"WAKESCOUR AROUND BRIDGE PIERS",

Abstract

When an alluvial stream is partially obstructed by bridge piers, the flow pattern in the channel around the bridge piers is significantly changed. The pier produces an adverse pressure gradient just upstream from the pier. The boundary layer that develops upstream of the pier undergoes a three dimensional separation resulting in the formation of horse- shoe vortex. The shear stress distribution around the pier is drastically changed due to formation of scour around the pier, which in-turn, changes the flow pattern and shear stress. The shear stress in the wake region of pier is different in rigid bed channel and in the alluvial channel with scour hole. A vast amount of literature is available on the topic of scour around bridge piers. Several investigators have studied effectiveness of devices as such as Caissons and Footings, Vanes, Piles, Collar etc. for control of scour around bridge piers. Very little effort has been made so far however to study the temporal variation of scour around bridge pier with such devices and no literature is available on the effect of riprap on the depth of scour in the wake region of bridge piers. In the present work, experimental investigation is made to study the effect of riprap on bridge upstream on scour reduction in wake region. Temporal variation of scour in wake region of pier and with riprap on the upstream is also studied. From the present study it was observed that there is reduction in scour depth when the riprap is provided on upstream of the bridge pier. The scour in the wake region is also observed to reduce due to riprap on pier upstream. The amount of scour depth reduction depends upon the extent of riprap with respect of the axis of bridge pier perpendicular to flow. The maximum reduction in scour depth is observed while the riprap is extended up to a length of b/0.25 from the centre line of pier, where b represents the pier diameter. The models proposed by Oliveto and Hager (2002 and 2005) and Kothyari et al. (2007) for the temporal variation of scour around bridge piers is applied after approximate modification to the case of scour around the bridge pier with riprap around it. The results produced by the modified algorithm compared well with the experimental observations.