Hedley et al

Evolution of the Irrawaddy delta regionsince 1850geoj_346 138..149

PETER J HEDLEY*, MICHAEL I BIRD*† AND RUTH A J ROBINSON**School of Geography and Geosciences, University of St Andrews, St Andrews, Fife KY16 9AL

E-mail: [email protected]†Present address: School of Earth and Environmental Science, James Cook University, PO Box 6811,

Cairns, Queensland 4870, AustraliaThis paper was accepted for publication in September 2009

We present a time series of coastline change for the Irrawaddy delta region of Myanmar usingthe earliest available navigation chart from 1850, and a set of topographic maps and satelliteimagery dating from 1913 to 2006. Despite the large sediment load delivered annually to thegulf by the Irrawaddy and Salween Rivers, the coastline has been largely stable for 156 years,advancing at an average rate of no more than 0.34 km per century since 1925. The long-termaverage rate of increase in land area across the study area between 1925 and 2006 is4.2 km2/year, but this masks a period of more rapid accumulation between 1925 and 1989(8.7 km2/year), followed by a period of net erosion at a rate of 13 km2/year until 2006. Lessthan 9% of the sediment load delivered to the study region by the Irrawaddy, Salween andSittoung Rivers has contributed to the observed progradation, with the remainder beingexported into the Gulf of Martaban to depths below low tide level, or filling any accommo-dation space created due to subsidence or sea level rise. In contrast to many deltasworldwide, we suggest that the coastline encompassing the Irrawaddy delta and the SalweenRiver is more or less in equilibrium, and that sediment deposition currently balancessubsidence and sea level rise. Myanmar has fewer large dams relative to its Asian neigh-bours, and the Salween is currently undammed. This is forecast to change in the next 5–10years with extensive damming projects on the mainstem of the Salween under considerationor construction, and the sediment retention will cause losses in sediment supply to the Gulfof Martaban, and retreat of the delta. This could impact the densely populated delta regionand Yangon, and further exacerbate the impacts of extreme events such as Cyclone Nargis in2008.

KEY WORD: Burma, Irrawaddy delta, Salween, sediment flux, coastal sedimentation

Introduction

Rivers are the major conduits for the transfer ofwater, particulates and dissolved material fromthe land to the ocean and more than 50% of

this flux enters the global ocean in the tropics (Milli-man and Meade 1983; Milliman and Syvitski 1992;Nittrouer et al. 1995). A significant proportion of thisflux is focused on, and delivered via, major deltacomplexes at the mouths of rivers such as the Amazon(Nittrouer et al. 1991), Ganges-Brahmaputra (Good-bred and Kuehl 1999), Orinoco (Warne et al. 2002),Niger (Sexton and Murday 1994), Mekong (Tanabeet al. 2003b), Yangtze (Yang et al. 2006), Chao Phraya

(Tanabe et al. 2003a), Fly (Palinkas et al. 2006) andIrrawaddy (Stamp 1940; Robinson et al. 2007). Suchdelta environments are geologically young, havingdeveloped since about 8000 years ago in response toa slowdown in the rate of post-glacial eustatic sea-level rise (Meade 1996; Stanley and Warne 19941997).

Deltas are dynamic environments shaped by theinteraction of sediment and water influx from rivers,tidal regime, sea level, wind patterns and ocean cur-rents, all of which vary on seasonal to millennialtimescales. The evolution of deltas also reflects anthro-pogenic changes in basin land use (Woodroffeet al. 2006). Tropical deltas are highly productive

The Geographical Journal, Vol. 176, No. 2, June 2010, pp. 138–149, doi: 10.1111/j.1475-4959.2009.00346.x

The Geographical Journal Vol. 176 No. 2, pp. 138–149, 2010© 2009 The Author(s). Journal compilation © 2009 The Royal Geographical Society

ecosystems both on- and offshore, and, as a result,commonly sustain high human population densitiesand agricultural productivity.They contain many of theremaining large mangrove areas and these are increas-ingly coming under pressure from high populationdensities, resulting in accelerated mangrove clearancefor wood, agriculture and aquaculture (Bird and Teh2006). This includes direct disturbance arising frommangrove clearance and embankment construction, aswell as activities such as deforestation and damming inthe river catchments that feed deltaic systems (e.g.Syvitski et al. 2005). In addition, deltas are likely torespond rapidly to both natural and anthropogenicclimate and sea-level change, with the potential forsignificant impacts on populations that live in deltaregions (IPCC 2007; Woodroffe et al. 2006).

The Irrawaddy delta is one of the major tropicaldeltas in the world in one of the poorest nations inSoutheast Asia; Burma currently has a low per capitaGDP ($379 in 2007) and low population density (esti-mated at 72 people/km2 for 2007) (United NationsDepartment of Economic and Social Affairs 2008).

This study seeks to quantify historic changes in theposition of the shoreline of the Irrawaddy delta, andthe coast eastwards around the Gulf of Martaban tothe Salween River (Figure 1) in order to elucidate theprocesses controlling the evolution of the delta, and tocompare it to other mega-deltas in Asia. Anthropo-genic activities, including mining, water and hydro-carbon extraction, sediment retention in dams, andengineering structures on waterways have led to thecurrent retreat of many mega-deltas in Asia and theplacement of large coastal urban populations atgreater risk (Syvitski 2008). Although Burma has abun-dant water resources, only ~18 km3 of water is cur-rently stored in reservoirs and tanks which represents~2.5% of the total annual discharge (all rivers) or~4.4% of the Irrawaddy discharge (Aung 2003;Robinson et al. 2007; Myanmar Irrigation WorksDepartment 2004). Although there are a considerablenumber of small–medium reservoirs and canalsystems on tributaries, Burma’s mainstem rivers arestill less regulated than other Asian rivers and thecurrent sediment supply to the Gulf of Martaban is

Figure 1 (A) Map of Myanmar showing its rivers and adjacent coastal area, including the Gulf of Martaban. Bathmetriccontours were provided by V. Ramaswamy and the contour intervals are in metres. The dashed thick line near the 40 m

contour off the Irrawaddy delta is the 36.6 m (20 fathom) contour derived from the 1850 map of Lt. W. Fell (see Figure 2).(B) Frequency of tropical storms and cyclones from the Indian Ocean for the past half century. Data are from the Climate

Explorer database (e.g. Oldenborgh and Burghers 2005). 1050 ¥ 667 mm (72 ¥ 72 dpi)

Evolution of the Irrawaddy delta region since 1850 139


less attenuated by damming. However, over the next5–10 years, major mainstem damming projects willprogress on the Salween River and further dams areunder consideration on tributaries of the Irrawaddyand Chindwin (Figure 1). This paper documents thehistorical and current status of the Irrawaddy deltaregion and serves as a reference for tracking howfuture changes in the delivery of riverine sedimentimpact on the position of the coastline.

Mangroves are a key element in the evolution oftropical deltas such as the Irrawaddy, acting assediment traps and primary colonisers of shallowsubmerged sand bars, and as bioshields against theimpacts of large storm events and tsunamis (Osti et al.2009). Almost 50% of the world’s mangrove forestshave been lost in the past 20 years and the ecologicalstatus of Irrawaddy delta mangroves is now in con-tinuous decline (Blasco and Aizpuru 2002) due toincreased rice production and harvesting of man-groves for fuel over the last 30 years (Oo 2002). Stamp(1925, 267), based on the late nineteenth to earlytwentieth century period of British involvement in theregion, commented that ‘very little damage . . . [tomangrove forests] . . . is caused by wind, andcyclones are unknown’. However, based on data fromthe last decades (Figure 1), while the number of tropi-cal storms per year in the Indian Ocean dropped inthe mid 1970s, the frequency and number of tropicalcyclones has increased, particularly since the mid-1990s (Oldenborgh and Burghers 2005). There is onlyone known Indian Ocean cyclone that had landfall inBurma prior to 2008; Mala hit the coast to the north ofthe delta (latitude 17.5°N) in 2006 (Fritz et al. 2009).Cyclone Nargis tracked eastward across the delta inearly May 2008, with winds reaching 165–212 km/hour (Fritz et al. 2009; NASA 2008). The cyclone, andaccompanying heavy rainfall (up to 600 mm), inun-dated more than 14 402 km2 of the delta causing cata-strophic damage and loss of life in the region (NASA2008), and has increased the vulnerability of the deltaduring storms due to the reduced density of man-groves. Our study additionally provides a baseline onwhich shoreline changes since Cyclone Nargis, andthe impacts of future reduction in mangrove forestdensity, can be evaluated.

Study area

The Irrawaddy River feeds one of the two largest deltasystems in Southeast Asia and the total catchment areaof rivers draining into the Gulf of Martaban is about0.7 ¥ 106 km2 (Robinson et al. 2007). The Irrawaddyand Chindwin Rivers (Figure 1) have been in existencesince at least the Eocene, and the sediments carried bythe palaeorivers from the Himalayas to the oceanhave gradually infilled the Burma Trough with about900 km of progradation over the last ~50 million years

(Nyi Nyi 1967; Bender 1983). The extensive wedge-shaped modern delta probably originated around7000–8000 years ago when other major deltas inSoutheast Asia developed (e.g. Tanabe et al. 2003a),and now comprises around 20 571 km2 of flat, low-lying fertile delta plain, with five major and manysmaller distributaries (Stamp 1940; Orton andReading 1993; Stanley and Warne 1994; Woodroffe2000). It is classified as a mud-silt, tide-dominatedsystem with a mean tidal range of 4.2 m (Hayes 1979;Orton and Reading 1993), and tidal influence extendsalmost 300 km inland as far as Henzada at the apex ofthe delta (Figure 1), with saline water penetrating upto 100 km upstream (Aung 2003). The maximumspring tidal range increases eastwards from DiamondIsland (2.8 m) to Elephant Point (6.4 m) (Figure 2).Tidal currents as high as 3 m/s have been reportedfrom the Bassein estuary (Figure 2) when ebb tidesand river flooding coincide (Volker 1966).

Much of the region was initially forested and man-groves grow in tidally influenced areas, but the totalmangrove forest area in the delta decreased from2345 to 1786 km2 between 1924 and 1995 as a resultof logging and clearance for agriculture and aquacul-ture (Oo 2002; Adas 1974). The population of thecoastal and delta areas grew from 9.6 million in 1983to 12.7 million in 1997 (27% of the country’s popu-lation for 1997 and a population density of 583people/km2), and the continued population increasehas affected land use and coastal resources (Oo2002); the majority of the population are involved inagriculture, and rice production accounts for thelargest proportion of the cultivated area (2 million hain 1996; Aung 2003).

A reassessment of the original data of Gordon(1879–80 1885) from surveys conducted throughoutthe 1870s suggests that the Irrawaddy delivers442 � 41 km3 of water containing 226–364 mega-tonnes (Mt) of sediment to the ocean via the deltaevery year (Robinson et al. 2007), carrying a prelimi-nary estimate of 3.1–5.2 Mt/year of organic carbon(Bird et al. 2008). Because of the monsoonal nature ofthe climate, approximately 80% of the water and 92%of the sediment discharge occurs between June andNovember. Most of the water debouches to the oceanvia the central distributaries of the delta, with com-paratively little entering via the western (Bassein) andeastern (Rangoon) distributaries (Figure 2). There areno major dams on the trunk stream of the Irrawaddy,but about 1300 km of embankments were built in thelate nineteenth and early twentieth century to protectagricultural land. These embankments, and the artifi-cial cutting of meander loops, have had the effect oflimiting overbank flooding and deposition of sus-pended load in the delta (Volker 1966; Stamp 1940).

The modern coast is marked by shore-parallel sandridges, with the offshore bathymetry characterised by

140 Evolution of the Irrawaddy delta region since 1850


a low gradient continental shelf about 120 km wide.To the east, the continental shelf increases in width toaround 250 km in the centre of the Gulf of Martaban,with the shelf break at 110 m depth. Most of the Gulfof Martaban is covered by modern muds and silts,with relict shelf sands dominating the outer shelf atdepths below 20–30 m (Rodolfo 1969a 1969b;Ramaswamy et al. 2004 2008; Rao et al. 2005).

The shelf in the centre of the Gulf of Martaban isincised by the Martaban Canyon (Figure 1), strikingroughly south to abyssal depths of over 2000 m in theAndaman Sea, and modern muddy–silty sedimentsextend further out onto the shelf to depths of at least100 m along the flanks of this feature (Rodolfo 1969a

1969b; Ramaswamy et al. 2004; Rao et al. 2005;Curry 2005). The present width of the canyon is150–200 km, and its position offshore of the SittoungRiver, which enters at the head of the gulf (Figure 1),indicates that the Martaban Canyon is a relict featureof a much larger palaeo-Irrawaddy drainage systemthat occupied the modern Sittoung River route untilthe early Miocene (Nyi Nyi 1967). The Sittoung Riverhas a low annual sediment load, and its outlet to theGulf of Martaban defines a funnel-shaped estuary,whereas the Salween delivers an estimated 110–180Mt/year of sediment to the Gulf of Martaban (Robin-son et al. 2007). In total, the Irrawaddy, Salween, Sit-toung and smaller tributaries deliver an estimated

Figure 2 Digitised maps of the Irrawaddy delta and the Gulf of Martaban from copies of maps produced by Lt. W. Fell(1850) and Mr G. H Barnett (1913). Numbers refer to locations mentioned in the text. 1109 ¥ 987 mm (72 ¥ 72 dpi)



752 km3 of water and 370–600 Mt of sediment peryear to the gulf (Robinson et al. 2007). It should benoted that these values are based on sediment con-centrations measured just above Henzada on theIrrawaddy and assume 100% transport efficiencythrough the delta. There are currently no availablequantitative data on sedimentation rates within thedelta to test these assumptions.

As a result of the seasonally reversing Asianmonsoon, circulation in the Andaman Sea is broadlycyclonic from May to September (southwestmonsoon) and anti-cyclonic from December to Feb-ruary (northeast monsoon). High suspended loadinputs from the rivers surrounding the Gulf and strongtidal currents lead to substantial sediment suspensionand re-suspension, resulting in the Gulf remainingturbid year-round, with surface suspended sedimentconcentrations up to 100 mg/litre persisting up to100 km from the coast (Ramaswamy et al. 2004). Awell-defined turbidity front oscillates north and southby 150 km in phase with spring–neap tidal cycles,with the area of the turbid zone varying from over45 000 km2 during spring tides to less than15 000 km2 during neap tides (Ramaswamy et al.2004).

Methodology

A number of cartographic resources of varying accu-racy are available for the region (Table 1). The earliestavailable map of the delta and Gulf of Martaban,including bathymetry, was produced in 1850 by Lt. W.Fell. Several more detailed maps of the delta wereproduced and periodically updated into the earlytwentieth century, with a version dating from 1914chosen for this study. The maps were digitised but arenot considered of a suitable quality for a quantitative

comparison with later maps and imagery; however,they still contain valuable information on the generalmorphology of the delta and the shoreline. Both mapswere provided as 600 dpi scanned TIFF files of origi-nal paper copies by the British Library.

The HIND series of 1:253 440 maps were producedin 1944–6 by the Survey of India from ground surveysconducted between 1919 and 1930. This is the earli-est series of maps that is sufficiently accurate and ofhigh enough quality (reproduction and preservation)to georeference. It is possible that data from aerialsurveys conducted in 1924 (Stamp 1925) until 1944are included in the maps, but for the purposes of thisstudy, these maps are assumed to relate to the situa-tion in 1925. The eight relevant map sheets wereprovided as 600 dpi scanned TIFF files of originalpaper copies by the British Library. From the 1970sonwards, cloud-free satellite imagery of the regionwas selected from the georeferenced Terralook JPEGimage archive, derived from the Landsat-MSS (1973),Landsat-TM (1989), Landsat-ETM+ (2000) and Terra-Aster (2006) sensors (Table 1).

In the case of the HIND topographic maps, the‘coast’ as identified on the maps was taken to be theseaward limit of the land and digitised using Terralook1.0 and ArcGIS. The ‘coast’ on the satellite imagerywas taken to be the seaward margin of vegetation. Inaddition, unvegetated exposed sediment further off-shore was digitised, although the area of exposedunvegetated sediment is critically dependent on tidelevel, which varies between scenes and years.

The source of uncertainties and errors that arisewhen comparing paper maps, aerial photographs orsatellite images include surveying errors, variations inthe degree of generalisation on maps, map registra-tion, distortion occurring to photocopied maps,and errors incurred during the digitising process

Table 1 Description of cartographic resources and satellite imagery used in this study

Publication date Acquisition mode Notes

1852 Hydrographic survey Chart of the coast of Pegu and Gulf of Martaban, published by J. Walker;British Library shelfmark: Maps 147.e.19.(65.); based on survey in 1850 byLt. Fell

1914 Ground survey Map accompanying report entitled ‘Note on the Protective Embankments inthe Irrawaddy Delta, 1862–1912’ by Mr C G Barnett; British libraryshelfmark: I.S.BU.53/14; based on ground surveys to 1913

1944–45 Ground and aerialsurvey

Burma 1:253,440. HIND first edition, unlayered; sheets 85L, 85P, 86I, 86M,94D, 94C, 94G, 94H; British Library shelfmark: Maps 58765.(27.);published by the Indian Survey based on surveys between 1919 and 1930

1973 Landsat MSS Individual scenes acquired for Jan and Mar 73; Jan 74; Nov 781989 Landsat TM Individual scenes acquired for Jan and Feb 19892000 Landsat ETM+ Individual scenes acquired for Dec 1999; Feb, Mar and Nov 20002006 Terra ASTER Individual scenes acquired for Jan, Feb, Apr and Dec 2006



(Heywood et al. 2006). In order to investigate thedegree to which any apparent changes in the delta canbe attributed to genuine land changes, rather thanerrors associated with map quality, registration of thesources and digitising, a comparison of the digitisedcoastlines located along the rocky western Arakancoast between the HIND series maps and the 1973satellite imagery was made to quantify the amount ofland change that can be attributed to the above errors.Between 1925 and 1973, an apparent total of30.7 km2 of land was gained, 2.1 km2 was lost, with anet change of 28.6 km2. Between 1973 and 2006,10.3 km2 was apparently gained, 7.1 km2 was lost,with an apparent net gain of 3.2 km2. We recognisethat this error analysis is spatially biased to thewestern edge of the maps and arises because there areno other fixed points common to all maps that allowmore spatial coverage of error. However, the errors areconsidered small in the context of total land changesas they represent 4.2% of the land gained and 0.5% ofthe land lost between 1925 and 1973, and 1.1% ofthe land gained and 0.8% of the land lost between1973 and 2006 (Table 2). The uncertainty in the cal-culation of coastline change is greatest for the oldermaps (4–5%) and less when based on the comparisonof more recent maps and satellite imagery (~1%).

To assess the areal extent of land gained and lostthrough time, the digitised maps and satellite imagerywere overlain, the coastline was digitised withinArcGIS, and difference polygons were created. Arealestimates of net land gained (Table 2) have been con-verted to sediment volumes assuming a verticaldimension equivalent to the maximum tidal range of6.4 m and this was converted to sediment massassuming a minimum density of 0.5 tonnes/m3 forfine-grained muds and a maximum density of 1.5tonnes/m3 for coarse-grained sands (Bird et al. 2004).

Estimates of annual sediment flux into the Gulf ofMartaban were taken from Robinson et al. (2007) andare used to compare the proportion of estimated sedi-ment mass deposited over the last 81 years to the totalriverine sediment flux over the same period. Theuncertainties described above are increased becausethe rate of vertical accretion and values of sedimentdensity are not quantified for the Irrawaddy deltaregion, and we have therefore adopted a simpleapproach by estimating the maximum and minimumsediment mass changes using the range of plausiblevalues for annual sediment flux and sediment density.The values used to estimate the percentage of sedi-ment mass deposited do not include any depositionrelated to filling available accommodation space(subsidence and sea level changes).

Results and discussion

The early maps from 1850 and 1913 are reproducedin Figure 2. While the maps are not of a quality thatallows a quantitative comparison, it is clear that themajor morphological features of the delta werepresent in 1850, while the more detailed map from1913 is qualitatively and broadly indistinguishablefrom subsequent maps and imagery up to the presentday (Figure 1). Purian Point (a 12 km long NNE-trending bedrock high) and Krishna Shoal are usedas fixed points of reference for the 1850 map, andthere appears to be no significant advances of thedelta between 1850 and 1925 based on a visual com-parison of the 1850 map and the 1920s surveys (uponwhich the HIND series topographic sheets publishedin the 1940s were based).

Quantitative comparisons of the georeferencedcoastlines from 1925, 1973 and 2006 are provided inFigure 3, as well as the area of unvegetated sediment

Table 2 Summary of gain, loss and net change over the time slices used in this study for both vegetated coastline andunvegetated sediments seaward of the vegetation line and offshore for the entire study area with cumulative values for1925 to the 2006 and 1973 to 2006. Figures marked with an asterisk refer only to the Irrawaddy Delta itself between

Purian Point and Elephant Point (see Figure 1)

Vegetated coast Unvegetated sediment

Time period YearsGain(km2)

Loss(km2)

Net change(km2)

Net change(km2/year)

Area(km2)

Net change(km2)

1925 – – – – – 2116 (1925) –1925–73 48 725 428 297 6 1746 (1973) -3701973–89 16 539 280 258 16 1794 (1989) 481989–2000 11 160 328 -168 -15 1291 (2000) -5032000–06 6 206 253 -47 -8 2015 (2006) 723Cumulative from 1925 81 1629 1289 340 (73*) 4.2 (0.90*) – -102Cumulative from 1973 33 905 861 44 (18*) 1.3 (0.55*) – 268



exposed at the time the satellite imagery wasobtained. A comparison of the maps and images con-firms little change in the coastline position fromPurian Point to Elephant Point between the 1920s and2006, with some significant areas of advance at theeasternmost end near Elephant Point. Table 2 providesa summary of the areas of coastline gained andlost between successive time slices, and the areaof exposed unvegetated sediment. Overall, theIrrawaddy delta gained 55 km2 of land betweenPurian Point and Elephant Point from 1925 to 1973,and an additional 18 km2 up to 2006, averaging lessthan 1 km2/year. This is equivalent to an average rateof advance distributed equally across the 250 kmcoastline of no more than 3.6 m/year or 360 m percentury. However, based on Chhibber (1934) (andlater citations of his work in Pascoe 1950; Rodolfo1969b; Bender 1983; Aung 2003), the Irrawaddy deltais rapidly advancing seawards, with quoted rates ofadvance ranging from 25 to 61 m/year. Chhibber(1934) calculated an average value of 4.1 km percentury from two bathymetric surveys in 1860–70 and1909–10. In the time between the surveys, the 10fathom (18.3 m), 15 fathom (27.4 m) and 20 fathom(36.6 m) contours advanced seawards by 16.5, 33 and41 km, respectively, east of longitude 95.5° E in theGulf of Martaban. The average value derived byChhibber (1934) would imply 6.4 km (4–10 km) ofadvance since the time of the earliest 1850 mapobtained for this study. We question the long-termapplicability of the rates quoted by Chhibber (1934)because the 36.6 m (20 fathom) contour from the1850 map overlies the present day 40 m contour fromRamaswamy et al. (2008), and therefore shows noappreciable seaward shift over 150 years (Figure 1). Itshould be noted that the estimates of Chhibber (1934)are based on a period of time that included extensiveembankment construction, river straightening andmeander loop blocking on the Irrawaddy River, andon measurements from near the margins of the Mar-taban Canyon. Since 1925, the progradation rate hasbeen, on average, about 10% of the Chhibber (1934)estimate for 1860–1910.

From our average calculation of 3.6 m/year we inferthat less than 5% of the sediment flux from theIrrawaddy River has contributed to sub-aerial deltaprogradation over this period. The sub-aerial deltaplain sediments at the coast form a series of coastparallel beach ridges of sand grain sizes (Stamp 1940)and are probably derived from river bedload whichhas never been measured in the Irrawaddy River.Therefore a component of the total sediment flux hasnot been quantified and it is possible that much of thesuspended load of the river is exported into the Gulf ofMartaban, consistent with previous qualitative obser-vations that little of this sediment is deposited withinthe delta itself (Stamp 1940; Volker 1966; Woodroffe

2000). Based on this analysis, it would appear likelythat the Irrawaddy delta section of the coastline isstable, protected in its current position by the bedrockhigh that comprises Purian Point and the offshoreAlguada Reef at its western end. It is more or less inequilibrium, with an unknown amount of sedimenta-tion balancing subsidence and sea level rise in thedelta. Progradation is also limited by efficient sedi-ment transport into the gulf region due to the strongoffshore tidal currents and surface water currents onthe shelf. There has been significant sediment redistri-bution in the shallow prodelta. Several areas south-west of the main Irrawaddy distributary channel,which are marked as shoals in 1850, are no longerevident. In addition, some small, partly vegetatedsandbars closer inshore that are now emergent cameinto existence between 1925 and 1973.

A comparison of each time slice for the region eastof Elephant Point into the head of the Gulf of Marta-ban and down to the mouth of the Salween River atKyaikkami (formerly Amhurst) suggests a considerablymore dynamic coastline. The coastline immediatelyeast of the Rangoon River has advanced since the1920s, with a mix of major advances and retreats ofseveral kilometres in the region at the head of the Gulf(Sittoung River mouth) and at the mouth of theSalween River (Figure 3). The majority of riverine sedi-ment delivered to the Gulf of Martaban occurs duringthe monsoon between late May and September (Rob-inson et al. 2007). Most of the sediment passesthrough the Irrawaddy distributary channel and theSalween River, with only a minor contribution fromthe Sittoung and Tavoy Rivers (Figure 1 and 2). It isthen transported by tidal currents and reversing oceancurrents (cyclonic in spring–summer and anti-cyclonic from November to May) into the Sittoungestuary and across the shelf in the Gulf of Martaban(Ramaswamy et al. 2008). This Sittoung mouth sectionof the delta is essentially replenished with sedimentdue to longshore drift processes and from marinesources, rather than by direct replenishment throughits river mouth.

Table 2 and Figure 4 suggest the total increase inland across the entire study area between 1925 and2006 amounts to 340 km2 at a long-term average rateof 4.2 km2/year, but this masks a more rapid rate ofadvance of 8.7 km2/year from 1925 to 1989, followedby net erosion at a rate of 13 km2/year up to 2006.There may be a link between the increased frequencyof Indian Ocean tropical cyclones since the mid1990s and increased erosion (Figure 1B), but only twocyclone events are on record as impacting the coast ofBurma (Fritz et al. 2009) and the effects of the IndianOcean tsunami on coastline erosion were small (Sweet al. 2006). It is very probable that sedimentdischarge to the Irrawaddy delta has been reducedsince the mid 1980s due to the increased construction



Figure 3 Areas of coastline gain (dark green) and loss (light red) for the period between 1925 and 2006. Areas ofunvegetated sediment in 2006 are also shown. 134 ¥ 280 mm (400 ¥ 400 dpi)



rate of small–medium sized dams and irrigation workson tributaries of the Irrawaddy and Chindwin. Storagecapacity has increased from 2.34 to over 18 km3 since1988 (Myanmar Irrigation Works Department 2004).Modern sediment discharge measurements quantify-

ing the annual Irrawaddy and Salween Rivers iscurrently work in progress.

The accumulation rate for the entire area is of thesame order as recent progradation rates of 5.5–16 km2/year measured on the Meghna delta (Allisonet al. 2003) and historical rates of 1.2 km2/year in partof the Mekong delta for the period between 1885 and1985 (Nguyen et al. 1999). The spatially averaged pro-gradation rate of 3.4 m/year for the last ~100 years islow in contrast to average rates of 8–15 m/year in theMekong delta and 50 m/year over the last 4000–6000 years for the Yangtze delta (Tanabe et al. 2003b;Saito 2001). However, although the mega-deltas ofAsia were in a constructional phase during the lastseveral thousand years, the Yellow (Huanghe), Yangtze(Changjiang), Red (Song Hong), Mekong and ChaoPhraya have all experienced net losses of land and theretreat of their deltas over the last decades due todam-related sediment discharge losses to the delta,subsidence due to resource extraction, and erosion(Saito et al. 2007; Syvitski and Milliman 2007; Wanget al. 2007; Yang et al. 2006).

Assuming that the sediment fluxes of the Irrawaddy,Salween and Sittoung Rivers all contribute to landbuilding in the study area, a maximum of 9% (370 Mt/

Figure 3 Continued

Figure 4 Total area of net gain or loss for all time slices ofvegetated coastline and unvegetated sediments. The values

are calculated relative to 1925 (solid lines) and 1973(dashed lines) coastlines. 769 ¥ 499 mm (72 ¥ 72 dpi)



year of sediment flux and 1.5 tonnes/m3 sedimentdensity) to a minimum of 2% (600 Mt of sediment fluxand 0.5 tonnes/m3) of the total annual sediment flux ofthese rivers may have been incorporated into newlyvegetated areas since 1925; the average value is 5.5%.The area of unvegetated sediment offshore has alsofluctuated dramatically in the head of the Gulf ofMartaban (Table 2). A quantitative interpretation ofnet sediment gains or losses is not possible for theunvegetated areas due to the large tidal range in theregion, with images acquired at different points in thetidal cycle. However, in general terms it would appearthat since 1925, there may have been overall losses or,at best, only relatively small gains in the area of off-shore unvegetated sediment across the study area.

The conclusion that possibly 91% of the total sus-pended flux delivered by rivers into the Gulf of Marta-ban is exported offshore is supported by the perennialpresence of a high turbidity region covering most of thegulf area (Ramaswamy et al. 2004) and by the exist-ence of an extensive mud belt covering the floor of theGulf (Figure 1) (Rao et al. 2005). Ramaswamy et al.(2008) demonstrate from carbon isotope analysis thatover 70% of the organic carbon in the surface sedi-ments of the Gulf of Martaban is of terrestrial origin andthis percentage increases to over 90% at the edge of theshelf in the gulf area (Figure 1).These results imply highrates of sediment accumulation at depths below thelowest tidal level in the Gulf.This sediment is predomi-nantly fine grained (Ramaswamy et al. 2004 2008) andis distributed over the maximum area of the highturbidity zone (45 000 km2). Assuming that it is rapidlycompacted to 1 tonne/m3 (Bird et al. 2004) implies thatthe sea floor has been raised 0.6–2 m since 1925 at arate of 0.7–2.5 cm/year. This calculation neglects thesediment deposited in any available accommodationspace created through subsidence and sea level rise,probable small losses to the Bay of Bengal, andunknown losses to the Martaban Canyon, but is never-theless consistent with the 2 cm/year estimate ofRodolfo (1969a).

Conclusions

The results of this study suggest that the distributarydelta section of the Irrawaddy coastline has beenstable since at least 1850, with a maximum averageprogradation rate of 0.34 km per century. The mostdynamic part of the coastline is in the estuarine part ofthe Sittoung River at the head of the Gulf of Martaban.The average progradation rate is less than 10% of mostprevious estimates (Chhibber 1934; Pascoe 1950;Bender 1983; Aung 2003), although the most authori-tative of these (6.1 km per century; Chhibber 1934),based on the seaward displacement of bathymetriccontours between 1860 and 1910, was considered amaximum for the delta. This is potentially a biased

record due to extensive Irrawaddy River embankmentconstruction in the late nineteenth century. Many ofthe Asian mega-deltas are in a destructive phase dueto sediment loss associated with damming (Saito et al.2007) and the stability of the Irrawaddy delta coast-line, including the Salween River mouth region, maynow be anomalous in Asia because of the current lowdensity of dams on the mainstems of both rivers. Thisis forecast to change in the next 5–10 years as theconstruction of a series of dams on the Salween Riverhas begun and construction of dams on tributaries ofthe Irrawaddy and Chindwin Rivers continues. Ourhistorical perspective on the apparent stability of theIrrawaddy to Salween coastline over c. 150 years is animportant baseline on which future changes to thecoastline in the aftermath of Cyclone Nargis can becompared, and the impact of future damming on theSalween (and Irrawaddy) can be evaluated.

The slow rate of progradation of the coastlineappears to be partially due to the effective transport ofthe bulk of the sediments carried by the IrrawaddyRiver through the delta plain, as noted previously(Stamp 1940; Volker 1966; Woodroffe 2000), the effi-cient dispersal of these sediments by strong tidal cur-rents (Ramaswamy et al. 2004; Rao et al. 2005), andby cyclonic oceanic currents during the dominantsouthwest monsoon season. The distributary deltasection appears to be protected at its current locationby coastal and offshore bedrock highs at its westernmargin and is unable to prograde significantlybecause of efficient longshore transport.

While a considerable flux of sediment is deliveredto the shelf of the Gulf of Martaban by the combinedIrrawaddy, Salween and Sittoung Rivers (Robinsonet al. 2007), 9% or less of the sediment deliveredsince 1925 has contributed to land-building along thecoasts of the study area. Most of this sediment accu-mulated prior to 1989, with erosion occurring at thehead of the Gulf since that time. The bulk of thesediment is accumulating as a wedge of mud coveringthe floor of the Gulf of Martaban (Ramaswamy et al.2004; Rao et al. 2005). A mass balance suggests thatthis sediment is likely to be accumulating offshore ata rate of 0.7–2.5 cm/year. This rapid rate of sedimen-tation suggests that an annually resolvable regionalrecord of environmental change for the last severalmillennia could be preserved in the Gulf of Martabanif there is not extensive re-working of the sediments.

Acknowledgements

Funding for this research was in part provided by aRoyal Geographical Society grant to Bird and Robin-son. V. Ramaswamy is gratefully acknowledged forallowing us to use his bathymetric contours inFigure 1. The paper was improved by the detailedcomments and suggestions from four anonymous



reviewers. This publication is a contribution from theScottish Alliance for Geoscience, Environment, andSociety (www.sages.ac.uk).

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