Our lithologic map was realized according to a vectorization of two geologic cards at 1/50,000 of Nabeul and Hammamet. With regard to the works of [7] - [10] , the studied land is composed mainly of sand, clay and sandstone (Figure 3).

-The Somaa formation, it is composed yellow or red sands intercalated with cong- lomeratic levels.

-The Beni khiar formation, presents a progressive passage of continental sandstones to oolitic limestone.

-The Oued El Bir formation is composed of sandstones, sand and azoic clays.

-The pottery clay is plastic with a grey color and bleu patina.

-The yellow sand of Nabeul is characterized by a varying granulometry with some conglomeratic levels.

-The sand and sandstones of Hammamet are yellow fin sand, sometimes containing clay.

-The Sicilian, its levels are composed generally by conglomerates poorly consolidated and dark grey sand.

-The Tyrrhenian outcrops a cord formed by sandstone and limestone and sandstone at the base dune large oblique stratifications at the top.

The lithologic nature and the consolidation state of each formation allowed us to predict the mechanical behavior of each formation.

This is checked out by the work of [11] that had proved the direct relation with the friction angle φ and the horizontal displacement of materials:

σh: horizontal stress.

σv: vertical stress.

Ka: thrust coefficient of land.

Applying this equation allowed us to classify the lithologic unites into 3 classes depending on their aptitude to the instability. To facilitate implementing this classification in the data base, some proportional indexes at each instability have been affected to the surface lithology (Table 1).

It includes the estimated and identified breaks at the level of our section. These breaks are generally oriented N120 to N170 (Figure 3). According to [12] , 1981 who affirm that break disposition is the most important criteria in our thematic, we have chosen to attribute:

-Index 1: to the estimated faults that affected sand and clayful lands or those which are not reactivated for a long time.

-Index 3: for the faults that represent a dip less to that designed breaks and also for the conjugated faults.

The obtained slopes map is a derived map through a digital elevation model (DEM) product following the extraction and processing of contour to 25 m not included on topographic maps at the scale 1/25,000 in the Fevilles of Hammamet and Nabeul NE, NW, SE and SW.

It shows that the study area is a peneplain except the Nabeul-Hammamet monoclinal where the slopes can exceed the value of 30% (Figure 4). The works of [13] [14] have showed that stability of slopes is maintained thanks to a balance between the propulsive forces and the resistance forces.

φ: friction angle, α: slope.

Applying this equation, which is in relation with the slope, allowed us to classify the topography into three classes depending on their contribution to instability (Table 2).

The same previous topographic maps were used as a data base during the vectorization

of the hydrographic network illustrated in the study area (Figure 5). The result has shown that the majority of the networks are in the form of trills which prove the preserve of permeable and friable land where water could dig and mobilize the particules easily [15] . In consideration with Strahler Classification to the whole network, land observation has proved that land instability is proportional is proportional to the order of the drain.

This is confirmed by the progressive increase of the solid charge in the drains from upstream to downstream. Hence the choice of indices is for different drains classes (Table 3).

The seismic map (Figure 6) of the average change in the intensity of earthquakes that affected the southern part of Cap Bon for the last thirty years which were registered by many geophones situated in Nabeul-Hammamet, Dar Chabaan, Bou Argoub and Grombalia. These data issued by National Meteorology Institute represented in the raster mode prove that the Nabeul-Hammamet region is seismically stable. In fact the maximum magnitude recorded was at the scale of 4.1.

The following Table 4 illustrates the adopted classification for the seismic factors.

The rainfall map (Figure 7) shows the special distribution of the interannual average rainfall for the last decade. The data base used in realizing this map is the monthly precipitations measured in the meteorological stations: Nabeul, Oued Souhil, Hammamet, Khangt El Hagag and Bou argoub.

Belonging to the Mediterranean climate which is characterized by its precipitations of low frequency and high intensity (Figure 8), it allowed us to attribute the instability index 2 to the whole area.

The land use map was created following the supervised classification of satellite Aster high resolution image acquired April 16, 2002, which essentially shows five classes of land cover (Figure 9). It is essentially exploited by agricultural land that covers more than 65%, the urban sites (Nabeul, Hammamet, Dar Chaabane and Beni Khiar), the forest of Jebel Nabeul-Hammamet, Jebel Ksous and waste Lands.

Every type of exploitation contributes to the land instability in a special way. This is illustrated through the following Table 5.

[16] has proved that the impact of fracturation sector on the land instability is generally localized at the level of a nearly 100 m thick corridor. This offered us a basis for creating “tampon” areas which are at the level of 50 m and an instability index to the present breaks in our data base.

In the case of hydrographic network this operation is a bit more complicated because we have attributed a specific distance for each chosen index:

- 25 meters for the index 1 drains

- 50 meters for the index 2 drains

- 75 meters for the index 3 drains

This choice was taken through land observation of the most sensitive areas to land instability around oued Essghir, El Kbir and Abid.

This operation associates the entities of an introductory these with the polygons of a recovering theme to produce a closing theme containing the attributes and the general view of the two themes.

As a result of our choice, we elaborated the synthesis map by working in vector mode by converting maps of the slopes, the soil exploitation, rainfall, seismicity and the tampon areas in vector files.

The first union operation was realized through lithologic and break themes, the result was afterwards combined with the slope theme.

Analogue operations were performed by integrating theme after theme the instability indexes. These were covered through:

- Rainfall,

- seismicity,

- drain classes,

- Soil exploitation.

After each association operation we have paid tribute to the data base to multiply the indexes of instability for the introductory theme by the recovering theme (Table 6).

The above box shows the use of new indexes produced through multiplying the above indexes in box 8 by the instability indexes linked to the slope factor (Table 7).

The final result of these special analysis operations and data combination is illustrated through the instability map of Nabeul-Hammamet (Figure 10).