Ruol, P & All (2007) Invited Lecture - Design Strategies and Management Coastal Protection Framework Environmental

8/3/2019 Ruol, P & All (2007) Invited Lecture - Design Strategies and Management Coastal Protection Framework Environme

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DESIGN STRATEGIES AND MANAGEMENT OF COASTALPROTECTION SYSTEMS IN THE FRAMEWORK OF

ENVIRONMENTAL SUSTAINABILITY

Piero Ruol1,

Barbara Zanuttigh 2 and Luca Martinelli 2

Aim of this note is to describe some coastal protection schemes with low environmental impact,

propose an optimal design strategy and encourage sustainable management of the Mediterranean

coastal protection system. It is seen that the procedure for designing an environmental friendly

coastal defence requires the interaction of several competences: ecology, economy, sociology

geology and engineering.

INTRODUCTION

Major threats for large stretches of European coasts are erosion and flooding,

which are mainly caused by: loss of river sediment load (due to hydraulic

works, bridles, crossbars, dams, on rivers); subsidence (natural or

anthropogenic, the latter due to extraction of water, gas , oil, etc.); inappropriate

interception of long-shore transport (presence of hard defence, works and

harbours along the coasts); dune decay (due to inappropriate management).

Effects of climate change (sea level rising, extreme storm events increasing their

frequency and intensity, shoreline receding, dune system destroying, low-land

flooding, etc.) concur in amplifying beach erosion and coastal vulnerability.

The Mediterranean areas will be among those most stricken by climate change

and a great deal of damages is to be expected given also the foreseen increase of

anthropic pressure along the coasts (318 to 584 cities from 1950 to 2005, about

70 million of people in year 2000 forecasted to be up to 90 million by 2025

Plan Bleu Report, 2005).

In this frame it is evident the need of a strategic and sustainable management of

sediments, paying attention to the new environmental aspects involved in the

related activities with a particular emphasis on Integrated Coastal Zone

Management (ICZM). It is noteworthy that the E.U. project EUROSION

stressed both the shortage of coastal sediments and the improperness of the

current Environmental Impact Assessment (EIA) practices... in addressing

coastal erosion matters.

Aim of this note is to describe some coastal protection schemes with low

environmental impact, propose an optimal design strategy and act as a spur for asustainable management of the Mediterranean coastal protection system.

1IMAGE, Padova University, Via Ogissanti 39, Padova, 35129, Italy

2DISTART, Bologna University, V.le Risorgimento 2, Bologna, 40136, Italy


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In the next Section, examples of pure nourishments, submerged, low crested and

innovative structures are given. Then, the design procedure for the choice of a

sustainable defence is suggested and conclusions are drawn.

COASTAL PROTECTION SCHEMES

Sustainable coastal development starts from the recognition of the beach

functions and from the evaluation of the economic and ecological components,

see for instance the results achieved by the recent Interreg IIIc project

"Beachmed-e". In Tab. 1, a synthesis of the beach functions, the various

characteristics of the associated uses and policies aiming at protecting beaches

are proposed.

Tab. 1 summarises why beach protection is required, while in Tab. 2 shows thepossible protection schemes, with a potential low environmental impact,

described in the following sub-sections.

Table 1. Beach functions and public policies with regard to beach protection(from Beachmed-e, ICZM Phase B Report, www.beachmed.it).

Direct use Indirect use

Market us e Beach protection topromote tourism activities

Environmental risks protection(agricultural land, dunes, etc.)Renewable natural resource protection (fish, etc.)

Non-market use Beach protection topromote leisure activities

Landscape conservationRenewable natural resource protection (fish, etc.)Habitat protection loss (posidonia, etc.)Biodiversity conservationSediment flows conservation

Marine floods assessment

Table 2. Possible design alternatives in case of large erosive problemsCross-shore dominatedtransport

Long-shore dominated transport

Alternative 1(most environmentalfriendly)

Heavy nourishment with suitable off-shore dredged sands

Alternative 2(consolidated)

Nourishment using suitablenear-shore dredged sands+Low crested structures

Nourishment using suitable near-shore dredgedsands+Submerged groins

Alternative 3(example ofinnovative solution)

Nourishment using suitablenear-shore dredged sands+

Algae or geosynthetics

Nourishment using suitable near-shore dredgedsands+

Filtering groin

Nourishment

It is generally accepted that pure nourishment is the scheme with lower

environmental impact on the defended area.

The main drawbacks are the lack of suitable material and the impact caused by

the works. In order to reduce cost and pollution and to avoid traffic, road

transport is substituted by transport via pipelines.


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Sand is mixed with seawater in a 1:3 - 1:7 ratio, and pumped through the pipes,

taking into consideration that the flow velocity needs to be high enough to avoid

depositions, but as low as possible to minimise head losses and pipe abrasion.

The sand is conveniently dredged form offshore mines (Fig. 1) or moved across

the shoreline with a permanent or temporary by-pass from accretion areas, such

as updrift the ports, to nearby suffering beaches (Fig. 2).

Offshore nourishment is cost effective only in case of large quantities, e.g.

dredgings of the order of 1'000'000 m3, to be carried to one or more beaches.

Example of such kind of works can be found in Burcharth et al. (2007),

Beachmed (2004) and Preti (2002).

Longshore movements are carried out by means of (temporary) by-passes.

A sort of funnel is built where bulldozers collect the sand. The hole where thepump suction head is placed is sometimes protected by sheet-pilings (Fig. 3) so

that vehicles can safely approach the borders. Additional pumping (or booster)

stations may be required for long pipeline reaches (Corbau et al. 2007).

Recently an interesting approach to management of sediment stocks has been

proposed through combined interventions of port dredging and beach

nourishment (Martinelli, 2008).

Figure 1. TSHD Dredger during suction operations.

Figure 2. Pipe outlet: the flow of mixed water and sand can be seen.


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Figure 3. Pump suction head attached to an hydraulic excavator.

Low crested structures

Structures are considered to be low crested (LCS) if they are submerged or

regularly overtopped when emerged. An example is given in Fig. 4.

Effectiveness of this kind of structures was recently subject of the European

project Delos. Design guidelines are given in Burchart et al. 2007.

In short, compared to traditional structures, they have a lower visual impact,

they induce a lower wave energy dissipation (dependently on their crestfreeboard) and a higher water recirculation in the protected cell. Therefore they

avoid deposition of fine material leeward of the structure and assure higher

water quality.

Figure 4. Low crested structures at Lido di Dante (RA).


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Overtopped water tend to accumulate in the protected area rising the water level,

thus causing the so called "piling up" (water level increase) (Ruol, 2004). In

presence of a careless design, piling up induced currents may cause deep erosion

holes at gaps and roundheads.

The LCS which are deeply submerged, i.e. if their height is lower than - say -

half the water depth even at ordinary low tide, behave in a rather different way.

The structure should be sufficiently wide to induce wave breaking on its crest

and achieve two main goals: the reduction of the energy of higher incoming

waves, moving slightly offshore the longshore currents, and the stabilisation of

the cross-shore profile.

An example of a successful scheme with submerged barriers is given in Fig. 5.

Figure 5. Deeply submerged barriers at Pellestrina (VE).

Filtering groinsRecent works in the Northern Adriatic coast suggested the use of filtering

groins, i.e. short groins made of an array of wood piles (see Fig. 6) or of

concrete piles (Fig. 7a, b) embedded in the soil up to a water depth of 2-3 m.

Figure 6. Filtering groins at Lido di Spina (FE).

Compared to traditional rubble mound groins, they are more porous and their

mutual distance is generally shorter. Consequently they entrap a lower quantity

of sediments, reducing the shoreline discontinuity and they are more effective in


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lowering long-shore currents close to the groynes heads, thus reducing the risk

of local scour developing.

Figure 7. Filtering groins under construction a) and in operation b) at Jesolo (VE)

Non traditional systems (e.g. vegetation, geosynthetics, etc.)

The principle of placing artificial algae (Fig. 8 a, b) on the bottom in order to

increase wave energy dissipation (Mendez & Losada, 2004; Tschirky et al.,

2000) follows from the observation of the effect of Posidonia Oceanica.

Natural phanerogams are unfortunately at high risk of extinction, mainly

because they cannot tolerate high turbidity and high deposition and/or polluted

water.

Figure 8 a). Artificial algae Figure 8 b). Deposition induced by algae

The use of geosynthetics is of increasing interest in designing coastal protection

structures (Ruol, 2004). Geotubes or geobags, for example, have a principle of

operation similar to the previous one, i.e. they induce wave energy dissipation,

creating less environmental impacts if compared with rigid (rock) structures

(Fig. 9a, b; Fig. 10a, b).

Among this category of non traditional systems of beach protection, many

other solutions have been proposed and analysed in recent years, but it is notaim of this paper to enter into the details of such solutions.

ENVIRONMENTAL DESIGN PROCEDURE

Traditional design of coastal defences essentially consists in the engineering

sizing of the structure (height, width, rocks or concrete units, etc.) to get a

desired beach protection, e.g. in case of a barrier parallel to the coast a certain

transmission coefficient and in case of a groyne system a certain amount of


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trapped sediment must be defined. This design, performed very often at local

scale (municipalities, or even touristic resorts) without accounting for the effects

on the adjacent beaches, produced for instance along the Northern Adriatic

coast, the disruption of the ecosystem equilibrium and conditions that in several

cases are actually unsustainable and would require an intervention in the close

future.

Figure 9 a). Sand bags groins Figure 9 b). Geo-bags

Figure 10 a). Geocontainers at Alassio (SV) Figure 10b). Placement

A careful design strategy shall consider several steps. Before planning anintervention, political (i.e. European, national, regional policies), technological

(for instance, availability of a given construction material), environmental (for

instance impact on water quality and species) and social (as the choice between

sand and gravel, or between emerged or submerged structures) constraints

should be identified.


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After a preliminary selection of design alternatives, each of these has to be

examined and compared with respect to its technical, socio-economical and

environmental performance.

2DH numerical prediction of the hydro-morphological consequences of

different defence schemes and their suitability to accomplish the design

objectives may assist designers in the selection of the best defence scheme or

in the optimization of existing ones (Burcharth et al., 2007). Estimated waves

and currents allow, for instance, to evaluate the inshore wave energy reduction

with the consequent level of beach protection; the water residence time inside

the protected cell to assess water recirculation (and thus also water quality) for

ecological purposes; the current patterns and intensities, in particular at gaps

and roundheads, to verify bathing safety. Estimated sediment transport allows,for instance, to evaluate the global sand volume balance for the protected cell, in

order to estimate if renourishment is necessary and, if it is, its quantity and

frequency; the formation of local scour that may produce structure instability, in

order to redesign a proper toe protection or structure extension; the

erosive/depositional patterns and their rate to identify the level of disturbance to

the assemblages for ecological purposes.

The results of analyses and numerical and/or physical modelling have to be

judged by different experts and then have to be synthesized defining appropriate

indicators such as: performance of the scheme for beach protection; initial and

maintenance costs; impact on habitats, species, ecosystem and their living

natural resources; cultural heritage of the coastline; recreational value. A

proper weight has to be assigned to each indicator and a mark for each

alternative is derived from the weighted sum of all indicators, providing anobjective selection of the optimum scheme.

An example of application of this design strategy, referred to Lido di Dante, a

small seaside resort in the Northern Adriatic Sea, 7 km from the town of

Ravenna, is investigated (details are published in Zanuttigh et al., 2005).

The beach of Lido di Dante is supposed to be protected only by the three

groynes (as in 1983) without the barriers and connectors built up in 1996 (aerial

picture in Fig. 11). This layout will allow to check the effects of different

design solutions.

Figure 11. Plan view of Lido di Dante in 1993. From Zanuttigh et al. (2005).


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The sandy beach of Lido di Dante has a concave shape and is more than 2500

m long. It can be divided into two parts: the Northern beach (almost 600 m

long) has been subject to strong erosion and therefore it has been protected by

groynes, nourishment and semi-submerged breakwater. In contrast, the Southern

beach has undergone slight erosion and is in a very natural state. Shoreline

retreat is mainly caused by the reduced sediment transport rates of the rivers and

by the anthropogenic and natural subsidence, which justifies a beach recession

rate of 3m/year.

Erosion has disrupted the equilibrium of the beach, with major damage when

storm surges are coupled with high tides. Littoral recession, such as erosion of

dunes and land subsidence, together with building of tourism facilities, has

altered and partially destroyed the maritime pinewoods behind the dunes.The main objective of the design is the maintenance of an adequate beach for

recreational bathing activity. The achievement of this objective also provides a

proper protection of land and infrastructures. It is indeed necessary to avoid

possible flooding, to protect residential properties and roads and all the human

activities on which the economy and safety of the village depends.

Several alternatives can be found, among which only the following were

examined, based on social and environmental constraints (Fig. 12):

0) pure beach nourishment with sand;

1) single submerged detached breakwater;

2) emerged multi-structure system;

3) extension of 2 of the existing groynes;

4) composite structure (extension of the edge groynes, to reach the submerged

detached breakwater).Some of the discarded solutions and the reason for which they were not

considered can be briefly mentioned. Beach nourishment with pebbles or gravel

would have contrasted with one of the social requirements, which is the use of

fine sand. Similarly, the choice of a revetment would have not provided a beach

for recreational use. Finally, sand filled geotextile bags cannot be considered as

a possible solution due to the fact that they have already been used in Lido di

Dante without success.

The designer has first to characterize the meteomarine climate in the area before

proceeding to a proper preliminary design of the selected alternatives based on

structure stability considerations.

In order to get reliable results in a reasonable time, the engineer has to collect

data of waves and tides and to elaborate a simplified wave climate, which isenergetically equivalent to the typical annual wave climate but is represented by

a limited number of conditions, lets say a set of 7 waves coupled with the

average tide in the area or, as in this case where tidal range is rather low, to no

tide (see Tab. 3). Preliminary design of the selected alternatives is then carried

out by using the specific formulae for structure stability for the main cross-

sections. Layouts are drawn considering shoreline evolution based on groyne

length, distance between barriers and shoreline and gap lengths.


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1 8 5

4 0

8 0

5 3 0

1 2 5

1 8 5

7 0

3 61 2 0

3

2

4

6 7 0

1

Figure 12. Plan view of the four selected design Alternatives (dashed line =

submerged). From Zanuttigh et al. (2005).

The four alternatives of Fig. 12 have been implemented in 2DH numerical

models, considering the anticipated simplified wave climate, in order to

determine their typical hydro-morphological response.

An example of the most interesting results that can be obtained for one of thedesign solution (i.e. the case of the emerged multi-structure of Fig. 12-4), is

shown in Fig. 13. What can the designer obtain from these simulations? From

Fig. 13 some conclusions can be drawn, just to have an idea of this procedure.

Wave agitation is almost null behind the barriers, whereas is still of importance

at gaps (Fig. 13c).


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Reduction of incident wave height on the shore (larger than 50%, even for the

highest wave attack Wave 6) is responsible for two opposite effects: one,

positive, the reduction of offshore sand transport from the emerged beach; the

other, negative, the landward reduction of wave agitation that induces

deposition of fine sediments.

Figure 13. Example application of 2DH hydromorphological simulations: a)

bathymetry with the emerged multi-structure; b) average erosive/depositional

trend per day induced by the annual sediment transport; c) reduction of wave

height for the most intense wave attack, Wave 6; surface elevation and current

velocity induced by Wave 6 (d) and in the almost calm condition, Wave 7 (e).

a) b)

c) d) e)


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Current speeds landward of the structures are in the range 0.1-0.3 m/s, whereas

at the groyne roundheads reach peaks 0.4-0.5 m/s but since are redirected

towards the beach do not cause problems to bathers (Fig. 13d). Due to the

reduction in water mixing induced by the decrease of both wave and current

intensities, the intervention will clearly increase the water residence time in the

protected cell (that can be calculated as the area of the protected cell divided by

the average current speed within it in the worst condition, Fig. 13e).

Table 3. Representative wave climate off-shore Lido di Dante beach (at -25 m depth)

Wave n Wavedirection []

Hos [m] Tm [s] Wind velocity[m/s]

Frequency [%]

1 45 1.5 5.0 12 4.742 45 4.0 8.0 20 0.533 90 1.5 5.0 12 5.864 90 3.5 8.0 18 0.815 135 1.5 5.0 12 4.806 135 3.5 8.0 18 0.477 120 0.3 3.0 5 40.00

Table 4. Evaluation rank of design alternatives, including factors to be judged andweights. Numbers refer to labels of different alternatives as in Fig. 12,

a part from 0 that means pure renourishment.

Beachprotection

Ecologicaleffects

Social effects Totalcosts

GlobalMark

Alterna

tive

Shoreline

maintenance

Effectsonadjacent

littoral

Ecologicalimpacts

Mitigation

effects

Recreationaluse

Aesthetic

impact

Swimming

safety

0 1 3 5 1 3 4 1 2 10.671 4 5 4 2 2 5 2 4 15.002 5 2 1 3 4 2 5 3 11.923 2 1 3 3 5 3 4 1 9.504 3 4 2 2 2 5 3 5 13.83

PartialWeight

1/2 1/2 2/3 1/3 1/3 1/3 1/3 --

GlobalWeight

1 1 1 1

The erosive/depositional trends per day due to the annual wave climate in

Fig.13b are derived by the weighted average (with frequencies shown in Tab. 3)of the erosive/depositional trends obtained for each wave. We say trends

since it is worth to remember that these results are derived from the composition

of the effects due to different waves on a fixed bottom bathymetry. Deposition

occurs in the protected area and, in average, at the shoreline, whereas gaps and

groyne roundheads are eroded. The mix of erosive and depositional patterns

inside the cell will certainly induce some disturbance to the assemblages.


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The erosion at gaps, close to the structure toe, may also create disturbance to the

organisms colonising the barriers, a rocky habitat in a natural sandy one that

will in time enhance species biodiversity. The accumulation process at the

shoreline may produce salients/tombolos as in other places protected by

breakwaters along the Emilia Romagna coast.

It is important to consider also the effects of the barriers on the adjacent littorals,

which will suffer from erosion, in particular the Southern one. The combination

of the sediment transport patterns due to the different waves as presented in Fig.

13b gives also a quantitative prediction of the average sediment losses/deposits

per year within the protected cell, of the average cross-shore and long-shore

transport rates and thus of the required nourishment volumes.

Similar considerations can be done for each design solution, leading to acomparative evaluation of their hydro-morphological performance. The

prediction of the hydro-morphological effects combined with the calculation of

the initial and maintenance costs of each defence scheme can guide the designer

to the selection of the preferred one, as it is shown in Tab. 4 where the same

weight in the final decision was assigned to beach defence, ecological impacts,

social impacts and costs.

After the selection of the most suitable scheme, the designer can proceed to

optimize the defence system, based on specific considerations regarding its

recreational use and aesthetic impact, and possible enhancement of living

resources (colonizing species, fishery farms, etc.), see for low crested structures

Burcharth et al. (2007).

CONCLUSIONSThis note briefly describes the procedure for an environmental design of coastal

protections. It is seen that the interaction of several competences are required in

order to set up a table with technical, ecological economic and social issues. The

choice of the most suitable scheme, that should include the zero option, is

therefore somewhat similar to an EIA evaluation, guided by technical

arguments.

ACKNOWLEDGMENTS

The support from the following projects is acknowledged: DELOS Fp5 project

contract EVK3-CT-2001-00041; Beachmed-e/GESA Interreg IIIc project BMe-

3S0155R-3.3; PRIN 2005-080197, "Cave sottomarine e ripascimenti:

modellazione morfologica e applicazioni".

The authors are also grateful to Ministero delle Infrastrutture - Magistrato alle

Acque di Venezia for giving some of the pictures used in this paper.


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Documents

Ruol, P & All (2007) Invited Lecture - Design Strategies and Management Coastal Protection Framework Environmental