The Floods in Arid Areas Case of Catchment of the Valley Béchar, South-West, Algeria

The present work is reserved to a presentation of the impact of climatic factors on the floods which, among the more serious, known to the region are the floods of October 2008 caused by intense precipitation with a height of 108, 40 mm. In addition, we are interested in the physical parameters such as the sections and the slope of the valley Bécharwhich is at the origin of the disaster. The profile along the valley shows a slope steeper, which produces an acceleration of flows, The calculation of the sections of the valley a various points also reveals the variations at the level of the slope, which sub-tends to bad behavior of the water flow at the level of the section reduced. Though it is as well, these investigations have guided and alert us once more on the need for a development integrated space taking account of regional specificities.


INTRODUCTION
Floods are a major risk in the contemporary world.They are contained in the first rank of natural disasters in the world, causing approximately 20.000 victims per year (Simona & Cedric, 2007).Algeria submitted to the floods that occur catastrophically thus constituting a major constraint for economic and social development, including the more deadly those of the last decade, such: as Ghardaïa (2008), Adrar (October 2004 andJanuary 2009), Béchar (2008Béchar ( /2014)),) Sidi Bel Abbots (April 2007), Bab El Oued.The catastrophic floods that have confronted the city of Béchar (south-west) are always related to the flooding of the Valley of Bechar (October 2008) following the rainfall during the days.A tire indicative, in October 2008 the precipitation reached more than 108mm, this is unusual for the region of Bechar and has been a determining factor in the overflow of the Valley of Bechar, the present work is reserved for the highlighting of the impact of the factors which have caused the process of the phenomenon of the Flood: -Climaticfactors and their variations.

Geographical characteristics and morphometrics of the region
Located in the southwest of Algeria, this basin is part of the Great Basin of the Saoura.It is understood between the Latitude 31°47'30.55"north and longitude 1°49'4.04"West, on a surface of approximately 6122 km2 and a perimeter of 698 km, oriented to the north (upstream) where it takes its source toward 1959 m of altitude in the Djebel Grouz South

RESULTS AND DISCUSSION
The profile in the length of the valley of bechar From the Profile in the length of the Valley of Bechar , we can show a change in the slope, The latter contains several parts, a first where it is slightly important, there is a decline of approximately 20 m, which is spread over a length of 08 km, the second part characterized by an alternation of the slope between stabilities, Increase and Low, that promote the stagnations of waters, in this part we note a decline of 15 m along 8 km, by against the third part is characterized by a decline of 35 m on a length of approximately 08 km.
On the other hand from the profile in long, it can be noted the changes that affect the latter, where it shows several states.In its upstream part the profile indicates a significant slope, it allows a flow of water toward the central area of the city.By against is in this last part where the profile takes a form of right almost straight, thus favoring the floods, In the last state the profile shows a slight slope straight sometimes also to foster floods.
The sections of the Valley of Bechar change from one point to another, which generates a behavior specific to this course of water.This calculated the sections and their spatial variations are indispensable.

The climatic
Monthly distribution of precipitation The variation of the pluviome tries monthly averages highlights that the rainiest months during the hydrologic year are September, October (flood 2008), November (flood 2014), March and May, the months of June, July and August are virtually dry.

The annual precipitation and its variability
The variations of precipitation interannual seem to a fluctuation and alternation in rainfall around the interannual average, the interannual average is (141,33 mm),the years (1993.1994, 1996, 2002, 2006, 2007, 2008, 2009), which are the most rainy with a maximum of 324,01 mm (2008), it is observed that the years (1995,1997,1998,2000,2001,2003,2004,2005,2010) are the driest years and year infertile in the series is 1999 (10.84 mm).Coefficient monthly rainfall It is a concept very expressive.The coefficients greater than 1 correspond to the months receiving more than their share of rain; those less than 1 to the contrary case; we distinguish at first sight the months "wet" (or "wet") and months "dry" (by agreeing to call wet, or rainy weather, and dry the months which have received more or less than their share) (Musset,1935).The previous stady shows the month of September.In October.In November.March and May, are considered rainy.The month of October is characterized by a coefficient maximum rainfall (1,86), which is greater than the threshold value 1, which allows us to consider as the wettest month, the stady below shows that precipitation reaches the values in the arid zones in October.

Numbers of rainy days
In our study, the histogram of variation of the number of days of rain shows that the maximum value of these days of rain is recorded in the fall season precisely in September and October, which is the period of flooding (year Flood 2008), these are the rainiest months, the months of the summer season are driest dry in its monthly averages.

Variation of seasonal pluviométries
To note also as regards the seasons of Winter and spring, which are considered as wet seasons due to the quantity of rain recorded in them, which vary with the other alternately everything depends on the quantity registered in each season.
Summer is the most sec among the seasons of the year because of the low quantity of rain recorded in it.

Study of the temperature
The temperature of the area acts directly with the other meteorological factors (precipitation, evapotranspiration, insolation, runoff, infiltration, etc.), it is a factor important climate which affects the balance sheet and the climate of a region.
The interannual average temperatures of the coldest year are recorded in 1993 (2 1.27°C.),whereas the warmest year is 2010 (23.50°C.)For our study we used the data in minimum temperatures, maximum, and medium of the station of Be char.

Average Variation Interannual temperature
The curve of interannual variation of temperatures represents an increasing variability which reached a maximum (23.50°C) in the year 2010 and a minimum registered in the year 1993 (2 1.27°C),This increase of the successive temperatures resulted in climate change.

Variation of monthly temperatures
Our region is characterized by a hot summer where high maximum temperatures can exceed 41° C in July, and a relatively cold winter with temperatures of the coldest month recorded in January (09,72°C.),an average temperature interannual 22.36°C.

Studies of evaporation
Evaporation is the refund of water to the atmosphere in the form of steam from the surface of the ground, regardless of its nature (soil, plant, or water-free).It is ,therefore, a very important element for the establishment of the hydrological balance, Evaporation is influenced by solar radiation, temperature of the air and the water, relative humidity of the air, wind speed and the quality of water.The evaporation affects all the masses of water.Flow of water molecules that have sufficient energy to vaporize and join the atmosphere since a water surface.These molecules saturate the air immediately above.The difference air saturated -and dry air from above affects the flow of water vapor (mm/day mm/month or mm/year).Since 1950, several formulas have been developed to estimate evapotranspiration: Thornthwaite (1944), Turc (1962), Blanney-Criddle (1950) and Penman-Monteith-FAO (1998).The use of these models requires the knowledge of certain climate data such as, maximum and minimum temperatures of the air, the relative humidities maximum and minimum, the global solar radiation and wind speed (Habaieb & Masmoudi-Charfi, 2003).

The potential evapotranspiration ETP
Method of Thornthwaite (1944) The formula from Thornthwaite is used when only the temperature is the only climate data.It expresses the potential evapotranspiration (ETP) by the following formula: (Agoussine et al., 2004) E = 16 (10   )
Formula of Turc (1962) (Bouchet, 1963) This method involves the monthly mean temperature, the overall radiation of the month considered, and the insolation relative.In this case, the ETP is expressed in mm/month according to the following formulas: If RH(The relative humidity) >50 %,It was: ETP = 0,4(I g + 50) T T + 15 (5) If RH (The relative humidity)<50 %, (Generally the case under our climates) It was: With ETP: Potential evapotranspiration monthly (in mm of water); T: mean monthly temperature of the air (in °C); Ig: overall radiation monthly average received on the ground (calorie/cm2/day); K: coefficient equal to 1 if the relative humidity RH is higher than 50%.The more air is hot the more it can contain water vapor (or humidity) (generally the case under our climates); Iga: radiation Theoretical overall for the month considers, that depends on the latitude, obtained has partiras table (source:N.O.M. Bechar) (in cal/cm2/day).H: The lasts an average of insolation in time.
H: Astronomical lasts the day in time, depends on the latitude and is obtained from the tables the coefficient 0.4 is valid for me s of 30 to 31 days for February will replace 0.4 by 0.37.
The values of ETP found by this formula should be corrected by a factor of corrections K ETP corrected = ETP.k The actual evapotranspiration ETR The actual evapotranspiration is the amount of water evaporated or transpired by the soil, plants and the free surfaces of a watershed, it is expressed in millimeters.

Formula of Turc
This method applies to all climates and it is a function of precipitation and average monthly temperatures.The formula is the following: With ETR: evapotranspiration Annual Actual in mm.P: Precipitation annual average, in mm; T: Annual average temperature.L=300+25T+0,05T3.

The indices general climate
Index of aridity of martonne (Iar) In 1926 Emmanuel de Martonne proposed a formula allowing climatological calculation of an index said aridity index (Iar), the calculates This index requires Knowing the height of the average precipitation interannual and the average Temperature Interannual Prediction (Martonne,1926).
With: P: Average precipitation interannual in mm.T: The average Temperature Interannual in °C.
Table 3. Parameters of the calculation of an index of aridity (Iar).

Station of Bechar P (mm) T (°C) Iar
141,33 23,50 4,21 For the station in Bechar, the index is equal to: have =4,21, and less than five (i<5), which allows you to say that the climate of the area of study is hyperarid.
The diagram of the emberger (Climagramme of emberger) Climagramme of Emberger is the classification the most often used to specify the climate of the region, where Emberger has sought a synthetic expression of the Mediterranean climate able to account for the drought, on which we have postponed the data relating to the station of Bechar in the abscissa the average temperature of the minima of the coldest month (in °C), and ordered Q2 given by the following formula: The maximum value of the days of rain is registered to the fall season precisely in October • The climate of our sector is characterized by a strong thermal amplitude and the degree temperature is to exeresis between an average of 41.23°C and 9.72°C corresponding to July and January.

•
A Temperature Interannual average in the order of 22.36 C°.

•
A potential evapotranspiration estimated by Turc and Thorntwaite respectively of the order of 828,29 mm 1114,78 mm, where the value given by Thorntwaite is very high compared to that of Turc.

•
The value of the ETR of Turc of the Order is 109.63mm.

•
The vas deferens climate indices general who classify the study area can be summarized as follows: • The climate index of Marton is classified the climate to hyperarid.

•
Climagramme of Emberger shows that our area of study to a Saharan climate with winter tempers.
In this study we have put in evidence that among the factors that direct the phenomenon of flooding the climate factor takes the large hand, The area of study is an arid zone characterized by large precipitation recorded in the fall season where the city of Be char subject to flooding which occurs catastrophically thus constituting a major constraint for the economic and social development, these floods which have become the challenges of environment the most serious problems are facing urban communities and imports their size, once the risk situation is described; it is possible to make a forecast of a wide range of facilities which may a priori to participate in the reduction of the consequences of the Flows causing floods, Among the facilities can be provided for in the primary networks and the major bed of water courses.

Reduction of the hazard
The purpose of the reduction of the hazard is to reduce waters in the areas with strong human issues and economic to the scale of the watershed: • recalibration of the course of the water, containment of the watercourse, reforestation, recalibration of works of art, and recalibration of the systems of evacuation of the water.• preservation of areas of expansion of floods, dams or withholding of water.• torrential correction.

Reduction of vulnerability •
The actions of reduction of the vulnerability can be envisaged to many scales of time: • Planning of the occupation of the soils (long-term).

CONFLICT OF INTEREST: None
FINANCIAL SUPPORT: None (downstream)  to 501 or he is going to lose in the Dayet Tiour, it is located at the foot of the southern slope of the Saharan Atlas, limited to the north by the mountainous chain Djebel Grouz, South is by the Great Western Erg, to the west by the Hamada of Guir and to the east by the Hamada Valley of Namous, this basin drains by the Valley of Bechar with a length of the main watercourse (Valley ofBe char) of 166 km, the Average slope of the Valley up to Dayet Tiour is approximately 6%.During the passage of the Valley toward their outfall, crossing the city of Be char in the middle along 20 km, and due to the high slope observed in this segment, adding the change of the section of the Valley who contract and reduced in this passage, produces an overflow of flow of waters in the vicinity of the neighboring districts of a bed of the Valley .

Figure 1 .
Figure 1.Geographical situation of the city of Bechar (southwestern Algeria) The flood of october 2008 The City of Be char (south-west) was known for a catastrophic flood linked to the flooding of the Valley of Bechar, in October 2008, Precipitation Falling the days the result of overflows of the Valley of Bechar and flooded the neighborhoods neighbors, esrepresented the Figure 2 under below.The following table represents the interventions of civil protection to the damage of this flood.

Figure 3 .
Figure 3. Climagramme of the Emberger According to the values in Table 4, we have postponed the data relating to the quotient of rainfall and the temperature of the minima (3.25; 12,59), The Climagramme of the Emberger (Figure 3), allows you to say that the study area has a climate Saharian with winter tempers.CONCLUSION The study area is part of the great watershed of Saoura, It is characterized by a climate Saharian arid with a cold winter and a hot summer and dry.Examination of the series of precipitation for the period (1993-2010) revealed an average rainfall of 141,33 mm for the watershed of the Valley of Bechar.The examination of the precipitation and temperatures of different positions in rainfall has given specific results for the watershed of the Valley of Becharwith: • a precipitation Annual Average (Bechar) of the order of

Table 1 .
The result of the interventions of the Civil Protection (Directorate of Civil Protection of Bechar, 2008) Type

Table 2 .
Basic Statistics of precipitation of the station of Bechar.