assessment of the flood occurrence potential in the upper teleajen

Year XXVI, no. 1/2016 (June), pp. 25-33
Article no. 261102-687
Analele Universităţii din Oradea, Seria Geografie
ISSN 1221-1273, E-ISSN 2065-3409
ASSESSMENT OF THE FLOOD OCCURRENCE POTENTIAL IN THE
UPPER TELEAJEN RIVER BASIN
Mihaela BORCAN
National Institute of Hydrology and Water Management, 97 Bucharest – Ploiesti Street,
013686, Bucharest, Romania, e-mail: [email protected]
Mihai RETEGAN
National Institute of Hydrology and Water Management, 97 Bucharest – Ploiesti Street,
013686, Bucharest, Romania, e-mail: [email protected]
Abstract: For a better management of the extreme situations generated by catastrophic
hydrological events, we used a methodology regarding the estimation of the flash-flood
occurrence potential in this particular river basin which integrates three physical elements (the
slope of the terrain, soil texture and vegetation cover) that relate to precipitation run-off. The
method also takes into account a fourth element that is the Land Cover/Use. For the purpose
of this paper, we have replaced it with the 1% occurrence probability maximum rainfall.The
results obtained by using this method is represented by an index which allows users to see
which places are more pre-disposed to flash flooding.
Key words: flood, rainfall, FFPI method, risk index, Teleajen
* * * * * *
INTRODUCTION AND STUDY AREA
The genesis of flash-floods in the Upper Representative Teleajen River Basin is primarily related to
the climatic conditions, but also to other factors such as: geology, through the degree of permeability of rocks,
soil, through its temperature and moisture content, vegetation, landforms, through the longitudinal slope of
the river beds as well as the form and the size of the river basin. All these features influence the concentration
time and the volume of the liquid run-off generated by a specific amount of rain. Within this context, the
present paper aims, by integrating the main physical factors controlling the surface flow, to estimate the areas
prone to accelerate run-off and therefore with high potential for flood occurance. The integrations has been
achieved in GIS and the result is a quasi-statistic numeral index of flash flood potential specific to a
geographic area. Four classes reflecting the flood occurence potential were established (very low, low,
moderate and high). The FFPI method was initiated by Smith (2003) and applied for case studies in the
United States (Abeyeta, 2009; Brewster, 2010; Kruzdlo & Ceru, 2010). In Romania it was adopted by
Mătreață and Mătreață (2010), Teodor and Mătreață (2011), Minea (2011), Prăvălie and Costache (2013) and
Zaharia & et al., (2013) who used it in order to estimate de flash flood potential on small and medium river
basins. Borcan & Achim (2011) applied the method on a larger basin (Ialomița).

Corresponding Author
http://istgeorelint.uoradea.ro/Reviste/Anale/anale.htm
26
Mihaela BORCAN, Mihai RETEGAN
Figure1. The position within the Romanian Territory of the Upper Teleajen River Basin (Cheia)
Figure 2. The main sub-basins of the Upper Teleajen River Basin (Cheia)
Assessment of the Flood Occurrence Potential …
27
The study area is represented by the experimental Upper Teleajen River Basin (Cheia),
located on the southern slopes of the Eastern Carpathians (The Curvature Group), around the
sources of Teleajen River, the most importanttributary of Prahova River (figure 1).
The river basin covers an area of 41.3 km², being situated on the southern slopes of Ciucas
Massif and ends at approximately 250 m downstream from the confluence of Tâmpa and Cheița
creeks, which merge to form the Teleajen river (figure 2). The analyzed basin comprises 8 subbasins with areas between 1.51 and 21 km² controlled by 8 river stations (table 1).
Cheia River Basin extends between 865 m and 1954 m, with a mean altitude of 1263 m.The
river network consists of two main creeks, Cheița and Gropșoare (Tâmpa), along with other small
tributaries (figure 2).
Table 1. Physical features of the river sub-basins compounded in the Upper Teleajen
Representative River Basin (Cheia)
Source: NIHWM database
River
Cheia
Cucu
Gropșoare
Zăganu
Baicu
Tâmpa
Ciobu
Teleajen
River
stations
Cheia
Cheia
Cheia
Cheia
Cheia
Cheia
Cheia
Cheia
Area
(km2)
21
1.2
8.82
2.87
1.18
13.9
1.78
41.3
Mean alt.
(m.a.s.l.)
1320
1096
1290
1073
1004
1213
1085
1263
DATA AND METHODOLOGY
In order to identify the vulnerable areas subject to processes generated by slope run-off, we
have used the Flash Flood Potential Index (FFPI) method.
The FFPI method was proposed by Smith (2003) and used in Romania by several scientific
researchers such as Mătreață and Mătreață (2010), Teodor and Mătreață (2011), Minea (2011),
Borcan and Achim (2011), Prăvălie and Costache (2013) and Zaharia et al., (2013).
FFPI method is based on cartographic algebra operations that result in the final thematic
raster layer representing the arithmetic mean of the four thematic raster layers, which, in their turn,
represent: maximum hourly rainfall amounts with an occurrence probability of 1%, the basin
slopes, soils texture and land use. These four thematic layers have a spatial resolution of 30 m and
represent the main physical-geographical factors that influence the catchment’s response.
We have obtained the values of the FFPI after integrating the spatial data in GIS software
(e.g. ArcGis) and completing several steps, as described below:
- the collection of the necesary data (vector and raster) and their geographical referencing
were geo-referenced in the Stereographic Projection 1970;
- slope: this layer was created using the Digital Elevation Model with a 30 m cell resolution
interpolated on the basis of topographic maps (Constantin, 2011);
- vegetation Cover/Forest density: Using the Corine Land Cover 2006 data set which
provides information regarding vegetation cover, we have determined the percentage of the entire
Upper Teleajen river basin area which is covered by forests and by other classes of land use, thus
obtaining a new thematic layer;
- soil type/Texture:The polygon type thematic layer representing soil texture was converted
into a raster type thematic layer with a 30 m cell resolution, as well. For that purpose we used the
pedological maps 1: 200 000;
- the maximum rainfall with a 1% occurance probability:the thematic layers representing
the maximum hourly rainfall with a 1% exceeding probability were added by the kriging
interpolation method;
28
Mihaela BORCAN, Mihai RETEGAN
- ArcMap software has the capability to re-classify raster datasets, which simply means
assigning a new range of values to the field contained in the raster;
- according to Smith`s classification (2003) and based on the existing literature (Chendeș,
2007) in order to determine the vulnerability to flash-flood occurrence we have associated to each
value of the above mentioned factors an integer-type value starting from a minimum value of 1
(representing the lowest risk) and increasing to a maximum value of 5 (representing the highest
risk). This has been achieved by a re-classification of the four thematic layers. Class 1 signified a
low participation in the flow formation, while class 5 corresponds to a high participation;
- by weight - averaging of all the factors we have already mentioned we obtained a new
final thematic layer which helped us determine the flash-flood occurrence possibility thorough the
Upper Teleajen river basin.
The integration of these parameters was accomplished in a GIS environment, through
multiple operations that include digitization, interpolation, cropping, conversion, classification, reclassification and cartographic algebra.
RESULTS AND DISCUSSION
By using the described methodology we estimated the FFPI values at the scale of the Upper
Teleajen river basin and consequently we were able to come up with a regionalization map.
Slopes are an important component in the triggering of flash floods, due to their potential
for determining a rapid run-off. Experience suggests that any slope exceeding 30% leads to
extremely quick run-off and a rapide response in local creeks and streams.
Figure 3. Slopes classification at 30 m resolution
Assessment of the Flood Occurrence Potential …
29
Regarding slopes, Cheia River Basin is characterized by extremes. In the northern part, slopes
are steep (25-30°). Very steep slopes can be encountered in the Zăganu Mountains. By contrast, the
mountains in the southern part are characterized by less steep slopes, with a tilt of 15-20°.
The least steep slopes (below 5°) are encountered in Cheia Basin as well as on the erosion
platforms located at 1500 – 1600 m (Chirușca Platform and Grohotișului Ridge).
Forest cover with land use are combined into one composite dataset. The values of this
dataset are binned into 5 classes, each class representing a 20% degree of forest cover.
In this river basin forests represent approximately 67% of the area and are located at
altitudes between 850 and 1400 m. Pastures, usually found at elevations higher than 1450 m, cover
around 33% of the territory and are mainly found in the Grohotiș Mountains. In the analysed river
basin, according to the degree of forest cover, the reduced-risk class for flash-floods occurrence
possibility has not been identified.Using GIS methods, we have obtained a classification of the
slopes and degree of forest cover correspondent to the associated vulnerability indexes at a spatial
resolution of 30 m (figures 3 and 4).
Figure 4. Classification of the degree of forest cover at 30 m spatial resolution
Mihaela BORCAN, Mihai RETEGAN
30
The correspondence between these 5 classes of values used for quantifying slopes, degree
of forest cover and the associated potential index is shown in table 2.
The classifying of slopes distribution and forest cover has been determined according to
Miță P. (1997) classification.
Table 2. The risk categories for slopes, degree of forest cover and Flash-Flood Potential Index
Slopes
(°)
Degree of forest
cover (%)
5 – 10
10 – 20
20 – 30
30 – 40
40 – 50
80 – 100
60 – 80
40 – 60
20 – 40
5 – 20
Flash Flood
Potential
Index
1
2
3
4
5
Sands and clay are very important components of soils when assessing run-off potential.
Since texture, considered as a hydro-physical feature of soils, is a major factor with
significant influence on the maximum flow, our analysis has taken into consideration the synthetic
table "Adapting the hydrological groups of soils to the Romanian soil texture classification"
(Chendeș, 2007, quoted by Mătreaţă, 2011) and has been achieved by adding a new group, group
E, composed of clayish textured soils and impervious areas (table 3).
Table 3. The adaptation of the hydrological groups of soils to the Romanian classification of texture
(Chendeș, 2007, quoted by Mătreață, 2011)
Group
A
B
C
D
E
Texture
Sandy; Sandy – sandy loam; Sandy – loamy sandy;
Sandy loam; Sandy loam – loam sandy; Loam sandy
Sandy – loamy; Sandy loamy – loamy; Loamy sandy –
loamy; Loamy; Varied texture.
Sandy loam – loam clayish; Sandy loam – clayish
loam; Loam clayish – clayish; Loam – loam clayish
Loamy – clayish; Loam clayish; Clayish loam – clayish
Clayish; Areas without infiltration
Flash Flood
Potential Transmission index
1
2
3
4
5
According to this table, soil types have been classified in 5 different classes, depending on
their texture (figure 5), and each class has been assigned an indicative, ranging from 1 (minimum
risk) to 5 (maximum risk). The maximum amount of rain fallen during a 24-hour period is a very
important characteristic of the pluvial regime, which can intensify the slope run-off and thus cause
flash-floods. As a result of the calculations which have been carried out in order to obtain the
maximum rainfall with a 1% occurrence probability, GIS helped us to classify the 24-hour rainfall
quantities into several categories (figure 6).
By calculating the weighted-average of the four grids, we have obtained the final layer grid
at a 30 m spatial resolution which shows the values of the index that help us estimate, taking into
account the influence of the main physical-geographic factors, the flash-flood occurrence
vulnerability potential (figure 7).
Please note that the results are obtained in the hypothesis of a quantity of rainfall with an
exceeding probability of 1%.
In these conditions in the Upper Teleajen River Basin (Cheia) the largest share is held by
average (34.7%) and high risk territories (30.3%), which cover extensive areas, being moderately
and highly vulnerable to flash-flood occurrence, while the minimum (7%), low (20.5%) and
maximum (7.5%) risk areas cover small, isolated territories (figure 7).
Assessment of the Flood Occurrence Potential …
Figure 5. Soil texture classification at a 30 m spatial resolution
Figure 6. The classification of the hourly maximum rainfall with a 30 m spatial resolution
31
32
Mihaela BORCAN, Mihai RETEGAN
Figure 7. Map of the Flash-Flood Potential Index spatial repartition
Assessment of the Flood Occurrence Potential in the Upper Teleajen River
We consider the results of FFPI method as being appropriate, as long as they were validated
by field observation. However, we must be awere of some limitations and possible errors which
derive from the density of forest vegetations; the resolution at which the various investigated
parameters are spatialized etc.
CONCLUSIONS
The estimation of the territories with accelerated surface run-off allows the identification of
areas susceptible to flash floods. A method that can be used for this purpose relays on the
determination of the FFPI as a syntetic index that integrates the main controls of surface flow.
The present study applied the FFPI method on the Representativ Upper Teleajen river basin
which shows a medium degree of vulnerability to the risk associated with slope run-off and floods.
The lack of forest cover, as well as the very steep slopes and the clayish soils may ease the
occurrence of flash-floods, especially if rainfall has a torrential character.
This method helped us to identify the areas which are not monitored from a hydrometrically
point of view but which can be affected by severe hydrological phenomena.
This method also helps specialists and authorities to adopt certain measures that would
mitigate the negative effects of flash-floods, by informing the exposed population regarding the
potential risk the area in which they live has.
Assessment of the Flood Occurrence Potential …
33
Mitigating the effects of flash-floods can be achieved by a continuous and viable
monitoring of the variations of the liquid flow, especially on small water courses, which are the
most susceptible to the occurrence of severe flash-floods.
Acknowledgements
The authors wish to thank to the National Institute of Hydrology and Water Management
for the kidness to put at our disposal the hydrological data.
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Submitted:
September 16, 2015
Revised:
December 12, 2015
Accepted and published online
January 27, 2016