LAND-USE CHANGE ON FLOOD HYDROLOGY (GUADALENTIN...
Transcript of LAND-USE CHANGE ON FLOOD HYDROLOGY (GUADALENTIN...
THE IMPACT OF LATE HOLOCENE CLIMATIC VARIABILITY AND LAND-USE CHANGE ON FLOOD HYDROLOGY (GUADALENTIN RIVER,
SE SPAIN)
G. Benito1, M. Rico2, Y. Sánchez-Moya3, A. Sopeña3, V. R. Thorndycraft4 and Barriendos, M.5
1Institute of Natural Resources, Consejo Superior de Investigaciones Científicas,
Serrano 115 bis, 28006 Madrid, Spain. 2Instituto Pirenaico de Ecología, CSIC, Zaragoza, Spain
3 Instituto de Geología Económica, CSIC-Universidad Complutense, 28040 Madrid,
Spain 4Department of Geography, Royal Holloway, University of London, Egham, Surrey,
TW20 0EX, U.K. 5Department of Modern History, University of Barcelona, B. Reixac s/n, 08028
Barcelona, Spain.
Abstract
The Guadalentín River, located in SE Spain, is considered one of the most torrential
rivers in Spain, as indicated by catastrophic events such as the 1879 flood that caused
777 fatalities in the Murcia region. In this paper, flood frequency and magnitude of
the upper Guadalentín River was reconstructed using geomorphological evidence,
combined with one dimensional hydraulic modelling and supported by records from
documentary sources at Lorca in the lower Guadalentin catchment. Palaeoflood
studies were conducted at a 2.5 km reach located at the confluence of the Rambla
Mayor (162 km2) and Caramel River (210 km2). These tributaries join at the entrance
of a narrow bedrock canyon, carved in Cretaceous limestone, which is 15-30 m wide
and 40 m deep. Six stratigraphic profiles were described, the thickest and most
complete corresponding to flood benches deposited upstream of the canyon
constriction. The stratigraphic and documentary records identify five main phases of
increased flood frequency. Phase 1, based on sedimentary palaeoflood evidence
alone, occurred at ca. 950-1200 cal AD with at least ten floods with minimum
discharge estimates of 15-580 m3s-1. Phases 2-5, identified through combined
sedimentary and documentary evidence occurred at: (a) AD 1648-1672, with eight
documentary floods and two palaeofloods exceeding 580-680 m3s-1; (b) AD 1769-
1802, comprising seven documentary floods, of which at least two events (>250 m3s-
1) are preserved in the sedimentary record; (c) AD 1830-1840, with four documentary
floods, and at least two events recorded in the stratigraphy (760-1035 m3s-1); and
finally (d) the AD 1877-1900 period that witnessed seven documentary floods, with
three palaeofloods (>880 m3s-1). The palaeoflood and historical flood information
indicate an anomalous increase in the frequency of large magnitude floods between
AD1830-1900, which can be attributed to climatic variability accentuated by
intensive deforestation and land-use practices during the first decades of the 19th
century.
Key words: Palaeoflood hydrology, documentary records, floods, paleodischarges,
Little Ice Age, environmental change, SE Spain,
1. Introduction The assessment of the response of hydroclimatic extremes to anthropogenic global
change is one of the main future uncertainties according to the latest IPCC report (in
Trenberth et al., 2007). The understanding of extreme event patterns in the context of
global change is crucial due to its social (e.g. vulnerability) and political (resilience
and adaptation) implications, in particular for those regions where extremes are an
intrinsic characteristic of the hydrological regime. This especially applies to the
Mediterranean as it is characterised by a fragile hydrologic and environmental
equilibrium linked to frequent droughts and/or severe flooding (Vita-Finzi, 1969;
Butzer, 1974, 1975; Lewin et al., 1995). Long-term changes of rare events are,
however, difficult to identify due to short gauge records and their limited spatial
distribution (Frei and Schär, 2001; Klein Tank and Können, 2003). These
instrumental records can be lengthened by hundreds to thousands of years by
reconstructing discharges of past floods using palaeoflood evidence (e.g. Baker et al.,
1983) and/or written descriptions of floods using historical, archival documents (see
Brazdil et al., 2006). Slackwater palaeoflood sediments (Baker et al., 2002), deposited
in flow marginal areas of bedrock canyons during high flood stages, have been
employed in many regions of the world for compiling long term flood magnitude and
frequencies of large floods over decades to millennia, with an emphasis on
understanding extreme event response to Holocene climatic variability (e.g. Ely et al.,
1993; Benito et al., 2003a; Sheffer et al., 2007; Thorndycraft et al., 2006).
During the Holocene, however, hydrological sensitivity was driven not only by
climatic variability (Ely, 1997; Gregory et al., 2006) but also by human activities
(Foulds and Macklin, 2006). The geographic location of south-east Spain at the
southern limit of influence of the polar front in the tropically sensitive Mediterranean
zone provides great potential for river systems to reflect short-term regional climatic
variability e.g. through changes in flood frequency and magnitude. Furthermore, the
region has a long history of anthropogenic land-use change, which may lead to
increasing rates of sediment yield and changes in flood magnitude and frequency (see
Knighton, 1998, pp.316-322). Dealing with Holocene fluvial sedimentary records it
is often difficult to disentangle the role played by climate and humans in driving the
changes observed in river channels and floodplain sediments (Foulds and Macklin,
2006). In contrast to alluvial floodplain depositional environments where
sedimentation may result from a range of overbank flood magnitudes, slackwater
flood deposits are usually emplaced by high stage floodwaters caused by extreme
rainfall events whose frequency can be related to changing climate conditions
(Hirsboeck, 1991; Benito et al., 2003a), whilst changes in sedimentological
characteristics may be linked to environmental changes at the basin scale
(Thorndycraft et al., 2004). A major limitation, however, in establishing the spatial
and temporal occurrence of these palaeoflood records are the uncertainties inherent in
geochronological techniques, such as measurement error and calibration in the case of
radiocarbon dating (Trumbore, 2000). The combination of palaeoflood and
documentary data sources has the potential to reduce these uncertainties and provide a
more holistic approach that can strengthen our understanding of past floods and help
elucidate the effect of climate and land-use changes on flood hydrology (Benito et al.,
2004, Thorndycraft et al., 2006). This study contributes to this debate through the
reconstruction of a 1000 year flood record from the Guadalentín River basin (SE
Spain), In this paper we highlight the potential of combined sedimentological and
archival research into past floods of European river basins. The main objectives are
to: (1) reconstruct centennial scale flood frequency using the stratigraphic record of
slackwater flood deposits; (2) analyse the palaeoflow hydraulics associated with these
floods and calculate estimates of peak flood discharges; (3) apply the historical flood
information compiled from archival sources to refine the palaeoflood chronology; and
(4) synthesise the collected date to gain a detailed understanding of changing flood
magnitude and frequency during the last millennium, and in particular to document
flood response to regional climatic and local environmental changes.
2. Study area The Guadalentín River (the name deriving from the Arabic Oued al Lentin meaning
river of mud) is located in SE Spain and is a major tributary of the Segura River, one
of the largest Mediterranean basins in Spain (Fig.1). The Guadalentín River is one of
the most torrential rivers in the Mediterranean region (Pardé, 1961) with over 2000
fatalities documented during the last 250 years (Muñoz, 1989; Bautista and Muñoz,
1986), including a dam break event that killed over 600 people in AD1802. The mean
annual precipitation ranges from over 1000 mm in the high mountains of the Sierra de
Maria to less than 300 mm in the lower catchment. The region is affected by: (a)
Atlantic frontal systems (zonal flow) during late winter (February-March) and spring
(May-June), when the highest annual precipitation values are registered; and (b)
autumnal (September to November) mesoscale convective systems born from
southwest flows at altitude. The latter generate the majority of floods in the region
(Capel, 1981; Benito et al., 1996). For example, in October 1973 convective storm
cells produced a daily rainfall in excess of 100 mm in Lorca and of 180 mm in the
Guadalentín headwaters (Capel, 1974). This rainfall, together with the steep gradients
in the catchment, resulted in severe flash flooding registering a peak discharge of
3544 m3s-1 at the Puentes reservoir. The event caused 83 fatalities in Puerto
Lumbreras and 13 in Lorca (Fig. 1). The nearest gauge station to the study area is
located at the Valdeinfierno dam and drains an area of 456 km2. The gauge record
covers the periods 1933-1949 and 1968 onwards, with the largest recorded daily flood
discharge being 178 m3s-1 in 1946, though it is important to note that most of this
gauge record corresponds to mean daily discharges. The most reliable gauge station is
located at the Puentes reservoir (15 km downstream of the study site) where the
catchment size is substantially increased to 1428 km2 by the inflow of the Corneros
(552 km2) and Turrillas (340 km2) rivers.
The palaeoflood study site (Fig. 1) was centred on a 2.5 km reach in the upper
catchment at the confluence of the Rambla Mayor (162 km2) and Caramel River (210
km2), both ephemeral streams. The geology is dominated by Mesozoic limestone and
marls of the Sub-Betic domain and the confluence of the two tributaries is located at
the entrance of a narrow bedrock canyon carved in Cretaceous limestones. The gorge
is 15-30 m wide with bedrock walls 40 m high and a gravel bed channel. During
flooding flow is hydraulically controlled by the narrow canyon entrance producing a
hydraulic ponding upstream of the constriction. This favours the accumulation of
flood deposit benches upstream of the gorge entrance. Palaeoflood sedimentation also
occurred along the canyon sides in small bedrock alcoves and in small tributary
gullies in the canyon.
3. Methodology
Palaeoflood hydrology using slackwater flood deposits as stage indicators of floods
(Baker & Kochel 1988, Baker et al. 2002, Benito et al. 2003a) were used to complete
a hydrological record of extreme events. Stratigraphic and sedimentological analyses
of the deposits were carried out both in the field and the laboratory, with sediment
peels of the stratigraphic profiles, measuring approximately 80 cm x 50 cm in size,
made in the field (Hattingh & Zawada,1996; Thorndycraft et al., 2005). Individual
flood units were identified through a variety of sedimentological indicators (Baker &
Kochel 1988, Benito et al. 2003b): the identification of clay layers at the top of a unit;
erosion surfaces; bioturbation indicating the exposure of a sedimentary surface;
angular clast layers, where local alcove or slope materials were deposited between
flood events; and changes in sediment colour. As well as identifying individual flood
units, sedimentary flow structures were also described in order to elucidate any
changing dynamics during a particular flood event and/or infer flow velocities that
can improve discharge estimation (Benito et al. 2003c).
Slackwater flood deposit chronology was determined using radiocarbon dating of
charcoal collected from individual flood units. Necessary preparation and pre-
treatment of the sample material for radiocarbon dating was carried out by the 14C
laboratory of the Department of Geography at the University of Zurich (GIUZ). The
dating itself was done by AMS (accelerator mass spectrometry) with the tandem
accelerator of the Institute of Particle Physics at the Swiss Federal Institute of
Technology, Zurich (ETH). Calibration of the radiocarbon dates was carried out using
the CalibeETH 1.5b (1991) programme of the Institute for Intermediate Energy
Physics ETH Zürich, Switzerland, using the calibration curves of Kromer & Becker
(1993), Linnick et al. (1986) and Stuiver & Pearson (1993). A summary of the
samples submitted for dating is presented in Table 1.
Documentary flood data was compiled to complement the palaeoflood data. Archival
flood evidence provides direct (e.g. flood date, duration, stage or spatial extent) or
indirect (e.g. post-flood damage repair) information of individual events, allowing
flood chronologies and socio-economic impacts to be compiled (Barriendos and
Coeur, 2004; Mudelsee et al. 2004; Brazdil et al. 2006; Thorndycraft et al., 2006). In
this study, the direct archival sources consulted included: 1) the Municipal Historical
Archive of Lorca (AHL); 2) the Municipal Historical Archive of Murcia; and 3) the
Cathedral Chapter Archive of Murcia. Further bibliographic sources used included
scientific and technical reports, local history works and non-systematic compilations
by historians (Hernández Amores, 1885; Couchoud, 1965; Diaz Cassou, 1977, 1993;
Fontana Tarrats, 1978; Torres and Calvo, 1975; Merino Alvarez, 1978; Barriendos
and Rodrigo, 2006).
The available information from the Municipal Historical Archive of Lorca dates from
the 1500s so any earlier data must be considered anecdotal in nature and probably
incomplete. At Lorca, the Guadalentín River is incised a few meters into its
floodplain, and the active channel forms a mixed anabranching and braided pattern
(according to the classification of Nanson and Knighton, 1996) with an ephemeral
channel of 100 m width and a gradient of 0.0051. The channel bed mobility of the
Guadalentín river at Lorca, as well as the history of urban change, prevents a precise
estimation of historical flood discharges. A classification of historical flood
dimensions was carried out according to the three categories proposed by Barriendos
and Coeur (2004), namely ordinary, extraordinary and catastrophic flooding, where
qualitative flood damage and descriptions of the severity of overbank flow are
combined. In ordinary flood situations water discharge is contained within the channel
and banks, with no damages. An extraordinary flood resulted in localised overbank
flow, resulting in damage but without major destruction. The classification of a
catastrophic flood depends on flood water inundating large floodplain areas resulting
in general damage and destruction of infrastructure.
Discharge estimates were calculated by hydraulic modelling of the 1800 m surveyed
study reach using HEC-RAS (Hydrologic Engineering Center 1995). The computation
procedure is based on the solution of the one-dimensional energy equation, derived
from the Bernoulli equation, for steady gradually varied flow. Subcritical flow
conditions were assumed along the reach, with critical flow selected as the boundary
condition at the most downstream cross-section where the channel narrows to pass
through a bedrock section where a deep pool has been scoured. Manning’s n values of
0.03 and 0.04 for the valley floor and margins, respectively, were assigned. A
sensitivity test performed on the model shows that for a 25% variation in the
roughness values an error of 4% was introduced into the discharge results.
Palaeoflood discharge estimation was based on the calculation of the step-backwater
profile that best fitted the surveyed geomorphological evidence of flood stage at
different sites along the longitudinal profile (O´Connor and Webb, 1988). Rating
curves relating individual flood unit elevations with flood discharges were established
at each site. High water marks (e.g. drift wood and pockets of fluvial sand) from the
largest instrumental flood that occurred in 1973 were also fitted to the model to
estimate flood discharges of this event. The accuracy of the discharge estimates
depends on the stability of cross-section topography through time. At the study reach
channel aggradation has infilled the bedrock channel with gravel deposits susceptible
to scour and fill. Assuming this channel bed aggradation post-dates construction of
the Valdeinfierno reservoir (3 km downstream of the study reach), the calculated
discharges of earlier floods are likely to underestimate the real peak flow.
4. Results
4.1. Slackwater stratigraphy and sedimentology
Nine sites of slackwater flood deposition were described throughout the 2.5 km study
reach (Fig. 2). The five thickest flood deposit sites (ES1, ES2, ES3, ES4 and RM)
form benches in backwater areas upstream of the abrupt canyon constriction, where
water is temporarily back-flooded during large floods (Fig. 2). Pockets of flood
sediments are common throughout this reach and occur in high crevices (RML),
shelves (RCS), slope depressions and upstream of tributary gullies (TL1, TL2).
A composite site of two trenches ES1 and ES2 shows the most complete palaeoflood
record. The site is composed of a flood bench of up to 7 m in thickness on the right
margin of the Caramel River, 150 m upstream of the gorge entrance (Fig. 2). The
stratigraphic sections contain spectacular sequences of multiple fine-grained flood
deposits (Fig. 3), providing evidence of at least 24 individual flood layers deposited
over the last 1000 years (Fig. 4). The lower ten flood units were dated to between cal.
AD 890-1160 and 1000-1210 suggesting a period of frequent relatively small floods,
on the basis of the very fine and fine sand grain size and the thin stratigraphic layers.
The top of this flood sequence is overlain by slope deposits which indicate a break in
flood sedimentation of at least 250 years. Overlying this are three flood layers (the
lower one dated to cal. AD 1450-1650), which also have a fine to very fine sand
grain-size (Fig. 4). These flood layers are capped by a 42 cm-thick colluvial layer
indicating a second break in flood sedimentation. The upper part of the sequence is
represented by nine modern flood deposits, with a basal date of AD 1630-1890
(81.5%), a middle date of cal. AD 1815 ± 80, and the upper flood layer most probably
left by the 1973 flood (Fig. 4). Field evidence suggests that the 1973 flood was the
largest event over the last 1000 years. Fresh-looking sand deposits most probably
related to this flood are approximately 3 m higher than the uppermost flood layer at
the ES2 section.
The sedimentary sequences described at sites ES1 and ES2 can be interpreted in terms
of flow energy and suspended load sediment conditions (see Benito et al., 2003c). The
sedimentological characteristics of the oldest deposits, with finer grain sizes and
thinner beds than the upper (younger) deposits, may be indicative of smaller floods,
lower sediment loads or a different channel position. The evidence for small-to-
moderate magnitude flooding is also supported by the two dominant sedimentary
sequences at the lower section comprising: (1) massive and highly bioturbated fine to
very fine-grained sand, with occasional granules, capped by a massive bioturbated silt
with mud cracks; and (2) parallel lamination, sometimes resting on an erosional
surface that includes granules or mud clasts, with ripples located either in the middle
of the sequence or more often towards the top of the bed. By contrast the upper flood
units of ES2 show a coarser grain size and flow structures indicative of higher energy
conditions. In particular, units 17, 21 and 22 all exhibit an erosive surface overlain by
coarse to very coarse-grained sand with parallel lamination and three dimensional
bedforms (Fig. 5). These sometimes resemble hummocky cross-stratification, and
occasionally include clay chips. The upper part of these units are characterised by a
fine-grained highly bioturbated sand. This sedimentary sequence is interpreted as
being generated by extreme floods with high stream flow velocity (>1ms-1) even in
these ineffective flow areas (Fig. 5). Units 23 and 24 contain parallel lamination and
climbing ripples in phase to/or climbing-in-drift developed with medium or coarse-
grained sand with parallel lamination, overlain by medium to fine-grained sand with
climbing ripples (Fig. 4). Climbing ripples, in-drift or in-phase, result from a vertical
variation in velocity and sediment fallout rate from flow with a high sediment load.
High velocity and lower aggradation rates produce climbing-in-drift whereas lower
velocities and higher aggradation rates produce climbing-in-phase (Ashey et al., 1982;
Jopling and Walker, 1968). The two breaks in flood deposition, during which
colluvial deposition occurred, are indicative of little, if any, flood deposition and
were, therefore, interpreted as evidence of a lack of extreme events during those
periods.
At the Rambla Mayor (RM) site, around 200 m upstream of the gorge entrance, flood
deposits accumulated in a large bench throughout the left margin of the river channel
(Fig. 2). Site RM is a 3 m-thick section preserving at least twelve flood units (Fig. 4).
A basal unit at this site yielded a date of cal. AD 1790 ± 90, a middle unit was dated
to cal. AD 1820 ± 80 and the upper deposits, separated from the previous ones by
slope wash sediments, were radiocarbon dated as modern (Fig. 4). The stratigraphy of
this site is equivalent in the number of preserved layers and flood chronology to the
upper section of site ES2, with seven flood beds (units 1 to 7) likely to be equivalent
to ES2 units 16 to 20, and five flood beds (units 8 to 12) probably corresponding to
ES2 units 21 to 24. Note (Fig. 4) that site RM is 5 m lower in elevation than ES2 and
a higher number of flood beds is expected for the same time interval. Although it is
difficult to correlate individual palaeoflood units between RM and ES2, the
sedimentary structures at units 5 and 6 indicate very high energy conditions during
deposition. The sedimentology is characterised by medium to coarse-grained sand
with trough cross bedding (3D), and occasionally small planar cross bedding (2D).
These units contain parallel lamination produced by aggradation on a plane bed and
avalanching faces developed infilling previous small hollows or topographic
depressions. This sequence is interpreted as vertical accretion produced by 3D dune
migration in response to an increase in stream power. Vertical aggradation of fine
sand produced by plane bed migration occurs under flow conditions of either
increasing velocity or decreasing flow depth. Commonly, fluid escape structures or
convolute bedding are developed within the sequence by density inversion. At the top,
sub-aerial exposure is indicated by bioturbation. In the upper section of the RM site,
the highest energy conditions were reflected in the sedimentary sequence of unit 10
(Fig. 4). This coarsening upwards sequence is characterised by fine-grained sand with
climbing ripples in phase to/or climbing-in-drift, overlain by medium to coarse-
grained sand with parallel lamination (18 cm in thickness). This unit, therefore, may
relate to a large flood such as the 1973 event.
Other evidence for the 1973 flood is found in high-elevation sediment pockets
deposited on the valley slope at RML next to the RM profile (Fig. 2). The narrow
canyon carved in limestone is devoid of relevant flood deposits probably due to high
energy conditions associated with extreme flood events. Only modern deposits were
found in pockets on small bedrock shelves (RCS in Figs. 2 and 4) and within tributary
gullies of the canyon (TL1 and TL2; Figs. 2 and 4).
4.2. Palaeoflood hydraulics and discharge estimation At the ES2 stratigraphic profile (ES2), the lower 10 flood units (cal. AD 890-1160
and 1000-1210) are associated with estimated discharges of 15-580 m3s-1. The second
set of three flood units (cal. AD 1450-1650) is associated with minimum discharges of
580-680 m3s-1. Finally at ES2 the upper nine flood units post-dating cal. AD 1630-
1890 (81.5% probability range) matched discharges of 730-1000 m3s-1. The upper
most unit of the RM stratigraphic profile is associated with a minimum discharge of
500 m3s-1. These discharges are minimum estimates since the water depth above the
flood units is unknown, and therefore, discharge values may be considered as
conservative. High water marks of the 1973 flood found along the study reach
provided a discharge calculation of 1615 m3s-1, which shows that “real” peak
discharges could be as much as 35% higher than the minimum estimates.
4.3. Documentary flood evidence
During the Medieval period, severe flooding is reported to have occurred in the
Guadalentín at Murcia in AD1143 (Santa Lucia flood), as well as during the 13th (2
events), 14th (3 events) and 15 th (10 events) centuries. Since the 1500s the Municipal
Historical Archive of Lorca (MHAL) provides a more complete documentary record
of flood events and damages, although the objectivity or continuity of the information
is not guaranteed. During the period AD1500-1900, 31 flood events were reported in
the Municipal Archive, of which 15 were classified as ordinary, 7 extraordinary and 9
catastrophic (Fig. 6; Table 2). In addition, two floods resulted from dam failures at
Puentes in 1648 and 1802 (Bautista and Muñoz, 1986). Since the third Puentes dam
was completed on October 2nd 1884, flood occurrence in Lorca decreased. As a result,
most of the large floods since AD 1884 were reported within the ordinary flood
category at Lorca, although the peak discharges recorded at the Puentes reservoir are
indicative of a large magnitude in the upper catchment (Table 2).
The temporal distribution of historical floods shows four major flood clusters at AD
1648-1672, 1769-1802, 1830-1840 and 1877-1900, with an anomalous number of
catastrophic and extraordinary category floods during the 19th Century (Fig. 6).
According to the archive descriptions the most severe floods occurred in AD 1651,
1653, 1879 and 1973 accounting for more than 2600 fatalities in the Guadalentín-
Segura catchment, including the AD 1802 dam failure (Table 2).
5. Discussion 5.1. Combining palaeoflood and documentary flood records
The value of the correlation of the palaeo- and historical flood data from the
Guadalentín basin is to test the degree of long-term agreement between these two
types of flood archive in terms of the number of floods (frequency) and relative
magnitude (energy and/or discharge). Four assumptions need to be made for such a
correlation: (1) flood benches were built up by successive flood layer deposition that
increased the flood elevation threshold required for further sediment accumulation
(self-censoring cf. House et al., 2002); (2) all floods exceeding the censoring elevation
of the previous flood deposits left preserved sedimentary evidence; 3) documentary
records have maintained the same flood perception levels through time; and 4)
relative flood magnitudes at Lorca and at the palaeoflood study reach are similar.
Assumptions (1) and (2) imply that documentary floods classified as ordinary may not
be represented in the palaeoflood record except during the first stages of sediment
deposition (the events at the base of the stratigraphic profile). At the ES2 site these
floods were dated to AD 900-1200, prior to the documentary flood record. Since AD
1500 it is likely that most palaeofloods will correspond to either catastrophic or
extraordinary historical floods recorded at Lorca (Table 2). Assumption (4) may not
always be true due to the differences in catchment size. A large event in the upper
basin (at the palaeoflood reach) may not always result in a large event at Lorca. This
is especially so after flood regulation by the Valdeinfierno and Puentes reservoirs that
would have modified flood discharges downstream at Lorca whilst the palaeoflood
reach maintained a natural flood regime.
The stratigraphic record at Estrecho 2, located at a protected low energy site, indicates
that the palaeoflood record here is likely to preserve evidence of all major floods over
the last 500 years and probably since AD 1000. The stratigraphy shows a cluster of at
least three palaeoflood events (units 13, 14, 15) post-dating cal. AD 1450-1650 and
capped by a slope deposit unit which can be interpreted as a period of reduced high
magnitude flooding (Fig. 4). The documentary record contains three catastrophic
floods within the palaeoflood radiocarbon age range (AD 1568, 1651 and 1653),
followed by a period of 50 years without major (catastrophic and/or extraordinary)
flood events (Table 2; Fig. 6). The second half of the 17th Century witnessed at least
five ordinary floods which were associated with a period characterised by a negative
precipitation anomaly in southern Spain according to the rainfall reconstruction
provided by Rodrigo et al (1999). This period coincides in time with the accumulation
of slopewash deposits above flood unit 15 at ES2 site, and incipient soil development
and carbonate precipitation. During the first half of the 18th Century, the total number
of documented floods decreased in relation to the previous period, although three
large floods were reported at Lorca (AD 1704, 1728 and 1733) (Fig. 6; Table 2). The
last three decades of the 18th Century show an increase of the frequency of ordinary
floods (7 events) associated with an above average precipitation anomaly. The
palaeoflood record of the Rambla Mayor (RM) section shows a similar number of
floods in the lower sedimentary sequence, units 3 and 5 dated to 1795 ± 85 cal AD
and 1790 ± 90 cal AD, respectively. Only one flood unit was deposited at the higher
elevation in the stratigraphic section ES2 (unit 16), dated with a calibrated age of AD
1775 ± 95. Interestingly, unit 16 at ES2 contains a high degree of bioturbation
indicating a period of surface exposure after deposition. We speculate, therefore, that
unit 16 may correspond to the catastrophic 1733 flood at Lorca (Table 2), the
threshold censoring level precluding deposition at this site of the extraordinary floods
in AD 1704 and 1728, and the later 18th Century ordinary floods.
Both documentary and palaeoflood records show an increase in frequency of
extraordinary and catastrophic floods during the 19th Century, in particular during the
second half of the century (Table 2; Fig. 6). The severity of these floods is reflected in
both the documentary record, with high economic losses and casualties (e.g. the 1838
and 1879 events), and the palaeoflood record where high energy sedimentary
structures (e.g. hummocky type cross-bedding) are evident (Fig. 5). Interestingly, this
is the first time in the palaeoflood literature that hummocky type cross bedding has
been described in sedimentary environments related to slackwater flood deposition.
The radiocarbon age resolution for the last 250 years unfortunately prevents a robust
correlation between the documentary and palaeoflood records. Therefore, palaeoflood
units at ES2 were assigned to documentary flood years on the basis of the correlative
sequenced number of floods (catastrophic and extraordinary), together with minor
adjustments to match the most extreme documentary floods with palaeoflood units
containing high-energy sedimentary structures (units 17, 18, 21 and 22). The most
severe documented floods in the Guadalentín occurred in 1838 and 1879 and were
assigned to units 18 and 21 respectively (Table 2). In between these floods, the
stratigraphy shows at least two flood units (units 19 and 20; the former <7 cm in
thickness), whereas the documentary records contains only one catastrophic flood
(1860). An additional documented flood in AD 1877 classified in Lorca as ordinary
was assigned to unit 20.
Since 1884 the documentary flood record was influenced by the construction of the
third Puentes dam, reducing flood magnitude and altering flood perception levels at
Lorca. However, the gauge station data at the Puentes dam enabled comparison
between the palaeoflood and systematic records. The largest flood at the end of the
19th Century occurred in 1891, with a peak discharge of 1890 m3s-1 at Puentes (Table
2). This flood was assigned to unit 22, associated with an estimated minimum
discharge of 940 m3s-1. The second largest flood in the 20th Century occurred in 1941
(palaeoflood unit 23) and the largest in 1973 (unit 24). This flood recorded a peak
discharge of 3544 m3s-1 at Puentes (1428 km2), whereas the Valdeinfierno gauge
station (456 km2) recorded only 108 m3s-1 (daily discharge). The palaeoflood evidence
provided a minimum discharge of 1035 m3s-1 but the ubiquitous high water marks and
sand pockets along the study reach indicated a flow of 1615 m3s-1.
5.2. Environmental and climatic changes in the upper Guadalentín basin
The combined palaeoflood (P) and documentary (D) records indicate that past floods
were clustered during particular time periods, namely at: AD 950-1200 (≥10 P
events); 1648-1672 (≥2 P & ≥8 D events); 1769-1802 (≥2 P & ≥7 D events); 1830-
1840 (≥2 P & ≥4 D events); and 1877-1900 (≥3 P & ≥7 D events). These flood
clusters may be explained by climate change altering extreme rainfall patterns
reaching the basin and/or land-use change affecting runoff generation conditions. The
effect of climatic variability on flood hydrology may be reflected by changing runoff
volumes, peak discharge and frequency (Changnon and Demissie, 1996). Land-use
changes may be reflected not only in runoff generation but also in terms of sediment
delivery to stream channels (Trimble, 1983). It is expected that the most dramatic
land-use changes in the basin are related to human activities during historical times.
Environmental history may facilitate identification of the onset of severe human
impact, after which environmental change should be considered in addition to climatic
drivers modifying flood hydrology.
Due to the small size of the study basin it is difficult to obtain palaeoenvironmental
data specific to the catchment. However, valuable historical information on human
population and land-use changes since the Middle Ages is available for the Almeria
province in general (Sánchez-Picón et al., 1996; García Latorre et al., 1998, 2001),
and for the Almeria mountains in particular (Lentisco, 1996). The economic system of
the Moorish settlements in Almeria (AD 711-1570) was based on irrigation
agriculture centred on floodplains. Marginal areas and forests were left for hunting,
gathering and grazing, explaining the existence of a rich forest and fauna of the
Almeria mountains as described by the traveller J. Münzer in AD 1494 (Münzer,
1495). This type of agriculture required much less land to be farmed than cereal
agriculture introduced later on. According to Lentisco (1996) and García Latorre et
al., (1998) the upper Guadalentin catchment (1141 km2) was sparsely populated with
around 2184 inhabitants in AD 1573 (2 inhabitants per km2). After expulsion of the
Moors in the 16th Century, the population rose to 10,459 inhabitants (9 inhabitants per
km2) in AD 1753 (Fig. 6), and a shift in agriculture to farming of cereal crops
occurred (representing 16% of total land-use at the Almería province according to
García Latorre et al., 1998). By AD 1860 population had increased to 22,890 (20 per
km2), with 25-30% of land-use under cultivation causing environmental
transformation of the study region (Sanchez-Picón et al., 1996; Lentisco, 1996). The
accelerated environmental change included massive deforestation of millions of trees
of a variety of species (e.g. Pinus nigra, Pinus halepensis, Pinus pinaster and
Quercus ilex) to clear areas for the cultivation of marginal areas, as well as for
charcoal production and domestic use of firewood (Madoz, 1845; Lentisco, 1996;
García Latorre et al., 2001). Given this historical information it is most likely that
widespread deforestation occurred in the upper Guadalentín basin in the late 18th and
early 19th centuries. It is interesting to note that the sedimentological characteristics of
the palaeoflood deposits dated to the 19th Century (units 17-22) illustrate a significant
change in the hydrological and environmental conditions of the upper Guadalentín
catchment. The sedimentary structures indicate very high energy conditions during
flooding (e.g. hummocky type cross-bedding), whilst the anomalous increase in grain-
size and unit thickness (>40 cm) of the slackwater flood deposits (Fig. 4) also suggest
increased sediment loads in response to human impact in the basin. The role of
climate still needs to be highlighted, however, as this period also covers the last phase
of cold conditions (at AD 1810-1850 after Flohn, 1993) during the Little Ice Age
(LIA), and the transition towards the warmer conditions of the 20th Century. Figure 6
shows that flood clusters (Phases 4 and 5) during the 19th Century occurred within a
range of precipitation anomalies similar to that experienced in previous periods of 17th
Century (Phase 2) and late 18th Century (Phase 3). In contrast, however, the number
of extraordinary and catastrophic floods over the 19th Century was remarkably high.
In earlier periods, climatic variability may be considered as the major controlling
factor on the recorded changes in flood magnitude and frequency. During the last
millennium the first palaeoflood period (AD 950-1200) broadly corresponds to the
late Medieval Warm Period (probably AD 900-1200, after Flohn, 1993) though some
authors doubt that this period can be clearly defined at the global and continental scale
(Hughes and Diaz, 1994). The concentration of floods at 1050-750 cal. BP (AD 950-
1200) is also apparent in palaeoflood and documentary evidence from Atlantic basins
(see Fig. 1) of the Iberian Peninsula (Benito et al., 1996), these events probably
associated with unusually wet winters. Flooding of the Tagus River from AD 1100 to
1200 was frequent as well as exceptional, and water stage data indicate that these
floods were the greatest of the available documentary record (Benito et al., 2003b). Of
particularly large magnitude were the floods of AD 1168 (also recorded for the rivers
Duero and Guadalquivir; Benito et al., 1996), 1178, 1181 and 1207 (Benito et al.,
2003b).
The following period 750-550 cal. BP (AD 1200-1500) was characterised by a lack of
slackwater deposition at profiles ES1, ES2 and ES3, and accumulation of slope
deposits (with root marks, bioturbation and carbonate precipitation) at ES2. This
period, therefore, represents a phase of reduced flood frequency, also evident in the
Spanish palaeoflood record (Thorndycraft and Benito, 2006a, 2006b) and the
documentary flood record (Benito et al., 2003b). This contrasts to the alluvial
stratigraphy of the lower terrace of the Guadalentín near Librilla (downstream of
Lorca) where initial aggradation was dated to ca. 730-540 cal. BP (Calmel Avila 2000
and 2002). This period coincides with the main recent phases of floodplain
aggradation of Iberian rivers (Vita-Finzi, 1976; Butzer et al., 1985, Thorndycraft and
Benito, 2006b; Uribelarrea and Benito 2008). This observation may reflect the fact
that the two depositional environments (slackwater deposits and alluvial floodplains)
represent different event magnitude and frequency relationships (Thorndycraft and
Benito 2006a,b). The palaeoflood record preserves evidence of high magnitude, low
frequency floods. Alluviation, however, may result from a range of event magnitudes
that cause overbank flooding and/or lateral aggradation, as well as being a response to
an increase in sediment supply that can be brought about by human activity, as shown
by van Andel et al. (1990) in Greece and the Aegean.
The second and third flood clusters (1648-1672 and 1769-1802) coincide with the
LIA (ca 1500-1850), generally believed to be a widespread event at the global scale.
In the palaeoflood record, the AD 1200-1500 hiatus was broken by a sequence of
three palaeoflood events (units 13, 14 and 15 at ES2) post-dating AD1505-1636, and
assigned to historical floods at AD 1568, 1648 and 1651. Documentary evidence from
other Mediterranean river basins shows an increase of severe flooding during AD
1580-1620, particularly in Catalonian rivers in NE Spain (Barriendos and Martín
Vide, 1998; Llasat et al., 2005). This period overlaps with one of the coldest phases
(AD 1590-1650) of the Little Ice Age, the Maunder Minimum, during which sunspots
were exceedingly rare (Vaquero et al., 2002). In southern Spain prevailing anomalies
were wet, with the wettest years centred on the decades of the 1590s, 1630s and 1640s
(Barriendos, 1997, Rodrigo et al., 1999 and Fig. 6), with large agricultural losses due
to excess rainfall noted in Murcia Province (Barriendos, 1997).
The flood cluster at AD 1648-1672 (2) is mainly composed of ordinary floods (with
the exception of the 1648 and 1651 events) and no links to anomalous behaviour of
regional climate conditions have been found. Similarly, seven documentary flood
events were described between AD 1769-1802 (3), overlapping the period AD 1760-
1800, which was noteworthy for its strong climatic variability in the Mediterranean
region with both floods and droughts occurring (Barriendos and Llasat, 2003). It can
be noted that flood generating conditions, in some decades of period 3, may be
enhanced under positive precipitation anomalies at the regional scale, although few
extraordinary and catastrophic floods were recorded from the 1500s to 1830s. In the
19th Century the anomalously high number of extraordinary and catastrophic floods,
occurred even during decades of negative or close to normal precipitation anomalies,
suggest that the climate signal of flood response was amplified by human disturbance.
This severe human impact shows the high sensitivity of Mediterranean basins to soil
and hydrological disturbance, which in the space of a few decades led to a high degree
of degradation in the upper Guadalentín catchment.
6. Conclusions
A combined palaeo and historical flood record provides evidence of extreme event
response to global change over the last 1000 years in the Guadalentín basin (SE
Spain). The stratigraphic and documentary records identify five main phases of
increased flood frequency. Phase 1, on the basis of sedimentary palaeoflood evidence
only, occurred ca. 950-1200 cal AD with at least ten floods with minimum discharge
estimates of 15-580 m3s-1. Phases 2-5 (Fig. 6), based on combined sedimentary and
documentary evidence, occurred at: (a) AD 1648-1672, recording eight documentary
floods and two palaeofloods exceeding 580-680 m3s-1; (b) AD 1769-1802, comprising
seven documentary floods, probably two included in the sedimentary record (at the
RM site) with minimum discharges of 250 m3s-1; (c) AD 1830-1840 recording four
documentary floods, with at least two events exceeding a discharge of 760-1035 m3s-1
recorded in the stratigraphy; (d) AD 1877-1900 includes seven documentary floods,
three of those exceeding 880 m3s-1 provided sedimentary evidence. In addition, two
flood units represented in the upper sequence of the ES2 stratigraphic profile are
probably related to the AD 1941 and 1973 floods, the latter being the largest flood in
the gauge record at Puentes. Discharge estimation at the palaeoflood study reach was
complicated by gravel deposition within the bedrock channel and subsequent
problems of scour and fill. As a result it is likely that for the older flood deposits the
calculated values are probably underestimates of the real discharges. Nevertheless, the
excellent flood frequency record at the site, in combination with the documentary
flood evidence, allows discussion of the roles of climatic variability and human
impact in driving the changes in flood frequency over the last 1000 years. Phase 1
coincides with the MWP, although detailed palaeoclimate reconstructions are limited
for this time period so it is not known whether floods relate specifically to a warming,
warm or cooling phase of climate. Greater precision can be attributed to phases 2-5,
which occurred during the LIA where both the flood record and climatic history is
better understood through corroboration between palaeoenvironmental and
documentary evidence. Phases 2 and 3 occurred during known cold periods of the
LIA, with wet anomalies (phase 3) in southern Spain, and correlate with flooding
elsewhere in Spain (Llasat et al., 2005), including large Atlantic basins such as the
Tagus (Benito et al., 2003a, 2003b). Phases 4 and 5 represent the most severe flooding
in terms of high energy conditions evident in the sedimentary structures as well as the
socio-economic impacts of the events at Lorca. This period not only represents the
last phase of the LIA and the change to 20th Century warmer conditions but also
coincides with historical evidence of major economic and land-use transitions in the
Almeria mountains, with high rates of population increase, major deforestation and an
increase in cultivated land area into marginal areas. Although the flooding cannot be
separated from climatic variability it appears that human impact has had a major
influence on the sensitivity of flood hydrology and the subsequent severity of the
events. This paper highlights many of the complexities associated with understanding
flood response to global change and shows the value of palaeo and historical flood
data for elucidating flood magnitude and frequency relationships under past global
change scenarios.
Acknowledgements
This research was funded by the EC through the projects “Systematic, Palaeoflood
and Historical data for the improvEment of flood Risk Estimation – SPHERE”
(contract no. EVG1-CT-1999-00010) and “FloodWater recharge of alluvial Aquifers
in Dryland Environments – WADE (contract no. GOCE-CT-2003-506680). The
Spanish Commission of Science and Technology grant HID99-0858, and the project
“Infiltration on channel beds and recharge of aquifers related to floods and
palaeofloods in ephemeral rivers- PALEOREC” (CICYT project no.CGL2005-
01977/HID). The authors are very grateful to Tim van der Schriek and Noam
Greenbaum for their critical review of the original manuscript and for their helpful
comments and suggestions.
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219.
River Flood unit Sample material
Lab code Age (yrs BP)
Calibrated age
(years AD)
Two sigma calibrated age
range
Known historical flood(s)
ES-1-T4
Charcoal UZ-4597/ ETH-24410
980±45
1080±45
AD 980-1190
ES-2-T14
Charcoal UZ-4598/ ETH-24411
945±45
1105±50
AD 1000-1210 1143 Santa Lucía flood
ES-2-T17
Charcoal UZ-4599/ ETH-24412
1020±50
1035±60
AD 890-1160
ES-2-T21
Charcoal UZ-4659/ ETH-24681
190±55
1790±95
AD 1630-1960
ES-2-T23
Charcoal UZ-4600/ETH-24413
340±45
1560±55
AD 1450-1650
ES-2-T29
Charcoal UZ-4601/ETH-24414
205±45
1775±95
AD 1630-1890 (81.5%)
1830
ES-2-T31
Charcoal UZ-4602/ETH-24415
120±45
1815±80
AD1670-1960
ES-3-T6
Charcoal UZ-4603/ETH-24416
1985±50
30±55
110 BC – AD 130 47 BC Julius Caesar flood
Caramel
ES-3-T9
Charcoal UZ-4660/ETH-24682
120±55
1810±80
AD 1660-1960
RM1-3
Charcoal UZ-4994/ETH-27650
175±45
1795±85
AD 1650-1890
RM1-5
Charcoal UZ-4995/ETH-27651
190±45
1790±90
AD 1640-1960
RM1-6
Charcoal UZ-4996/ETH-27652
105±45
1820±80
AD 1670-1960
Rambla Mayor
RM1-11 Charcoal UZ-4997/ETH-27653 Modern Table 1. Radiocarbon dating samples and results, including calibrated ages calculated by the CalibETH 1.5b programme using the calibration curves of Kromer and Becker (1993), Linick et al. (1986) and Stuiver and Pearson (1993).
Year Date Relative Magnitude
Comments Flood unit in profile
Estrecho 2
Q (m3s-1) recorded in
lower catchment (Puentes)
Minimum Q (m3s-1) at upper
catchment (this study)
1568 17th September Catastrophic - 13 - 625 1648 5th August Ordinary Damage on Puentes Dam
foundation - - -
1651 14th October Catastrophic San Calixto flood 1000 deaths in Murcia region Destruction of > 1000 houses
14 - 640
1653 4th November Catastrophic San Severo flood In lowland areas of Murcia: >250 deaths in Murcia region Destruction of 4000 houses out of 6000
15 - 680
1704 27th August Extraordinary San Leovigildo - - - 1728 29th October Catastrophic - - - - 1733 3rd September Catastrophic Nª Sª de los Reyes flood 16 - 730 1802 30th April Ordinary 2nd Breakdown of Puentes
Dam; 608 deaths Catastrophic only downstream of the dam
- - -
1830 3rd September Extraordinary - - - - 1831 19th October Extraordinary La Diforme flood 17 - 760 1838 3rd October Catastrophic San Francisco flooding
Several deaths in Lorca 18
- 810
1860 17th September Catastrophic - 19 - 870 1877 19th September Extraordinary San Cosme and San
Damian- 20 - 880
1879 14th October Catastrophic Santa Teresa flood 761 deaths in Murcia; 13 in Lorca, 2 in Librilla and 1 in Cieza
21 1510 915
1880 28th August Catastrophic - - - - 1884 22nd May Catastrophic Ascensión flood - 1136 - 1891 11th September Ordinary* San Jacinto flood 22 1890 940 1898 16th January Extraordinary San Fulgencio flood - - - 1900 27th June Ordinary* San Aniceto flood - 1295 - 1921 24th September Ordinary* Virgen de las Mercedes
flood - 1005 -
1941 28th June Ordinary* - 23 1378 980 1973 19th October Catastrophic 300 mm in 24 h; 13 deaths
in Lorca and 83 in Puerto Lumbreras
24 3544 1035-1616
Table 2. Chronology of the major Guadalentín floods and assigned flood units of the ES 2 profile (number refers to stratigraphic profile in Fig. 4). Relative flood magnitude follows the classification by Barriendos and Coeur, 2004. Minimum discharges were estimated using sedimentary evidences as palaeostage indicator. *Flood hydrograph attenuated at Lorca by the Puentes reservoir.
Figure captions
Figure 1. Location of the Guadalentín river catchment and the study area.
Figure 2. Geomorphological sketch map of the study reach illustrating the location of slackwater flood deposits and the stratigraphic profiles. Figure 3. View of the ES 2 stratigraphic profile showing the spectacular sequence of slackwater flood deposition reaching a thickness of ca. 7 m. Flood unit numbers used in the text are annotated. Figure 4: Stratigraphic profiles at the study reach (radiocarbon dates in calibrated years AD) and proposed correlations between sections. Figure 5. A : Photograph of lacquer peel (50 cm high) of slackwater flood deposits
(units 16, 17 and 18) from the ES2 site; B: Grain size (VC: very coarse, C; coarse, M;
medium, F; fine, Vf; Very fine, L; Silt ); C: Flood sequence number (see ES2 profile,
Fig. 4); D: Sketch drawn from lacquer peel; and E: Inferred flood velocity
Figure 6. Upper: (i) Reconstruction of precipitation anomalies 1501–1990 for southern Spain (10 year moving average) after Rodrigo et al. (1999). The anomalies for AD 1500-1791 were estimated from documentary records, and since 1791 from instrumental data. (ii) Normalised annual flood series (this study) smoothed with a Gaussian filter (11 and 30 years, respectively). Lower: (a) Accumulated number of documentary floods compiled from the Municipal Archive of Lorca. The relative magnitudes of the events are indicated (classified according to Barriendos and Coeur, 2004). (b) Accumulated number of palaeoflood events. Palaeoflood units 17, 18, 21 and 22 (profile ES2) containing high-energy sedimentary structures are identified; other units of ES2 follow a correlative order. (c) Time framework of slope accumulation in the sedimentary record. (d) Demographic evolution (inhabitants/km2) of the upper Guadalentin region (1141 km2) according to Lentisco (1996). Shaded areas show the temporal distribution of flood clusters (Phases 2-5) described in the text. Phase 1, not shown, relates to the palaeoflood evidence of increased flood frequency dated to cal. AD950-1200.
Fig.1
Fig. 2
Fig. 3
24
23
21
22
20
18
17
16
15
13
14
11
70m
0.5
1
89
10
687,5 m
17
23
1615
1413
7
6
9
1
Jurassic
CO3=
CO3=
AD 1105 50
AD 1035 60
AD 1560 55
AD 1775 95
AD 1815 80
AD 30 55
692,2 m
684,99 m
1
2
3
56
79
8
Rambla
?
??
Rambla
Rambla
12
11
10
9
8
7
6
5
43
21
687,39 m
693,5 m
1
692,9 m
686,7 m
1
112
3
14C
Fresh sand, likely 1973 flood
14C
14C
14C
14C
14C
14C
14C
14C
14C
14C
14C= Modern
soil
8
76
5
4
2
3
1
680,85
LEGEND
Trough (3d)Cross bedding
Bioturbation
Ripples lamination
HummokyCross bedding
Roots
Climbing ripples
Parallel lamination
Climbing ripplesIn phase
Planar (2d)Cross bedding
Charcoal14C
Slope washflow
CarbonatesC03
=
Bedrock
Bedrock
Bedrock
Bedrock
BedrockBedrock
Gravel levels
Mud cracks
1 2
2
Fresh sand, likely 1973 floodFresh sand, likely 1973 flood
AD 1080 45
C+sl = Clay-Silt
S = NUMBER OF SEQUENCE( )each represents one flood only
Clay
C+sl Sand Gravel
vf f m c vc
GRAIN SIZE
AD 1080 45 = RADIOCARBON CALIBRATED AGE
Soil
RAMBLA CARAMELSHELTER (RCS)
ESTRECHO 2 (ES2)
ESTRECHO 1 (ES1)
ESTRECHO 4 (ES4)
ESTRECHO 3 (ES3)
TRIBUTARY L2 (TL2)TRIBUTARY L1 (TL1)
RAMBLA MAYOR (RM)
RAMBLA MAYOR L (RML)
AD 1810 80
AD 1795 85
AD 1790 90
AD 1820 80
Fig. 5
Fig. 6