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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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Recommended