Environmental impacts of heavy flash floods, Golestan province flood (10, 11 Aug 2001) case study

摘要

After a heavy rainfall in 10 and 11 Aug. 2001 in upper part of Gorgan river watershed, this watershed suffered very big flood and more than 650000(10~6 Rial) damage. Gorgon River has more than 490000 ha watershed area if we consider Golestan dam as an outlet of watershed. Golestan dam is in the middle part of Gorgan river watershed. Gorgan River is one of the three important big rivers in Golestan province that end to the Caspian Sea. Its direction is from east to west and there are many kind of land use such as conservation forest, agriculture lands, ranges, urban area, in the different part of it. The heavy rainfall that caused very big and exceptional flood measured in different stations in Gorgan river watershed. The most rainfall was 450 mm in Park station and 150, 150, 118, 94, and 84 in Dasht, Tangrah, Ramian, Nodeh, and Cheshmehkhan station in 12 hours respectively. The flood peak in different subwatershed was different, in Dogh station 2000 m~3/sec, in Nodeh station 300m~3/sec in Oughan station 200m~3/sec and in Golestan dam 3017 m~3/sec. Gorgan River was 10-15 m but after this heavy flood increased to 300-400 m. Also in the upper part of watershed that is covered by forest land more than 8,000,000 m~3 forest wood destroyed and transported to the lower part and Golestan reservoir. This wood closed the opening of bridges and caused bridges destruction in many parts. In this flood more than 1,500 ha forest in two sides of main channel of river destroyed. Unfortunately more than 250 people dead in this event. Golestan dam in this flood had a very important role in reducing peak flood in the lower part of itself. Maximum capacity of Golestan dam is 90,000,000 m~3 that there was 10,000,000 m~3 of water in it before this flood, and the remaining of capacity filled rapidly in less than 10 hour. Also after filling of the dam the overflow over the spillway was 240 m~3/sec and maximum outflow from lowest gate was 86 m~3/sec. This heavy rainfall had many other environmental impacts in different part of the watershed that is very important and good case for different environmental aspects.