Towards an improved understanding of erosion rates and tidal notch development on limestone coasts in the Tropics: 10 years of micro-erosion meter measurements, Phang Nga Bay, Thailand

Towards an improved understanding of erosion rates and tidal notch

development on limestone coasts in the Tropics: ten years of micro-

erosion meter measurements, Phang Nga Bay, Thailand

Cherith Moses1, David Robinson1, Miklos Kazmer2 and Rendel Williams1

1Department of Geography, University of Sussex, Brighton, GB

2Department of Palaeontology, Eotvos University, Budapest, Hungary

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/esp.3683

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Abstract

Knowledge and understanding of shore platform erosion and tidal notch

development in the tropics and subtropics relies mainly on short-term studies

conducted on recently deposited carbonate rocks, predominantly Holocene

and Quaternary reef limestones and aeolianites. This paper presents erosion

rates, measured over a ten year period on notches and platforms developed

on the Permian, Ratburi limestone at Phang Nga Bay, Thailand. In so doing it

contributes to informing a particular knowledge gap in our understanding of

the erosion dynamics of shore platform and tidal notch development in the

tropics and subtropics - notch erosion rates on relatively hard, ancient

limestones measured directly on the rock surface using a microerosion meter

(MEM) over time periods of a decade or more.

The average intertidal erosion rate of 0.231 mm/yr is lower than erosion rates

measured over 2 – 3 years on recent, weaker carbonate rocks. Average

erosion rates at Phang Nga vary according to location and site and are, in

rank order from highest to lowest: Mid-platform (0.324 mm/yr) > Notch Floor

(0.289 mm/yr) > Rear notch wall (0.228 mm/yr) > Lower platform (0.140

mm/yr) > Notch roof (0.107 mm/yr) and Supratidal (0.095 mm/yr). The micro-

relief of the eroding rock surfaces in each of these positions exhibit marked

differences that are seemingly associated with differences in dominant

physical and bio-erosion processes. The results begin to help inform

knowledge of longer term shore platform erosion dynamics, models of marine


notch development and have implications for the use of marine notches as

indicators of changes in sea level and the duration of past sea levels.

Keywords Micro-Erosion Metre, shore platform, tidal notch, limestone, marine

erosion, sea-level indicator, Thailand


Introduction

This paper presents supratidal, above high water, and intertidal, between high

water and low water, notch and shore platform erosion rates measured over a

period of ten years on hard, crystalline Permian Limestone in a tropical setting

at Phang Nga Bay, on the Andaman coast of Peninsular Thailand. In doing so

it addresses a particular knowledge gap in our understanding of the erosion

dynamics of marine notches and platforms more broadly in the tropics –

marine erosion rates on relatively hard, crystalline, limestones that are much

older than weaker Quaternary and Holocene reef limestones and aeolianites

on which, to date, most data have been gathered, and which are measured

directly on the rock surface using a microerosion meter (MEM) over a time

period of more than two to three years (Moses, 2013). The results contribute

to knowledge and understanding of longer-term marine notch erosion

dynamics, models of marine notch development and the use of marine

notches as indicators of sea level change and the duration of past sea levels.

Marine notches on rock coasts in the tropics

Rock coasts in the tropics exhibit a suite of morphological features that

appear to differ in the extent of their development from those of other climatic

regimes (Emery and Kuhn, 1982), (Sunamura, 1992), (Trenhaile, 1987), (Bird,

2004), (Fairbridge, 2004), (Finkl, 2004). These include cliffs with intertidal

notches and quasi- or sub-horizontal platforms that often terminate with a

steep low tide cliff or ramp (Sunamura 1992), (Trenhaile, 1980), (Trenhaile,

2002), (Trenhaile, 2010). Occasionally at the foot of the low tide cliff there is a


notch whose origin is often ascribed to a prior sea level (Sunamura, 1992),

(Dickson, 2006).

Particularly steep, vertical to sub-vertical, coastal cliffs are often associated

with drowned limestone tower landscapes, which are common in the tropics

and widespread throughout Southeast Asia (Bird, 2004), (Gillieson, 2005). In

inland settings the steep slopes of tropical limestone towers, Turmkarst, are

maintained by the development of basal notches that undercut the sides and

are usually formed by solution (McDonald and Twidale, 2011). In coastal

settings, however, the steep tower sides are considered to be maintained by

marine undercutting through the development of deep cliff foot notches e.g.

(Kiernan, 1994), (Waltham, 2005). The rate of notch development influences

the development and frequency of occurrence of rockfalls (Kogure, et al.

2006) (Kogure and Matsukura, 2010).

The intertidal notches and platforms of tropical rock coasts are commonly

used to provide evidence of former sea levels for the reconstruction of

Quaternary and late Holocene sea-levels and to estimate tectonic movements

e.g. (Pirazzoli, 1996), (Tija, 1996), (Woodroffe and Horton, 2005), (Bhatt and

Bhonde, 2006), (Smithers, 2011), (Evelpidou et al., 2012), (Pirazzoli and

Evelpidou, 2013). In such cases they are generally considered to be

associated with wave erosion, e.g. (Blanchon and Jones, 1995), although

weathering, particularly solution in the case of limestone, is sometimes given


more importance e.g. (Trenhaile, 2002), (Trenhaile, 2004), (Trenhaile, 2008),

(Trudgill, 1983), (Trudgill, 2011a, b).

On limestone coasts in the warm, micro and mesotidal seas of the tropics

marine notches typically develop in the mid-littoral zone. As many as six notch

profile types have been recognised e.g. (Takenaga, 1968), (Föcke, 1978), but

Pirrazoli (1986) identifies two main types of mid-littoral notch: tidal notches,

associated with sheltered coasts and cut wholly or partly in the intertidal zone,

and surf notches, usually cut above high tide level and associated with

exposed sites and the presence of a surf bench that protrudes seawards by

up to as much as two metres above high tide. Tidal notches, with their

characteristic recumbent V-shaped or U-shaped profile forms are the most

common and most useful sea level indicators (Evelpidou et al., 2012),

(Pirazzoli and Evelpidou, 2013).

Marine notches developed on limestone coasts are thought to be more

directly and closely associated with sea level than on other rock types, e.g.

sandstone or granite (Twidale and Bourne, 1976), (Kelletat, 1988), (Twidale,

1986), (Wray, 1997), (Twidale et al., 2005), (Pedoja et al., 2011) where the

level of the notch may be controlled by rock structure or the erosion of

weathered regolith rather than by direct marine action. Hence they are

considered more reliable as indicators of stillstands, sea-level change and of

tectonic movement, e.g. (Pirazzoli, 1996), (Tija, 1996), (Nunn et al., 2002),

(Ramírez-Herrera et al., 2004), (Pedoja et al., 2011), (Evelpidou et al., 2012),


(Pirazzoli and Evelpidou, 2013), than are notches developed on other rock

types. Because of their role in the development of rockfalls and consequent

modification of cliffs and their global use as indicators of past sea levels and

stillstands it is important to improve understanding of the rates and modes of

development of tidal notches. This is particularly the case in the warm seas of

the tropics where existing knowledge is based on predominantly short term

studies conducted on relatively young lithologies the majority of which are reef

limestones and aeolianites.

Rates of erosion and of marine notch development on rock coasts in the

tropics

Rates of erosion and of notch development in the tropics have been

measured directly and indirectly yielding averages ranging from 1.3 to 3.03

mm/yr, depending on the method used (Moses, 2013). The most reliable rates

are likely to be those derived from direct measurement using erosion pins and

Micro Erosion Meters (MEMs), although these are usually specific to a very

short time period of no more than two to three years’ duration. The lower

average intertidal erosion rate of 1.3 mm/yr is calculated on the basis only of

direct measurements (N = 10; Moses, 2013).

In addition to being based principally on erosion rates that have been

measured over only short time periods of two to three years, present

knowledge of tropical intertidal erosion rates is predominantly limited to


intertidal notches, neglecting shore platforms and is almost entirely limited to

relatively young lithologies, such as beachrock and reef limestone, although

the nature of the limestone is not always clearly described or dated. Only one

study of marine notches in the tropics has measured intertidal erosion rates

over more than ten years, and this is for a thirteen year period on Quaternary

reef limestone on Aldabra, Indian Ocean (Viles and Trudgill, 1984; Table 1).

A series of studies, using the MEM and Traversing MEM (TMEM, t-MEM),

have measured marine notch development in the warm, but extra-tropical,

Adriatic Sea and whilst most of these are short term studies two include data

collected over almost 17 years (Furlani et al., 2011) and up to 22 years

(Furlani et al., 2009; Table 1). Unfortunately the data collected over different

time periods are not differentiated in such a way that they can be extracted to

compare values collected over shorter time periods of 1 – 2 years with the

longer term datasets of > 15 years. However, the average intertidal erosion

rates recorded on these Cretaceous limestones in a Mediterranean setting are

generally an order of magnitude lower than those collected on more recent

reef limestones in a tropical setting. It is unclear if this difference is due to the

rock type, the data collection period or some other factor. The current study is

designed to help unravel these factors through an ongoing, long term MEM

measurement programme of erosion rates on intertidal notches and platforms

developed in Permian limestone at Phang Nga Bay, Thailand. This paper

presents a report of the first ten years of measurements.


Field Site description

Phang Nga Bay is located on the Andaman coast of peninsular Thailand at

8.2833° N, 98.6000° E (Figure 1a). The lithology is Ratburi limestone, which is

approximately 500-800m thick and Permian in age. It is a bioclastic packstone

and grainstone (Dill et al., 2005). The Ratburi limestone is considered by

Kiernan (1994) to be broadly equivalent to the Mawlamyine Limestone of

Southern Burma and the Chuping Limestone of Malaysia. It is more massive

than the Carboniferous to Devonian limestones of Ha Long Bay in Vietnam

(Fenart et al., 1999), (Waltham, 2005). The towerkarst of the Phang Nga and

Ao luk region are the best known of the Thai towerkarst. The karst is

characterised by strike ridges, trending N-S or NE-SW parallel to the principle

faults, and isolated towers of limestone up to 600 m high and with very steep

to vertical sides. The isolated towers are thought to be related to a series of

transverse NW-SE trending faults. Inland the towers rise from alluvial plains

and at Phang Nga Bay, and further offshore, they project out of the sea. The

slopes of the towers are steeper on the coast than inland and those of the

limestone islands in the bay are even more steep. This is considered to reflect

the importance of marine erosion and notches occur at present day sea level

and up to heights of 10 to 15 m above mean sea level (Kiernan, 1994).

Average compressive strength of the Ratburi limestone, measured using a N-

type Schmidt Hammer on smooth rock surfaces is more than 600 kg/cm2 .

The annual average rainfall at Phuket, just to the west of Phang Nga, is 2388

mm, with a maximum of 6606 mm, which falls almost entirely during the

southwest monsoon between May and August. The yearly mean diurnal


temperature is 260 to 300 C, with maximum temperatures reaching 37 0 C in

March and April falling to just under 200 C in the cool season during October

to January. The mean tidal range in Phang Nga Bay during spring tides is 2.5

to 3.0 m, with a maximum of approximately 4 m. Waves reach the west coast

from the west, northwest and southwest for most of the year but from the

south in July. Because the site is protected by Phuket Island which extends

into the Andaman Sea on the western side of Phang Nga Bay, the fieldsite is

affected only by waves from the south and southwest associated with the

southwest monsoon. During this period significant wave heights, hs are 0.3 to

1.5 m and significant wave periods, Ts, range from 4.5 to 7 seconds. The

2004 tsunami had a wave height of 5 – 8 m on the west coast of Phuket but

Phang Nga bay was protected. The bathymetry of Phang Nga bay is a

network of drowned valley systems that lie approximately 23 to 26 m beneath

present day sea level. This causes an increase in wave height, particularly of

waves entering the bay from the southwest and, at nearby Laem Pho, wave

energy on the coast has been calculated to range from 4,500 to 7,200 kg/m2

(Kiernan, 1994), (Weesakul et al., 1997).

Methodology and study locations

Three Study Locations on Phang Nga Bay were selected on the basis of their

notch occurrence at present day sea level, orientation and the presence or

absence of sand Figure 1b, 2). At Study Location 1 the platform, if it is

present, is obscured by fine-grained sand and the floor of the notch that is

visible has a biological accretion. The platform at Study Location 2 is

obscured by reef rubble and the floor of the notch has a biological accretion.

At Study Location 3, the platform is visible, is partially covered by a coarse


grained sand beach and in places has a biological accretion. The morphology

of each study location was profiled using a Leica Disto D8 handheld laser

distance meter and the methodology is described by Kázmér and Taboroši

(2012a).

Erosion rates have been measured using a standard MEM, of the type

described in Swantesson et al. (2006), based on the original design of High

and Hanna (1970), which, by rotating the three legs of the MEM on studs fixed

into the rock, provides three measurement points at each measurement site.

In 2002 40 MEM sites were installed across the three Study Locations in

different positions above and within the tidal notch, giving 120 points in total.

Large, round-headed brass screws were fixed into the rock surface to act as

the fixed points onto which the legs of the instrument are placed to take

readings. Screws have been used in preference to ball-bearings used by High

and Hanna (1970) as they have been found to last longer in marine

environments (Viles and Trudgill, 1984). Although brass is softer than

stainless steel, the authors have found brass to be more resilient in the

marine environment, where regular inundation by seawater sometimes

causes stainless steel bolts and screws to rust, and brass screws have been

used successfully in storm wave environments for a number of years (Foote

et al., 2006), (Moses and Robinson, 2011), (Swantesson et al., 2006). Except

where heavily abraded adjacent to coarse gravel beaches, brass does not

suffer significant abrasion and the dome screws remain in good condition for

many years. Figure 4 a and b show brass screws in good condition after more

than ten years of exposure.


Approximately 50% of these measurement points were readable after a

decade located as follows: supratidal, on the top of a large boulder set back

approximately ten metres from the notch (4); roof of notch (9); back wall of

notch (14); notch floor (19); mid platform (8); lower platform (8). The MEM

sites that could not be read had either lost one of the brass studs; the probe

no longer reached the rock surface, or one or more of the studs was unstable

making any reading unreliable. At a few of the measurement points on the

rear wall of the notch the MEM gauge faced downwards and could not be

read. One MEM site, at Study Location 2, was completely covered by

biological encrustation and could not be measured. The erosion rates

reported in this paper are based on the measurement of the 62 points detailed

above over the period 2002 to 2012.

For practical reasons such as the height of the notch roof, the presence or

absence of a platform, or accessible sub-aerial surfaces, it was impossible

from the outset to install a full range of measurement sites at each location.

This situation was exacerbated over the 10 years of monitoring by the loss of

50% of measuring points. Thus, Study Location 3 has the greatest number

of monitoring points, with 39, followed by Study Location 2 with 14 and Study

Location 1 with 9). Only the notch floor was measured at all three Study

Locations, the notch roof at two Study Locations, the rear of the notch, the

platform and a sub-aerial location at just one Study Location each. In 2011 a

further 15 MEM measurement sites, 45 measurement points, were installed,


and in 2014 a further 36 sites, 115 points, to improve the spatial distribution of

montitoring sites and ensure continuation of the measurement programme

over the longer term, but data from these points are not discussed in this

paper.

Taking into account environmental factors, instrument wear and operator

error, individual MEM readings are thought to be correct to 0.05 mm if the

instrument is carefully calibrated (Spate et al., 1985; Swantesson et al., 2006).

When mean values for locations and sites within locations are derived from

individual measurement points, the level of precision of the mean is

influenced by the number of measurement points used to calculate the mean

value and these should preferably be greater than ten (Trenhaile and Lakhan,

2011). Designed originally for use on flat or gently sloping surfaces, MEM use

on more steeply sloping and vertical or near vertical surfaces is more

controversial. It has been used on steeply sloping and vertical surfaces by a

number of authors, with no real discussion of accuracy when used on non-

horizontal surfaces e.g. (Sharp et al., 1982), (Trudgill, 1976), (Trudgill et al.,

1989, 1990, 1991, 2001), (Viles and Trudgill, 1984) (Smith et al., 1995),

(Furlani et al., 2009). Stephenson and Finlayson (2009) suggest that the

effectiveness of the Kelvin clamp is reduced when the bolts are not on a

horizontal plane and do not recommend placing the MEM on sloping or

vertical surfaces. No data are presented to substantiate this recommendation.

Sunamura (2014) encountered difficulty in applying a conventional MEM to a

steep slope of 800 and so modified it to include three gauges but does not

explain how this addresses the difficulty. Furlani et al. (2009, p77) draw a


distinction between the use of standard and traversing MEM’s on non-

horizontal surfaces stating that ‘The sites located in the vertical walls, a rather

frequent case in the coastal sector, are analysed by using the MEM, as the t-

MEM can lead to incorrect readings’.

Given the uncertainty, a test of the accuracy of readings of the standard MEM,

used in this study, when placed repeatedly on the studs in horizontal, vertical

and inverted positions was carried out. Using the calibration plate, sets of 10

individual MEM readings were taken as follows: 6 sets with the with plate and

the MEM positioned horizontally with the three legs located on the studs in

identical orientation; 6 sets after rotation of the MEM to place the legs in the

second orientation and a further 6 in with the legs in the final orientation. This

was repeated with the plate and MEM in the vertical and inverted positions.

This provided 9 sets of 6 readings for each of the horizontal, vertical and

inverted positioning of the MEM. In all cases the standard error of the

measurements for repeat measurements in each positioning plane were very

small, <0.005 mm, and there was no significant difference in the errors

obtained between repeat measurements taken in each plane (Table 2). In

obtaining this level of repeat accuracy the legs of the MEM were always

relocated first on the hemispherical hollow, then the wedge and finally the flat,

and two persons were used to take the measurements, one to locate the legs

and hold the MEM firmly on the studs, the second to take the readings. This is

the procedure that is followed when taking readings in the field.


Throughout the study, at each measurement point readings were repeated

until three consecutive identical values occurred and this was the value

recorded. The instrument was calibrated before and after readings were

recorded. Both the instrument and calibration plate were given sufficient time,

prior to the calibration and commencement of readings, to adjust to ambient

temperatures.

For each of the locations above and within the notch, and on the platform in

front of the notch at Study Location 3, the rock surface micro-relief was

measured using a simple mechanical contour gauge, of the type described by

McCarroll (1992), McCarroll and Nesje (1996), Robinson and Moses (2002),

Moses et al. (2014). The gauge used was 200 mm in length and for each

profile, the height difference (Hdiff) and spacing (Hs) between each successive

high point (Hmax) and low point (Hmin) was measured to produce three indices:

i. Mean Surface Relief = (Hdiff) averaged across each Hmax – Hmin

sequence identified on each profile

ii. Mean Spacing = Length of Profile (200) / ∑ Hs

iii. Mean Surface Roughness = Mean Surface Relief x Mean

Spacing

Results

The notches range from 2.5 to 4.5 m in depth and 2.0 to 3.5 m in height, both

distances being measured along a vertical line down from the outer edge of


the notch roof, and have a common biological zonation with littorinid snails

present in the upper intertidal notch roof (Figure 3, 4a and b), chitons and

oysters present in the mid-intertidal, back of notch (Figure 4c), and boring

bivalves, predominantly Lithophaga sp., present on the floor of the notch

(Figure 4d). Algae are present on the rock surface in the supratidal and

intertidal locations, although Study Location 3 has a barren zone on the

platform where a coarse sand beach is present and also for approximately 10

cm above the platform at the junction with the back of the notch (Figure 4e).

Shallow runnels are present on the middle and lower platform at Study

Location 3 (Figure 4f).

The erosion measurements, presented in Table 3, suggest that erosion at

Study Location1 is less than at Location 2 which in turn is less than at

Location 3, however the uneven distribution of measurement points makes it

impossible to determine whether this is variation between locations or

between sites. Only at Study Location 3 are there sufficient points to

statistically test the difference in erosion rates between sites, and here

analysis of variance indicates that a significant difference exists between the

two platform sites, the notch floor and the rear of the notch. The primary

reason is the low erosion rate on the lower platform that, from observations

over the study period, is frequently protected by a covering of sand. If this site

is removed from the analysis, the other three sites are not significantly

different from each other. Erosion rates at Study Locations 1 and 2 suggest

that erosion of the roof of the notch is slower than the floor, but the differences


are not significantly different because of the low number of measurement

points and variation within the data sets.

The maximum micro-relief on any 200 mm profile was 12.37 mm (Table 4),

and the maximum roughness 43.1, both of which were on the rear wall of the

notch. The surfaces with the lowest micro-relief were on the platform and the

floor of the notch. These two surfaces were, however, markedly different in

their roughness values, the platform having small numbers of rounded peaks

and hollows, whilst the floor of the notch had a close spacing of many small,

steep peaks and hollows averaging only 16 mm from peak to peak. There was

considerable difference between the relief of all surfaces except for the

platform and the notch floor but differences in the roughness index are less

marked. This is because some surfaces, such as the notch floor exhibit low

micro-relief with frequent spacing whilst others such as the back wall and

supratidal sites have a higher relief with the lower spacing.

Discussion

The biological zonation identified at each of the Phang Nga study locations

are comparable to those identified on tidal notches elsewhere in the tropics

e.g. (Trudgill, 1976), (Spencer, 1981), (Spencer, 1988), (Kázmér and

Taboroši, 2012). Although bioerosion has not been measured directly it is

likely that chitons contribute, in the manner described by Spencer (1988), to

the relatively high erosion rate of 0.228 mm/yr in the back of notch location.


The absence of organisms from the platform fronting the notch at Study

Location 3 suggests that abrasion by the coarse sand beach is dominant and

this explains also the relatively smooth micro-relief. Erosion rates on the notch

floor (0.289 mm/yr), which at this site forms the upper platform, and mid-

platform (0.324 mm/yr) are greater than on the lower (0.140 mm/yr) platform,

with the greatest average rate being recorded from the mid-platform sites.

Significant variation in erosion rates with position down-platform agrees with

variation in rates that have been identified on rock shore platforms in extra-

tropical macrotidal regions, where evidence of a zone of high erosion, mid-

platform, has also been identified (Foote et al., 2006), (Moses and Robinson,

2011), (Stephenson and Kirk, 1998), (Stephenson et al., 2010). This poses

particular problems for the long term evolution of platforms which generally

dip seaward at low angle. In the case of the platform at Phang Nga it appears

to result from the mid-platform being the zone of maximum abrasion by a thin,

mobile layer of coarse grained sand. Lower sites are covered by a thicker,

less-mobile, and therefore potentially less abrasive layer whilst the upper

platform has only an intermittent layer of sand that is discontinuous over time

and space. As the platform develops over time the zone of maximum abrasion

will gradually move up-platform and erosion of the lowered sections decrease

as the sand cover increases.

The back of notch erosion rate of 0.228 mm/yr is a little lower, but not

significantly different to the rates of lowering of the middle platform and the

floor of the notch, which are 0.324 mm/yr and 0.289 mm/yr, respectively. This

suggests that the three are evolving in some form of equilibrium. However, the


rear of the notch exhibits a significantly different micro relief to the notch floor

and platform. This suggests the erosion processes most probably differ.

Whilst the notch floor and the platform are surfaces of low micro-relief (Figure

4d and e), the notch rear wall exhibits high micro-relief (Figure 4c). The

platform also exhibits runnels in the mid-lower platform but these have not

been captured by the micro-relief measurement method reported here

because they are > 200 mm in size. The extent to which the differential micro-

relief measured is the result of grazing chitons and oysters remains to be

investigated as does the variation in larger scale relief features including

runnels. Although the micro-relief of the platform and floor of the notch are

very similar, as noted in the results section they differ significantly in

roughness due to a much closer spacing of high and low points. This appears

to be the result of boring into an otherwise smooth abraded surface of the

notch floor by bivalves (Figure 4d).

The supratidal and notch roof sites exhibit the lowest erosion rates, the

supratidal site averaged 0.095 mm/yr and the notch roof sites averaged 0.107

mm/yr. The rock surface on both the supratidal and notch roof sites is coated

in algae, and is very dark grey to black in colour compared with the light grey

colour of the actively abraded platform visible at Study Location 3 (Figure 4a,

b, e). The dominant influences in the supratidal zone are likely to be rainfall,

salt spray and algae. In the notch roof, which is influenced by wave action

only at the highest tide levels, occasional wave wash, salt spray, algae and

grazing Littorinid snails are likely the dominant influences on erosion rates

(Figure 4a, b).


Mean rates for the two locations where the notch roof was measured

averaged 0.055 and 0.134 mm/yr respectively. These rates are considerably

lower than the respective values for the erosion of the notch floor at each of

these locations. Further measurements are needed to ascertain whether this

is a significant difference. However they exhibit very different surface forms,

are occupied by different biological organisms and the floor is likely also to be

subjected to much higher levels of marine abrasion by sand.

The overall average intertidal erosion rate of 0.231 mm/yr recorded at Phang

Nga is at least half an order of magnitude lower than the average or range for

comparable studies at other marine notches in the tropics (Table 1). This may

be because the average for other sites is calculated from MEM

measurements recorded over short time periods of 2 – 3 years on recent reef

limestones and calcarenites. It fits with Viles and Trudgill’s (1984) suggestion

that there may be a tendency for short term higher erosion rates to become

lower rates in the longer term, and calcarenites are known to erode relatively

rapidly via a process of solutional disintegration that attacks the high

magnesium cements leading to the collapse of the rock fabric (Trudgill, 1976),

(Spencer, 1981). However, on the Malaysian coast on limestones of

comparable age to those of Phang Nga, Tija (1985) reports an average

intertidal erosion rate of 1 – 1.5 mm/yr, which is also half an order of

magnitude higher than the average intertidal erosion rate recorded in this

study. Tija (1985) did not, however, measure the erosion rate directly but

estimated it by assuming that a notch of 2 – 3 m depth had formed over a time


period of 2000 years and so it is likely to be much less accurate than the MEM

measurements reported in this paper. Even so, when compared with average

erosion rates measured over an equivalent time period of at least ten years in

tropical seas, the Phang Nga rates are lower (Table 1). Despite a much higher

mean annual rainfall of 2388 mm at Phang Nga (Kiernan, 1994), the

supratidal erosion rate of 0.095 mm/yr is much lower than subaerial erosion

rates for Grand Cayman (0.38 mm/yr; Spencer, 1981)and Aldabra (0.39

mm/yr; Trudgill, 1976) where mean annual rainfall is 1495 mm and 1142 mm

respectively (Spencer, 1981) and the average intertidal erosion rate at Phang

Nga is an order of magnitude lower than the intertidal erosion rates at both

Aldabra and Grand Cayman, including the long term rate measured over

eleven years at Aldabra (Table 1). Grand Cayman has a much lower tidal

range (spring tidal range 0.55 m; Spencer, 1981) than either Phang Nga or

Aldabra, which are comparable. Phang Nga mean spring tidal range is 2.5 to

3.0 m, (Kiernan, 1994), (Weesakul et al., 1997); Aldabra’s mean spring tidal

range is 2.74 m (Farrow and Brander, 1971). Wave data are not available for

Aldabra but Grand Cayman and Phang Nga have similar wave environments.

At Phang Nga the significant wave heights, Hs, range from 0.3 to 1.5 m and

significant wave periods, Ts, range from 4.5 to 7 seconds (Kiernan, 1994),

(Weesakul et al., 1997). Grand Cayman has an Hs of 1.0 m and Ts of 6 s.

Differences in wave energies do not therefore explain the differences in

average intertidal erosion rates at these locations. This is particularly the case

because nearshore topography at Phang Nga is known to increase wave

height at the shoreline (Weesakul et al., 1997), whereas at Grand Cayman

there is a reef in front of the shoreline that reduces wave heights across the


shelf by c. 20 per cent (Spencer, 1981), (Spencer, 1985) yet the intertidal

erosion rates at Phang Nga are much lower than those measured at Grand

Cayman.

The average erosion rate for the shore platform at Phang Nga is more broadly

comparable with intertidal erosion rates recorded on older lithologies but in

different climatic and tidal settings e.g. on the upper Cretaceous Turonian

limestone of Istria and the Gulf of Trieste in the Adriatic Sea (Table 1). In high

energy macro-tidal settings the erosion rates recorded on shore platforms on

older lithologies are also comparable to the intertidal erosion rates recorded at

Phang Nga. In order from oldest lithologies: on Carboniferous limestone rates

of 0.033 mm/yr (Oxwich Point, Gower, South Wales; Shakesby and Walsh,

1985) and 0.145-0.383 mm/yr (The Burren, Co. Clare, Ireland; Trudgill et al.,

1981) are recorded but over a very short time period of 1 year; on Cretaceous

limestone and Jurassic dolomitic breccias rates of 0.09 mm/yr are recorded

over a 3 year period (Mallorca, Spain; Swantesson et al., 2006) and on

Cretaceous chalk intertidal erosion is much more rapid at 3.650 mm/yr

(Channel Coast of UK and France; Foote et al., 2006); longer term studies on

Palaeocene Amuri limestone & Oligocene mudstone report rates of 0.900

mm/yr (Kaikoura Peninsula, New Zealand; recorded over 10 years;

Stephenson et al., 2010) and 1.090 mm/yr (Kaikoura Peninsula, New

Zealand; recorded over 30 years; Stephenson et al., 2010). There is no

discernible clear pattern of variation in intertidal and platform erosion rates in

relation to the age of the rock on which the notches or platforms have formed.


Unfortunately hardness values, with which to compare the values recorded at

Phang Nga, are not included with most published erosion rates and so it is not

possible at present to assess the importance of rock hardness in determining

intertidal notch erosion rates. However, in temperate environments, rock

strength has been shown to be an important determinant of platform erosion

rate (Moses et al., 2006) and such a relationship may be more important in

determining platform erosion rates than climatic and other environmental

contrasts within and between different tropical, Mediterranean and temperate

coastal environments.

Some authors, e.g. (Verstappen, 1960), (Hodgkin, 1970), (Tija, 1985),

(Pirazzoli, 1986), have used notch depth divided by ‘age’ of the notch to

calculate a variety of erosion rates ranging from 0.4 to 5 mm/yr (Moses,

2013). The back of notch erosion rate of 0.228 mm/yr at Phang Nga indicates

that tidal notches of 1 to 2 m deep notches may have formed in the Permian

limestone during the Holocene. The notches at the study locations range in

depth from 2 to 4.5 m, suggesting that the long term MEM measured erosion

rates may be an underestimate. Alternatively, the recent stillstand may not be

responsible for forming the Phang Nga tidal notches in their entirety and they

may be reoccupied notches inherited from a previous Quaternary sea level. It

is possible also that the present day tidal notches began as solutional

notches, formed by terrestrial processes in the manner described by

McDonald and Twidale (2011) at the base of the limestone towers when sea

levels in the region were lower. Marine modification of the notches is

evidenced by the fact that the limestone towers formed on the Ratburi


Limestone in the area become progressively steeper towards the coast

(Kiernan, 1994). When the Phang Nga MEM measurement sites were being

installed in 2002 a large slab rockfall occurred from the seaward facing side,

and including part of the roof of a notch, of a tower at Study Location 2,

suggesting that active marine erosion continues to maintain the steeper

slopes of the coastal towers.

Another possible explanation for the low notch erosion rates at Phang Nga is

that sea level rise is currently outstripping notch erosion. For tidal notch sites

in Greece Evelpidou et al. (2012) suggest that when the rate of notch erosion

is less than the rate of sea level rise then the notch will cease to form and will

disappear over millennial time periods. This has not been considered in the

presentations of intertidal erosion rates for sites in the tropics or subtropics

e.g. the two most intensively studied sites of Aldabra Atoll, Indian Ocean

(Trudgill, 1972), (Trudgill, 1976), (Trudgill, 1983), (Viles and Trudgill, 1984)

and the Cayman Islands, West Indies (Spencer, 1981), (Spencer, 1985,

(Spencer, 1988). Blanchon and Jones (1995), however, use the intertidal and

subtidal erosion rates presented by Spencer (1985) to suggest that a

submerged notch at -18.5 m on Grand Cayman was drowned by a very rapid

eustatic sea level rise of 5 - 8 m in less than 200 years. The regional average

sea level rise for the north Indian Ocean, of which the Andaman Sea is a part,

is estimated to be 1.29 mm/yr (Unnikrishnan and Shankar, 2007). The

intertidal erosion average of 0.231 mm/yr measured at Phang Nga is therefore

likely to be outstripped by this according to the principal used by Evelpidou et

al. (2012).


Lal et al. (2005) have used cosmogenic radionucliedes, 26Al and 36Cl, to

measure long term subaerial erosion rates on Pleistocene reef terraces of

Barbados and Puerto Rico. Their conclusion is that the subaerial rates

measured over periods of up to 2 years on Aldabra by Trudgill (1976) and on

Grand Cayman by Spencer (1985) overestimated erosion rates by an order of

magnitude when projected over the time period since the last interglacial

period. The implication is that this is also likely to be the case for the intertidal

erosion rates but those were not measured by Lal et al. Nonethless their study

calls into the question the applicability of short term measures of erosion rate

to longer timescales, reinforcing the importance of conducting long term

monitoring programmes particularly on older lithologies.

Conclusion

Erosion rates on the Ratburi Limestone of Permian age at Phang Nga,

Thailand, appear to vary between Study Locations and between

measurement sites within Study Locations in ways that require further data

before any significance can be ascertained. Rates obtained for the different

sites and locations are, as with most MEM rates published to date, based on

a relatively small number of individual measurement points. Trenhaile and

Lakhan (2011) suggest that there is a need for at least 10 measurement

points per site to provide a reasonable estimate of rates of surface change on

slowly eroding rocks. The values should therefore be treated with some

caution and further data are required in order to improve the confidence in


differences between locations and sites. Overall, the long term intertidal

erosion rates at Phang Nga are half to one order of magnitude lower than

most published MEM rates for tropical limestone coasts (Table 1). It remains

uncertain whether the lower than average rate is because the Permian

limestone at Phang Nga is more resistant to erosion than these other rock

types or if it is for some other reason such efficacy of the processes causing

the erosion or the time period over which the erosion values are recorded.

The fact that the results reported from Phang Nga show considerable

variation both between locations and measurement sites suggests that great

care needs to be taken when making comparisons with results from other

studies. Rates should be compared only if measured at equivalent inter-tidal

sites, are based on a significant number of measurement points and collected

from several separate locations.

The micro-relief of rock surfaces, in terms of their relief, the spacing of relief

elements and the resulting roughness exhibit clear differences between the

platform, the floor, the rear wall and the roof of the notch, and supratidal

locations. These differences appear to represent differences in the dominant

erosion processes acting on the rock in the different locations, including,

importantly, bioerosion processes. Colonisation of the different sites is

dominated by a different species or species group and this is reflected in the

resultant surface form.


The results reported here are an interim part of a longer term study and, in

many respects, raise a number of questions rather than providing any clear

answers to the knowledge gap that exists for tidal notch development on hard,

crystalline, ancient limestones of the tropics. It remains unclear, for example,

if the measured erosion rates are lower because of the rock hardness or the

longer time period over which they were recorded. This has significant

implications for studies concerned with Quaternary and Holocene sea level

changes as evidenced by the presence of emerged and/or submerged

notches, many of which use MEM measured erosion rates recorded on

young, relatively soft, reef limestones and calcarenites in the context of

harder, older, crystalline limestones.

There remain, therefore a number of research needs: because the rates are

so low there is a need to have some very long term monitoring sites; due to

the dearth of information on rates of erosion on tidal notches in rock types

other than limestone, this long term monitoring needs to be conducted on a

range of rock types, including basalts and harder sedimentary rocks; these

measurements should comprise a considerable number of measurement

points at each site and location to ensure reasonable levels of confidence in

the values obtained. For this particular study site, measurements of erosion

rates continue and particular issues to be addressed using the data include:

assessing the importance of tidal range and degree of exposure to wave

action; assessing the role of algal cover and organisms on erosion rates in

supratidal and intertidal settings; examining rates collected over shorted and

longer time periods in order to assess the importance of monitoring period on


rates measured. This paper has focussed solely on erosion, but accretion

processes also occur at Phang Nga, which have resulted in the loss of one

complete MEM measurement site at Study Location 2, which was completely

crusted over. The distribution and rates of such accretion and its interaction

with erosion are other areas for future study.


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Tab

le

1.

Inte

rtida

l er

osio

n ra

tes

mea

sure

d on

tid

al n

otch

es d

evel

oped

in

limes

tone

in

war

ms

seas

of

the

tropi

cs a

nd t

he

Med

iterra

nean

.

Lo

cati

on

E

nvir

on

me

nt

Ero

sio

n r

ate

(mm

/yr)

Su

bs

tra

te

Me

as

ure

me

nt

peri

od

(ye

ars

)

No

. o

f m

ea

su

rem

en

t

sit

es (

no

. o

f

me

as

ure

me

nt

po

ints

)

Au

tho

r &

co

mm

en

ts

Alda

bra

Atol

l, In

dian

Oce

an

Inte

rtida

l 0.

5 – 3

.0

Ree

f lim

esto

ne

(Ple

isto

cene

) 2

Tr

udgi

ll 19

76.

0.09

– 2

.7

13

5 (2

1 to

tal)

Vile

s &

Tru

dgill,

198

4

0.00

2 – 7

.5

2

5 (2

1 to

tal)

Vile

s &

Tru

dgill,

198

4

Gra

nd C

aym

an,

Car

ibbe

an S

ea

Supr

atid

al

0.78

(0.0

9 - 1

.77)

< 2

4

(up

to 2

7 pe

r site

) Sp

ence

r 198

5; 1

7 m

onth

s

Inte

rtida

l 1.

23

(0.3

1 - 3

.01)

< 1

5

(up

to 2

7 pe

r site

) Sp

ence

r 198

5; 7

mon

ths

Subt

idal

1.

79

(0.2

9-3.

67)

< 2

37

(up

to 2

7 pe

r site

) Sp

ence

r 198

5; 1

7 m

onth

s

Gul

f of T

riest

e,

Adria

tic S

ea

Supr

atid

al

0.11

(0.0

7 - 0

.155

)

Turo

nian

lim

esto

ne

(upp

er C

reta

ceou

s)

1 10

(up

to 1

25 p

er s

ite)

Toru

nski

, 197

9; 1

2 m

onth

s


Inte

rtida

l 0.

627

(0.1

3 - 1

.114

)

1 10

(up

to 1

25 p

er s

ite)

Toru

nski

, 197

9;

Schn

eide

r and

Tor

unsk

i, 19

83; 1

2 m

onth

s

0.

09 -

0.2

< 2

24 (n

ot s

peci

ficie

d)

Cuc

chi e

t al.,

200

6; 1

7 m

onth

s

0.

011-

2.96

6 <

22

17 G

ulf o

f Trie

ste;

12

Istri

an c

oast

(20

per

site

)

Furla

ni e

t al.,

200

9; 0

.7 -

21.9

yea

rs

0.

011-

0.97

0 <

17

> 50

0 (2

2 pe

r site

) Fu

rlani

et a

l., 2

011;

178

-617

2 da

ys

0.

007

- 1.2

3 Al

mos

t 5 y

ears

>

600

(25

per s

ite)

Furla

ni a

nd C

ucch

i, 20

13 (r

emov

able

ro

ck s

lab)


Table 2. MEM orientation sensitivity test.

a. Means and Standard Errors of readings taken with the MEM in horizontal,

vertical and inverted positions

b. Analysis of variance of the standard errors of repeat readings taken with

the MEM in the three different positions.

Each count comprises 6 sets of 10 readings with the MEM on each of the

three leg positions i.e. 180/reading position, 540 readings in total.

Reading position Count

Average Standard

Error

Variance of the

Standard Errors

Upside down (1) 18 0.000158 7.901417

Vertical (2) 18 0.000260 8.310341

Horizontal (3) 18 0.000177 4.402292 Table 2a

ANOVA Source of

Variation Sum of

Squares df Mean

Square F P-value F crit

Between Groups 1.063677 2 5.31838 0.77399391 0.466508091 3.17879767

Within Groups 3.504388 51 6.87135

Total 3.610756 53 Table 2b


Tab

le 3

. E

rosi

on ra

tes

reco

rded

at t

he th

ree

Stud

y Lo

catio

ns a

t Pha

ng N

ga, T

haila

nd.

Sam

ple

sit

es

Stu

dy

Loca

tio

n 1

St

ud

y Lo

cati

on

2

Stu

dy

Loca

tio

n 3

A

ll St

ud

y Lo

cati

on

s

No

. p

oin

ts

me

asu

red

er

osi

on

m

m/y

St

an

da

rd

erro

r

No

. p

oin

ts

me

asu

red

e

rosi

on

m

m/y

St

an

da

rd

erro

r

No

. p

oin

ts

me

asu

red

er

osi

on

m

m/y

St

an

da

rd

erro

r

No

. p

oin

ts

me

asu

red

er

osi

on

m

m/y

St

an

da

rd

erro

r

Sup

rati

dal

4

0.0

95

0

.02

88

4 0

.09

5

0.0

28

8

Ro

of

of

no

tch

3

0.05

5 0

.01

34

6 0.

1340

0

.06

00

9 0

.10

7

0.0

41

1

Re

ar o

f n

otc

h

14

0.22

8 0

.04

13

14

0.

228

0.0

41

3

Flo

or

of

no

tch

2

0.11

5 0

.05

70

8 0.

3250

0

.10

49

9

0.29

8 0

.06

36

19

0.

289

0.0

53

7

Mid

pla

tfo

rm

8 0.

324

0.0

31

5

8 0.

324

0.0

31

5

Low

er p

latf

orm

8

0.14

0 0

.01

89

8

0.14

0 0

.01

89

Loca

tio

n m

ean

9

0.0

860

0

.01

76

14

0

.24

30

0

.06

83

3

9

0.2

45

7

0.0

23

9

62

0

.22

2

0.0

22

5

Inte

rtid

al m

ean

5

0.07

90

0.0

244

0.24

30

0.0

68

3

0.

2457

0

.02

39

58

0.

231

0.0

23

5

Sup

rati

dal

4

0.0

95

0

.02

88

4 0

.09

5

0.0

28

8

No

. p

oin

ts

me

asu

red

9

14

39

62


Tab

le 4

. R

ock

surfa

ce m

icro

-relie

f (N

is th

e nu

mbe

r of p

rofil

es)

Sit

e

N

Sp

ac

ing

S

t E

rro

r R

eli

ef

St

Err

or

Ro

ug

hn

es

s

St

Err

or

No

tch

ro

of

10

6.40

0

.163

4.70

0

.26

29.9

7 1

.821

No

tch

re

ar

wall

5 5.

40

0.6

78

7.98

1

.372

39.9

1 3

.033

No

tch

flo

or

5 9.

90

0.7

95

2.53

0

.413

30.2

2 4

.328

Pla

tfo

rm

20

4.40

0

.366

1.78

0

.215

6.74

0

.423

Su

pra

tid

al

10

12.2

0 0

.800

2.92

0

.272

29.3

1 4

.02


Figure 1. Location of the Study

a. Phang Nga Bay

b. Study site locations


Figure 2. Study site locations

a. Situdy Location 1, facing southwest, fine sand present

b. Study Location 2, facing northwest, sand absent

c. Study Location 3, facing southwest, coarse sand present


Figure 3. Study site morphology, MEM positions and ecology.

a. Study Location 1

b. Study Location 2

c. Study Location 3


Figure 4. Micromorphology and ecology of MEM measurement locations

A. Roof of notch: fretted. Littorinid snails grazing on a pitted rock surface in

the spray zone (Study Location 1)

B. Roof of notch: rippled. Littorinid snails grazing on a rippled rock surface

pitted by inactive patellid grazing trace (Study Location 3). White spots

forming a triangle are the brass screws for the MEM.

C. Back of notch: chitons and oyster crust. Chitons grazing the dissected rock

surface. Widespread oyster crust present (Study Location 1 and 2).

D. Floor of notch: Boring bivalves, Lithophaga sp., present in boreholes

(Study Location 2).

E. Upper platform: note the abrasion at the cliff platform junction and on the

upper platform indicated by the white rock surface that has no biological cover

(Study Location 3).

F. Middle and lower platform: runnels are present on the middle to lower

platform surface and in the lower platforms they are covered by an algal mat

(Study Location 3).


Documents

Towards an improved understanding of erosion rates and tidal notch development on limestone coasts in the Tropics: 10 years of micro-erosion meter measurements, Phang Nga Bay, Thailand