Evaluating the Structural-Hydromorphological Status of Tajan River Based on German River Structural Quality Mapping Method (LAWA-OS)

All around the world, surface waters are dramatically vulnerable to the Impacts of various human activities, as sustainable river systems and animal and plant habitats are widely endangered, and the land use and the quality of water has been changed. Structural-hydromorphological characteristics are of great importance in the ecological status of rivers, so that the study of their status as one of the basic components in the sustainable management of the river basin and the revitalization of river conditions Is going. In Europe, simultaneous with the introduction of the Water Framework Directive, the status of surface water has received much attention and the urgent need for an accurate monitoring system, as a first step towards achieving a minimum good ecological status, was introduced. The Literature review shows that, the focus of studies in Iran are mostly on the morphological status of rivers, but so far, the assessment of the hydromorphological status of rivers has not been studied. Hence, the present paper studies the structural-hydromorphological status of 12 km of Tajan River using the German river structural quality mapping method.

 The main focus of the German river structural quality mapping method is to identify the morphological pressures caused by human interventions, in order to determine appropriate measures for re-establishing natural and semi natural condition within the assessment and monitoring system. This method classifies the current condition of the structural-hydromorphological of the river based on a scoring system in four phase including, data collection, definition of river sections and remote sensing, field survey and status assessment based on scoring of 25 single parameters. The study area is divided into 100-meters reaches and each reach was evaluated in two phases of remote sensing using the catchment topographic, soil, vegetation, and land use maps, digital elevation model of the region, the Landsat 8 satellite image of 2019, and the field survey from April to June 2019.

 The results showed that 44% of reaches evaluated as structural quality class 4-Distinctly changed. While no Unchanged (completely natural) reaches were observed, but 4% strongly changed. In addition, the distribution percentage of each structural quality class of the single and main parameters were investigated, and the results showed that the riverbank protection of the study area has not a suitable condition in comparison to the other parameters. In order to have a more detailed investigation, Structural quality classes percentage of three subarea including, before city of Sari (BS), city of Sari (SS) and after city of Sari (AS) was examined. The results showed that the situation of Tajan River along Urban area, due to interventions such as change in bed structure, more than two km concrete bank protection and construction transverse structures like robber dam, Ramp structure, without fishway consideration, especially at Mellal park area, led to completely change of river bed and channel width.

In the German river structural quality mapping method, geographical condition has a strong relation to the landscape units and defined river types, the form of the river valley, bed structure and vegetation. Which is to a large extent compatible with the geographical condition of northern Iran, especially with Tajan River that has three mountain, plain and coastal landscape units. Besides, Structural quality classes percentage of two single parameters, 2-5 bank protection with 49% and  3-5 special bank features with 67%, are in completely changed situation (class 7), and plays the role of status decreasing parameters for Tajan river at the studied area, which is identified as the priority in restoration measurements of Tajan River.