.e. beach level, hydrodynamic measurement) may be compromised by the high level of activity in the area. Some construction process necessitates a great amount of excavation work which, in turn,


results in unusually high levels of suspended sediment concentrations. In such circumstances the plume development must be monitored. In cases of soft bottom substrate compaction/ subsidence should be monitored, during and after the construction.

The careful monitoring of the early post-construction period is of the utmost importance. Beach level measurements need to be intensified, both in spatial and temporal scales, in order to capture the immediate response of the beach system. Such measurements will also provide information for the sediment budget and the morphodynamic evolution; for this reason an accurate evaluation of the volume changes close to the scheme is important. Irish et al. (1997) demonstrated that the error in computing beach volumes from beach profiles is increased with increasing profile spacing. The recommended spacing, in the literature, both for pre and post-construction monitoring seems to be 30 m; in practice 300 m spacing is used from the majority of Local Authorities in their monitoring programmes (Kana and Andrassy, 1995). However, a certain level of flexibility in the spacing of beach profiles was to be adopted, especially in the area of the scheme, as all of the major features of the system (i.e tombolos, salients) have to be monitored. Such flexibility is rather difficult in beach profiling procedures, whereas a 3D beach level measurement, using a total station or kinematic GPS systems, can provide faster beach coverage and better accuracy in the morphological representation. The time interval between successive measurements needs to be more frequent (more often than seasonal measurements), incorporating fast response monitoring after storm events. Offshore bathymetric surveys also have to be undertaken in order to investigate the offshore morphodynamic influence of the scheme. Standard field measurements of sediment distribution, hydrodynamic condition and sediment transport have to be continued as in the pre-construction period. Furthermore, these measurements have to be intensified closer to the LCS for the identification of specific processes taking place (i.e. wave diffraction reflection at the structures, wave energy behind the structures) and the evaluation of their performance. Again the data can be used for the calibration ofhydrodynamic and morphodynamic models.

In the vicinity of the breakwater scour measurements at the head and the trunk sections of the structures have to be performed. Although a considerable amount of research has been undertaken in laboratories considering scour development and prediction, field measurements of scour are very rare and difficult. For the long time monitoring of the scour around coastal structures the most common method is the use of scour rods (Dean et al., 1997). Rods are tubes with relatively small diameter and long enough so they can be placed firmly in the study area. A movable disk is placed around the tube on the sand surface and when erosion takes place the disk follows the sands elevation; then the sand is excavated down to the disk and the maximum scour depth is obtained. The disadvantage of this method is that only the maximum scour depth is obtained with no information on the time scale of the process or the shape of the scour hole.

On the regional scale, following construction, the monitoring programme should provide data for the evaluation of significant changes in the adjacent coastlines. These can be done in terms of accretion/erosion and sediment budget calculations. The spatial spacing of beach profiling in the adjacent coastlines should be kept low for a more accurate estimation of sediment volume changes (Irish et al., 1997); such estimations will provide evidence on the probable blockage of longshore sediment transport. For better understanding of the sediment dynamics of the area the regional transport pathways have to be established.

LCSs would be expected to have environmental impacts on short (largely associated with construction) and immediate responses to altered sediment regimes. Thus detailed monitoring


needs to be made for 1-2 years. Subsequent ecological effects are likely to be long term and to date have not been measured. Thus programme of biannual survey of sediment infauna (early spring, early autumn) needs to be run around the structure using sample locations selected on the basis of hydrodynamics/sediment modelling. Particular attention should be given to the sampling at various distances to the seaward/landward side of the structure, at least two control areas outside the influence of the structure (ideally on either side). Samples should also be located at the round heads (simple structures) or gaps (multiple structures). At the end of the 2nd year the number of station can be minimised on the basis of experience. Within the sediments granulometry, organic matter and chlorophyll are the minimum environmental data required. The infauna should be sampled on 0.5 mm sieve and identified to highest taxonomic level possible. Data can be processed using appropriate univariate, bivariate and multivariate statistics.

Depending on resource value surveys of fish and shellfish can be made around the structures using appropriate methods (nets, traps, visual transects). Such survey should be made at least four times per year to allow for seasonal variation.

The ecology of the hard substrates can be monitored using broad-scale rapid assessment methods (biotope mapping) compiled with more detailed stratified random non-destructive sampling of major species and categories (percentage cover of canopy forming algae, ephemeral algae, algal turfs, barnacles, mussels, number of grazers and predators (especially winkles, limpets and whelks). In addition where mussels occur biomass can be evaluated. If there are exploitable resources, then yields should be estimated by recording fishing activities. Structures should be censused 1, 3, 6, 12, 18, 24 months after construction bioannually for at least 5 years. Each survey is estimated to take 2 people times 2 days for a single structure.

8 . 8 M A I N T E N A N C E P L A N

(Lamberti, Zanuttigh & Martinelli, UB; Burcharth, AA U)

Structures built for local shore protection and the accompanying beach fill must be maintained to preserve the project functionality. The maintenance plan should be part of the design procedure and should include periodic scheduled interventions (ordinary maintenance) as well as sporadic interventions after exceptional storms (extraordinary maintenance).

It is necessary to identify: - possible ~failure modes>> of the intervention; - state indicators to monitor the first signs of these ~failure modes>>; - threshold values of these state indicators to trigger maintenance actions; - the type of maintenance to be performed. The plan is site specific and based on the information obtained from preliminary surveys

of the site (see Section 8.7): - historical records of natural shoreline evolution (regression) and of shore response to

similar defense schemes; - general environmental conditions of the littoral (tide, wind, waves, ecology); - records of subsidence of the coastal zone including the submerged beach; - sediment characterization and sediment budget of the protected cell; - coast vulnerability to sea ingression. The use of morphological/morphodynamic simulations allows:


- to quantify the frequency and the sand volume for re-nourishment; - to anticipate local erosions close to the structures that may require reinforcement of

toe protection. The necessity of structure/beach maintenance is made evident by comparison of the state

indicators with the threshold values. For instance a failure mode may be beach erosion beyond a limit that cause damage to

landward structures (dunes, seawall, buildings .... ). Beach width or beach volume are appropriate indicators; they can be evaluated from surveys of the shoreline position or from bathymetric and topographic surveys of the submerged and emerged beach; the volume might be preferred because it is insensitive to temporary displacement of sand from the emerged beach to submerged bars and therefore less noisy than the beach width. A target and a threshold value of the beach width can be defined; if erosion continues so that the beach width falls below the threshold value a nourishment has to be carried out and the necessary sand volume can be estimated from the difference between the target and actual beach width (or from the loss of beach volume).

If scour holes of the order of twice the stone diameter are shown by bathymetric surveys, toe berm stability may be compromised and toe protection should be reinforced and widened.

In the Mediterranean Sea, cross-shore profiles of the structures frequently documented structure settlement. Field observations in Ostia, Pellestrina and Lido di Dante (see the description of the sites in Chapter 11) show a barrier settlement variable in the range 3 to 15 cm/year, with the greatest values occurring immediately after the works on fine sandy bottoms. Since LCS effectiveness is very sensitive to submergence, settlement can easily bring the structure out of the acceptable functioning domain and rock recharge has thus to be planned.

In case of flooding, dune maintenance (planting and fertilizing dune stabilizing vegetation and/or installing proper sand fences) should be performed.

If beach recreational value is affected by organic deposits on the beach (for instance, algae grown on the structure and drifted during storms), periodic removal of these deposits has to be done, even daily in the holiday season.

Attention has to be paid to the fact that maintenance of water and sediment quality is extremely difficult and costly compared to a design that avoids this negative effects of the intervention.

Maintenance works produce disturbance to the surrounding ecosystem; it is therefore suggested to moderate the maintenance frequency. Re-nourishment should hence be planned with a frequency not greater than once every 3rd year and the maintenance of a rocky structure is suggested to be even more rare, i.e. once every 10-20 years.

Chapter 8. Detailed design of preferred scheme. 8.4. Designs mitigation measures

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