LOW LATITUDE ISLAND BEACH PROCESSES.
by
Munesh Munbodh.
Project Report
submitted to
Marine Resource Management Program
School of Oceanography
Oregon State University
1981
in partial fulfillment of
the requirements for the
degree of
Master of Science.
LOW LATITUDE ISLAND BEACH PROCESSES.
by
Munesh Munbodh.
Project Report
submitted to
Marine Resource Management Program
School of Oceanography
Oregon State University
1981
in partial fulfillment of
the requirements for the
degree of
Master of Science.
Contents
Introduction
1
Uses of the beach
1
The dynamic nature of beaches
3
Islands and beaches
8
Beach composition
9
Grain size of beach materials
11
Sediment formation from the reef
13
The littoral sand budget
15
Erosion , 17
Effects of storms on beaches
20
Effects of recreation on beaches
22
Other processes
23
Beach and shoreline protection
24
Effects of sand mining
28
Mauritius coast and beaches
30
Concluding remarks
39
Appendix
•
LOW LATT2UDE ISLAND BEACH PROCESSES
Introduction
Beaches form the interface between the land and the sea
in most parts of the world except where the coast ends
abruptly in cliffs. The relation of the beach to its environment is at first sight not apparent and , we may be tempted to
think of it in isolation. However, the beach is an integral
part of the coastal zone and must be understood in the context
of that system.
This paper describes beach processes in general with
emphasis on subtropical and tropical regions. A section on
the effects of sand dredging in the lagoon is included as
this activity is very important due to its direct and indirect effects on the beach.
Uses of the beach
The beach is a constant attraction for people on holidays
and those just touring the country. Pleasant weather conditions prevailing in subtropical and tropical areas make it
possible for people to enjoy the beach year-round. Creation
of facilities for people visiting the beach has resulted in
much of the coastal development found in many places. - The
development of good internal and external transportation
systems has made it easier for more people to spend some
2
time on the coast. Holidays and vacations have become
closely linked with spending some time on the beach for many
people.
Some people use the beach for sunbathing. Others find
it a convenient place to relax and listen to the swash of
the waves. Children often find pleasure in digging and
making sand castles in the sand. Other people still enjoy
strolling along the beach and beachcombing for flotsam. Sea
shells and coral debris attract their attention and are
collected. The value of the beach to swimming, boating, sunbathing, surfing and diving activities are, of course,
obvious. The beach provides a very interesting environment
for scientists, students and other people who are inquisitive
about physical and biological processes on the seashore.
Poets and artists have often found the beach environment a
source of inspiration.
Such demandsupon and activities on the beach bring it
under tremendous pressure. It may be noted that there is
only 13 cm of shoreline for every person in the world
(Inmann and Brush, 1973). The coastline considered included
Artie and Antartic shores which, we would agree, are not
shores generally available for a desirable form of recreation.
The beach length per person is reduced further if only
coastlines suitable for recreational purposes are considered,
not to mention that private ownership makes many prima
3
recreational beaches unavailable to the general public
(Burka, 1974).
The dynamic nature of beaches
Beaches are dynamic features and there is a continuous
flux of materials between the beach and its surroundings.
They are continually changing in composition, structure
and volume seasonally, yearly and over longer periods of
time. These phenomena are related by a complex series of
equilibria with the rates of sand production, longshore
transport and loss. Combinations of these processes may
lead to accretion, erosion or equilibrium of the beach. A
typical beach profile and terminology used is shown in Fig. 1.
Figure •1 The terminology
used to describe the beach
profile. (From Komar,
1976).
Cyclic fluctuations may be most pronounced between
seasons both in the vertical profile (Fig. 2) and the horizontal configuration (Fig. 3). Diurnal, semidiurnal and
•
4
fortnightly fluctuations are also common. Noncyclic changes
due to tsunamis, hurricanes, storms or cyclones are also
often observed.
swell (summer) profile
swell profile shoreline
Figure 2, The storm beach profile with bars versus the
profile with a pronounced
berm that occurs, under swell
wave conditions, kFrom,
storm profile shoreline
erm
Sea
Cliff
meon water level
---
...-
bar
trough /
//
bar
storm (winter) profile
Komari1976).
HEADLAND
WINTER
ACCRETION
SUMMER
EROSION
SUMMER ACCRETION
WINTER EROSION
PLA N
Fig.3.TYPICAL SEASONAL
FLUCTUATIONS OF BEACH
sal:=4:7
ma/5=A
(From U.S. Army C or Ps of Engineers,1979).
In summer or during swell conditions the beach profile
is characterized by a wide shoreward portion (the berm) and
a smooth offshore profile with no bars. In winter the berm
is narrow and bars are formed offshore. Clearly these profile
5
changes involve movement of large volumes of sand. Komar
(1976) gives a good review of the various mechanisms proposed
for these changes. Mention will be made here of the explanation proposed by Bascom (1980). Waves with . steepness
(steepness being the ratio of wave height to wave length)
smaller than 0.025 have been observed not to form bars. Such
waves move sand shoreward with orbital currents. Sand
particles are picked up, movedforeward and set down. Although the orbiting water returns seaward an equal distance, the
sand particles do not. Friction with other sand particles
and existence of non-turbulent flow at the bottom keep the
sand from moving quite as far as the water does and thus
completing the orbit. The net motion of sand is, therefore,
landward when wave steepness is small.
In storm conditions, wave steepness increases and the
energy expended on the beach by waves is considerably higher
than during swell conditions. There is a general flow of
water shoreward at the surface. This flow is balanced by an
equal flow seaward at the bottom. The high frequency of
waves arriving at the beach keep the bottom saturated and not
much water is lost into the sand. The bottom flow carries
with it a sandy suspension which is deposited in.the breaker
zone where landward-flowing currents are generated. The
berm is thus reduced in width and offshore bars are formed.
Waves set up onshore-offshore transport of sediments.
6
As waves shoal on the beach they steepen. This steepening
causes their crests to be separated by wide troughs.
Particles in these waves do not move in closed orbits as in
deep water waves. There is a mass transport in the direction
of wave advance. Orbital motion under the crests are of
short duration but high in velocity while the return flow
under troughs are slower but longer in duration. The shoreward component will be more effective ih carrying coarser
materials toward the beach because of the higher power
exerted on the bottom. Finer particles like sand and silt
move in both directions equal distances. More sand is
transported shoreward on the beach because of frictional
drag on the wave swash and water percolation into the beach.
Gravity tends to move sediment particles seaward as a slope
is formed. At equilibrium the two opposing forces are equal
such that the slope of the beach is,
tan IS = tan cp ( 1-c )
where tan
(1)
is the coefficient of internal friction of
shearing of the beach material and
c = local offshore energy dissipation.
local onshore energy dissipation
Waves also play an important role in the longshore transport of sediments. They cause particles to move above the
bottom in a to and fro motion. If a unidirectional current
7
is superimposed on this motion, the suspended material will
be carried in the direction of motion. Such a current can
be generated when waves arrive at the beach at an angle or
can be due to winds and tides. Komar (1976) gives a relationship from Bagnold (1963) in which
8 = K' w
Ue
u;
where i is the immersed-weight sediment transport rate per
unit bed width in the direction A determined by the unidirectional current U e . w is the available power from wave
motion which supports the sediment above the bottom and U0
is the orbital velocity of wave motion. K' is a dimensionless
constant.
Another form of littoral transport known as swash transport occurs when waves break on beaches 'at an angle to the
shoreline. Longshore currents are generated which transport
suspended particles (set up by orbital motions of the waves)
before the particles settle out again. Repetition of this
process results in a saw-tooth transport of sediment along the
beach (Fig. 4).
•
8
ZIG ZAG MOVEMENT
OF SAND PARTICLES
RESPONDING TO UPRUSH
& OOWNRUSH OF WAVES
PLAN
Fig.4.
LONGSI-IORE LITTORAL TRANSPORT
(From U.S. Army Cor p s of Engineers,1979).
Islands and beaches
Beller (1973) has emphasized the ecological, economic
and cultural fragility of islands. The small size of an
island fosters close critical relationships between its
cultural heritage, economic status and ecosystems. This
leads us to suppose that island beaches would be more
vulnerable to change than their continental counterparts,
given the small sizes involved.
Island beaches in the tropical regions cannot be viewed
in isolation from the lagoon and coral reefs. Most of these
beaches are derived from material almost wholly from the
reefs and the lagoon. The latter acts as a sediment
reservoir. The presence of fringing reefs around islands
protect the shore from large oceanic swells; they influence
the flow of the inshore water thereby affecting the
deposition of sand which is needed for forming and maintaining the beach.
Beach composition
"The beach is an accumulation of unconsolidated sediment (sand, shingle, cobbles and so forth) extending
shoreward from the mean low tide line to some physiographic
change such as a cliff or dune field or to the point where
permanent vegetation is established." (Komar, 1976).
The range of composition of the beach is very broad.
In temperate regions rivers and streams supply most of the
sediments found on beaches and the nature of the hinterland
has a direct control on their composition. Quartz and
feldspar grains are found on most beaches of continents.
They are derived from granitic-type rocks--schists and
gneisses that are abundant on continents. In localized areas
erosion of seacliffs and headlands can become a major source
of sediments for beaches (Katz and Gabriel, 1977).
Beaches on volcanic islands may be composed.entirely
of volcanic debris derived from andesites and basalt lavas.
In some cases sediments may be derived from olivine or
basalt glass (Moberly et al., 1975). Quartz and potash
10
feldspars, very common minerals of sands in other parts of
The world, are absent along Hawaiian shores. Beaches on
Hawaii and other volcanic islands range from black volcanic
sands to white sand composed entirely of calcium carbonate.
Moberly et al. (1965) found that most Hawaiian beaches are
highly calcareous. Foraminifera predominate in most beaches,
followed by mollusks, red algae and echinoids. They remarked
that "coral sand" is a misnomer for Hawaiian beaches as sand
derived from coral was fifth in general order of composition
of the beaches. Foraminifera made up from 3 to 20% of
grains in the biogenous fraction (Inmann et al. 1963).
Guilcher (1969) also found that calcareous algae such as
Halimeda and Porolithon, foraminifera, echinoderms, bryozoans,
gastropods, crustaceans and sponges all contribute to sediment
supply to the lagoon and beach. Other supplies come from
sand transport by wind or calcareous oolites formed by precipitation.
Studies of sediments on tropical island beaches led
Guilcher (1969) to hypothesize that the content of terrigenous and organic particles depends on the ratio of the
catchment area on the island to the surface area of the
lagoon surrounding islands with barrier reefs. Thus beaches
bordering the relatively small central island in the Society
Islands have carbonate percentages ranging from 88 to 99%
even in places where basalt outcrops immediately behind or
11
above the beach. The influence of the central volcanic
island is practically negligible. Terrigenous particles are
much more numerous at Tahiti (a larger island) and the
calcium carbonate content from the inner lagoon ranges from
25 to 94%.
Grain size of beach materials
Grain size in beach sediments can vary from very fine
sand (0.125 mm) to particles larger than boulders (256 mm).
Boulders and cobbles can be found in pocket beaches, especially in the vicinity of rocky headlands. However, the more
familiar type of beach is made up of sand size particles , of
0.25 to 2.0 mm.
The mean grain size of beach sediments is controlled by
the sediment source, the wave energy level and the general
offshore slope on which the beach is constructed (Komar,
1976). The largest sediment particles are usually located
at the plunge point of the breaking waves with a decrease in
size both toward deeper water and shoreward across the surf
and swash zones. Various explanations have been given for
this, the most notable being that of Miller and Zeigler
(1958). They visualized the breaker zone to have a net
vertical movement of water and sediments. Finer particles
are easily lifted and carried up the beach face whereas
coarser ones stay in the breaker zone. Komar - (1976) further
•
12
remarks that the relationship between source and grain size
is more apparent in carbonate than in quartz-feldspar beaches.
This is because the carbonate is derived from biological
activity and often grain size corresponds to certain species
of plants and animals.
Folk and Robbles (1964) observed that sediments found
on the west coast of Isla Perez, Yucatan, are composed of
the following constituents: well-sorted coral sticks with a
mean length of about -5.5 cp** (45 mm); well-sorted 0
(P
(1.0 mm) sand, largely Halimeda segments and well-sorted 2
(i)
(0.25 mm) sand largely coral grit. Carbonate mud is produced by the black snails, Battileria minima, browsing on
coral and forming faecal pellets and abrasion of Halimeda
particles. The grain size distributions of Isla Perez
beaches are, therefore, often multimodal, reflecting their
diverse biological sources as well as physical processes.
The latter may sort out particles at some locations such
that particles are 100% Halimeda fragments or 100% coral
debris.
The mean diameter of sand samples on Kauai, Hawaii,
were 0.25 to 0.8 mm and significantly coarser in places more
exposed to wave action (Inmann et al., 1963). Bimodality of
The diameter in cp units is equal to the negative lo:garithum
to the base 2 of the diameter in millimeters.
13
grain distribution was attributed to mixing of sediments from
different parts of the coast. Grain diameters of 0.2 to
0.5 mm have been noted on the Oregon coast. The source of
sediments here is the erosion of seacliffs composed of
Pleistocene terrace sands.
Grain size of sand particles may determine their rate
of transport by currents. Komar (1977) notices selective
transport rates of different size of sand grains along
El Monero Beach, Mexico. The fraction centered round 1.19 mm
moved alongshore the fastest with a mean advection rate of
0.31 cm/sec. The rate of movement of sand grains of 0.30 mm
diameter was four times less. Finer sand grains swash up
high on the beach face and move at slower rates. Komar (1978)
also found that 75% of total drift in the surf zone is due to
bedload transport. The remaining 25% is transported in
suspension.
Sediment formation from the reef
Beaches in most tropical areas are accumulations of
detritus of organic origin. Studies have shown that the
coral reef is the principal source of sediments in such
situations. The processes involved are mainly biological
but physical processes are not to be disregarded, especially
storms.
Guilcher (1969) found that large amounts of sand are
14
produced by coral reef fishes such as the trigger fish,
Balistapus undulatus, the algal browsing fish Acanthurus
achilles, the snapper, Monotaxis grandoculis and puffer fish
Arothron nigropunctatus. Similar bioerosion processes
caused by parrot fish, echinoids, endolithic algae, fungi
and other boring organisms are reported for Carribean reefs
(Stearn and Scoffin, 1977; Frydl and Stearn, 1978). Stearn
and Scoffin (1977) studied the carbonate budget of a fringing
reef in Barbados. Productivity was estimated at 163 x 10 6 g
per year. 3% of the destruction of the reef was caused by
worms such as sipunculids, polychaetes and cirripeds. The
most important agents of hard tissue boring were the sponges,
particularly Cliona species, but the sea urchin Diadema was
the most important erosive agent. Diadema grazes on corals
for epilithic algae. Estimated bioerosion was 189 x 10 6 g
per year of which half was due to Diadema. Hunter (1977)
found that the echinoid D. antillarum produced about 97 metric
tons/ha/year of sand on a Barbados fringing reef. Approx
imately 65% of this was very fine sand to silt size and was
quickly winnowed out of the sediment. The remainder was fine
to coarse sand and probably the largest source of sediment
on the reef.
Waves and tidal currents are important transport,
mechanisms of sediment from the reef into the lagoon. Studies
of Clack and Mountjoy (1977) on sediment transport from the
15
reef in Cariacou, West Indies, showed that sand particles are
first carried away from the reef by waves into the lagoon
where tidal currents become the major means of transport and
distribution.
The littoral sand budget
The relationship between the quantity of sediment on the
beach and its environment can be better explained if the
sources and sinks of the sediment are understood on a
quantitative basis. As we have seen, sources of input are
longshore transport, river transport, sea Cliff erosion,
onshore transport, biogenous deposition, wind transport onto
the beach and artificial beach nourishment. Losses of sand
from an area can be through longshore transport, deposition
in submarine canyons, aeolian transport inland, paralic
sedimentation (e.g. lagoonal, shallow neritic, transportation
of sand into deep water via submarine canyons), beachrock
formation, solution, abrasion and mining activities. The
balance of these factors will be either accretion, erosion
or equilibrium of the beach.
Littoral cells may be identified (Fig. 5) which limit
the length of the beach and are usually found between two
rocky promontories. Sand budgets can be formulated for each
cell. Where no such compartments exist, it would be
necessary to choose arbitrary boundaries.
•
16
F0
0
LU
BIOLOGICAL ACTIVITY
(Reef Contribution)
u_
(+)
(,)
cc
01)11111,
11 11
(+)
Iti
COASTAL '4
D EROSION(+)
PARAL1C
SEDIMENTATION H
BEACHROCK
(-)
FORMATION
ea
(-OH
WIND
COASTAL STREAMS
( ) (Hinterland Contribution)
LOSSES FROM LITTORAL
SAND BUDGET
PARALIC SEDIMENTATION
BEACHROCK FORMATION
WIND
FIG.
H=(-1-)
CONTRIBUTIONS TO THE LITTORAL
SAND BUDGET
COASTAL STREAMS
COASTAL EROSION
BIOLOGICAL ACTIVITY
WIND
5. Hawaiian littoral cell. (From Chamberlain,1968).
Particles produced by the reef flora and fauna are
frequently transported through a complex series of reef
environments before being brought onto the beach as observed
by Chamberlain (1968) for Hawaii. He identified two types
of reservoirs through which the sand passes as it is transported from the reef: the nearshore and beach reservoirs.
Any channel or depression across the nearshore zone acts as
a trap for the sand. The largest channels across the reef,
some of which are ancient river courses, contain reservoirs
17
of sand that can be measured in millions of cubic yards.
Depressions in the reef flat also trap sand and the volume
of sand found in locations such as the Waimanalo Reef in
Hawaii can be several millions of cubic yards per square
mile of reef. Sand reservoirs can also be found in the
river mouths of islands.
It has been suggested (Tait, 1972) that sediment loss
from the reef could be due to water circulations set up by
breaking waves on the reef. This model showed that water
levels on the outer edge of sea-level reefs may be raised
by as much as 20% of the incident wave height above the
mean water level just seaward of the reef. This wave set-up
creates water circulation through passages in the reef. This
hypothesis is supported by observations of Inmann et al.
(1963) who suggested that the channel through Kapaa Reef on
Kauai, Hawaii was subject to scour from rapid currents
heading out to sea. However, Moberly (1968) has attributed
channels to lower sea stands of sea-level.
Erosion
The phenomenon which is most striking and of concern to
people living near the beach is erosion. The factors which
lead to erosion have already been mentioned above and they
may act singly or in combination. In tropical areas, erosion
may be observed if there is a decrease in biological activity
•
18
of coral reef organisms leading to a reduction in sediment
supply. Low precipitation on the hinterland may result in a
reduction in transport of weathered basaltic materials that
streams carry to the beaches. Abrasion can be a significant
process for losing sand (Moberly, 1968) as experiments have
shown that calcium carbonate particles are easily abraded
(Kuenen, 1966).
Longshore transport can be significant in erosion in
certain places. McGowen et al. (1977) reported that about
60% of the Gulf shoreline of Texas was in an erosional state.
Erosion was more rapid along peninsulas and deltaic headlands
because waves approach them at an angle. Longshore currents
are thus generated which transport sand away from the area.
As wave direction is closely related to wind direction,
shifts in wind direction can cause changes in the shoreline.
Emery (1963) observed that erosion of certain sand beaches on
the east side of Kauai, north side of Oahu and Maui, Hawaii,
was probably due to a shift in wind directions as noted
between 1908 and 1925 and 1925 and 1943. Harvay (1978) noted
an average dune recession between 1948 and 1977 of 27 metres
at Waihi Beach, Bay of Plenty, New Zealand. Sand-loss rates
averaged 3.4 m 3 per metre of beach per year. The basic
reason for beach erosion here was attributed to a lack of
sediment to supply the littoral drift.
Natural processes at some beaches, however, can maintain
19
them in long-term equilibrium. Campbell (1972) made an
assessment of selected Hawaiian beaches for the period 1962
to 1972. A plot of total volume of beaches against time
(Iig. 6) showed no major long-term change on any of the
islands. Individual gains and losses tended to balance out
for each island.
2.0
a
a
•
1 .0
0.8
0.6
0.4
0.2
4
8
1962
12
4
8
1963
12
12
1971
4
8
1972
MONTH OF MEASUREMENT
FIGURE 6. Size of selected beach-sand reservoirs,
Hawaiian Islands. Measurements of
1962-63 compared with those of 1971-72.
(From Campbe11,1972).
20
Effects of storms on beaches
Storms can have far-reaching geomorphological effects on
coasts and beaches. During storm surges waves_can break
material or erode beaches
further - up the beach and pile u p
depending on circumstances.
Severe erosion of Siletz spit,on the Oregon_coast during
the winter of 1972-73 resulting in the destruction and
damage to houses was reported by Komar and Rea (1975). The
causes of erosion werestorm-generate d rip currents hollowing
out large embayments on the beach. Siletz beach is a pocket
beach and it was suspected that sand mining had disrupted
the natural beach sand budget. As the volume of beach was
•
decreased it was unable to protect coastal property from wave
attack. Storm erosion of Siletz spit was again reported in
1976 (Komar and McKinney, 1977).
There are many reports of changes brought to shorelines
in tropical and subtropical areas by storms. Ogg and Koslow
(1978) found the following changes on the beaches of Guam
after the passage of a typhoon on the island in 1976. The
north and eastern sides were exposed to wave heights of
4.5 to 8.0 metres above MLLW. Most of the vegetation to a
height of 3 to 4 metres above MLLW was removed. Preztyphoon
beach slopes of 8° to 10° decreased to gentle 3° to 5° slopes
and the beach extended seaward by 5 to 10 metres. Fig. 7
shows the modification and recovery of a typical beach profile
21
on Guam. An estimated 20 m 3 of material per metre of beach
face was removed. Morton (1976) reported that erosion due
to a hurricane at Florida was as much as 30 m 3 per metre of
beach face and Tanner (1976) found an average lowering of
the beach profile of over 0.8 metre.
PEREZ
BEACH PROFILES
RECOVERY NEARLY
COMPLETE
10°
4
2
10
20
30
40
50
6}0
70
80
90
100
110
120
METERS
FIGURE 7: Typhoon modification and stages of recovery of typical beach profile. The typhoon waves swept Banc
seaward to form a wide, gently sloping beach. Surf and wind action had begun to move the displaced sand landwarC
at the time of the survey, creating a beach ridge. This landward transport will continue until the beach regains iti
pretyphoon profile. (Prom Ogg una KoSIOW,1978).
Formation of a nearly continuous rubble rampart 18 Km
long on the outer edges of the south-eastern reef flats on
Funafuti Atoll was reported by Maragos et al. (1973) after
the passage of a cyclone. An estimated 1.4 x " 10 6 m 3 of
22
material originating from submarine reef slopes was moved.
However, only 5% of this was derived from recently living
reef corals Acropora, Pocillopora and Pavona. The material
which was separated from the island by a moat 2 to 50 metres
wide was gradually being moved ashore and it was presumed
this process may play a significant part in accretion of
and formation of atoll islets.
Effects of recreation on beaches
The beach is not only a place for recreation for people
but often it also becomes a parking lot for vehicles. Though
not apparent immediately such uses may affect beach processes.
Carter (1980) studied the effects of recreation pressure on
geomorphic processes on the Northern Ireland coast. He found
that both people and vehicles alter the beach environment.
Disturbance of the sand caused changes in moisture content,
salt cohesion between small grains, increased compaction and
resulted in affecting wave run-up and tidal translation. The
beach was thus rendered more susceptible to surge and spring
tides and resulted in foredune erosion. In undisturbed areas
natural beach erosion and accretion tended to balance out.
Adverse effects of off-road vehicles on beach and dune
ecology are also reported by Godfrey and Godfrey (1980) for
Cape Cod.
23
Other processes
Coastlines in tropical areas are sometimes beach and
dune deposits or reefs and shallow sediments that have been
indurated by intertidal cementation subaerially. Notches
found at the base of such structures have been attributed
However,the
to waves and solutional processes (Russel, 1963).
(196 )show that these factors are only
of
Neumann
s
studies
secondary and came to increase erosion already started
through the activities of marine organisms. He noticed that
the carbonate cliffs in Harrington Sound, Bermuda, were
undercut by as much as 4 to 5 metres by a notch whose flat
roof coincided closely with the level of extreme low tides.
r
The notches could not have been formed by physical processes
as wave energy at that position would be relatively low.
- mp ,
oratory experiments showed that the sponge Cliona la
Lab
found in the notches, is capable of destruction rates of
2 per 100 days or an erosion rate of 1 metre
to
7
Kg
per
m
6
in 70 years. Fig. 8 is an attempt to show typical shore
profiles, zonations of physical and chemical processes
affecting carbonate coastlines. Profiles on the left are
typical of the higher energy coastline of Bermuda's north
and south shores.
24
1._JBA.ER I AL
ZONE
LITHO! OGY
SPRAY
ZONE
'INTERSTITIAL
Cti ALGAE
BEACHROCK
REEFROCK
EOLIANITE
MARINE LMS.
----..,
ALGAE
r-P,'
BRYOZOANS ,
WORMS A e-'‘T----. tg'.
CORALS L ' ,..__ r,;;- 4-''''''',-'1,:,..
c.,,..r.--:.--..:
MOLLUSCS
LOW
ENERGY
t—
1.:f.e.„..—
FIG. 8. Illustration of the general coastal morphology and zonation observed at Bemnida including
notation of biological agents and processes associated with coastal erosion. (Fr om Neumann, 1
9 66 )
Beach and shoreline protection
There are many beach and shore protection structures
which have been evolved including seawalls, groins and
breakwaters and each structure can be used in many different
forms. The U.S. Army Corps of Engineers (1979) categorize
erosion-control measures into non-structural and-structural.
Non-structural measures include no-action, regulation of
shoreland uses, relocation of existing buildings away from
eroding land, beach fill and nourishment, providing piling
25
support for a house and planting vegetation. Structural
solutions include revetments, seawalls, groins, breakwaters
and other devices which protect the shoreline from direct
wave attack and control sand transport.
Nordstom and Allen (1980) divide beach protection
measures into static and dynamic categories. They remark
that shorefront residents continue to retain a preference
for static measures like seawalls and groins which give a
feeling of permanence and appear to be a good return on
investment. Yet an increased emphasis for improving beaches
for recreation and wildlife habitat, as well as for protection,
demands the consideration of more environmentally compatible
methods. The major problems of static structures pointed out
are that they often result in an irreversible commitment of
resources, reduce attractiveness of the shoreline for
recreation and many of them do not favor the formation of a
beach.
Dynamic measures are distinguished from static ones
in that their forms are allowed to be freely worked by
waves, currents, winds and biological processes to achieve
a dynamic equilibrium between the features and the environment.
The habitat and recreational functions of the beach are not
destroyed. Nordstom and Allen (1980) summarize data for
static and dynamic methods for beach and shoreline protection
in Tables 1 and 2. They propose beach fill as a solution to
Table
Static measures of shore protection. (From i. ordstom and. Alleu,
1980)
Form of
Protection
Function
Traditional Method
of Construction
Traditional
Construction
Materials
Groins
Barrier to movement of
sand alongshore. Intended to reduce rate
of longshorc transport.
Creates wider beach on
updrift side and
starves downdrift side.
Impermeable, extending
perpendicular to
shoreline from backshore
into water normally
beyond breaking waves.
Stone, riprap,
concrete,
wood, sheet
pile, cribs
Prevent undermining and
slumping of backshore
surface. Protect backshore from attack by
swash and small waves.
Stabilize shoreline
position. Do not
favor beach creation.
Impermeable, parallels .
shoreline at contact
between beads and
upland.
Same as
groins.
None
Same as above plus chemical
soil solidification, synthetic nylon mat, plastic
erosion control fabric,
woven wire netting.
Prevent attack of backshore by large waves
and stabilize the
shoreline position.
Do not favor beach
creation.
Same as bulkhead.
Usually, riprap
or concrete but
other materials
as above.
None
None-few of the construction
materials above offer
sufficient strength.
Revetments
Dissipate wave or swash
energies on sloping,
immobile surface. Secondary function as seawall. Do not favor
beach creation.
Variable
Same as seawalls
None
See groins and bulkheads
Breakwaters
Energy filter designed
to dissipate wave energies and reduce erosive
effects of waves. Energy shadow favors deposition from updrift
sources and starves
beach downdrift.
Offshore in depths
which cause waves to
be reflected without
breaking or submerged
to allow larger waves
to break (not directly on the structure).
Usually stone
riprap but can
be other materials as indicated under
groins.
Construction with unconsolidated material
(see "mounds" in Table 3).
Floating breakwater,
bubbling breakwaters,
artificial seaweed.
Materials mentioned above,
sand mounds, stretched polypropylene foam strands
(artificial seaweed), plastic
reeds attached to concrete
base, floating tires.
Foreshore
obstructions
(Perched Beach)
Low cost, beach parallel
structure designed to
create swash zone
deposition.
Development stage
N/A
Located within swash
zone (some permeable)
creating perched
beaches.
Concrete blocks, gabions,
filled bags, Longard tubes.
Bulkheads
Seawalls
Non-Traditional Method
of Construction Impermeable or very
• low, creating baffle
for deposition or allowing some transport
to downdrift beach.
New Construction
Material
Gabion mesh baskets filled
with stone, Longard tubes
(permeable polyethylene),
acrylic or nylon bags filled
with sand or grout, asphalt,
nami rings, artificial vegetation, compressed solid
waste, junk cars, ships or
barges are possible but
unsightly.
•
27
Table 2 Dynamic methods of shore protection (see CERC, 19771 for jnore complete dis-
cussim( From
Nordstom and Allen,1980).
Purpose
Construction Materials
Beads fill
Increase protection afford. Hydraulic pipeline or
ed by beach and provide trucking with
recreational space
bulldozing.
Sand
Dunes
Barrier against flooding,
reservoir of sand to
replace beach losses.
Sand (plus structures)
to interrupt air
stream
Offshore mound
Dampen wave energies,
Dumping from barges Sand (larger particles
provide a reservoir of
would be static)
sand for eventual onshore
migration
Vegetation
Stabilize slopes, make
Planting
unconsolidated sediments
more resistant, damper
wave, swash, and wind
energies, trap sand,
improve habitat
Provide wind break
(fences, vegetation),
bulldozing
Seedlings
the severe erosion problem occuring at Sandy Hook, New
Jersey. This beach is losing sand at the rate of 270,000 m3
per year. Dunes 6.1 m high are planned as a source of
beach material during periods of erosion and for protection
of property on the backshore. These recommendations agree
with those of Carter (1980) who suggested that some flexibility
must be retained in beach protection measures to allow both
physical and biological processes room to operate. He noted
that erection of solid barriers only leads to further changes
of the shoreline.
The following quotation from Burka (1974) is worth
noting here: "In no area of coastal management is the futility
of man's efforts more evident than in his struggle to control
28
the shape of the shoreline. He has erected concrete
barricades against the sea; he has constructed jetties far
out into the ocean; he has piled rocks in the water; he has
tried to trap precious sand with groins of every design;
and he has even diverted the course of rivers. His few
successes in shaping the contours of the seashore are
insignificant compared to the untold acres of beach which
have been lost because of man's activities. Erosion is a
function of natural processes--primarily the supply of sand
and the intensity of wave action--and the only truly
effective means of controlling erosion is to cooperate with
these natural processes."
Effects of sand mining
Sand recovery from the beach can have direct effects
on it by a reduction in its volume as not only is the sand
budget disturbed but the ability of the beach to give
protection to the backshore is reduced as observed by Komar
and Rea (1975). Sand recovery from the lagoon can have
direct and indirect effects on the lagoon and the beach.
Indirect effects will result from the alteration of the
lagoon and reef environments thereby adversely affecting
flora and fauna contributing to sediment formation.
Sand dredging activities appear primarily to affect the
environment by turbidity increases of the water and deposition
•
29
of fine particles. Coral growth has been observed to be
reduced by suspension of particles (Dodge, 1974). Brock et
al. (1966) found that there was a significant decrease in
the growth of corals, echinoderms and fishes in the lagoon
of Johnston Atoll. Suspension of fine sediments continue
long after the end of the dredging activity and cause coral
mortality (Grigg, 1970). Preliminary observations (S.
Seeneevassen-MacKay, personal communication) on the effects
of sand dredging in the lagoon in Mauritius indicate that
the flora and fauna are adversely affected by fine silt
deposition and anaerobic conditions. Conditions were
observed to improve after the passage of a cyclone, thestorm flushing the area affected.
Salvat et al. (1979) made a comprehensive study of the
effects of sand dredging on the lagoon of Moorea, Society
Islands, French Polynesia. They found direct and indirect
effects. Madreporite corals were slowly buried and could
not resist the high load of fine sediments in the water.
Algae readily colonized the modified environment. The
diversity of molluscs decreased, certain species like Tridacna
maxima having disappeared; herbivorous species, viz. Cypraea
obvelata, C. moneta and Cantharus fumosus increa s ed considerably. Fish species diversity declined with increase in
numbers of Pomacentridae, Mullidae, and Scorpaenidae.
Nursery areas of lagoon fish were certainly damaged and could
30
have adverse repercussions on adult fish stocks.
Use of the Submarine Sand Recovery System (SSRS)
developed by Casciano (1973) for sand dredging in Hawaii
did not cause any turbidity or siltation problems (Maragos
et al., 1977). The effects were localized and temporary.
But mechanical damages were done to the coral communities
by the settling and dragging of anchors and steel cables
used to anchor the platform and buoys during the operation.
No direct harm was done to fish populations and no erosional
effects were noted on the beaches.
The contrasting observations of Salvat et al. (1979)
and Maragos et al. (1979) lie in the different methods used
for sand recovery, the one used in Hawaii being adapted to
lessen effects on the environment. Sand dredging in the
lagoon should be looked at with skepticism as the method
which may be used is most likely to be less environmentally
compatible than that used in Hawaii.
Mauritius coast and beaches
Mauritius is located in the South-East Trades belt and
eighty percent of the wind blows from the Southeast and east.
Swells, therefore, arrive from these directions generally
but can also Come from the west due to the presence of
anticyclonic cells formed in the subtropical high (also
called the Mascarenes High) during winter. The fetch is
•
31
exceptionally long from any direction as the island occupies
an isolated position in the ocean.
A number of cyclones pass near or over the island every
year and the associated wind systems generate swells which
may arrive at the coast from any direction.
The coastline of Mauritius is 208 km long. Fig. 9
shows the different coastal forms that can be found on the
57',7
I
30'
I
I
1
Mauritius
x
45'
I
xv„ix.
x x
I
x
COASTAL FORMS
x
xx
•
Z 0̀.— ...,,;,7.71
7
,
.,...- '''Cals''
Alternate low boulder headland
lalheureux __,
L' i.,..
1
coral sand pockets . .-.F-K,
;
4,
F_:---1 Low angle shingle - mud flat
L7,g High cliff coast
_
1:::?7:.:]
Coral sand beaches
.d
.X----°
x—
..14
x — 0: oa
x.<2.5-;„..
L
Late
Younger
I Early }volcanic series
I
_23.
)77X
x
Esperance
--.:-:i
s?.
Older volcanic series.:
••
—
e"--
x
l!..
n
ilco;
4", x
.cy
PORT LOUIS
A col ianite
.-,:
I
•
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•
Ca
erPsl'...
c
?
..Beach -'J
'Y
t4
rock •
-70
'' A 0111110e
(5!),.e;N TISO:a4b
-•
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15
VA
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n—
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4.,
,
7Tho
, "\ ahebourg
0
,.._
LE,-1----:
-.'C:1:1 -. 4
----I l' x X x
X
'General gctikNv after Sannson. 19701
5, :7
418V
:
,..
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x
—s.
CU REPIPE '.
%
glittoyo
'/
,,,,
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-
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37
)1.
. ,,:S,
o
/.."
I
Fig.9..Mauritius, index map.(
o
Imma=m•r. _
I
—30'
I
”.
From McIntire ,
1 961 ) .
32
island. Coastal processes are dominated by the presence of
a fringing reef which encircles the island almost completely
except for gaps on the south and west sides. The reef flat
forms a complete barrier between the lagoon and the ocean
as shown in the cross-section (Fig. 10) of a windward reef
on the south-east of the island at Mahebourg. Oceanic swells
break and lose their energy on the reefs, which thus protect
the beaches and coast from these high energy waves. Semidiurnal tidal ranges are small with a mean range of 1.1 feet
and spring range of 1.6 ft. at Port Louis.
OUTER SLOPE GROOVE-SPUR ZONE
REEF
FLAT
BOULDER
ZONE
ACROPORA
ZONE
PAVONA ZONE
SEA GRASS. ZONE
H5
a
2'31
s
r11.10
Pavan° divaricata
Pavona decussata
Galatea fascicular's
lyinadoCoa
Syringoalvm 130ttlfallurfi
o
°
NWS
a
:WS
"<\
Paroti than onkOdes
Pia ty9yra
Leptorio
Pot illopora
Acropora
Favi tea
Porto
O
SOm
Fig. 10. onation of a windward reef, near Mahebourg, Mauritius. [After Pichon (1967).]
‘From Jones and Endean,1973).
Beaches consist of sand, pieces of coral (Plate 1),
basaltic gravel, boulders and mud. Terrigenous sediments
BEACH
33
are important on the east coast in calm bays (Baissac et al.,
1962). Beach rock formations are found in certain places.
The volume of sand on beaches seems to be directly
related to the presence of coral reefs in Mauritius. Beach
development is most pronounced where the distance to the reef
is shortest (Plates 1 and 2 ). Where the reef is distant from
the coast, as in the south-east of the island, beach volumes
are smaller and mud flats may occur in small bays. Pocket
beaches are, however, present between headlands.
Cliffy coasts exist in sectors along the south and west
coasts where coral reefs are absent.
Beach erosion in Mauritius has not been studied in a
regular fashion partly due to lack of people trained in this
field. Spectacular changes are reported in the news, however.
Erosion of the Flic-en-Flac beach and foreshore was reported
earlier in 1980 when spring tides coupled with swells from
a distant cyclone laid the roots of Casuarina trees bare down
to depths of three to four feet.
The only published data on beach erosion are from
McIntire and Walker (1961) who visited the island about five
months after the passages of two cyclones near and on
Mauritius in 1960. Tropical cyclones Alix and Carol passed
20 miles west of and across Mauritius respectively during
January and February, 1960. The effects of Carol on the
geomorphology of the island was much greater than for Alix.
Plate 1: Sandy beach with coral debris,Trou aux Biches,Mauritius'i*
Fe
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6
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Plate 2:Pronounced beach development,Tamarin,Mauritius.
,
• V.4
.1..ro,
36
During the passage of Carol, barometric pressure dropped to
942 mb and the tide record at Port Louis showed a rise of
2.75 feet. The total rise in water level over predicted
high tide can be accounted for by the reduction in barometric
pressure (taking 35 mb drop in barometric pressure as
equivalent to a rise of 1 foot in water level). Flotsam
heights measured round the island varied between 5 and
12 feet (Pig. 11). Highest levels occurred in places where
reefs were absent in the south.
Surveys of beach profiles were made around the island
and compared with prestorm profiles. Fig.°12 shows the
results obtained on some selected beaches.
It was noted that the distance between the reef and
coast was important. For short distances considerable
erosion occurred. Beaches of low profiles such as a and c
in F ig. 12 were overtopped by waves and sand was carried
inland reducing the volume remaining on the foreshore.
Where overtopping did not occur, the foreshore was widened
and flattened during the cyclone.
As the cyclone abated and the sea-state returned to
normal, sediments accumulated on the foreshore. Gravel and
small blocks of coral were transported and covered many
poststorm beaches, in some cases, to the upper limit of wave
wash. The wind transported sand to the backshore and deposited it in a thin veneer up to several hundred feet inland.
Pre•storm Profile
Post.storm Profile,
Pori•storre Profile
Post-
tiorm
Pod
FEET
storm
Beach Crest
.
q0
ins
60
FEET
.
VS/.
.
.
. . ' ..•
•.•...
Souillac
30
.
.
. • .
:. • :
.:*
''''' •
h. Pte.d'Ariambel
10
I
0
Bench Rock
I.
MAURITIUS:.
o
Le Morne
iclad
0
. Fite cn Flacq
10
k. Pte.aux Piments
„.„
11►
• Crrta Sect. CS .LS11
poqstom
-
bench. profiles with accompanying maps to show location( and relationships
McIntire and talker 1°64).
.1
9
.10
' etween
the shore and reef-front distance.
Carte Sect..CSI.LSU
I. Cap Malheureux
•
39
Concluding remarks
General
The beach is one element in the complex system of the
coastal zone. In low latitudes it is intimately related to
the lagoon and coral reef surrounding islands. The beach
is both nourished with sediments from the reef and protected
from high energy oceanic waves by the reef and lagoon. Beach
protection on these islands becomes synonymous with protecting
the lagoon and reefs, that is, the coastal zone. The components of the coastal zone are in delicate balance and any
human activities imposed upon them should be such that they
minimize the effects on the system. However, as Dahl (1977)
has remarked, most tropical islands face an acute lack of
understanding of their coastal processes. Ways of rapidly
collecting information on these processes and making sound
management decisions on coastal resources remain a challenge
for such islands.
Mauritius
With a resident population of 900,000 inhabitants and
a yearly influx of more than 150,000 tourists from foreign
countries the available beach length per person is about 20 cm.
Due to private occupation of beach fronts about less than
10 cm of beach is available to the general public.
40
Recreational activities bring lots of off-road vehicles
on dunes and the beach scarp. Vegetation such as the filao,
Casuanina equisetifolia and the grass, Stenotaphrum
dinidiatum play crucial roles in stabilizing sand dunes but
they are coming under increasing pressure from people and
vehicles.
Though sand mining directly from the beach is prohibited,
sand quarrying in old dune deposits has been permitted, the
sand being extensively used in the construction industry.
The reef flat and sand bars behind the reefs are also exploited for sand in some localities especially on the east and
southeast coast. However, as old dune deposits are becoming
exhausted, the attention of companies is moving towards the
lagoon sand reservoirs and mechanical sand removal using
suction pumps. This will change the whole picture as not
only will the volume of sand removed be at a greater rate
but also widespread suspension of sediments may occur leading
to unpredictable changes in lagoon environment and ecology.
The present equilibrium between the lagoon sand reservoirs
and the beach will be disturbed leading to erosion of beaches
as the beach-lagoon system attempts to attain a new equilibrium level. Shorefront property will come under direct
attack of waves and storm conditions will bring about even
more destruction.
41
It would be advisable, therefore, to study our beaches
now before any change is brought so as to understand their
processes and predict any changes. Beach profiles, seasonal,
yearly and irregular (due to cyclones) can be studied.
Beach composition can be studied to find out relative
importance of sources of sediments. Lagoonal circulations
will have to be studied to predict effects of currents on
sediment transport. These studies can be carried out
jointly by the Ministry of Fisheries and the University of
Mauritius. Suggested methods of study (Emery, 1961 and
Stephen, 1977) for beach profiles are given in the appendix.
In the meanwhile other sources of sand for the construction industry must be looked into. Rock sand is at
present produced from crushing basalt boulders but sold at
a price which is about 30% more than for "natural" sand.
Moreover, we can expect the price of sand to increase as the
sources on land are exhausted making rock sand more competitive on the market. Another possibility is to look for sand
resources on the outer slope of the reef where sand recovery
is expected to have no effect on the lagoon and beach
environments.
42
Acknowledgments
I would like to express my gratitude to my advisor,
Dr. V. T. Neal for his help, suggestions and advice on
working on this project. I am grateful also to Dr. P. D.
Komar of the School of Oceanography for his comments and
for reading the manuscript. My thanks to Joseph Farrell
(former student in the Marine Resource Management Program) for
his help and encouragement. In particular, my thanks to
the Sea Grant Marine Advisory Program of the University of
Hawaii, Honolulu, Hawaii, for having sent me literature
related to this project.
Thanks also to the Oregon State University and the
Intergovernmental Oceanographic Commission (I.O.C.) of
UNESCO for having sponsored my studies at this university.
PP•
APPENDIX.
&,teL'it*
(0 A SIMPLE METHOD OF MEASURING BEACH PROFILES
( Emery ,K C
For at least 15 years the writer has measI tired profiles of beaches using a simple rapid
method. Other students of beaches have
frequently inquired about the method and
/. its accuracy, indicating that the method is
not well known. This summary is intended
to satisfy both kinds of requests.
The method requires .only two wooden
rods, each 5 ft long and about one inch by
fr.. 1. Sketch of equipment—two wooden
,- 5 ft long and marked off in feet and tenths of
;4 _41sed for measuring profiles of beaches.
It .k. beach. The observer holding the land! rod rod aligns his eye with the top of the
award rod and the horizon. He then reads
' of records the distance down from the top
I; hi; own rod of the point which is inter' ,-f ed by this line of sight ( Fig. 1), interpo: e to hundredths of a foot. Assuming the
,..,e of sight to be level, this distance is a
x asure of the difference in elevation of the
the two points that are 5 ft apart ( in
riiple
of Figure 1, the difference in
t
,
esil
ch
;'
.irent elevation is 0.53 ft 1. Where the
l
,:^
,i.j el, has a backslope, the difference in die.j. ion can be read by the same observer
aligning the top of his own rod with the
,Y uen and reading the intersection on the
,ward rod. To continue the profile, one of
r ods is moved to a point 5 ft on the oppo...side of the other rod and a second read-.
..•is made. Profiles can be measured either
,t
-., ine
rods toward the sea or away
...., movin g
the
sea.
For fixing the direction of
....0
qe the convention has been adopted of
.ii,,,4 the differences in elevation as minus
I ...bts according to whether the leading rod
, lo wer or higher than the following one.
,nail)', the differences in elevation are
,..tined up and plotted against horizontal
;,-13nCe in order to obtain a profile across
", m iole width of the beach.
csually the profiles extend from the base
j; sea cliff or a point on the landward side
41 beach to below water level. True eleva0 sometimes can he determined from a
...enientl)encli mark, but more commonly
. ,,,,st lx' estimated from the depth of water
, 1 96 1 ) .
one inch in cross-section. Notches are cuts`
1-ft intervals along each rod, with smalls
notches at 'So-ft intervals throughout!
least a foot at one end. A minor elaboration
is a small wooden pad about 4 in. squao
nailed to one end of each rod to prevent the
rod from sinking into loose sand.
In use, the rods are held vertically one rod
length apart in a line to be extended acrog
at the seaward end of the profile as compared with a table of predicted tide. Where
comparison of profiles at different times is
desired, it is often sufficient merely to relate
each set of measurements to a stake or other
ix•rmanent reference point.
As it test of the reproducibility and accuracy of the method profiles were measured
three times, With three different readers,
over the same line across a beach near Santa
Monica, California ( Fig. 2 ). The results
(Table .1) show a surprising consistency,
with readings across individual 5-ft sections
having a mean difference of 0.013 ft and a
maximum of 0.05 ft. The greatest differences
occurred on the soft upper part of the beach
where the rods could sink slightly, on the
steeper part where a small difference, of rod
position made a large difference in elevation, or in the swash zone where water movement undermined the rods. When summed
up as complete profiles (Table 1), the mean
difference at any point was 0.035 ft and the
maximum was 0.18 ft. The difference generally becomes greater with distance from
the starting point because of the accumulative tendency of the errors; however, the
maximum difference is less than twice the
width of the line representing the profile on
Figure 2.
For somewhat more precise work an
allowance should he made for the fact that
the line of sight to the horizon is not quite
horizontal, Owing to curvature of the earth
surface. The angle between the horizon and
a level line increases according to the
height of the observer above the water, but
on beaches this height is such that the angle
is only a few minutes (Table 2 ). When
curvature correction is applied to the measured profile, it is obvious that the true slope
is slightly greater than the measured apparent slope. For the profile of Table 1 and
Figure 2 the true slope is steeper by 0.12 ft
in a distance of 160 ft, or by less than 0°0:3'.
On profiles of several hundred feet length
this correction may become important.
Where the horizon cannot be seen, as from
a lake beach, the true slope can still be obtained, hilt the approximate distance to the
opposite shore or other reference point must
92
NOTES AND COMMENT
TABLE 1.
Horizontal
distance
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
12.5
130
135
140
145
150
155
160
165
Comparison of beach profiles made with rods and and with plane table (1:
Rod readings
A
B
Elevations from rods
C
-.36 -.35 - .35
_ .90 _ .23 - .24
- .12 - .13 - .13
-.05 -.06 - .06
+.0 +.01 +.01
- .13 - .13 - .13
- .08 - .05 - .06
- .20 - .20 - .21
- .21 - .20 - .20
.49 - .50 - .50
- .69 - .68 - .68
- .76 -.78 - .77
- .70 - .70 - .70
- .76 - .75 - .75
.64 -.64 - .66
- .95 - .93 - .94
- .92 - .90 - .95
- .72 - .74 - .74
- .68 - .67 - .69
.63 - .64 - .66
- .57 -.56 - .57
- ..57 - .57 - .58
- .52 - .52 - .52
- .46 - .45 - .46
- .42 - .41 - .41
- .38 - .38 - .38
- .33 - .33 - .33
-.30 - .30 - .31
-29 - .30 - .29
24 - .21 - .20
- .22 .19
- .17 - .17 - .16
- .14 - .17 - .14
Varia- Corr. for Cumu- Corrected Elevation E.ev. by
lion
elevation lative
memo
elevation
by
per 5 ft
Corr.
C
plane table Eev. b1 F.
A
B
C
A, B, C
15.00
14.64
14.92
14.30
14.25
14.26
14.13
14.05
13.85
13.64
13.15
12.46
11.70
11.00
10.24
9.60
8.65
7.73
7.01
6.33
5.70
5.13
4.56
4.04
3.58
3.16
2.78
2.45
2.15
1.86
1.62
1.40
1.23
1.09
15.00
14.6.5
14.42
14.29
14.23
14.24
14.11
14.06
'13.86
13.66
13.16
12.48
11.70
11.00
10.2.5
9.61
8.68
7.78
7.04
6.37
5.73
5.17
4.60
4.08
3.63
3.11
2.84
2.51
1.11
1.91
1.70
1.50
L33
1.16
15.00
14.65
14.41
14.28
14.22
14.23
14.10
14.04
13.83
13.63
13.13
12.45
11.63
10.98
10.23
9.57
8.63
7.68
6.94
6.25
5.59
5.02
4.44
3.92
3.46
3.05
2.67
2.34
2.03
1.74
1.54
1.35
1.19
1.05
0.00
.01
.01
.02
.03
.03
.03
.02
.03
.03
.03
.03
.02
.02
.02
.04
.05
.10
.10
.12
.14
.15
.16
.16
.17
.17
.17
.17
.18
.17
.16
.15
.14
.11
he known in order to make the computations.
A check on the profiles which were measured with rods was made by a telescopic
TABLE. 2. Correction for curctiture of earth surface
( From Boss-ditch, American Practical Navigator )
Eye height
(ft)
Angle to
horizon
Vertical difference for
5 ft horizontal distance.
0
5
10
15
0°00Ur
0°02'11"
0°03'06"
0°03'48"
0°04'23"
0°04'54"
(1.0000
.0030
.0045
.0055
.0063
.0072
20
2,5
0.005
.00.5
.005
.005
.005
.005
.005
.005
.003
.005
.005
.005
.005
.005
.004
.004
.004
.004
.004
.003
.003
.003
.003
.003
.002
.002
.002
.002
.001
.001
.001
.001
.001
.001
0.00
.01
.01
.02
.02
.03
.03
.04
.04
.05
.05
.06
.06
.07
.07
.08
.0t, .
.08
.09
.09
.09
.10
.10
.10
.11
.11
.11
.11
.11
.11
.12
.12
.12
.12
15.00
14.69
14.40
14.26
19.20
14.20
14.07
14.00
13.79
13.58
13.08
12.39
11.62
10.91
10.16
9.49
'8.55
7.60
6.85
6.16
5.50
4.92
4.34
3.82
3.35
2.94
2.56
2.23
1.92
1.63
1.42
1.23
1.07
15.00
14.64
14.40
14.2.5
14.18
14.18
14.02
13.95
13.72
13.49
12.99
12.30
11.51
10.80
9.99
9.39
0.00
.00
.00
.01
6.10
5.46
4.86
4.29
3.76
3.30
2.88
250
2.14
1.87
1.50
1.36
1.20
0.95
.06
.04
.06
.05
.06
.05
.06
.06
.09
.05
.13
.06
.03
.12
.02
.05
.05
.07
.09
.09
.11
.11
.17
alidade on a plane table and a stadia rc
The result (Table 1) shows differences th-:
are the same order as those between r
results obtained with rods by differe:
observers. The use of a surveyor's alid2:
or level avoids- the problem of aecumulat:
error inherent in use of the rods, but it
more subject to error of reading off elev,
tion. Usually where an alidade or a level
used, the beach profile is drawn on the bps
of fewer points than are obtained with tr
5-ft rods. In addition to their greater spec.
the wooden rods are useful in remote are;
where heavier and expensive surveyor's i:
struments may not he easily available.
93
15
0
ZS
.
50
DISTANCE IN FEET
75
100
I
125
150
i
J
td
la
..J
W t:
ol
—
.
\
\
Z
W
I
W
C
'''..'........„............„..„___......._,._.....,
I.Id
Li)
LL
SWASH ZONE
0
I
;
I
1
F ic . 2.. Profile of beach at Will Rogers Beach State Park about two miles northwest of Santa Monica,
califomia, the site of repeated measurements of Table I. Rased on measurements of Table I by observer
Cuxorr-xted for earth curvature.
It is possible that other workers may find
the simple wooden rods useful for measuring profiles of beaches to determine seasonal
I nd other cyclic changes with respect to
graves, to relate slope to grain size of sand,
Ind for other purposes. For most such objectives the method appears to possess sufficient accuracy, particularly in view of the
fact that the presence of cusps and other
irregularities produce local variations in
profile which are greater than the error of
measurement by the rods.
K. 0. EMERY
University of Southern California
Los. Angeles, California
JOURNAL OF SEDIMENTARY PETROLOGY, VOL. 47, No. 1, P. 860-863
Flo. I, JUNE 1977
Copyright Q 1977, The Society of Economic Paleontologist. and Mineralogists
(if)
ONE-MAN PROFILING METHOD
861
A ONE-MAN PROFILING METHOD FOR BEACH STUDIES'
W. J. STEPHEN'
Terrain Sciences Division, Geological Survey of Canada, Ottawa, Ontario, Canada K IA 0E8
AssntacT: An instrument system is described whereby beach profiles may be surveyed by one man
without the aid of a field assistant. The accuracy of the method is tested and shown to give results
within about i .07 feet vertically with traverse legs less than .10
in Itoriconl it l distance. 'the
procedure has additional appeal because the equipment Is Inexpensivefret
and portable and the profile can
usually be completed more quickly than by using a level and rod.
INTRODUCTION
EQUIPMENT
Studies of the coastal zone nearly always inThe equipment (Fig. 1), consists of a dozen
clude measurements of changes to the subaer- lengths of thin gauge aluminum tubing, each
ial beach profile over time. Most commonly, 2/3 inches long by 1/4 of an inch in diameter. An
these are carried out at times of low tide, with inch of brightly colored tape is wound around
a telescopic level and a surveying rod. Breaks
the top of each tube for easy visibility, and the
in slope along the shore-normal profile are tops are numbered from one to twelve.
marked. Starting with a backsight to a bench- Number 8 fencing wire is run through each of
mark on the backshore to establish a "height the tubes so that about 8
inches protrude from
of instrument," the survey rod is sighted at the bottom, and a loop is formed
on the top of
each break in slope as the rod man progresses the wire to prevent it front falling through the
seaward as far as is prudent. Elevations are tube. The sighting instrument
is a clinometer
calculated by subtracting the rod readings manufactured by Suunto of Helsinki,
arid is
from the "height of instrument," and horizon- widely available. It is of the floating card type,
tal distances between the stations are calcu- graduated both in degrees and percent grade.
lated front the interval between the upper and
A brass frame with a ball joint on the bottom
lower stadia hairs.
is mounted on a standard camera tripod from
This method, with only minor modifica- which the pan head ltas been removed. The
tions, is the one in most widespread use and frame holds the clinometer by means of a
has much to recommend it. It is simple, rea- knurled grub screen, and is hinged so that the
sonably fast, and accurate. K. 0. Emery has clinometer may be tilted with a slow motion
also developed a useful method of profiling tangent screw. A stainless steel spring holds
that uses two men and two wooden rods,
two hinged sections together, and provides
eliminating the need for a telescopic level atheresistance
against which the tangent screw
(Emery, 1961).
turns. The brass frame is constructed so that
The purpose of this paper is to describe an
the hinge axis intersects the optical axis of the
alternative method of obtaining beach profiles instrument.
In this way, the "height of instruwhich, while not intended to replace either of ment" remains constant as the clinometer is
the techniques mentioned above, offers impor- b rought onto the target. The
hinge pin is tapped
tant advantages III that it is quicker and requires only one man with a minimum of to accept a small brass machine screw
from the center of which extends a 12 inch
equipment to carry out the survey.
length of nylon monofilament. A small glass
' Manuscript received January 8, 1976; revised spirit level, encased in plastic tubing for proSeptember 22, 1976.
tection and scaled at each end with nylon
l'resent address: Beak Consultants Limited,
plugs
3530—I IA Street N. E., Calgary, Alberta T2 E 61117. at-Dent is permanently mounted on the moitufil-
the system will show that it is important that
FIELD METHOD
The beach profile is surveyed by walking it the hinge axis of the instrument be horizontal
and that its elevation be coincident with the
four times.
On the first leg, the number one rod is run top of each rod. Coarse adjustment to these
into the ground at the base station so that the requirements consists of simply pushing the
bottom of the tube rests on the station, and the appropriate tripod leg deeper into the ground.
rod is vertical. At the first break in slope the Fine adjustment of the horizontality of the
number two rod is similarly placed so that the hinge axis is done by means of the ball joint
bottom of the tube rests on the ground. The which is then locked in place. The instrument
remaining rods are placed consecutively down is then raised or lowered to the same elevation
the profile at each break of slope until a rod as the top of the aluminum rod using the rack
has been placed at the top of the swash zone. and pinion on the center post of the tripod. On
An extra rod is left there for later placement clear days the sea-sky horizon is used as a
shore-parallel horizontal reference. On cloudy
seaward of the swash limit.
On the second leg, which is a return trip to days, or when the horizon is obscured, the
the base station, the slope distances between monofilament is laid across the top of the alustations are chained and entered in the field- minum rod and the center post racked up or
book. A short length of wire attached to the down until the bubble in the spirit level is cenend of the chain and run into the ground at tered. The top of the next rod on the profile is
each station anchors it firmly enough for the then sighted and the clinometer brought on
tape to be tensioned. After the slope distance is target by means of the tangent screw. The verrecorded, a light pull is sufficient to release the tical angle is read to the nearest one tenth of a
degree and entered in the fieldbook. These
tape front the ground.
On the third, seaward kg, the instrument is steps are followed at each station until the one
set close to the right-hand side of each alumi- at the top of the swash zone is reached. here
num rod. A consideration of the geometry of the instrument is set up as before . and the most
•
862
W. J. STEPHEN
TABLE I.-Levelled
and clinometer-derived station elevations
(51
Base
10
20
30
40
50
60 •
(2)
A
a
100.00
98.50
98.43
97.71
97.99
97.69
97.66
98.54
98.46
97.79
98.01
97.60
97.75
98.52
98.46
97.74
97.87
97.60
97.75
98.48
98.46
97.64
98.01
97.69
97.75
(5)
98.51
98.38
97.79
97.87
97.60
97.64
98.52
98.35
97.79
98.01
97.69
97.54
(4)
(5)
High
Low
M
98 .51
+.01
-.01
+.04
-.04
-.05
+.03
+.04
+.03
+.08
+.02
0.0
+.09
-.02
-.08
-.07
-.12
-.09
-.12
.02
.04
.07
.06
.05
.08
98.42
97.75
97.95
97.64
97.69
(I) Stations.
471 Levelled elevations for each 'lotion.
(3) Llevation. calculated 1,0111 tIlitometer readings for five observers,
(41 Mean station elevations calculated from five clinometer readings,
(51 Deviations of mean station elevations from levelled elevations.
(6) Mall)11111111 deviations of clinometer-derived elevations from levelled elevations.
(7) Alvan values of the absolute difference' between the levelled elevation for each station and Its elevation La
derived by clinometer.
seaward station is placed and chained as the
backwash recedes. Depending upon a number
of variables including the swash period, the
solidity of the lower foreshore sediments, the
beach slope, and the nimbleness of the field
man, the vertical angle to the top of the last
rod can often be read before the next swash
advance.
This completes the surveying of the profile,
and all that remains on the fourth and final leg
is to pick up the equipment. At this time, of
course, any notes on surface texture, vegetative cover, occurrence of cusps, etc. may also
be made and related to each station.
The field notes contain vertical angles and
slope distances between each pair of numbered
stations. These represent one angle and the
hypotenuse, respectively, of a series of right
angled triangles. The horizontal distance between stations may easily be found by multiplying the slope distance by the cosine of the
vertical angle, and the difference in elevation
by multiplying the slope distance by the sine of
the vertical angle.
DISCUSSION
•
The amount by which the station elevations
along the profile depart from their true value
depends primarily upon the accuracy with
which the vertical angle is read. Specifically,
at 10 feet, an angular error of one tenth of one
degree subtends a difference in elevation of
.0175 feet, Accordingly a good deal of care is
necessary in reading the vertical angle. Unfortunately, the Suunto clinometers used by the
writer commonly had significant index errors.
Despite scale divisions of one degree which
863
ONE-MAN PROFILING METHOD
with care can be estimated to one or two tenths
of a degree, the writer found that even new
units are sometimes in error by as much as two
degrees. Although regrettable, this problem is
easily remedied. By considering up-slope
readings to be positive and down-slope
readings to be negative, it follows that the
algebraic suns of two reversed readings taken
on any plane surface represents twice the instrumental error. Correction factors for the
range of slopes to be measured which cancel
this error can therefore be calculated by halving the instrumental error and changing the
sign. The corrected angles are then used in all
subsequent calculations.
The practicability of using this method of
surveying was confirmed on a number of natural beaches, but in order to empirically quantify both the precision and accuracy of the
technique at various sighting distances, and
hence its potential for application under a
wider variety of topographic conditions, a
simulated traverse was set up consisting of a
base station and six profile stations located at
successive ten-foot intervals down the section.
Each of the profile stations was sighted in turn
from Use instrument setup at the base station.
Other than this uniform station spacing, the
equipment used and the procedures followed
were us described above. Arbitrarily setting
the elevation of the base station to 100.00 feet,
the elevation of each profile station was determined to within ±.01 feet with a rod and
telescopic level. Five observers, none of whom
had previous familiarity with the technique,
were then asked to survey Iho profile timing the
clinometer, The surveys were run intlepen-
dently. The calculated results, with the values
expressed in feet, are shown in Table I.
Column 5, which may be regarded as the
residual error in elevation at each station
which remains after operator bias has been
minimized, shows that the mean of the
clinometer-derived values are in fairly close
agreement with the levelled values. However,
inasmuch as operator bias can not usually be
reduced in a set of observations, column 7 is
more indicative of the actual errors that can be
expected ut Increasing distances down the profile. For example, at the 60-foot station, although the mean elevation of all five observers
Is reasonably accurate in that it falls within .03
feet of the levelled elevation (Column S), the
precision of the measurements is rather low.
Deviations about the levelled elevation range
from .09 feet too high, to .12 feet too low (Column 6). The value of .08 in column 7 is the
mean of these deviations from the levelled
value for the five observers at this station. A
comparison of columns 7 and 5 shows that,
although the method possesses a certain
amount of inherent accuracy out to 60 feet
(Column 5), losses of precision (or visual acuity) appear to become important between 20
and 30 feet from the instrument (Column 7). If
errors in elevation are to be kept to a minimum
(i.e., <.07 feet), then the sighting distance
from the instrument should be less than 30
feet.
It will be clear from the foregoing discussion
that as far as coastal work is concerned, this
method of profiling is especially applicable
either to high wave energy coasts where substantial volumes of material are reworked on
the foreshore, or to coarse textured beaches
where accurate surveying to within .01 feet is
pointless when the beach face is composed of
pebbles and cobbles.
CONCLUSION
In the light of other techniques for mapping
beach profile configurations this method offers
three main advantages.
First and most important, the surveying can
be done without the aid of a field assistant.
This reduction in manpower in itself probably
justifies its use in situations where either there
is a scarcity of personnel or where it is desirable to map a number of profile sites contemporaneously. Second, a small amount Of inexpensive equipment is all that is required for
the work. It is portable and both rugged and
relatively maintenance-free. Finally, compared to three wire levelling, in all but those
cases where only one instrument setup is required, the survey can be completed in less
(line with the clinometer than it can with a
level and rod.
ACKNOWLEDGMENTS
The methods described here were developed
by the writer while he was a Ph.D. student at
the University of Canterbury in Christchurch,
New Zealand. Sandy Gall of the Geology Department at Canterbury deserves thanks for
building the brass frame of the instrument.
The writer is also grateful to C. F. M. Lewis of
the Terrain Sciences Division, Geological Survey of Canada, for a critical reading of the
manuscript.
REFERENCES
K. 0.,
1961, A simple method of measuring beach profiles: Linmol. and Oceanog., v. 6,
EMERY,
p. 90-93.
STEPHEN, W. J., 1974, Wave processes and beach
responses on a coarse grained gravel delta: unpublished Ph.D. thesis, University of Canterbury, Christchurch, New Zealand, 395 p.
Fr. 1 . Sketch of equipment—two wooden
,- 5 ft long and marked off in feet and tenths of
—.....u.secl for measuring profiles of beaches.
ar beach. The observer holding the land::rd rod aligns his eye with the top of the
,award rod and the horizon. He then reads
' A,irecords the distance down from the top
/ hi own rod of the point which is inter' ,-ted by this line of sight (Fig. 1), interpo,,c t o hundredths of a foot. Assuming the
,..,, of sight to be level, this distance is a
..i .sure of the difference in elevation of the
;ea ch at the two points that are 5 ft apart ( in
example of Figure 1, the difference in
cent elevation is 0.53 ft 1. Where the
-,:"
,,a, has a hackslope, the difference in ele,eion can be read by the same observer.
„ aligning the top of his Own rod with the
wiiz on and reading the intersection on the
cjward rod. To conthlue the profile, one of
dis moved to a point 5 ft on the opporods
ro
.,.. 5 ide of the other rod and a second read... i s
... made., Profiles can be measured either
;,, movin
g
rods toward the sea or away
;.:,,,, t he sea. For fixing the direction of
;Iv t he convention has been adopted of
.i i n g t he differences in elevation as minus
j ..Aus according to whether the leading rod
ilwer or higher than the following one.
;-r,.iliy, the differences in elevation are
..--,I
Hned
up and plotted against horizontal
stance in order to obtain a profile across
,,
– „.hole width of the beach.
usuall
C
so y the profiles extend from the base
j; sea cliff or a point on the landward side
4 , beach to below water level. True eleva;11 sometimes can he determined from a
..venient bench mark, but more commonly
•,sust he estimated from the depth of water
,
f
at the seaward end of the profile as compared with a table of predicted tide. Where
comparison of profiles at different times is
desired, it is often sufficient merely to relate
each set of measurements to a stake or other
permanent reference point.
As a test of the reproducibility and accuracy of the method profiles were measured
three times, with three different readers,
over the same line across a beach near Santa
Monica, California (Fig. 2 ). The results
(Table 1) show a surprising consistency,
with readings across individual 5-ft sections
having a mean difference cf 0.013 ft and a
maximum of 0.05 ft. The greatest differences
occurred on the soft upper part of the beach
where the rods could sink slightly, on the
steeper part where a small difference of rod
position made a large difference in elevation, or in the swash zone where water movement undermined the rods. When summed
up as complete profiles (Table 1), the mean
difference at any point was 0.03.5 ft and the
maximum was 0.18 ft. The difference generally becomes greater with distance from
the starting point because of the accumulative tendency of the errors; however, the
maximum difference is less than twice the
width of the line representing the profile on
Figure 2.
For somewhat more precise work an
allowance should be made for the fact that
the line of sight to the horizon is not quite
horizontal, Owing to curvature of the earth
surface. The angle between the horizon and
a level line increases according to the
height of the observer above the water, but
on beaches this height is such that the angle
is only a few minutes (Table 2 ).
When
curvature correction is applied to the measured profile, it is obvious that the true slope
is slightly greater than the measured apparent slope. For the profile of Table 1 and
Figure 2 the true slope is steeper by 0.12 ft
in a distance of 160 ft, or by less than ,0°03'.
On profiles of several hundred feet length
this correction may become important.
Where the horizon cannot be seen, its from
a lake beach, the true slope can still be obtained, but the approximate distance to the
opposite shore or other reference point
must
•
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