An Integrated Study for the Evaluation of Natural and Anthropogenic

Zeitschrift für Geomorphologie Vol. 56, Suppl. 1, 045–067 Article Stuttgart, January 2012

An integrated study for the evaluation of natural and anthropogenic causes of ﬂ ooding in small catchments based on geomorphological and meteorological data and modeling techniques: The case of the Xerias torrent (Corinth, Greece)

Efthimios Karymbalis, Petros Katsafados, Christos Chalkias and Kaliopi Gaki-Papanastassiou, Athens

with 9 ﬁ gures and 2 tables

Summary. This study investigates the natural (hydro-meteorological and geomorphological) and anthropogenic factors responsible for fl ooding events in the ungauged Xerias torrent drainage basin, which is located in northeastern Peloponnesus, Greece. The study focuses on the analysis of the meteorological and hydrological processes of the most severe fl ooding event of the torrent, which happened on January 11th and 12th, 1997. The combined effects of spatially-varied precipitation and catchment characteristics on surface runoff were examined for this fl ash-fl ood event. The major meteorological feature of this event is associated with the passage of a cyclonic system accompanied by a cold front. 176 mm of rainfall in 10 hours was recorded at the meteorological station closest to the fl ooded area, demonstrating the severity of the event. In order to investigate the development and evolution of this cyclonic system, a numerical simulation was performed using a non-hydrostatic limited area atmospheric model on a very fi ne spatiotemporal resolution. Detailed atmospheric and soil parameters derived from the atmospheric simulation were imported into an integrated Geographical Information System (GIS) for further hydrological analysis and estimation of hydrographs throughout the catchment area. Additionally, the quantitative geomorphological characteristics of the Xerias torrent drainage basin were estimated and studied. The hierarchical drainage by stream order was investigated and the longitudinal profi les, as well as the stream power diagrams of the main stream and seven of its major tributaries, were constructed and analysed. The estimated maximum discharge for the outlet was ~610 m3/sec at almost 21:00 (12/12/1997), while this value for Solomos (9 km upstream from the river mouth) and Soussana (13.6 km upstream from the mouth) were estimated as ~540 m3/sec and ~410 m3/sec, respectively. Among the most important natural fl ood causes are extreme rainfall and the geomorphological characteristics of the drainage network. These features include irregularities in the number of channels that drain directly into streams of higher order, as well as the high bifurcation ratio values between the fourth and the fi fth order streams and high channel gradients in the upper reaches of the tributaries. A crucial fl ood factor is also human interference, expressed by artifi cial confi nement of the channels and the construction of ineffi cient bridges and pipes to facilitate the discharge.

Keywords: Xerias torrent, ﬂ ash-ﬂ ood event, Geographical Information System (GIS), runoff simulation, stream power

Zusammenfassung. Eine zusammenfassende Studie über die Beurteilung der natürlichen und anthropogenen Gründe für Flutereignisse in kleinen Einzugsgebieten, basierend auf geomorphologischen und meteorologischen Daten sowie Modelltechniken: Der Xerias Strom (Korinth, Griechenland) Diese Studie untersucht die natürlichen (hydro-meteorologische und geomorphologische) und anthropogenen Faktoren, die für das Hochwasser in der Umgebung des Xerias Stroms (nordöstliche Pelopon- nes, Griechenland) verantwortlich sind. Die Studie konzentriert sich auf die Analyse der meteorologischen und hydrologischen Prozesse der schwersten Hochwasserkatastrophe vom 11. und 12. Januar 1997. Sowohl die räumlich variierende Niederschlagsmenge als auch die Beschaffenheit des Oberﬂ ächenabﬂ usses wird

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hierfür untersucht. Der wesentliche meteorologische Bestandteil dieses Ereignisses ist der Durchzug eines Tiefdruckgebietes, das von einer Kaltfront begleitet wird. Eine Niederschlagsmenge von 176 mm in 10 Stun- den wird von der nahegelegenen meteorologischen Station gemessen. Um die Entstehung und Entwicklung dieses Zyklons zu erforschen, wird eine numerische Simulation benutzt, unter Verwendung eines nicht hyd- rostatisch begrenzten Modells mit einer sehr feinen spatiotemporalen Aufl ösung. Detaillierte Parameter der Luft sowie des Bodens sind das Ergebnis der Simulation. Diese wurden für weitere hydrologische Analysen und Einschätzung von Ganglinien im gesamten Einzugsgebiet in ein integriertes geographisches Informati- onssystem (GIS) eingegeben. Die Deutung dieser Ganglinien trägt zur dynamischen Auswertung des Ereig- nisses, sowie zur quantitativen Schätzung der Höchstentladungen bei. Zusätzlich wurden die quantitativen geomorphologischen morphometrischen Eigenschaften des Xerias Strom-Entwässerungsgebietes geschätzt und studiert. Die hierarchische Entwässerung durch Verdampfung wurde erforscht und die Längsprofi le des Hauptstromes und der sieben Nebenfl üsse wurden konstruiert und analysiert. Der geschätzte maximale Abfl uss war ~610 m3/s um 21:00 (12/12/1997) für den Abfl usskanal, ~540 m3/s für Solomos (9 km von der Mündung fl ussaufwärts) und ~410 m3/s für Soussana (13,6 km von der Mündung fl ussaufwärts). Die Ursachen dieser Flut sind extremer Niederschlag und die geomorphologischen Eigenschaften des Entwäs- serungsnetzes. Dazu zählen Unregelmäßigkeiten in der Anzahl der Kanäle, die direkt in Ströme höherer Ordnung fl ießen sowie der hohe Anteil an Gabelungen zwischen der vierten und fünften Ordnung von Zufl üssen. Ein entscheidender Faktor ist auch der menschliche Eingriff durch künstliche Anordnung von Kanälen sowie ineffi zienten Brücken und Rohren, die das Entladen erleichtern.

Résumé. Une étude intégrée pour l’évaluation des causes naturelles et anthropogéniques amènent à des inondations en de petits bassins de drainage, basée sur de données géomorphologiques – météorologiques et de techniques de modélisation. Le cas du torretorrentnt de Xerias (Corinthe, Grèce). Cette étude vise à examiner l’environnement hydro-météorologique et géomorphologique aussi bien que les facteurs humains responsables pour les inondations dans le secteur plus large du bassin de drainage du torrent Xerias, situé au NE du Péloponnèse, Grèce. L’étude est concentrée sur l’analyse des processus mé- téorologiques et hydrologiques concernant les inondations les plus graves qui ont eu lieu les 11 et 12 Janvier 1997. A l’occasion de cette inondation exceptionnelle, la distribution spatiale de la précipitation ainsi que les caractéristiques du bassin de drainage ont été examinées. Les caractéristiques météorologiques majeures de cet événement ont été liées au passage d’un système cyclonique accompagné d’un front froid. Presque 176 mm de pluie ont été enregistrés en dix heures à la station météorologique la plus proche de la région étudiée. Les paramètres atmosphériques détaillés et du sol sont le résultat de la simulation atmosphérique utilisée comme données dans un système d’information géographique intégré (GIS) pour l’analyse hydrologique et l’évaluation des hydrogrammes directs et des décharges maximales aux endroits spécifi ques du bassin de drainage. En plus, les caractéristiques morphométriques géomorphologiques quantitatives du bassin de drainage de torrent de Xerias ont été estimées et étudiées. L’hiérarchisation du drainage par ordre de tributaires a été étudiée ainsi que les diagrammes de puissance du torrent et les profi les longitudinaux du torrent principal et de ses sept tributaires majeurs ont été également étudiés. La décharge maximale estimée à l’embouchure était de 610 m3/sec le 12/12/1997 vers 21:00 h, alors que ces valeurs pour Solomos (à 9 km en amont) et Soussana (à 13,6 km en amont) étaient respectivement estimées à 540 m3/sec et 410 m3/sec. Parmi les causes les plus importantes de la provocation de l’inondation fut la précipitation exceptionnelle et les caractéristiques géomorphologiques du réseau de drainage. Ces caractéristiques incluent l’hiérarchisation du drainage par ordre de tributaires, la valeur élevée du ratio de bifurcation entre le quat- rième et le cinquième ordre tributaires et la grande pente à l’amont des tributaires. Le facteur crucial provo- quant l’inondation est également les interférences humaines qui se manifestent par l’arrangement artifi ciel du canal et la construction des ponts et des conducteurs insuffi sants pour faciliter le déchargement d’eaux.

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Introduction

The Mediterranean region is a transition zone between the temperate, humid northern latitudes and the dry southern latitudes. Despite the diversity of environments included in the term “Medi- terranean”, it still denotes a particular combination of complex climatic, structural and geomorphological factors (CAMARASA-BELMONTE & SEGURA-BELTRAN 2001). In this geographical context, ephemeral streams with torrential behaviour are common fl uvial systems. Rainfall and runoff processes in torrential streams have been studied in various morphoclimatic zones, including arid, semi-arid and the Mediterranean (DE VERA 1984, SCHICK 1988, MARTINEZ-MENA et al. 1998, MAR- TIN VIDE et al. 1999). In Greece, many drainage basins are relatively small with steep slopes, confi gured by tor- rents with braided main channel morphology. These systems are usually dry, but have extreme fl ash fl ood events of low frequency, but high magnitude. Such exceptionally high runoff may be a source of signifi cant damage to human infrastructure. Despite the importance of these fl oods, the hydrological analysis of ephemeral streams in Greece has been especially diffi cult due to the lack of precipitation and discharge gauges. Generally, the fl oods in the Mediterranean area are linked to storm events, but there are additional factors that can intensify fl ooding. The most important are the pattern of the drainage network, the morphology of the catchment and human interventions. Previous studies have tried to establish the link between the hydrological response of a catchment and descriptors of its physical attributes (among others, POST & JAKEMAN 1996, 1999, RUNGE & NGUIMALET 2005). The basic scientifi c concept of this study is to test the coupling between state-of-the-art meteorological modeling and surface GIS-based runoff modeling with standard geomorphological analysis in order to simulate a real fl ash fl ood event in a small catchment of Greece. These kinds of events in small, ungauged catchments in Mediterranean and semi- arid environments are expected to become more frequent due to global climate changes. The aim of this study is to investigate the natural (meteorological and geomorphological) as well as the anthropogenic factors responsible for fl ood events in the broader area of the ungauged drainage basin of Xerias torrent in Northern Peloponnesus, Central Greece (Fig. 1). The study focuses on analysis of the meteorological and hydrological processes of the most recorded fl ood event of the torrent, which occurred on January 11th and 12th, 1997. The surface meteorological station closest to the affected area recorded a rainfall rate of 176 mm in 10 hours and a total precipitation of 299.3 mm in 24 hours, which is almost half of the mean annual precipitation in Northern Peloponnesus. This extremely intense event has been simulated with advanced modeling techniques in order to analyse the dynamics controlling the behaviour of complex fl ooding processes. Simulation is assumed as the appropriate method to derive quantitative estimates of various atmospheric and hydrologic parameters, especially in cases involving a lack of reliable and accurate measurements of precipitation and fl ow rates. The rainfall patterns over the catchment area were estimated by simulating the atmospheric synoptic conditions using a non-hydrostatic limited area model on a fi ne spatiotemporal resolution. Quanti- fi cation of the hydrologic response of the drainage system was attempted utilizing GIS-supported hydrological analysis (OLIVERA & MAIDMENT 1999). Additionally, the geomorphic characteristics of the drainage network and the catchment area were also quantitatively analyzed and discussed. Human interference along the channels was recorded and its infl uence on the occurrence of the extreme fl ood event was assessed.

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Fig. 1. Topographic map of the Xerias torrent drainage basin.

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Study area

The Xerias torrent, located in Northern Peloponnesus (Central Greece), is an ephemeral fl ow and drains an area of 164.2 km2 (Fig. 1). The drainage basin is elongated along an almost S-N trending axis and reaches an elevation of 1137 m at its southern end (Trapezona Mt.). The main channel fol- lows a S-N fl ow direction for about 32 km and discharges into the Gulf of Corinth. At the mouth of the river, a delta fan has developed covering an area of about 1.9 km2 where the city of Corinth, with a population of ~3,3000 people, is located. Northern Peloponnesus experiences a typical Mediterranean climate, with annual tempera- tures averaging 15 °C. The mean annual precipitation in the basin ranges between 550 mm in the northern low-lying coastal region and 700 mm in the southern mountainous region (KATSAFADOS et al. 2009). Rainfall is distributed relatively unevenly with about 75 % of it occurring between the months of October and March. The Northern Peloponnesus is tectonically active and the form of the Xerias torrent drainage network and the morphology of its catchment are controlled by local tectonics. A series of fi ve well-preserved Pleistocene marine terraces located in the catchment area of the Xerias torrent are indicative of the tectonic uplift of the region (ARMIJO et al. 1996). The Xerias catchment can be morphologically divided into three areas: the southern mountainous area of rough relief and the highest altitudes encompassing the mountains Trapezona (1,137 m) and Psili Rachi (1,078 m); an intermediate semi-mountainous area where lower mountain ranges, Onia Mountains (562 m) to the east and the hills named Profi tis Ilias (701 m) and Tsouba (621 m), are interrupted by smoother relief; while the third area lies in the northern part of the basin (north of the village of Examilia) and is comprised of lower elevations and gentle slopes. This zone of low elevation is associated with the surfaces of two marine terraces corresponding to the last interglacial (stage 5 e) and the Holocene (CHAPELL & SHACKLETON 1986, ARMIJO et al. 1996). The drainage basin consists of Alpine and post-Alpine sediments (Fig. 2). The Alpine formations possess transition characteristics between the units of Pindos, Subpelagonic and Parnassos geotectonic units (BORNOVAS et al. 1971), occupy the southern part of the basin and consist mainly of intensively deformed limestones, which exhibit enhanced secondary permeability and karstifi cation. The Plio-Pleistocene formations (marls, sandstones and conglomerates) are developed at the northern part of the basin and their hydrogeological behaviour varies due to lateral transitions and alternations. Marls are the most abundant formation and exhibit small to very small permeability, thus facilitating surface runoff. The drainage network pattern refl ects the infl uence of the lithological and tectonic regime of the area. The elongation of the main channel is the result of headward erosion due to the tectonic uplift of the area. In the northern region, impermeable marls cause a dense drainage network with a signifi cant number of fi rst and second order streams, while the drainage density in the southern region of the basin is much lower, mainly due to the presence of limestone formations.

Methodology

The atmospheric conditions for the period of 10 –13 January 1997 were simulated with the Weather Research and Forecasting limited area model with the embedded dynamical core of the Non-

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Fig. 2. Simpliﬁ ed lithological map of the Xerias torrent drainage basin (based on geological map of Institute of Geology and Mineral Exploration, BORNOVAS et al. 1971 and ﬁ eld observations).

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hydrostatic Mesoscale Model (WRF-NMM). In an effort to examine the synoptic and mesoscale atmospheric patterns, a nested domain on a very high horizontal grid increment was applied. Domain geometry included a coarse 305 × 273 mesh with 0.09° × 0.09° (about 10 km) grid spacing and 38 vertical levels with a fi ne nest of 249 × 269 grid points on 0.03° × 0.03° (about 4 km) of horizontal resolution covering the eastern Mediterranean. The structure of the domain followed the Arakawa E-staggered grid and it was centered over 39.50° N and 14.95° E. ECMWF analyses on a 0.25° × 0.25° (almost 30 km) horizontal grid increment, and 11 isobaric levels were used for the model initial and boundary conditions. For this study, the simulated period began at 00:00 UTC January 10th 1997, and continued for 72 hours up to January 13th at 00:00 UTC. The WRF model consists of advanced schemes for the simulation of the entire atmospheric and soil processes. The more signifi cant modules of the model are: – the Ferrier scheme for the microphysics (FERRIER et al. 2002), – the GFDL scheme for the estimation of both longwave and shortwave radiation budget, – the Betts-Miller-Janjic cumulus parameterization scheme (JANJIC et al. 2001), – the Monin-Obukhov-Janjic scheme for the simulation of the surface layer processes, – the Mellor-Yamada-Janjic turbulent kinetic energy scheme (JANJIC 1996) for the estimation of the planetary boundary layer physics and – the unifi ed NOAH land-surface model ( CHEN & DUDHIA 2001) for the simulation of soil moisture and temperature. A major part of the proposed methodology is based on GIS functions and the integration of various data for the study area. For this purpose, a spatial database was designed and implemented. This geodatabase consists of the primary GIS layers, including contour lines, elevation points (point topology), stream network (line topology), mainland, land cover, geological formations (polygon topology), precipitation and meteorological simulation data. The main sources of these datasets were analogue maps provided by national cartographic agencies, such as the geological maps from the Institute of Geology and Mineral Exploration of Greece (IGME), and topographic maps from the Hellenic Military Geographical Service (HMGS) at the scale of 1:50,000. The maps were georeferenced and then onscreen digitization took place, in order to create the aforemen- tioned information layers. The post processing of these layers within a GIS environment produces secondary thematic layers, e.g., a Digital Elevation Model, as well as its products (hillshade, fl ow direction, fl ow accumulation, fl ow length, stream network from DEM, stream ordering, drainage basins). The digital elevation model (DEM) of the study area is one of the most critical datasets for the GIS-based direct runoff modeling (HOLMES et al. 2000, KNEBL et al. 2005, GALLOW et al. 2007). In this study, a 50 m × 50 m resolution DEM was constructed with the use of the available topographic maps of the Xerias basin and the ANUDEM interpolation algorithm (HUTCHINSON 1989, 2003). This interpolation algorithm produces hydrologically consistent outputs and maintains landscape integrity (GALLOW et al. 2007). In order to produce an accurate DEM – proper for further analysis – this method was repeated with additional elevation data from more detailed maps until the performance of the model for the replication of stream network and sub-basin delineation was acceptable. DU et al. (2009) suggested “Good overall results could be attained with a grid size of less than 100 m…”. The selection of the cell size (50 m × 50 m) for the fi nal DEM is based on previous studies in catchments with similar characteristics.

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In the context of GIS-based hydrological analysis, a method is proposed for routing spatially- distributed excess precipitation over a watershed to produce runoff at its outlet. The fi rst step was the discretization of the watershed under investigation. For this purpose, the Digital Elevation Model (cell size 50 x · 50 m) of the study area was constructed as mentioned above. GIS analysis provides the terrain-based derivables, such as a depression-less DEM, fl ow direction, upslope drainage area (fl ow accumulation) and fl ow length (TRIBE 1992, O’CALLAGHAN & MARK 1984). Next, the outputs of the meteorological model (spatiotemporal variation of the excess rainfall and surface runoff) were integrated into the GIS spatial database. Afterwards, a routing function was defi ned for each cell of the DEM in order to determine water fl ow from cell to cell. All these derived maps were used to calculate the travel-time to outlet maps and the isochrone maps were constructed corresponding to the real rainfall event. Finally, a convolution technique was implemented in order to construct the direct runoff hydrographs at selected locations along the Xerias torrent main channel. The creation of direct runoff hydrographs is based on the spatially-distributed unit hydrograph method and its alterations (among others, MAIDMENT et al. 1996, MUZIK 1996, OLIVERA & MAIDMENT 1999, AJWARD & MUZIK 2000, SARANGI et al. 2006). An alteration of this method was attempted by considering the spatiotemporal variation of rainfall in the study area, according to the previously-described state–of-the-art meteorological modeling. This general modeling approach is applicable in the GIS environment and its use is still acceptable (DU et al. 2009). The model uses raster datasets in grid format. The fi rst category of these datasets is related to the background terrain properties (e.g., topography, land cover, soil types, etc.), while the second corresponds to hydrological features and are used as secondary datasets (fl ow direction, fl ow accumulation, stream network, fl ow length, slope etc). These secondary grids can be created with the use of standard GIS terrain analysis functions. For the calculation of the maximum discharge at specifi c defi ned outlets, the GIS-based unit hydrograph derivation method was adopted. According to this concept, the defi nition of isochro- nes (fl ow traveling to the watershed outlet) at time intervals Δt is the key point for the construction of time-area diagrams (CHOW et al. 1988). The spatiotemporal distribution of the rainfall inside the study area was used for the creation of the direct runoff hydrographs (MAIDMENT 1993). The direct runoff at time t = n·Δt is assumed as the aggregation of the runoff pulses from each of the time zones properly lagged in time: ⋅ n i AP i Q = ∑ j n Δ =1i t

where Pij is the total excess rainfall for all cells in the isochrone zone during time interval j and Ai is the area of each isochrone zone. The general concept is to incorporate GIS-based hydrological analysis functions and background spatial data for the construction of the ﬂ ow time layer. Next, this layer is integrated with rainfall simulation data for the creation of the direct runoff hydrograph by convolution. Direct runoff hydrographs were estimated in three of the most affected locations of the ungauged drainage network and were correlated with both the reported indirect discharge observations after the event and the structural damage. The selection of these locations (apart from the outlet to the Gulf of Corinth) was based on the geomorphological analysis. These are locations downstream from the conﬂ uence of major branches to the main channel.

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Additionally, the quantitative geomorphological and morphometric characteristics of the Xe- rias torrent drainage basin were estimated and studied. The drainage network derived from the DEM was numbered according to A. STRAHLER’s (1957) system and quantitatively analysed, and the hierarchical drainage by steam order was also investigated. The constructed spatial database and GIS technology were utilized for the quantitative measurements. Stream power (the energy of a stream at specifi c location) is highly related to the transport of sediments in fl uvial channels (PETIT et al. 2005), differentiations of fl oodplains (NANSON & CROKE 1992) and of channel patterns (ALABYAN & CHALOV 1998). Accordingly, the total stream power profi les along main branches of the study area were constructed. The longitudinal profi les of the main stream and seven of its major tributaries were also drawn and analysed. The stream power diagrams for the particular fl ood event were used in combination with the corresponding longitudinal profi les in order to interpret the energy potential for each part of the channels under investigation. In order to calculate the total stream power diagrams for each main stream of the study area, the following equation (KNIGHTON 1999) was used: Ω = γ · Q · S where γ is the specifi c weight of the water (9,807 N/m3), Q is the discharge (m3/sec) and S is the energy slope derived from the DEM (m/m). Since there are no available discharge recordings, the Q parameter was measured by maps with the use of fl ow accumulation area (from the analysis of the DEM) and the maximum active rainfall for 24 hours from the simulation of meteorological data. Finally, a record of the human activities along the channel and especially at the lower reaches of the river through fi eldwork was attempted in order to estimate the contribution of human impact on the fl ood event. The methodological scheme described above combines various quantitative and qualitative approaches for the integrated study of the fl ooding event in the small catchment under investigation. Meteorological simulation, spatial database creation, GIS hydrological analysis, geomorphological evaluation of the stream network and qualitative assessment of human interaction are the main aspects of this integrated approach.

Results and Discussion

The event The city of Corinth, which is located at the fan delta of the torrent, has often suffered extensive damage during extreme rainfall events. In 1972, four people lost their lives and in 1990, the torrent experienced another extreme discharge event. The most severe ﬂ ood happened on January 11th and 12th of 1997 and caused six people to lose their lives and extensively damaged houses and cultivations regionally, especially in the city of Corinth (GAKI-PAPANASTASSIOU et al. 2008). In some locations (for example the village of Solomos), the water level increased up to 13 m, causing severe damage (LEKKAS et al. 1998). As the drainage basin of the Xerias is not meteorologically and hydrologically monitored, there are no available measurements of precipitation and water discharge for this event. Thus, the exact amount and spatial distribution of precipitation in the catchment during the event, as well as the ﬂ ood hydrograph causing the inundation, are unknown due to the lack of rainfall and hydrometric stations.

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Synoptic analysis of the atmosphere

The synoptic conditions were associated with the passage of a cyclonic system accompanied by a cold frontal zone over Southern Greece (KOTRONI et al. 1999). The storm initially developed over the cyclogenetic area at the Gulf of Genoa on January 9th, 1997, and over the next several days it moved in a southeastern direction towards Sicily and the Ionian Sea. According to the simulated distribution of the geopotential height at 500 hPa on January 12th at 00:00 UTC, a cut-off low was

Fig. 3. Temperature (°C) at 500 hPa (grey scale shading) superimposed by the geopotential height (gpm) at 500 hPa (solid line) valid for the January 12th, 1997 at 00:00 UTC.

Fig. 4. Mean sea level pressure (solid line) for January 12th 1997 at 00:00 UTC and 3 hr accumulated precipitation (mm) for the period January 11th at 21:00 UTC to January 12th, 1997 at 00:00 UTC (grey scale shading).

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located southeast of Sicily (Fig. 3) and it was associated with a surface low pressure system of 1010 hPa (Fig. 4). As it passed over Southern Greece, the system was further enhanced by upper air heat and humidity advection. The unstable air masses were forced to orographic lifting in an axis with edges defi ned by the topographic obstacles over northeastern Peloponnesus and southeastern central Greece, producing severe convection. Intense precipitation was recorded by the entire network of surface meteorological stations located east of the main orographic barrier of the Greek peninsula. Specifi cally, the meteorological station near the city of Lamia recorded 100 mm within 24 hours, while the station located at the site of the Hellenic National Meteorological Service in Athens accumulated 54 mm. However, the major storm cell with the most severe characteristics occurred over northeastern Peloponnesus and southeastern central Greece. Extreme precipitation rates of almost 176.8 mm in 10 hours were recorded by the meteorological station closest to Corinth, while the 24 hour accumulated precipitation was 299.3 mm and is defi ned as the decadal maximum for the period 1987–1997. In the same period, the Argos station, which is located 35 km southeast of the affected area, accumulated 83.4 mm of rainfall.

Atmospheric model analysis

The WRF model simulated the entire synoptic and mesoscale features of the storm and estimated valuable atmospheric and soil parameters, such as precipitation height and surface runoff for the affected area of the event. The implementation of the nested grid allowed to resolve the spatiotemporal distribution of the accumulated precipitation and revealed the areas with the higher amount of precipitation. Indeed, the maxima of the simulated precipitation were located over Corinth and Lamia with 225.3 mm in 24 hours for the period from January 12th at 00:00 UTC up to January 13th at 00:00 UTC (Fig. 5).

Fig. 5. Model nested grid accumulated precipitation (mm) for the period January 12th at 00:00 UTC to January 13th, 1997 at 00:00 UTC (grey scale shading).

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Fig. 6. Timeplots of the observed (dark grey) and the simulated (light grey) precipitation at (a) Argos and (b) Corinth meteorological stations.

In order to provide a quantitative assessment of the model forecast skill, accumulated precipitation at the location of 2 meteorological stations close to the affected area was used for the point-to-point comparison between model-generated values and observations (Fig. 6). Based on these results, the model accurately simulated the general pattern of precipitation, especially in the case of the Argos station, where the observed and simulated precipitation was 83.4 mm and 78.2 mm, respectively for the period from 18:00 UTC on January 11th to 18:00 UTC on January 12th (Fig. 6a). In the same period, the meteorological station in the vicinity of the city of Corinth accumulated 299.3 mm of precipitation, while the model-derived amount was 192.3 mm (Fig. 6b). A more detailed investigation of the rainfall temporal variability at the Corinth station indicated an underestimation of the total amount of the precipitation rate by the model, setting the main core of

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the simulated precipitation 6 hours later (12:00 –18:00 UTC on January 12th instead of 06:00 –12:00 UTC). The underestimation of the total precipitation is partially explained by the fact that each grid point over the model domain corresponds to an area equal to 4 km × 4 km = 16 km2. There- fore, the simulated values represent average values over an area rather than a single point value. Furthermore, the underestimation can possibly be attributed to the model’s inadequate resolution of the steep orography of the Northern Peloponnesus, smoothing the terrain-induced mechanisms, which triggered the convective motions of the air masses. However, the temporal and spatial variability of discrete fi elds, such as precipitation, is diffi cult to analyse objectively with a sparse rain-gauge network. The temporal evolution of the event in terms of simulated precipitation and surface runoff timeplots reveal the catchment areas with the maximum contribution. Soussana, Solomos and Athikia were the most severely affected areas, with almost 55 mm accumulated in 3 hours maximum rain rate, while the estimated peak of accumulated surface runoff exceeded 20 kgr/m2. Despite the fi ner nest and the high spatiotemporal resolution of the model, which was able to resolve the entire synoptic and mesoscale patterns of the atmosphere, the model’s domain is assumed to be too coarse to accurately represent the main physiographic characteristics of this small catchment area and therefore to resolve in detail the main hydrological features. To this end, specifi c model outputs, such as precipitation and surface runoff, were used as background data in the GIS, confi gured at a horizontal resolution of 100 m for the fi ner estimation of surface hydrological parameters.

The response of the catchment

Direct runoff hydrographs were reconstructed at three specifi c locations along the Xerias torrent main channel where extreme water levels and associated extensive damage were observed during the event. Location 1 is the outlet of the river at the apex of the fan-delta where the torrent enters the city of Corinth (Fig. 7). Location 2 is 9 km upstream from the river mouth in the area of Solo- mos, where the water level increased up to 13 m and Location 3 is in the area of the Soussana plain, 13.6 km upstream from the mouth at the confl uence of the Voukina and Klissoura, which are the two main mountainous branches where water level increased up to 6 m. The analysis of the model-derived hydrographs for the Xerias ephemeral stream is relatively simple since there is no base fl ow. In all three locations, the shape of the hydrographs produced is typical of a fl ash fl ood. They are sharp, with a relatively short time base and steeply rising limbs. Fig. 7 displays a combined diagram of the hydrograph produced by the applied model that corresponds to the apex of the fan-delta with the hyetograph temporally defi ned for the total duration of the event (x-axis). The hyetograph, in the form of an upper axis histogram, is comprised of the 3-hourly accumulated precipitation, spatially integrated over the entire catchment area. This model-derived hydrograph seems to follow the mean rainfall pattern above the basin with three discharge peaks. The diagram shows that the basin’s response to the precipitation was very quick. The fi rst peak (~100 m3/sec) occurred on January 12th, 1997 at about 03:00, almost six hours after the fi rst precipitation maximum (20.04 mm), which occurred on January 11th, 1997 at 21:00. The main peak with the maximum discharge of ~610 m3/sec was estimated on January 12th, 1997 at the 3-hour time interval between 20:00 and 22:00, almost three hours after the occurrence of the

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Fig. 7. Direct hydrographs for three locations along the main channel of Xerias torrent for the extreme event occurred on January 11th and 12th, 1997. 3 hourly stream discharges were obtained from the GIS-based hydrological analysis. The hydrograph of Location 1 corresponds to the outlet of the torrent and includes also the 3 hourly mean rainfall above the catchment area.

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mean maximum rainfall (51.67 mm in 3 hours) over the catchment area, which was associated with the passage of the frontal system. One of the reasons for this high discharge was the oversaturation of the soil with a signiﬁ cant amount of moisture, since a large percentage of the total precipitation was accumulated during the previous day, prior to the arrival of the main rainfall wave. The hydrographs at the other two locations have a similar shape to the one at the outlet of the network. Maximum discharges at Solomos and Soussana are estimated to be ~540 m3/sec and ~410 m3/sec, respectively. Some small differences in the shape between the three hydrographs can be attributed to the spatially-uneven distribution of the precipitation and the synchronization of the tributaries. Validation of the produced hydrographs is impossible since there are no gauging records. However, the ﬁ ndings of the proposed methodology are in agreement with the results of previous approaches to this event. LAZARIDOU et al. (2004) estimated a wide range of peak discharge values (between 150 m3/sec and 1,000 m3/sec) at the outlet of the basin. Additionally, BALOUTSOS et al. (2000) simulated the event with the Soil Conservation Service’s (SCS) method of “Runoff Curve Number” and estimated ~600 m3/sec as the maximum discharge at the outlet (~20:00 pm, January 12th, 1997) and ~470 m3/sec at Soussana plain at the same time.

Table 1. Relation between the numbers of streams, mean cumulative channel length and mean drainage basin area per order for the Xerias torrent drainage network. Stream order (u) Streams Bifurcation ratio Mean ratio Ideal stream Divergence (%) Number (Nu) Rb number 1 387 410 – 5.6 2 94 4.1 91 + 3.2 3 26 3.6 4.5 20 + 28.4 4 6 4.3 5 + 20.0 5 1 6.0 1 0.0

Table 2. Number of streams, channel length and basin area for each order that drain directly into streams of higher order for the drainage network of Xerias torrent. Stream order Number of % Channel length % Basin area % streams 1st to 2nd 308 79.2 130.8 76.6 78.6 79.4 1st to 3rd 53 13.7 25.1 14.7 13.7 13.8 1st to 4th 14 3.6 8.6 5.1 3.6 4.8 1st to 5th 12 3.5 6.2 3.6 3.1 2.0 2nd to 3rd 75 79.8 165.8 77.5 75.1 78.6 2nd to 4th 10 10.6 28.1 13.1 13.2 13.9 2nd to 5th 9 9.6 20.2 9.4 7.1 7.5 3rd to 4th 18 69.2 187.0 78.6 93.1 85.5 3rd to 5th 8 30.8 51.0 21.4 15.9 14.5 4th to 5th 6 100.0 248.1 100.0 130.9 100.0

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Fig. 8. Hill shade map of the catchment showing the 4th and 3rd order streams (and corresponding basins) that fl ow directly into the 5th order main channel of the Xerias torrent. Locations 1, 2 and 3 correspond to the places along the Xerias torrent main channel where direct hydrographs for the fl ood event were estimated through combined modeling and GIS techniques. The map also depicts the locations that were affected by the intense rainfall of January 1997. Numbers correspond to the maximum height of the water level observed during the fl ood event.

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The role of geomorphic features A large number of descriptions have been used in the literature in search of connections between the hydrological response of a catchment and some observable indices of its physical properties (among others, SEFTON & HOWARTH 1998, KOKKONEN et al. 2003). The quantitative geomorphological analysis of the drainage network showed very high bifurcation, channel length and basin area ratios between the 4th and 5th order streams (Table 1). This means that there are four 4th order channels that drain directly into the 5th order main stream of the Xerias (Fig. 8). These junctions are located about 0.4, 7.9, 11.4 and 12.5 km upstream from the river mouth. Thus, during extreme rainfall events, the surface runoff of an area of 88.7 km2 is directly added to the main channel discharge. Table 2 includes the number of streams, channel length and basin area for each order, which drain directly into higher order streams. The most important irregularities concern the 3rd order streams that drain directly into the 5th order main channel of the Xerias. For the 3rd order streams, 21.4 % of their total channel length and 15 % of their drainage basin area join the main torrent channel at its lower reaches before the entrance of the Xerias into the city of Corinth, enhancing its discharge (Fig. 8). This assumption is supported by a comparison of the direct hydrographs estimated for the particular event at locations 1 and 3 (Fig. 7). A mean discharge of about 200 m3/sec during the main wave was fed by the four 4th and eight 3rd order tributaries that join the main channel of the Xerias between Soussana (Location 3) and Corinth (Location 1) (Fig. 8). Longitudinal profi les (Fig. 9) revealed that most of the tributaries contain an upper mountainous region with signifi cantly high slopes (9.9 –18.6 %) and a lower region near the junction with the main channel with much lower values (1.9 – 3.7 %). Intense and abrupt variations in the relief result in the decrease of water velocity in low gradient areas and the consequent accumulation over a short time period of a vast volume of water, which the main stream channel cannot accommodate. The most interesting stream power diagrams along the main channels of the drainage network, for this particular fl ash fl ood event, are the ones that correspond to the main channel of the Xerias and those of the Platania and Klissoura tributaries (numbers 1, 5 and 7 in Fig. 9 respectively). These streams release a large amount of power, reaching up to almost 13,000 watts/m through high gradient sites, as is shown by the combined longitudinal profi le-stream power diagrams (Fig. 9). High power values were also observed along the stream at the confl uence of major 3rd and 4th order tributaries with the main channel (especially at its lower reaches of the Xerias) due to their high contribution of surface runoff to the main channel.

Human interference Although it is hard to separate the role of human activities from natural fl ood causes, it is important to qualitatively describe human impacts along the channels of the catchment. The most severely affected locations, as observed during the event and from observations reported by local people, are depicted in Fig. 8. A series of direct and indirect human interferences can be observed in the area of the Xerias torrent catchment. Some of them lead to the reduction of the stream cross- section or impede water fl ow. These human impacts are closely related to cultivations and constructions observed along the streambed, debris disposal and the artifi cial incision of the stream. Although the main river channel forms relatively wide valley bottoms, crops occupy almost all of the overbank areas. These human activities decrease the width of the active riverbed by a few

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metres. In locations along the main streams, vines and olive trees are planted within the channel and reduce the potential channel width, which signifi cantly increases the fl ood risk (GATSIS et al. 2001). The plain of Soussana, where the riverbed is relatively wide, but intensely cultivated with vines and olive trees, was one of the most severely affected areas. Part of the Klissoura streambed, near the alluvial plain, has been cultivated; while along the plain, the whole riverbed is cultivated with vines and olive trees. One of the reasons for the extensive damage during the 1997 fl ood event was the fact that constructions (mainly bridges associated with roads) were ineffi cient at facilitating the fl ow of rain. Such concrete bridge constructions exist in the villages of Athikia and Solomos, the plain of Sous- sana and the apex of the fan delta at the entrance of the Xerias into the city of Corinth. In the area of Athikia, on the road that links the village with Chiliomodi, a small low concrete bridge with a small oblong-shaped opening blocked the fl ow of rainwater. Apart from its small dimensions, it was also chocked full of boulders during the extreme discharge event, causing destruction of the road and the construction (LEKKAS et al. 1998). Along the part of the main channel of Klissoura stream between the villages of Chiliomodi and the area of Soussana, small constructions (mainly bridges with pipes of small cross-section) exist. These bridges were blocked by debris carried by the stream and fi nally failed to reach channel runoff. In the broader area of the Soussana plain at the junction of the two main streams (the Klissoura and Voukina), there are two concrete bridges that were inadequate to cope with extreme discharge. Debris and a bus that was carried away by the torrent blocked these two bridges. At Solomos village, another arch-shaped bridge 6 m high and 20 m wide did not cope successfully with the extremely high torrent sediment discharge. Logs blocked the torrent bed and the water level increased up to almost 2 m above the top of the bridge. In fact, for the exceptional fl ood event of 1997, the presence of the above-mentioned constructions became an obstacle to the water movement and generated both backwater with local fl ooding and steep waves resulting from the sudden opening of the partial obstruction by the transported fl oating material. Fine sand and silt of considerable thickness was deposited upstream from these manmade structures, indicating the high sediment load of the drainage network during the event. According to LEKKAS et al. (1998), during the time interval between the two precipitation maximums, fi ne material derived from the southern mountainous parts of the catchment was deposited in the low- lying area of Susana. This newly deposited sediment was washed away by the second phase of the heavy rainfall. At the time of the event, the torrent channel within the city was artifi cially incised along two- thirds of its course in a rectangular cross-sectioncross-section concrete canal. In this region, three concrete bridges were destroyed due to the torrential fl ow and the cars that were swept away. Due to the damage associated with the 1997 fl ood, the Greek government made plans to repair destroyed infrastructures and build new protective structures (embankments, canals, dikes) at the lower reaches of the torrent where people and buildings are potentially under threat. After the event, the channel was artifi cially widened for the section between the national motorway and the border of the city, while the entire channel through the city has been totally covered by the road network. ᭣ Fig. 9. Longitudinal profi les and stream power diagrams of Xerias torrent and seven of its major tributaries. Dotted lines correspond to the stream power along the channels for the investigated extreme fl ood event. Insert map shows the location of each stream in the catchment.

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It is clear that the case of the 1997 extreme fl ash fl ood and its hydrological characteristics should be the reference fl ood event for management planning and designing fl ood protection works for the city of Corinth (LAZARIDOU 2004). This approach attempts to provide an overall view of the response of the basin to this extreme rainfall event using modeling, as there are no rainfall and discharge gauges within the catchment. Extreme fl ood events like the one under investigation are expected to occur more often in the future. The deforestation that has taken place during the last decades due to logging, grazing, agriculture, as well as forest fi res, are the main reasons for this increase (GATSIS et al. 2001). The natural vegetation was dramatically reduced after the wildfi res of July 1998 and the forest fi res during the summer of 2007 almost destroyed the vegetation in a large part of the drainage basin, eliminating some of the last pine forests of the area. Lack of vegetation reduces infi ltration capac- ity and increases the velocity of surface fl ow. Thus, the development of an integrated scheme for the evaluation of extreme fl oods is expected to be more valuable in the future.

Conclusions

Among the most important natural causes that triggered the 1997 fl ash fl ood event was the intense rainfall, while geomorphic features of the drainage network and human interference intensifi ed the phenomenon. The meteorological station closest to the affected area surface recorded a rainfall rate of 176 mm in 10 hours and a total precipitation of 299.3 mm in 24 hours, which is almost half of the mean annual precipitation in Northern Peloponnesus. According to synoptic analysis, the extreme weather event was produced by the passage of a cyclonic system on January 11th to 12th, 1997, which was accompanied by a cold frontal zone over Southern Greece. As it passed over Southern Greece, the system was enhanced by upper air heat and humidity advection and the unstable air masses were forced to orographically lift over the mountainous of northeastern Peloponnesus and southeastern central Greece, which produced severe convection. The lack of a meteorological and hydrological monitoring system for this small torrential catchment revealed the necessity of simulating the atmospheric patterns associated with this extreme fl ooding event. To this end, an advanced modeling system was applied for the analysis of the entire synoptic and mesoscale atmospheric conditions. In order to assess the skill of the model to simulate the characteristics of the storm, a point-to-point comparison between model- generated and observed rain rates was performed. According to the extracted timeplots, the model satisfactorily simulated the general pattern of precipitation in the case of the Argos station, but it underestimated the total precipitation at the Corinth station. This can possibly be attributed to the model’s inadequate resolution of the Northern Peloponnesus’ steep orography, which smoothed the terrain-induced mechanisms that triggered the convective motions of the air masses. Based on the model outputs, Soussana, Solomos and Athikia were the most severely affected areas, with a maximum accumulated precipitation rate of 55 mm/3 hrs during the time interval 15:00 –18:00 UTC on the 12th of January. Precipitation timeplots over the above-mentioned areas suggested that almost 30 % of the total precipitation accumulated on January 11th and enriched the soil with a signifi cant amount of moisture prior to the arrival of the cold front. The next day, the precipitation associated with the frontal passage accumulated over an already saturated soil, resulting in signifi cant fl ooding.

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The hydrological (GIS-supported) analysis was used for the development of direct runoff diagrams for specifi c high-risk locations. This analysis showed that the maximum discharge for the Corinth outlet was ~610 m3/sec at almost 21:00 (01/12/1997), while this value for the Solomos and Soussana outlets were estimated as ~540 and 410 m3/sec, respectively. The estimated time lag between the maximum precipitation and the maximum discharge was very short (about less than 3 hours) suggesting that the response of the basin to the extreme rainfall was very rapid. The only available data for model sensitivity analysis and validation of the hydrographs produced by the applied model are the outputs of previous modeling approaches, as well as spo- radic rough estimations of the fl ooding event. Simulation with the use of the “Runoff Number Curve” method proposed by the Soil Conservation Service (SCS) (BALOUTSOS et al 2000) estimated ~600 m3/sec as the maximum discharge at the outlet and ~470 m3/sec in the Soussana area at ~20:00 pm, January 12th, 1997. From the same study, the maximum discharge for the Soussana region was estimated by using fi eld data measurements and Manning’s formula to be up to 400 m3/ sec. Additionally, rough estimations from other studies (LAZARIDOU et al 2004) indicate a wide range of peak discharge values at the main outlet (lower limit: 150 m3/sec, upper limit: 1,000 m3/ sec). The fi ndings of this study are in general agreement with the fi ndings of these previous studies. A critical future study is to check and calibrate this methodology in a gauged catchment with the use of real fl ood event data. The geomorphological analysis showed that the pattern and characteristics of the drainage network enhance fl ash fl oods. These features include irregularities in the hierarchical drainage by stream order, the high value of the bifurcation ratio (6) between the 4th and 5th order streams and high channel gradients in the upper reaches of the tributaries. Human interference in the main channel of the torrent, particularly with respect to its path through the city of Corinth, seems to be detrimental during extreme fl ood events. Human interference is expressed by artifi cial confi nement of the channels and constructions (mainly bridges), which become an obstacle for water movement because they cannot adequately cope with the extremely high torrent discharge. This type of approach is important because qualitative observations, along with estimates of extreme stream-fl ow in ungauged rivers, are a prerequisite for solv- ing a number of engineering and environmental problems, including design of bridge structures, fl ood control and protective measures, stream habitat assessment and land use planning within the catchment area.

Acknowledgements

We would like to thank Prof. Irena Tsermegas, two anonymous reviewers and the Editors of the Special Issue Prof. Zbigniew Zwolinski and Prof. Beylich Achim for their helpful suggestions, comments and corrections that signiﬁ cantly improved the paper.

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Addresses of the authors: EFTHIMIOS KARYMBALIS, Department of Geography, Harokopio University, 70 El. Venizelou Str. Kallithea 176-71, Athens, Greece. email: [email protected] PETROS KATSAFADOS, Department of Geography, Harokopio University. email: [email protected] CHRISTOS CHALKIAS, Department of Geography, Harokopio University. email: [email protected] KALIOPI G AKI-PAPANASTASSIOU, Department of Geography – Climatology, Faculty of Geology & Geo- environment, University of Athens. email: [email protected]

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