bridge and bridge piers effects on hydraulics parameters of the rivers flow (study on nazloo river in west azarbaejan)

general kind of encroachment of river is constructing a bridge on it. purpose of encroachment of river is occupation part of river and flood plain for constructing bridge. often being economical of bridge innovation affirmated that most of encroachment take place to river and flood plain, which this encroachment can be in form of embankment in flood plain of river or into the main bed of river which cause decreasing of bridge length or appearant in form of bridge piers in main bed of river. in this state, long term or short term reactions of hydraulics, environmental, alluvium, geomorphological and biological aspects are settled in bound of river engineering projects that each of mentioned cases must be analyzed specializely. in this paper used hec- ras model for considering and analysis of river changes in hydraulics aspect. hec-  ras model simulate the steady flow characteristics only in main direction of flow (1 dimention). this model have been widely accepted to give satisfactory results for simulation of main characteristics of flow (i. e. depth, velocity, shear stress, …), using field data.this paper considered three different kinds of bridge. 1: natural and exist conditions in river. 2: state which omitted only bridge piers. 3: state which total of bridge (abutment and bridge piers) is omitted. obtained results showed that in lower descharges, the effect of bridge embankment and bridge piers is low and don’t make problems. for example, in flow rate 28.81 m 83/s, it is seem that insignificant changes in most of hydraulics parameters until 100 meters upstream of the bridge. but in higher and flood descharges, effects of bridge embankment and bridge piers increases, so that, appearance of different dangerous is possible until more distances of bridge. for example, in flow rate 180 m 83/s, changes of hydraulics parameters is seen until 400 meters of upstream of the bridge. highest of effects intensity is happen in upstream and downstream sections of the bridge (s4 and s5).