Geomorphology and environmental dynamics in Save River delta, Mozambique A cross-timescale perspective

Geomorphology and environmental
dynamics in Save River delta, Mozambique
A cross-timescale perspective
Elídio Massuanganhe
Department of Physical Geography
Stockholm University
Stockholm 2016
In memory of my mother Laurentina Samuel.
To my wife Leopoldina, and my children
Wendy and Elvy.
–
c Elídio Massuanganhe, Stockholm University 2016
c CNES 2011, Distribution Spot Image S.A., France,
Cover illustration: includes material SICORP, USA, all rights reserved
ISBN: 978-91-7649-311-3
ISSN: 1653-7211
Type set with LATEX using Department of Physical Geography thesis template
Published articles typeset by respective publishers, reprinted with permission
Printed by: Holmbergs, Malmö, 2016
Distributor: Department of Physical Geography, Stockholm University
Abstract
Long-term perspectives on the evolution of river deltas have provided useful knowledge capable
of responding to pending questions related to the ongoing climate and environmental changes.
Increasing utilization pressure on delta environments has necessitated increased attention to protect the socio-economic and ecological values. As a result, multiple local initiatives have been
designed, aimed at mitigating environmental deterioration and implementing adaptive measures,
but many such initiatives have shown limited success. This thesis uses a case study of Save River
delta in Mozambique to explore the relation between geomorphological evolution and socioecological system dynamics in delta environments. In addition, key environmental variables that
concern the society today are highlighted and discussed in a management perspective. The results
of the study show the development of Save River delta from the mid-Holocene to the present.
The geomorphological settings of the delta suggest a faulted coastline over which subsequent
deposition of fluvial sediments has formed a protruding delta front. Between c. 3000 and 1300
years ago, fine-grained sediments accumulated on top of the delta-front in the proximal part of
the delta. This type of material was deposited under intertidal conditions and supported the formation of mangrove habitat. The geographical distribution of the mangrove deposit was driven
by successive stages of back-barrier swamp formation and sea-level change as the delta evolved.
From c. 1300 years ago, the river delta started to receive fluvial sediments from pulses of floods
forming an alluvial floodplain. These sediments have accumulated mainly on the fine-grained
mangrove wetland deposit. All the geomorphological features have evolved in a shorewardshifting pattern over time. Centennial to decadal changes observed in the delta have followed a
predictable geomorphological pattern, which is also part of the millennial evolution. The mangrove system, the base for the socio-economic system, is consequently, and strongly affected
by the geomorphological development of the area. An increasing sensitivity of socio-ecological
systems to environmental stressors, e.g. floods, cyclones and erosion, has motivated multiple initiatives to work towards a sustainable management of delta environments. This thesis highlights
the need for interplay between geomorphology and ecology, considering both long- and shortterm dynamics of delta environments. Hitherto, management initiatives have been concentrated
on fragmented interventions of controlling water flow, which have disrupted the natural dynamics by obstructing the sedimentation-erosion cycle. To change this trend, coastal planners need
to consider the significance of natural processes, e.g. cyclones, floods, erosion and accretion, for
the long-term ecological and social sustainability of delta environments.
Keywords: Save River delta, deltaic wetlands, biogeomorphology, climate change,
landscape evolution, coastal management, socio-ecological systems.
Sammanfattning
Studier av utvecklingen av floddeltan har gett betydelsefull och användbar kunskap om pågående
miljöförändringar. Ett ökat antropogent tryck på deltamiljöer har krävt åtgärder för att skydda de
socioekonomiska och ekologiska värden som deltamiljöerna hyser. Lokala initiativ har vidtagits
för att mildra miljöförstöring och för att identifiera och implementera lokala anpassningsåtgärder.
Emellertid har många sådana initiativ visat begränsad framgång. Denna avhandling presenterar
en studie av Save River delta i Moçambique vars syfte är att undersöka kopplingen mellan deltats
geomorfologiska utveckling och socio-ekologiska systemdynamik. Miljövariabler som berör dagens samhällen i deltat diskuteras i perspektiv av naturresursförvaltning. Resultaten visar den
geomorfologiska utvecklingen av Save River delta från mitten av holocen till idag. Vid den
ursprungliga förkastningskusten har fluvial sedimentation utvecklat en avancerande deltafront.
Mellan ca 3000 och 1300 år sedan avsattes finkorniga sediment ovanpå deltat under dess tillväxt
ut från kusten. Detta finkorniga material deponerades under tidvattenbetingelser och skapade ett
lämpligt habitat för mangroveskog. Den geografiska fördelningen av detta habitat styrdes av successiva stadier av våtmarksbildning i laguner på landsidan av kustnära landformer som uddar och
dyner, samt av havsnivåförändringar under deltats utveckling. Från och med ca 1300 år sedan,
påbörjades deposition av fluviala sediment i form av pulser i samband med översvämningar.
Dessa sediment utgör idag en alluvial flodslätt, främst avsatt på tidigare finkorniga mangrovehabitat. Deltats landformer har utvecklats gradvis i riktning mot havet. I ett tidsperspektiv av
sekler och decennier har de observerade förändringarna i deltat följt ett förutsägbart geomorfologiskt mönster, som också är en del av deltats utveckling i perspektiv av tusentals år. Mangrovehabitatet, basen för det socio-ekonomiska systemet, påverkas följaktligen starkt av deltats
geomorfologiska utveckling. Det socio-ekologiska systemets ökande sårbarhet för stressfaktorer
i miljön, t.ex. översvämningar, cykloner och erosion, har resulterat i initiativ för en hållbar
förvaltning av deltamiljöer. Denna avhandling belyser behovet av samspelet mellan geomorfologisk och ekologisk forskning, i beaktande av både den långsiktiga och kortsiktiga dynamiken
i deltamiljöer. Hittills har förvaltningsinitiativ fokuserat på insatser att kontrollera vattenflöden. Dylika åtgärder hindrar de sedimentations- och erosionscykler som utgör en del av deltans
naturliga dynamik. För att ändra denna trend måste planeringen gällande deltamiljöer ta hänsyn
till betydelsen av naturliga processer, t.ex. cykloner, översvämningar, erosion och sedimentation,
för en ekologiskt och socialt hållbar utveckling i deltamiljöer.
Thesis content
This doctoral compilation dissertation consists of a summarising text and the 5 articles
listed below.
I
Massuanganhe, E.A., Westerberg, L.-O., Risberg, J., Preusser, F., Bjursäter, S.,
Achimo, M. Geomorphology and landscape evolution of Save River delta,
Mozambique. Manuscript
II
Massuanganhe, E.A., Berntsson, A., Risberg, J., Westerberg, L.-O., Christiansson,
M., Preusser, F., Bjursäter, S., Achimo, M. Palaeogeography and dynamics of
the deltaic wetland of Save River, Mozambique. Manuscript
III
Massuanganhe, E.A., Westerberg, L.-O., Risberg, J. Morphodynamics of deltaic
wetlands and implications for the coastal ecosystem – A case study of Save
River delta, Mozambique. Manuscript
IV
Macamo, C.C.F., Massuanganhe. E., Nicolau, D.K., Bandeira, S.O., Adams, J.B.
Under review at Aquatic Botany. Mangrove’s response to cyclone Eline (2000):
what’s happening 14 years later. Under review at Aquatic Botany.
V
Massuanganhe, E.A., Macamo, C., Westerberg, L.-O., Bandeira, S., Mavume, A.,
Ribeiro, E. (2015) Deltaic coasts under climate-related catastrophic events – Insights from the Save River delta, Mozambique. Ocean & Coastal Management,
116, 331–340.
Author contributions
The contributions from listed authors are divided as follows for each article.
I
My contribution: I designed and conceived the study, led the fieldwork and wrote
the earlier versions of the paper. I processed the aerial photos and SPOT images to
generate the geomorphological map. I was responsible for the interpretation of the
map, for creating and editing all the illustrations in the paper. I led the discussions.
Others’ contributions:L-OW and JR supervised the methodology, participated in
the fieldwork, discussed and co-edited the paper. SBj performed OSL dating and
presented the preliminary dates. FP supervised the OSL dating, improved the age
calculation, and together with SBj wrote the respective method section. MA contributed with discussions during fieldwork and supervised the part of the sample
collection.
II
My contribution: I designed and conceived the study, carried out sample collection
and field description and wrote the main part of the paper. I was responsible for
editing all the diagrams and I led the discussions.
Others’ contributions: AB, supervised by JR, carried out preparation of slides
for siliceous microfossil analysis, analyzed the samples and wrote the respective
methodology and result section. Jointly, AB and JR contributed to the edition and
discussion of the manuscript. L-OW supervised the study, contributed with discussions throughout all stages of the paper. MC analyzed the siliceous microfossils,
contributed with discussion during fieldwork. SBj analyzed the samples for OSL
dating under supervision of FP, and they were responsible for describing the respective section. MA participated in the fieldwork, and contributed with discussion
to the conception of the paper, and supervised the sample collection of at the sampling site “P5”.
III
My contribution: I designed the study, performed all the described methods and
wrote the earlier versions of the paper. I was also responsible for all the illustrations
in the manuscript and for leading the discussions.
Others’ contributions: L-OW and JR contributed to the discussion of the results
and to the overall language edition of the paper.
IV
My contribution: I jointly participated in one of the fieldworks and contributed with
discussions around the topic. I was responsible for processing the SPOT images to
calculate and compare the NDVI and I wrote the respective methodology and result
sections.
Others’ contributions: CCFM conceived and designed the study with inputs and
supervision of SBa. CCFM wrote the main part of the paper and led the discussions.
DKN and JBA contributed to the discussion of the paper.
V
My contribution: I conceived and designed the study under guidance of L-OW and
SBa. I wrote the main part of the paper and I took part of the multidisciplinary fieldwork. I led and coordinated the discussion and the contribution of the co-authors in
the paper.
Others’ contributions: L-OW contributed to the discussion part of the paper and
supervised the final editing. CM and SBa participated in the fieldwork and organized the focus group meeting in Nova Mambone and contributed to the edition and
discussion of the manuscript. AM searched for cyclone data and contributed with
his knowledge to discussion. ER undertook socio-economic survey and contributed
with the data to the discussion.
Contents
1
Introduction
1.1 Scope and objectives . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2
2
The Save River delta
5
3
Material and methods
3.1 Remote sensing and GIS (Papers I, III and IV)
3.2 Fieldwork (Papers I-V) . . . . . . . . . . . .
3.3 Laboratory Analysis (Papers I-III and V) . . .
3.4 System analysis (Paper V) . . . . . . . . . .
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4
Results
4.1 Summary of papers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Delta development . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Discussion
5.1 Chronological aspects of the delta . . . . . . . . . . . . . . . .
5.2 Timescale evolution of the delta . . . . . . . . . . . . . . . . .
5.3 Interdisciplinary aspects of research on river deltas . . . . . . .
5.4 Challenges for sustainable management in deltaic environments
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Conclusions
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Acknowledgements
7.1 Financial support . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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References
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1 Introduction
River deltas are depositional environments formed at the interface between land and sea
by a seaward progradation of river sedimentary bodies (Mathers and Zalasiewicz 1999,
Saito et al. 2000, Ta et al. 2002a, Gani 2005, Bird et al. 2007). They constitute a vital,
multifaceted natural resource. Their geological origin and sedimentological properties
have made them good reservoirs for natural resources with importance to the economy,
such as oil, gas, and heavy minerals (Suter 1995, Sidi et al. 2003). The extensive
plains of the deltas are in many cases also highly productive agricultural areas, ensuring
food supply for a large number of people (Wright 1978, Suter 1995, Stanley and Chen
1996, Mañosa et al. 2001, Atahan et al. 2007). In the lower areas, river deltas host
wetland systems with vital ecosystem services (Mitsch and Gosselink 2000, Costanza et
al. 2008, Day et al. 2008). In tropical climate regions, deltaic wetlands are dominated
by mangrove ecosystems, which in addition are important in a global climate context for
their carbon sequestration (Post et al. 1990, Donato et al. 2011, Breithaupt et al. 2012,
Hopkinson et al. 2012). The attractiveness of the deltas has been evidenced in early
human history, by e.g. Neolithic agricultural civilizations in the Yangtze delta and Nile
delta (Hassan 1985, Stanley et al. 1999, Yu et al. 2000, Stanley et al. 2003). Today, river
deltas continue to be densely populated and heavily exploited (Stanley and Warne 1998,
Ericson et al. 2006, Syvitski and Saito 2007, Syvitski et al. 2009, Seto 2011).
Most of the modern river deltas (cf. Giosan and Bhattacharya 2005b), started developing in the early Holocene, induced by the global deceleration of postglacial sea-level
rise (Stanley and Warne 1994, Hori et al. 2004, Hori and Saito 2007, Wanner et al.
2008, Tamura et al. 2009, Walker et al. 2012). The relatively stable sea-level during the Holocene enabled a general seaward progradation of sedimentary bodies, and
an ensuing reworking of river sediments by waves, currents and tides (Kroonenberg et
al. 2005, Overeem 2005, Vakarelov et al. 2012). Over time, various morphodynamic
processes have shaped different sub-aerial features and also different geometries of the
sedimentary bodies (Giosan 2005, Storms et al. 2005, Olariu and Bhattacharya 2006).
The different morphologies presented by deltas have intrigued geomorphologists and
sedimentologists who have been interested in understanding the main driving factors of
delta development (e.g. Galloway 1975, Wright 1978). Morphodynamic processes at
different scales have been interpreted from imprints left on the sub-aerial morphology
and from the internal architecture and geometry of the sedimentary bodies (Gani 2005,
García-García et al. 2006, Hanebuth et al. 2012). Further, facies analysis, initially
developed for fluvial environments (Miall 1977, 1985), has been applied in the interpretation of deltaic environments (Stanley and Warne 1998, Ta et al. 2002a, Ta et al.
2002b, Maselli et al. 2014). With the advent of seismic, radar and laser surveys, a more
complete picture of the internal geometry and architecture of the sedimentary bodies in
deltaic environments has been obtained. Comparisons of these interpretations with deep
cores have produced consistent models, which have been applied to reconstruct ancient
deltas (Bhattacharya and Giosan 2003, Tanabe et al. 2003, Davies et al. 2005, Gani
1
Geomorphology and environmental dynamics in Save River delta, Mozambique
2005, Olariu et al. 2005, Olariu and Bhattacharya 2006, Tamura et al. 2012, Hodgetts
2013) and used in petroleum geology.
Observed variations of the lithostratigraphic characteristics of deltas represent changes
in depositional conditions or in sediment supply (Miall 1985, Gani 2005, Bianchi and
Allison 2009, Riboulot et al. 2012). Depositional conditions in the deltas are linked to
e.g. climate variations and sea-level fluctuations (Tanabe 2003, Longhitano and Colella
2007, Maselli et al. 2014). Deltaic environments have therefore been the target for paleoclimatic reconstructions, using a variety of proxies (Riboulot et al. 2012, Di Bella et
al. 2013, Maselli et al. 2014, França et al. 2015, Sarti et al. 2015). By reconstructing
processes of delta development, paleoenvironmental conditions have been inferred (Liu
et al. 1992, Saito et al. 2001). Part of these data have been applied by archaeologists to
interpret the interaction between the early civilizations and the environment (Stanley and
Chen 1996, Stanley et al. 1999, Berendsen and Stouthamer 2000, Yu et al. 2000, Atahan
et al. 2007). These studies have shown the impacts of climate change on delta civilizations, impacts that remain relevant for the modern society in the face of prognosticated
climate change.
Despite the current understanding of delta evolution, the rate and the direction of
recent changes in deltaic environments are of societal concern. Many river deltas are
currently being affected by sea-level rise, which has been responsible for degradation
of part of the deltas, thereby affecting the socio-ecological system (Stanley and Warne
1998, Aung et al. 2013, Woodruff et al. 2013). In addition, compaction of recently
deposited sediments, coupled with exploration of natural resources, e.g. gas, oil, and
groundwater, has contributed to subsidence of river deltas (Jelgersma 1996, Blum and
Roberts 2009). In the list of the human and climate-related factors that negatively affect deltas, cyclones and floods are currently the most devastating ones (Webster 2008,
Seekins 2009, Woodruff et al. 2013). The vulnerability of deltas to these phenomena
is elevated because of high population densities and exploitation of natural resources
(Kuenzer and Renaud 2012). One of the deadliest cyclones ever, recently registered in
delta environments, was the tropical cyclone Nargis, which in 2008 made landfall over
the Irrawaddy deltaic plain in Myanmar, causing more than 130,000 fatalities (Webster
2008, Fritz et al. 2009). Comparable scenarios with different magnitude have been observed in other deltas, particularly in south-eastern Asia (Nguyen et al. 2007, Yang et
al. 2015). The south-eastern coast of Africa, where the Save River delta is located, has
also been affected by tropical cyclones (Mavume et al. 2009, Goni et al. 2010), which
have caused negative impacts on the society. Some of these cyclones have made landfall
directly on the Save River delta, causing substantial environmental changes (Massuanganhe et al. 2015). Under the scenario of a predicted magnitude increase of climaterelated events, the carrying capacity of the deltas may be compromised (cf. Arrow et al.
1995, Hopfenberg 2003). The incidence of cyclones and floods in coastal zones poses
challenges to preset coastal management plans. Examples of initiatives that have been
put in place are plans for adaptation to future climate changes
1.1
Scope and objectives
The available geological studies of modern river deltas have produced detailed understanding on their origin and also on the multiple functionalities that these environments
may offer to the society. In addition, researchers in many environmental sciences have
dedicated their research to river deltas and recommended management procedures to decision makers (e.g. Cui et al. 2009, Grumbine et al. 2012, Moser et al. 2012, Wei et al.
2015). However, one of the concerns is that knowledge produced has not yet integrally
2
Elídio A. Massuanganhe
answered questions related to recent environmental changes in river deltas (Giosan and
Bhattacharya 2005b). In addition, the relative prosperity that has characterized delta
inhabitancy is threatened by the socio-ecological vulnerability to environmental and climate change. Therefore this thesis uses a case study of Save River delta in Mozambique
to integrate geological, geomorphological and environmental data with the overall aim
to improve the understanding on the dynamics of river deltas. With this aim the thesis
intends to answer questions related to causal links between processes and environmental changes in the Save River delta. A secondary aim is to discuss the environmental
changes of the delta within a management perspective. Therefore, the thesis converges
on the following objectives:
i Systematize the evolution of the Save River delta across different timescales;
ii Analyse the implication of morphodynamic processes for the socio-ecological systems;
iii Analyse the interdisciplinary interplay of research in river deltas;
iv Identify possible alternatives to improve the current management policies of deltaic
environments.
By reaching these objectives the thesis will elucidate whether ongoing changes are part
of the natural dynamics of the deltas or if they are within the context climate change.
Furthermore, under either scenario, the study may contribute to identify links through
which natural stressors affect the environment. Thereby, the thesis falls within the “new
directions” for deltaic studies proposed by Giosan and Bhattacharya (2005a) and within
the new agenda of the United Nations that prioritizes research towards the sustainable
use of the natural environments (Lu et al. 2015).
3
Geomorphology and environmental dynamics in Save River delta, Mozambique
4
2 The Save River delta
The Save River delta is located in the south-central Mozambican coastal plain (Figure
2.1) and is one of the largest deltas in south-east Africa. Geologically, the study area
is part of the Mozambique sedimentary basin, which started to form between the Carboniferous and the Triassic with the fragmentation of Gondwana (McElhinny and Briden
1971, Salman and Abdula 1995). At its initial stage the basin was characterized by volcanic activities, which were responsible for deposition of volcanic rocks as continental
drift started (Förster 1975). Along the evolution process, transgressions and regressions
have controlled the deposition of marine and continental sediments within the basin.
Later consolidated, some of these rocks are represented by limestone of the Jofane Formation (Förster 1975), mapped in some areas along the river valley (DNG 2006). Part
of the surface of the surrounding area consists of Quaternary reworked sediments that
cover the Jofane Formation.
The catchment area of the river is calculated to c. 102,000 km2 and most of the
area is located over the Zimbabwe Craton. In Mozambique the river runs on Cretaceous,
Paleogene, Neogene and Quaternary formations. At the distal part the, river splits into
two distributary channels (Figure 2.1). During the rainy season (October to March) the
high precipitation in the catchment area increases the water flow of the river, occasionally causing floods in the delta. The most severe floods are generally associated with
the landfall of tropical cyclones and storms. At least three major cyclones have made
landfall in the study area over the last 15 years: Cyclone Eline in 2000, Cyclone Japhet
in 2003, and Cyclone Favio in 2007. From the listed cyclones, Eline was one of the most
deadly in the region (Vitart et al. 2003, Jury and Lucio 2004, INGC 2009). Cyclones
and floods have also caused environmental changes, with special emphasis on fluvial
erosion and mangrove dieback (Figure 2.2). The study area and the whole southern region of Mozambique are under the influence of south-eastern winds. These winds affect
the surface morphology of the region, which is characterized by coastal dunes pointing to northwest. The wind prevalence has also induced northward longshore currents,
which are responsible for a general drift of shore sediments. This drift pattern has also
influenced the morphodynamics of the shore (Massuanganhe and Arnberg 2008). The
tides in the delta are semi-diurnal and macrotidal and they induce bidirectional currents
in distributary and tidal channels.
In the study area, there are two rural villages located by the river, one on each side:
Nova Mambone and Machanga (Figure 2.1). These are headquarters of Govuro and
Machanga districts. In 1999, the total population for both villages was projected to
reach 46,000 in 2002 (INE 1999). The main activities of the villagers are fishery, timber collection (for domestic use), livestock and agriculture. Fishery is practiced along
the channels and also in the open sea. Fishermen use small boats to run their activities,
and work in small groups that camp for days or weeks in the mangrove area or in other
remote sites along the shore (Figure 2.2 D).
5
Geomorphology and environmental dynamics in Save River delta, Mozambique
Figure 2.1. Geographic location of the study area showing in A – the geographical
settings of the Save River Basin (in grey colour) in reference to the southern region of
Africa. B – SRTM topographic elevation model overlaid with the limits of the Save River
basin, and with the simplified network drainage system. C – SPOT image showing the
Save River delta and the location of the villages. D – Spot image zoomed in from the
white dashed rectangle on “C” showing the sampling sites.
Despite the presence of the two villages in Save River delta, the human impact on
the natural system is minimal. Activities are controlled by the authorities, local leaders
and by the community. For example, villagers are allowed to cut mangrove timber for
household purposes but not for sale or export. Furthermore, villagers are encouraged to
collect driftwood from dead mangrove to use as firewood instead of cutting living mangrove. Agriculture is currently practiced on the deltaic floodplain at subsistence level
and for small-scale commercial purposes. In the uppermost part of the delta, however,
an irrigation project is being engineered, covering c. 1000 ha of arable land. It is esti6
Elídio A. Massuanganhe
Figure 2.2. A – Effects of riverbank erosion in Nova Mambone. B – Young mangrove
forest flourishing in the distal sector of the delta. C – Dead mangrove trees on the lower
deltaic plain of Save River. D – Campsite for fishermen in the mangrove area. Photos
by: Massuanganhe.
mated that this project will demand c. 4.13 million m3 of water per year with a maximum
monthly capacity of 910 thousand m3 . This volume of water will be pumped from the
Save River, c. 30 km upstream the river outlet.
7
Geomorphology and environmental dynamics in Save River delta, Mozambique
8
3 Material and methods
3.1
Remote sensing and GIS (Papers I, III and IV)
In this thesis one time series of black and white analogue aerial photos and five time
series of SPOT images covering the study area were used for map production. The aerial
photos, produced in 1963 and 1964, were purchased from CENACARTA (Centro Nacional de Cartografia e Teledeteção), a national institute responsible for the geographical data in Mozambique. The four SPOT images, from 1999, 2000, 2007 and 2011,
were provided by Planet Action (http://www.planet-action.org/) under the WIOMSA
joint project between Kenya and Mozambique and the image from 2014 was purchased
from MapMart by Stockholm University. The SPOT images were already georeferenced
by the providers on WGS84 datum and projected to UTM, zone 36 S. The images had
the four typical spectral bands, except the one from 2000 as detailed in Table 3.1. The
aerial photos and SPOT images were interpreted using different methods in the different
stages of the study. In the first stage, as part of the desk study, the interpretation was
undertaken with focus on planning for the fieldworks. Subsequently, detailed maps were
produced to respond to specific objectives of each Paper. Therefore, aerial photos were
digitized in 300 dpi, georeferenced, and projected according to the SPOT images. After
georeferencing, the aerial photos displayed a mosaic according to the references given
to each aerial photo. Simultaneously analogue photos were analysed under stereoscope,
and the result of the analysis was vectorised on the displayed mosaic. Each SPOT image
was classified with ISODATA classifier to generate 15 classes which were subsequently
grouped to match the geomorphological features interpreted from the aerial photos and
the SPOT images. Successive comparisons of the resulting maps were made using ArcGIS to identify the patterns of the changes (Paper III) and interpreted together with
results from field observations (Paper I, II and III).
Table 3.1. Details of the remote sensing data used in the study. Green (G), Red (R),
Near-Infrared (NIR), Short-Wave Infrared (SWIR).
Data
Date taken
Spectral bands
Aerial photos
September 5, 1963 and- (Black and white; 1:20 000
November 16 and 27, 1964 greyscale)
SPOT image
June 23, 1999
G, R, NIR, SWIR
20 m
SPOT image
May 26, 2000
G, R, NIR
20 m
SPOT image
April 27, 2007
G, R, NIR, SWIR
10 m
SPOT image
April 13, 2011
G, R, NIR, SWIR
20 m
SPOT image
October 1, 2014
G, R, NIR, SWIR
6m
9
Spatial scale or
resolution
Geomorphology and environmental dynamics in Save River delta, Mozambique
For Paper IV, Normalized Difference Vegetation Index (NDVI) was calculated from
each SPOT image to compare the progressive development or mortality of the mangrove trees in the forest. The calculation of NDVI excluded areas covered by terrestrial
vegetation which coincide with the coastal dunes and alluvial floodplain in the upper
part. Simultaneously, a 10x10 m square polygon fishnet covering the area of maps and
the respective centroids were created in ArcGIS 10.2. The centroids were used to read
the values of each NDVI map and export to the attribute table of the fishnet. Further
arithmetical calculations were undertaken, aiming to compare in continuous scale the
difference of the NDVI in different years
3.2
Fieldwork (Papers I-V)
Field trips were organized during multiple campaigns between 2011 and 2015. The first
visit to the study area was undertaken by a multidisciplinary team consisting of socioeconomists, ecologists, oceanographers and geologists/geomorphologists. In the field,
observations were made with focus on the landscape characteristics previously analysed
and described in the desk study, and on changes that have occurred within the study
area. These changes were discussed in the field and correlated with the possible impacts
of climate-related catastrophic events that have affected the study area. In the mangrove
area the research team conducted informal interviews with fishermen and other people
frequenting this area to get their perception about changes in the mangrove area.These
informal interviews were extended to semi-structured interviews with the villagers to
understand how they had faced the changes in the area. Furthermore, a meeting with
administration representatives, local youth association representatives (AJOAGO – Associação de Jovens e Amigos de Govuro), fishery representatives, and local leaders was
organized in Nova Mambone. The objective of the meeting was to brainstorm about
climate-related catastrophic events and find out how the communities and the authorities
shared the responsibilities to manage impacts caused by the calamities.
From 2012 onwards, fieldworks focused more on the process-based geomorphological and sedimentological aspects of the delta. Sixteen sampling sites were considered
for detailed descriptions (Figure 2.1). These sites included cut-banks of the river, coastal
dunes and other characteristic features within the study area (Figure 3.1). The geomorphological description consisted of a characterization of the different landscape features,
relating them to the maps interpreted in the desk study. Simultaneously, sedimentological descriptions were undertaken in open sections along the riverbanks. For other locations in the floodplain, cores were retrieved using a piston corer and peat and auger
samplers (Figure 3.1 A). The sediments were described macroscopically and some of
them were selected for further laboratory analysis. Samples for Optically Stimulated
Luminescence (OSL) dating were collected in open pit (Figure 3.1 B) and open sections
by forcing in a plastic PVC tube of 5 cm diameter. The samples were retrieved under
dark conditions using a black plastic cover.
3.3
Laboratory Analysis (Papers I-III and V)
Samples collected in the field were sent to different laboratories for analysis. Siliceous
microfossil analysis was undertaken at Stockholm University, and the process consisted
of two main stages: preparation of samples and analysis of slides under microscope. In
the laboratory, 10% HCl were added to the samples to remove carbonates, and samples
were then boiled in 17-35% H2 O2 to oxidize organic matter. After the boiling ceased, a
solution of NH3 was added to dissolve clay aggregates. The residue consisting of the silt
10
Elídio A. Massuanganhe
Figure 3.1. A – Piston corer equipment on the supratidal flat in the lower part of Save
River delta. B – Collection of OSL sample from a dune, at c. 1.5 m depth. C – Example
of cores retrieved by piston corer.
R
fraction was spread on slides and embedded in Naphrax
to increase the refraction index
(Peabody 1977). Subsequently, microfossils have been identified and counted under
microscope using immersion oil to further increase the refraction index. The statistical
results have been processed using Tilia 1.7.16 software. Samples consisting of plant and
root remains and bulk sediments were dated in the Poznan Radiocarbon Laboratory, and
a shell sample was dated in the Ångström Laboratory, Uppsala University. The results
reported by the Laboratories were calibrated using OxCal 4.2 according to ShCal 13.
OSL dating was performed at Stockholm University. Background measurements for
the OSL analyses were carried out in Laboratory for Radionuclide and Environmental
Analyses (VKTA), Dresden.
3.4
System analysis (Paper V)
A system analysis approach was used to structure the interaction of socio-ecology and
climate-related catastrophic events in river deltas. This methodology consisted first
of a literature-based listing of socio-ecological variables relevant for the livelihood of
Nova Mambone and Machanga villagers (e.g. INGC 2009, Menomussanga and Matavel
2011). These variables showed the basic social settings of the study area and the incidence of the climate related catastrophic events in the region. Further, literature focusing
on climate-related catastrophic events in southeastern Africa was reviewed (e.g. Jury and
Lucio 2004, Mavume et al. 2009, Arndt et al. 2010, Manhique et al. 2011, Matyas and
Silva 2011). Preliminary results from interviews in the field were also added to the list
of variables, for the purpose of identifying links that operate in this environment. Finally, part of the data used for systems analysis was obtained from informal interviews
and from focus group meetings held in Nova Mambone. After listing the variables to
11
Geomorphology and environmental dynamics in Save River delta, Mozambique
be used in the study, a Causal Loop Diagram (CLD) was constructed using Vensim PLE
software. The construction of the CLD aimed at establishing cause-effect links between
the listed variables and at systematically grouping them according to their similarity. The
linking arrows were assigned a plus (+) sign to indicate the same direction of change,
or a minus (-) sign to show opposite directions of change. Feedback loops were identified using the Vensim PLE, and further discussions on the relevance of the loops were
undertaken in harmony with all the interplaying variables.
12
4 Results
4.1
Summary of papers
Paper I
Massuanganhe, E.A., Westerberg, L.-O., Risberg, J., Preusser, F., Bjursäter, S., Achimo,
M. Geomorphology and landscape evolution of Save River delta, Mozambique. Manuscript.
The aim of this paper is to interpret and map the geomorphological characteristics
of the Save River delta and to discuss its evolution. Therefore, aerial photos and SPOT
images were interpreted to produce a geomorphological map of the study area. Sedimentological characteristics were described in open sections of the riverbanks and from
sediment cores. In addition, samples were collected for radiocarbon and OSL dating.
The results of this study are presented as a geomorphological map showing a seaward
protruding alluvial floodplain, which follows the current position of the main river channel. This feature is superimposed on pre-Holocene and Holocene deposits and crosses
almost perpendicularly, the straight line boundary between the deposits. This line suggests the existence of a fault and indicates a pre-Holocene shoreline. Mangrove wetland
occupies the lower part of the delta and encompasses aligned dunes in the southern part
of the delta. These dunes were most likely formed as a response to the wave pattern refracted progressively by the delta-front during the stages of the delta progradation. The
development of the dunes created sheltered back-barrier areas where fine-grained sediments started to deposit. Close to the current position of the river channel, delta-front
deposits are found underneath the fine-grained sediments that have been correlated with
the current mangrove deposit. In the lowermost stratigraphic position in the wetland
area, radiocarbon dates indicate that the deposition of brackish marine sediments commenced c. 2200 cal. yrs BP at c. 6 m depth. At this depth the brackish-marine deposit
ends as indicated by the occurrence of limestone, which has been interpreted as part of
the basement of the delta. Overall, the development of the delta has been dominated by
fluvial processes and controlled by waves.
Paper II
Massuanganhe, E.A., Berntsson, A., Risberg, J., Westerberg, L.-O., Christiansson, M.,
Preusser, F., Bjursäter, S., Achimo, M. Palaeogeography and dynamics of the deltaic
wetland of Save River, Mozambique. Manuscript.
The aim of the study in Paper II is to reconstruct the distribution of mangrove wetland
deposits over millennial and centennial timescale in the Save River delta. The distribution was correlated with the recent situation where mangrove ecosystems are currently
developing. The central idea is to back-track the evolution of mangrove deposits and
to identify processes that have influenced their dynamics. Therefore, lithostratigraphic
13
Geomorphology and environmental dynamics in Save River delta, Mozambique
descriptions were undertaken at eight sampling sites located in open sections of the cut
banks of the river and across the delta plain. In addition, microfossil analysis was performed to interpret the paleosalinity and the paleoenvironmental conditions that the deposit might have experienced during its development. In this study, mangrove deposits
have been correlated with an identified fine-grained layer. At the upper part of the delta,
this layer is interbedded in-between sand layers while in the lower part of the delta it
occupies the uppermost stratigraphic position. This layer is in general characterized by
brackish-marine and freshwater diatoms in its lowermost part while the upper part is
marked by a scarcity of diatoms. For the sampling sites located in the transition area between the alluvial floodplain and the mangrove wetland the uppermost part of the cores
is characterized by phytoliths. The fine-grained layer has been developing as part of
the delta progradation and it started to form c. 3000 years ago and continues to form
today in the current mangrove wetland. Around 1300 years ago massive sedimentation
of sand over the wetland deposits may have started and progressively caused permanent
degradation of the wetland. The massive sedimentation has been interpreted as a result
of pulses of high discharge of water during flooding events in the delta. Towards the
edges of the accumulated sediments gully erosion has removed the overlaying alluvial
clay and exposed the once extinct mangrove habitat which is currently recolonized by
a new generation of mangrove. In this paper it is concluded that the current mangrove
ecosystem has been evolving spatially from former mangrove habitat developed as part
of the delta progradation and influenced by sea-level change. In addition, it has been
observed that the mangrove ecosystem can reclaim the extinct habitat when necessary
conditions of water flow and sea-level are created.
Paper III
Massuanganhe, E.A., Westerberg, L.-O., Risberg, J. Morphodynamics of deltaic wetlands and implications for the coastal ecosystem – A case study of Save River delta,
Mozambique. Manuscript.
Paper III aims to assess the pattern of changes of the deltaic wetland of Save River
under an interdecadal timescale. The study focuses on the links between the morphodynamic processes and their effects on the mangrove wetland ecosystem. Time series of
aerial photos and four SPOT images covering the study area were interpreted and processed in GIS. The aerial photos used in this study are from 1963/4 and the SPOT images
from 1999, 2007, 2011 and 2014. The remote sensing data were processed to generate
time series maps showing the landscape characteristics of the lower deltaic plain, with
particular emphasis on the mangrove and the coastal dunes. Each map was overlapped
on the subsequent one, to outline spatial changes between each pair of maps. The resulting four change-detection maps were interpreted considering the morphodynamic
processes of fluvial and coastal processes in the delta. Ground-truthing was carried out
in the study area and additional field-based descriptions were performed to support the
change-detection maps. The results show a general increase of mangrove wetland area
over the time period in analysis. However, the rate at which the mangrove area has increased, has reduced over time, and between 2011 and 2014 the rate was negative. The
observed change on the mangrove wetland area has followed a morphodynamic process
typical of the deltaic wetland. Part of the accreted mangrove area is located in an avulsed
distributary channel, where accumulation of fine-grained sediments has occurred. Mangrove has also expanded at the back barrier sectors of the delta, sheltered by coastal
dunes. The development of a barrier spit has also contributed to mangrove dieback as
14
Elídio A. Massuanganhe
the spit shifted landward in response to the prevailing southeasterly winds. Hence, it is
concluded that the expansion and retreat of mangrove wetland area result dominantly
from natural morphodynamic processes of the delta.
Paper IV
Macamo, C.C.F., Massuanganhe. E., Nicolau, D.K., Bandeira, S.O., Adams, J.B. Under
review at Aquatic Botany. Mangrove’s response to cyclone Eline (2000): what’s
happening 14 years later. Under review at Aquatic Botany.
Paper IV investigates the impacts of cyclones on the mangrove forest in the deltaic
wetland of Save River. This study gives particular emphasis on the impacts of the cyclone Eline in 2000, one of the most devastating cyclones ever registered in the region.
Furthermore, this study has investigated the dynamics of the mangrove forest in the study
area, over the subsequent 14 years. Hence, SPOT images taken in 1999, 2000, 2007,
2011 and 2014 were used to calculate the NDVI for each year in analysis. A 10x10 fishnet and the respective centroids covering the study area were generated and the centroids
were used to read the NDVI values on each map. The NDVI values were exported to a
table of attributes of the polygon fishnet. To assess the possible changes in the NDVI
values between 1999 (before Eline) and 2000 (after Eline) the arithmetical difference
between the NDVI from both years was calculated. The study was also supported by
ground-truthing for measurements of mangrove structure in areas that were considered
as impacted by the cyclones. The results show a considerable area of mangrove impacted
by the cyclone Eline. Most of the mangrove forest area has been defoliated by this cyclone, but in the subsequent years part of the mangrove has regenerated. This study has
confirmed that the degradation of the mangrove habitat can be followed by regeneration
if the necessary conditions of water exchange are created.
Paper V
Massuanganhe, E.A., Macamo, C., Westerberg, L.-O., Bandeira, S., Mavume, A.,
Ribeiro, E. (2015) Deltaic coasts under climate-related catastrophic events – Insights
from the Save River delta, Mozambique. Ocean & Coastal Management, 116, 331–
340.
Paper V summarizes and documents climate-related catastrophic events and their
chain of impacts that have occurred in deltaic coasts. Specifically, the paper assesses the
interaction between these events and the socio-ecological system within the perspective
of management. The study applies a case study on the Save River delta, making comparisons with other examples to interpret the complex links between the processes that
characterize the deltaic coasts under climate-related catastrophic events. A field visit
to the Save River delta was undertaken by a multidisciplinary research team to assess
the impacts of the climate-related catastrophic events that have been reported, including
the impacts of the cyclone Eline in 2000. During this visit, semi-structured interviews
were conducted and a focus group meeting was organized to discuss the perception of
the local communities on climate-related catastrophic events and their implications on
the communities. The outcomes were structured in a Causal Loop Diagram (CLD) to
show the typical interactions between multiple variables that are involved in the process. The results show that cyclones and floods are the most devastating climate-related
catastrophic events in the deltaic coast. These events have caused fatalities in most of
15
Geomorphology and environmental dynamics in Save River delta, Mozambique
the affected deltas and degraded the ecosystem that the communities rely on for their
subsistence. The adaptation measures used in the study area to cope with climate-related
catastrophic events include the sensitization of the communities for a sustainable use of
the ecosystem. This measure is highly effective as the communities rely on the mangrove
ecosystem for their livelihood. If, on the one hand, the local communities are interpreted
as stressors on the deltaic environments, on the other hand, well-sensitized communities
can be seen as contributing to the preservation of the natural ecosystem.
4.2
Delta development
This study proposes the first insights on the geomorphological development of Save
River delta and its environmental dynamics. A general model of the delta development,
summarized in longitudinal cross-section alongside representative photographs from the
different sedimentary units, is presented in Figure 4.1. Fluvial sediments from the deltafront dominate the lower part of the stratigraphy. Sandy sediment containing shells
were found in this layer at P12 (cf. Figure 2.1), indicating a marine influence over
the delta-front platform. The intermediate layer is the fine-grained layer, which has been
interpreted as mangrove deposit in Paper II. In the proximal sector of the delta the finegrained layer is superimposed by fluvial sediments accumulated during flooding events.
This layer is labelled as an alluvial floodplain and constitutes the higher ground on which
settlement and agricultural lands are situated.
The sedimentation pattern in the Save River delta has varied according to different
geomorphological settings. Fine-grained sediments have accumulated more continuously in comparison with the coarse-grained sediments. In back-barrier sectors for example, current accumulation occurs during daily high tide conditions. Coarse-grained
sediments in the alluvial floodplain have accumulated during seasonal flooding events,
occurring with irregular frequency and intensity. Thus, while the accumulation of finegrained sediments (Figure 4.1 A) occurs with high predictability, the accumulation of
coarse-grained sediments on the alluvial plain is difficult to predict in specific sites, owing to the complex variables that operate during flooding events. This is illustrated by
sandbags, used to mitigate the effects of erosion during one flooding event. In subsequent
years, more than 60 cm of coarse-grained sediments with trough cross-stratification accumulated in the same site (Figure 4.1 B).
16
Figure 4.1. A – Idealized cross-section showing the main lithostratigraphic features identified in the Save River delta. B – Fine-grained sediments, mostly
clay, superimposed on nearshore sand in a back-barrier sector where new mangrove is developing. C – Sandbags previously used to mitigate erosion, covered
by trough cross-stratified sand (interpreted by dashed line in the figure). D – Open section at P1 showing coarse-grained layer (sand, and sand and silt) of
the alluvial floodplain, superimposed on the fine-grained layer (clay). Photos by: Massuanganhe.
Elídio A. Massuanganhe
17
Geomorphology and environmental dynamics in Save River delta, Mozambique
18
5 Discussion
The timescale in Figure 5.1 covers the time period during which the modern river deltas
of the world have developed (cf. Hori et al. 2004, Bird et al. 2007, Hori and Saito 2007,
Tamura et al. 2009). Over the logarithmic timescale, the time period covered by each
paper in this thesis is placed. Figure 5.1 shows that the different studies that compose
this thesis partially overlap, and that they together cover a representative portion of the
evolution of modern river deltas. Exploring the bridges between the papers is a potential key towards an integral understanding of the development processes of the deltas.
Paper I covers the long-term evolution of the Save River delta, discussing the primary
driving factors in a geological perspective. Although the chronology obtained in this
paper records back to c. 3000 years ago, insights for the entire evolution process can
be deduced. Paper II and Paper III employ both millennial and centennial timescales to
broach the evolution perspective using the morphodynamic approach. Both papers make
a bridge between the millennial timescale of Paper I and the current processes discussed
in Paper IV and Paper V. The appended Papers represent different disciplines, which are
to some extent fragmented. Interweaving these disciplines is relevant to understand the
dynamics of deltas.
Figure 5.1. Representation of the appended papers with regards to timescale of analysis
and disciplinary domains. The intensity of the greyscale of the papers and the domains
indicates their respective position.
19
Geomorphology and environmental dynamics in Save River delta, Mozambique
5.1
Chronological aspects of the delta
The combination of radiocarbon dates, OSL dates and the comparative analysis of the
lithostratigraphy is essential to set the timing of the landscape development. Dates obtained in this study fall within the mid- and late-Holocene (cf. Walker et al. 2012).
Although the dates do not cover the full stratigraphy of the delta, owing to the shallowness of the cores, the depths reached are sufficient to support the lithostratigraphical
interpretation of the upper layers and the development model of the delta. The inverted
position of the radiocarbon dates in reference to the stratigraphy at P2 and P11 indicates
that the obtained ages are not coeval with the deposition of the layer (Paper I and Paper
II). At P2, the four radiocarbon dates from the c. 3 m fine-grained layer (clay to silty sand
and fine sand) indicate ages between c. 2050 (Poz-53660) and c. 750 cal. yrs BP (Poz53659). The layer rests on coarse sand with an age of c. 250 years (OSL), i.e. similar to
the OSL-age of the sand that covers the fine-grained layer. In this particular example the
OSL-ages are likely to be more reliable than the radiocarbon ages, if the error margins
are considered. The radiocarbon-dated material is interpreted to have been brought from
multiple sources, resulting in reworking of old organic material (cf. Stanley and Hait
2000). Considering the young age of both the underlying and superimposing sand, the
accumulation of the fine-grained layer must have been rapid. The rapid accumulation is
also supported by convolute bedding structures that characterize the interface between
the lowermost part of the fine-grained layer and the underlying coarse sand.
The inverted radiocarbon dates in P11 occur in the lower part of the section, where
dates vary between c. 4900 (Poz-60013) and c. 1500 cal. yrs BP (Poz-60015). Most
of the dates are obtained from plant and root remains incorporated within the interstratified sand. Therefore, multiple sources of the dateable material are likely. However, the
lowermost date, c. 2200 cal. yrs BP (Poz-60016) (Paper I), was obtained from plant
fragments incorporated in a clay layer, with reduced possibility to have been reworked.
This date is close to the OSL-age (c. 2050 years) of the stabilized dune at P10 (Paper
I), interpreted as part of a dune barrier that sheltered the back-barrier sector at P11. This
supports the radiocarbon date and thus the time when nearshore sediment accumulation
took place. Under the scenario of multiple sources and reworking of accumulated organic material, OSL dating is more reliable than radiocarbon dating (Huckleberry and
Rittenour 2014). Thus, the OSL dates are interpreted to be coeval with deposition of the
dated layer. The dates are in agreement with the evolution model proposed in this study.
Hence, they can be used to support radiocarbon dates, such as the example given above,
and as at P1, where an OSL date (c. 1300 years) and a radiocarbon date (c. 1000 cal. yrs
BP) show near-overlap within their error margins (Paper II). The two dated layers are
stratigraphically close, which suggests that the radiocarbon dates at P1 may be coeval
with their respective layer.
5.2
Timescale evolution of the delta
The millennial timescale in which the geological and geomorphological features have
been interpreted gives a generalized outlook regarding driving processes. Geological interpretations, for example, have given relevance to the relative ages between the mapped
units. Each unit in turn, has been assigned an absolute age or time interval based on the
geological timescale. Similarly, although the geomorphological map of Save River delta
(Paper I) can give an impression of being composed of static features with defined ages,
the cyclic accumulations of fluvial sediments that compose the alluvial floodplain have
shown the continuity of its development until today. These dynamics apply also for the
20
Elídio A. Massuanganhe
mangrove wetland, standing as a well-defined unit but revealing progressive changes,
discerned by short time-interval sedimentary records and by morphodynamic changes.
Another cross-temporal example of the delta evolution is the dune (chenier) within the
mangrove area at P12. This dune was formed c. 950 years ago, and at this site, the
stratigraphic arrangement of dune sand, fine-grained layer and fluvio-marine sand (Figure 5.2 B) is interpreted to represent different stages of the delta formation. Current
nearshore morphodynamic processes illustrate how the stratigraphical arrangement at
P12 may have been formed (Figure 5.2 A). Here, at the active shoreline, the biogeophysical driving factors are more discernible (Paper III) and give an impression of erratically
occurring, rapid changes. However, the comparison of the two sites indicates that such
changes are part of the long-term delta dynamics.
The dunes and spits at the shoreline are elongated in a northward direction. Upon
reaching a certain height, dunes shift landward owing to the prevailing south-easterly
winds. As the dunes shift, they cover mangrove wetland causing conspicuous damage
on the ecosystem. However, the spits and dunes also create back-barrier conditions,
Figure 5.2. Diagrammatic representation of geomorphological processes in the lower
deltaic plain of Save River delta analysed under different timescale perspectives; A – A
current northward growing, and landward shifting dune (chenier) (Paper III), covering
the fine-grained layer that composes the mangrove habitat; B – Stabilized dune (chenier)
superimposed on the fine grained layer within the mangrove area, a process dated to c.
950 years ago (Paper I).
21
Geomorphology and environmental dynamics in Save River delta, Mozambique
thereby promoting the accumulation of fine-grained sediments that constitute the mangrove habitat. At the shoremost back-barrier mangrove area fine-grained sediments have
been accumulating for a time period of approximately 50 years (Paper III). Although the
thickness of these sediments is unknown, the rapid colonization of mangrove at this site
indicates a rapid accumulation, probably comparable with the rate of fine-grained sediment accretion at P12, where OSL dates (with error margins considered; Paper I: Table
2) indicate that 1.5 m of fine-grained material accumulated during a surprisingly short
period. Hence, it can be inferred that the delta evolution, including mangrove habitat
formation is strongly influenced by coastal processes forming beach ridges such as spits
and dunes (cf. Roy et al. 1995, Otvos 2000, Van et al. 2000, Rao et al. 2015).
The importance of applying a time perspective on the morphodynamic processes is
also evident from a cut bank in Nova Mambone. At this site, accelerated erosion has
occurred during each flood (Figure 2.2 A), and has been considered as one of their consequences. This has also been observed elsewhere and from laboratory experiments (e.g.
Kesel et al. 1974, Tal and Paola 2007), and like the morphodynamic processes along the
sea shore, the cut bank erosion contributes to reinforce the general understanding that
deltas are highly dynamic environments (cf. Ericson et al. 2006, Nicholls et al. 2007,
Satyanarayana et al. 2011). However, in both examples the rapid changes can be integrated within the long-term perspective of delta evolution.
5.3
Interdisciplinary aspects of research on river deltas
The multiple disciplines that have produced knowledge on river deltas have acted within
specific timescales (Haber 2004, Dahdouh-Guebas and Koedam 2008). In the geological
domain (Figure 5.1), the Quaternary Geology leads a number of studies under millennial timescales. Most of these studies have been undertaken under reciprocal interdependence with the geomorphological interpretation (e.g. Paper I and Paper II). Both domains
have produced comprehensive data on the long-term evolution process of the deltas (e.g.
Woodroffe 1993, Van et al. 2000, Vandenberghe 2002, Amorosi et al. 2008). In Paper I
and Paper II, on the one hand, the geomorphological interpretations are correlated with
the depositional processes, which in turn are related to the deposited sediments. On the
other hand, questions from the geological and sedimentological perspective, such as the
cyclic sediment sequences with darkening upward trend, find answer from geomorphological interpretation of the alluvial floodplain. The reciprocity between both domains
has been strengthened by the similarity in their origin as earth sciences. Archaeology
for example has easily interplayed with Quaternary Geology exploring the timescale that
both disciplines share (e.g. Dickinson et al. 1998, Stanley et al. 1999, Berendsen and
Stouthamer 2000, Stanley et al. 2003). However, the interaction has been limited to the
application of geological information to understand the origin of certain archaeological
artefacts and the dynamics of the civilizations. The application of archaeological studies
to understand the dynamics of geology is weak, possibly because the implications of
anthropogenic activities under millennial timescale are negligible.
For the short timescale, research on river deltas is dominated by environmental studies, which have been based primarily on the dynamics of the mangrove ecosystem (cf.
Jimenez et al. 1985, Smith III et al. 1991, Aung et al. 2013, Alongi 2014, Giri et
al. 2015). Within the environmental sciences domain, the ecology is a central discipline. Like many studies in ecology (cf. Escobar 1998, Macintosh and Ashton 2002,
Butchart et al. 2010, Barrett et al. 2011), Paper IV and Paper V are aligned with the
principle of “conservation ecology” in which mangrove dieback is often interpreted as
a negative impact. These examples have been focusing on the current status of the suc22
Elídio A. Massuanganhe
cessfulness of the environment, which is measured by the current and recent state of
the ecosystem. However, the mangrove dynamics follow a long-term landscape dynamics associated with the morphodynamics of the delta (Paper II and Paper III). Within the
long-term perspective, the morphodynamic processes interplay with essential parameters
(e.g. sedimentation, salinity, bioturbation, water exchange) used in the ecosystem field
(cf. Burchett et al. 1984, Macintosh and Ashton 2002, Gilman et al. 2008, Bandeira et
al. 2009). Therefore, the interplay between the ecology and geomorphology is essential
for integral understanding of landscape dynamics. Multidisciplinary studies involving
both fields have met in an emerging field termed “Biogeomorphology” (e.g. Thom et al.
1975, Phillips 1995, Naylor 2005, Haussmann 2011), but although holistic knowledge
has been produced under this integrated field, the contribution from ecology is less than
the contribution from geomorphology (Haussmann 2011, Stine and Butler 2011). One
of the reasons for this might be the different agendas that the two disciplines are called
to fulfil. The conservation perspective of the ecology and the high expectations of the
society on the ecosystem services (cf. Butchart et al. 2010) is a hinder for many studies
to acknowledge geomorphologists’ view that natural dynamics may be responsible for
the perceived environmental degradation. Relevant support for the field of biogeomorphology have been developed using modelling platforms of the intertidal environments
(e.g. Overeem 2005, D’Alpaos et al. 2007, Mazda and Wolanski 2009, van Asselen et
al. 2009), but much more is needed to accommodate the complex variables driving the
self-organization process in this landscape.
5.4
Challenges for sustainable management in deltaic environments
Currently, most of the management initiatives of deltas are based on a human, short-term
perspective. Therefore, management has targeted biogeophysical components with the
aim to prevent what has been observed as negative for the environment, often without
consideration of long-term delta evolution. Contrary to long-term delta dynamics, recent
conditions include variables such as infrastructure development and population increase
(Stanley and Warne 1998). Moreover, management of river deltas is even more complicated, because on the one hand environmental managers have to deal with environmental
problems that have implications for communities, and on the other hand they have to deal
with the conservation of natural ecosystem. Therefore, the common ground used by the
managers is to safeguard both the ecosystem and the communities (Paper V) which is, in
practice, a challenging task. One of the additional difficulties is how the impacts of the
climate-related events are interpreted; often seen by the society as negative processes for
river deltas. Unquestionably, these events have concerned coastal managers and caused
enormous losses in the last two decades, but on the other hand they are important to the
natural cycle of deltas.
In a short timescale, cyclones and floods are the most disastrous processes affecting
river deltas (cf. Paper IV and Paper V), and they have implications for human livelihood
and for the progressive dynamics of river deltas. Present management practices in river
deltas focus on controlling fluvial erosion, through constructing artificial dykes, by using dams in river catchments, or by diverting rivers to control the amount water reaching
the delta (cf. Hudson et al. 2008, Kolker et al. 2012). These anthropogenic activities
aim to reduce the vulnerability of socio-ecological systems, but in a long-term they affect negatively the natural cycle of the river deltas by e.g. reducing sediment transport,
and by preventing erosion of cut banks, which is needed for the self-organization of the
deltas (Paper III). The ground where mangrove ecosystems develop today is a result of
23
Geomorphology and environmental dynamics in Save River delta, Mozambique
previous sedimentation that has built the deltaic platform. In some cases, these sediments were supplied from the eroded cut banks that are today protected from erosion
to accommodate the current interests of the society. Stanley and Warne (1998) have
warned about a “destruction phase”, which the Nile River delta is undergoing because of
human exploitation since the 19th century. In this example, they have demonstrated that
environmental problems related to water and soil pollution are caused by agricultural
and industrial activities coupled with water regulation and flood control. The example
of adaptation measures shown in Nova Mambone and Machanga villages (Paper V) is
part of sustainable solutions to cope with climate-related catastrophic events. However
the application of such measures in other deltas would require observing site-specific
conditions. Mitigation and adaptation measures that consist of moving the communities
temporarily to places not affected by climate-related catastrophic events may bring both
short- and long-term benefits to the deltaic environment. Examples of these benefits include the restoration of the required nutrient levels for agricultural lands and dilution of
pollutants from agriculture and industrialization (cf. Stanley and Warne 1998). However,
in heavily populated deltas, such as some South-Asian deltas, the logistics involved in
temporary transfer of people to safe places would be excessively complex (cf. Douglas
2009). For a successful implementation of such management strategies, there is a need
for a continuous commitment by decision makers and by the communities that live in
these areas to adopt more flexible measures. The fragmented way in which most of the
management policies have addressed environmental problems in deltaic environments,
represents a threat for the long-term benefits of living in delta environments. To take a
step ahead, towards the improvement of management policies, it is necessary to apply
the lessons learnt from studies of long-term processes of delta evolution under multiple timescales. Under such a paradigm, managers should acknowledge that some of the
climate-related events that have been responsible for the long-term natural development
of the deltas may indeed be an important prerequisite for the delta to function.
24
6 Conclusions
The Save River delta has well-preserved geomorphological and stratigraphic features
showing the evolution process over multiple timescales. This study proposes a development model implying that the current landscape of the delta was formed during the
late-Holocene. Prior to c. 3000 years ago, the proximal sector of the delta, i.e. in the area
around P1, was dominated by fluvial sedimentation that built a delta-front deposit, which
successively protruded seawards. From this age onwards, fine-grained sediments started
to accumulate on the delta-front deposit, most likely induced by a regional sea-level rise.
As the delta-front deposit protruded seawards, coastal dunes developed in the southern
sector of the delta. Most likely the dunes were built up on beach ridges formed by waves
refracted by the most shoreward position of the delta-front. The aligned dunes mark
a shoreline position at different stages of the delta development and have successively
created back-barrier swamps where fine-grained sediments have accumulated, promoting the mangrove habitat. In the proximal sector of the delta (at P1), accumulation may
have ceased around 1300 years ago, possibly as a response to a sea-level highstand and
the subsequent sea-level drop. From this time onwards, the delta started to receive fluvial sediments supplied by floods, which accumulated on the mangrove deposit forming
a protruding alluvial floodplain. The delta evolution continued over time following a
general shoreward progradation, probably in connection with a drop in sea-level, which
has been registered during the last millennium. Over centennial to decadal timescale the
delta experienced morphodynamic processes caused by the combined effect of fluvial
and costal processes with emphasis on cyclic floods, prevailing south-easterly winds
and longshore currents. The episodic erosion and sedimentation that characterize the
Save River delta today are likely part of natural processes and they have been observed
throughout its evolution.
There is a strong link between the morphodynamic processes and the socio-ecological
aspects of river deltas. Erosion and accretion in the study area have created both negative and positive impacts on the development of the mangrove ecosystem and also on the
livelihood of the population. An example of negative impact is the erosion that continuously affects infrastructure located along cut banks. In the ecological system, mangrove
habitats have been destroyed by fluvial and coastal erosion, and by dune migration. Positive impacts include the development of mangrove habitat in the recently accreted areas
in the lower deltaic plain and the development of new back-barrier swamps. Positive impacts of the morphodynamic processes are not easily identified in the short-term, but in
a long-term perspective the continuous evolution of the delta is unquestionably a benefit
as it ensures renewal of the natural resources that have always attracted people to the
deltas.
Environmental sciences have responded positively to calls for sustainable management of socio-ecological systems. The discipline of ecology has produced useful knowledge which has been applied to set up management initiatives of deltaic environments
and ensured the ecosystem preservation which has contributed directly with multiple
25
Geomorphology and environmental dynamics in Save River delta, Mozambique
services to the society. However, the emergence of questions related to the impacts of
climate change challenges these disciplines for wider time-perspective outlook. This thesis has contributed in this direction by discussing the links through which climate-related
events may affect the deltaic environments and by interpreting how similar events play a
pivotal role in the delta evolution.
Management of river deltas would improve by considering multiple timescale aspects in order to support decision making. It is important to consider that river deltas
are dynamic environments. Their long-term development started in the early Holocene,
and much of the recent transformations of deltas may be ascribed to natural dynamics.
There is a need to let environmental processes run their course in order to renew the
basic components of the wealth that the deltas provide. This situation calls for a change
in the way that the deltas are viewed by the society today. An integrated approach that
accommodates the full evolution of the deltas in different timescales could result in an
improved and sustainable management.
26
7 Acknowledgements
This PhD was fulfilled thanks to the support and contribution from people to whom I
would like to express my gratefulness.
First of all, I would like to thank my supervisors Senior Lecturer Lars-Ove Westerberg and Associate Professor Jan Risberg for the supervision of my PhD studies and
for the fruitful discussions in the field. It is thanks to your recommendations that I have
chosen the Save River delta as my study area, instead of the entire coast of Mozambique,
and in the delta I have found complexities that make my understanding of coastal zones
even deeper. I enjoyed the discussions in Govuro during the fieldwork. I thank my cosupervisor Göran Alm for good discussions related to the GIS aspects which have been
crucial components of this study and for the productive discussions during the fieldwork
in Govuro. I express my gratefulness to Professor Karin Holmgren for having contributed to make my studies in Sweden a reality and for coordinating the project that this
thesis is part of. In your memory, Associate Professor Wolter Arnberg, I am thankful for
having shared your knowledge before your eternal departure.
During the time spent at Stockholm University I had a first class assistance from all
the administration team, and I am very glad for that.
I had great support from my colleagues and sponsors at Eduardo Mondlane University. I am thankful to Professor Alberto Mavume, the coordinator of my project.
Professor Mavume, my thanks go beyond the role that you played as coordinator; you
have introduced me to the wonderful world of multidisciplinary studies and much more.
I am thankful to you, Professor Salomão Bandeira, for having invited me to visit the
Govuro District, and for your vision of multidisciplinary studies. I would like to address my thanks to you, Professor Mussa Achimo, for the fruitful discussion in the field
and for your support during my studies. I am thankful to Célia Macamo, for the fruitful discussions about the Save River delta ecosystem and for the discussions during the
fieldwork. I address my thanks to you, Sandra Sitoe, for your support during my studies.
My acknowledgements are extensive to the financial Department of Eduardo Mondlane
for troubleshooting tasks during my activities. I address my thanks to you Dr Orton
Malipa and to Eng. Benedito Zavalane for your attention.
I thank you Annika Berntsson for the discussions on aspects of Quaternary Geology
for this thesis in general and for your co-authorship in Paper II in particular. You have
given new dynamics to the thesis. I thank Marie Christiansson for the joint fieldwork in
the Save River delta, and for helping me to analyse samples for this thesis. Thank you
Stefan Bjursäter, for helping to analyse and for discussing the OSL dating.
I thank the administrative and local authorities of Govuro district for receiving me
and authorizing my work within this jurisdiction area. My thanks in Govuro are particularly to Mr. Albino Chidala helping and troubleshooting complicated tasks in Nova
Mambone.
I would like to thank my family for all the support during my studies and for undertaking my familiar duties while in Sweden. Particular thanks to my brothers César,
27
Geomorphology and environmental dynamics in Save River delta, Mozambique
João, Mário, Rito, Bernardo and André for the motivation and support. To my twin soul
Leopoldina Charles for playing multiple roles, some of them on my behalf, during my
studies. I thank you Wendy and Elvy for motivating me in this journey; I challenge you
also to follow the same steps.
7.1
Financial support
This PhD project was supported by Swedish International Development Cooperation
Agency (SIDA) under the cooperation Program between Mozambique and Sweden (SIDA
Decision No. 2011-002102). The project was implemented under cooperation between
the Department of Geology of Eduardo Mondlane University in Mozambique and the
Department of Physical Geography of Stockholm University in Sweden.
28
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