CLUB DE REGATAS LIMA Sede Chorrillos Rehabilitation of Playa 3

Transcription

CLUB DE REGATAS LIMA
Sede Chorrillos
Rehabilitation of Playa 3
October 2013
PUM N etherlands senior experts
Mail address: PO Box 93078
2509 AB The Hague
The Netherlands
Telephone:
+31 (0)70 349 05 55
E-mail:
[email protected]
Website:
www.pum.nl
Date:
Project number:
Name expert:
Name applicant:
Address applicant:
Contact person:
Project dates:
Status of the report:
25 October 2013
64047 M PE
Casper Veeningen
Club de Regatas Lima
Av. Chachi Dibós 1201 Chorrillos - Lima
Gabriel Seminario De La Fuente
6 - 26 October 2013
Final
Table of contents
Summary............................................................................................................................. 1
1
Introduction .................................................................................................................. 2
2
Beach development at the Club de Regatas ................................................................. 2
3
Natural coastal phenomena .......................................................................................... 4
4
Causes of erosion at Playa No.3 .................................................................................. 5
5
Alternative solutions ..................................................................................................... 6
6
Conclusions and recommendations ............................................................................ 12
References ........................................................................................................................ 14
Annex A: Comparison of depth contours 1999 and 2012 ................................................... 15
Annex B: Improvement of wave conditions in San Antonio ................................................. 16
Summary
Since many years the Club de Regatas de Lima, location Chorrillos, has attempted to
maintain three beaches of sufficient width to accommodate the members of the Club. The
beaches No.1 and No.2 have been stable during the years. At beach No.3, however, only a
narrow strip of sand is left. During high water the waves are reaching and even damaging the
wall that is separating the beach from the esplanade behind the beach and the adjoining
parking area. During the last 40 years many hydrographic and bathymetric data have been
collected, and various solutions were proposed. Some of these solutions have been
implemented, without achieving the desired results, however.
In the present report an assessment has been made of the causes of erosion and of the
various options that can be considered to rehabilitate beach No.3. Two alternative solutions
have been presented. The effects to be expected from each of the alternatives is discussed.
One option is the construction of a submerged breakwater parallel to the beach, at about
100m from the protective wall. The other option consists of an extension of the existing
breakwater at the southern extremity of the Club area. Because the beach will not grow in a
natural way, an artificial sand replenishment on the beach is required in both options.
The second solution (breakwater extension) in principle opens the possibility to create two
additional beaches (No.4 and No.5) southwest of beach No.3. In particular for the possible
development of a future beach No.5 more bathymetric data are required to form a good
opinion on the feasibility of such a project.
At the request of the Client, some provisional comments are made on a possible
improvement of the wave conditions at the location San Antonio of the Club de Regatas (see
Annex B)
PUM Netherlands senior experts
1
Club de Regatas de Lima, October 2013
1
Introduction
The Club de Regatas Lima is a private non-profit organization, established in 1875, presently
counting almost 18,000 members. The main facilities are located in Chorrillos, Lima. Other
locations are La Punta (Callao, Lima), San Antonio, La Cantuta (Chosica) and Villa Deportiva
(Villa El Salvador). The main objectives of the Club are to enable, promote and organize
activities related to sports, recreation and culture, and to facilitate social contacts between
the members.
In addition to all other facilities, the availability of three beaches is an important asset for the
Club. Whereas the beaches No.1 and No.2 have sufficient width and are in a stable
condition, only a narrow strip of sand is left at beach No.3 (Playa 3). Various efforts have
been made to rehabilitate this beach, without achieving the desired result.
The Club de Regatas has requested the assistance of PUM Netherlands Senior Experts to
make an assessment of the methodologies that are available to restore the beach, and to
formulate recommendations on the technical feasibility of possible solutions. The need for
additional studies, if any, should also be considered.
2
Beach development at the Club de Regatas
During the past 50 years many developments have taken place along the coastline at the
Club de Regatas, situated between the Playa Pescadores and La Punta Chorrillos. Various
structures have been built during these years, all of them with the objective to create or
maintain usable beaches of sufficient width. Figure 1 shows the present situation in a
schematic way.
Fig. 1: Existing situation
It is reported that already in the early 1960’s erosion occurred in the eastern area of the Club,
where the present Playas 1 & 2 are situated. The existing short piers (‘Espigones No.2 and
No.3’), were extended and no further erosion was observed. It appeared that accumulation of
sand in the area between the Muelle Regatas (E-1, at the eastern boundary of the Club area)
and the two piers took place in a natural way. Apparently, no artificial placement of sand was
required. Playa 3 did not yet exist in those years.
In later years, upto the year 2000, the piers were further extended; in particular Espigon No.3
with the objective to reclaim land for a parking area and for the present Playa No.3. At the
southern side of Playa 3 another small pier was constructed (Espigon No.4).
2
Whereas Playas 1 and 2 remained in good condition, Playa 3 suffered from erosion. In the
year 2001 the Espigon No.3 was extended by another 40 metres, with the objective to
improve Playa.3 by catching some of the sand moving along the shore in a northern
direction. A comparison of bathymetric charts prepared by SCRL 1 in 1999 and 2012 shows
indeed a considerable accumulation of sand south of the pier (Figure 2 and Annex A). The
sea bottom in this area has been raised by 1.5 to 2 metres. However, these sand deposits
did not move towards the beach. In the area between the High Water and Low Water Lines
some further erosion could even be observed. A study by Controlamar in 2002 already
showed the initial tendency of these phenomena [Controlamar, 2002].
Fig. 2: Sedimentation near Espigon 3
More recent developments near Playa 3 are the artificial seaward extension of the rocky
outcrop (‘acantilado rocoso’) at Punta Chorrillos in 2005/2006, followed in 2008 by the
construction of a small groyne in the middle between Espigon 4 and Punta Chorrillos, and a
breakwater extending over a length of 60m from Punta Chorrillos in the northeastern
direction. The short seaward extension of the Punta was meant to block the movement of
stones into the beach area. The main objective of the other structures was to improve the
conditions for the development of new beaches southwest of the existing Playa 3. It should
be noted that the breakwater at the Punta was intended to be much longer than 60m, but no
permission was obtained to complete this structure.
In this period an attempt was also made to reduce the wave energy by constructing a
submerged breakwater of geotubes at about 80m seaward of the beach. Unfortunately the
geotextile was not properly fabricated, causing a failure of the structure.
Due to the urgency for the Club to make sufficient beach area available for their members, a
temporary solution was recently adopted for Playa No.3. In 2011 a low breakwater consisting
of geotubes was placed parallel to the beach, in the vicinity of the Low Water Line. In the
same time a sandfill was deposited on the beach. However, the waves breaking across the
geotubes caused erosion behind the breakwater, and the tubes were also locally damaged
by the forces of the breaking waves (Figure 3).
1
Servicios y Estudios Hidrográficos SCRL
3
Fig. 3: Geotubes at Playa 3 (October 2013)
3
Natural coastal phenomena
In this section some basic phenomena are described, that will be used in the evaluation of
the coastal developments observed at the Club the Regatas.
In general the coastal transport of sediment is caused by two phenomena:
● Longshore transport along the coast, caused by breaking waves approaching the
shoreline under a small angle.
The breaking waves are generating currents in the breaker zone, parallel to the coastline,
(Figure 4). These currents are relatively strong. In shallow nearshore areas the waveinduced currents are often much stronger than the tidal currents or other ocean currents. In
most situations the wave-induced currents are therefore the main factor causing sand
transport along the coast (the so-called “littoral drift”). .
Depending on the direction of the incoming waves, the transport of sediment along the
Peruvian coast can take place in a northern or a southern direction. In the area near Lima the
resulting net transport is northward, because the waves are predominantly coming from
southwesterly directions.
Fig. 4: Currents induced by breaking waves
4
Fig.5: Onshore-offshore sand transport
● Transport of sediment perpendicular to the coast, equally caused by waves.
In general, strong wave action will cause erosion of the upper part of the beaches and the
formation of a sand bar at some distance from the shore (Figure 5). During periods of
moderate waves, the sand movement is directed towards the beach. As a result sand will be
eroded from the bar and move back towards the beach. At beaches not disturbed by human
intervention, this will result in a so-called dynamic equilibrium. However, if structures are built
at the upper part of a beach (e.g. seawalls to protect properties behind the beach), the
erosive forces are aggravated due to reflection of the waves against the structure, during
high water. In those situations permanent erosion may be expected.
Another factor of importance is the question whether from the point of view of sediment
transport the coastline has to be considered as one unit over a longer distance, or that the
coastline is subdivided in smaller units, separated by rocky outcrops (such as the ‘Puntas’).
In the latter case the beaches are characterized by a stable curved shoreline and the
movement of sand past the protruding rocky headlands may be small or even negligible. A
similar effect is obtained artificially by the construction of a so-called groyne (pier, ‘espigon’)
perpendicular to the shore. Updrift from a groyne the sand is trapped and will not continue
moving along the coast. Because the sand transport is blocked, this will lead to erosion
further down the coast. This may be a temporary erosion, because after some time the area
directly updrift from the groyne is filled with sand, so that the sand can start passing the
groyne in a downdrift direction, thus re-establishing the sand transport along the coast. As
mentioned, near Lima the net sand transport along the coast is in the northern direction, due
to the predominance of waves from southwestern directions.
Specific data on local tides, currents, waves and bathymetry can be found in the references
listed at the end of this report. This information has been used for the present assessment,
but has not been repeated in this report.
4
Causes of erosion at Playa No.3
With a reference to Section 3 an assessment has to be made whether the erosion is caused
by a decrease in longshore transport of sediment, by transport of sand in a seaward direction
perpendicular to the coast, or by a combination of both.
Longshore transport
Immediately south of the project area, at La Herradura, the sandy beach has eroded, leaving
a beach consisting of gravel and stones. People claim that human interventions further south
at La Chira in the 1980’s are likely to be the cause of this erosion. An assessment of this
problem is beyond the scope of the present study, but it may be assumed that a decrease in
the quantities of sand passing La Punta Chorrillas in the northern direction has occurred.
Nevertheless, it is my opinion that a decrease in longshore sand transport can hardly be the
main cause for erosion of Playa 3. This opinion is based on two considerations, of which the
first one is the most obvious consideration:
(i) After the extension of Espigon No. 3 by 40 metres considerable quantities of sand, moving
along the coast in a northward direction, have been trapped (Figure 2). However, apparently
these quantities of sand did not contribute to an improvement of Playa No.3. This would
mean that a decrease in the sand transport in the northern direction would not have a
noticeable effect on Playa 3 either.
(ii) Due to the presence of various rocky headlands along the coast south of Lima there is a
natural tendency that the waves will try to form stable beaches between those headlands.
Mainly during heavy wave attack the sand will be transported seaward, forming nearshore
bars. From there the northward transport may continue to some extent. In such cases
5
interruption of the sand transport in the shallow area close to the beach may have an effect
at the adjoining beach, but the effect on the overall sand movement will be moderate only.
[Note: During periods of high waves a long line of breaking waves can be observed offshore
from the beaches of the Club de Regatas, probably indicating a shallow area that may
enable the movement of sand in a northern direction towards the Bay of Miraflores]
Onshore/offshore transport
Visual observations clearly show that the wave action near Playa 3 is much stronger than
near the Playas 1 and 2. The frequently occurring waves from southwestern to western
directions turn around the Punta Chorrillos towards the beach both by refraction and
diffraction 2. It may be that the extension of Espigon 3 by 40m is the cause of some increased
wave action due to reflection of the waves against the southern slope of the breakwater. The
orientation of Playa 3 is such that this beach is directly facing the incoming waves, which are
breaking with considerable force at the moment they arrive in the shallow breaker zone. The
other beaches are more or less sheltered from the southwesterly waves.
Nevertheless, a dynamic equilibrium could in theory still be possible (see Section 3), on the
condition that sufficient sand is present on the higher parts of the beach. In fact, such an
equilibrium is more or less existing, because photographs taken since the year 2000, as well
as bathymetric charts since 1999, do not show a significant progression of the erosion.
However, it is a fact that the beach is much narrower than it used to be in the past.
An important development to be taken into consideration is the following:
Although some erosion occurred from time to time at Playa 3 during periods of high waves,
the sand generally moved back towards the beach in a natural way. Until early 1985 a wide
beach could be observed (see Figure 6). Then it was decided to extend the parking area and
to move the ‘malecon’ in a seaward direction, removing the upper part of the beach (Figures
7 and 8). The remaining beach was much smaller, and moreover during the highest High
Water levels, combined with high waves, these waves could reach the malecon. This
resulted in a more or less permanent loss of sand on the beach, in particular on the southern
part. Bathymetric charts show the presence of a sand bar in the area between approximately
80 and 130m seaward from the High Water line, where the sand from the beach has been
deposited. Although during moderate wave conditions this sand may partially move towards
the beach again, it will not be deposited high on the beach due to the frequent periods of
increased turbulence near the malecon.
Summarizing, it may be concluded that the transport of sand perpendicular to the coast
during periods of strong wave action is the main cause of erosion, and not a deficit in sand
supply from the south. Another conclusion is that the present situation is fairly stable.
5
Alternative solutions
During the past decades, various solutions have been proposed for a rehabilitation of Playa
3. Generally speaking these solutions would all be able to contribute in some way to an
improvement of the beach. The main purpose of the present study is to make an assessment
of the effects to be expected from the various alternatives, in order to enable a well-balanced
final decision.
2
Refraction: The phenomenon that waves turn towards the coast when entering the shallow water near the coast.
Diffraction: When waves are passing a structure, the wave crests are turning around the structure (decreasing in
height).
6
Fig. 6: Playa 3, 1985 – 1: Before extension of parking area
Fig. 7: Playa 3, 1985 – 2: Work under construction
Fig. 8: Playa 3, 1985 – 3: After extension of parking area
7
In principle there are three basic alternatives to be considered for rehabilitation of Playa 3:
A. Artificial nourishment of sand to the beach
B. Submerged breakwater at some distance from the beach
C. Extension of the existing breakwater at Punta Chorrillos
A. Artificial nourishment of sand
Beach nourishment will immediately provide a wider beach. If the quantities are sufficiently
large, it could even re-establish the natural dynamic equilibrium. It is necessary in that case
that sufficient sand will be available to permit erosion during periods of strong wave attack
without completely losing the beach. Then it may be expected that the erosion will not be
permanent, because it is normal that during calm periods most of the sand will move back
towards the beach, as described in Section 3.
However, the present infrastructure at Playa 3 (malecon, parking area and restaurant) does
not provide space for sufficient storage of sand to restore the natural equilibrium for the
prevailing wave conditions. Therefore, sand nourishment should be considered as a
temporary solution. It must be repeated at intervals of several years (though on a smaller
scale), if no other measures are taken simultaneously.
B. Submerged breakwater
A submerged breakwater can improve the beach conditions in two ways:
- The higher waves will break at the location of the breakwater, so that the beach will be
affected by smaller waves only.
- If seaward transport of sand is occasionally occurring, most of this sand will remain at the
landward side of the breakwater, and will not be lost to the beach system.
In fact, a new beach equilibrium will be established, in harmony with the reduced wave
conditions. Because there is still a variability of wave conditions, this is again a dynamic
equilibrium with sand movement alternating in offshore and onshore directions.
It is important that the submerged breakwater is located at a sufficient distance from the
shore, in order to leave enough space for the movement of sand towards the coast and away
from the coast. A position near the head of the small Espigon 4 is recommended (Figure 9),
preferably even a few metres further seaward.
Fig. 9: Alternative B - Submerged breakwater
8
The crest of the breakwater could be just below the Low Water level at Springtide (Nivel de
Bajamares de Sicigias Ordinarias). Then the height of the breakwater is about 1.5m. This
means that occasionally, at extremely low tides, the breakwater will be visible. A lower crest
level is not recommended, however, because the effect on the waves during high water
would be less.
The construction of a geotube breakwater at this location has been attempted already in
2006. Unfortunately, the structure was demolished by the waves. In principle, this would
have been a useful measure, however.
The present geotubes on Playa 3 have been placed close to the Low Water line (Figure 3). In
principle, this is too close to the beach to have a lasting positive effect. In fact, the structure
cannot be considered as a submerged breakwater, reducing the waves that are approaching
the beach. In particular at high water, it can be observed that the waves are breaking against
or across the geotubes, in the latter case eroding the beach directly behind the structure.
Sand, brought in suspension by the turbulence of the waves, is then transported in a
seaward direction, and lost to the beach. This solution may function as a very temporary
one, with the objective to more or less maintain the (artificially replenished) beach during one
summer season.
The choice for geotubes or for rocks as a construction material for the breakwater is a matter
of economics. It is worth mentioning, however, that in most cases where geotubes have been
applied for submerged breakwaters, the tubes are covered with a layer of stones, sometimes
in combination with a fill of concrete or asphalt between the stones.
The submerged breakwater will reduce the wave energy and create more moderate wave
conditions close to the beach. However, it will not create a wider beach. In order to obtain a
beach of a sufficient size, it will be necessary to artificially replenish the beach. Figure 10a
shows some typical cross-sections of Playa 3. It is estimated that about 8,000 m3 of sand is
required, in order to shift the High Water line in a seaward direction over a distance of at
least 30m (Figure 10b; LAM = Linea Alta Marea). Precise figures cannot be given, because
the final shaping of the beach profile will be a natural process. The estimate of the eventual
shape of the beach cross-section after the sandfill is based on the beach slopes observed at
Playa 2 (see Figure 11). It is important that the average grain size 3 of the sand placed on the
beach is at least equal, but preferably larger than the grain size of the existing sand on the
beach. Coarser sand will create a steeper beach slope, so that less sand is moving to deeper
water. Instead of sand supply from land-based sources, dredging from the nearby seabed
may be considered, provided that sand of a suitable grain size can be found.
It is obvious that the solution of a submerged breakwater is beneficial for Playa 3 only. The
wave conditions in the area between Espigon 4 and the breakwater at Punta Chorrillos will
not improve. Hence, for the development of further beaches or a marina in this area, other
solutions have to be considered.
3
Characterized by the median diameter D50.
9
Fig. 10a: Typical beach cross-sections at Playa 3
Fig. 10b: Artificial replenishment of sand on Playa 3
Fig. 11: Typical beach cross-sections of Playa 2
10
C. Extension of the existing breakwater at Punta Chorrillos
Another alternative, already considered by the Club, is the extension of the existing
breakwater at the SW extremity of the Club area. This existing breakwater is 60m long and is
oriented in the direction N43E. Originally the proposed extension was 200m, with an
orientation N15E (Figure 12). However, for creating moderate wave conditions, a length of
about 170m will also be sufficient. A comprehensive study has been completed in 2012 with
the objective to assess the effects of the breakwater extension (of 200m) on both other
beaches of the Club and on the beaches further to the North [GGM, 2012]. There appear to
be no significant effects with respect to waves, currents and transport of sediment in the area
North of the breakwater. This could indeed be expected because the extended breakwater is
not affecting the ocean currents and is not blocking the transport of sediment. On the
contrary, it may even be expected that sand, instead of being deposited directly south of
Espigon 3 as observed in the present situation, will move further northward after
implementation of the breakwater extension. The study of 2012 was made for a breakwater
extension of 200m. A shorter breakwater is even less interfering with the overall hydraulic
conditions.
Fig. 12: Alternative C – Extension of existing breakwater
After construction of the breakwater, the basin between Playa 3 and the breakwater will be
sheltered from the predominant waves, approaching from SSW to SW. Although this is
beneficial for the beach with a view to erosion, it also means that no natural growth of the
beach can be expected. Therefore, this solution should also be combined with artificial
nourishment of sand, in sufficient quantities to obtain a beach of the desired width,
comparable to Alternative B. An important advantage of this solution is that the hydraulic
conditions in the whole basin will be very calm, so that virtually no sand can be lost to deep
water.
If Alternative C would be implemented, it opens in principle the possibility to create new
beaches West of Playa 3 (provisionally referred to as Playa 4 and Playa 5). Detailed
recommendations cannot be given because insufficient data are available. However,
provisional estimates indicate that at the location of Playa 4, a sand nourishment of 6,000 m3
could possibly be sufficient for the creation of a beach width between 20 and 30 m,
depending on the water level (tidal variation). The beach fill is shown in Figure 13. The crosssection of the existing seabed in this figure has been plotted, using the bathym etric chart
from the report [Controlamar, 2003]. For Playa 5 such an estimate could not be made, due to
11
lack of recent bathymetric data for that area. If the larger depths near Playa 5 would require
much more sand, an initial fill of the deeper areas by rocks could be considered.
Fig. 13: Artificial sand nourishment for creation of new Playa 4
6
Conclusions and recommendations
1. The movement of sand in an offshore direction due to strong wave action is considered to
be the main cause of erosion at Playa 3. If the upper part of the beach would have been
undisturbed by human interventions, a dynamic equilibrium could be expected, even in case
of high waves. However, it is likely that the extension of the parking area in 1985, has
affected the beach in a negative way. The construction of a new malecon on the beach, at
the seaward side of the restaurant, has reduced the width of the beach, thereby interfering
with the natural movement of sand in offshore and onshore directions.
2. It is not likely that a reduction of sand supply from the beaches south of Punta Chorrillos
will have had a significant effect on Playa 3. Since the extension of Espigon 3 by 40m in
2001, a considerable sedimentation has been observed directly south of the extended pier.
Apparently, there was an ongoing sand movement from south to north, which was partially
blocked by the extended pier. However, this sand did not reach Playa 3 during the period of
12 years after the pier extension.
3. Although Playa 3 is far too narrow nowadays for a proper use, the present situation may
be considered as a stable one.
4. The geotubes recently placed on the beach near the Low Water line are too close to the
beach to have a permanent positive effect. In combination with sand nourishment it should
be considered as a very temporary solution only.
5. Artificial nourishment of sand (Alternative A) would immediately create a wider beach.
However, if no other measures are taken, this sand nourishment has to be repeated from
time to time, although on a smaller scale.
6. A solution for restoration of Playa 3 could be the construction of a submerged breakwater
parallel to the beach, combined with artificial nourishment of sand (Alternative B). The sand
12
nourishment is required to create a beach of sufficient width; the breakwater should
sufficiently reduce the waves approaching the beach, in order not to lose significant
quantities of sand to deeper water. A position of the breakwater about 100m from the
malecon, near the head of the small Espigon 4, is recommended.
6. An alternative solution is an extension of the existing breakwater at the western extremity
of the Club area by about 170m in a direction N15E (Alternative C). This solution requires a
similar artificial beach nourishment as the previous alternative.
Although this option is much more expensive, it has several advantages:
- Because the basin will be completely sheltered from the dominant waves, this solution is
safer with respect to avoiding possible sand losses during extreme wave conditions.
- A larger beach is possible because the dimensions of the beach are not restricted by the
presence of a submerged breakwater relatively close to the beach.
- The sheltered basin offers a possibility to create new beach areas to the west of Playa 3
(Playas 4 and 5).
7. The following actions are recommended as a follow-up to this report:
● Detailed design of the structures to be implemented in Alternatives B and C, including
cost estimates, by a firm of consulting engineers. This has to be done for both alternatives,
assuming that the costs play a role in the selection process.
● Grain size analysis of 10 sand samples from Playa 3 and 25 samples from the seabed in
the area between Playa 3, Espigon 3 and the breakwater, including the area close to the
projected Playas 4 and 5.
● Grain size analysis of 5 sand samples from each of the Playas 1 and 2, for comparison.
● Investigation of sources where suitable sand can be obtained for beach replenishment. In
addition to sources on land, dredging from the nearby seabed may be considered. The grain
size of the sand to be procured should be equal, or preferably larger, than the size of the
sand on the existing Playa 3.
● Cost estimates for procurement of sand for beach replenishment.
●
Bathymetric measurements of the whole area between Playa 3, Espigon 3 and the
existing breakwater at Punta Chorrillos, with particular emphasis on the area close to the
projected future Playas 4 and 5.
13
References
The following documents, made available by the Club de Regatas Lima, have been used to
gather information on the development of the beaches of the Club, and to obtain relevant
hydrographic, bathymetric and oceanographic data.
The documents have been listed in chronological order.
Livesey & Henderson del Perú S.A. Estudio de rehabilitación de playas , 1983
Servicios y Estudios Hidrográficos SCRL. Estudios Hidro-oceanograficos espigones del Club
de Regatas Lima, Bahía de Miraflores – Chorrillos, Enero 1999.
Controlamar. Estudio del comportamiento morfológico de la Playa No.3 del Club de Regatas
Lima, Sede Chorrillos, Levantamiento topográfico batimétrico en temporada de invierno,
Julio 2002
Controlamar. Estudio del comportamiento morfológico de la Playa No.3 del Club de Regatas
Lima, Sede Chorrillos, Levantamiento topográfico batimétrico en temporada de verano, Abril
2003
Enrique VALDIVIA García. Opinion técnica en relación con la problemática de la
recuperación de la Playa No.3, Mayo 2005
Marina de Guerra del Perú, Dirección de Hidrografía y Navegación. Modelamiento numérico
del transporte de sedimentos para el Club Regatas Lima, Bahía de Miraflores, Julio 2007
Luis Alberto HUAMÁN Amasifuén. Club Regatas Lima, Sede Chorillos, Playas 3 y 4,
Levantamiento batimétrico y topográfico, Noviembre 2007
Luis Alberto HUAMÁN Amasifuén. Club Regatas Lima, Sede Chorillos, Levantamiento
batimétrico y topográfico para calculo de volumen en la Playa 3, Octubre 2008
COPAG S.A.C. Club Regatas Lima, Estudio hidrografico y oceanografico para el diseňo y
operatividad de un rompeolas, Noviembre 2009
Luis Alberto HUAMÁN Amasifuén. Club Regatas Lima, Sede Chorillos, Levantamiento
batimétrico y topográfico en la Playa 3, Octubre 2010
Servicios y Estudios Hidrográficos SCRL. Levantamiento batimétrico en la zona de mar
frente a la Playa No.3 del Club de Regatas Lima, Sede Chorillos, Bahía de Miraflores –
Chorrillos, Agosto 2012
GGM Ingeniería Portuaria, Costas y Hidráulica. Estudio de dinámica litoral de las playas del
Club Regatas para la ampliación de rompeolas de protección de costa, 2012
Servicios y Estudios Hidrográficos SCRL. Estudio hidro-oceanografico para la instalación de
estructuras para protección de costas, San Antonio – Cerro Azul – Lima, Febrero 2013
14
Annex A: Comparison of depth contours 1999 and 2012
Depth contours 1999 (grey lines):
Depth contours 2012 (black lines):
Based on bathymetric survey reported in [SCRL,1999]
Based on bathymetric survey reported in [SCRL, 2012]
15
Annex B: Improvement of wave conditions in San
Antonio
At the specific request of the Client some remarks are made on the beach conditions at the
San Antonio location of the Club de Regatas, 80 km south of Lima. These remarks are based
on a half-day reconnaissance visit to the area on 15 October, 2013, and some hydrographic
data from the report [SCRL, 2013]. It is obvious that, based on this scarce information, no
firm conclusions can be drawn. The remarks may serve, however, to develop some initial
ideas on the methodologies to be adopted for improvement of the beach conditions.
Club de Regatas, location San Antonio
The facilities of the Club de Regatas at San Antonio are still under development. The length
of the coastal strip between the two headlands is about 1.8 km. The sandy beach is very
wide and the protective wall (malecon), constructed by the Club, is at a sufficient distance
from the High Water line so that it is not interfering with the natural variations in the beach
profile.
During a considerable part of the year the beach is suffering from strong wave attack, with
high waves approaching more or less perpendicular to the shoreline. According to
computations in the report [SCRL, 2013], based on wave observations in deep water further
offshore, the significant wave height Hs 4 in the nearshore zone would be about 2.7m (at 4m
depth) and the maximum significant wave height at that depth would be almost 5.3m. The
predominant wave periods are between 12 and 14 seconds.
The beach is apparently in a stable condition, although in general the waves are higher than
near Playa 3 at Chorrillos. It may serve as an example of a dynamic equilibrium (see Section
3 of this report). This situation illustrates the benefits of leaving sufficient space of sandy
beach between the infrastructure built by the Club and the High Water line.
The high waves, breaking with considerable force in the shallower water close to the beach,
create very rough conditions in the water. Bathing and swimming conditions are not
4
Significant wave height Hs is defined as the average of the highest one third of the waves
16
comfortable or even dangerous for many people, especially for young children. The Club
would like to find a solution to create calmer wave conditions in the nearshore area.
Since the waves are approaching the coast from a direction more or less perpendicular to the
shoreline, it is obvious that the construction of groynes (espigones) perpendicular to the
shoreline will not have a reducing effect on the waves. The solution should be found in
structures parallel to the shoreline which are able to form a partial shelter against excessive
wave attack.
A submerged breakwater will in principle be an adequate solution to create calmer wave
conditions nearshore. One could consider a submerged breakwater over the whole length of
the beach. This would be a very expensive solution. An attractive alternative may be to
construct a series of shorter breakwaters (also submerged). In that case a construction in
phases is possible. A start could be made, for example, with the construction of a first
breakwater of 170 – 180m length extending from the rocky promontory at the southern end of
the beach. (see Figure B-1). At present this is the most developed area, where the residential
facilities are situated.
Fig. B-1 Schematic sketch of San Antonio beach with submerged breakwaters
The underwater slope of the beach is very gentle (about 1:30, compared to about 1:14 at
Playa 2, Chorrillos). This means that the breakwater can be placed in relatively shallow water
and nevertheless be at a sufficient distance from the shoreline in order to leave sufficient
space for the natural onshore/offshore movements of sand. As a very tentative indication a
position at the depth contour of approximately -2.5m can be given, and a breakwater height
of about 2.0 to 2.3 m (see Figure B-2). This is a position about 150m seaward from the Linea
Alta Marea (LAM) and a crest level just below the bathymetric reference level, which is the
Low Water level at Springtide (Nivel Medio de Bajamares de Sicigias Ordinarias, NMBSO).
It should be noted that the wave conditions in the case of one short breakwater will not be
representative for the situation with more breakwaters, due to wave diffraction around the
northern head of the breakwater. A reasonable judgment of the effect can only be made after
construction of at least one second breakwater. Tests in a mathematical or physical hydraulic
model will be useful to predict the effectiveness of a solution with submerged breakwaters at
different locations and with different crest levels, so that the optimum layout of the structures
can be defined.
Furthermore, it could be attractive for the users of the beach that wave conditions are
different at different parts of the beach. Therefore, also from the point of view of the users, it
may not be necessary to protect the whole coastal stretch against the high waves.
17
Fig. B-2: Typical beach cross-section San Antonio Beach (South)
Finally, it is repeated here, that the content of this Annex is not based on comprehensive
studies. It should merely be considered as a basis for further development of ideas for
solutions to mitigate the wave conditions at San Antonio Beach.
18

CLUB DE REGATAS LIMA Sede Chorrillos Rehabilitation of Playa 3

Transcription

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