At present, a number of the rural settlements in the north of Tehran and tourist destinations have developed along the valleys and the main basins. In addition, the urban limit line has expanded to include the land beyond Tehran’s northern fault and on the low northern slopes. These areas are threatened by slope instability, and torrents and immersion.

2. Tehran alluvial fans: Almost the whole city is laid on alluvial fans formed from the alluvial deposits of the rivers in the southern slopes of mountain. The texture of the deposit, its penetrability, its topography and appropriate slope for urban drainage, these are the main conditions that alluvial morphology has provided for the urban development of Tehran. Geomorphologically, the alluvial structure of Tehran from the north to the south can be divined into four distinct parts as follows based on topography, tectonic influences and usages:

A) Hezardarreh landform in the north of Tehran (first stage alluviums). As stated above, Hezardarreh formations, belonging to Eocene age, are the oldest alluvial formations of Tehran region. These formations are wider in the east of Tehran, especially around Ghoochak defile, and are as high as 2000 meters. These deposits are formed from big and small alluviums and among them there are pieces as big as several meters. These alluvial fans have emerged at the time of the last tectonic activities of the Alborz mountains in the Paliocene age. For this reason, their layers are highly sloped; in some parts the slopes vary between 35 to 45 degrees. Sat present, most of these landforms have hill like morphology with high drainage density.

B) Conglomeratic hills in the northern half of Tehran (second stage alluviums). A great part of the hills scattered in the city belong to this category. These hills include: Lavizan hills, international exhibition hills, Milad Tower hill, Taleghani park, Pardisan park, Parvaz park, Chitgar parks along Karaj road.

C) Recently developed alluvial fans (third stage). The sediment from the upper watersheds and the material from the destruction of old alluvial fans and Hezardarreh formation in the lower parts of the old fans have appeared as new sediments, which forms a great part of Tehran. The city has a fairly smooth topography. A mils slope (1 to 10%) from the south to the north is the surface feature of these alluvial fans.

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D) New alluviums and the flood plain of rivers (fourth stage). The most recent sediments of new alluvial fans have been formed along the flood plain of rivers and at the end of young fans in the southern areas of Tehran. 3. Alluvial plains of the south of Tehran. The alluvial plains of the south of Tehran are located in the base of the new fans and their surface texture has been covered with sand, silt and fine grain sediments. The surface topography of this area is very smooth, mostly with a slope of less than one degree. 4. Sorkheh Hesar mountains. Sorkheh Hesar mountains are located in the east of Tehran as pre-mountains of Alborz in the south. The greatest height of these mountains is about 2000 meters. Geomorphologically, no certain order is observed in these mountains. The prominence of Sorkheh Hesar mountain with a warping down toward the west has created a special irregularity in the morphology of Tehran and limited the development of the city toward the east.

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Geomorphologic overview map of the area of Tehran and its surroundings

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Figure 1.3  Block diagram of geomorphology within Tehran limits

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