Proc. of the gth WSEAS Int. Conf. on Mathematical and Computational Methods in Science and Engineering, Trinidad and Tobago, November 5-7, 2007

The influence of hydrological parameters on the stream floods of amountainous area in Thessaly, Greece

P. LOKKASI, S. KOTSOPOULOS2, J. ALEXIOU2, G. GRAVANISI, V.VASSILOGLOUI. S. MAGALIOS3. V. KASSOS4

1: Department of Civil Engineering Infrastructure Works, Technological EducationalInstitute (TEI) of Larissa, GREECE

2: National Agricultural Research Foundation, Institute of Soil Classification andMapping of Larissa, GREECE

3: MScE Surveyor Eng, Larissa, GREECE 4: Electro-Mechanical Eng, Larissa, GREECEp. lokkas @tei lar. gr http ://users. teilar. gr I -p. lokkas

Abstract: - The sloping area on the North and North-East of Mount Ossa, opposite Mount Olympus, in theregion of Thessaly, Greece, receives considerable rainfalls. However, despite its extended natural vegetation,severe problems have arisen due to floods. The existence of these problems is mainly based on two reasons: a)on the peculiarity of rainfall events combined with the local conditions (hydrologic, geologic, land use, etc.)and b) on the existing infrastructure works. In order to analyze and interpret the phenomena of local floods, inthe frame of the research program 'oArchimedes" which is materialized by the TEI of Larissa under the title"Spatial mapping and estimation of hydrological risk, emphasizing the floods and draughts in urban and nonurban areas of Thessaly and their environmental impacts" it is essential to record, evaluate and validatevarious hydrologic parameters of the corresponding study area. The Geographic Information Systems (GIS) isutilized to analyze and validate the information about topographic, hydrologic, geologic and soil data, alongwith land cover and infrastructure. Finally, the GIS are also used to produce and map complex hydrologicparameters, which, in tum consist input data for hydrologic models as well as presentation of results.

Key-Words: - Hydrology, geology, vegetation, stream,

1 IntroductionThe region of the North and North East slopes of themount Ossa, in Thessaly, Greece, is covered by richand dense natural vegetation, forest trees, shrubs etc,especially chest nut trees, which, locally, isconsidered a valuable means for agriculturaldevelopment. The level of rainfalls in the area isconsiderably high; almost three times highercompared to those taking place on the main part ofEastern Thessaly. This situation, combined with thelocal hydrologic conditions and the existinginfrastructure, has resulted in serious problemsconceming damages due to excessive flood events.

In the frame of the research program"Archimedes - EPEAEK II", co-funded by theEuropean Social Fund & National Resources andrealized by the Dept. of Civil Engineering of TEI ofLarissa, Greece, the above region has been selectedas a study area. The work is aiming at a fluentevaluation of hydrological parameters, which arerelated to the estimation of flood discharges.

These flood discharges, which actually constitutethe main problem, take place occasionally during thewinter and are due to the large amount of water inthe streams. The problem therefore, is originating to:

flood, infrastructure.

. the particularity of theregion,

rainfall events in this

r the local conditions, i.e. hydrologic, geologic,land use. etc and

o the lack of the necessary infrastructure works.The environmental consequences of the above

floods are highly crucial for both the cultivated areasand the local delta of Pinios river.For recording and management of the necessary forthis study local data, the Geographic InformationSystem GIS-ArcView has been utilized. The use ofGIS [1] is advisable for any presentation of thespatial variables and for this reason it has beentreated as the main tool for a large number ofapplications, like soil-water resources managementL2l,l3l, [4], [5] etc.

As far as estimation of flood discharge isconcemed, it has to be noted that there are spatialparameters, that after processing, create input datafor flood simulation [6] through hydrological models[7], [8], like soil data, data ofvegetation cover, landuse, relief, stream network characteristics etc.The processing of the above data, may directly leadto hydrological parameters, like the area of

Proc. of the gth WSEAS Int. Conf. on Mathematical and Computational Methods in Science and Engineering, Trinidad and Tobago, November 5-7 ,2007 168

hydrologic basins, the slopes ofthe basins and theirwater streams, runoff coefficients etc.

2 ProcedureFor recording and plotting both the existing

information and the resulted conclusions, theGeographic Information System GIS-ArcView hasbeen utilized, so that the spatial variability ofdifferent parameters may easily be presented. TheGIS is a powerful technology approachable andaccessible to many users. This technology allows forautomation in many procedures, therefore workingin a complementary way for many analyticalmethods, which are utilized for aquatic resourcesmanagement [].

For analysis, estimation and evaluation of floodevents on the study area, the collection and use ofavariety of data is necessary. Some of this data mayconcern hydrological, soil and geologicalcharacteristics, use of land, vegetation cover, andterrain, along with the existing infrastructure works.

The last one may comprise the road network, thecross section of natural water streams. technicalworks and crucial cross- sections that affect the flowof water.

For this reason the following tools have beenutilized:

. Maps of Hellenic Army Geographical Service(HAGS), scaled at l:5000 and l:50,000,

. Geological maps of the Institute of Geologyand Mineral Exploration (IGME), scaled at1:50,000,

. Soil maps of the Ministry of AgriculturalDevelopment and Foods (MADF), scaled atl :50,000,

. A silvicultural map scaled at 1:20,000 ando A map of mount Ossa's soil cover, i.e. borders

of forest vegetation species, scaled at1:350,000.

Apart from the above official data it wasnecessary to collect data from local heldmeasurements; for this reason the followingactivities were realized:

r Soil samples taken from different locations allover the study area. The depth of samples mayvary in the ranges of 0-30, 30-60 and 60-90cm. Every location of sampling was recordedthrough a Geographic Positioning System(GPS).

o Elements of technical works, having the formof stream bridges, along the axis-roadOmolion - Stomion - Karitsa, were recorded,i.e. their cross sectional details alons with the

position of each work, through GPS, weresaved in corresponding files. There weretotally kept 46 positions of stream bridges,ranked from very small to very large. Eachone of the corresponding streams was mainlyconnected to one or more watersheds.

o Topo€traphic data for plotting the slopes,along with the corresponding cross - sectionalareas of the main streams that belong to thestudy region.

All the above cartographic information, related tothe study area of mount Ossa is shown in Fig. l,where, the watersheds were drawn with a boldcontour. The information was then digitized andconverted to GIS-ArcView format file, where, theinter-correlations among the geographic elements,i.e. their topological relations [1] along with theirdatabase connected to them were developed.

On the top of them, in GIS files there werecreated secondary information, like:

o the borders of watersheds of the mostimportant water streams, along with their area,average slope and data base,

o the database of water streams accompanied bytheir length, average slope and the correspondinglocations of the selected cross sections, along withtheir geometrical characteristics,

r the database of soil samples, taken out ofselected locations ofthe study area.

All these data were then combined and workedout to give results, which in tum became new datanecessary to simulate the natural system.

The elaboration of elements comprises:. Topological processing of hydrological and

topographic data, i.e. hydrographical network,watershed areas, slopes of earth and waterstreams, as well as length and cross sectionsof water streams.

. Topological processing of pedologic mapsalong with their corresponding data.

. Topological processing of silvicultural mapsand maps of vegetation cover.

The average slope of the watersheds has beencalculated through the equation of Gregory &Walling, as described by Tsakiris [10]:

\ ,L .D^ S ^ - - . ( r ,. E

where S" is the average slope of the watershed, )Z isthe total length ofthe contour lines, D is the contourinterval of lines and E is the area of the watershed.

The outflow of the watershed could be estimatedthrough the Soil Conservation Service (SCS)method, where, an estimation of the runoff curvenumber, Cl{ is required [9], [10], [1 1], [12], [13].

proc. of the gth WSEAS Int. Conf. on Mathematical and Computational Methods in Science and Engineering, Trinidad and Tobago, November 5-7,2007 169

Fig. 1: Topographic map of the local study area with the contours of watersheds. Scale l:165,000

N

A

This coefficient is given in tables and variesaccording to the land use, to the management orpractise, to the hydrological conditions and to thetype ofearth [9] , [10], [1 1], [12], [13].

In the case that there are more than one land useswithin the watershed, then, the so-called compositecurve numbero CN", must be taken into account. Thisnumber is an areal weighted average of thewatershed sub-regions and is calculated through theequation

I cl'r, 'r nCN,=#=lCN, .e , , (2)

, L : i = lH

where Ci[ represents the runoff curve number forthe sub-region i, E; is the watershed area for sub-region i, and e; is the relative area of sub-region l,over the total watershed area (Iar: 1).

The composite curve number, CN", for everywatershed has been calculated through the GIS-ArcView program, provided, all the partial runoffcurve numbers for every soil cover category withineach watershed, have been taken into account.

3 ResultsIn the above study area there are various species

of forest vegetation and pastures as shown in Fig. 2.

The elaboration of cartographic data, along withthe data and types of earthy plotting, result in theestimation of:

o the area along with the average slope of thehydrologic watersheds,

o the slopes, the lengths and the cross sectionsof water streams.

Figures 2 and 3 represent graphically the aboveresults.

An analysis of the soil samples taken from depthsup to 90 cm, wherever feasible, leads to theconclusion that, according to the final infiltrationrate estimates, the majority of the hydrologic soilgroups belong to type A [9], [0], [11].

In relation to the hydrological conditions, theircharacterization as normal, moderate or poor, itresults from the combination of the earth slopes, thedepths, the earth profile and the density ofvegetation cover.

Therefore, for each sub-region, i.e. region whichis covered by different kind ofvegetation, the valueof runoff curve number, CN of SCS is defined [9],[10], [117, ll2], [3], and then, through a topologicprocessing, the composite value of CN, CN", ofevery watershed is obtained (Fig. 4).

Proc. of the gth WSEAS Int. Conf. on Mathematical and Computational Methods in Science and Engineering, Trinidad and Tobago, November 5-7 , 2007 170

, 1 S A M P L I N G P O I N T - B o U N D A R Y o F B A S I N, 0.22 AVERAGESLOPE OFWATERSHED - WATER COURSE

N/ CONTOUR LINES 1OOF'N FIR FOREST DENSE N:] OAK FOREST DENSE

rii:!r? FIR FOREST SPARSE 'm CHESTNUT FQREST DENSErT- BEECH FOREST DENSE [TIq-,: 6HESTNUT F6REST SPARSEr----r BEECH FOREST SPARSE F.,j.:r PASTURE,@ EVERGREEN FOREST ffi CULTIVATED OR BARE LAND

(SCRUBBY)

Fig. 2: Part of the study area with categories of soilcover and soil sampling locations.

Scale l: 80.000

- BOUNDARYOF BASIN- WATERCOURSE

/\./ CONTOUR LINES 1000-0.25 VALUES OF AVEMGE SURFACE SLOPESr.-r s0.25 tr ', ' :r 0.41-0.50r- 0.2$0.40 w >o50

Fig. 3: Map of average surface slopes per categoryofsoil cover. Scale l:80,000

Fig. 4: Map of composite runoff curve numberc CNcper watershed. Scale l:80,000

4 ConclusionThe Geographic Information Systems have been

proven a very useful tool for the above hydrologicalapplication in the region of the study area, locatedon the North and North East slopes of mount Ossa inThessaly, Greece. It has been especially useful fortracing a mass of cartographic data, i.e. terrain,hydrographical network, vegetation cover - or landuse from the geological or pedologic point of view.

It is also useful for process, management andvaluation of hydrological parameters, along with thearea, the slopes ofhydrologic watersheds, the slopes,lengths and cross sections of water streams as wellas the runoff curve number CNof SCS.

More and probably more useful conclusions onthe above research are expected to be obtained aftera full completion of the program, when, the spatialvariation of flood risk in the study area, is going tobe refereed.

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information systems for land resourcesassessment, Clarendon Press, 1986

[2] Kalfountzos D., I. Alexiou, S.S. Magalios and P.

Vyrlas, Integrated water resources management

- BOUNDARY OF BASIN

- WATERCOURSE

/\/ CONTOUR LINES 1OO

25,0 COMPOSITE RUNOFFCURVE NUMBER

Proc. of the gth WSEAS Int. Conf. on Mathematical and Computational Methods in Science and Engineering, Trinidad and Tobago, November 5-7,2007 171

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