geographia - Studia

AHMAD KHORSANDI, SEYED HOSSEIN GHOREISHI, MANIJE ABDALI, CACOVEAN HORIA
the Plain. These are collapse sinkholes, while in other areas they are of subsidence sinkhole
and Ponor type.
In the region of Lar Dam, 9 old and young sinkholes were formed in the alluvium
and the lake deposits (fig.4).
Areas influenced by the Hamedan sinkholes and subsidences extend to a minimum
of 3 m and a maximum of 2000 m (table 1). In the region of Lar Dam,9 old and young
sinkholes were formed in alluvium and the lake deposit. These sinkholes occurred from
1980 to 1989 or a period of 9 years after priming of the Dam. The specifications of sinkholes in
Lar Dam region are shown in table 2. The crush zone of the faults and the high speed of
carbonated bedrock decomposition in the Plain (M. Haydari şi colab., 2003).
There is a number of underlying reasons for sinkholes in the Hamadan plain such as:
- the tectonic condition of the plain, location of the sinkholes along the fault in the
cabonateed rock (G. Saadati, 2003);
- the crushed zone of the fault, and the high speed of carbonated bedrock
decomposition in the plain (Haydari, 2003);
- the geological specifications of the Plain’s bedrock, the sand discharge from
wells, the gas emission from wells, rapid drop in groundwater tables (M. Amiri, 2003);
-the rapid drop in groundwater and the intersection of the wells’ drop cones;
- the circulation of pneumatolytic solutions in the karstic system of the Hamedan
Plain’s bedrock (M. Amiri, 2005);
- the collapse of the wells’ beds and the increase in their depths until the karstic
bedrock, as well as the infiltration of CO 2 to groundwater.
The factors influencing sinkholes in the Lar Dam region are:
- as a geologist Knill has associated the sinkholes to the faults passing beneath the
dam (Wolofsky, 1973);
-Lar Consultants have associated sinkholes with karstic effects, stating the
infiltration of bedrock by water as reason for their formation.
- the existence of karstic bedrock under the lake’s deposits and the existence of a
fault in the bedrock have been stated as reasons for sinkholes (A. Khorsandi, 2003);
- the influence of faults in forming and location of sinkholes, and determination of
fault activity by sinkholes have been proposed in an article (A. Khorsandi, 2007).
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4. RISK ASSESSMENT
The occurrence of sinkholes in the Hamedan Plain to date has caused problems in the
agricultural lands and irrigation systems and has resulted in the turbidity of well water, gradual
land subsidence, sudden collapses accompanied by terrifying sounds and tremor and finally the
appearance of tension joints on the ground surface. Risks associated with sinkholes include the
collapse of connection roads, specially the main Hamedan –Tehran road, the reduction of
agricultural surface area, landslides in rural and residential areas, the collapse and landslide in
the area of Hamedan power station and the risks of death and injury of residents. In the region of
Lar Dam the sinkholes have occurred in the sedimentary soil of the lake accompanied by
landslide and a terrifying sound. The location of the sinkholes in the sedimentary soils to the
north of the lake depicts water escaping from the main factors i.e. karstic bedrock and faults.
The risks in the region of the dam are associated with active faults passing beneath the Dam,
which influence the occurrence and location of the sinkholes. Their movement during an
earthquake will result in damages to the dam’s structure.