EFFECT OF ROAD BRIDGE ON WATER SURFACE PROFILE

Abstract

Bridges are constructed to ensure and facilitate the communication over the flow of waterways conveniently. However, these structures have detrimental effects on the hydrology and morphology of the adjacent area of the rivers/streams as the waterways get constricted. In this study, the effect of constriction of waterways due to bridge construction is reviewed through various literature available. A significant portion of waterways is occupied by bridge construction by bridge pier in case of small rivers compared to large rivers. The construction or renovation of bridges may require placement of bridge piers in the channel or floodplain of natural water ways. These piers will obstruct the flow i.e. the flow strikes piers and causes an increase in water levels upstream of the bridge for subcritical flows and hence considerable scours in bridge piers and its downstream side. The increase in water level is called as backwater or afflux. The extent of backwater caused by piers depends mainly on their geometric size, shape, their position in the stream or their spacing, the flow rate and the amount of channel blockage. Here the subject of study is to know how bridge influence channel flow and its effect in the water level variation at upstream and downstream of the bridge. The effect of different parameters of Road Bridge on water surface profiles has been studied using popular HEC-RAS model. The variables under study are Manning’s coefficient for different soils of the waterway, bridge pier shape and size, their spacing, contraction and expansion widths towards the flow direction. For this, the HEC-RAS model has been setup for water surface profiles for showing variation of afflux with variation of bridge parameters both upstream and downstream sides of the bridge under steady flow condition. There has been found a significant change in water surface elevation with and without bridge condition in the river. The data of Alaknanda river reach starting from Devaprayag (a confluence of river Bhagirathi and Alaknanda) up to Lachmoli, which is about 10 km from Devaprayag are used in the study. First the water surface elevation has been determined at the proposed bridge locations at various River Stations (RS) with HEC-RAS programming at various cross sections of river. Then the comparison has been made between the Water Surface Elevations for the same river iii | P a g e cross sections and at the same River Stations after placing the Bridge for the various values of steady flows (Discharge), Manning’s coefficient n, Sizes of bridge piers, Spacing of piers, Piers geometry (shape), Contraction and expansion width of river etc. Then the corresponding water surface elevations are noted which are later used for plotting the water surface profiles. When there is no bridge at the proposed RS and for steady flow, for the constant discharge when Manning’s coefficient n increases Water Surface Elevation increases, Velocity decreases, Flow area at Bridge site increases, and thus Froude no. (Fr) decreases. But after placing the bridge at the same location (RS), the bridge piers obstruct the flow at the upstream and downstream sides of the bridge and hence there is significant changes in water surface elevations at both sides of the bridge. For the same Discharge, when Manning’s coefficient n increases Water Surface Elevation increases, Velocity decreases, Flow area at Bridge site increases, and thus Froude no. (Fr) decreases. Also, providing the same Discharge, when bridge pier spacing increases, Water Surface Elevation decreases, Velocity increases, Flow area at Bridge site decreases, and thus Froude no. (Fr) increases. For different pier sizes i.e. 2.0, 2.5, 3.0 m piers @ 10 m spacing and for the same Q, when the bridge pier size increases, Water Surface Elevation increases, Velocity decreases, Flow area at Bridge site increases, and thus Froude no. (Fr) decreases. Similarly, for same pier size (i.e. taking 2 m pier) at different spacing@ 10, 15, 20 m spacing and for the same steady flow Q, when the bridge pier spacing increases, Water Surface Elevation decreases, Velocity increases, Flow area at bridge site decreases, and thus Froude no. (Fr) increases. Now, according to the Momentum method considering the shape of piers (or pier geometry), when the coefficient of discharge Cd (Cd = 0.60 for Elliptical piers with 2:1 length to width, 1.0 for Triangular nose with 30 degree angle, 1.2 for circular, 1.33 for Elongated with semicircular ends, 1.39 for Triangular nose with 60 degree angle and 2.0 for square nose) increases, Water Surface Elevation at u/s side of bridge increases whereas it is constant at d/s side, Velocity at u/s side decreases and there is no change at d/s side, similarly flow area at u/s bridge location increases but no change at d/s side, and thus Froude no. (Fr) at u/s side decreases and is constant at d/s side. iv | P a g e To determine the effect of Yarnell coefficient k (k = 0.90 for semi-circular nose and tail, 0.90 for 90 degree nose and tail and 1.25 for square nose and tail), for the same steady flow Q, it has no effect on any of the above parameters. Providing the constant k i.e. for a particular pier when the flow Q increases, there is no change in Water Surface Elevation, Velocity, Flow area and Froude no. (Fr). Also for the same coefficient of discharge Cd and n at the same time, when the Discharge Q increases, Water Surface Elevation increases, Velocity also increases, Flow area at Bridge site increases but the Froude no. (Fr) at u/s side increases while it is decreased at d/s side. And for the constant n, when flow Q increases, Water Surface Elevation increases, Velocity also increases, Flow area at Bridge site increases but the Froude no. (Fr) at u/s side increases while it decreased at d/s side of the bridge. Also for the particular soil having constant n and same pier size, if Spacing of piers increases, Water Surface Elevation decreases, u/s velocity increases but d/s velocity decreases, Flow area at u/s decreases and increases at d/s side, thus Froude no. (Fr) at u/s side increases while it decreased at d/s side of bridge. But for the multiple bridges, the effect of backwater is of local nature and does not extend largely towards the u/s and d/s sides of bridge (here the spacing between the each successive bridges is 1500 m) but it should be considered for the flood, river training and other hydraulic effects of the river as well as bridge.