Assessment of Geomorphic Characteristics and Analysis at the Existing Bridge Site in Puerto Rico

Abstract

The scour on bridges is considered one of the main reasons for bridge failures. Scour is the consequence of the erosion action that is caused by the water flow, which at the same time excavates and carries away material from the bed and banks of streams particularly on the piers and abutments of the bridges. There are three scour components that are added to obtain the total scour at a pier of the abutment, which are classified by long-term degradation of the river bed, contraction scours at the bridge, and local scour at the piers or abutments. Federal regulations require all proposed bridges to be designed for scour resistance and all existing bridges to be evaluated for scour vulnerability. Scour evaluations are typically based on the 100-year recurrence of flood events. Bridges are determined to be unstable due to observed scour or lateral movement of the river. Various equations to evaluate scour are available, however many of them are considered conservative and lead to overestimation of the scour depths. Due to a possible overestimation, it is wise to consider an extremely important geomorphologic analysis of the river during the pre-construction analysis which can give a better understanding of how the river will act because most of the streams that highways or street cross are alluvial. Hurricane Maria hit Puerto Rico and led to a catastrophic flooding event to the magnitude of a 100-year recurrence flood and higher. During the hurricane, the natural conditions for which the bridges were designed were put to task. It is important to consider that the bridges analyzed in this geomorphic analysis were all affected by scours. An analysis of evaluated bridges against observed scour within Maria’s track was inspected and compared as a case study to auscultate any direct relation to the scour on the evaluation results. After the geomorphic analysis, the outcome showed that 88% of the bridges were meandering, and sinuous, and only 46% were classified as fair due to the river stability rating analysis which led to a 42% poor classification, and only 12% as good classification on the geomorphologic analysis. The results of this study could give an impact on the Puerto Rico Bridge Program to take into consideration when planning a road that it is crossing an alluvial stream to consider a geomorphic analysis prior to the design to prevent possible scour occurring on the bridge, which shows that in Puerto Rico there are 495 scour critical bridges, all requiring flood monitoring and, consequently, greater resources. Key Terms – Bridge Scour, Geomorphologic Analysis, Stability Rating Analysis.