Local Scour Downstream of Grade Control Structures in Urban Stream Restoration

Abstract

Urban streams need restoration structures, such as sills, in order to prevent excessive erosion of the bed, which could undermine the stability of other structures, such as bridge piers or embankments. An interesting example of the restoration process performed on an urban stream is represented by that performed on the Tevere River in Rome. This process started in 1870, as soon as Rome became the capital of the Kingdom of Italy, and consisted in building high embankments along the urban reach of the Tevere, in order to contain floods. As a matter of fact, until 1870 no important works have been done on the Tevere, in order to prevent flood hazards, although important floods occurred 20 times from the 13 to 19 century (approximately three important floods every century) [1]. The restoration works lasted approximately sixty years; in 1930 the construction of the embankments along the urban stretch of the river, together with the implementation of other important works for the protection of the reclaimed lands in the proximity of the mouth of the river, was fully completed. As a consequence of the construction of the embankments and their quays, the erosion of the bed started to increase dramatically. Visentini [2] estimated that during the twelve years 1930-1942 the average stream bottom subsided 0.81 m. Later Torzilli et al. [3] showed that from 1942 to 1961, the erosion process continued without slowing down. The stability of ancient bridges was in danger and urgent measures had to be adopted to control the erosion process. In particular it was decided that several sills were to be positioned along the urban stretch of the river. As a consequence, ten years after building the first sill, in 1976, it was noted that the erosion process was not only arrested but even reversed [4]. The positive effects of the sills were however counterbalanced by local scour, which occurred downstream of the sills and which could affect the stability of both the structure itself and other structures in the vicinity (bridge piers, embankments, etc.). Some of the sills had to be removed and located in different cross sections of the river. It clearly appeared that the proper design and position of the sill, as is the case for other grade control structures as well, must account for the scouring process, which is determined by the flow over the sill structure and can seriously influence the stability of the latter.