Zonation and Investigating the Morphological Effects of Flooding on Zarrineh-Roud River (From Sariqamish to Noruzlu Dam)

Floods are among Earth's most common and most destructive natural hazards. Floods create geomorphic hazards via changes in sediment transport and channel configuration (e.g. channel width, lateral migration, planform changes, etc). In this context, floodplain zoning and its application in spatial planning is important in non- structural measures in order to reducing flood damages. One-dimensional models are the simplest option existing for modeling the flow conditions within a river channel. HEC-RAS, a commonly used one-dimensional hydrodynamic model, has the capability to perform both steady and unsteady state simulations. HEC-RAS is a hydraulic model developed by the Hydrologic Engineering Center (HEC) of the U.S. Army Corps of Engineers. The model results are typically applied in floodplain management and flood insurance.
The objectives of the current paper are flood hazard zoning and evaluating the geomorphological effects of flooding on Zarrineh-Roud River. This river is located in the northwestern Iran. It drains a watershed of 11788 km2. Zarrineh-Roud River, the most important river in the Urmia lake basin, supplies about 48 percent of the lake’s water.
Material and The West Azerbaijan Regional Water Authority topographic maps (1:2000 scale) are base data in the present study. Also, data from Sari-Qamish hydrometric station located in the main stream and Qureh-Chay and Janaqa stations on tributaries were used for calculation of return periods and discharge – stage relation. To determine the friction coefficient distribution of channel and floodplain, land cover maps was generated using Google Earth satellite imagery.
HEC-RAS uses a number of input parameters for hydraulic analysis of the stream channel geometry and water flow. For steady, gradually varied flow, the primary procedure for computing water surface profiles between cross-sections is called the direct step method. The basic computational procedure is based on the iterative solution of the energy equation. Given the flow and water surface elevation at one cross-section, the goal of the standard step method is to compute the water surface elevation at the adjacent cross-section. The flow data for HEC-RAS consists of flow regime, discharge information, initial conditions and boundary conditions (HEC, 2010). Hydraulic modeling of floodplains requires accurate geographic and geometric data for both river channel and floodplain. Geographic Information Systems (GIS) allow collection and manipulation of geographic or geometric data. Model geometry input and model output were done using the HEC-GeoRAS extension for ArcGIS. HEC-GeoRAS is a set of procedures, tools, and utilities for processing geospatial data in ArcGIS. Therefore, the GeoRAS software assists in the preparation of geometric data for import into HEC-RAS and processing simulation results exported from HEC-RAS (Cameron and Ackerman, 2012). Finally, we used the stream power index to evaluate the geomorphological effects of floods, which is a measure of the main driving forces acting in a channel and determines a river’s capacity to transport sediment and perform geomorphic work.
The studied reach of Zarrineh-Rud River was divided into two sub-reaches according to geomorphological characteristics: reach (1) from the beginning to Mahmudabad city and reach (2) from Mahmudabad city to Noruzlu dam. In the reach (1), due to the narrow floodplain, flood prone areas is limited. In this reach, a recurrence interval of 25 year flood, approximately covers the entire floodplain. In the reach (2), coincides with increasing of floodplain width, flood prone areas becomes wider. Floods with significant increases in stream power, play an important role in the morphological changes of river channel. In general, a decreasing trend could be seen in the studied river from upstream to downstream, due mostly to gradient reduce and therefore the decrease of flow velocity and shear stress of river channel.Although, potential perform geomorphic work of stream power in the reach (1), particularly in meander bends, is too much, but for the most part, river bed consisting of pebbles and cobbles; as a result, capability to perform geomorphic work is limited. Also, because of the armoring bed, ability for bed incision is very low. The same is true in the case of bank erosion, the channel banks in this reach, either formed from coarse sediment, which is often well-cemented, or as a result of the migration of the river channel bends, are directly connected to the mountains and hills. Mountain unit in the studied reach consists mainly of various types of conglomerate and limestone which are considered as a major obstacle in the channel changes. In this reach, sedimentary point bars are very limited. Therefore, it can be said that the limited sedimentary point bars are evidence of dominant the sediment transport process and limited deposition in many parts of studied reach. In the reach (2), although the stream power is lower than the upper reach, but, what is of utmost importance, is high erodibility of bed and banks of channel in the many parts of the reach. Thus, in this reach the flooding plays a major role in bank erosion and lateral changes of channel. So that, the erosional effects related to bankfull and overbank floods can be seen in abundance in the river margins. In this reach, the erosional and depositional features frequently can be seen adjacent to each other, which can be attributed to local changes in the stream power. Significant increase of stream power in meander bends associated with high erodibility of banks, leads to severe erosion during floods and large amounts of sediment entered in river channel. Conversely, in parts where the stream power is reduced, the deposition process occurs. Abundance of point bars, either in and side of convex banks of the meander bends evidence that during floods, large volumes of sediment entered into the river channel, which are not able to move all of them. So that, in some parts, river channels show threshold behavior (transition from meandering pattern to braiding pattern).
The results show that the river floods in the studied reach no threat to settlements, because, cities and villages are located at the piedmont and high terraces. However, the floods are a serious threat to agricultural activities existing on the floodplain. For example, nearly 1713 hectares of agricultural land in the floodplain was inundated with a recurrence interval of 25 years flood. In upper reach, although the power stream is high during floods, because of low erodibility of the bank materials and bed armoring, ability for forming is low and the dominant process in the reach is sediment transport. But in the reach (2), in addition to increasing inundated areas due to erodibility of banks materials, lateral dynamic of channel is high.