Flow-altering countermeasures against local scour at bridge piers

The ability to protect bridge piers and abutments against scour is critical to bridge safety.\nExcessive pier scour can cause high maintenance costs or even bridge collapse resulting in the\ninterruption of traffic and possibly death.\nUp to now, various types of countermeasures have been recommended to reduce scouring\naround pier foundations. The countermeasures against pier scour can be broadly classified into\ntwo categories: (1) flow-altering and (2) bed-armouring countermeasures.\nThis thesis focuses on the first category of countermeasures. Based on a comprehensive\nreview of the up-to-date studies on various types of flow-altering countermeasures, these can be\nclassified into four main groups based on their shapes and performances: (1) openings through\npiers, (2) pier attachments, (3) bed attachments and (4) other devices. After evaluation and\nconsidering all the countermeasures, a few countermeasures were recommended for further\ninvestigations. Among them, five countermeasures were selected for analysis in the present\nthesis. They are transverse sacrificial piles, collar, threading, pier slot and bed-sill.\nThe first part of the experiments, were performed in clear-water scour condition, near\nthreshold condition of sediment movement. In this part, countermeasures were tested\nindividually. Afterwards, countermeasures that showed appropriate efficiency in scour depth\nreduction were combined one by one. Results of this part showed that a collar, pier slot and\nsacrificial piles may show efficiency of about 30% in scour depth reduction; however, threading\ncannot be assumed as a reliable pier scour countermeasure. Results show that the blockage ratio\nof sacrificial piles is an important parameter, since by increasing the number of piles from three\nto five, the efficiency reduces. In this part also optimum size and location of a pier slot were\ncriticised and best configuration based on previous and present studies was proposed. In fact, the\nbest configuration can be suggested as a pier slot half as long as the flow depth plus maximum\nscour depth. In this case, the pier slot near the water surface may not significantly increase the\nefficiency. A combination of sacrificial piles with a collar and sacrificial piles with a slot showed\nthat they cannot reduce the scour depth significantly with respect to each individual\ncountermeasure. In particular, a combination of a collar and sacrificial piles increases the rate of scouring with respect to a single collar. At the end of this part, a combination of a bed-sill and\ncut-collar was selected as the best configuration. Two forms of this countermeasure were\nproposed for further studies. In the first proposed countermeasure, the use of a sill as wide as the\nchannel width was recommended. This form, although reliable to prevent souring at the upstream\npart of the bed-sill, may not be applicable for some practical purposes. Therefore, in another\nattempt, a shorter bed-sill was also proposed, protected with two side-walls, in order to reduce the\nrisk of scour intrusion in front of the bed-sill. These two proposed countermeasures showed\nefficiencies of 68.5% and 64.5% in scour depth reduction, however owing to the use of a border\naround the collar rim, the scour did not reach the pier body.\nIn next part, the later form of proposed countermeasure that consists of a shorter bed-sill\nand seems to be more practical, was evaluated in unsteady flow tests with the peak flow intensity\nslightly greater than the threshold for the inception of sediment movement. In this section of the\nthesis, circular and round nose and tail rectangular piers were tested in both configurations of\nsingle and two in-line piers. The results of unsteady flow tests also show that the proposed\ncombined countermeasure can protect pier foundation during a flood event. Specifically, it seems\nthat the proposed countermeasure is more effective when it is applied to round nose and tail\nrectangular piers than circular ones