Flood Zonation and Morphological Analysis of Qaraso River using HEC-RAS Hydrodynamic Model (From Pirazmeyan Village to the Confluence of Ahar-Chay River)

Nowadays floods are considered as one of the greatest threats in terms of social security and sustainable development which affect 20 to 300 million peoples each year (Hirabayashi & Kanae, 2009). In recent decades, many studies have been carried out on floods which have mainly approached to flood risk management. Flood inundation models are defined as the tools which could simulate the rivers hydraulic and also occurred floods in flood plains (Horrit, 2007). In other words, flood inundation models are useful tools for flood plain management. This ability in flood prediction, and also flood damage reduction by protecting of near river land use, improving the knowledge of indigenous people of flood plain, and preventing the construction in banned area of flood plain is very useful (Di Baldassarre, 2010). In this study the risk of flood occurring for different return period along 72 Kilometer in Qaraso River have been investigated.

This research is based on filed studies and 1:2000 scale topographic maps that provided by Ardabil regional water authority. The 1:100000 scale geological maps, satellite imagery such as IRS and Google Earth and also hydrometric data of Dostbaglo and Arbab Kandi hydrometric station, considered as essential data. The HEC-RAS model could calculate the water surface profile in stable flow gradual variable in rivers and artificial channels in the subcritical, supercritical and complex regimes. The calculation of water surface profile carried out from one cross section to other cross, step by step, solving energy equation in standard way.

In this study in order to estimate the frequency distribution of floods we used Pearson Type III distribution in logarithm base 10, which provided by USA federal organizations of rivers reconstruction. Then flood peak discharges with different return period and its probability of occurrence has been calculated for important hydrometric station namely: Dostbaglo and Arbab kandi. According to the flood-prone zonation map, we observed the maximum wide of flood-prone area in reach2 to reach4 of river (from Qadirlo Village to Lalganj Village) in different period. In these reaches the width of flood-prone area in 25 years return period is about 500 meter on average. In first and some parts of reach6 and reach7, due to narrowing of channel and some obstacle the wide of flood-prone area was decreased and instead the water depth has been increased. Regarding to the placement of the Qadirlo, Kangarlo, Agh¬ Darag and Dostbaglo villages in the margin of the Qaraso River, the flood zonation in this rivers should be considered. The main landuse of Qaraso riparian is assigned to agriculture and orchards. By overlay the flood zonation maps with satellite imagery we observed that the main portion of this agriculture lands, has been located in the river bed and riparian zone (according to 25 years return period), are at risk of flooding. This could lead to heavy damage for farmers. For example, the flood events with 25 years return period, affected about 1085 hectare of farmland and orchard that located beside Qaraso River and lead to heavy damages.

Regarding to notable encroachment by farmers in the Qaraso river bed, the Flood risk is very important. The flood-zoning of Qaraso River shows that wide part of farmlands and orchards and also part of the facility located around the Qaraso River (nearby the village of kangarlo) have been exposed to floods with 25 years return period. Therefore, according to article 10 of regulation of bed and riparian of rivers the regional water authority of Ardabil should determine the condition of this high risk area. Accordingly, the floodplain management should be considered as an important and essential issues.