The effect of cross construction (bridge) on river morphology (case study, Jurband village, Chamestan, Mazandaran)

Humans alter river systems directly and indirectly. Construction of bridge and culvert is one of the anthropogenic interventions which take place. Road-stream crossing (RSC) is essential to be installed on the river lines for roadway transportation, while incorrect and structurally poor RSCs often negatively influence the geomorphology and ecology of river systems. Culvert and bridges can increase stream flow velocity, shear stress, turbulence of flow, degradation and aggradation, development of deep scours, channel braiding and downstream bank erosion. Jones et al., 2000; Wemple et al., 2001 & Blanton and Marcus, 2009, previously worked on the impact of roads on fluvial geomorphology with changing floodplain’s form and function. The difficult issue in survey study is gathering, up-to-date and easy to use data with high accuracy, therefore short-rang photogrammetry was used in this research.

In this investigation, the effect of bridge on river system was analyzed in Jurband village in Mazandaran province. Documentary references, remote sensing (UAV) and survey are 3 methods used. Longitudinal and cross-section profiles were drawn on digital elevation model. The model was generated from UAV with high resolution (pixel size: 4.6 cm). Longitudinal profile was obtained from survey along 400m from upstream to downstream and 8 cross-section sites were selected for detailed study in upstream (U) and downstream (D) of river channel (on bridge section, 25m, 50m, 100m, 200m).

The channel slope is smooth rend. After bridge cross section, the channel bed has collapsed at a rate of 8m gradually. In investigated reaches, all of cross sections had no change in width index except in 25U section. According to coefficient of variation index, parameters were 18.47% upstream and 2.8% in downstream. Maximum depth was in bridge cross section (1.40m) and in 200D (1.18m). The minimum amount of depth maximum and mean was in 25D (respectively 0.74m and 0.37m). There was a significant difference in W/d ratio in upstream to downstream, about 22m decrease in downstream. Cross-section area varies from 25D (65.7m) to bridge section (79.26m). The significant difference of bank-full area was 42% in all selected reaches. According to information, amount of flow velocity altered from 38.1 m/s in bridge cross-section to 18.4 m/s in 25D cross-section. Also, the maximum and minimum rate of Froude number and power stream were obtained at 25D and bridge cross-section respectively. Sinuosity index (SI) of cross-sections were equal with 1 except in 200U (1.30) , 50D (1.20) and 200D (1.10).

  Based on The principle of flow continuity, it can be stated that the amount of discharge was the same in reaches and variation in channel parameters is due to bridge construction. Because of scour in upstream over the time, altitude differences has changed in river bed and bank side between 3-4 m. After bridge construction, degradation in upstream has stopped due to local bed level foundation but sedimentation and widening of channel has increased about 2-3 meter lower than bank side. Nevertheless, degradation and scouring has increased in downstream and bed collapsing has occurred up to 8m after bridge section.